Method of, and Apparatus for, Measuring the Pressure of a Gas

ES 2 467 697 T5

DESCRIPTION

Method and apparatus for measuring the pressure of a gas.

The present invention relates to an apparatus for measuring the pressure of a gas. More particularly, the present invention relates to an apparatus for measuring the pressure of a gas using a piezoelectric oscillator.

The apparatus described herein is particularly applicable to systems where relatively high pressure fluids (for example about 10 bar or more) may be present, such as for example gas supply from high pressure cylinders or manufacturing plants. using high pressure gases. The present invention is particularly concerned with clean gases, ie gases with little or no impurities or contaminants, such as water vapor or dust.

A compressed gas cylinder is a pressurized container designed to contain gases at high pressures, that is, at pressures significantly greater than atmospheric pressure. Compressed gas cylinders are used in a wide variety of markets, from the general low cost industrial market through the medical market to higher cost applications such as electronics manufacturing using specific high purity, corrosive gases, toxic or pyrophoric. Commonly pressurized gas containers comprise steel, aluminum or composites and are capable of storing compressed, liquefied or dissolved gases with a maximum fill pressure up to 450 bar gauge for most gases, and up to 900 bar for gases such as hydrogen and helium.

The present invention is particularly applicable to permanent gases. Permanent gases are gases that cannot be liquefied by pressure alone, and for example can be supplied in gas cylinders at pressures up to 450 bar gauge pressure. Examples are argon and nitrogen. However, this should be taken as limiting and the term gas can be considered to encompass a wider range of gases, for example a permanent gas and a vapor of a liquefied gas.

Liquefied gas vapors are present on top of the liquid in a compressed gas cylinder. Gases that liquefy under pressure when compressed to fill in a cylinder are not permanent gases and are more precisely described as liquefied gases under pressure or as vapors of liquefied gases. As an example, nitrous oxide is supplied in a cylinder in liquid form, with an equilibrium vapor pressure of 44.4 bar gauge at 15 ° C. Such vapors are not permanent or true gases as they are liquefiable under pressure or temperature around ambient conditions.

In order to efficiently and controllably dispense gases from a gas cylinder or other pressurized container, a regulator is needed. The regulator can regulate the flow of the gas in such a way that the gas is dispensed at a constant or variable pressure by the user.

Measurement of pressure in such systems is well known in the art and there are a variety of devices that work to measure pressure. The most conventional type uses an elastic diaphragm equipped with strain gauge elements. Another commonly used pressure gauge is a Bourdon gauge. Such a meter comprises a thin-walled, closed-ended flattened tube which is connected at the hollow end to a fixed pipe containing the pressure of the fluid to be measured. An increase in pressure causes the closed end of the tube to arc.

While these types of pressure gauges are relatively low in cost, they tend to be relatively large in size, and have a mechanical structure that is relatively complex and expensive to make. Additionally, such meters comprise delicate components that make them vulnerable to damage from environmental factors, such as exposure to high pressure.

For example, a conventional pressure gauge designed to operate reliably at pressure between 0 - 5 bar will be irreparably damaged if exposed to significantly higher pressures such as 200 bar. If this occurs, the meter will need replacement. Also, the meter can fail dangerously and can leak. This is a particular matter if flammable or combustible gases are present.

A situation where such a meter could be inadvertently exposed to excessively high pressures is known as creep. Consider an arrangement whereby a pressure gauge is provided at the outlet of a high pressure regulator of a high pressure gas cylinder, and the output is switched off. In this case, the gas cylinder can be, for example, at 300 bar internal pressure. When left for a period of time, even a small gas leak through the valve seat of the regulator can lead to pressures between the regulator and the closed outlet that are close to and possibly equal to the internal pressure of the gas cylinder. Such pressures can damage a conventional pressure gauge and become irreparable.

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As another example, consider a 300 bar fixed pressure regulator having an inlet connected, through a high pressure isolation valve, to a high pressure gas cylinder. The regulator outlet connects to a low pressure gauge. Such fixed pressure arrangements are configured to provide a constant outlet pressure of, for example, 5 bar. However, when the high pressure isolation valve is first opened, the pressure will briefly pulse to a much higher value before the regulator diaphragm is able to adjust to regulate the pressure. This brief pulse of high pressure can damage the gauge.

An alternative type of device used to measure the physical properties of gases is a piezoelectric device such as a quartz crystal. Quartz crystals show piezoelectric behavior, that is, the application of voltage to them results in a slight shrinkage or stretching of the solid, and vice versa.

The document A Precise And Robust Quartz Sensor Based On Tuning Fork Technology For (SF6) - Gas Density Control by Zeisel et al, Sensors and Actuators 80 (2000) 233-236 describes an arrangement whereby a quartz crystal sensor is used to measure the density of SF6 gas in high and medium voltage electrical equipment. Measurement of the density of SF6 gas is critical to the safety of the appliance. Therefore, this description does not concern pressure measurement.

US 4,644,796 describes a method and apparatus for measuring fluid pressure using a quartz crystal oscillator housed within a variable volume housing comprising an arrangement of bellows. The internal volume of the housing varies due to compression / expansion of the bellows by the pressure of the external fluid. Consequently, the density of the fluid within the housing varies as the internal volume of the housing varies. The density within the housing can be measured using a quartz crystal oscillator. However, the quartz crystal oscillator is not in contact with the fluid being measured and instead indirectly measures the gas pressure by changes in the internal volume of the housing.

US-A-4,734,609 describes a gas density transducer that compares the resonant frequency of a reference tuning fork enclosed crystal oscillator with the resonant frequency of a sensing tuning fork crystal oscillator exposed to surrounding gas. The oscillation frequency of the exposed detector crystal oscillator varies according to the gas density. The frequency of the detector oscillator is compared to the frequency of the reference oscillator to determine the gas density.

US-A-5,471,882 describes a pressure transducer assembly for measuring fluid pressure. The transducer assembly includes a thicknessshear mode resonator pressure sensor and a thickness shear mode resonator temperature sensor for pressure sensor temperature compensation. Document EP-A-0273649 describes pressure sensing means in thickness shear mode and a temperature sensor. The apparatus can work to compensate for temperature gradients.

According to a first aspect of the present invention, there is provided a pressure gauge according to claim 1.

By providing such a pressure gauge, an accurate over pressure proof gauge can be provided. The piezoelectric oscillator is a solid-state device that is resistant to high pressures, sudden changes in pressure, or other environmental factors. This allows the piezoelectric oscillator to be entirely submerged in the gas and to be invulnerable to creep or creep or other over-pressure situations. This is unlike conventional gauges (such as a Bourdon gauge) which require a pressure differential in order to function and which is permanently damaged by over-pressure situations.

In one arrangement, the sensor assembly further comprises a temperature sensor for measuring the temperature of the gas within said housing.

In one arrangement, the sensor assembly comprises a driver circuit for driving said piezoelectric oscillator at said resonant frequency.

