RKI Instruments 65-2435RK - Manual (Page 17)

65-2435RK CO Transmitter • 17
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.
Resetting the Zero Reading
The adjustment of the null potentiometer may result in a change in the zero reading, so the zero
reading may need to be adjusted.
NOTE If you can verify that the CO transmitter is in a fresh air environment, you do not need to
apply zero air to the detector before adjusting the zero reading.
1. Screw the regulator into the zero air calibration cylinder. Gas will automatically begin to flow.