WO1997049985A1

WO1997049985A1 - Device for measuring the partial pressure of gases dissolved in liquids

Info

Publication number: WO1997049985A1
Application number: PCT/EP1997/003177
Authority: WO
Inventors: Michael Dieckmann; Rainer Buchholz
Original assignee: Euroferm Gmbh
Priority date: 1996-06-21
Filing date: 1997-06-18
Publication date: 1997-12-31
Also published as: EP1036312A1; CA2259275A1; AU3260497A; AU732530B2; JP2000512758A

Abstract

The present invention relates to a device for measuring the partial pressure of gases dissolved in liquids. Said device contains (a) a measuring point (15) which is partially separated by a gas-permeable membrane (11) permeable to the gas to be determined; (b) a light emission source (6) for producing a light ray interacting with the liquid in the measuring point (15) and having a wave length which is absorbed by the gas to be determined; and (c) a measuring arrangement (7) for determining the light leaving the measuring point (15). Said device is characterised in that (d) the measuring point (15) is in contact with the interface of a photoconductive element (14), and (e) the light is guided through said element (14) in such a manner that total reflection thereof is reduced at the interface.

Description

Device for measuring the partial pressure of gases dissolved in liquids

The present invention relates to a novel device for measuring the gas partial pressure in liquid media according to P 4445 68.9.

Especially in the field of fermentation technology, there has been an increasing need to measure gases by determining the partial pressure. For example, special probes have been developed for determining the oxygen and carbon dioxide partial pressures. A common example of this are the so-called Severinghaus electrodes. These devices work with membrane-covered single-rod pH electrodes (DE-OS 25 08637, Biotechnol. Bioeng. 22 (1980), 2411-2416, Biotechnol. Bioeng. 23 (1981), 461-466). In this system there is an electrolyte solution or paste between the gas selective membrane and the pH electrode. The measuring principle is based on the fact that carbon dioxide forms carbonic acid in aqueous solution, which dissociates into a bicarbonate anion and a proton. This process causes a change in pH in the electrolyte solution, which is measured using the pH probe. The disadvantage of this measuring principle is the fact that carbon dioxide is not measured directly, but its ionic form. Since the proportion of the ionic form is less than 0.1%, this method is not sufficiently precise. Apart from this, other volatile acidic or basic gases interfere with the pH measurement. Furthermore, a very high level of maintenance is required.

Furthermore, pCO2 optodes are known from the prior art. This is also a membrane-covered sensor system (SPIE Vol. 798 Fiber Optic Sensors II (1987) pp. 249-252; Anal. Chim. Acta 160 (1984) pp. 305-309; Proc. Int. Meeting on Chemical Sensors, Fukuoka, Japan, Elsevier, pp. 609-619, 1983, Talanta 35 (1988) 2 p.109-112, Anal.Chem. 65 (1993) pp. 331-337, Fresenius Z. Anal. Chem. 325 (1986) pp. 387-392). In the case of the pH optodes, pH indicators which change their absorption or fluorescence properties as a function of the proton concentration are used as the indicator phase (Anal. Chem. 52 (1980), pp. 864-869, DE-OS 3 343 636 and 3 343 637, U.S. Pat. Appl. 855,384). If the indicator is separated from the material to be measured using a gas-permeable membrane, only gases, for example carbon dioxide, can penetrate through the membrane to the indicator phase and cause a change in pH value there by hydrolysis. Such carbon dioxide optodes work analogously to the Severinghaus electrodes. The disadvantages of optical pH and therefore pCO2 measurements lie in the very limited analytical measuring range and the dependence on ionic strength. The wide use of the optodes is also opposed by the disadvantages already mentioned with regard to the Severinghaus electrodes.

A differential pressure measuring device for the determination of carbonic acid is known from German laid-open specification 2435493. However, this device can only be used in flowing media. It is therefore particularly unsuitable for use in conventional stirred or fixed bed reactors, as are used in particular in the fermentation industry.

A device for the continuous measurement of the content of dissolved carbon dioxide in liquids is known from German Offenlegungsschrift 2926138. The measuring principle is based on the determination of the conductivity difference. The device is equipped with a membrane which is flown on one side by the liquid containing dissolved carbon dioxide and on the other side by a neutral or basic measuring liquid. One conductivity sensor each is arranged in the conduit of the measuring liquid before and after the permeable membrane. The disadvantage of the measurement is that it is not suitable for liquids whose chemical and physical properties change.