In one embodiment, the sensor assembly comprises one or more of: a driver circuit, a processor, and a power source.

In one embodiment, the driver circuit comprises a Darlington pair arranged in a feedback configuration from a common emitter amplifier.

In one embodiment, said piezoelectric oscillator comprises a quartz crystal oscillator.

In one embodiment, the quartz crystal comprises at least one spike. In a variation, the quartz crystal comprises a pair of flat prongs.

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In one embodiment, the quartz crystal is AT cut or SC cut.

In one variation, the surface of the quartz crystal is exposed directly to the gas.

The sensor assembly comprises a driver circuit. In a variation, the sensor assembly comprises a driver circuit comprising a Darlington pair arranged in a feedback configuration from a common emitter amplifier.

In one embodiment, the sensor assembly comprises a power source. In one arrangement, the power source comprises a lithium ion battery.

The sensor assembly comprises a processor.

According to a second aspect of the present invention, there is provided a pressure reducing device comprising the pressure gauge of the second aspect.

In one embodiment, the pressure reducing device is in the form of a pressure regulator.

In one embodiment, the pressure reducing device is in the form of a valve or a valve with an integrated pressure regulator.

In one embodiment, the pressure regulator has a pressure range between 0 to 5 bar.

In one embodiment, the pressure regulator is an electronic pressure regulator and the pressure gauge can function to control the electronic pressure regulator.

In one embodiment, the electronic pressure regulator comprises a solenoid valve, the sensor assembly can function to control, in use, the solenoid valve.

In one embodiment, the pressure regulator has a pressure range between 0 to 5 bar.

Embodiments of the present invention will now be described in detail with reference to the accompanying drawings, in which:

Figure 1 is a schematic diagram of a gas cylinder and regulator assembly;

Figure 2 is a schematic diagram showing an upper part of a gas cylinder, a regulator, and a pressure gauge arrangement;

Figure 3 is a schematic diagram showing an upper part of a gas cylinder, a regulator and a pressure gauge arrangement according to an embodiment of the invention;

Figure 4 is a schematic diagram of a driver circuit for use with one embodiment of the present invention;

Figure 5 is a schematic diagram showing an alternative to the drive circuit for use with one embodiment of the present invention;

Figure 6 shows a graph of quartz crystal frequency (kHz) on the Y axis as a function of density (kg / m³) for several different gases;

Figure 7 shows a graph of the frequency change (in kHz) on the Y axis as a function of pressure (bar gauge) on the X axis for a quartz crystal oscillator immersed in Ferromax 15 (comprising 82.5% Ar, 15% CO2 and 2.5% O2) for low pressures;

Figure 8 shows a graph of the frequency change (in kHz) on the Y axis as a function of pressure (bar gauge) on the X axis for a quartz crystal oscillator immersed in Ferromax 15 (comprising 82.5% Ar, 15% CO2 and 2.5% O2) for high pressures;

Figure 9 is a flow chart illustrating a method;

Figure 10 shows a graph of the frequency behavior of different types of crystal;

Figure 11 is a schematic diagram showing an alternative sensor assembly comprising two quartz crystals; Y

Figure 12 is a schematic diagram showing a further alternative sensor assembly comprising two quartz crystals; Y

Figure 13 shows an alternative arrangement using a remote electronic data unit.

Figure 1 shows a schematic view of a gas cylinder, regulator, and pressure gauge assembly 10. The gas cylinder assembly 10 comprises a gas cylinder 100 having a gas cylinder body 102 and a valve 104. The gas cylinder body 102 comprises a generally cylindrical pressure container having a flat base 102a arranged to allow The gas cylinder assembly 10 stands unsupported on a flat surface.

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The gas cylinder body 102 is formed of steel, aluminum, and / or composite material and is adapted and arranged to withstand internal pressures up to about 900 bar gauge. An opening 106 is located at a proximal end of gas cylinder body 102 opposite base 102a and comprises a thread (not shown) adapted to receive valve 104.

The gas cylinder 100 defines a pressurized container having an internal volume V. Within the gas cylinder 100 any suitable fluid can be contained. However, the present embodiment is related to, but not exclusively limited to, purified permanent gases that are free from impurities such as dust and / or moisture. Some non-exhaustive examples of such gases can be: Oxygen, Nitrogen, Argon, Helium, Hydrogen, Methane, Nitrogen Trifluoride, Carbon Monoxide, Krypton or Neon.

Valve 104 comprises a housing 108, an outlet 110, a valve body 112, and a valve seat 114. Housing 108 comprises a complementary thread for engagement with opening 106 of gas cylinder body 102. The outlet 110 is adapted and arranged to allow the gas cylinder 100 to connect to other components in a gas assembly; for example, hoses, tubes or pressure valves or additional regulators. Valve 104, optionally, may comprise a VIPR (Valve with Integrated Pressure Regulator). In this situation, regulator 150 (described below) can optionally be omitted.

Valve body 112 can be adjusted axially relative to valve seat 114 by rotation of a grip 116 grip to selectively open or close outlet 110. In other words, movement of valve body 112 toward or away The valve seat 112 selectively controls the area of ​​the communication conduit between the interior of the gas cylinder body 102 and the outlet 110. This, in turn, controls the flow of gas from within the gas cylinder assembly 100 to the external environment.

Downstream of outlet 110 is a regulator 150. Regulator 150 has inlet 152 and outlet 154. Inlet 152 of regulator 150 connects to inlet tube 156 that provides a communication path between outlet 110 of cylinder gas 100 and regulator 150. Inlet 152 of regulator 150 is arranged to receive high pressure gas from outlet 110 of gas cylinder 100. This can be any suitable pressure; however, generally, the pressure of the gas exiting outlet 110 will be greater than 20 bar and more likely to be in the region of 100-900 bar.

Outlet 154 connects to outlet tube 158. A coupling 160 is located at the distal end of outlet tube 158 and is adapted for connection to additional tubes or devices (not shown) for which gas is needed.

A gauge arrangement 200 is located in communication with outlet tube 158 between outlet 154 and coupling 160. Gauge arrangement 200 is located immediately downstream of regulator 150 and is arranged to determine gas pressure downstream of the regulator. 150.

Regulator 150 and gauge arrangement 200 are shown in more detail in Figure 2.

In this embodiment, regulator 150 comprises a one-diaphragm regulator. However, one skilled in the art will be readily aware of variations that could be used with the present invention; for example, a two-diaphragm regulator or other arrangement.

Regulator 150 comprises a valve region 162 in communication with inlet 152 and outlet 154. Valve region 162 comprises a valve stem 164 located adjacent a valve seat 166. The stem valve 164 is connected to a diaphragm 168 which is configured to allow translational movement of the stem valve 164 toward and away from the valve seat 166 to respectively close and open an aperture 170 therebetween.

