Vaisala HMD70 - Manual (Page 12)

HMD70U/Y Transmitters
Operating Manual
U303en-1.1
8 1997-07-31
4. CALIBRATION
4.1 One-point humidity calibration
The accuracy is recommended to be checked at least once a year; the interval
depends on the operating conditions and the required accuracy of the
measurement. The transmitter calibration can be conveniently checked with
the HMI41 indicator equipped with an appropriate probe and optional
calibration cable. If adjustment is needed, use the one-point calibration
potentiometer. If you prefer to calibrate the HMD70U/Y transmitters against
saturated salt solutions, use LiCl (11 %RH) and NaCl (75 %RH) solutions.
4.2 Two-point humidity calibration
The calibration can also be done with salt calibrators, or the instrument can be
sent to Vaisala or a Vaisala representative.
4.2.1 Two-point humidity calibration procedure
•
Leave the calibrator and the transmitter for at least 30 minutes in the
same space so that their temperatures have time to equalize.
•
Place the probe into the calibration hole of the LiCl bottle in the
humidity calibrator.
•
Wait for 10 minutes.
•
Use the RH offset potentiometer (Figure 3.4.) to adjust the output
signal to the value given in the calibration table (Chapter 4.3).
•
Place the probe into the calibration hole of the NaCl bottle in the
calibrator.
•
Wait for 10 minutes.
•
Check that the reading corresponds within the desired accuracy to the
reading given in the calibration table. If not, adjust the reading with
the RH gain potentiometer (Figure 3.4).
•
Check again the reading at the first point and adjust if necessary.