Modern controlled environment cabinets with temperature control specifications to ±0·4°C and humidity control at 20°C of ± 3 % between 55 % and 90 % are expensive. The standard cabinets available for biological work can limit fluctuations at ‘constant’ temperatures to ±1 °C. Humidity control is primitive and the range of humidities attainable and the accuracy with which they can be held is unacceptable. Studies in experimental biology recognize the importance of a closely controlled atmospheric temperature and moisture content in work ranging from translocation and product storage in plants to the physiology and behaviour of pests and other animals. In many of these studies saturated salt solutions are used. These are unsuitable since they can only be used with small volumes of air, access to the experimental subjects is incompatible with the maintenance of the stated humidity levels, and controlled changes in humidity are difficult to achieve.
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