Diaphragm 168 is resiliently biased by a spring 172 located around a stem 174. A grippy handle 176 is provided to allow a user to adjust the biasing force of spring 172, thereby moving the position of diaphragm 168 , and, as a result, adjusting the balance spacing between the valve stem 164 and the valve seat 166. This allows adjustment of the dimensions of the opening 170 through which the high pressure gas flow from the outlet 110 can pass.

Regulator 150 can function to receive gas from outlet 110 at full cylinder pressure (eg 100 bar), but to deliver gas at substantially constant fixed low pressure (eg 5 bar) to outlet 154. This is accomplished by a feedback mechanism whereby gas pressure downstream of opening 170 can function to act on diaphragm 168 in opposition to the biasing force of spring 172.

Therefore, if the gas pressure in the region adjacent to the diaphragm 168 exceeds the specified level, the diaphragm 168 can function to move upward (relative in Figure 2). As a result, the valve stem 164

ES 2 467 697 T5 moves closer to valve seat 166, reducing the size of opening 170, and consequently restricting gas flow from inlet 152 to outlet 154.

Gauge arrangement 200 comprises a housing 202 and a sensor assembly 204. Housing 202 can comprise any suitable material; for example, steel, aluminum or composites. The housing has an interior 206 that is in communication with the interior of the outlet tube 158 through a short feed tube 208. Consequently, the interior 206 of the housing 202 is at the same pressure as the interior of the outlet tube 158 . During use, the housing 202 is generally sealed and isolated from the external atmosphere.

Alternatively, housing 202 could be provided as part of outlet tube 158. For example, a portion of outlet tube 158 could be widened to house sensor assembly 204. Alternatively, within tube 158 only part of the tube assembly may be located. sensor 204, and the rest being located outside or spaced from it.

Additionally, housing 202 may form an integral part of regulator 150. For example, sensor assembly 204 may be located entirely within output 154 of regulator 150. One of ordinary skill in the art will be readily aware of variations and alternatives that fall within the scope of scope of the present invention.

Sensor assembly 204 comprises a quartz crystal oscillator 210 connected to a driver circuit 212, a temperature sensor 214, and a battery 216. These components are located within housing 202.

The drive circuit 212 and the quartz crystal oscillator 210 will be described in detail below with reference to Figures 4 and 5. The temperature sensor 214 comprises a thermistor. Any suitable thermistor can be used. A high precision thermistor is not needed. For example, a precision of 0.5 ° C is suitable for this embodiment. Consequently, cheap small components can be used. However, in certain circumstances the temperature sensor 214 may be omitted. For example, in situations where the temperature is likely to be known well (for example at room temperature) or if the accuracy of the temperature measurement is not critical to the application. (for example the temperature can be assumed to be within a particular range).

In this embodiment, the quartz crystal oscillator 210 is located in communication with the gas from the high pressure gas source. In other words, the quartz crystal oscillator 210 is in contact with and exposed to the gas from the gas source. A processor 230 (shown in Figure 3) is also provided, either separately or as part of the drive circuit 212. This is described below.

In this arrangement, the quartz crystal oscillator 210 is constantly under isostatic pressure within the housing 202 of the gauge arrangement 200, and consequently does not experience a pressure gradient. In other words, the mechanical stress caused by the pressure difference between the external atmosphere and the internal components of the gauge arrangement 200 is expressed through the housing 202.

In Figure 2, the entire sensor assembly 204 is located within housing 202. Thus, quartz crystal oscillator 210, drive circuit 212 (and processor 230), and battery 216 are located within interior 210. of housing 202 of gauge arrangement 200. In other words, all components of sensor assembly 204 are completely immersed in the gas and are under the isostatic pressure of gas within housing 202.

The inventors have found that only a few components of sensor assembly 204 are sensitive to high pressure. In particular, larger components such as batteries can be susceptible to high pressures. However, lithium ion batteries have been found to perform particularly well at high pressures found within gas cylinder 100. Consequently, battery 216 comprises lithium ion cells. However, one of ordinary skill in the art could easily contemplate alternative suitable power supplies.

The location of the sensor assembly 204 entirely within the housing 202 provides additional flexibility in configuring the regulators 150. In particular, the location of relatively fragile electronic components entirely within the strong metal or composite walls of the housing 202 provides considerable protection. against accidental or environmental damage. This is particularly important, for example, in storage areas or stations, where gas cylinders 100 comprising regulators 150 are located adjacent to other gas cylinders 100, heavy machinery or rough surfaces.

Additionally, the internal location of sensor assembly 204 protects these components from environmental conditions such as salt, water, and other contaminants. This would allow, for example, the use of a high impedance circuit that is highly sensitive to salt and water damage as part of the sensor assembly 204.

The benefits of the internal location of the sensor assembly 204 are unique to solid-state sensor devices such as the quartz crystal oscillator 210. For example, a conventional pressure sensor such as a Bourdon meter cannot be located in this manner. . While a glass-based sensor can operate fully submerged in gas at constant pressure, a conventional pressure sensor is not capable of measuring isostatic pressure.

ES 2 467 697 T5 and requires a pressure gradient to function. Consequently, a conventional manometer must be located between the high pressure to be measured and the atmosphere. This increases the risk of damage to the external components of the pressure gauge.

An embodiment of the invention is shown in Figure 3. Features of the embodiment shown in Figure 3 that are common with Figure 2 are assigned the same reference numerals and will not be described again here.

In the embodiment of Figure 3, the regulator 250 differs from the regulator 150 of Figure 2 in that the regulator 250 is arranged to provide automatic control of the gas from the outlet 154 by means of a solenoid valve 252. The solenoid valve 252 comprises an armature 254 that is movable in response to an electrical current through the coils (not shown) of solenoid valve 252. The armature 254 is movable to open or close the stem valve 164, and consequently the opening 170.

The solenoid valve 252 shown in Figure 3 is in the normally open state. In other words, in the absence of an electrical current through the solenoid valve 252, the armature 254 is in an extended position such that the stem valve 164 is open, that is, the opening 170 is open. If a current is applied to solenoid valve 252, armature 254 will retract and poppet valve 164 will close.

One of ordinary skill in the art will be readily aware of alternative solenoid variations that could be used with the present invention. For example, instead of acting directly on stem valve 164, armature 254 could act directly on a diaphragm such as diaphragm 168 shown in Figure 2. Alternatively, armature 254 could control flow through a narrow conduit in communication with outlet 154 in order to regulate movement of diaphragm 168. Such an arrangement is known as a diaphragm-piloted valve. Alternatively, the stem valve could be eliminated and a diaphragm could be the valve member that directly controls the flow of gas from inlet 152 to outlet 154.

The embodiment comprises a gauge arrangement 260. Components of gauge arrangement 260 common with gauge arrangement 200 are assigned the same reference numerals for clarity.

