Note: Descriptions are shown in the official language in which they were submitted.
-
CA 02208~97 1997-06-23 .
~LE, ~!N T~5~S
TE~TR~SLA~
Device for measuring the partial pre~ure of ga~es
olved in liguids
The present invention relates to a novel device
for measuring gas partial pre~sure in liquid m~
There is an increasing need, primarily in the
field of fermentation technology, to measure gases by
determ; n; ng their partial pressure. Special probes have
therefore been developed for det~rm;n;ng the partial
pressure of oxyye~ 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 cause~ 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 propor-
tion of less than 0.1%, this method is not sufficientlyaccurate. Apart from this, other acidic or basic volatile
gases vitiate the measurement of the pH. Furthermore,
very high outlay on maintenance is needed.
PCO2 optodes are also known from the prior art.
Once again, these involve 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, Fllkllok~, Japan,
Elsevier, pp. 609-619, 1983, Talanta 35 (1988) 2 pp. 109-
112, Anal. Chem. 65 (1993) pp. 331-337, Fresenius Z.
Anal. Chem. 325 (1986) pp. 387-392). In pH optodes, pH
indicators, which change the-ir absorption or fluorescence
properties as a function of the proton concentration, are
used as indicator phase (Anal. Chem. 52 (1980) pp. 864-
CA 02208~97 1997-06-23
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 mea~ured, 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 PC02 measurements reside in the very
limited analytical measuring range and the dep~n~nce on
ionic strengths. This, as well as the disadvantages
already mentioned with regard to the Severinghaus elec-
trodes, is a hindrance to wide application of optodes.
It is also known to det~rm;ne the CO2 concentra-
tion in liquids using attenuated total reflection (TheChemical Engineer 498 (1991) p. 18) In a continuous
measuring cell for fluid substances, for example beer, a
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 reflec-
tion. On each reflection, the radiation travels several
~m 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.
DE-A 2435493 discloses a differential-pressure
measuring instrument for the determ;n~tion of carbonic
acid. However, it is only possible to use this instrument
in flowing media. It is therefore unsuitable, in particu-
lar, 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 det~rm;n;ng
conductivity difference. The instrument is equipped with
CA 02208~97 1997-06-23
a membrane, one side of which has the liquid cont~;n;ng
dissolved carbon dioxide flowing over it, and the other
side of which ha~ a neutral or basic measuring liquid
flowing over it. There i~ a con~llctivity meter arranged
in the flow path of the measuring liquid both before and
after the p~rm~hle membrane. A disadvantage with the
measurement is that it is unsuitable for liquids whose
chemical and physical properties are not constant.
Furth~more, European Patent Application 0462755
discloses the det~rr;n~tion of gases, for example CO2, by
measuring infrared absorption. In this case, the infrared
light beam is passed through the fluid to be measured.
The light beam i~ split into two or more components.
These split light beams are then measured. A disadvantage
with this measuring arrangement i8 that it does not allow
partial pressures to be determ;n~ 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, G.: Einsatz von LED-Strahlungs-
quellen in Analysengeraten [Use of LED Radiation Sources
in Analyzers]).
These instruments and methods have in common the
fact that they only determine concentrations. The sub-
stance 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 det~m;ne~ using the described
optical methods. Neither i~ it pos~ible to use them for
varying physical composition and liquids cont~ining
particles giving rise to turbidity.
CA 02208~97 1997-06-23
The object of the present invention is therefore
to provide a device for mea~uring the partial pressure of
gase~ dissolved in liquids using optical methods which
does not have the abG~ ~ntioned 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 meA; A 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 perme~hle to the
gas to be determ;neA, from a sample space which
contains the li~uid and, dissolved therein, the gas
Io be ~e~P~m.nP~,
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
determ; n~A, and ~ _
c) a measuring arrangement for det~rm; n; ng the light
beam emerging from the measuring chA~ber.
According to the invention, the measuring
chamber, the light-emission source and the measuring
arrangement are arranged in a rod-shaped 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 pr~m;nAntly 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 according to the invention are
solubility membranes. When they are introducted into the
CA 02208~97 1997-06-23
,
-- 5
sample space (10), they can establish equilibrium between
the ~ample liquid and the internal mixture.
According to the invention, 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 determ;ne~, which diffuses through
the membrane into the measuring chamber. To this end,
suitable liquid~ or gases may equally well be used. The
nature of the said fluids is dictated by the gases which
are to be measured.
According to the invention, light-emitting diodes
are preferably used as the light source. Using these
device~ has the following advantages: -
The emission has a relati~ely narrow band, which
means that it is not absolutely necessary to use inter-
ference filters in order to determ;ne the correspo~A;ng
gas selectively. By virtue of the relatively low power
consumption, it is in principle possible to design the
measuring structure as portable with battery operation.
A decisive advantage over conventional infrared ~ources
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 light-
emitting 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 ~h~rm~l and merh~n;cal stresses of
steam sterilization.
In the method according to the invention, 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).
CA 02208~97 1997-06-23
Furthermore, according to the inventionr 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 predomi-
nantly 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
deterr;n;ng the unabsorbed light intensity. According to
the invention, the measuring arrangement is preferably
connected to a special circuit for evaluating, storing
and displaying the signals. Because of this, the device
according to the invention is suitable, in particular,
for the automation of systems. When an integrated evalua-
tion unit is used, all the data can be acquired
automatically and a control process can be carried out.
