Flowmeter primary element with sensors

0001The present invention relates to measuring the flow rate of a process fluid in an industrial process. More specifically, the present invention relates to a flow transmitter.

0002Measuring flow rates based on differential pressure measurements is common in this field and there are many types of fluid flow meters. For example, the Pitot tube senses the upstream (ie "stagnation") and downstream (including "static" or "suction") pressures of the flowing fluid and the flow rate of the fluid hitting the Pitot tube. Causes a differential pressure value associated with. The average pitot tube includes a pressure port leading to the fluid plenum in the pitot tube body. The impulse line then transmits the fluid pressure to a flow calculator such as an industrial process variable transmitter.

0003The process variable transmitter includes at least one sensor that receives differential pressure. For example, a differential pressure sensor is used, which receives and responds to upstream and downstream pressures from a Pitot tube type bluff body, in response to which electrical is associated with the differential pressure between these two pressures. Provide output. The circuit in the transmitter is configured to calculate the flow rate in response to the sensed differential pressure.

<p num="0004"> The functionality of existing flowmeters, consisting of an average pitot tube and a differential pressure sensor, has been proven and evaluated to be effective in many applications. However, functionality can be increased by measuring multiple pressures. In addition, additional measurements allow for plenum clogging, pipe clogging, installation problems, sensor failures, and other diagnostics.</p><p num="0005"> As mentioned in the background section, Pitot tube type flow sensors typically operate by generating a differential pressure. A differential pressure sensor can be used to sense the differential pressure that correlates with the flow rate of the process fluid through the Pitot tube. Typically, the pressure is transferred to the differential pressure sensor through the plenum in the Pitot tube. It is known that more accurate flow measurements can be obtained when the upstream and downstream pressures obtained from the Pitot tube are the average pressures obtained across the diameter of the flow tube. This provides a more accurate flow measurement, but loses information related to pressure at a particular location in the flow pipe. Such additional information is useful to give the flow meter additional functionality. For example, it is possible to detect anomalies in the flow profile to detect clogging, information about the density of process fluids, deterioration or corrosion of components in the flow pipe, failure or deterioration of differential pressure sensors, and the like.</p>

<p num="0006"> The present invention provides a flow meter that measures the flow rate of a process fluid through a conduit using a Pitot tube or other bluff body that extends into the conduit. At least one sensor is located upstream and / or downstream of the Pitot tube / bluff body to provide flowmeter information. This information can be used to determine the flow rate and / or to give the flow meter additional functionality. The operation of the embodiments of the examples of the present invention will be described later.</p><p num="0007"> The process variable transmitter is configured as a flow meter for measuring the flow rate of process fluid through the conduit. The transmitter includes a pitot tube that extends into the conduit and in the process fluid creates a differential pressure due to the flow of the process fluid. The upstream process variable sensor is mounted on the Pitot tube and couples with the flow of the process fluid to detect the upstream process variable of the process fluid. The downstream process variable sensor is mounted on the Pitot tube on the downstream side of the upstream process variable sensor and couples with the flow of the process fluid to sense the downstream process variable of the process fluid. The measuring circuit determines the flow rate of the process fluid based on the upstream and downstream process variables and / or performs the diagnosis. In other forms, process variable sensors are placed laterally on both sides of the Pitot tube and are used to determine flow rate and / or to perform diagnostics.</p>

<p num="0008"> By providing redundant flow measurement, the degree of reliability of the measurement is increased, the reliability of the result is improved, and preventive maintenance is facilitated. Further, the pressure sensor held on the Pitot tube can be used instead of the differential pressure sensor in the form of the prior art.</p>

