Method and device for rapidly ascertaining the parameters of a sample medium.

The invention relates to a method for quickly determining the parameters of a sample medium, in particular a flowing sample medium, according to the preamble of claim 1 and an arrangement for carrying out the method. Such a method is known, for example, from DE-OS 3 148 830.

For numerous areas of application, it is necessary for the oxygen concentration in gases, liquids and the like to be determined, for example, with an extremely short response time of a few ms and without dead time, and that no oxygen is consumed during the measurement. For example, when treating patients with respirators, it is desirable to monitor the oxygen concentration in the exhaled air.

The principle of luminescence quenching of layers excited with light of a specific wavelength to luminescence has proven to be promising for measurements with short response times. Studies have now shown that the luminescent properties of the layers are influenced not only by oxygen, but also by other parameters. For example, the temperature changes the intensity of the luminescent light in the same way as oxygen, ie with increasing temperature the amplitude of the emitted light decreases. In addition, rising temperature also causes a wavelength shift towards longer wavelengths. Furthermore, moisture or water have an influence on the luminescent signs.

In the special case of respirator treatment, the influence of anesthetic gases such as laughing gas or halothane are also of interest.

In general, there are a large number of substances that influence certain luminescent layers. It will often depend on the combination of carrier material and luminescent dye chosen. Essentially flowing sample media such as gases or liquids can be considered as sample media. However, this does not exclude other sample media. It is essential for the measurement of a parameter with the aid of the luminescence quenching that the luminescence layer is influenced by the parameter of the sample medium to be determined.

The additional changes in the luminescent light due to other parameters have hitherto been regarded as insurmountable difficulties for the rapid measurement of oxygen concentration. In order to eliminate the influence of moisture, the layers have been shielded against the sample medium with a membrane (US Pat. No. 4,003,707). As a result, however, the setting time for the layers used was greatly extended, so that this known measuring arrangement is not suitable for the rapid determination of the oxygen concentration, for example in patient monitoring.

The influence of temperature on the luminescent layers has so far been investigated practically only for pure temperature sensors. When determining oxygen in breathing gases, the temperature effect was regarded as disruptive and, for example, an attempt was made to eliminate it by bringing the breathing gas to a predetermined temperature before the analysis. Under certain circumstances, a certain humidity was also set at the same time. As a result, these measures also lead to an undesirably long response time.

From DE-OS 3 148 830 mentioned at the outset, luminescent layers with water-repellent carrier material are already known, the luminescent properties of which are no longer influenced by moisture, but the temperature effects remain.

The present invention is based on the object of specifying a method of the type mentioned at the outset which, despite the difficulties mentioned, enables a reliable determination of a parameter of the sample medium with an extremely short response time even when there are several parameters influencing the luminescent properties.

Another object of the invention is to be able to determine at least one further parameter at the same time as this first parameter.

According to the invention, this object is achieved by the features specified in the characterizing part of claim 1.

The invention takes advantage of the influencing of the luminescent properties by various parameters, which is regarded as disruptive in specialist circles, in order to enable the sufficiently rapid measurement of a parameter, for example the oxygen concentration, in the first place and, in addition, to even create the possibility of one or more others To determine parameters. In contrast to the prior art, attempts are no longer made to suppress the influence of the other parameters. In contrast to this, the additional influencing of luminescence is also determined by the further parameters and used to determine the parameter sought.

To carry out the method, the intensity of the luminescent light must be determined for at least as many different wavelength ranges as there are parameters that influence the luminescent properties. At the same time, the wavelength ranges must be selected so that at least one parameter influences the layer properties differently in the different wavelength ranges. In order to keep the measurement effort and also the effort for determining the parameter from the measurement results within reasonable limits, it is possible to cleverly select different luminescent layers that react to different parameter combinations and whose properties are independent of some of the parameters. For example, it is possible to select luminescent layers whose luminescent properties only depend on two parameters, for example the oxygen content and the temperature of the sample medium. If such a layer is combined with another layer which can also be influenced by moisture, two parameters can already be determined via the first layer, which can then be assumed to be known for the second layer. This reduces the effort to also determine the third parameter.

It is known from US Pat. No. 4,003,707 to carry out the measurement of the luminescence quenching several times at different wavelengths. The wavelength of the excitation light and that of the detected luminescent light are changed simultaneously. The various measurement signals are fed to a signal processing device, which, however, determines only one parameter therefrom and disregards interference from other parameters. The various measurements only eliminate optical errors in the system, scattered light effects and individual measurement errors.

