Measuring probe for amperometric determination of gases and/or n

Details Measuring probe for amperometric determination of gases and/or n Measuring probe for amperometric determination of gases and/or n

1991-12-23

1993-08-24

Harvey, Jack B.

Electricity: measuring and testing

Determining nonelectric properties by measuring electric...

204415, G01N 27403

Patent

active

052392570

DESCRIPTION:

BRIEF SUMMARY
TECHNICAL FIELD

The invention concerns a measuring probe for amperometric determination of gases and/or nonionic compounds contained in a measured medium, as well as the use of the measuring probe for the amperometric determination of oxygen in gas mixtures or fluids, and the use for amperometric determination of chlorine or hydrogen in gas mixtures or fluids.


BACKGROUND OF THE INVENTION

Membrane-covered measuring probes, specifically amperometric sensors are increasingly used for monitoring biological and biochemical processes as well as in the beverage and brewing industries, for determination of gases and/or nonionic compounds contained in a measured medium, for instance volatile ingredients of the measured medium or gases, specifically oxygen. To achieve an optimum function of such measuring probes, for instance a short response time, high sensitivity, low detection limit and good long-term stability, it is necessary to ensure that, e.g., pressure differences caused by temperature fluctuations between the measured medium and the interior of the measuring probe surrounding its sensitive parts will be extensively avoided or compensated for. Especially necessary for such measuring probes is to allow steam sterilization, so that the pressure differences occurring during steam sterilization will not affect the function of the measuring probe, i.e., measures are to be taken by which a deformation of the membrane due to high pressure differences between the measured medium and the probe interior will be prevented. In the case of measuring probes with a nonreinforced gas-permeable membrane, for instance as described in U.S. Pat. No. 2,913,386, a sufficient pressure stability can be achieved only with high equipment expense. To counteract the undesirable membrane deformation, various approaches were taken and the following measures applied: interior of the measuring probe;
The pressure equalization cited under a) is accomplished, for instance with the oxygen electrode described in U.S. Pat. No. 4,252,627 featuring an electrolyte-filled interior, in such a way that the probe housing is provided with an air gap via which the probe interior can communicate with the measured medium. A swift and exact pressure equalization can be achieved thereby; but it risks that volatile compounds can unimpededly penetrate the probe interior from the measured medium. This may cause a change of the electrolyte composition or, if no electrolyte is present, a change of the gas phase present in the probe interior, with the result of possible measuring errors in both cases.
The pressure equalization mentioned under a) may also be accomplished with the use of a flexible membrane or with the aid of a fluid drop, as described in U.S. Pat. No. 4,455,213, or by means of a piston moving in the probe housing. Besides, the pressure equalization can be accomplished by means of a valve which opens at a specific overpressure in the probe interior. This allows keeping the interior pressure within certain limits, but providing a valve causes a considerable equipment expense and, additionally, counter-acts a miniaturization of the probe, which for numerous applications is desirable.
The adjustment of a pressure gradient as mentioned under b) can be accomplished, e.g.. in that the probe interior is open toward the atmosphere causing the membrane to be forced on a backing, by the pressure of the measured medium. The membrane suffers then hardly a deformation, provided the roughness of the backing required for maintaining an electrolyte film is not selected excessively high and the pressure does not exceed a specific maximum value. In the case of probes where the interior is gas-filled, a satisfactory function is given only if the measurements will not be influenced by the penetration of atmospheric matter.
From the above comments it is evident that a sufficient pressure stability, and thus the prevention of an undesirable membrane deformation, can be accomplished with the measures cited under a) and b) only with considerable equipment expense. Th

REFERENCES:
patent: 3577332 (1971-05-01), Porter et al.
patent: 3718562 (1973-02-01), Haddad
patent: 4252627 (1981-02-01), Ohashi et al.

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