The gauge arrangement 260 is substantially similar to the gauge arrangement 200 of the first embodiment. However, the gauge arrangement 260 further comprises an electronic solenoid driver 262 connected to the solenoid valve 252 and to the sensor assembly 204. The solenoid driver 262 is arranged to receive a signal from the sensor assembly 204 and to control the solenoid valve. 252 in response to that signal. Consequently, the gauge arrangement 260 can function to control the flow of gas through the regulator 250. In other words, the gauge arrangement 260 and the solenoid valve 252 form a feedback loop that allows precise and remote pressure regulation, downstream of exit 154. This may be particularly applicable to situations where remote pressure flow management is needed, for example in automated applications such as welding machines.

Solenoid driver 262 may comprise any suitable driver circuit for controlling solenoid valve 252. A suitable circuit may be an op amp arrangement having input from sensor assembly 204 to the negative terminal of the op amp. Consequently, a variable resistor could be connected to the positive terminal. The variable resistor can be arranged to provide a constant reference level and act as a comparator. The reference level can be varied automatically or manually.

An input from sensor assembly 204 to solenoid driver 262 will cause solenoid valve 252 to operate. For example, if the input signal from sensor assembly 204 (or alternatively processor 230) exceeds a particular threshold level , solenoid driver 262 may energize solenoid valve 252. Solenoid valve 252 may be controlled in a digital manner (ie active or inactive) in which a DC voltage is varied between a maximum and a minimum value. Alternatively, the DC voltage from solenoid driver 262 can be continuously variable to precisely adjust the position of stem valve 164.

Additionally or alternatively, solenoid driver 262 may control solenoid valve 252 via a DC output comprising an AC component. Since the extension of armature 254 from solenoid valve 252 is roughly proportional to the applied current, this causes armature 254 to oscillate from solenoid valve 252. Such oscillations mitigate static friction of armature 254, that is, they help prevent the 254 frame is stuck or jammed.

Alternatively, other control arrangements, such as FETs, processors, or ASICs as appropriate may be used to control the operation of solenoid valve 252. Additionally, solenoid valve 252 may operate in digital (i.e. active / inactive) or analog (i.e. say continuously variable) to allow precise movement of the stem valve 164 or the like.

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The main components of the gauge arrangement 260 are shown separately from the regulator 250 in Figure 3. In such a situation, the regulator 250 can be remotely controlled via wireless communication between the sensor assembly 204 and the solenoid driver 252. However, this need not be the case. For example, gauge arrangement 260 could be fully integrated into regulator 250 and form an integral part thereof. Thus, the gauge 260 and regulator 250 arrangement can form a self-regulating unitary component that could be positioned at the outlet to a gas source and that could automatically and remotely control the pressure of the gas flowing therefrom.

Sensor assembly 204 will now be described in more detail with reference to Figures 4 and 5. Quartz crystal oscillator 210 comprises a small thin section of cut quartz. Quartz shows piezoelectric behavior, that is, the application of a voltage across the crystal causes the crystal to change shape, generating a mechanical force. On the contrary, a mechanical force applied to the crystal produces an electrical charge.

Two parallel surfaces of the quartz crystal oscillator 210 are metallized in order to provide electrical connections through the raw crystal. When a voltage is applied across the glass via the metal contacts, the glass changes shape. With the application of an alternating voltage to the crystal, the crystal can be made to oscillate.

The physical size and thickness of the quartz crystal determine the resonant or characteristic frequency of the quartz crystal. Indeed, the resonant or characteristic frequency of crystal 210 is inversely proportional to the physical thickness between the two metallized surfaces. Quartz crystal oscillators are known in the art and thus the structure of the quartz crystal oscillator 210 will not be described further here.

Additionally, the resonant frequency of vibration of a quartz crystal will vary depending on the environment in which the crystal is located. In a vacuum, the crystal will have a particular frequency. However, this frequency will change in different environments. For example, in a fluid, the vibration of the crystal will be dampened by the surrounding molecules and this will affect the resonant frequency and energy required to oscillate the crystal at a given amplitude.

Additionally, the deposition of surrounding materials on the crystal will affect the mass of the vibrating crystal, altering the resonant frequency. This forms the basis for commonly used selective gas analyzers where an absorbent layer forms on the glass and increases in mass as the gas is absorbed.

However, in the present case, no coating is applied to the quartz crystal oscillator 210. Adsorption or deposition of material on the quartz crystal oscillator 210 is certainly not desirable in the present case since the accuracy of the measurement may be affected.

The quartz crystal oscillator 210 of the present embodiment is tuned fork shaped and comprises a pair of tines 210a (Figure 4) approximately 5 mm long arranged to oscillate at a resonant frequency of 32.768 kHz. The barbs 210a are formed in the flat section of quartz. The fork tines 210a normally oscillate in their fundamental mode, in which they move synchronously toward and away from each other at the resonant frequency.

Fused (or non-crystalline) quartz has a very low temperature-dependent coefficient of expansion and a low coefficient of elasticity. This reduces the dependence of the fundamental frequency on temperature and, as will be shown, the effects of temperature are minimal.

Additionally, it is desirable to use AT cut or SC cut quartz. In other words, the flat section of quartz is cut at particular angles so that the temperature coefficient of the oscillation frequency can be set to be parabolic with a broad peak around room temperature. Therefore, the crystal oscillator can be arranged such that the slope at the top of the peak is precisely zero.

Such crystals are commonly available at a relatively low cost. Unlike most quartz crystal oscillators that are used in a vacuum, in the present embodiment the quartz crystal oscillator 210 is exposed to the gas under pressure in the housing 202.

The drive circuit 212 for driving the quartz crystal oscillator 210 is shown in Figure 4. The drive circuit 212 must meet several specific criteria. First, the quartz crystal oscillator 210 of the present invention can be exposed to a range of gas pressures; Potentially, pressures can range from atmospheric pressure (when gas cylinder 100 is empty) to around 900 bar gauge if the gas cylinder contains a pressurized gas such as hydrogen. Thus, the quartz crystal oscillator 210 is required to operate (and restart after a period of non-use) under a wide range of pressures.

Consequently, the quality factor (Q) of the quartz crystal oscillator 210 will vary considerably during use. The Q factor is a dimensionless parameter related to the damping rate of an oscillator or

ES 2 467 697 T5 resonator. Equivalently, you can characterize the bandwidth of a resonator with respect to its center frequency.

In general, the higher the Q factor of an oscillator, the lower the rate of energy loss relative to the stored energy of the oscillator. In other words, the oscillations of a high Q factor oscillator decrease in amplitude more slowly in the absence of an external force. Sinusoidally driven resonators that have the highest Q factors resonate with higher amplitudes at the resonant frequency but have a lower frequency bandwidth around that frequency for which they resonate.

The drive circuit 212 must be able to drive the quartz crystal oscillator 210 despite the changing Q factor. As the pressure in the gas cylinder 100 increases, the oscillation of the quartz crystal oscillator 210 will become more and more dampened, and the Q factor will fall. The falling Q factor requires that a higher gain be provided by an amplifier in drive circuit 212. However, if too high amplification is provided to drive circuit 212, the response from quartz crystal oscillator 210 can become difficult to distinguish. In this case, the driver circuit 212 may simply oscillate at an unrelated frequency, or at the frequency in a non-fundamental mode of the quartz crystal oscillator 210.