In order to increase the measurement accuracy, the invention provides for the spectral distribution and / or the intensity of the light directed onto the luminescent layer to be kept constant by means of a control loop. This enables the use of simpler ones and, in particular, for a series^pproduction of more advantageous light sources such as light-emitting diodes (LED). Additionally or alternatively, provision is made to determine the intensity of the light coming from the luminescent layer in the wavelength range of the excitation light and to use it as a reference signal for correcting the luminescence intensities in the selected wavelength ranges. Even if the intensity of the light source is constant, this does not guarantee that the same amount of light always falls on the layer. Especially with the relatively small intensity differences that are caused by the parameters to be determined, it is therefore important to optimally eliminate all interference effects.

It is advantageous for the method if the light directed onto the luminescent layer is pulsed. As a result, drifts in the detectors and / or the signal processing device can be avoided, for example, by resetting the corresponding components to zero during the breaks, for example. Furthermore, in particular when using LEDs, it is avoided that they heat up too much and thus receive a different emission spectrum. The light pulses can also be generated by means of a constantly operated light source and an optical or mechanical chopper.

For the determination of two parameters with the aid of the signal processing device, the approach is advantageously taken that the detector signals S_i consist of a subsurface component, a component that is linearly dependent on one of the parameters and a reciprocal component that depends on the other parameter according to the formula:<maths id="math0001" num=""><img file="EP0175352A2_D0001.tif" /></maths>where 1_i the luminescence intensity at the wavelength λ_i and with a certain value of the parameter X, (especially with X₁= 0), a_i the underground intensity and k_i, b_i Are approximation coefficients. The coefficient k_i is a measure of the luminescence quenching by the parameter X₁if no other parameters affect luminescence. The coefficient b_i specifies how strong the parameter X₂ the signal S_i The index i runs from 1-2 when measuring with two luminescence wavelengths.

However, to improve the measurement accuracy, it is possible to use more than two wavelengths, only increasing the effort for signal evaluation.

Experiments have shown that two parameters can be determined very precisely using this formula. If there are more than two parameters, a different formula must be set up.

In addition to this method of calculating two parameters from the two detector signals, it is also possible to iteratively determine the parameters from the detector signals, as will be shown in more detail below using an example.

In order to obtain an arrangement for carrying out the method according to the invention that meets the high requirements for reliability, particularly in the field of patient ventilation, it is provided that at least one light-emitting diode (LED) is provided as the light source. A parameter of the LED is determined and used to control the diode (s) in such a way that the temperature of the active diode region has a defined value or The aim is to ensure that the intensity and spectral distribution of the emitted light do not change as far as possible. In order to select the correct excitation wavelength from the emission spectrum of the LED, at least one filter is arranged between the LED and the luminescent layer.

It should be noted at this point that, if conditions permit, other light sources can be used as well. Because of its high intensity and monochromaticity, a laser is always an excellent light source, and filters on the input side can then be dispensed with. Laser diodes are also conceivable if they are available for the required excitation wavelengths and with sufficient stability and service life.

If the intensity and possibly also the spectral distribution of the emitted light fluctuate, these variations may have to be determined and used to correct the detector signals.

Further filters are arranged between the layer and the detectors. It is also essential for the arrangement that a device is provided via which the sample medium is brought directly or indirectly to the layer.

In order to increase the intensity of the excitation light when using LEDs and to compensate for fluctuations in the number of individual LEDs, it is advantageous to provide a number of LEDs connected in series or in parallel. At the same time, this improves the signal / noise ratio.

The light-emitting diodes can be operated continuously, the voltage drop across the light-emitting diodes being used for the control, for example, and the current flowing through the light-emitting diodes being set via this value. In order to avoid drifting in the components used, it is possible to briefly switch off the light-emitting diodes at arbitrarily predetermined time intervals and to adjust all components during these pauses.

In order to avoid, in particular, excessive heating of the light-emitting diodes, a further development of the invention provides that they are operated in a pulsed manner. These can be constant current pulses. In this case, the light emitting diodes can be regulated via the pulse duration. In this case, it is provided that the detectors only register the intensity of the luminescent light during part of this pulse duration. The length of the excitation pulse therefore has no influence on the measurement signals. Another option for regulation is to vary the current profile during the pulses.

In an advantageous development it is provided that the intensity of two wavelength ranges of the light-emitting diodes is measured and the quotient is used as a control parameter.

In order to also eliminate other possible interference influences such as dirty filters or the like, it is provided that the registration time is regulated in dependence on a reference signal. The light coming from the luminescent layer in the wavelength range of the excitation light can serve as a reference signal. For this purpose, it is advantageous to provide a further detector and a filter between this and the layer. The measurement signal from this detector can be used directly to control the registration time. For this purpose, it is possible to integrate the signal of the detector and to end the registration when a predetermined integration value is reached. This practically means automatic standardization.

