65-2435RK CO Transmitter • 17
CAUTION:
The null potentiometer must be set whenever the complete detector assembly is
changed or the hydrogen compensation may not work properly.
12. Calibrate the replacement detector as described in the Calibration section of this manual.
13. Secure the junction box cover to the junction box.
Setting the Null Potentiometer
When an old sensor is replaced, the null potentiometer must be adjusted so that the CO
transmitter’s hydrogen compensation will work properly with the new sensor. The procedure
basically involves applying hydrogen to the transmitter and using the null potentiometer to
cancel out the hydrogen response. This adjustment is made only once after the sensor is
replaced and does not need to be made again until the sensor is replaced again. A
concentration of 4,000 ppm or 5,000 ppm hydrogen is recommended for setting the null.
Perform the following procedure to adjust the null pot after changing a sensor.
Preparing for the Null Adjustment
1. Screw the calibration cup onto the bottom of the CO detector.
2. Use the sample tubing to connect the fixed flow regulator to the calibration cup.
NOTE:
Do not screw the regulator into the hydrogen cylinder at this time.
3. Set a voltmeter to measure in the millivolt (mV) range.
4. Remove the junction box cover, then plug the voltmeter leads into the test points on the
amplifier.
Plug the positive lead into the test point labeled
TP+
; plug the negative lead into the test
point labeled
TP-
.
NOTE:
Adjusting the null potentiometer may cause alarms. Be sure to put the controller into
its calibration program or disable external alarms before continuing.
Adjusting the Null Potentiometer
1. Screw the regulator into the hydrogen calibration cylinder. Gas will automatically begin to
flow.
2. Allow the gas to flow for two minutes, then adjust the null potentiometer clockwise and
counterclockwise until you find the adjustment that
results in the lowest mV reading at
the voltmeter
. This adjustment position will be somewhere in the middle of the
adjustment range and not at extreme clockwise or extreme counterclockwise.
3. Unscrew the regulator from the hydrogen cylinder. Wait two minutes before proceeding.
Leave the sample tubing connected to the regulator and the calibration cup.
