3. H
2
O
2
Measurement
3.1 Operating Principle of H
2
O
2
Measurement
When there is H
2
O
2
in the probe's environment, the probe must
always be powered on. When powered on, the PEROXCAP sensor is heated,
which permits using the probe in condensing H
2
O
2
conditions, maintains
measurement performance, and lengthens the probe's lifetime. When the probe
is powered o, exposure to H
2
O
2
condensation can break the PEROXCAP sensor
within a day, and the sensor will not recover.
CAUTION!
PEROXCAPâ sensor technology works using measurements from two Vaisala HUMICAPâ
sensors. HUMICAP sensors guarantee quality and reliability, with their reputation for
repeatability, accuracy, excellent long-term stability, and negligible hysteresis – even in the
most demanding high-concentration H
2
O
2
applications in atmospheric pressure.
HUMICAP sensor is a
thin-film polymer sensor consisting of a substrate on which a thin
polymer film is deposited between two electrodes. The film absorbs or releases vapor
according to humidity changes in the environment. As the humidity changes, the dielectric
properties of the polymer
film change, and so does the capacitance of the sensor. The
instrument’s electronics measure the capacitance of the sensor and convert it into a humidity
reading.
PEROXCAP measurement uses two composite HUMICAP sensors, one with a catalytic layer
and one without. The catalytic layer catalyzes H
2
O
2
from the vapor mixture. Therefore, the
HUMICAP sensor with this layer only senses water vapor, providing a measurement of partial
water pressure, i.e. relative humidity (RH). The sensor without the catalytic layer senses both
hydrogen peroxide vapor and water vapor in the air mixture. The
dierence between the
readings from these two sensors indicates the vapor concentration of H
2
O
2
.
HPP272 User Guide M211972EN-B
14
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