From European patent application 0462755 it is also known to determine gases, for example CO2, by infrared absorption measurement. Here, the infrared light beam is sent through the fluid to be measured. The light beam is divided into two or more components. These split light beams are then measured. The disadvantage of this measuring arrangement is that it does not allow the determination of partial pressures and is sensitive to scattering particles in the sample liquid.

A division into two beam paths is already known from GB 2194333. With this method, only one light beam is passed through the material to be measured. The rest of the radiation is used as a reference light to also increase the accuracy.

Another publication describes a so-called chopped gas analysis device which also works with luminescent diodes (Laser and Optoelectronics 17 (1985) 3, pp. 308-310, Wiegleb, G .: Use of LED radiation sources in analysis devices).

What these devices and methods have in common is that they only determine concentrations. The material to be measured is placed directly into the beam path and measured. This is possible for gases and liquids without scattering particles with a constant composition, in which disturbances can be detected by a blank value. However, partial pressures cannot be determined with the optical methods described. Nor can it be used for media-variable compositions and liquids containing opacifying particles. The analysis method of Attenuated total reflection, short ATR has been known and described in the prior art (in English ^* Attenuated Total Reflectance ").

The measurement takes advantage of the phenomenon of the formation of evanescent waves or surface waves at the interface of two optically differently dense media. In a medium with a high refractive index, a light beam at the interface to an optically thinner medium is reflected back into the optically denser medium if the angle between the incident light beam and the solder on the interface exceeds the critical angle of total reflection. Some of the light waves penetrate a few wavelengths into the surrounding thinner medium and are only reflected back from there into the optically denser medium. If there are light-absorbing substances in the area of this short light path, the reflected portion of the light is reduced. This weakening can be detected and correlated with the amount of the absorbent. A large number of configurations for the use of this light absorption phenomenon are state of the art today. Most ATR devices contain crystals, mostly trapezoid cut prisms. The simplest geometric shapes for the ATR element are described in German Offenlegungsschrift DE 42 27 813. Simple, commercially available, plano-convex microlenses made of glass and plastic, which have the shape of hemispheres, are used.

A cube corner reflector in the form of a triple prism is used in German Offenlegungsschrift DE 44 18 180. The advantage of this arrangement is its compact construction. The light emitted is thereby redirected around 1800. This enables an arrangement in a thin rod. The supply of the light to be radiated and the removal of the residual light is solved constructively by the use of optical fibers. The German published patent application DE 4038354 describes an ATR probe with no prisms, lenses or other components. The light is also transported via light guides. In the invention, the incoming and outgoing light guides and the actual ATR sensor consist a common optical fiber. In the area of the sample to be examined, the jacket of the light guide has been removed. The optical waveguide is mechanically supported and arranged in a probe body in a measuring room so that it is in contact with the medium to be examined.

It is also known to determine the CO 2 concentration in liquids by means of attenuated total reflection (The Chemical Engineer 498 (1991) p. 18). A sapphire ATR (attenuated total reflection) crystal is arranged in a flow measuring cell for fluid substances, for example beer, perpendicular to the direction of flow. The infrared light, which is fed to the crystal on one side, passes through the crystal and is totally reflected several times. With each reflection, the radiation enters the sample liquid by several μm and is weakened by the presence of carbon dioxide. The amount of residual light at the other end of the crystal is measured. The disadvantage of this method is that no partial pressures can be measured. On the other hand, with changing fluids, the results can be falsified by changing the reflective properties.

All the arrangements described have in common that they are in direct contact with the medium containing the substance to be determined. Because of this arrangement, however, only concentrations can be determined. It is not possible to determine the partial pressure of gases dissolved in liquids. Nor can it be used for media with a changing composition, in particular with changing particle concentrations that interfere with the measurement.

The basic application P 4445 68.9 on which this application is based has set itself the task of providing a device for measuring the partial pressure of gases dissolved in liquids by means of optical methods which no longer has the disadvantages described of the devices known from the prior art and which in particular the gas partial pressure measurement with longer long-term stability of the device allows precise and changing chemical-physical composition in media as well as in clear, cloudy and changeable cloudy media.

