Short Communication Modifications to a Controlled Environment Cabinet to Gain Proportional Control over Temperature and Humidity

Modern controlled environment cabinets with temperature control specifications to + O4°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. This paper describes modifications to a Sherer Growth Chamber (Model No. Cel 37—14: purchased 1969) which improve its operational characteristics. The specification and performance of the original and the modified machine are outlined in Table 1 and Fig. 1. The cabinet has been used to study the control of rhythmic behaviour in woodlice. A ventilated psychrometer was constructed following British Standard 4194: 1967. The wet and dry loaded thermocouples were set into a tube continuously ventilated at 1 m s~, and were used for monitoring the conditions in the chamber. Three thermistors (two dry, TH1, TH2 and one wet, TH3) were also incorporated into the tube and used as the sensors for the new control systems. Temperature control was established by continuously refrigerating the cabinet while providing compensatory heating by proportional zero voltage switching of the power supply to the heating elements. A total of 3-1 kW of heating was installed to permit rapid transition from one temperature regime to another. The