Humidity Sensing using Soft Ferrites

Abstract
The electrical properties of polycrystalline soft ferrites, namely Cu-Zn system are considerably changed by doping controlled amounts of Lithium and Calcium salts. The room temperature resistivity is found to exceed 100 MΩcm at 45% relative humidity. Variation of resistivity with percentage relative humidity is found to be linear in the range above 60% RH and is of the type ρ=ρ₀-MRH. M being a slope of the curve which is humidity sensitivity. In the range 45 to 60% RH, log ρ Vs % RH exhibits linear relation and is governed by the nature ρ=ρ₀e^-krh. K being humidity sensitivity. The statistical tools are employed for the analysis of the curves and all the parameters are tested giving best results. The response time in the disc form, for both negative and positive step is of the order of 6 to 7 seconds for 5% change while it is of the order of 7 to 8 minutes for -ve step and one minute for +ve step for 45% to 85% change in RH. The material does not exhibit any aging effect. In the entire range of humidity, the pellets do not become watery. Basically, ferrites are temperature sensitive and therefore, humidity and temperature sensing both can be done in a single device, because there is no any effect of humidity on the magnetic properties of these materials.