Principle of Operation

The measurement system is made up of a meter connected to a probe. The probe measures capacitance, a capacitor with a polymer or plastic dielectric material with a fixed dielectrical constant from 2 to 15. Increased humidity causes the dialectric to dilate, hence distancing the plates with consequent variation of the capcitor's geometry and reduction of it's capacitance. These capacitance variations in turn cause a frequency change in the instrument's electronics, resulting in a frequency modulation which is a function of relative humidity. The frequency is then converted into a relative humidity value displayed on the LCD.

The hygrometers precision essentially depends on how insusceptible it is to the following three factors: the first is the 'linearity error' caused by the typical non-linearity of RH (Relative Humidity) sensors. Hanna hygrometers compensate for the effects of this error. It is advisable, however, to calibrate the meter periodically to reduce the probability of this error reoccurring.

The second factor is the 'temperature error' caused by the variation of the hygroscopic properties of the sensor's dialectric material as a function of temperature. In fact, the ratio between the quantity of water vapour present in the dialectric and the relative humidity is not directly proprtional, but varies with temperature.

The third is the 'calibration error' caused by an incorrect calibration procedure.

There are many 'do-it-yourself' calibration kits on the market. Most of these are made of a receptacle containing two sealed chambers and two different types of salt. It is possible to simulate a particular RH value by filling each chamber with the correct distilled water and salt solution.

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