General Monitors 610A - Check Points for Calibration and Operation; Frequency of Calibration; Background of Combustible Gases; Replacing a Sensor

Model 610A
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3.9 Check Points for Calibration and Operation
3.9.1 Frequency of Calibration
GMI recommends that the calibration be checked on each sensor at least every ninety
(90) days. If a sensor is installed where it may be subjected to splashing water, mud or
dirt accumulation, or adverse gases, more frequent calibration is recommended. The
exact frequency can vary with the severity of conditions and must be established in the
field.
3.9.2 Background of Combustible Gases
In some applications, there is an occasional or continuous presence of “background”
combustible gases. Generally, this is a very small % LEL. Usually, it is advisable to zero
out the background gas concentration during calibration.
To Zero the Background Gas Concentration:
1. Isolate the sensor from the surrounding air by placing your hand tightly over the
sensor.
2. Observe the reading on the “%” LEL display. A gradual drop in reading indicates the
presence of background or combustible gases. Keeping the hand over the sensor,
wait for the reading to stabilize and start the calibration without the calibration cap.
3. When “AC” displays, remove your hand from the sensor, place the cap over the
sensor and continue the calibration.
3.9.3 Replacing a Sensor
When a sensor is replaced, the new sensor must be calibrated. To avoid false alarms,
General Monitors recommends disabling all the alarm circuits until the sensor is
calibrated. For better results, the sensor should be powered up at least one hour before
calibration.
CAUTION: Extended exposure of a sensor to a high concentration of combustible
gases can introduce stress in the sensing element, which may seriously
affect performance. Re-calibration should therefore be performed after an
alarm due to a high concentration of gas and the sensor should be
replaced, if necessary. A display reading of “99”% LEL, or high off scale,
may mean an explosive concentration of gas is present.
NOTE: The previous warning is applicable to all catalytic bead sensors.
3.10 Special Options
3.10.1 Optional Model 610A Controller for Zone Control (Voting)
The Model 610A Controller can be supplied with a special voting option, when specified
on the original order for the equipment. A system supplied for voting use, must be the
common relay version and will require that two channels reach the High alarm set point