Page:NIOSH Manual of Analytical Methods - Chapter H.pdf/2

 b.

Electrochemical sensors are available for, e.g., SO2, H2S, NO2, COCl2, CO and O2 [1,2,3]. Except for O2, the widest application for electrochemical sensors has been as alarm/dosimeter systems rather than as continuous monitors. Because of the low power requirements and small size, the electrochemical sensor is ideally suited for use in combination monitors, that is, those that are able to monitor two or more substances at once. Many combination monitors are available, including in one package the sensors for oxygen deficiency, combustible gas, and toxic gas. The oxygen and toxic gas sensors are usually electrochemical. Electrochemical sensors may be located several meters away from the electronics/readout unit in order to facilitate remote or pre-entry monitoring.

Because of the low power requirements of these devices, it is possible for them to be used in lightweight, personal monitor/alarm devices. Electrochemical sensors for oxygen deficiency, H2S, HCN, and others have been designed into monitor/dosimeter/alarm packages that are small enough to fit into a shirt pocket, that weigh less than one pound (0.45 kg) and that operate continuously for as long as four months without changing the replaceable battery. Also, because of the low power required, it is relatively easy to design them to be intrinsically safe.

c.

The environmental conditions (temperature, relative humidity, barometric pressure) of the monitor at the time of calibration should be as near as possible to those that will be encountered during use. Of these three, temperature is most important because changes in temperature are most often encountered in the field and can cause bias in the readings obtained. Even with the temperature compensating circuitry employed in most sensors, some time is required for equilibrium to be reached. If it is not possible to calibrate at the working temperature, the user must allow sufficient time for field equilibration of temperature. Changes in barometric pressure are usually less significant than temperature changes and so are of less concern to the user. Oxygen monitors with pressure compensating circuitry should be employed whenever pressures differing by 5 kPa (0.05 atmosphere) or more from the calibration pressure [1,4] are encountered.

4. DATA ACQUISITION AND TREATMENT

a. The most simple example of field calibration of an electrochemical sensor is the oxygen monitor which may be calibrated by placing it in fresh (outdoor) air and adjusting the calibration potentiometer to make the readout meter read 20.9% O2. To determine if it responds to oxygen deficiency, hold the breath for a few seconds, then slowly exhale, directing the exhaled breath to the sensor. If it is functioning properly, the meter will deflect downscale and the alarm circuit will be activated.

For electrochemical sensors used to monitor other chemicals (e.g. H2S, CO, NO2, SO2), stable cylinders of calibration gases in the concentration range of interest as well as other, less convenient chemical generation systems (e.g., permeation tubes) may be used for calibration. These sensors may also be zeroed in fresh air or zero air from a compressed gas cylinder or clean air prepared by filtration. The frequency of calibration cannot be prescribed exactly, but a good rule is to calibrate at least once a day at the start of a shift. Manufacturers instructions or user experience may dictate more frequent calibration.

Always carry out the calibration procedure for toxic gases in a well-ventilated area,