A new formula for accurate calculation of water activity in sorption calorimetric experiments

A sorption calorimetric method proposed by Wadsö and Markova allows determination of water activity. Calculation of water activity is based on Fick’s first law. The correct form of the Fick’s law takes into account not only vapour diffusion itself but also bulk flow of vapour. A new formula, which uses the correct form of the diffusion law for direct calculation of water activity, is proposed.