Humidity Calibration and Adjustment (RHCAL) - DMT132

DMT132 User Guide

Document code
M211289EN
Revision
B
Language
English (United States)
Product
DMT132
Document type
User guide

The procedure below describes a humidity measurement calibration and adjustment using serial line commands and the Vaisala Humidity Calibrator HMK15. You can use the procedure to do a one-point calibration (offset) or a two-point calibration (offset and gain).

  • To use the HMK15 humidity calibrator with the DMT132, you need the special cover set for HMK15 (Vaisala order code 230914)
  • One-point calibration requires a < 50 %RH reference
  • Two-point calibration requires additionally a > 50 %RH reference, with over 30 %RH difference between the two points
Even though the transmitter outputs dew point, the humidity calibration procedure is performed using relative humidity (RH) values. You can use the Humidity Calculator to convert between dew point and relative humidity. The Humidity Calculator is available at: www.vaisala.com/humiditycalculator.
Figure 1. Calibrating DMT132 Using HMK15 and USB Cable
  1. Disconnect the cable from connector I of the DMT132 (if connected).
  2. Connect the USB cable to connector II on the DMT132, and to a USB port on your PC.
  3. Start a terminal application and set the service port connection settings (19200 8 N 1).

    Remember to check which COM port the USB cable is using.

  4. Verify that the connection works by giving the ? command:
    >?
    If the transmitter does not respond with device information:
    • Check your serial line settings and cable connections
    • Reconnect the USB cable, restart the terminal application, and try again
  5. When your serial connection is working, use the L command to see the current user adjustment parameters.

    Write down the RH offset and gain values so you can restore them manually using the LI command, if necessary.

    >l
  6. Use the ERRS command to see that no errors are active:
    >errs
  7. Install the HMK15 calibrator fitting over the LiCl salt chamber of the humidity calibrator (11 %RH).

    Make sure it sits tightly over the salt chamber.

  8. Insert the DMT132 into the chamber. Do not remove the filter.

    Make sure all unused holes are plugged and the transmitter is installed tightly on the thread.

  9. Give the RHCAL command.

    The output from the command shows the currently measured RH value, and prompts you to enter the value of the first reference (Ref1). This is the reference on the dry end, which produces the offset correction for the measurement.

    >rhcal
    RH: 10.840536 Ref1 ?
  10. Wait for 20 ... 40 minutes for the reading to stabilize.

    Press ENTER without typing anything to update the value shown by the RHCAL command.

    RH: 10.890510 Ref1 ?
    RH: 10.920197 Ref1 ?
    RH: 10.930460 Ref1 ?
    
  11. When the measurement shown by RHCAL is stable, you can either apply the correction or exit the procedure:
    • To apply the correction, press ENTER one last time to update the value, type 11 and press ENTER again (assuming you are using the HMK15 and the LiCl salt).
    • To exit the RHCAL command without adjusting the measurement, press the ESC key and exit this procedure.
  12. If you only want to perform a one-point calibration (offset correction), press the ESC key after entering the correction. The RHCAL command will exit, and you can move to 16.
    RH: 10.950116 Ref1 ? 11
    Press any key when ready
    >

    To continue with the calibration of the wet end (gain correction), press ENTER.

  13. Insert the DMT132 in the NaCl salt chamber (75 %RH). Make sure the salt chamber is sealed and the transmitter sits tightly.
  14. Wait for 20 ... 40 minutes for the reading to stabilize. Press ENTER without typing anything to update the value shown by the RHCAL command.
    RH: 75.251050 Ref2 ?
    RH: 75.200621 Ref2 ?
    RH: 75.198200 Ref2 ?
  15. To apply the correction, press ENTER one last time to update the value, type 75 and press ENTER again (assumes you are using the HMK15 and the NaCl salt).
    RH: 75.175200 Ref2 ? 75
    OK
    Saving settings...done
    >

    The RHCAL command is now finished and both adjustments have been applied.

  16. Use the L command to verify that the new user adjustment parameters are in use:
    >l
  17. Use the SEND command to verify that the measurement is OK:
    >send
    Tdf= -9.33'C
    >