A further advantage according to the invention is
the possibility of the 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 pres-
sures of 200 bar. ~referably, the probe is used at
pressures of up to 20 bar. In the case of use for fermen-
tation processes, it is merely necessary to ensure that
the probe withstands the elevated pressures which occur
under sterilization conditions.
A further subject of the present invention is a
method for measuring the partial pressure of gases
dissolved in liquids. In this method, the device accord-
ing to the invention is immersed in the liquid present in
the sample space in such a way that the m~hrane 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 w~ve~uides. The gas diffus-
CA 02208~97 1997-06-23
ing into the latter absorbs ~ome of the radiation. The
unabsorbed part of the light beam is fed to the measuring
arrangement, via an optical wd~e~uide, for det~m;n;ng
the partial pressure of the gas. Using correspo~;ng
evaluation, storage and display devices, the measurement
of the unabsorbed light beam can be used to determ;ne and
evaluate the partial pre~sure of the gas.
According to the invention, use is preferably
made of electromagnetic radiation generated by light-
emitting diodes. The infrared range is quite particularlypreferred.
The device according to the invention and the
method according to the invention are suitable, in
particular, for use in measuring the partial pre~sure 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
refr~sh;ng taste. Most deterr;n~tions are currently
carried out with simultaneous pressure and temperature
monitoring.
For optimum process control of biotechnology
processes, measurement of the partial pressure of carbon
dioxide is likewise al~o necessary. An important fact in
connection with this is that the supply of the micro-
organisms with gases and their inhibitory properties are
a function of the corresps~;ng 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 determ;n~tion 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
- CA 02208~97 1997-06-23
-- 8
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 determ;ne
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, det~rm;n~-
tion from the waste air gives concentrations and notpartial pressures. Concentrations can be converted into
partial pressures, and vice versa, using Henry-~s law. In
contrast to oxy~e~ 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
of the measured signal when approaching a new equili-
brium. Signals of this type can be misinterpreted as
changes in metabolism.
When the device according to the invention 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 sub-
stance to be measured, in order substantially to avoid
pressure fluctuations. According to the invention,
however, the device is not restricted to a particular
carrier liquid. Instead, the composition and chemical
nature of the latter are dictated by the type of gas to
be measured and the working conditions for the probe.
The invention will be expl~;n~ in more detail
CA 02208~97 1997-06-23
below with reference to the figure, according to which
the device in accordance with the invention consists of
the 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 ~hould not corrode.
The probe (1) has a connector (2) which makes it
po~sible 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 correspon~;ng connector to the connector (2).
This structure affords the possibility of sub-
]ecting the probe head to steam sterilization and usingit in sterile operation.
A light source (6) and a measuring arrangement
(7) are present inside the probe (1). 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 (8j. 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 photo-
detector (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 material, for example silver halides and
chalcogenides, are therefore preferable.
These optical w~e~ides can withstand thermal
stresses and are theretore suitable for use in a steam-
sterilizable environment.
- CA 02208~97 1997-06-23
.
-- 10 --
The measuring space (14) is located at the tip o$
the head of the probe (1). In the example according to
the invention, this space i8 filled with a chemically and
biologically inert fluid which has a high physical
absorption capacity for carbon dioxide. In fermentation
methods, the fluids chosen have a melting point which is
selected in such a way that pressure fluctuations do not
occur during the sterilization.
The measuring space (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 steam-
sterilizable material. According to the invention, poly-
tetrafluoroethylene and/or silicone are preferred for
this.
The dissolved gas diffuses through the membrane
(;i) into the sample space (lû) unril an equilibrium is
established. Since the diffusion of gases through a
membrane is controlled by partial pressure, the probe (1)
det~rm;ne the partial pressure. The probe therefore
measures a biologically me~n;ngful parameter, since the
supply to the microorganisms i8, 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 W/VIS or in the
infrared range. For carbon dioxide, this wavelength is
preferably 4.3 ~m.
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 ~nh~nces the detection
and minimizes effects such as DC drift.
The emitted radiation is fed, via the optical
waveguide (12), to the measuring space. The gas which is
present specifically attenuates the emitted radiation.
-~ ~ . CA 02208~97 1997-06-23
Some of the attenuated light i8 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 m~ ted light is used, the electrical signal may
likewise be modulated.
The length of the measuring chamber (14) corre-
sponds to the optical path length. An optimum optical
path length is selected in the measuring space (14), 80
that the probe (1) acquires the entire measuring range.
The measuring range is inversely proportional to the path
length. Thus, the smaller the path length o~ the measur-
ing chamber (14) of the probe (1), the greater is the
detectable range and the ~maller is the resolution.
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
simultaneous knowledge of temperature and pressure, as
well as the properties of the media. The latter is
difficult, in particular when using fer~enta-ion ~.edia.
Further~ore, the long-term stability, accuracy and
measuring range are increased in comparison with pH-
sensitive partial pressure probes.
The probe according to the invention can be used
particularly well both in the drinks industry and in
biotechnology. Probes for measuring ranges of up to
10 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,
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
CA 02208~97 1997-06-23
- 12 -
longer be carried out subsequently. A further advantage
in this field of application is that, during steriliza-
tion, the probe withstands therm~l 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.
