Electrochemical sensors are
extremely sensitive to changes in the gas pressure, even when the O2
concentration remains constant. LuminOx from SST Sensing (UK) is available in
two versions: LOX-01 (without built-in pressure compensation) and LOX-02 (which
contains a barometric pressure sensor).
LOX-01 measures oxygen
partial pressure between 0 and 300mbar. If the total gas pressure changes, then
the ppO2 changes and LuminOx reports this.
LOX-02 reports both oxygen sensor (ppO2 in mbar) and O2 concentration (vol. % O2), the latter being
calculated from the total gas pressure and the ppO2. This means that LuminOx
will continue to read the correct oxygen concentration, even during rapid
pressure changes. As for the temperature measurement, LuminOx is able to
transmit the total gas pressure to the host via the two-wire communications
interface.
The three sensors were
supplied with 21% O2 and the gas temperature was held at 20 degrees C. A pump
was used to introduce a brief pulse of positive pressure of around 250mbar. The
blue line in the graph below is the pressure measured inside the test
enclosure. The pressure is read on the axis to the right of the graph. The O2%
reported by the three sensors is measured on the left axis.
A similar test was
performed with smaller (20mbar) positive and negative pulses.
LuminOx is affected
slightly but only briefly during the pressure event. The electrochemical
sensors are affected significantly (the O2% error is as large as 34%) and the
disturbance in the output lasts for a long time.
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