Felix Technology Inc.

- Model FT3122 - Hygrometer Used to Measure the Relative Humidity



The hygrometer FT3122 is used to measure the relative humidity by means of a special prepared hairstring element, with direct indication of relative humidity and electrical transmission. The temperature range is from -60 to +70°C and the humidity range is from 5% to 100%. Measuring the relative humidity is based on the fact that the dimensions of hygroscopic materials will change with the ambient humidity. Selected and pre-treated human hair as well as “PERNIX”-elements, which are manufactured by a special process, are such hygroscopic materials. They are particularly suited for measuring purposes, because their longitudinal expansion and contraction are very large, and the measured values are consistent. Furthermore the changes will be independent of the change in temperature. The sensor contains a number of 'PERNIC'-elements, combined in bundles and framed in three sections in order to increase the displacement force.

The upper end of the measuring elements is connected through a lever to the axle of the pointer so that changes in length of the elements will cause the pointer to turn. The relative humidity may in this way be read on the dial which is scaled form 0 to 100 %. If for example the pointer indicates a value of 60 %. This means that the ambient air contains 60 % of the maximum possible amount of water vapor (in relation to the saturation pressure).

The transmission of the measured data is achieved by converting the variations in length elements into resistance values. For this purpose the instrument is equipped with a potentiometer having its wiper connected to the pointer axle. The wiper divides the winding of the potentiometer into two branches whose resistances change with the position of the wiper, i.e. with the relative humidity.

The sensor FT3112 has a linearized potentiometer output. This model is especially used for the automatic data acquisition. It has a potentiometer winding by which a linear correlation of the resistance values between A and R and between E and R in relation to the relative humidity is achieved. The local scale thereby retains a characteristic for hygrometers almost as logarithmic division. Humidity sensors with a linear resistance output are specially calibrated to unit values. Hereby the replacement of the measuring elements or the receiving equipment is possible without additional re-calibration.

The total range from 0...100 % relative humidity results in a resistance variation of 100Ω.
Every 10 %-variation in humidity therefore yields a change in resistance of + or - 10Ω in each branch of the potentiometer.

The winding of the potentiometer and the wiper are made of gold alloys. By this, a corrosion-resisting quality is obtained. The potentiometer may receive an electric load of up to 0.5Ω with the wiper taking a maximum current of 60 mA (DC).

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