Operating principle of H2O2 measurement - HPP272

HPP272 User Guide

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CAUTION When there is H2O2 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 H2O2 conditions, maintains measurement performance, and lengthens the probe's lifetime. When the probe is powered off, exposure to H2O2 condensation can cause the PEROXCAP sensor to fail within a day, and the sensor will not recover.

PEROXCAP® sensor technology works using measurements from two Vaisala HUMICAP® 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 H2O2 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 H2O2 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 difference between the readings from these two sensors indicates the vapor concentration of H2O2.

Figure 1. Operating principle of PEROXCAP measurement
HUMICAP sensor with a catalytic layer (under the probe filter). This sensor only senses water vapor.
HUMICAP sensor without a catalytic layer (under the probe filter). This sensor senses the air mixture with both hydrogen peroxide vapor and water vapor.
Catalytic layer over the thin film polymer. This layer catalyzes hydrogen peroxide into water and oxygen and prevents it from entering the sensing polymer.
Thin film polymer between two electrodes.
Alumina substrate.

In addition to measuring vaporized H2O2 concentration, HPP272 provides measurement for relative saturation, relative humidity, and temperature. Relative humidity (RH) is a parameter that indicates the humidity value derived from water vapor only, whereas relative saturation (RS) indicates the humidity percentage derived from water vapor and H2O2 vapor together.

When relative saturation reaches 100 %RS, the vapor mixture starts to condense.