0009<figref num="1">It is a figure which shows the cross section of the flow rate measuring system and process piping of this invention.</figref><figref num="2">It is a simplified block diagram of the flow rate measuring system and the flow rate transmitter according to the embodiment of an example of this invention.</figref><figref num="3">FIG. 6 is a cross-sectional view of a probe in a flowmeter according to the present invention, including a process variable sensor.</figref><figref num="4">FIG. 5 is a partial view of a Pitot tube with a process variable sensor according to an embodiment of the present invention.</figref><figref num="5">FIG. 3 is a partial view of a Pitot tube with a process variable sensor according to an embodiment of another example of the present invention.</figref><figref num="6">FIG. 3 is a partial view of a Pitot tube isolated from a process fluid using a process variable sensor in the cavity and an isolation diaphragm.</figref><figref num="7">FIG. 5 is a partial view of a Pitot tube with a process variable sensor according to an embodiment of the present invention.</figref>

0010FIG. 1 is a schematic view of a process control system 10 showing an example of an environment according to an embodiment of the present invention. The flow measurement system 12 is connected to the control chamber 14 (modeled as a voltage source and resistor) by a process control loop 16. In loop 16, an appropriate protocol can be used to communicate flow information between the flow transmitter 12 and the control room 14. For example, the process control loop 16 operates according to a process industry standard protocol, such as Highway Addressable Remote Transducer (HART "registered trademark"), FOUNDATION Fieldbus, or any other suitable protocol. The process control loop 16 may also consist of, for example, a wireless process control loop that wirelessly communicates information using a wireless HART® communication protocol according to the IEC62591 standard. Other technologies, including Ethernet® or fiber optic coupling, can be used like any other communication technology.

0011In one form, differential pressure is used to determine the flow rate of the process fluid based on the pressure difference sensed upstream and downstream of the Pitot tube probe inserted into the flow of the process fluid. In another example form, pressure is perceived laterally on both sides of the probe, approximately at right angles to the direction of flow. These lateral pressures vary as a function of vortex shedding. The frequency and / or amplitude of this change can be used to determine the flow rate of the process fluid, as described below.

0012FIG. 1 further shows a cut portion of a process fluid vessel, such as a tube or closed conduit 18 with a differential pressure measuring probe 20 inside. The probe 20 includes a Pitot tube type bluff body 22 that spans the inside of the tube 18 in the radial direction. The turn signal arrow 24 in FIG. 1 indicates the direction of the fluid flow in the pipe 18. The fluid manifold 26 and the flow transmitter housing 13 are mounted at the outer end of the Pitot tube 20 as shown. The transmitter housing 13 can include an optional pressure sensor 28 that is fluidly coupled to the probe 20 through a passage. In addition, FIG. 1 shows an auxiliary sensor coupling 27 used to couple the process variable sensor carried by the bluff body 22 to a circuit within the flow transmitter housing 13. The operation of the process variable sensor will be described in more detail below.

0013FIG. 2 is a system block diagram of the flow rate transmitter 12. The flow transmitter 12 includes a flow transmitter housing 13 and a differential pressure measuring probe 20. The flow transmitter 12 can be coupled to a process control loop such as loop 16 and is adapted to communicate process variable outputs related to the flow rate of the process fluid flowing through the tube 18. The transmitter 12 includes a loop communication device 32, an optional differential pressure sensor 28, a measurement circuit 34, and a controller 36.

0014The loop communicator 32 can be connected to a process control loop such as loop 16 and is adapted to communicate on the process control loop. Such communication may follow a suitable process industry standard protocol, such as the protocol described above.

0015When the optional pressure sensor 28 is used, the first and second ports 38, 40 are connected through the passage 30 to the optional first and second plenum 42, 44 of the probe 20, respectively. The sensor 28 may be any device as long as it has electrical properties that change in response to changes in applied pressure . For example, the sensor 28 can be a capacitive pressure sensor whose capacitance changes in response to the differential pressure applied between ports 38 and 40.

0016The measurement circuit 34 is connected to the sensor 28 and is configured to provide a sensor output associated with the differential pressure between ports 38 and 40. The measurement circuit 34 may be any electronic circuit as long as it provides an appropriate signal related to the differential pressure. For example, the measurement circuit may be an analog / digital converter, a capacitance / digital converter, or any other suitable circuit.

