Capacitance Adjustment (CRH)

CAUTION Adjusting the capacitance measurement affects measurement accuracy across the entire temperature range. This command should be used only when the sensor has been replaced, or the measurement has drifted significantly. After sensor change, you must perform the two-point capacitance adjustment. For normal adjustment, use the RHCAL command instead.

CAUTION Write down the current capacitance adjustment parameters (visible with the L command) so that you can restore them manually if necessary. Be aware that if you revert the transmitter to factory settings using the FRESTORE command, you will clear all of the user adjustment parameters, including the capacitance offset and gain that are set by the CRH command. See Restore Factory Settings.

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

One-point adjustment requires a < 50 %RH reference.
Two-point adjustment requires additionally a > 50 %RH reference, with over 30 %RH difference between the two points.

Disconnect the cable from connector I of the DMT132 (if connected).
Connect the USB cable to connector II on the DMT132, and to a USB port on your PC.
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.
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.
When your serial connection is working, use the L command to see the current user adjustment parameters. Write down the Cp offset and gain values so you can restore them manually using the LI command, if necessary.
```
>l
```
Use the ERRS command to see that no errors are active:
```
>errs
```
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.
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.
Give the CRH 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.
```
>crh
RH: 11.39800112 Ref1 ?
```
Wait for 20 ... 40 minutes for the reading to stabilize. Press ENTER without typing anything to update the value shown by the CRH command.
```
RH: 11.39800112 Ref1 ?
RH: 11.38377201 Ref1 ?
RH: 11.37988329 Ref1 ?
```
When the measurement shown by CRH 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 CRH command without adjusting the measurement, press the ESC key and exit this procedure.
If you only want to perform a one-point adjustment (offset correction), press the ESC key after entering the correction.
The CRH command will exit, and you can move to 16.
```
RH: 11.35423321 Ref1 ? 11
Press any key when ready
>
```
To continue with the adjustment of the wet end (gain correction), press ENTER.
Insert the DMT132 in the NaCl salt chamber (75 %RH). Make sure the salt chamber is sealed and the transmitter sits tightly.
Wait for 20 ... 40 minutes for the reading to stabilize. Press ENTER without typing anything to update the value shown by the CRH command.
```
RH: 74.87782214 Ref2 ?
RH: 74.89157890 Ref2 ?
RH: 74.91556023 Ref2 ?
```
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: 74.95221002 Ref2 ? 75
OK
>
```
The CRH command is now finished and both adjustments have been applied.
Use the L command to verify that the new user adjustment parameters are in use, and write them down for later reference:
```
>l
```
Use the SEND command to verify that the measurement is OK:
```
>send
Tdf= -9.33'C
>
```

Capacitance Adjustment (CRH) - DMT132

DMT132 User Guide