Vaisala HMT330 - Manual (Page 27)

Chapter 2 ___________________________________________________________ Product Overview
VAISALA ________________________________________________________________________ 25
Warmed Probe HMT337
Temperature difference between the probe and external environment can
cause a risk of condensation on the sensor. A wet probe cannot observe
the actual humidity in the ambient air. If the condensed water is
contaminated, the life span of the probe may shorten and calibration may
change.
HMT337 probe shall be used in applications where condensation can
occur due to high humidity and rapid humidity changes. The warmed
probe is heated continuously so that its temperature is always higher than
in environment. This prevents condensation on the probe. The power
consumption of the warmed probe is slightly higher than other probes.
Filter Options
There are several filter types for HMT330. All filters are 12 mm in
diameter with a female thread, and are compatible with all HMT330
probe models. The filters recommended for a probe type can be selected
on the corresponding HMT330 order form.
Fore more information, see section Spare Parts and Accessories on page
178
Catalytic VHP Filter
Vaporized hydrogen peroxide (VHP) is a gaseous form of hydrogen
peroxide that is commonly used in decontamination and sterilization.
Hydrogen peroxide has relatively low saturation pressure which increases
the possibility of saturation when used in humid conditions. When
saturation conditions occur, liquid hydrogen peroxide will form on
surfaces in the decontaminated space.
Vaisala HUMICAP® sensor is compatible with direct VHP exposure
even at high concentrations and hundreds of VHP cycles up to saturation.
The long term performance is very good at concentrations higher than
typically used for sterilization. However, for the applications where full
saturation is possible, Vaisala recommends using the catalytic VHP filter
to protect the sensor and extend the calibration interval.
The catalytic filter is based on a generally known method where the H
2
O
2
is split to H
2
O and O
2
. This reaction is produced by the catalyst layer that
has been applied to the filter. As the filter prevents the VHP from
reaching the humidity sensor, the sensor responds to humidity only. The
catalytic filter affects the reading approximately +1 %RH at 300 pmm
VHP, or +3 %RH at 900 ppm VHP (at 23 °C / 73.4 °F).