How to Field Calibrate Humidity Sensors

ACI’s A/RH Duct Series Sensor is a Relative Humidity Transmitter designed for use with electronic controllers in commercial HVAC building management systems. The RH Duct Transmitter can be powered with either AC or DC supply voltage and is field selectable with a 4-20 mA, 0-5 VDC, or 0-10 VDC output signal that is equivalent to 0 to 100% RH.

Calibration

Although ACI sensors are designed and engineered to drift less than other competitive humidity sensors on the market, over time, factors like environmental conditions and wear and tear can affect the performance of any sensor.

With proper calibration, the sensing element will not need to be replaced and the A/RH Duct Series Sensor will continue to provide reliable and accurate measurements – reducing the long-term operating cost of these sensors.

Calibration Steps

Using the dipswitch, the user can calibrate the sensor entirely through the software. To start, set SWITCH 4 to ‘ON’ – this puts the transmitter into setup mode, allowing the increment and decrement to work.

SETUP MODE

Once in setup mode, the output will change to 50% (2.5 VDC for 0-5 VDC, 5 VDC for 0-10 VDC, 12 mA for 4-20 mA). Each increment or decrement step will cause the output to change by 0.1 VDC for 0-5 VDC, 0.2 VDC for 0-10 VDC, and 0.32 mA for 4-20 mA.

• Increment RH Output
  To increase RH output by 0.5%, set SWITCH 5 to ‘ON’. The increase goes into effect each time SWITCH 5 is set to ‘ON’ – and only if SWITCH 4 remains set to ‘ON’ (SEE ABOVE).
• Decrement RH Output
  To decrease RH output by 0.5%, set SWITCH 6 to ‘ON’. The decrease goes into effect each time SWITCH 6 is set to ‘ON’ – and only if SWITCH 4 remains set to ‘ON’ (SEE ABOVE).
• Reset RH Output
  This will reset the RH output back to the original calibration. With SWITCH 4 set to ‘ON’, toggle SWITCHES 5 and 6 ‘ON’ then ‘OFF’. After 5 and 6 are ‘OFF’, slide SWITCH 4 ‘OFF’.  

• Calibration Benefits

  Using these options, the user can check how far offset the transmitter is. To reaffirm the starting point, set SWITCH 1 to ‘ON’. This will show the 50% output again. When the unit is out of setup mode, the output will go back to RH output. The maximum offset is 10% and there can be, in total, 20 increments.