0017The controller 36 is connected to the measurement circuit 34 and the loop communication device 32. The controller 36 is adapted to provide a process variable output to the loop communicator 32, which process variable output is associated with the sensor output provided by the measurement circuit 34. The controller 36 may be a microprocessor or other suitable device. Normally, the controller 36 converts the differential pressure into an output related to the flow rate of the process fluid. The controller can perform compensation using, for example, curve matching techniques for adjusting the non-linearity in the relationship between differential pressure and flow rate. Additional factors can be used for compensation of flow measurements, including compensation for changes due to temperature, perceived process fluids, absolute pressure, etc.

0018Although the loop communicator 32, measurement circuit 34, and controller 36 have been described as separate modules, they may be combined, for example, as an application specific integrated circuit (ASIC). Similarly, various software components in a microprocessor-based system can provide functions as a measurement circuit 34, a controller 36 and a loop communicator 32.

0019The differential pressure measuring probe 20 is coupled to the transmitter housing 13 by a passage 30. As a result, the port 38 of the sensor 28 is coupled to the first plenum 42, while the port 40 of the sensor 28 is coupled to the second plenum 44. A "plenum" is a passage, conduit, pipe, etc. through which a fluid of a particular characteristic or pressure is directed or contained, through which the pressure of the fluid is guided or conveyed.

0020In the illustrated embodiment, the first (upstream) plenum 42 comprises at least one impact opening 48 and transmits pressure from the probe's impact (ie upstream) surface 46 to port 38 of the sensor 28. Arranged like this. The opening 48 may be of any suitable shape, the opening 48 comprising a vertically elongated component, and in some embodiments, sufficiently long enough to be substantially aligned with the vertical axis of the bluff body 22. The second (downstream) plenum 44 includes a non-impact (ie, downstream) surface 50 spaced downstream from the impact surface 46. The non-impact surface 50 includes at least one non-impact opening 52 arranged to transmit the pressure from this non-impact surface to the port 40 of the sensor 28 via the plenum 44. If the second plenum is not used, a pressure tap is provided. The location of the measured pressure is for illustration purposes only and the present invention is not limited to this form.

0021On the one hand, the present invention comprises at least one process variable sensor 60 held by the probe 20 of the flow meter. The process variable sensor 60 is used by the transmitter 12 to provide additional functionality. For example, additional pressure measurements can be obtained to obtain temperature measurements and the like. When a plurality of sensors are provided, additional information can be obtained across various positions of the probe 20. FIG. 2 shows the process variable sensors 60L and 60T, which are coupled to the measurement circuit 34 as shown. The sensor 60L is located on the front edge of the probe 20 and the sensor 60T is located on the trailing edge of the probe 20. The detailed technique used by the process variable sensor 60 may follow any suitable technique. In addition, any number of process variable sensors 60 can be used. These additional process variable sensors can be used, for example, in performing diagnostics to provide redundant techniques for flow measurement. This diagnosis includes, for example, verifying clogging of the plenum opening, that is, clogging of the flow tube, verifying the formation of deposits on process variable components, monitoring noise through the process fluid, and so on. Including. Several forms are described below. Also, the invention is not limited to embodiments that require a plenum and / or separate pressure sensor. In some embodiments, the sensor is only located on the probe itself. For example, two absolute pressure sensors are used and the difference is calculated to determine the differential pressure.

0022As described above, the flow rate from the probe 20 is determined from the differential pressure signal between the high pressure side and the low pressure side of the main element of the bluff body.

0023In addition to creating a differential pressure, the bluff body in the flow of fluid escapes a vortex with a frequency proportional to the velocity of the fluid. The formula for the frequency of vortex separation is as follows.

0024<maths num="1"><img id="000002" he="12" wi="43" file="JP6239727B2_D0001.tif" img-format="tif" img-content="drawing" /></maths>

0025Where S is the Struhal number, V is the fluid velocity, d_pIs the probe width and f is the vortex separation frequency.