This object is achieved in that the device a) is separated from a measuring chamber, which is permeable to the gas to be determined by means of a gas-permeable membrane, from a sample space which contains the liquid with the gas to be determined therein, b) a light emission source for generating a light beam passing through the measuring chamber with a wavelength that is absorbed by the gas to be determined, and c) a measuring arrangement for determining the light beam leaving the measuring chamber.

The measuring chamber, light emission source and measuring arrangement are arranged in a rod-shaped probe. If this is in the field of biotechnology, e.g. used in fermentations, beverage production or wastewater treatment, it is designed as a sterilizable device. Since the field of fermentation technology is mainly sterilized using steam, the probe materials must be adjusted to these conditions. That is why the membrane materials that have proven themselves in this area are primarily used. This primarily includes polytetrafluoroethylene (silicone and other fluoride polymers). Solubility membranes have proven successful as gas-selective membranes. When used in the sample chamber, these can establish a balance between the sample liquid and the inner mixture.

The measuring chamber is preferably filled with a chemically and biologically inert fluid. This is selected so that it absorbs the gas to be determined, which diffuses through the membrane into the measuring chamber. For this Suitable liquids or gases can be used for the same purpose. The type of fluids mentioned depends on the gases to be measured.

Luminescent diodes are preferably used as the light emission source. The use of these devices has the following advantages:

The emission is relatively narrow-band, i.e. the use of interference filters is not absolutely necessary to selectively determine the corresponding gas. Due to the relatively low power consumption, it is possible in principle to make the measurement setup portable with battery operation. A key advantage over conventional infrared sources is the high level of performance. Therefore, it may be possible to do without a comparison route or to set up compensation circuits without moving parts. Such a system is mechanically less sensitive. At the same time, the high level of performance guarantees long operation without recalibration. The luminescent diodes are dimensioned so small that the light can easily be coupled into the optical waveguide. The sensitive parts can thus be positioned externally and are not subject to the thermal-mechanical loads of steam sterilization.

In the method according to P 4445 68.9 it is also possible to work with different wavelengths, preferably two different wavelengths, in order to increase the accuracy. The methods for increasing the measuring accuracy and compensating for fluctuations in the electronic components are generally known and published (Meas.Sci.Technol. 3 (1992) 2 191-195, Sean F. Johnston: Gas Monitors Employing Infrared LEDs).

The detectors compatible with the luminescent diodes are also used. Photodiodes, photoresistors and lead selenide photodetectors (PbSe detectors) are particularly suitable as such. The latter work predominantly in the infrared range and are particularly suitable for the determination of carbon dioxide.

Optical fibers are used to guide the light waves from the light emission source to the measuring chamber. The same applies to the conduction of the light from the measuring chamber to the measuring arrangement for the determination of the non-absorbed light components. The measuring arrangement is preferably connected to a special circuit for evaluating, storing and displaying the signals. Because of this, the device is particularly suitable for the automation of systems. Using an integrated evaluation unit, all the data can be automatically recorded and fed to a control process.

The possibility of a pressure-resistant design of the device is also advantageous. It is only necessary to adapt the housing construction of the probe accordingly. In this way, the device can be used at pressures of 200 bar. The probe is preferably used at pressures up to 20 bar. When used for fermentation processes, it is only necessary to ensure that the probe can withstand the increased pressures that occur under sterilization conditions.

Another subject of P 4445 68.9 is a method for measuring the partial pressure of gases dissolved in liquids. In this method, the described device is immersed in the liquid present in the sample space in such a way that the membrane is completely wetted with sample liquid. As a result, the gas to be determined can now selectively diffuse through the membrane into the measuring chamber. A light beam is guided through the measuring chamber by the light emission source via optical waveguides. The gas diffusing there absorbs some of the radiation. The non-absorbed part of the light beam is fed via an optical waveguide to the measuring arrangement for determining the gas partial pressure. By appropriate evaluation tion, storage and display devices can be determined and evaluated on the basis of the measurement of the non-absorbed light beam.

Electromagnetic radiation generated by luminescent diodes is preferably used. The infrared range is very particularly preferred.

