ELECTROCHEMICAL

This monitor uses an electrochemical voltammetric sensor or polarographic cell to provide continuous analyses and electronic recording. In operation, sample gas is drawn through the sensor and absorbed on an electrocatalytic sensing electrode after passing through a diffusion medium. An electrochemical reaction generates an electric current directly proportional to the gas concentration. The sample concentration is displayed directly in parts per million, % oxygen or % LEL (lower explosive limit). Since the method of analysis is not absolute, prior calibration against a known standard is required. Tests have shown the method to be linear; thus, calibration at a single concentration, along with checking the zero point, is sufficient. The oxygen meter displays the concentration of oxygen in percent by volume measured with a galvanic cell. Other electrochemical sensors are available to measure carbon monoxide, hydrogen sulfide, and other gases. Some units have an audible and/or visual alarm that warns of low oxygen levels, LEL or malfunction. These pieces of equipment generally rely on the passive diffusion of air into the detector, however, some applications will require the user to attach a mechanical pump to actively draw air into the sensor.

Calibration:

Calibrate the direct-reading gas monitor with the appropriate calibration (span) gases before and after each use in accordance with the manufacturer's instructions. The monitor should be calibrated at the altitude at which it will be used. Changes in total atmospheric pressure caused by changes in altitude will influence the instrument's response. The unit's instruction manual provides additional details on the calibration of sensors.

Special Considerations:

Interference from other gases can be a problem (see manufacturer's literature).

If the span gas is directly fed into the instrument from a pressurized cylinder equipped with a regulator, the pump must be disconnected from the sensor to avoid sensor damage and the span gas flow rate should be set to match the sampling rate of the pump.