Vaisala HMP240 - Manual (Page 44)

HMP240
User's Guide
M210300en
42
6. CALIBRATION
The HMP240 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 of the
transmitter have changed. The adjustments of the temperature measurement
channel and the analogue outputs are particularly stable; in normal
circumstances there is no need to recalibrate them. Humidity calibration
should be performed at least once a year.
NOTE
During the calibration procedure, the HMP240 is like
any standard RH transmitter and is therefore calibrated
against known RH values. The warming function is not
active and corrective calculations are not made. It is
therefore essential that the sensor head is allowed
enough time to stabilize to the ambient conditions in
order to ensure the highest possible accuracy in
calibration.
Either a one-point or a two-point calibration is possible. The security lock
jumper has to be disconnected at the moment of taking the transmitter to the
calibration room in order to make sure that the temperature of the sensor head
stabilizes correctly and the warming function does not interfere with the
stabilization. The stabilization of the humidity sensor head can be monitored
by the RH reading or by the T reading: the T reading is actually that of the
humidity sensor head stabilizing to the ambient temperature.
When the security lock jumper is disconnected, the serial port and analogue
outputs show the RH reading and T reading measured directly from the
humidity sensor head (instead of the calculated values). This means that the
readings (RH, T, a, x, dT or Tw) are erroneous until the humidity sensor head
has stabilized to the ambient temperature; the Td reading is correct also during
stabilization. It is recommended that the transmitter is disconnected from the
process during the calibration.
6.1 Humidity calibration
The calibration of the HMP240 transmitter can be checked with a calibrated
Vaisala humidity meter, e.g. the HM34. The HMI38 humidity data processor
with an appropriate reference connection cable can be used as a field cali-
brator. A two-point calibration can be done with the HMK15 or the HMK13B
calibrator, or the instrument can be sent to Vaisala. We recommend
recalibration at least once a year. The instruments must be recalibrated every
time the HUMICAP

HC sensor is changed.