The device and the method are particularly suitable for using the measurement of the carbon dioxide partial pressure. Carbon dioxide is a significant production factor in the food industry, particularly in the beverage industry. In the beverages themselves, carbon dioxide is responsible for the shelf life and the refreshing taste. Most determinations are made today via simultaneous pressure and temperature control.

A carbon dioxide partial pressure measurement is also required for optimal process management of biotechnical processes. Of importance in this context is the fact that the supply of gases to the microorganisms and their inhibitory properties are a function of the corresponding partial pressures and not of the concentrations. Despite this knowledge, the carbon dioxide partial pressure has not been sufficiently taken into account to date. A satisfactory solution to its determination has not yet been found. The main problems when choosing a suitable determination method are the lack of equipment and the high chemical stability of the carbon dioxide. Carbon dioxide is the highest oxidation state of carbon and is therefore very inert at room temperature. In solution, unlike other heterogeneous gases, it does not form hydrogen bonds. With a dissociation constant for carbonic acid of 2xl0- ^ M, only a very small part is in the form of dissolved ions. A measuring probe that is based on the determination of the ionic form is therefore already faulty afflicted. For an exact method, it is therefore necessary to determine the dissolved carbon dioxide directly. The absorption measurement of carbon dioxide is available for a measurement at room temperature. Absorption measurement in the infrared range is state of the art with the existing exhaust gas analyzers. However, the determination from the exhaust air provides concentrations and no partial pressures. With the help of Henry's law, concentrations can be converted into partial pressures and vice versa. The conversion of concentrations into partial pressures is more difficult for carbon dioxide in contrast to oxygen, since the Henry constant is influenced by the pH value and media components. Fluctuations in the pH value lead to changes in the carbon dioxide concentration in the exhaust air over time. Particularly in basic fermentations and in large reactors, the carbon dioxide storage of the media leads to the measurement signal overshooting when a new equilibrium is approached. Such signals can be misinterpreted as a change in metabolism.

Using the described device according to P 44 45 68.9 solves in particular the problems of carbon dioxide partial pressure measurement shown. In this case the measuring chamber is filled with a carrier fluid for carbon dioxide. This fluid must have a solubility for carbon dioxide. Another condition is that it is chemically and biologically inert. For steam sterilization, it is also advantageous if the fluid has a higher boiling point than the material to be measured, in order to largely avoid pressure fluctuations. However, the device is not limited to a specific carrier liquid. Rather, their composition and chemical nature depend on the type of gas to be measured and the conditions under which the probe is used.

A disadvantage of a system according to P 4445 68.9 is the relatively high response time to changes in the partial pressure in the sample space, which is of the order of magnitude because of the required diffusion into the measuring chamber n

Seconds to minutes. Furthermore, use at very high partial pressures is problematic, since in this case too much light is absorbed and the measurement signal consequently becomes too weak.

The present invention has for its object to provide a device for measuring the partial pressure of gases dissolved in liquids by means of optical methods, which no longer has the disadvantages described of the devices known from the prior art and which in particular the gas partial pressure measurement for a long time Long-term stability of the device allows precise chemical and physical composition changing in media as well as in clear, cloudy and changeably cloudy media. Compared to P 4445 68.9, the device should have considerably shorter response times and, in particular, should also be usable at high partial pressures of the gas.

This object is achieved by a device which a) has a measuring location which is partially separated by means of a gas-permeable membrane which is permeable to the gas to be determined, b) a light emission source for generating a light beam with a wavelength which interacts with the liquid in the measuring location , which is absorbed by the gas to be determined, and c) includes a measuring arrangement for determining the light leaving the measuring location, and which is characterized in that d) the measuring location (15) is in contact with the interface of a light-conducting element, and e ) the light is guided through this element in such a way that attenuated total reflection occurs at the interface.

The element showing the attenuated total reflection is referred to below as the "ATR element" (for "Attenuated Total Reflectance"). According to the invention, the ATR element, light emission source and measuring arrangement are arranged in a rod-shaped probe. If this is used in the field of biotechnology, it is designed as a sterilizable device. Since the field of fermentation technology is mainly sterilized using steam, the probe materials must be adjusted to these conditions.

The type of light emission source, its arrangement, its use, the type of detectors, its arrangement, its use, the type of optical waveguide and the pressure-resistant design corresponds to P 4445 68.9 as described above.

