DE3939166A1 - Rapidly calibrating metal oxide sensors for oxidising gas - using effusion cells, emitting gases in controlled concn. - Google Patents

Abstract

Metal-oxide sensor, when heated, undergoes sensitive resistance changes in response to the presence of oxidising gases. The prolonged period needed for stabilising for the calibration of such sensors is considerably shortened by disposing near the sensor a gas effusion cell periodically induced by a control and evaluation unit emitting a defined electrical current causing emission of a defined concn. of gas, e.g. H2. The sensor element, esp. of tin oxide type, which may receive a stepped series of impulses corresp. to different concn. values, change an electrical parameter so that the control and evaluation unit pref. microprocessor controlled, can produce calibration based on two or more points. USE - Include alarm signals, esp. for fires, requiring min. power and operable for long periods. Calibration time is reduced from possible weeks to hours. The effusion cells used are readily available.

Description

It is known that metal oxide sensors that are heated sensitive to the property with a change in resistance reaction of oxidizable gases.

Tin dioxide sensors have become known after the above ge called principle, it has become known that these Sen stable operating condition after a few days to reach.

It follows from this that a stable zero point during short-term Operation is not achieved.

The reason for this behavior lies in the fact establishes that the moisture in the sensor surface activity with the moisture in the air must balance, with water as OH Adsorb groups on the surface of the sensor.

A publication by FIGARO (Sensors and actuators 14 (1988) 223-232) found that a stable balance after about 100 hours of operation sets.

Applications in which one of the long stabilization time is not available.

The invention described below turns on gabe, a problem-free application even under short-term guarantee conditions.

The invention takes advantage of the fact that electro chemical effusion cells that are commercially available e.g. B. from from Simatec / CH-Herzogenbuchsee are available from Applying an electrical voltage z. B. hydrogen free put.  

There is a linear relationship between the current, which flows through the chemical cell, and the developed one Gas rate.

Published in vacuum technology 22, 1973, page 106, electrochemical cells that release halogens, wherein the electrochemical mechanism in principle with that of Hydrogen cells is comparable.

According to the invention, it is therefore proposed to use such chemical cell with a gas sensor in a housing like this to unite that at the request of the chemical Cell released gases directly onto the gas sensor Act. In the preferred embodiment, below however, a metal oxide sensor, preferably consisting of Tin dioxide.

The basic structure is shown in FIG. 1:

The electrochemical cell ( 1 ) is arranged in the immediate vicinity of the gas sensor ( 2 ). The signals from the gas sensor are processed by a control unit, which is preferably microprocessor-controlled ( 3 ).

Periodically at freely definable intervals, the evaluation and control unit ( 3 ) generates an electric current which causes the electrochemical cell ( 1 ) to release a specific amount of gas. In the preferred embodiment that would be hydrogen.

Figs. 2 shows the basic sequence:

Immediately after switching on, the sensor drifts for several days. The baseline ( 4 ) accordingly shows a gradient.

According to the invention, a comparison is now proposed short time after switching on, e.g. B. at a distance less minutes, via the electrochemical cell a defi nated concentration of z. B. release hydrogen.  

According to the invention, the concentration should assume different levels in steps in a short sequence ( 5, 6, 7 ).

A gradient can be calculated from these levels, which on the one hand allows the zero line to be calculated and on the other hand allows the sensitivity to be determined. In a ge time afterwards ( 8 ), the z. B. 5 minutes, the sensor for gas detection can be used on the basis of the sensor data thus determined, advantageously also taking into account the influence of rela tive humidity and other interfering gases by this measure.

The invention thus advantageously solves the problem of auto matic calibration, especially because of electrochemical Effusion cells relatively cheap on the market are.

The teaching of the invention is particularly advantageous when using multi-sensor systems, such as those used for. B. has been described in European Patent Application 8 91 14 409.9. In such applications in particular, the reproducibility of the characteristic curves is crucial. The measures described by the present invention can be used to ensure that the conditions for the measurement phase ( 8 ) are always stable in terms of calculation. It is of course possible to dispense with the integration into the sensor housing and in a further embodiment of the invention with the help of the teaching of the invention to design a calibratable gas generator which enables ge to calibrate or test existing sensor systems.

In a special embodiment of the invention, a Use within fire alarm systems, often under Use of tin dioxide sensors work in such a way that the Smoldering gases or combustion gases a change in resistance cause that is evaluated electrically.  

It has been shown that due to the very high diffusion speed, hydrogen gases first reach the sensor, even before other gases reach the sensor. So it is sensible, a function check of the sensor with the help of generated by the teaching of the invention hydrogen gas respectively.

This makes it possible to con device at any time structure that an automatic function test is possible. The fire alarm sensors that can be created according to this principle get a significant increase in quality.

It is also noted that it is sensitive to hydrogen gas sensors based on the MOS-FET principle. The practical Among other things, the applicability of these sensors is given so far because of zero, because the zero points and working points drift considerably. Advantageous these new sensors, however, is the fact that hardly own energy consumption takes place. Because the electroche mix cells also only with a very low energy dispose of it, it is possible according to the teaching of the invention Handy, handy and portable and battery-operated devices to construct with a long service life, the most diverse Applications, e.g. B. portable fire alarm devices etc., can be assigned.

Claims (2)

1. Apparatus and method for sensing gaseous substances using gas sensitive layers, for. B. tin dioxide or MOS-FET or heat-toning pelistors or electrochemical mixing cells, characterized in that an electrochemical effusion cell is spatially assigned to the gas-sensitive sensor element so that gases emitted by the electrochemical effusion cell act on the gas sensor, with a control and evaluation unit ( 3 ) the electrochemical cell is subjected to a defined current at time intervals, which causes the electrochemical cell to emit gas in a defined concentration, whereby the sensor element ( 2 ) changes its electrical parameters and thus the evaluation and control unit ( 3 ) gives the opportunity to calibrate the sensor element ( 2 ). 2. Apparatus and method according to claim 1, characterized in that the electrochemical cell does not emit a pulse with a defined concentration, but a gradation of pulses of different concen tration to a two or multi-point calibration by the electrical evaluation and control unit ( 3 ) to be able to make.