Vaisala HUMICAP HMP155, User Manual, Page: 17 / 102

In some applications the sensor gain may decrease gradually due to an interference caused,
for example, by a chemical present in the measured gas. The following figure illustrates the
decrease of sensor gain due to an interfering chemical and the effect of the chemical purge
process.
Figure 2 Decrease of sensor gain
The sensor polymer absorbs the interfering chemical which reduces the ability of the polymer
to absorb water molecules and consequently the sensor gain decreases. In chemical purge,
heating the humidity sensor to approxi180 °C (356 °F) for several minutes evaporates
the interfering chemical.
You can set chemical purge (startup purge) to start within 10 seconds from HMP155 startup.
The purge has a heating phase and a cooling phase. After the purge cycle of 6 minutes the
sensor returns to normal mode.
Chemical purge function locks the output values for about 6 minutes.
Avoid using the purge function in below freezing temperatures. Low
temperatures decrease the effect of purge. It also takes the sensor longer to
return to normal function.
CAUTION!
Chapter 2 – Product overview
15