Shortcut to tinman description: water vapour flux generator

Sorption curves

The sorption curves were measured by weighing fragments of materials suspended in net bags from a carousel contained in a chamber whose RH is controlled by mixing dry and saturated air streams.

[photos and exact description here]

Diffusion coefficient

The water vapour diffusion coefficient was not measured directly. The standard method measures the amount of water penetrating a disc of material under a constant RH difference. In the usual setup, the material forms the lid of a cup containing a desiccant or a saturated salt solution. The cup is placed in a moving air stream of constant RH. The cup is weighed at intervals and the permeability is measured over several periods after the weight change has reached a constant rate.

A defect of this process is that it says nothing about the diffusion process when the environmental RH is changing. At a steady state, the water content of the material is constant at each level through the specimen and at equilibrium with the interstitial RH at that level, so there is no interaction between the permeating water molecules and the material. One is really measuring the permeation of an inert gas of molecular weight equal to that of water.

Response to water vapour flux

The material, or assemblage, is exposed in a chamber which contains a water vapour flux generator. This device injects into the air space a programmed flux of water vapour. It does not impose a particular RH. The RH within the chamber is a result of the competing effects of injecting water vapour from the flux generator and absorbing the water vapour into the test material. For a good buffer, the RH in the chamber will remain nearly constant, even though a lot of water vapour is being exchanged through the surface of the material. This exchange is measured by weighing the rate of escape, or condensation, of water in the flux generator. The material is not weighed, because the change of weight will be small compared with the weight of the material. The chamber is made airtight, so the weight loss of the water reservoir, corrected for a slight change in RH of the air in the chamber, is equal to the weight gain of the material under test.