AU732530B2 - Device for measuring the partial pressure of gases dissolved in liquids - Google Patents

Description

BIO 9401 1 PCT 23.05.1997 EUROFERM GmbH Gustav-Meyer-Allee 13355 Berlin Device for measuring the partial pressure of gases dissolved in liquids The present invention relates to a novel device for measuring gas partial pressure in liquid media according to P 44 45 68.9.

There is an increasing need, primarily in the field of fermentation technology, to measure gases by determining their partial pressure. Special probes have therefore been developed for determining the partial pressure of oxygen and carbon dioxide. A common example of these is constituted, for example, by so-called Severinghaus electrodes. These devices operate with membrane-covered single-rod pH electrodes (DE-A 25 08 637, 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, in aqueous solution, carbon dioxide forms carbonic acid, which dissociates into a bicarbonate anion and a proton. This process causes a change in the pH in the electrolyte solution, and this change is measured using the pH probe.

A disadvantage of this measuring principle is the fact that carbon dioxide is measured not directly, but in its ionic form. Since the ionic form is present in a proportion of less than this method is not sufficiently accurate. Apart from this, other acidic or basic volatile gases vitiate the measurement of the pH. Furthermore, very high outlay on maintenance is needed.

pCO 2 optodes are also known from the prior art.

Once again, these involve a membrane-covered sensor 145 system (SPIE vol. 798 Fiber Optic Sensors II (1987) pp. 249-252; Anal. Chim. Acta 160 (1984) pp. 305-309; i Proc. Int. Meeting on Chemical Sensors, Fukuoka, Japan, Elsevier, pp. 609-619, 1983, Talanta 35 (1988) 2 pp. 109- 2 112, Anal. Chem. 65 (1993) pp. 331-337, Fresenius Z.

Anal. Chem. 325 (1986) pp. 387-392). In pH optodes, pH indicators, which change their absorption or fluorescence properties as a function of the proton concentration, are used as indicator phase (Anal. Chem. 52 (1980) pp. 864- 869, DE-A 3 343 636 and 3 343 637, US Pat. Appl.

855 384). If a gas-permeable membrane is used to separate the indicator from the substance to be measured, only gases, for example carbon dioxide, can penetrate the membrane to reach the indicator phase, where they cause a change in the pH through hydrolysis. Carbon dioxide optodes of this type operate in similar fashion to Severinghaus electrodes. The disadvantages of optical pH and therefore pCO 2 measurements reside in the very limited analytical measuring range and the dependence on ionic strengths. This, as well as the disadvantages already mentioned with regard to the Severinghaus electrodes, is a hindrance to wide application of optodes.

DE-A 2435493 discloses a differential-pressure measuring instrument for the determination of carbonic acid. However, it is only possible to use this instrument in flowing media. It is therefore unsuitable, in particular, for conventional stirred or fixed-bed reactors, as used, in particular, in the fermentation industry.

DE-A 2926138 discloses a device for continuously measuring the dissolved carbon dioxide content in liquids. The measuring principle is based on determining conductivity difference. The instrument is equipped with a membrane, one side of which has the liquid containing dissolved carbon dioxide flowing over it, and the other side of which has a neutral or basic measuring liquid flowing over it. There is a conductivity meter arranged in the flow path of the measuring liquid both before and after the permeable membrane. A disadvantage with the measurement is that it is unsuitable for liquids whose chemical and physical properties are not constant.

Furthermore, European Patent Application 0462755 discloses the determination of gases, for example C0 2 by measuring infrared absorption. In this case, the infrared measuring infrared absorption. In this case, the infrared 3 light beam is passed through the fluid to be measured.

The light beam is split into two or more components.

These split light beams are then measured. A disadvantage with this measuring arrangement is that it does not allow partial pressures to be determined and it is sensitive to scattering particles in the sample liquid.

Splitting into two beam paths has already been disclosed by GB 2194333. In this method, only one of the light beams is guided through the substance to be measured. The rest of the radiation is used as reference light, so as likewise to increase the accuracy.

A further publication discloses a so-called chopped gas analyzer, which likewise operates using light-emitting diodes (Laser und Optoelektronik 17 (1985) 3, p. 308-310, Wiegleb, Einsatz von LED-Strahlungsquellen in Analysengerhten (Use of LED Radiation Sources in Analyzers]).

