Design and development of electrochemical gas sensors
Historically, electrochemical gas sensors had suffered from several drawbacks such as poor temperature coefficient, leakage, susceptibility to shock and vibration and orientation sensitivity, which led to poor field reliability. In the present work these problems have largely been overcome by superior design, drawing on field experience in fuel cell and battery technology. The culmination of a sensor design embodying a number of unique concepts has revolutionised electrochemical gas sensor analysis and has pioneered the way for many new and hitherto difficult applications. The main features are: (a) A capillary diffusion-limiting barrier, based on gas-through-gas diffusion, with a theoretical temperature coefficient of 0.17% of signal per °C (at 20°C). (b) Very active fuel cell-type Pt black electrodes with large activity reserves giving rise to low span temperature coefficients, wide dynamic measurement ranges and enhanced long-term stability. (c) A close-wick sandwich arrangement of the electrodes conferring very good stability, to the extent that the sensors are substantially immune to shock and orientation problems. The sandwich design also enables the sensors to be very compact. (d) Use of strong sulphuric acid electrolyte in balance with ambient relative humidity (RH) - about 65% on average in temperate climates - in conjunction with a wick dipping into an expansion reservoir, giving maintenance-free, continuous dynamic range of operation between 20% and 90% RH and very long residence times outside these limits -several weeks in zero RH and several months in 100% RH at 20°C. (e) Extensive use of porous polytetrafluoroethylene (PTFE) membrane sealing techniques, which have dramatically improved cell integrity to the extent that leakage is virtually eliminated.(f) Matched sensing and reference electrodes in conjunction with zero bias cell operation, which allows the sensing and reference electrodes to be shorted out when the instrument is switched off; this gives almost instant warm-up when the instrument is switched on and the cell has excellent (NULL) stability under all conditions. (g) Since the sensor does not need to be powered-up when the instrument is switched off, there is a considerable saving on battery power in portable, hand-held instruments. (h) Inclusion of a second sensing (auxiliary) electrode, which enables the cancellation of partially reacting cross-interfering gases such as hydrogen. The auxiliary electrode can also substantially offset baselines; this is especially beneficial in biased sensors which generate large baselines. (i) Use of inboard chemical filters, which can remove cross interfering gases such as NO, N02, SO2, C12, NH3 and C2H4 by chemical reaction/adsorption.