Vaisala HMP230 - Manual (Page 151)

HMP230 SERIES
M210225en-B Appendix 9: Re-gaining
145
1. GENERAL
In some applications the sensor gain may decrease gradually due to an
interference caused by some chemical present in the ambient (see Figure 1).
The sensor polymer absorbs the interfering chemical; this reduces the
polymer's ability to absorb water molecules and so decreases the sensor gain.
In re-gaining the interfering chemical is evaporated by heating the humidity
sensor to a temperature level of approximately +160 °C. Automatic re-gaining
(if active) is made repeatedly after time intervals defined by the user. Re-
gaining can also be started manually using either LED, display/keypad or
serial commands.
Calibration
values
Humidity
Re-gaining
Measured values
after chemical
exposure
Calibration
values
Humidity
Re-gaining
O
ut
put
s
igna
l
Fig. 1 Decrease of the sensor gain due to an interfering chemical and
the effect of the re-gaining process.
The re-gainable sensor is a composite sensor in which the HUMICAP ® and Pt
100 temperature sensors are glued together. Re-gaining option requires that
the sensor is protected either with a PPS grid with stainless steel netting (part
no. 16720HM) or with a stainless steel sintered filter (part no. 16452).
The power supply module (optional for the HMP230 series) cannot be used
when the transmitter includes the re-gaining option.