The ATR element can be chosen arbitrarily. This includes the use of prisms, lenses or optical fibers. For use under steam sterilization conditions, they must be able to withstand thermal loads. In particular, quartz glass is available for the UV to NIR range, and Sapphire for longer-wave light. If an optical waveguide is used, quartz glass fibers are suitable for the UV to NIR range, and chalcogenide, fluoride or silver halide fibers are particularly suitable for the longer-wave range.

In principle, the membrane can be arranged in two different ways in relation to the ATR element. If the membrane material shows no or constant absorption for the wavelength range, the membrane can be applied directly to the ATR unit. If this is not the case, a gap in the order of a few wavelengths of light can be left between the membrane and the ATR element. This gap is then filled according to the invention with a chemically and biologically inert fluid. This is selected so that it absorbs the gas to be determined, which diffuses through the membrane into the gap. Suitable liquids or gases can be used for this purpose in the same way. The type of fluids mentioned depends on the gases to be measured.

So while in the first case the measuring point is still inside the membrane (on the side facing the ATR element), in the second there is Fall your own fluid-filled space / gap, between the ATR element and the membrane. In any case, due to its small thickness of only a few micrometers (= penetration depth of the total reflected light) and the immediate proximity to the membrane, the gas to be determined can diffuse out of the sample within a very short time. Partial pressure changes in the sample are therefore registered with an extremely short response time in the range from milliseconds to seconds. In contrast, the diffusion for a device according to P 4445 68.9 takes a time in the range of minutes.

Furthermore, due to the thin measuring location, the device according to the invention is particularly suitable for the measurement of high partial pressures at which the measuring signal is absorbed too strongly in conventional systems. In particular, the arrangement of a fluid that absorbs the gas between the ATR element and the membrane means that very low partial pressures can also be measured, since the gas accumulates in this fluid.

The membrane consists of steam sterilizable materials. Proven membrane materials are primarily used in this area. These include above all silicone, polytetrafluoroethylene and other fluorinated polymers. For application to fibers as an ATR element, these must be liquefiable or sprayable, in particular polytertrafluoroethylene.

Another object of the present invention is a method for measuring the partial pressure of gases dissolved in liquids. In this method, the device according to the invention is immersed in the liquid present in the sample space in such a way that the membrane is completely wetted with sample liquid. As a result, the gas to be determined can now selectively enter the membrane in the event that the membrane is applied directly to the ATR element and in the event that a gap containing fluid is arranged between the ATR element and the membrane diffuse into the gap. The gas diffusing there absorbs part of the radiation. The not absorbed 7/49985 PC17EP97 / 03177

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Part is fed to the measuring arrangement for the determination of the gas partial pressure via an optical waveguide. Appropriate evaluation, storage and display devices can be used to determine and evaluate the gas partial pressure based on the measurement of the non-absorbed light beam.

The device and the method according to the invention are particularly suitable for using the measurement of the carbon dioxide partial pressure. In particular, the specific problems of carbon dioxide partial pressure measurement described above are solved. Depending on the measuring range, a gap can be provided which is filled with a carrier fluid. The short response time and suitability for determining high partial pressures are particular advantages.

The invention is described in more detail below with reference to FIGS. 1 to 4.

1 shows the entire probe.

Fig.2 shows the probe tip with gap

Fig. 3 shows the probe tip with optical fiber as an ATR element without

gap

1 shows the device according to the invention in the form of a probe 1. In the example according to the invention, the probe body is made of stainless steel. However, it is possible to manufacture from any other material. As a rule, however, these are corrosion-free substances.

The probe 1 has a connection piece 2, which allows the probe 1 to be inserted pressure-tight into the pipeline or the wall 5 of a vessel. The connector 2 and the O-ring arrangement 3 allow the probe 1 to be fastened in a sealing manner in an access pipe 4 to the wall 5. The Access pipe 4 has the corresponding connector to connector 2.

This construction gives the possibility to subject the probe head to steam sterilization and to use it in sterile operation.

A light source 6 and a measuring arrangement 7 are present within the probe 1. In the example according to the invention, the light source 6 is a luminescence diode and the measuring arrangement 7 is a photo receiver. Both parts of the device are connected to the electrical cables

8 and 9 provided. The luminescence diode 6 is supplied with current via the line 8. The photo receiver 7 transmits a signal pulse over the line

9 to a means for amplifying and recording the signal.