0026The probe 20 of the Pitot tube type flow meter functions as an object placed in the flow of the process fluid, thus leaving the vortex proportional to the flow rate of the process fluid. The sensor 60 connected to the probe can be used to measure the differential pressure and also to sense the pressure generated during the vortex break, thereby determining the frequency of the vortex break. Figure 3 shows the top of a T-shaped probe 20 as provided by the ANNUBAR (registered trademark) average pitot tube, available from Rosemount Inc., Chanhassen, Minnesota, for example. It is a cross-sectional view.

0027The flow rate obtained from the measured frequency of vortex separation is compared to the flow rate determined from the differential pressure signal between the high pressure side and the low pressure side of the main element. Differences in these flow measurement results can be used to trigger alarms for sensor services. If one mode of flow measurement is found to be unreliable, the other can be used. You can also use multiple sensors to optimize performance, increase redundancy, and enhance turndown.

0028The sensor 60, located at the location shown in FIG. 3, can be used to measure fluctuating pressure and also senses fluctuating elements of drag force. This drag variation occurs at twice the frequency of vortex separation. Similarly, by subtracting one of the two sensors at the rear (immediately after) of the vortex break from the other, a variable element of lift force can be obtained. It exists at the frequency of vortex separation. Generally, the fluctuating lift element has a larger amplitude than the fluctuating drag element.

0029The density of the flow medium can be determined by comparing the velocity calculated from the vortex separation equation with the velocity calculated from the main element equation.

0030<maths num="2"><img id="000003" he="13" wi="96" file="JP6239727B2_D0001.tif" img-format="tif" img-content="drawing" /></maths>

0031Where Q_aIs the volumetric flow rate as a gas, F_naIs the unit conversion coefficient, K is the flow coefficient of the main element, D is the inner diameter of the pipe, Y is the gas expansion coefficient of the main element (Y = 1 for incompressible fluids such as liquids), F_aaIs the coefficient of thermal expansion, ρ_fIs the flow density, h_wIs the differential pressure.

0032The volumetric flow rate of the fluid is equal to the area inside the tube multiplied by the fluid velocity. Therefore, the tentative density value in the mean pitot equation can be determined or modified using the fluid velocity (V) calculated through the following vortex equation.

0033<maths num="3"><img id="000004" he="6" wi="38" file="JP6239727B2_D0001.tif" img-format="tif" img-content="drawing" /></maths>

0034Or

0035<maths num="4"><img id="000005" he="12" wi="34" file="JP6239727B2_D0001.tif" img-format="tif" img-content="drawing" /></maths>

0036Here, A is the area of the pipe.

0037Substituting the right-hand side of equation 2 of the main element into equation 4 gives the following equation.

0038<maths num="5"><img id="000006" he="14" wi="63" file="JP6239727B2_D0001.tif" img-format="tif" img-content="drawing" /></maths>

0039Expressing this in relation to the diameter, the following equation is obtained.

0040<maths num="6"><img id="000007" he="16" wi="63" file="JP6239727B2_D0001.tif" img-format="tif" img-content="drawing" /></maths>

0041Rewriting Equation 1 yields Equation 7.

0042<maths num="7"><img id="000008" he="13" wi="37" file="JP6239727B2_D0001.tif" img-format="tif" img-content="drawing" /></maths>

0043Assuming that the right-hand side of Equation 6 is equal to the right-hand side of Equation 7, Equation 8 is obtained.

0044<maths num="8"><img id="000009" he="19" wi="78" file="JP6239727B2_D0001.tif" img-format="tif" img-content="drawing" /></maths>

0045Simplifying and rewriting this yields Equation 9 and Equation 10.

0046<maths num="9"><img id="000010" he="19" wi="72" file="JP6239727B2_D0001.tif" img-format="tif" img-content="drawing" /></maths>

0047<maths num="10"><img id="000011" he="16" wi="83" file="JP6239727B2_D0001.tif" img-format="tif" img-content="drawing" /></maths>

0048This relationship is related to the fluid density ρ in Equation 2._fAnd the following equation for volumetric flow rate is obtained.

