Gas Analysis Sensor for Measuring Concentration of Gas Constituent

The invention relates to gas analysis sensors for measuring concentration of a gas constituent, and more particularly to electronic equipment utilized in such sensors.

In order to determine the proportions of gas such as, for example, of oxygen, nitrogen oxides, etc., in a gaseous atmosphere to be monitored or of combustible constituents in a fuel-gas atmosphere, use is made, for example, of analyzer cells or sensors which operate on the basis of electro-chemical oxidation. With this measuring procedure, the gaseous constituent to be measured in the gas to be analyzed (test gas) produces in the analyzer cell or in the sensor which, for example, contains two electrodes and an electrolyte, an electro-chemical reaction by means of which there is produced a voltage (EMF) which is proportional to the concentration of the gas to be measured, the said voltage being fed to an indicating device and converted into a concentration indication of the gas to be measured.

With apparatus equipped with analyzer cells or sensors, which operate on the above-mentioned principle, the voltage produced in the sensor is proportional to the concentration or to the partial pressure of the gas in the medium to be analyzed. Should changes take place in the atmospheric pressure, the indicating instrument will follow the change in the partial pressure of the gas, with the consequence that indicating errors must be taken into consideration when making measurements at depths (below sea level) or at high elevations. Until the present, in the event that the elevation was different, an adjustment was made at the measuring site or the indication error was allowed for by a precalibration, or computed and evaluated with the aid of tables. It is obvious that a procedure of this type has its drawbacks and is problematical, especially when measurements have to be made to determine the amounts of gas in a gaseous atmosphere to be monitored below ground.

Consequently, the invention's basic task is to create an electronic measuring device for a sensor whose output is decisively influenced by the partial pressure of the substance to be measured, and to so construct the device that the sensor in independent of external influences, such as all pressure fluctuations, and ensures an automatic matching of the indication to the changing environmental pressure at the measuring site.

This task is accomplished in accordance with the teachings set forth in the appended claims.

The invention is suitable for all sensors such as, for example, those which are also incorporated in infrared gas analyzers and by means of which, in order to determine the concentration of a particular gas constituent in a gaseous atmosphere to be monitored, the current developed in the sensor is proportional to the partial pressure of the gas in the medium being analyzed.

The advantages to be gained with the invention reside, in particular, in the fact that, on using the apparatus at the measuring site, it is unnecessary to carry out any adjustments on the apparatus which allow for changes in the atmospheric pressure, such as adjustments by means of calibrating screws which could subject the apparatus to mechanical stresses of too strong a nature. Dispensing with adjustments of this type constitutes, on the one hand, a simplification in the manipulation of such equipment and, on the other hand, provides a greater reliability and accuracy in the indication, which is especially advantageous when the elevation or the depth of the measuring site is unknown.

An embodiment of the invention will now be described in detail by way of example on the basis of the appended drawing.

The instrument consists essentially of a pressure probe or module 1, a series-connected amplifier 2, a sensor 3 with a series-connected differential amplifier or microprocessor 4 and of an indicating device 5.

A power supply 6 is connected in parallel with the pressure probe or module 1. The pressure detected by the pressure probe 1 is converted into an electric signal which, adjusted via the amplifier 2, is fed to the measurement amplifier 4. The adjustment of the signal is necessary because electric signals of undesired amplitude are emitted by the pressure probe 1 and these values must correspond and be matched to the pressure changes of the partial pressure of the gas in the sensor 3. The input A of the amplifier 4 is connected to the output C of the pressure probe amplifier 2. A change in the value of the input to the input A of the amplifier 4 produces a change in the value of the amplifier's output D and thus a change in the indication in the indicating device 5. With constant pressure relationships, no voltage change takes place via the amplifier branch C, A, D. In the event of changes in the atmospheric pressure such as occur, for example, as the result of increases in pressure at a depth, the sensor gives a higher EMF with the same gas content. This increase in voltage is now compensated by the simultaneous voltage increase at the input A of the amplifier 4, that is, the pressure probe 1 with the series-connected amplifier 2 changes the voltage at the input A of the amplifier 4 by the same amount, corresponding to the higher EMF of the sensor 3. By this means, the difference between the two measuring signals from the pressure probe 1 and from the sensor 3 at the inputs A and B of the amplifier 4 is maintained, so that the output signal D likewise remains unchanged and provides, in the indicating device 5, a true unfalsified measured value which has matched the prevailing pressure relationships.

The normal gas concentration of the gas constituent to be measured is calibrated via the amplifier 4 by means of a potentiometer 7 connected in series with the sensor 3.