Abstract

Electrolytic or coulometric hygrometers (Dunmore element) utilize a cell coated with a thin film of phosphorous pentoxide (P₂O₅), which absorbs water from the gas under measurement. When an electrical potential (voltage) is applied to the electrodes and incorporated in the measurement cell, the water vapor absorbed by the P₂O₅ is dissociated into hydrogen and oxygen molecules, generating a finite current. This reaction takes place at a voltage of at least 2 V (dc). The resulting electrolysis current is a measure of the sample’s moisture content based on Faraday’s Law. The amount of current required to dissociate the water is proportional to the number of water molecules presented in the sample gas. It was established that the coulometric sensor is a robust and cost-effective tool for determination of the trace humidity. This chapter is devoted to the consideration of such sensors. In particular, one can find the description of the design and operation principles of these sensors, as well as advantages, disadvantages, and possible areas of their use. Modern P₂O₅-based humidity sensors and the polystyrene surface resistivity–based sensor, the so-called Pope’s cell, are also discussed in this chapter. The Pope’s cell is structurally similar to the Dunmore element but, instead of lithium chloride solution, it uses sulfuric acid.