As a further limitation, the driver circuit 212 must be low power to be able to run on small low power batteries for a long time with or without supplemental power, such as photovoltaic cells.

The drive circuit 212 will now be described with reference to Figure 4. In order to drive the quartz crystal oscillator 210, the drive circuit 212 essentially takes a voltage signal from the quartz crystal oscillator 210, amplifies it, and inputs that signal. signal again in quartz crystal oscillator 210. The fundamental resonant frequency of quartz crystal oscillator 210 is essentially a function of the rate of expansion and contraction of quartz. This is generally determined by the cut and size of the crystal.

However, external factors also affect the resonant frequency. When the energy of the generated output frequencies matches the losses in the circuit, an oscillation can be maintained. Drive circuit 212 is arranged to detect and maintain this frequency of oscillation. The frequency can then be measured by processor 230, can be used to calculate the appropriate gas property needed by the user, and, if necessary, can be derived for a suitable means of exposure (as will be described later).

Drive circuit 212 is powered by a 6V battery 216. Battery 216, in this embodiment, comprises a lithium ion battery. However, alternative power sources will be readily apparent to the person skilled in the art; for example, other types of battery, rechargeable and non-rechargeable, and a solar cell arrangement.

The driver circuit 212 further comprises a Darlington pair Common Emitter amplifier 218. The Darlington pair comprises a composite structure consisting of two NPN bipolar transistors D1 and D2 configured in such a way that the current amplified by a first transistor is further amplified by the second. This configuration allows for a higher current gain when compared to each transistor taken separately. Alternatively, PNP bipolar transistors can be used.

Darlington pair 218 is arranged in a feedback configuration from a single transistor Common Emitter 220 (T1) amplifier. A junction NPN bipolar transistor is shown in Figure 5. However, one skilled in the art will be aware of alternative transistor arrangements that can be used; for example, a junction PNP bipolar transistor or Metal Oxide Semiconductor Field Effect Transistors (MOSFETs).

As a variant, automatic gain control could be implemented in the feedback circuit between Darlington pair 218 and Common Emitter amplifier 220 (not shown). This can take the form of a potentiometer, variable resistor, or other suitable component located in place of, for example, the rightmost 22k resistor shown in Figure 4.

Automatic gain control allows compensation for changes in Q factor with pressure and changes in supply voltage (for example, under low battery conditions). Automatic gain control can be particularly applicable for low pressure applications.

The drive circuit 212 comprises a further NPN emitter follower transistor T2 which acts as a buffer amplifier 222. The buffer amplifier 222 is arranged to function as a buffer between the circuit and the external environment. However, this feature is optional and may not be necessary; for example, a FET could be connected directly to drive circuit 212.

ES 2 467 697 T5

A capacitor 224 is placed in series with the quartz crystal oscillator 210. The capacitor 224, in this example, has a value of 100 pF and allows the drive circuit 212 to drive the quartz crystal oscillator 210 in situations where the glass has been contaminated, for example by salts or other deposited materials.

Additionally, the drive circuit 212 can be optimized for fast startup of the quartz crystal oscillator 210. To achieve this, an additional resistor and an additional capacitor can be connected between the base of the transistor D1 and ground. These components can comprise, for example, a 10 MO resistor and a 10 nF capacitor.

An alternative drive circuit 240 will now be described with reference to Figure 5. The drive circuit shown in Figure 6 is configured similarly to a Pierce oscillator. Pierce oscillators are known from IC digital clock oscillators. In essence, the driver circuit 240 comprises a single digital inverter (in the form of a transistor) T, three resistors R1, R2, and Rs, two capacitors C1, C2, and the quartz crystal oscillator 210.

In this arrangement, the quartz crystal oscillator 210 functions as a highly selective filter element. Resistor R1 acts as a load resistor for transistor T. Resistor R2 acts as a feedback resistor, biasing inverter T in its linear region of operation. This effectively enables the T inverter to function as a high gain inverting amplifier. To limit the gain and to damp unwanted oscillations in the circuit, another resistor Rs is used between the output of the inverter T and the quartz crystal oscillator 210.

The quartz crystal oscillator 210, in combination with C1 and C2, forms a Pi network band-pass filter. This allows for a 180 degree phase shift and voltage gain from output to input of approximately the resonant frequency of the quartz crystal oscillator. The driver circuit 240 described above is reliable and inexpensive to manufacture since it comprises relatively few components.

The gain of the drive circuit 240 is generally less than that of the drive circuit 212. Lower gain can make it more difficult to reset the quartz crystal oscillator 210 when the quartz crystal oscillator 210 is exposed to high pressures. However, in the present application, circuit 240 is particularly attractive due to a generally low pressure environment in which gauge arrangements 200, 260 are likely to be used.

As mentioned above, sensor assembly 204 may include a processor 230 that receives inputs from quartz crystal oscillator 210 and drive circuit 212. Processor 230 may comprise any suitable arrangement. Processor 230 may comprise a microprocessor or central processing unit (CPU), or may comprise an Application Specific Integrated Circuit (ASIC) or Field Programmable Gate Array (FPGA). Alternatively, processor 230 may simply be a collection of logic gates or another simple processor configured to perform the necessary computation required in the above-described embodiments.

When used with the quartz crystal oscillator 210, the processor 230 can be configured to measure the frequency f or the period of the signal from the drive circuit 212. This can be accomplished, for example, by counting oscillations at a fixed time, and converting that frequency to a density value that uses an algorithm or look-up table. This value is passed to processor 230.

Processor 230 can optionally be designed for mass production in order to be identical across gauge 200 arrangements, with different features in software and hardware enabled for different gases.

Additionally, processor 230 can also be configured to minimize power consumption by implementing standby or sleep modes that can encompass processor 230 and additional components such as driver circuit 212 and quartz crystal oscillator 210.

Multiple schemes can be implemented; for example, processor 230 may be on standby for 10 seconds out of every 11 seconds. Additionally, processor 230 can control quartz crystal oscillator 210 and drive circuit 212 such that these components are put on hold for most of the time, turning on only the most power-hungry components during¹Z second every 30 seconds.

Additionally, the gauge arrangement 200 may be connected to an antenna (not shown) for remote communication with, for example, a base station. This is mentioned later. In this case, the antenna can be located internally or externally of housing 202 and connected to sensor assembly 204 by means of a cable or equivalent connector. The antenna itself can be adapted and arranged to use some suitable communication protocol; For example, a non-exhaustive list may be RFID, Bluetooth, Infrared (IR), 802.11 wireless, frequency modulated (FM) broadcast, or a cellular network.

ES 2 467 697 T5

Alternatively, single wire communication can be implemented. Single-wire communication needs only one metal conductor to communicate - the circuit's 'return' path is provided by capacitive coupling through the air between communicating devices. One skilled in the art will be readily aware of alternatives to the antenna (and associated transmission hardware) that could be used with the embodiments mentioned herein.

