Vaisala HMP240 - Manual (Page 122)

HMP240
Appendix 7: Re-gaining
M210300en
120
1. GENERAL
In some specific applications the sensor gain may decrease gradually due to an
interference caused by some particular 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. Automatic re-gaining (if active) takes place repeatedly with 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
u
tp
u
t
si
g
n
al
Figure 1.1 Decrease of the sensor gain due to an interfering chemical and
the effect of the re-gaining process.
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).