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HYDROGEN SULFIDE DETECTION:

Electrochemical vs. Solid State Semiconductor Sensors

Technical Discussion (cont'd)

variations in chip manufacturing and the process of metal oxide deposition. A sensor operating too cold will respond very slowly; a sensor operating too hot will respond very quickly but will also demonstrate a tendency toward non-repeatability. However, only a few manufacturers address the critical importance of this issue by thermostatically controlling the heater to prevent surface temperature fluctuations. Jhe temperature effects on the sensitivity of the electrochemical sensor are predictable and repeatable from cell to cell. From 0 °C to 40 °C, the span changes by less than ± 10% of the reading given at 20 °C. From 0 °C to -40 °C, there can be a further fall of 5% of the reading. The zero shifts by less than 3 ppm for a temperature change from 20 °C to 40 *C. Below 20 °C, baseline shifts are negligible. The electrolyte does not freeze at -40 °C.

The currents generated by the oxidation reaction are quite low—typically 0.4 jiA/ppm H_;S. However, the intrinsically low background current and low noise output of the cell results in excellent repeatable sensitivity to H;S.

External Influences on Response

Sensor response may also be influenced by a variety of environmental conditions. Solid state semiconductor and electrochemical detectors vary quite differently in how they are affected by these often uncontrollable environmental factors. Lack of H₂S Exposure - A common trait of solid state semiconductor sensors is their tendency to "go to sleep" when exposed to H?S free air for prolonged periods of time. This effect is generally caused by the localizing of electrons within the metal oxide film, which drives the zero resistance into the high meg ohm range. This normally high zero resistance prevents a quick or repeatable response. Most electrochemical cells have an absolute zero, which eliminates the "going to sleep" syndrome Rain and Humidity - The metal oxide film on a solid state semiconductor sensor is vulnerable to changes caused by exposure to water. A gradual conversion of the film to a metal hydroxyl state occurs, which eventually deadens the sensor's sensitivity to H;S.

Even brief exposures to water through rain or washdown will cause surface changes that prevent absorption of hydrogen sulfide; recallbration of the sensor is advisable after exposure to moisture. To minimize susceptibility to moisture damage, surface temperature should be greater then 100 °C. This is often accomplished with a built-in heater. Changes in humidity or direct exposure to moisture have little effect on the electrochemical cells. All reactions take place at the working industrial electrode, where moisture is continually present. The electrolyte reservoir has sufficient overcapacity to allow the cell to accommodate all but the most prolonged periods of very high or very low humidity. The cell has been tested for three months at a relative humidity of 0% without III effects. In addition, once the RH was increased, the cell reabsorbed all lost moisture. Temperature - Maintaining a constant chip surface temperature is important to keeping the response time and stability of solid stale sensors within acceptable limits. For this reason, heaters are often built into the sensor surface.

Equally important is the need to set each individual sensor's operational temperature to compensate for

Operating Considerations

There are certain operational costs affected with any gas industrial monitoring system. Calibration and power consumption are two of the major factors that should

Calibration - In general, the manufacturers of solid state sensors recommend monthly or three-month calibration intervals. However, many companies have adopted a daily calibration schedule for these systems. This is due primarily to the lack of repeatability caused by the sensor's vulnerability to surface changes. In addition, and as previously mentioned, calibration at two or more concentration levels is required to assure proper response all along the logarithmic response curve; this multi-concen trail on calibration process often requires

The linear nature of the electrochemical sensor, along with its absolute zero and excellent repeatability, allows single point calibration at up to six-month intervals.

Reproduceability between electrochemical cells is outstanding Direct replacement without calibration will produce a response within 10% of the original; with calibration, the response will be within 2% of the original.

Power Consumption - The sensor heater and associated circuitry is a major power draw in solid state semiconductor systems This can be major drawback in applications for portable monitors or for fixed systems needing solar power or battery backup. Power consumption for systems using the electrochemical sensor is significantly reduced.