Water Vapour Calibration using Salt Hydrate Transitions

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Abstract
There is a thermodynamic equilibrium between hydrate pairs of a salt and the water vapour pressure of the surrounding gas. The vapour pressure depends on the nature of the salt hydrate pair and on the salt temperature. It is shown that the transition of ferrous sulphate from the heptahydrate to the tetrahydrate will humidify flowing gas to this equilibrium vapour pressure, provided that the gas is in contact with the crystals for about 12 s. The relationship between the equilibrium water vapour pressure and crystal temperature has been established over the range 10–40 °C (with an accuracy of ±0-1 °C at 16 °C). At 20 °C the equilibrium relative humidity is 55.8%. A practical humidity calibrator based on this hydrate transition has been built and tested. Using two-way valves, the input supply of dry air is divided so that part goes to the exit directly, and the rest to the exit via a ferrous sulphate column maintained at a constant and measured temperature. By varying the fraction passing through the ferrous sulphate, the humidity of the emergent air may be varied between zero and the humidity in equilibrium with the ferrous sulphate.
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