However, remote communication is possible without the explicit need for an external antenna. For example, communication may be accomplished by means of acoustic transmission from within housing 202. Acoustic transmission may be effected by a transmitter located within housing 202. The transmitter may comprise, for example, a simple fixed frequency piezoelectric resonator.

A supplemental receiver is also needed and this component may be located remotely from the gauge arrangement 200 and may comprise hardware such as, for example, a phase locked circuit tone detector integrated with a microphone. That type of acoustic arrangement provides the advantage that no feed path is required (as is the case for an external antenna) and that all electronic components can be located entirely within the housing 202 of the gauge arrangement 200.

The theory and operation of the gauge arrangement 200 will now be described with reference to the

Figures 6 to 8.

The quartz crystal oscillator 210 has a resonant frequency that depends on the density of the fluid in which it is located. Exposure of an oscillating tuning fork type crystal oscillator to gas leads to a shift and damping of the crystal's resonant frequency (when compared to the crystal's resonant frequency in a vacuum). There are many reasons for this. While there is a damping effect of the gas on the crystal oscillations, the gas adheres to the vibrating tines 210a of the tuning fork crystal oscillator 210 which increases the mass of the oscillator. This leads to a reduction in the resonant frequency of the quartz crystal oscillator according to the movement of a one-sided, fixed, elastic beam:

Δω

Where ^ωθ is the relative change in resonant angular frequency, ρ is the gas density, t is the thickness of the quartz oscillator, ρq is the density of the quartz oscillator and w is the hairpin width, ci and c2 are geometrically dependent constants and θ is the thickness of the gas surface layer as defined by:

2)

Where η is the temperature-dependent viscosity of the gas.

The two parts of equation 1) relate to a) the additive mass of the gas in the tines of the quartz crystal oscillator 210 and to b) the shear forces arising in the outermost surface layer of the tines during the oscillation.

The equation can be rewritten in this way as a function of frequency and simplified to:

Where A =

3)

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Δ / = Λρ + Byfp + C and C is a compensation constant. is the natural resonant frequency of the crystal in vacuum.

The inventors have found that a suitably good approximation can be obtained by approximating:

ES 2 467 697 T5

4) Δ / ~ τ4ρ

Consequently, to a good approximation, the change in frequency is proportional to the change in the density of the gas to which the quartz crystal oscillator is exposed.

Figure 6 shows, for several different gas / gas mixtures, that the resonant frequency of the quartz crystal oscillator 210 varies linearly as a function of density.

In general, the sensitivity of the quartz crystal oscillator 210 is that a 5% change in frequency is seen with, for example, Oxygen gas (having a mass number of 32) at 250 bar when compared to atmospheric pressure. Such gas pressures and densities are typical of storage cylinders used for permanent gases, which are typically between 137 and 450 bar manometric for most gases, and up to 700 or 900 bar manometric for helium and hydrogen.

Additionally, the quartz crystal oscillator 210 is particularly suitable for use as a sensor for commercially supplied gases. First of all, to correctly perceive the density of a gas, it is necessary that the gas be free of dust and droplets of liquids, which is guaranteed with commercially supplied gases, but not with air or in general pressure monitoring situations. .

The aforementioned illustrates that the frequency response of the quartz crystal oscillator 210 is, to a good approximation, proportional to the density. However, in order to measure pressure, a relationship between pressure and density needs to be derived. This is determined from:

5) PV = nRT

Where P is the gas pressure, V is the volume of gas, n is the number of moles of gas, R is the gas constant, and T is the temperature. Following on:

6)

MW =

7) where MW is the molecular weight of gas and M is the mass of gas. Therefore, substituting V in equation 5) leads to:

_P~ P^RT

Consequently, for a known molecular weight of gas (or average molecular weight of gas in the case of a known mixture), the gas pressure can be accurately derived from the density of the gas and the temperature of the gas.

The above approximations assume that the compressibility of the gas, Z, is equal to one. In conventional arrangements, this approach is only held for low pressures in cases where a direct pressure measurement is made. At high pressures, the compressibility Z is not proportional to the gas pressure as expected in an Ideal gas. Therefore, a conventional manometer such as a Bourdon meter must be corrected for compressibility in order to correctly read the content - mass of gas - of a gas cylinder at high pressures. It was previously shown that quartz crystal oscillator 210 is Intrinsically corrected for Z compressibility when density is measured. But when measuring pressure at high pressure values, a quartz gauge must be corrected for Z.

Figures 7 and 8 illustrate the frequency response of the quartz crystal oscillator 210 as a function of pressure. Figure 7 shows a graph of the frequency change (in kHz) on the Y axis for the quartz crystal oscillator 210 as a function of the pressure (bar gages) on the X axis for pressures in the range 0-6 bar gages. . Figure 8 shows a graph of the frequency change (in kHz) on the Y axis for the quartz crystal oscillator 210 as a function of the pressure (bar gages) on the X axis for pressures in Interval 0.

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- 300 bar manometric. In both cases, the gas used was Ferromax 15, which comprises 82.5% Ar, 15% CO2 and 2.5% O2.

As illustrated in Figures 7 and 8, to a good approximation, the change in frequency Af of the quartz crystal oscillator 210 is linear with pressure over two orders of magnitude of pressure. Therefore, if the temperature and molecular weight of the gas are known, then the quartz crystal oscillator 210 can function as an accurate manometer.

As described above, temperature can be easily measured using inexpensive and widely available components, such as a thermistor. Furthermore, in the case of permanent gases supplied to consumers packed in gas cylinders, the molecular weight of the gas (or average molecular weight of a homogeneous gas mixture) is generally very well known.

Therefore, while the approach described above may be imprecise if the gas is not uniform - for example, if the gas is a non-uniform mixture such as a cylinder partially filled with fluid or a recently prepared and insufficiently mixed mixture of light gases and heavy, such a situation is unlikely to occur in most bottled gas applications.

Additionally, it is surprising that the quartz crystal oscillator 210 can operate in a pressure range between 0 to 300 bar gauge, while being accurate enough to accurately measure pressure values ​​two orders of magnitude lower than the upper limit of this range. . This property makes the quartz crystal oscillator 210 particularly suitable for use as a gauge as part of the gauge arrangement 200.

This is because the gauge arrangement 200 is capable of reliably and accurately measuring small pressure variations as can typically be measured in a low pressure application (eg between 0 and about 5 bar gauge) such as immediately downstream of the regulator 150.

The arrangement as described above is particularly suitable for measuring low pressures where there is a risk of high pressures occurring during use. Since quartz crystal oscillator 210 is a solid state component and can operate at pressures up to 900 bar, if an initial over-pressure condition occurs in outlet tube 158, sensor assembly 204 will not be affected. and it will continue to work as needed. In other words, the inventors have developed an accurate low pressure gauge that is entirely resistant to exposure to high pressures.