0049<maths num="11"><img id="000012" he="14" wi="63" file="JP6239727B2_D0001.tif" img-format="tif" img-content="drawing" /></maths>

0050Here, the terms in parentheses are measured values, and the terms in square brackets are constants or values determined during the manufacturing process. There are numerous means of calculating various terms using the sensor 60, where, as in Equation 2, in a different direction with respect to the flow direction than that used in conventional differential pressure (DP) flow calculation. The differential pressure is measured, and on both sides of the Pitot tube, the frequency of vortex separation used in Equation 7 is sensed to determine the fluid velocity, and then the volumetric flow rate is calculated immediately. Alternative methods can demonstrate the performance of the instrument.

0051Using the sensor of the present invention, in addition to being able to calculate the volumetric flow rate, it is also possible to calculate the mass flow rate. The mass flow rate can be calculated directly from the measured values associated with vortex separation. By rewriting the above equation 1 related to the frequency of vortex separation so as to give the fluid velocity, the following equation is obtained.

0052<maths num="12"><img id="000013" he="13" wi="40" file="JP6239727B2_D0001.tif" img-format="tif" img-content="drawing" /></maths>

0053Also, in the present invention, the amplitude of vortex separation can be used to determine the mass flow rate. Mass flow rate is defined as follows.

0054<maths num="13"><img id="000014" he="9" wi="50" file="JP6239727B2_D0001.tif" img-format="tif" img-content="drawing" /></maths>

0055Furthermore, the amplitude of the vortex signal is proportional to the dynamic pressure according to the following relationship.

0056<maths num="14"><img id="000015" he="10" wi="58" file="JP6239727B2_D0001.tif" img-format="tif" img-content="drawing" /></maths>

0057Where Ampv is the amplitude of the vortex signal, ρ_fIs the fluid flow density, C is the coefficient of proportionality (presumed to be fluid independent, but may require calibration), and V is the velocity of the fluid.

0058Rewriting Equation 14 yields Equation 15.

0059<maths num="15"><img id="000016" he="10" wi="47" file="JP6239727B2_D0001.tif" img-format="tif" img-content="drawing" /></maths>

0060Next, by substituting the expression of the fluid velocity V of Equation 12 into Equation 15, Equation 16 is obtained.

0061<maths num="16"><img id="000017" he="15" wi="60" file="JP6239727B2_D0001.tif" img-format="tif" img-content="drawing" /></maths>

0062Substituting the values of ρf and V into Equation 13 gives the following equation representing the mass flow rate.

0063<maths num="17"><img id="000018" he="15" wi="67" file="JP6239727B2_D0001.tif" img-format="tif" img-content="drawing" /></maths>

0064Here, the terms in parentheses are measured values, and the terms in square brackets are constants or values that can be determined during the manufacturing process. Also, some terms can be thought of as constants (eg S and C), and in fact they are a function of Reynolds number. The amendment can be made using means known in the art.

0065Therefore, unlike typical prior art vortex separation sensors that have sufficient frequency bandwidth to detect the frequency of vortex separation, but do not allow quantitative measurement of vortex amplitude, the sensor 60 of the present invention has a mass flow rate. Provides sufficiently accurate amplitude and frequency information to determine.

0066In addition to determining process variables such as mass flow, additional process variable sensors of the invention can be used to perform diagnostics. For example, the impulse tube 30 is a small inner diameter pipe that transmits a pressure signal from the tube to the pressure sensor. In the flow measurement, the impulse tube transmits the impact pressure from the upstream side of the main component and the static pressure from the downstream side of the main component to the differential pressure (DP) sensor 28. The square root of the value of DP caused across the main component is proportional to the flow rate in the pipe.

0067Improperly designed impulse tubes can cause problems in flow measurement systems, including clogging, leaks, liquid traps in gas flow measurements, gas traps in liquid flow measurements, and more. An additional process variable sensor 60 can be used to measure the differential pressure between the upstream and downstream sides of the Pitot tube. The relationship between these measurements and the perceived differential pressure can be used to examine potential problems with impulse tubes. This allows the operator to replace the service or tubing prior to a critical failure. Similarly, additional process variable sensors can be used to verify degradation in the tube, thereby compensating for differential pressure measurements. Similarly, the sensed differential pressure can be compensated for clogging, blockage, or other degradation detected in these tubes. This allows the flow meter to continue operating until the impulse tube is replaced, sealed, or repaired, even if the flow meter becomes less accurate. Compensation can be performed, for example, based on the compensation coefficient stored in the memory in the controller 36.