These instruments and methods have in common the fact that they only determine concentrations. The substance to be measured is placed and measured directly in the beam path. This is possible for gases and liquids which do not contain scattering particles and have constant physical composition, in which noise can be quantified using a blank measurement. However, partial pressures cannot be determined using the described optical methods. Neither is it possible to use them for varying physical composition and liquids containing particles giving rise to turbidity.

The ATR (Attenuated Total Reflectance) analysis method is known and has already been described in the prior art.

The measurement makes use of the phenomenon of the formation of evanescent waves or surface waves at the interface between two media with different optical densities. In a medium with high refractive index, a light beam is reflected back, into the optically denser 1 medium, at the interface with an optically thinner medium Sif the angle between the incident light beam and the -normal to the interface exceeds the critical angle for 4 total reflection. However, a fraction of the light waves penetrates a few wavelengths into the adjoining thinner medium and is only from there reflected back into the optically denser medium. If there are substances which absorb light in the region of this short optical path, then a lower fraction of the light will be reflected.

This attenuation can be detected and correlated with the amount of absorbing substance.

A large number of configurations for the use of this light-absorption phenomenon are now state of the art. Most ATR devices contain crystals, usually trapezoidally sectioned prisms. DE-A 42 27 813 describes the simplest geometrical shapes for the ATR element. Simple commercially available planoconvex microlenses made of glass and plastic, which have the shape of hemispheres, are employed.

DE-A 44 18 180 uses a cube corner reflector in the form of a triple prism. The advantage with this arrangement is its compact design. The emitted light is thereby deviated through 1800 [sic]. This makes it possible to have an arrangement in a thin rod. The supply of the incident light and the removal of the residual light is achieved in the design by the use of optical waveguides.

DE-A 40 38 354 describes an ATR probe completely avoiding the use of prisms, lenses and similar components. The light is once again transported via light guides. In the invention, the supplying and removing light guides and the actual ATR sensor consist of a common optical fibre. In the region of the sample to be investigated, the cladding of the light guide is removed.

The optical waveguide is mechanically supported and arranged in a measuring space in a probe body, so that this measuring space is in contact with the medium to be examined.

It is also known to determine the CO 2 concentration in liquids using attenuated total reflection (The SChemical Engineer 498 (1991) p. 18). In a continuous omeasuring cell for fluid substances, for example beer, a 5 sapphire ATR (Attenuated Total Reflectance) crystal is arranged perpendicularly to the flow direction. The infrared light fed to one side of the crystal passes through the crystal and undergoes repeated total reflection. On each reflection, the radiation travels several ym into the sample liquid and is attenuated by the carbon dioxide which is present. The residual light intensity at the other end of the crystal is measured. A disadvantage with this method is that it is not possible to measure partial pressures. Furthermore, in the case of fluids which undergo changes, variations in the reflection properties can lead to errors in the results.

One point which all the described arrangements have in common is that they are directly in 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. Neither can they be used for media with changing composition, in particular with changing partial concentrations which interfere with the measurement.

The object of the application P 44 45 68.9 on which this application is based is to provide a device for measuring the partial pressure of gases dissolved in liquids using optical methods which does not have the abovementioned disadvantages of the devices known from the prior art, and which, in particular, allows the partial pressures of gases to be measured accurately, with extended long-term stability for the device, and in media whose physico-chemical composition may change, as well as in clear or turbid media or media whose turbidity varies.

This object is achieved in that the device consists of a) a measuring chamber which is separated, by means of a gas-permeable membrane which is permeable to the gas to be determined, from a sample space which contains the liquid and, dissolved therein, the gas to be determined, 6 b) a light-emission source for generating a light beam which passes through the measuring chamber and has a wavelength which is absorbed by the gas to be determined, and c) a measuring arrangement for determining the light beam emerging from the measuring chamber.

The measuring chamber, the light-emission source and the measuring arrangement are here arranged in a rodshaped probe. When it is intended to be used in the field of biotechnology, for example for fermentation, the production of drinks or waste-water purification, it is designed as a sterilizable device. Since, in the field of fermentation technology, sterilization is predominantly carried out using steam, the materials of the probe should be tailored to such conditions. For this reason, membrane materials which are tried and tested in this field are also primarily to be used here. In particular, these include polytetrafluoroethylene (silicone and other fluoride polymers). Gas-selective membranes which have proved successful are solubility membranes. When they are introduced into the sample space, they can establish equilibrium between the sample liquid and the internal mixture.

