Estimation of Humidity with Cobalt Thiocyanate Papers and Permanent Colour Standards

The method of measuring relative humidity by matching the colours of tissue paper impregnated with cobalt thiocyanate has now been made more convenient by the commercial production of impregnated paper and coloured glass standards. Although the method as used straightforwardly is not at all precise, it is especially useful for the measurement of humidity in small or relatively inaccessible spaces, e.g. , cracks in floors, air spaces in stored grain, under bark, or inside small tubes, etc., and for general use when elaborate equipment cannot be employed. A piece of the paper is exposed to the humidity to be measured, preferably for two hours, then mounted on white opal glass in oil (liquid paraffin) and matched, in a simple comparator, against the coloured glass standards. When matching is done at a temperature other than 20°C., corrections are required as tabulated. Tables of error are given, showing the range of variation from different causes, with estimates of 95 per cent, confidence limits. To cover all the sources of variation normally affecting the measurements under various conditions, limits up to ±5 are allowed for relative humidities down to 70 per cent.; these limits increase at lower humidities to a maximum of ±15, about 30 per cent. Various ways of avoiding errors are described, and it is shown that if special precautions are taken the method can be used with considerable accuracy, particularly at relative humidities above 65 per cent.