The luminescence diode 6 and the photo receiver 7 are arranged outside the liquid space 10. They are used via the extrinsic optical waveguides 12 and 13, which are used to transmit the light 12 from the liminescence diode 6 and the non-absorbed light to the photo receiver 7. The optical waveguides can be made from any materials suitable for the transmission of light. In the example according to the invention, work is carried out in the infrared range. Therefore, light guides made of transparent material, e.g. from silver halides and chalcogenides. These fiber-optic cables can withstand thermal loads and are therefore suitable for use in steam-sterilizable environments.

ATR element 14 is located at the tip of the head of probe 1. In the example according to the invention, this is a sapphire crystal.

The ATR element 14 is separated from the sample space 10 via the gas-permeable membrane 11. In the example according to the invention, the membrane 11 is a thermally stable membrane, which is made of steam sterilizable material is made. According to the invention, polytetrafluoroethylene or / and Teflon is preferred for this.

The dissolved gas diffuses into the membrane 11 until equilibrium is established. Since the diffusion of gases into a membrane is partially pressure controlled, the probe 1 determines the partial pressure. The probe thus measures a biologically important parameter; because the supply of the microorganisms, like all transport processes from the cells or into the cells, is controlled by partial pressure and not by concentration.

The luminescence diode 6 emits narrow-band light, which is selectively absorbed by the gas to be determined. The wavelength can be in the UV / VIS as well as in the infrared range with respect to the gas to be examined. For carbon dioxide, this is preferably 4.3 μm. The emitted wavelength range can be limited by a heat radiator with an interference filter or preferably by a narrow-band luminescence diode. The particular advantage of using the luminescent diode is that the radiation can be modulated, which increases detection and minimizes effects such as DC drift.

The emitted radiation is fed to the ATR element 14 via the optical waveguide 12. The gas present specifically attenuates the radiation emitted. The weakened light is partially received by the optical waveguide 13 and fed to the photo receiver 7. This measures the attenuated light and produces an electrical signal proportional to the attenuated light. If modulated light is used, the electrical signal can also be modulated.

A device according to P 4445 68.9 could be created from the arrangement according to FIG. 1 by removing the ATR element 14. In this case, a fluid-filled chamber would remain behind the membrane 11 through which the measuring light can be guided. Figure 2 shows the tip of the probe 1 in the event that the membrane 11 absorbs light at the corresponding wavelength. The ATR element 14 is not arranged flush with the probe head (as in FIG. 1), but rather is sunk somewhat so that a gap 15 is formed. The gas located in the sample space 10 then diffuses through the membrane 11 into the gap until equilibrium prevails and it can be determined without additional absorption through the membrane 11. The same arrangement is chosen for the case where there are low partial pressures. In this case the gap is filled with a carrier fluid which has a high physical absorption capacity for the gas. This configuration can also be selected if the ATR element 14 consists of a stripped fiber and a gap is used.

Figure 3 shows the configuration of the probe tip in the event that an optical fiber is used as an ATR unit and is operated without a gap. The incoming and outgoing optical fibers 12 and 13 and the ATR element 14 consist of one fiber. The actual ATR element 14 is an optical fiber stripped on this piece. A membrane 11 is applied to this. In order to protect the exposed fiber against mechanical stresses from the medium, a cage 16 is attached to the tip of the probe.

The advantages achieved by the invention consist in particular in the fact that, in particular in the case of carbon dioxide partial pressure measurement, separation of the measurement space from the sample space does not result from the presence of turbid and changing particles in their concentration. Furthermore, the implementation of the membrane guarantees the measurement of the partial pressure. In principle it is possible to convert concentration into partial pressures using Henry's law. But it requires the simultaneous knowledge of temperature and pressure as well as the media properties. The latter is particularly difficult when using fermentation media. Furthermore, the long-term stability, Accuracy and the measuring range compared to pH-sensitive partial pressure probes increased.

The extremely short response time and the suitability for determining high partial pressures are particularly advantageous in the system according to the present invention. In addition, the probe structure is simplified since no separate light emitters and light receivers need to be arranged, in which problems with adjustment and sterilization can occur.

