Humidity meter with capacitive sensor. Scheme, description

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ENCYCLOPEDIA OF RADIO ELECTRONICS AND ELECTRICAL ENGINEERING
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Humidity meter with capacitive sensor. Encyclopedia of radio electronics and electrical engineering

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Encyclopedia of radio electronics and electrical engineering / Measuring technology

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The capacitance of a capacitor depends on the dielectric between its plates. For the selected bulk material (grain, sugar, raisins, building materials, etc.), the capacitance of the capacitor sensor depends on the moisture content of the material.

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The meter allows you to evaluate the moisture content of bulk materials and contains a master oscillator made according to the multivibrator circuit on transistors V1, V2, a measuring circuit, a calibrator and a sensor. The capacitive type sensor C1 is connected in parallel with the capacitor C3. The multivibrator is balanced by the resistor R4 according to the zero reading of the arrow of the RA1 device. With an increase in the capacitance of the sensor C1, the symmetry of the multivibrator is violated and the arrow of the measuring device deviates. Calibration of the device is carried out using the voltage regulator on the transistor V3, for this, when the key S1 is pressed, the resistor R9 sets the arrow to the last division of the scale.

The sensor consists of two single-sided foil textolite plates 105 x 40 mm in size, spaced 15 mm apart. The foil layer on the plates is removed from the edges of the plate by 3 mm. The calibration of the device is carried out according to an industrial device or by trial measurements of the mass of the material during the drying process.

Details. Transistors of the type KT315, KT312, KT306 or assembly K125NT1 with a transfer coefficient greater than 70; mica, paper, film capacitors with low TKE; measuring device with a scale of 100 μA.

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