The measuring chamber is preferably filled with a chemically and biologically inert fluid. This fluid is selected in such a way that it absorbs the gas to be determined, which diffuses through the membrane into the measuring chamber. To this end, suitable liquids or gases may equally well be used. The nature of the said fluids is dictated by the gases which are to be measured.

Light-emitting diodes are preferably used as the light source. Using these devices has the following advantages: The emission has a relatively narrow band, which means that it is not absolutely necessary to use interference filters in order to determine the corresponding gas selectively. By virtue of the relatively low power 14/ consumption, it is in principle possible to design the measuring structure as portable with battery operation.

7 A decisive advantage over conventional infrared sources is that the power is extremely constant. It may therefore be possible to make do without comparison paths or to construct compensation circuits without moving parts. A system of this type is mechanically robust. At the same time, the fact that the power is very constant ensures extended operation without recalibration. The lightemitting diodes are small enough for the injection of light into optical waveguides to be straightforward. The sensitive parts can thus be located externally, and are not subjected to the thermal and mechanical stresses of steam sterilization.

In the method according to P 44 45 68.9, it is also possible to operate with different wavelengths, for example two different wavelengths, in order to increase the accuracy. The methods for increasing the accuracy of the measurement and for compensating for fluctuations in the electronic components are widely known and published (Meas. Sci. Technol. 3 (1992) 2 191-195, Sean F.

Johnston: Gas Monitors Employing Infrared LEDs).

Furthermore, the detectors which are compatible with the light-emitting diodes are used. Suitable examples are, in particular, photodiodes, photoresistors and lead selenide photo-detectors (PbSe detectors). The latter operate predominantly in the infrared range and are suitable, above all, for the determination of carbon dioxide.

Optical waveguides are used to guide the light waves from the light-emission source to the measuring chamber. The same is true for guiding the light from the measuring chamber to the measuring arrangement for determining the unabsorbed light intensity. The measuring arrangement is preferably connected to a special circuit for evaluating, storing and displaying the signals.

Because of this, the device is suitable, in particular, for the automation of systems. When an integrated evaluation unit is used, all the data can be acquired automatically and a control process can be carried out.

A further advantage is the possibility of the 8 device having a pressure-proof design. It is merely necessary to tailor the design of the housing of the probe accordingly. In this way, the device according to the invention can be used at pressures of 200 bar.

Preferably, the probe is used at pressures of up to bar. In the case of use for fermentation processes, it is merely necessary to ensure that the probe withstands the elevated pressures which occur under sterilization conditions.

A further subject of P 44 45 68.9 is a method for measuring the partial pressure of gases dissolved in liquids. In this method, the device described is immersed in the liquid present in the sample space in such a way that the membrane is fully wetted with sample liquid.

Because of this, the gas which is to be determined can then diffuse selectively through the membrane into the measuring chamber. Using the light-emission source, a light beam is guided through the measuring chamber via optical waveguides. The gas diffusing into the latter absorbs some of the radiation. The unabsorbed part of the light beam is fed to the measuring arrangement, via an optical waveguide, for determining the partial pressure of the gas. Using corresponding evaluation, storage and display devices, the measurement of the unabsorbed light beam can be used to determine and evaluate the partial pressure of the gas.

Use is preferably made of electromagnetic radiation generated by light-emitting diodes. The infrared range is quite particularly preferred.

The device and the method are suitable, in particular, for use in measuring the partial pressure of carbon dioxide. Carbon dioxide represents a considerable production factor in the food industry, in particular in the drinks industry. In the drinks themselves, carbon dioxide is responsible for the shelf life and the N refreshing taste. Most determinations are currently carried out with simultaneous pressure and temperature monitoring.

