A fifty-channel digital recorder for thermocouple psychrometers

Outputs from fifty thermocouples are recorded by converting the movement of a galvanometer light spot into a series of electrical impulses which are then amplified, shaped, counted, and finally printed on a paper strip. The thermocouples act as wet- and dry-bulb thermometers to measure the relative humidity of vapour in equilibrium with small samples of soil or plant material and the principle of Peltier cooling is exploited to deposit water on the wet bulbs. Passage of the cooling current, switching of the thermocouple circuits and the initiation of counting and printing operations are controlled by a programme timer and a selector which allows any combination of psychrometer units to be scanned repetitively. The precision of the system (equivalent to one digit) is ±0·0007 degC or 0·007% in relative humidity at 25°C or 10 J kg −1 (1 metre) in water potential.