Coating thickness gauge. Encyclopedia of radio electronics and electrical engineering

Encyclopedia of radio electronics and electrical engineering / Measuring technology

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When checking the quality of painting and priming of flat ferrous metal products, as well as when examining the condition of car bodies, it is often necessary to measure the thickness of the paint coating.

Similar devices have already been described in the magazine "Radio" [1; 2], however, the first of them requires power from a 220 V network, and the second requires the presence of a multimeter capable of measuring capacitance. The simple meter offered to our readers is free from these shortcomings and provides acceptable measurement accuracy.

The meter allows you to control the thickness of the paint coating applied to ferrous metal products. The limits of the measured thickness are from 0 to 0,8 mm. The measurement accuracy for thicknesses from 0 to 0,4 mm is ±0,02 mm, and from 0,5 to 0,8 mm - ±0.05 mm. The device is powered by a Krona battery, the current consumption does not exceed 25 mA, the functionality of the device is maintained when the battery voltage is reduced to 7 V. The operating temperature range is from +10 to +30 C. The device is assembled in a plastic box measuring 95x65x25 mm.

The master oscillator, assembled on the DA1 timer (see diagram in Fig. 1), produces rectangular pulses with a frequency of 300 Hz and a duty cycle of 2. The R3C2R4R5 low-pass filter selects the first harmonic of oscillations, which improves the measurement accuracy. The signal level regulator - trimming resistor R5 - sets the optimal mode of the measuring transformer T1. The signal amplitude at the output of ultrasonic sounder DA2 is approximately 0,5 V.

(click to enlarge)

The W-shaped plates of the measuring transformer are butted together, but without a package of end plates. The role of the magnetic contact here is played by the metal base, on which the studied paint and varnish coating is applied. The thicker it is, the larger the non-magnetic gap in the magnetic circuit of the measuring transformer. A larger gap corresponds to a smaller connection between the windings, therefore, a lower voltage on the secondary winding of the transformer. Circuit R6C4 is an additional filter that eliminates high-frequency components of the signal. Capacitor C5 is a separation capacitor.

Microammeter PA1 shows the current of the secondary winding of the transformer rectified by diode VD1. Voltage stabilizer DA3 allows you to maintain stability of the gain of the ultrasonic frequency device DA2 when the degree of discharge of the power battery GB1 changes. Resistor R8 and push-button switch SB2 allow you to periodically check the battery voltage. The measurement is carried out with the SB1 button pressed.

The parts of the device are placed on a printed circuit board (Fig. 2), made of fiberglass foil on one side with a thickness of 1 mm.

All fixed resistors are MLT-0,125, trimmers are SPZ-276. Capacitors C1, C2, C4 - KM-6 (or K10-17, K10-23), capacitors C3, C5, C6 - K50-35. The PA1 microammeter serves as a recording level indicator from the "Electronics-321" tape recorder (frame resistance 530 Ohms, total needle deflection current - 160 µA).

Transformer T1 is wound on a magnetic circuit Ш5Х6 (an output or matching transformer from pocket receivers is used), the primary winding contains 200 turns of PEL 0,15 wire, the secondary winding contains 450 turns of the same wire. Only W-shaped plates are required. During assembly, they are lubricated with epoxy glue; after the glue has dried, the ends of the bag are leveled with a velvet file. The transformer is glued from the inside into a rectangular hole in the device box so that the working ends of the magnetic circuit protrude beyond the box by 1...3 mm.

The KR1006VI1 timer can be replaced with LM555, and the KR1157EN502A stabilizer with 78L05.

To establish the device, set the slider of the resistor R7 to the left position according to the diagram. Transformer T1 is located at a distance of at least 5 cm from metal objects. After turning on the power, resistor R5 sets the pointer of the RA1 microammeter to a position corresponding to 3 ... 5% of the total deflection of the pointer. After that, the transformer is applied with the working end of the magnetic circuit to a flat, clean surface of a steel sheet and, with a resistor R7, the arrows are transferred to the final division of the microammeter scale. Laying between the transformer and the metal surface sheets of paper 0,1 mm thick (density 80 g / m2), calibrate the device.

Resistor R8 is selected so that with a fresh battery, when you press both buttons SB1 and SB2, the microammeter needle deflects to the final scale division. Having connected a battery discharged to 7 V to the device, repeat the measurement on the microammeter scale and make a mark corresponding to the discharged battery.

When measuring the thickness of the coating, the device is applied to the controlled surface, the SB1 button is pressed, slightly rocking and turning the device, the maximum deflection of the arrow is achieved and the thickness value is read. The thickness of car body coating with regular paint is in the range of 0,15...0,3 mm, and with metallic paint - from 0,25 to 0,35 mm.

When using the device in low ambient temperatures, it is advisable to keep it in your inner clothing pocket, removing it immediately before measurement.

Literature

1. Velsky A. Assessment of paint coating thickness. - Radio, 2002, No. 2, P. 57.
2. Chekhovskoy I. Control of the thickness of the enamel on the body. - Radio, 2004, No. 1, C 47.

Author: Yu.Pushkarev, Achinsk, Krasnoyarsk Territory

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