Vaisala HMP240 - Manual (Page 58)

HMP240
User's Guide
M210300en
56
7. MAINTENANCE
7.1 Reference measurements
Reference measurements are necessary for the verification of whether the
transmitter readings are within specifications. This way the user can check if
the transmitter needs calibration or service. If the transmitter can be taken out
of the process or the control system, perform the reference measurement in a
laboratory with stable conditions.
The reference meter should preferably use the same technology as the instru-
ment checked, i.e. transmitters as HMP240 which use a capacitive RH sensor,
should be checked with instruments that also use capacitive RH sensors. This
minimizes the risk of errors caused by different measurement techniques.
Whatever technique is used, make sure that the reference instrument is at the
same ambient conditions as the checked instrument in order to avoid errors
caused by temperature differences. The reference measurement should be
made as close to the checked sensor as possible and the readings should be
read at the same time, when possible.
When an RH reference meter is used, disconnect the security lock jumper and
wait until the temperature of the probe has stabilized to the same temperature
with the reference meter. When the security lock jumper is disconnected, the
HMP240 is like any other RH transmitter outputting the RH and T values (for
further details, see Chapter 6).
7.2 Self-diagnostics
The HMP240 transmitter goes through a self-diagnostics procedure when the
power is switched on. If the procedure does not reveal any errors or faults, the
transmitter starts operating normally. If errors or faults are found, first check
whether the humidity and temperature sensors are damaged. If they are intact,
send the transmitter to Vaisala or a Vaisala representative for repairs. The
error messages the transmitter outputs are listed in Appendix 4.
If any errors occur during operation, the error messages are output on the local
display if the transmitter displays measurements; if the menus are used, error
messages are not output. The LEDs indicate errors at all times. During opera-
tion, however, the error messages are not output automatically through the se-
rial interface. If there is any reason to doubt that there is something wrong
with the transmitter, use command ERRS:
ERRS <cr>
If there are no error messages, only a prompt is displayed:
>ERRS <cr>