THE EFFECT OF HYPERBARIC PRESSURE ON CATALYTIC AND ELECTROCHEMICAL GAS SENSORS

Sensors commonly used in workplace multi-gas monitors, and normally op erating at atmospheric pressure, have been evaluated under the pressur ized (hyperbaric) atmospheres typically found in tunnelling operations , i.e., 1-4.5 bar (100-450 kPa) static pressure and pulses of up to 0. 3 bar (30 kPa) above ambient pressure and of less than 30 s duration. Three types of sensor have been tested: catalytic sensors (pellistors) for methane, electrochemical fuel cells for carbon monoxide and elect rochemical metal-air batteries for oxygen. All the sensors are affecte d by pressure to some degree. Pellistors show a positive pressure effe ct (response increases by up to 50% at 3 bar), while pulses up to 0.3 bar produce transient responses equivalent to 0.2% methane. The electr ochemical carbon monoxide sensors are unaffected by pressure in clean air but there is a positive effect in carbon monoxide/air (the sensor reads approximately 11% high at 3 bar). During compression and pulsing in carbon monoxide/air larger effects are observed. The electrochemic al oxygen sensor behaves similarly to the carbon monoxide sensor: at 3 bar the sensor reads 12% high, while pulses of the order of 0.1 bar a bove ambient induce a 25% increase in response. The effects of static hyperbaric pressure can be overcome by recalibration of the sensors at the working pressure. Transient effects on electrochemical sensors ca n be reduced by careful siting of the sensor away from pressure-pulse sources or possibly by the incorporation of auxiliary diffusion barrie rs into the sensor. The effects of pressure on the sensor characterist ics are compared with theoretical models.