For optimum process control of biotechnology 9 processes, measurement of the partial pressure of carbon dioxide is likewise also necessary. An important fact in connection with this is that the supply of the microorganisms with gases and their inhibitory properties are a function of the corresponding partial pressures rather than of the concentrations. In spite of this knowledge, the partial pressure of carbon dioxide has not to date been taken into account sufficiently. A satisfactory solution to its determination has not yet been found. The main problems in choosing a suitable determination method are the lack of available equipment and the high chemical stability of carbon dioxide. Carbon dioxide represents the highest oxidation state of carbon and is therefore very unreactive at room temperature. In contrast to other heterogeneous gases, it does not form hydrogen bonds in its dissolved form. With a dissociation constant for carbonic acid equal to 2 x 10- 4 M, only a very small proportion is present in the form of dissolved ions. A measuring probe which relies on determination of the ionic form therefore has an inherent shortcoming. For an accurate method, it is therefore necessary to determine the dissolved carbon dioxide directly. For a measurement at room temperatures, it is possible to measure the absorption of carbon dioxide. Measuring absorption in the infrared range is, with existing exhaust-gas analysis instruments, part of the prior art. However, determination from the waste air gives concentrations and not partial pressures. Concentrations can be converted into partial pressures, and vice versa, using Henry's law. In contrast to oxygen, the conversion of concentrations into partial pressures proves more difficult for carbon dioxide, since Henry's constant is influenced by the pH and the constituents of the media. Fluctuations in the pH lead to variations in the concentration of carbon dioxide in the outlet air over time. In particular with basic fermentations, and in large reactors, the accumulation of carbon dioxide in the media leads to temporal overshoots

S

of the measured signal when approaching a new equilibrium. Signals of this type can be misinterpreted as 10 changes in metabolism.

When the device described according to P 44 45 68.9 is used, the abovementioned problems in measuring the partial pressure of carbon dioxide are solved in particular. In this case, the measuring chamber is filled with a carrier fluid for carbon dioxide. Carbon dioxide must be soluble in this fluid. A further prerequisite is for the fluid to be chemically and biologically inert. For steam sterilization, it is furthermore advantageous if the fluid has a higher boiling point than the substance to be measured, in order substantially to avoid pressure fluctuations. However, the device is not restricted to a particular carrier liquid. Instead, the composition and chemical nature of the latter are dic- 15 tated by the type of gas to be measured and the working ooe conditions for the probe.

SOne disadvantage with a system according to P 44 45 68.9 is the relatively long response time to changes of the partial pressure in the sample space, S 20 which because of the required diffusion into the measuring chamber are of the order of seconds to minutes.

ooooo S"There are also problems with use at very high partial pressures, since in this case too much light is absorbed and the measurement signal consecuently becomes too weak.

S: 25 The present invention advantageously provides a device for measurang the partial pressure of gases dissolved in liquids, by means of optical methods, which no longer presents the aforementioned disadvantages of the devices known from the prior art and which, in particular, permits measurement of gas partial pressures, with greater long-term stability of the device, precisely and in media with changing chemical/physical composition and in clear, turbid and varyingly turbid media. In comparison with P 44 45 68.9, the device is intended to have considerably shorter response times and, in particular, to be usable even when the partial pressures of the gas arn hiah.

p: OPER MU32604-97.doc- 19/0201 11- According to one aspect of the invention there is provided a device for measuring the partial pressure of gases dissolved in liquids including: a measuring location which is partially separated by means of a gas-permeable membrane, which is permeable to the gas to be determined; a light-emission source for generating a light beam which interacts with the liquid in the measuring location and has a wavelength which is absorbed by the gas to be determined; and a measuring arrangement for determining the light leaving the measuring location; wherein the measuring location is in contact with the interface of a light-guiding element; the light is channelled through the light-guiding element in such a way that attenuated total reflection is brought about at the interface; and the measuring location is filled with a chemically and 20 biologically inert fluid for absorbing the gas to be determined.

There is also provided a method for measuring the ~partial pressure of gases dissolved in liquid using a device as described in the immediately preceding paragraph, wherein: the membrane of the device is immersed in the liquid present in the sample space, the gas which is present in the liquid and is to be determined diffuses via the membrane into the measuring location, a light beam having a wavelength which is absorbed by the gas to be determined is guided by a light-guiding R element in such a way that attenuated total reflection is brought about at the interface between this element and the 0measuring location, and the unabsorbed light is fed to the measuring arrangement.

P:\OPERAxd32604-97.doc-19/2A I -11A- The element exhibiting attenuated total reflection will be referred to below as the "ATR element".