Another possibility is that the measurement signals of the detectors registering the luminescent light of different wavelengths are relativized by the signal of the further detector, ie each measurement signal is divided by the signal of the further detector before it is fed to the signal processing device.

An advantageous further development of the method according to the invention with pulsed light-emitting diodes results from the fact that a parameter of the light-emitting diodes is determined during the pulse and in particular at the beginning of the pulse and is used to form the same and that, depending on the value of this parameter, the temporal position of the measurement duration, in which the detectors measure the luminescence intensity. If you measure the voltage drop across the diodes at the beginning of the pulse as a parameter, this provides information about the temperature of the LEDs. A specific spectral distribution arises depending on this temperature. If this temperature is too low, the pulse can be extended or the current strength increased during the pulse. Both lead to an increase in the diode temperature. If the temperature is too high at the beginning, you go the opposite way. In both cases, a diode temperature that corresponds to the desired spectral distribution will occur within the pulse at a certain point in time. If, depending on the parameter measured at the beginning of the pulse, the time span for the luminescence measurement is placed in the range in which the light-emitting diodes have the desired temperature and thus spectral distribution, the same relationships always result, at least on average.

In order to reduce the effort for avoiding interferences, it is provided in a constructive embodiment of the invention that the LEDs, the filters, the luminescent layer and the detectors are arranged in a light-tight housing.

An exemplary embodiment of the invention is described and explained in more detail below with reference to 5 figures. It shows or show<ul id="ul0001" list-style="none"><li>1 schematically shows the basic structure of an arrangement for determining two parameters,</li><li>2 shows the time course of the switch-on times of the LEDs and the detectors according to FIG. 1,</li><li>3 shows an emission spectrum for a luminescent layer and</li><li>Fig. 4.5 schematic detector waveforms for two wavelengths as a function of the oxygen partial pressure and the temperature.</li></ul>

Fig. 1 shows schematically and partially in section an arrangement for the simultaneous determination of the oxygen concentration and the temperature of a gas. In a light-tight housing 1 there are a number of LEDs 2 connected in series, a transparent luminescent layer 3 and filters 4-7, which are arranged together with the layer in a holder 8, and four photodiodes 9-12. The housing 1 and the holder 8 are provided with a gas inlet 13 and a gas outlet 14. The direction of the gas flow is indicated by arrows. It is of no importance for the functioning of the arrangement. In this way, the gas is conducted directly past the luminescent layer, which is only indicated in principle and not to scale in FIG. 1. So that the parameter to be measured can be varied quickly at layer 3, the gas inlet or gas outlet should be large and the volume between filter 4 and layer 3 should be small.

The LEDs are operated in a pulsed manner via a power supply 15. The upper part of FIG. 2 shows the current profile over time when pulsing with constant current. The part of the impulses that can be varied is shown in dashed lines. If necessary, as already mentioned at the beginning, it is also possible to choose other pulse shapes. The regulation takes place via the power supply 15 as a function of a diode parameter. Different options are alternatively indicated in FIG. The intensity of the emitted light can thus be determined via the detector 12 and fed to a servo device 16, which in turn, as indicated by the arrow 17, controls the power supply 15.

Another possibility is to determine the voltage drop across the LEDs using a measuring instrument 18 and to supply this signal to the servo device for control purposes, as indicated by the dashed line 19.

A third possibility consists in measuring the intensity of the light passing through the luminescent layer and a filter adapted to the wavelength range of the excitation light by means of the detector 9. and in turn feed this signal to the servo device, as indicated by dashed line 20.

Regardless of the type of control, it can be achieved that the spectral distribution and the intensity of the emitted light remain constant or at least have a defined course.

6 and 7 are two filters for different wavelength ranges of the luminescence spectrum. The light passing through these filters is registered by the detectors 10 and 11, respectively.

Amplifiers 21-23 are directly connected to the detectors 9-11, which may additionally contain integration, differentiation and other elements. At the same time, they determine the pulse duration for the registration of the signals coming from the detectors, as shown in the lower part of FIG. 2.

If the signal from the detector 9 is determined with the amplifier 21, this can be passed via lines 24, 25 to the amplifiers 22 and 23, in which the signal value of the detectors 10 and 11, which is normalized to the intensity of the excitation light, is then formed.

As shown in the lower part of FIG. 2, the pulse width for the registration of the luminescent light is smaller than the smallest pulse width of the excitation light.