0068In yet another example embodiment, an additional process variable sensor 60 can be configured as a pressure sensor and used to completely remove the impulse tube itself. Eliminating the impulse tube in differential pressure flow applications eliminates the problems commonly associated with impulse tubes. In such a form, the pressure measurement value from the downstream side of the Pitot tube is subtracted from the pressure measurement value on the upstream side to obtain the differential pressure. This subtraction can be done by an analog electronic circuit or, for example, digitally in the controller 36 or the measurement circuit 34. As mentioned above, the sensor 60 can also be used to measure the frequency of vortex breakout.

0069A Pitot tube with a plurality of process variable sensors 60 located outside the upstream and downstream sides can directly measure the velocity flow profile in the tube. By using this information in the measurement circuit 34, it is possible to correct the error caused by the fluctuation of the flow profile as well as the execution of the diagnosis. For applications where the process fluid is swirling or has two-dimensional profile distortion, two or more Pitot tube-type probes, each with a distributed process variable sensor, are used to better detect changes in the flow profile. It can be used as an obstacle in the pipe, thereby correcting errors in flow measurements or identifying potential problems during the process. The accuracy of the flow profile correction is related to the number of sensors placed along the probe. A large number of sensors can provide more information related to changes in flow profile, thereby obtaining more accurate flow measurements.

0070When placing a pitot tube in an active fluid stream (hot tapping), it is difficult to determine if the tip of the pitot tube is in contact with the opposite wall of the conduit. is there. This can be a serious problem as the cantilevered main element is substantially weaker than the one supported in contact with the opposite wall. Partially inserted devices that have been exposed to fluid flow for a long time are prone to failure. In addition, the insertion mechanism, which can break components if overtightened, has significant mechanical advantages. The cantilevered Pitot tube produces a resonant frequency that can be detected by the sensor 60. In another embodiment, a sensor 60 located at the tip of the probe can sense the opposite tube wall when the probe is seated. This measurement allows the distinction between supported and unsupported tips. An indicator on the transmitter can be used to provide the operator with instructions that the tip has been seated once.

0071FIG. 4 is a partial view of the Pitot tube 20 containing the process variable sensor 60 supported on it. In FIG. 4, the process variable sensor 60 is arranged as a pressure sensor. In this form, the sensor 60 is composed of a brittle, substantially incompressible material with properties suitable for long-term exposure to process fluids. For example, the sensor 60 is configured by joining two sapphire pieces together, for example, by melt bonding. A cavity 104 is formed between the two sapphires, for example supporting a capacitive plate (not shown) in it. When pressure from the fluid is applied to the sensor 60, the cavity 104 deforms slightly. This deformation is perceived based on the change in capacitance between the two capacitive plates. FIG. 4 shows an electrical connection extending from the capacitive plate to the measurement circuit 34 shown in FIG. In such a form, the sensor 60 was issued on July 18, 200, entitled "Elongated Pressure Sensor for Pressure Transmitters" by Roger Tee, Flick et al., For example, transferred to Rosemount Inc. Works similarly to the pressure sensor shown and described in US Pat. No. 6,089,097.

0072In the form illustrated in FIG. 4, the sensor 60 is directly exposed to the process fluid. To provide sufficient space for the sensor 60 to be placed, the sensor 60 can be deflected vertically (perpendicular to the paper) along the length of the pitot tube 20. The sensor 60 can be attached through the hole in the tube 20 and can be fixed using brazing 105 or the like. The two sensors 60T located on the downstream side of the Pitot tube 20 are located on both sides of the Pitot tube, whereby vortex detachment can be detected. The upstream sensor 60L is located in the center of the Pitot tube 20 for more accurate sensing.

