The use of thermocouples for psychrometric purposes

The paper deals with certain factors affecting the use of thermocouples for psychrometric purposes, and considers some of the advantages possessed by instruments of this type. An investigation is made for the purpose of determining the manner in which the wet-bulb depression depends on the degree of ventilation, the diameter of the wire, the thickness and length of the water film covering the junction of the wet thermocouple, and the relative positions of the wet and dry thermocouples. It is shown that the need for forced ventilation of the wet thermocouple decreases as the diameter of the wetted wire is diminished. A typical example is given showing that for a psychrometer composed of nichrome and constantan wires of 40 s.w.g. mounted in a still air enclosure, the percentage relative humidity derived from its readings is only two or three units greater than that given by a fully ventilated instrument, when psychrometric tables appropriate to the latter condition are used in each instance.