According to the invention, the ATR element, the light-emission source and the measuring arrangement are arranged in a rod-shaped probe. If the latter is to be employed in the field of biotechnology, it will be designed as a sterilizable device. Since, in the field of fermentation technology, sterilization is predominantly carried out using steam, the materials of the probe should be tailored to these conditions.

The type of light-emission source, the way in which it is arranged and the way in which it is used, the type of detectors, the way in which they are arranged and the way in which they are used, the type of optical 15 waveguides and the pressure-proof configuration corre- Sspond as described above to P 44 45 68.9.

Any design may be chosen for the ATR element.

i This includes the use of prisms, lenses or optical waveguides. For use under steam-sterilization conditions, they must be able to withstand high temperatures. For the UV to NIR range, quartz glass is in particular available, and sapphire, in particular, for longer-wave light. If an "optical waveguide is used, quartz-glass fibres are suitable for the UV to NIR range, and chalcogenide, 12 fluoride or silver halide fibres, in particular, are suitable for the longer-wave range.

In terms of design and principle, the membrane may be arranged in two different ways with respect to the ATR element. If the membrane material exhibits no absorption, or constant absorption, for the wavelength range the membrane may be applied directly to the ATR unit. If this is not the case, a gap measuring of the order of a few wavelengths of the light may be left between the membrane and the ATR element. This gap is then, according to the invention, filled with a chemically and biologically inert fluid. This fluid is chosen in such a way that it absorbs the gas which is to be determined and diffuses through the membrane into the 15 gap. For this purpose, suitable liquids or gases can be ooo used in the same way. The type of the said fluids depends on the gases to be measured.

While, in the first case, the measuring location 0: is thus still inside the membrane (on the same side as S 20 the ATR element), in the second case there is an individual liquid-filled space/gap between the ATR element and the membrane. In any case, the gas to be determined can diffuse out from the sample into the measuring location go• in the shortest time because of the very small thickness of the latter, measuring only a few micrometres penetration depth of the light which undergoes total reflection) and the immediate proximity with the memo brane. Partial pressure changes in the sample are therefore registered with an extremely short response time, in the range of milliseconds to seconds. In contrast, diffusion for a device according to P 44 45 68.9 requires a time in the region of minutes.

Furthermore, because of the thin measuring location, the device according to the invention is especially suitable for measuring high partial pressures, RA4 at which excessive absorption of the measurement signal S takes place in customary systems. In particular by the -O arrangement of a fluid which absorbs the gas between the ATR element and the membrane, it is however 13 possible to measure even very low partial pressures, since the gas becomes enriched in this fluid.

The membrane consists of steam-sterilizable materials. Membrane materials tried and tested in this field will primarily be used. These include, above all, silicone, polytetrafluoroethylene and other fluoropolymers. For the application to fibres as the ATR element, they must be liquefiable or sprayable, in particular polytetrafluoroethylene.

The invention furthermore relates to a method for measuring the partial pressure of gases dissolved in liquids. In this method, the device according to the invention is dipped in the liquid present in the sample space in such a way that the membrane is fully wetted with the sample liquid. As a consequence of this, the gas to be determined can then diffuse into the membrane, in :the case when the membrane is applied directly to the ATR element, and through the membrane selectively into the gap, in the case when a fluid-containing gap is arranged 20 between the ATR element and the membrane. The gas which diffuses there forms a fraction of the radiation. The unabsorbed fraction is guided via an optical waveguide to the measuring arrangement for determining the gas partial "pressure. By means of corresponding evaluation, storage and display instruments, the gas partial pressure can be determined and evaluated with the aid of the measurement of the unabsorbed light beam.

The device according to the invention and the method according to the invention are suitable, in particular, for use in the measurement of the partial pressure of carbon dioxide. In this case, in particular, the aforementioned specific problems of measuring the partial pressure of carbon dioxide are solved. Depending on the measurement range, a gap may be provided which is filled with a carrier fluid. Particular advantages are, in this case, the short response time and the suitability for determining high partial pressures.

The invention will be described in more detail below with reference to Figures 1 to 4.

14 Fig. 1 shows the probe overall.

Fig. 2 shows the probe tip with a gap.

Fig. 3 shows the probe tip with an optical waveguide as an ATR element without a gap.

Fig. 1 shows the device according to the invention in the form of a probe 1. In the example according to the invention, the body of the probe is made of stainless steel. It is, however, possible to make it from any other desired material, but in general the material should not corrode.

