Vaisala HMP228 - Manual (Page 27)

HMP228
M210282en-A Operating Manual
21
4.3 Moisture calibration
The HMP228 transmitter has been fully calibrated at the factory so there
should be no immediate need for recalibration. The transmitter should be
calibrated only if there is reason to believe that the adjustments have changed.
The optimal interval for moisture calibrations depends on the process and the
recommended interval varies from 3 months to 2 years.
NOTE
The HMP228 transmitter measures water activity in
liquid/oil and the calibration is performed as for relative
humidity transmitters. In calibration mode the
transmitter automatically outputs relative humidity as
the calibration parameter. It is essential to clean the
sensor surface of oil before calibration; use e.g.
instrument air or nitrogen to blow off the oil. Even the
smallest amount of oil may destroy the salt solution.
NOTE
As oil performs a membrane over the sensor surface,
the sensor's capability of absorbing moisture is slowed
down. The stabilization time is thus at least double the
time of a clean sensor; pay special attention to
controlling the stabilization time.
A two-point calibration can be performed with Vaisala’s HMK15 or HMK13B
Calibrator or the instrument can be sent to Vaisala. The instrument has to be
recalibrated each time the moisture sensor is changed.
A Ø 13.5 adapter must be used when calibrating with the HMK13B Calibrator.
The adapters (part no. 16611) can be ordered from Vaisala or Vaisala
representatives.
Calibration can be performed by giving the commands using the press
switches inside the housing, through the serial bus or through the menus on
the local display.
When LED commands are used, relative humidity is output instead of water
activity. When the transmitter is calibrated at two points, the points must be
either 50 %RH or 50 °C apart from each other.
NOTE
If the sensor has been changed, perform a calibration according to Chapter
4.4.1.