RH for H2O2 calibration and adjustment - HPP271

HPP271 User Guide

Document code
Document type
User guide

PEROXCAP sensor technology works using measurements from two HUMICAP humidity sensors: one sensor with a catalytic layer and one without a catalytic layer. The catalytic layer catalyzes hydrogen peroxide from the vapor mixture, preventing it from entering the sensor.

Figure 1. PEROXCAP operating principle

Vaisala algorithm transforms the observations from the humidity sensors into a measurement reading of H2O2 concentration. Any drift in the humidity sensors shows as drift in the measurement reading. Therefore, removing the drift from the humidity sensors with calibration and adjustment improves H2O2 measurement performance.

Figure 2. Adjustment removes drift

Even though the probe does not measure relative humidity during normal operation, you can still calibrate and adjust the probe's H2O2 measurement with relative humidity. To be able to calibrate and adjust relative humidity, which is a temperature-dependent parameter, the PEROXCAP sensor also contains two internal temperature sensors (one for each humidity sensor) that are used for calibrating and adjusting RH for H2O2 and in certain measurement calculations. In normal measurement, the PEROXCAP sensor is heated to allow it to be used in condensing H2O2 conditions, which sets limitations for the use of the internal temperature sensors. During RH for H2O2 calibration, however, the probe switches into a calibration mode that temporarily turns off the sensor heating.

CAUTION Do not expose the probe to H2O2 vapor when the probe is in calibration mode and sensor heating is off. Exposure to H2O2 condensation can cause the PEROXCAP sensor to fail within a day, and the sensor will not recover.

RH calibration and adjustment for H2O2 is a two-point adjustment. The two reference points must be at least 40 %RH apart from each other, and the second reference point must be at least 50 %RH. The following RH levels are recommended for optimal results:

  • Reference point 1: 0 ... 35 %RH
  • Reference point 2: 65 ... 85 %RH

The recommended calibration and adjustment temperature is 20 ... 30 °C.