About this task
The CO transmitter features a simple snap-mount pre-calibrated sensor PCB. You can replace the entire sensor PCB with a new calibrated PCB without the removal of the enclosure. This sensor swap requires no tools and you can complete it in seconds. Disconnect the device wiring, remove the old sensor PCB, snap in the new PCB, and reconnect the device power. There is no need to make any adjustments or apply gas to the transmitter using the sensor swap method.
If necessary, calibrate the device or verify with CO gas. This requires a field calibration kit that consists of a bottle of gas, for example, 250 ppm CO in air, a tank pressure regulator with flow restrictor, and the necessary tubing with a calibration cap to cover the sensor. You can calibrate at 68°F to 81°F (20°C to 27°C).
You can verify with gas without the removal of the device cover. Use the calibration cap attached directly to the port on the cover to apply gas and monitor the output value. See Figure 1. Remove the device cover to perform an actual calibration. In this case, the gas calibration cap attaches to the sensor fixture inside the enclosure. See Figure 2.
For local display, on a device with a 0 VDC to 10 VDC output, use a voltmeter to measure between the OUT and COM terminals to measure the output. For a device with 4 mA to 20 mA, place an ammeter in series of the output. Disconnect the signal wire from the OUT terminal of the CO sensor. Connect the + lead of the ammeter to the OUT terminal of the CO sensor and connect the COM lead of the ammeter to removed signal wire. Set ammeter to read a 20 mA signal.
Continuously power the sensor for at least 30 min prior to calibration. To calibrate the sensor, complete the following steps:
- Calibrate the sensor first in clean air with no CO gas present. Adjust the ZERO pot on the sensor board until you obtain a 0 VDC or 4 mA output depending on device. For this case, adjust the output slightly above 0 VDC or 4 mA and slowly reduce the output signal to 0 VDC or 4 mA, depending on the device.
- Monitor the output signal with the voltmeter or ammeter that connects to the OUTPUT and COMMON terminals.
- Attach the gas supply.
- Turn the regulator on/off knob fully off and attach it to the 250 ppm gas bottle and firmly tighten it by hand.
- Moisten the sponge and squeeze out any excess water.
- Place the sponge in the cap so that it does not touch the sensor and does not plug the hole in the side of the cap.
- Attach the cap to the fixture over the sensor.
- Slowly turn the valve knob on the regulator to let the gas flow. The regulator restricts the flow rate to the specified 200 ml per min and the sponge ensures the gas is in the right humidity range.
- Wait for 5 min and adjust the SPAN pot on the sensor board until the output value reads 250 ppm. For the 0 ppm to 300 ppm device span, 250 ppm equals 8.33 VDC or 17.33 mA, depending on the model.
- Close the valve on the tank and take the cap off from the sensor. This completes calibration.
- If the gas cap is too loose on the fixtures, wrap electrical tape around the cap to tighten it.