The output signals of the two amplifiers 22 and 23 are sent to a signal processing device 26, which uses them to determine the values ​​for the oxygen concentration and the temperature.

For a better understanding, the emission spectrum of a luminescent layer and its dependence on the parameters oxygen and temperature are shown in the following FIG. 3. The conditions for the different curves are indicated in the left area of ​​the figure. The luminescence spectrum has two emission maxima, one at approximately 655 and the other at 720 nm. The excitation light has a shorter wavelength. As this Fig. can be seen, strong luminescence quenching occurs through oxygen. Rising temperature causes a rectified but weaker effect. Furthermore, with increasing temperature, the luminescence maxima shift to higher wavelengths regardless of the oxygen concentration. Since rapid respiratory fluctuations of several degrees Celsius can occur during the respiratory treatment of patients in the exhaled air, rapid oxygen concentration measurement, for example in the expiration phase, was previously not possible.

The method according to the invention now provides for measuring the luminescent light in two wavelength ranges. The upper band edge 61 of the filter 6 and the lower band edge 71 of the filter 7 are indicated by dashed lines in FIG. If, for example, a temperature increase now occurs, the first luminescence maximum migrates out of the transparency area of ​​the filter 6 and the second maximum into the transparency area of ​​the filter 7. As these considerations show, there are therefore quite different changes in the signals of the detectors 10 and 11 when the temperature changes. According to the formula for two parameters, in this case the oxygen concentration, which at known pressure is caused by the oxygen^{fabric pa}rtialdruck P₀₂ expresses, and the temperature T, the following detector signal S results₁:<maths id="math0002" num=""><img file="EP0175352A2_D0002.tif" /></maths>

The same applies to the detector signal S.₂. If the coefficients are determined using different calibrations, the values ​​for the two parameters can be obtained from these two equations^T and P₀₂ can be determined in the signal processing device. It should be noted that the coefficients in the formula for S₂ can assume different values ​​than in the formula for S₁. Certain coefficients have to be determined by calibrations. Others can always have fixed values.

As already mentioned, the two parameters can also be determined iteratively. This will be briefly explained with reference to Figures 4 and 5. In both figures, the course of the measurement signals of the detectors 10 and 11 is plotted as a function of the oxygen partial pressure and, for the sake of simplicity - since it is only about the principle - a linear relationship is assumed. The five straight lines in both figures apply to five different temperatures between 20 and 30 degrees Celsius. The detector signals decrease with increasing temperature.

It is now assumed that each detector delivers a measurement signal. Because of the indefinite temperature, these correspond to the P₀₂ -Axis in A 1 or A 2 area. If the smaller range, in the present case Δ1 1, is used in the diagram according to FIG. 5, it can be seen that not all temperatures are then permissible. In other words, the possible temperature range is reduced. This in turn shrinks according to Fi^G.⁴ the P₀₂ Range with which the temperature is then reduced again in FIG. 5, etc. By repeatedly using this method, the two parameters can also be determined with sufficient accuracy.

Which method you will choose depends, among other things, on whether a sufficiently precise formula can be set up for certain parameters, especially if there are more than two, and the computing effort that can be allowed to obtain the parameter values ​​in just a few ms.

In addition to the oxygen concentration, one also measures or determines the oxygen uptake, the C0₂- concentration, the C0_2-Production, pressure, gas flow and the respiratory quotient, you get a complete picture of the conditions in a patient's ventilation.

The method according to the invention and the arrangement for carrying it out can advantageously also be used for other measurements. If, for example, the percentage oxygen content is constant - there are such conditions in the earth's atmosphere - the amount of oxygen coming into contact with the luminescent layer varies with the pressure. In this case, the method can measure the gas pressure in addition to the temperature. The arrangement is therefore an excellent barometer.

If you choose a layer whose luminescent properties depend on moisture, you can get a very fast and exact hygrometer in a corresponding way.

By choosing the excitation wavelength, the filter characteristics and especially the layer properties, good conditions can be created for all measurements.

With the help of water-insensitive layers, for example, the blood oxygen content can be quickly determined using a small catheter. Integrated into a pacemaker electrode, a physiological control parameter could thus be measured.

The invention is not restricted to the methods and arrangements specified by the exemplary embodiments, but can be varied within a wide range with the aid of normal, professional skill without leaving the scope defined by the claims. For example, there is the possibility of using luminescent layers that consist of several combinations of dye / carrier material, each combination reacting to one or some of the parameters of interest. The different combinations can lie separately in the luminescent layer. The excitation can take place via a common light source or via different ones.