The probe according to the invention can be used particularly well both in the beverage industry and in biotechnology. For use in food technology, probes for measuring ranges of up to 10 bar can be created.

When used for carbon dioxide partial pressure measurement in the field of fermentation technology, it is advantageous that a pre-calibration is possible. Because due to the inhibiting influence of carbon dioxide on most organisms, recalibration can no longer be carried out. Another advantage in this area of application is that the probe can withstand thermal loads during sterilization and can easily withstand temperatures of 150 ° C. Finally, it is advantageous that, in contrast to the previous methods with absorption measurement, interference by substances which also absorb in the infrared range is excluded.

Claims

Expectations

1. Device for measuring the partial pressure of gases dissolved in liquids according to P 44 45 68.9, containing a) a measuring point (15) which is partially separated by means of a gas-permeable membrane (11) which is permeable to the gas to be determined, b ) a light emission source (6) for generating a light beam interacting with the liquid in the measuring location (15) and having a wavelength which is absorbed by the gas to be determined, and c) a measuring arrangement (7) for determining the light leaving the measuring location (15) , characterized in that d) the measuring point (15) is in contact with the interface of a light-conducting element (14), and e) the light is guided through this element (14) in such a way that there is an attenuated total reflection at the interface.

Device according to claim 1, characterized in that the measuring location (15), the light emission source (6), and the measuring arrangement (7) are arranged in a rod-shaped probe (1).

3. Device according to claim 2, characterized in that the probe (1) can be sterilized.

4. The device according to claim 3, characterized in that the probe (1) can be sterilized by means of steam.

5. Apparatus according to claim 1 to 4, characterized in that the membrane consists of polytetrafluoroethylene.

Apparatus according to claims 1 to 5, characterized in that the membrane is a gas-selective solubility membrane, within and over which an equilibrium is established between the sample space (10) and the measurement location (15).

7. The device according to claim 1 to 6, characterized in that the measuring location (15) is filled with a chemically and biologically inert fluid for absorption of the gas to be determined.

8. The device according to claim 7, characterized in that the fluid is a liquid or a gas.

9. The device according to claim 1 to 7, characterized in that in it an optical waveguide (12) for guiding the light beam from the light emission source (6) to the light-conducting element (14) and from there to the measuring arrangement (7) is arranged.

10. The device according to claim 1 to 9, characterized in that the light emission source (6) is a luminescent diode.

11. The device according to claim 1 to 10, characterized in that the measuring arrangement (7) is a photo diode, a photo resistor or a lead selenide photo detector.

12. The apparatus of claim 1 to 11, characterized in that the measuring arrangement (7) is connected to a circuit arrangement for evaluating, storing and displaying the signals.

13. The apparatus according to claim 1 to 12, characterized in that it is designed pressure-resistant.

14. The apparatus according to claim 13, characterized in that it is designed for operation under pressures up to 200 bar, preferably up to 20 bar.

15. The device according to one of claims 1 to 14, characterized in that the light-conducting element (14), at whose interface to the measuring location (15) the attenuated total reflection takes place, consists of sapphire.

16. The device according to one of claims 1 to 15, characterized in that the measuring location is wholly or partially within the membrane (11).

17. A method for measuring the partial pressure of gases dissolved in liquids with a device according to. one of claims 1 to 16, characterized in that a) the membrane (11) is immersed in the liquid present in the sample space (10), b) the gas to be determined, which is present in the liquid, via the membrane (11) in the Measuring site (15) diffuses, c) a light beam with a wavelength that is absorbed by the gas to be determined is passed through a light-conducting element (14) such that it is at the interface between this element (14) and the measuring site (15 ) attenuated total reflection occurs, and d) the unabsorbed light is fed to the measuring arrangement (7).

18. The method according to claim 17, characterized in that the measurement is carried out by means of infrared radiation.

19. The method according to any one of claims 17 or 18, characterized in that the light beam is brought up to the interface between the light-guiding element (14) and the measuring location (15) several times so that there is attenuated total reflection.

20. Use of the device according to one of claims 1 to 16, for determining the partial pressure of oxygen or carbon dioxide.

21. Use of the device according to one of claims 1 to 16, for measuring, controlling and regulating fermentation processes, beverage production processes and wastewater treatment plants.