The probe 1 has a connector 2 which makes it possible for the probe 1 to be fitted in pressure-proof fashion into the pipe or the wall 5 of a vessel. The connector 2 and the O-ring arrangement 3 allow the probe 1 to be fastened in leaktight fashion in an access tube 4 on the wall 5. The access tube 4 has the corresponding connector to the connector 2.

This structure affords the possibility of subjecting the probe head to steam sterilization and using it in sterile operation.

A light source 6 and a measuring arrangement 7 are present inside the probe i. In the example according to the invention, the light source 6 is a light-emitting diode and the measuring arrangement 7 is a photodetector.

Both instrument parts are provided with electrical leads 8 and 9. The light-emitting diode 6 is supplied with electricity via the lead 8. The photodetector 7 transmits a signal pulse, via the lead 9, to a means for amplifying and recording the signal.

The light-emitting diode 6 and the photodetector 7 are arranged outside the liquid space 10. They are used via the extrinsic optical waveguides 12 and 13, which serve to transmit the light 12 from the light-emitting diode 6 and the unabsorbed light to the photodetector 7.

The optical waveguides may be made of any materials suitable for the transmission of light. In the example according to the invention, operation is carried out in the infrared range. Light guides made of transparent Smaterial, for example silver halides and chalcogenides, 15 are therefore preferable. These optical waveguides can withstand thermal stresses and are therefore suitable for use in a steam-sterilizable environment.

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

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

The dissolved gas diffuses into the membrane 11 until an equilibrium is established. Since the diffusion of gases into a membrane is controlled by partial pressure, the probe 1 determines the partial pressure. The probe therefore measures a biologically meaningful parameter, since the supply to the microorganisms is, like all transport processes out of or into cells, controlled by partial pressure rather than concentration.

The light-emitting diode 6 emits narrow-band light which is absorbed selectively by the gas to be determined. In accordance with the gas to be examined, the wavelength may be either in the UV/VIS or in the infrared range. For carbon dioxide, this wavelength is preferably 4.3 gm. The emitted wavelength range can be restricted by a heat radiator with interference filter, or preferably by a narrow-band light-emitting diode. The particular advantage in using light-emitting diodes is that the radiation can be modulated, which enhances the detection and minimizes effects such as DC drift.

The emitted radiation is fed, via the optical waveguide 12, to the ATR element 14. The gas which is present specifically attenuates the emitted radiation.

Some of the attenuated light is picked up by the optical waveguide 13 and fed to the photodetector 7. The latter measures the attenuated light and produces an electrical signal proportional to the attenuated light. If modulated Z/ light is used, the electrical signal may likewise be 16 modulated.

A device according to P 44 45 68.9 could be obtained from the arrangement according to Fig. 1 by removing the ATR element 14. In this case, a fluid-filled chamber, through which the measurement light can be guided, is left behind the membrane 11.

Figure 2 shows the tip of the probe 1 for the case when the membrane 11 absorbs light at the corresponding wavelength. The ATR element 14 is arranged not flush with the probe head (as in Figure but somewhat set back, so as to create a gap 15. The gas in the sample space then diffuses through the membrane 11 into the gap, until equilibrium is set up, and it can be determined without additional absorption by the membrane 11. The same arrangement is chosen for the case when there are low partial pressures. For this case, the gap is filled with a carrier fluid which has a high physical absorption capacity for the gas. This configuration may also be chosen if the ATR element 14 consists of an unclad fibre and is operated with a gap.

Figure 3 shows the configuration of the probe tip for the case when an optical fibre is used as the ATR element and is operated without a gap. The optical fibres 12 and 13 supplying and removing the light, as well as the ATR element 14, consist of a fibre. The actual ATR element 14 is an optical fibre unclad in this portion. A membrane 11 is applied to this fibre. In order to protect the exposed fibre against mechanical stresses resulting from the medium, a cage 16 is fastened on the probe tip.

The advantages achieved with the invention consist, in particular, in that, primarily in the case of measuring the partial pressure of carbon dioxide, separating the measuring space from the sample space avoids effects due to the presence of particles which give rise to turbidity and have a concentration which varies. Furthermore, implementation of the membrane ensures that the partial pressure is measured. Although it is in principle possible to use Henry's law to convert concentration into partial pressures, this requires 17 simultaneous knowledge of temperature and pressure, as well as the properties of the media. The latter is difficult, in particular when using fermentation media.

