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.
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.
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.