Humidity Calibration (CRH) - HMM100

HMM100 User Guide

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Adjusting the capacitance measurement affects humidity measurement accuracy across the entire temperature range. Take care when making this adjustment.

Be sure to write down the current capacitance adjustment parameters (visible with the L command) so that you can restore them manually if necessary.
The following procedure provides a calibration procedure using the 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).
  • 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.
  1. Connect the USB cable to the service port on the module, and to a USB port on your PC.
  2. Start a terminal application and set the correct connection settings. The default serial settings are 19200 8 N 1. Remember to check which COM port the USB cable is using.
  3. Verify that the connection works by giving the ? command:

    If the module does not respond with device information:

    1. Check that the module is powered
    2. Try the ?? command in case the module is in POLL state, open the line using the OPEN command if necessary
    3. Check your serial line settings and cable connections
    4. Power off the module, unplug the USB cable, restart the terminal application, and try again
  4. When your serial connection is working, use the L command to see the current user adjustment parameters and the status of the trimmers. Write down the Cp offset and gain values so you can restore them manually using the LI command, if necessary.
  5. Use the ERRS command to see that no errors are active:
  6. Use the PASS command to enable the advanced commands:
    >pass 1300
  7. Turn both trimmers to zero using the Phillips head screwdriver, and use the ZERO command to set their correction to exactly zero:
  8. Insert the probe in the LiCl salt chamber of the humidity calibrator (11 %RH), and wait for 20 – 40 minutes for the reading to stabilize.
  9. Give the CRH command. The output from the command shows the currently measured RH value, and prompts you to enter the value of the reference 1. This is the reference on the dry end, which produces the offset correction for the measurement.
    RH: 10.971112003 Ref1 ?
  10. You now have three choices:
    1. To apply the correction, type 11 and press ENTER (assuming you are using the HMK15 and the LiCl salt)
    2. To exit the CRH command without adjusting the measurement, press the ESC key and exit this procedure
    3. To refresh the measured value without typing in the correction, press ENTER.
  11. If you only want to perform a one-point calibration (offset correction), press the ESC key after entering the correction. The CRH command will exit, and you can move to 14.
    RH: 11.0800360 Ref1 ? 11
    Press any key when ready

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

  12. Insert the probe in the NaCl salt chamber (75 %RH) and wait for 20 – 40 minutes for the reading to stabilize.
  13. Type 75 and press ENTER to apply the correction (assumes you are using the HMK15 and the NaCl salt).
    RH: 75.00134550 Ref2 ? 75

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

  14. Use the L command to verify that the new user adjustment parameters are in use.
  15. Use the SAVE command to save the changed settings.
    Saving settings...done
  16. Measure the value of the analog output to verify that the measurement is OK.