Furthermore, the long-term stability, accuracy and measuring range are increased in comparison with pHsensitive partial pressure probes.

A quite particular advantage with the system according to the present invention is the extremely short response time and the suitability for determining high partial pressures. At the same time, the probe structure is simplified since it is not necessary to provide separate light emitters and light detectors, for which problems may occur with alignment and sterilization.

The probe according to the invention can be used particularly well both in the drinks industry and in e:s biotechnology. Probes for measuring ranges of up to 1 0 bar can be made for use in food technology.

For use in measuring the partial pressure of carbon dioxide in the field of fermentation technology, 20 it is advantageous for precalibration to be possible.

This is because, in view of the inhibiting effect of carbon dioxide on most organisms, calibration can no longer be carried out subsequently. A further advantage in this field of application is that, during sterilization, the probe withstands thermal stresses and is readily stable at temperatures of 150°C. Lastly, it is advantageous that, in contrast to the prior methods involving the measurement of absorption, interference by materials which likewise absorb in the infrared range is ruled out.

The reference to any prior art in this specification is not, and should not be taken as, an acknowledgment or any form of suggestion that the prior art forms part of the common general knowledge in Australia.

Claims (14)

1. 3. Device according to Claim 2, wherein the probe is sterilizable.
2. 4. Device according to Claim 3, wherein the probe can be sterilized using steam.
3. 5. Device according to any one of Claims 1 to 4, wherein the membrane consists of polytetrafluoroethylene.
4. 6. Device according to any one of Claims 1 to 5, wherein the membrane is a gas-selective solubility membrane, within Pwhich and through which equilibrium is established between the sample space and the measuring location. P: OPER\AUMd(-I3267-9.dc- d-/I)2 -19-
5. 7. Device according to Claim 6, wherein the fluid is a liquid or a gas.
6. 8. Device according to any one of Claims 1 to 6, including an optical waveguide for guiding the light beam from the light-emission source to the light-building element and from there to the measuring arrangement.
7. 9. Device according to any one of Claims 1 to 9, wherein the light-emission source is a light-emitting diode. Device according to any one of Claims 1 to 9, wherein the measuring arrangement is a photodiode, a photoresistor or a lead selenide photo-detector. .11. Device according to any one of Claims 1 to 10, wherein the measuring arrangement is connected to a circuit arrangement for evaluating, storing and displaying the 15 signals.
8. 12. Device according to any one of Claims 1 to 11, wherein it is of pressure-proof design.
9. 13. Device according to Claim 12, wherein it is designed for operation under pressures of up to 200 bar, preferably 20 up to 20 bar.
10. 14. Device according to any one of Claims 1 to 13, wherein the light-guiding element at whose interface with the measuring location the attenuated total reflection takes place consists of sapphire.
11. 15. Device according to any one of Claims 1 to 14, wherein the measuring location lies fully or partially within the membrane.
12. 16. Device substantially as hereinbefore described with reference to the accompanying drawings.
13. 17. Method for measuring the partial pressure of gases dissolved in liquids using a device according to any one of Claims 1 to 16, wherein a) the membrane of the device is immersed in the liquid present in the sample space, P:\OPER\A.d32604-97.doc- 19/}2 I b) the gas which is present in the liquid and is to be determined diffuses via the membrane into the measuring location, c) a light beam having a wavelength which is absorbed by the gas to be determined is guided by a light-guiding element in such a way that attenuated total reflection is brought about at the interface between this element and the measuring location, and d) the unabsorbed light is fed to the measuring arrangement.
14. 18. Method according to Claim 17, wherein the measurement is carried out using infrared radiation. i 19. Method according to Claim 17 or 18, wherein the light *beam is repeatedly delivered to the interface between the light-guiding element and the measuring location in such a way as to bring about attenuated total reflection there. Use of the device according to any one of Claims 1 to 16 for determining the partial pressure of oxygen or carbon dioxide. 20 21. Use of the device according to any one of Claims 1 to 16 for measuring, controlling and regulating fermentation eeee processes, methods for the production of drinks, and waste- water purification plants. DATED THIS 19th day of February 2001 Euroferm GmbHO by DAVIES COLLISON CAVE Patent Attorneys for the Applicants