AbstractThe measurement of the relative humidity (RH) at high levels to the desired accuracy is still a problem, especially for levels above 90% RH. The reasons for this are:iThe accuracy of the standard apparatus is lower at high RH than at moderate and low RH;iiThe need for accurate measurement of the RH in moisture controlled rooms is greater at high RH than at lower RH, because for moisture sensitive products, such as fibreboard boxes and certain horticultural products, the influence of a minor deviation in RH on the properties of these products is considerably greater at high RH than at moderate and low RH.