In contrast, a conventional manometer such as a Bourdon gauge will be permanently damaged and may fail if exposed to even a brief burst of high pressure gas, such as can occur during creep or creep conditions or when a gas cylinder is made run for the first time.

Additionally, the arrangement of the present invention allows pressure measurements with very high precision with a definition of parts per million. Together with the linear response of the 210 quartz crystal oscillator with density and / or pressure, the high precision allows even very light gases such as H2 and He to be accurately measured.

Also, if compressibility is taken into account, then the same gauge is capable of reading even higher pressures without any modification. In contrast, a conventional pressure gauge would only be suitable for a particular range of pressures and would have to be replaced in order to read a different range of pressures. A method is now described with reference to Figure 9.

Step 300: Initialize Measurement

In step 300, the measurement of the gas pressure downstream of the outlet 158 ​​is initialized. This can be activated, for example, by a user pressing a button on the outside of the gas cylinder 100. Alternatively, the measurement It can be initiated by means of a remote connection, for example a signal transmitted over a wireless network and received by the gauge arrangement 200 through an antenna.

Alternatively or additionally, the gauge arrangement 200 may be configured to be initialized remotely or on a timer. The method proceeds to step 302.

Stage 302: Boost the quartz crystal oscillator

Once initialized, drive circuit 212 is used to drive quartz crystal oscillator 210. During initialization, drive circuit 212 applies a noise random AC voltage across crystal 210. At least a portion of that voltage The random will be at a suitable frequency to make crystal 210 oscillate. Crystal 210 will then begin to oscillate in sync with that signal.

ES 2 467 697 T5

By means of the piezoelectric effect, the movement of the quartz crystal oscillator 210 will then generate a voltage in the resonant frequency band of the quartz crystal oscillator 210. The driver circuit 212 then amplifies the signal generated by the quartz crystal oscillator 210. , such that the signals generated in the frequency band of the quartz crystal resonator 202 dominate the output of the driver circuit 212. The narrow band of resonance of the quartz crystal filters out all unwanted frequencies and the drive circuit 212 then drives the quartz crystal oscillator 210 to the fundamental resonant frequency f. Once the quartz crystal oscillator 210 has stabilized at a particular resonant frequency, the method proceeds to step 304.

Step 304: Measure resonant frequency of quartz crystal oscillator

The resonant frequency f depends on the pressure conditions within the housing 202. In turn, the pressure conditions within the interior 206 of the housing 202 are representative of the pressure conditions downstream of the outlet 154 of the regulator 150.

In the present embodiment, the change in resonant frequency Δί is, to a good approximation, proportional in magnitude to the change in gas pressure inside 206 of housing 202 and will decrease with increasing pressure.

In order to make a measurement, the frequency of the quartz crystal oscillator 210 is measured over a period of about 1 s. This is to allow the reading to stabilize and for enough oscillations to be counted to determine an accurate measurement. Frequency measurement is carried out in processor 230. Processor 230 may also record the time, T1, when the measurement was started.

Once the frequency has been measured, the method continues to step 306.

Step 306: Measure gas temperature

In step 306, the temperature sensor 214 measures the temperature of the gas within the housing 202. This measurement is carried out for the purpose of calculating the pressure from the change in frequency measured in step 304.

The temperature measurement does not need to be particularly accurate. For example, if the temperature sensor 214 is accurate to 0.5 ° C, then this corresponds to an error of only about one part of six hundred (assuming normal atmospheric temperatures) in the absolute temperature value required for the pressure calculation. at step 308.

However, in certain circumstances the temperature sensor 214 may be omitted. For example, in situations where the temperature is likely to be known well (for example at room temperature) or if the accuracy of the temperature measurement is not critical to the application. (for example the temperature can be assumed to be within a particular range). In this case, the temperature determination in step 306 can be considered to be the assignment of a particular temperature value stored by the processor 230 and used in the pressure calculation in subsequent steps.

Step 308: Determine the gas outlet pressure

Once the frequency of the quartz crystal oscillator 210 has been successfully measured in step 304 and the temperature measured in step 306, the processor 230 then calculates the gas pressure within the interior 206 of the housing 202.

This is done using equation 8) above where the pressure P of the gas can be calculated directly from the density, temperature, and molecular weight of the gas in question. Therefore, knowing the resonant frequency as measured in step 304, the known temperature T of the gas in housing 202 measured in step 306, and the known molecular weight of the gas (or average molecular weight of a mixture of gases), an accurate pressure measurement can be made. The method then proceeds to step 310.

Step 310: Communicate and store results

Gas pressure is exposed in various ways. For example, a screen (not shown) connected to housing 202 or regulator 150 could display gas pressure downstream of outlet 154 of regulator 150. In the alternative, the pressure measurement could be communicated remotely to a base station. or to a meter located in an adjacent fixture as will be described later.

Once the gas pressure has been determined, this can also be recorded in an internal memory associated with the processor 230 of the gauge arrangement 200 for later retrieval. As an alternative

Still further ES 2 467 697 T5, the gas pressure at time Ti could be stored in a local memory of said processor 230 to generate a time record.

The method then proceeds to step 3i2.

Step 312: Turn off the sensor assembly

It is not necessary to keep the gauge arrangement 200 operational at all times. Rather, it is beneficial to reduce power consumption by turning off the gauge arrangement 200 when not in use. This prolongs the life of the 216 battery.

The configuration of the drive circuit 212 allows the quartz crystal oscillator 210 to be reset regardless of the gas pressure in the housing 202. Therefore, the gauge arrangement 200 can be disconnected as and when necessary in order to save battery power.

Variations from the above embodiments will be apparent to one of ordinary skill in the art. The precise configuration of hardware and software components may differ and still fall within the scope of the present invention. The person skilled in the art will be readily aware of alternative configurations that could be used.

For example, the embodiments described above have used a quartz crystal oscillator having a fundamental frequency of 32.768 kHz. However, crystals operating at alternative frequencies can be used. For example, quartz crystal oscillators operating at 60 kHz and 100 kHz can be used with the embodiments described above. A graph showing the frequency change with density for different crystals is shown in Figure 10. As a further example, a crystal oscillator operating at a frequency of 1.8 MHz could be used.

Higher frequency operation allows the frequency to be monitored more frequently because a shorter period of time is required to sample a given number of cycles. Additionally, higher frequency crystals allow a lower duty cycle to be used in a sleep mode than a crystal. By way of explanation, in most cases the crystal and drive circuit will be idle most of the time, only activated for a second or so when a measurement is needed. This can happen, for example, once in a minute. When using a higher frequency crystal, pressure can be measured faster. Therefore, the time in which the crystal is operational can be reduced. This can reduce power consumption and concomitantly improve battery life.

A further variation is described with reference to Figure 11. A sensor assembly 400 is shown in Figure 11. The sensor assembly 400 comprises a first quartz crystal oscillator 402 and a second quartz crystal oscillator 404. The first The quartz crystal oscillator 402 is driven by a driver circuit 408. The second quartz crystal oscillator 404 is driven by a driver circuit 410.

