Electrolytic Hygrometer

Author: Hans-Jürgen Schwarz

English Translation by Sandra Leithäuser

back to Air Humidity Measurement

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Abstract

The electrolytic hygrometer uses the electric current decomposition (????) of absorbed water, for the measurement of humidity. The value depends on the humidity content of the air to be measured. The electrolytic-hygrometer permits the continuous measuring of water vapor traces in gases down to a dew point of -80°C. This corresponds to less than 1mg of water per m³ gas.

The sample gas flows past a Pt-electrode assembly, at a known constant flow. The Pt-electrode assembly is coated with phosphorus pentoxide. The exceptionally hygroscopic phosphorus pentoxide dehumidifies/dehydrates/withdraws nearly all (the water from) the gas that flows past it. The small amount of water left, is decomposed by the DC current into H₂ und O₂. According to Faraday’s law, the electrolysis current is a measure for the water to be absorbed and therefore the absolute gas humidity. The sensor is a directly operating sensor, i.e. it does not need to be calibrated.

Phosphorus pentoxide as an electrolyte allows the sensor to work in the humidity range of 0- 1000 ppm, to an accuracy of ± 5%. Higher concentrations result in a loss of the linearity, because the water cannot be absorbed completely. Phosphorus pentoxide dissolves at high humidity levels.

describes a measure of opposition to alternating current, comparable with the resistance to direct current in Ohm’s law.