0073FIG. 5 shows a form similar to that of FIG. 4, in which the protective piece 106 is arranged around the upstream sensor 60L. The protective piece 106 may be of a barrel, perforated barrel, or other shape and serves to protect the upstream sensor 60L from the flow of process fluid without interfering with pressure detection. FIG. 6 shows another embodiment in which the sensor 60L is held in an oil-filled cavity 110 formed in a tubular structure. In FIG. 6, the cavity 110 formed in the tubular structure 114 is isolated from the process fluid using an isolation diaphragm 112. The cavity 110 is filled with a substantially incompressible fluid, whereby the pressure applied to the isolation diaphragm is transmitted to the sensor 60L via the isolation fluid. The sensor 60L is fixed in the tube 114 by using a brazing method or the like. The diaphragm 112, tube 114 and sensor 60L are assembled separately and attached to Pitot tube 20 as required. For example, the assembly is welded to the Pitot tube 20. Although not shown in Figure 6, the Pitot tube can hold additional assemblies. This form allows the sensor 60 to be isolated from the process fluid, thereby protecting the sensor 60 from damage. In another embodiment, the sensor 60 is supported within the plenum 42 or 44. In this form, the sensor 60 is also protected from the impact of any small piece carried in the process fluid. Although the form of a particular type of pressure sensor has been described here, any suitable pressure sensing technique can be implemented.

0074In yet another example of the form shown in FIG. 7, the differential pressure sensor 60 is located between the plenum 42 and 44, which removes the impulse tube 30. In some embodiments, a process variable sensor that directly measures the differential pressure is a more accurate differential pressure determination value than using two absolute pressure sensors to determine the differential pressure by subtracting the measured values between the two sensors. I will provide a. In such a form, the differential pressure sensor 60 is directly exposed to the process fluid in the plenum 42 and 44. In another form, the isolation diaphragm 43 is placed within the walls of the plenums 42 and 44, which isolates the differential pressure sensor 60 from the process fluid. The isolation fluid is used to transfer the pressure in the plenum 42, 44 from the diaphragm to the differential pressure sensor.

0075Although the present invention has been described with reference to preferred embodiments, one of ordinary skill in the art will recognize that embodiments and details can be modified without departing from the spirit and scope of the invention. The present invention involves arranging one or more sensors along the length of a probe inserted into the flow of process fluid. The probe can be configured as a pitot tube that transmits the pressure from the process fluid flow to an external pressure sensor. In such configurations, process variable sensors supported on the probe can be used for diagnostic purposes to improve accuracy and sensor measurements, and for calibration purposes to determine flow profiles, etc. can do. However, the present invention is not limited to this form. The present invention may be in the form in which all the sensors are held by the probe itself without using any external sensor. Such a form does not require an internal plenum in which a typical Pitot tube is used. As used herein, the term "Pitot tube" generally means a probe that is inserted into a fluid stream. The "Pitot tube" does not require an internal passage to guide pressure from within the process fluid flow to the external pressure sensor. In some embodiments, the process variable sensor 60 comprises a pressure sensor, including a temperature sensor.

007610 ... process control system, 12 ... flow transmitter, 13 ... flow transmitter housing, 14 ... control room, 16 ... process control loop, 18 ... conduit, 20 ... Pitot tube (probe), 22 ... Bluff body, 24 ... Direction indicator arrow, 26 ... Fluid multi-tube (manifold), 27 ... Auxiliary sensor coupling, 28 ... Pressure sensor (differential pressure) Sensor), 30 ... passage, 32 ... loop communication device, 34 ... measurement circuit, 36 ... controller, 38,40 ... port, 42,44 ... plenum, 43,112 ... Isolation diaphragm, 46 ... impact surface, 48 ... impact opening, 50 ... non-impact surface, 52 ... non-impact opening, 60,60L, 60T ... process variable sensor, 104 ... cavity , 105 ... Brazing, 106 ... Protective piece, 110 ... Oil-filled cavity, 114 ... Tubular structure