The first quartz crystal oscillator 402 and a second quartz crystal oscillator 404 differ in their sensitivity coefficients σ, where p

Where Af is the change in frequency of the quartz crystal oscillator 402, 404 and ρ is the pressure of the gas being measured. The first quartz crystal oscillator 402 may have a large sensitivity coefficient σ1, which provides a large change in frequency with pressure. However, such a glass may not be suitable for high pressure operation, where excessive damping (ie a loss of the Q factor) reduces the performance of such a glass. Therefore, the second quartz crystal oscillator 404 is provided which has a lower sensitivity coefficient σ2 (where σ1&gt; σ2) that enables high pressures to be reliably measured.

Another situation where having two crystals can be helpful is in the event that there is a danger that one or both crystals will be contaminated either permanently or temporarily. Here the use of two identical crystals is recommended. Contamination will affect both crystals, but due to their different position in the gas path this will almost always differ slightly.

In correct operation the two will give the same frequency. However, in the case of contamination, they will both indicate an incorrect frequency, but, because of their different levels of contamination, different incorrect frequencies: this discrepancy may be indicated to the user as a warning that the sensor assembly needs cleaning or replacement in the case of permanent contamination and that the pressure indication may in any case be inaccurate.

ES 2 467 697 T5

An electronic switch 412 may be provided that allows one of the quartz crystal oscillators 452, 454 to be selected, depending on whether a low or high pressure measurement is to be made. Such adaptability cannot be achieved with a conventional manometer such as a Bourdon gauge, which must be replaced with a different gauge to measure different pressure ranges.

Additionally, the above embodiments have been described by measuring the absolute frequency of a quartz crystal oscillator. However, in self-contained electronics incorporated in a regulator associated with a gas cylinder, it may be advantageous to measure the change in sensor frequency by comparing that frequency with a reference crystal of the same type but enclosed in a vacuum package. or pressure. The pressurized pack may contain gas with a selected density, gas under atmospheric conditions, or it may be open to the external atmosphere of the gas cylinder.

A suitable sensor assembly 450 is shown in Figure 12. Sensor assembly 450 comprises a first quartz crystal oscillator 452 and a second quartz crystal oscillator 454. The first quartz crystal oscillator 452 is a reference crystal. which is located within a vacuum sealed container 456. The first quartz crystal oscillator 452 is driven by a drive circuit 458.

The second quartz crystal oscillator 454 is a crystal similar to crystal 210 described in the earlier embodiments. The second quartz crystal oscillator 454 is exposed to the gas environment within the housing 202. The second quartz crystal oscillator 454 is driven by a driver circuit 460.

This comparison can be made using an electronic mixer circuit 464 that combines the two frequency signals and produces an output at a frequency equal to the difference between the two crystals. This arrangement makes it possible to cancel out small changes due, for example, to temperature.

In addition, the circuitry used in sensor assembly 204 can be simplified because only the frequency difference needs to be measured. Furthermore, this approach is particularly suitable for use with a high frequency (MHz) crystal oscillator, where it can be difficult to directly measure crystal frequency.

Additionally, all the electronics needed to measure and display density, mass, or mass flow need not be mounted on or within housing 202. For example, electronic functions could be divided between units permanently mounted on the cylinder and units mounted. at a station for customer use or temporarily mounted on the cylinder outlet such as the position normally used for a conventional flow meter.

An example of this arrangement is shown with reference to Figure 13. The arrangement comprises a gas cylinder assembly 50 comprising a gas cylinder 500, a regulator 502, and a pressure gauge arrangement 504. The gas cylinder 500, the regulator 502 and gauge arrangement 504 are substantially similar to gas cylinder 100, regulator 150, and gauge arrangement 200 as previously described with reference to prior embodiments.

In this embodiment, the gauge arrangement 504 comprises a quartz crystal oscillator and a drive circuit (not shown) similar to the quartz crystal oscillator 210 and the drive circuit 212 of the first embodiments. An antenna 506 is provided for communication by any suitable remote communication protocol; for example, Bluetooth, Infrared (IR), or RFID. Alternatively, single wire communication can be used.

As a further alternative, acoustic communication methods can be used. The advantage of such methods is that remote communication can be accomplished without the requirement for an external antenna 506.

A connecting tube 508 connects to the outlet of the gas cylinder 500. The connecting tube is terminated with a quick connection 510. The quick connection 510 allows connecting pipes or components to be easily and quickly connected and disconnected from the gas cylinder. gas 500.

A quick connect unit 550 is provided for connection to gas cylinder 500. A complementary quick connect connector 512 is provided for connection to connector 510. In addition, quick connect unit 550 is provided with a data unit 552. The data unit 552 comprises a display 554 and an antenna 556 for communication with the antenna 506 of the gauge arrangement 504. Display 554 may comprise, for example, an LCD, LED, or light-readable display to minimize power consumption and maximize display visibility.

Data unit 552 may record various parameters as measured by sensor assembly 502 of gas cylinder assembly 50. For example, data unit 552 could record pressure versus time. Such a note could be useful, for example, for welding contractors who want to check that there was sufficient pressure during lengthy gas welding procedures on critical components, or to supply data to a company about a particular customer's usage.

ES 2 467 697 T5

Alternatively, the data from data unit 550 may be output to a computer enabled welding machine (for welding applications) or other equipment using gas, to allow calculation of derived parameters, along with warning messages.

Additionally, data unit 550 may be arranged to provide the following functions: contain and display gas pressure data, i.e. what types of welding, what types of welded metal, or provide links so that mobile phones or computers can collect detailed data; providing multi-mode operation, for example a provider / provider mode and a client mode; expose different amounts to the customer than the gas company that fills the cylinders; allow the contribution of data; Provide data such as a cylinder number, the type of gas, a certificate of analysis, a customer history (who had the cylinder on a few dates), the cylinder can carry in summary form safety data and functional tips.

Alternatively, all of the above examples can optionally be processed, stored, or obtained from a system located entirely on (or within) the gas cylinder 500 as mentioned in terms of the gauge arrangement 200, 502.

While the above embodiments have been described with reference to the use of a quartz crystal oscillator, one of ordinary skill in the art will be readily aware of alternative piezoelectric materials that could also be used. For example, a non-exhaustive list may include crystal oscillators comprising: lithium tantalate, lithium niobate, lithium borate, berlinite, gallium arsenide, lithium tetraborate, aluminum phosphate, bismuth germanium oxide, titanate ceramic polycrystalline zirconia, high-alumina ceramic, zinc-silicon oxide compound, or dipotassium tartrate.

Embodiments of the present invention have been described with particular reference to the illustrated examples. Although specific examples are shown in the drawings and described in detail herein, it should be understood that the drawings and detailed description are not intended to limit the invention to the particular form disclosed. It will be understood that variations and modifications can be made to the examples described within the scope of the present invention.