2. Problem formulation
The reliability of aerological stations and sensor networks could
be significantly improved by addressing the problem of sensors
interoperability, i.e. when a sensor can successfully detect the gas
concentrations in 0–100% range. This can be done by improving the
“informativeness” of the sensors which employ the catalytic principle
of operation towards detecting low concentrations defined by
sanitary standards, as well as with high concentrations up to 100%.
Nowadays, low gas concentrations are detected using IR and
semiconductor sensors. IR sensors are sensitive, expensive, and
bulky. Their sensitivity can significantly decrease due to moisture
condensation on the optical path at high humidity conditions
when, for instance, the ambient temperature changes rapidly.
Semiconductor sensors have another important disadvantage – low
selectivity – which results in frequent false detections.
There is a widespread notion that the measurement of low concentrations
of combustible gases and their vapors with catalytic
sensors is not acceptable due to the high level of thermo and electrical
noise comparable with the response signal and a number of
uncontrollable parameters, such as humidity (see Fig. 1).
Fig. 1 shows the response of catalytic sensors embedded in
Wheatstone circuit. The experiments have been conducted in a
chamber which emulated different conditions: wet atmosphere,
dry atmosphere with and without 100 ppm and 1% of methane. We
note that methane in this experiment, as well as in the experiments
presented later in Fig. 3 and Fig. 4, is diluted by dry air. Fig. 1b shows
that the response signal V1 in atmospheric air (humidity = 79%)
is approximately 1.5 mV, which is much higher than the response
signal V2 at 100 ppm of CH4. This analysis demonstrates that the
measurement approach based on the Wheatstone circuit cannot
control the humidity influence. The sharp peak near the ‘0’ level
is associated with the circuit and the graphical transient processes
from zero value towards the value to be measured.
To overcome this problem, we propose the modified Wheatstone
circuit and a gas diffusion control mechanism described in
the next sections.