Water level meter. Encyclopedia of radio electronics and electrical engineering

Encyclopedia of radio electronics and electrical engineering / Measuring technology

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In the water supply system of a country house, a 160-liter polyethylene barrel installed in the attic serves as a storage element [1]. A very useful addition to the system is the water level (volume) meter in the barrel described in this article.

The sensor of the proposed meter is a capacitor formed by two plates of foil strips, fixed on the outer surface of the plastic barrel vertically from opposite sides. The capacitance of such a sensor almost linearly depends on the water level: from 7 pF for an empty barrel, up to 270 pF for an almost full one. The indicator can be a multimeter or pointer microammeter.

To obtain an electrical signal proportional to the capacitance, a well-known principle is used - rectangular pulses are passed through the measured capacitance of the sensor and a diode rectifier assembled according to the voltage doubling scheme. If the load resistance of the rectifier is small and the voltage drop across it is significantly less than the amplitude of the pulses, the average rectified current is proportional to the capacitance in the first approximation: I = U f C_д, where U is the amplitude of the pulses minus the voltage drop across the rectifier diodes; f is the pulse repetition rate; WITH_д - sensor capacitance.

This current can be applied to a microammeter and the pulse frequency can be selected so that its readings are directly proportional to the volume of water in liters or as a percentage (of the maximum volume). If you install a resistor at the rectifier output and connect a digital voltmeter (multimeter in voltage measurement mode) to it, you can get the same information in digital form.

Rice. 1. Meter scheme (click to enlarge)

The scheme of such a meter is shown in fig. 1. It consists of a rectangular pulse generator assembled on three logic elements DD1.1 - DD1.3, a buffer stage on the DD1.4 element and a rectifier on diodes VD1 and VD2 with a smoothing capacitor C4. The generator operates at a frequency of about 100 kHz. With a supply voltage of 9 V, a voltage drop across two diodes of about 1,2 V, and a sensor capacitance of 270 pF, the output current calculated using the above formula will be I = (9-1,2)x100x10³h270h10^-12 = 210х10^-6 = 210 µA. Therefore, the calculated voltage drop across the resistor R5 will be U_R5 = 210х10^-6x820 = 170 mV. The actually measured voltage with an almost full barrel is less - about 150 mV.

The meter is designed to work together with widely used digital multimeters of the M83xx series, on the right in Fig. 1 shows the connection points to it. If you set the measurement limit of 200 mV in the multimeter, the resolution of the meter (the weight of the least significant digit) will be 0,1 l. Of course, the error in measuring the amount of water will be greater, primarily due to the non-cylindrical shape of the barrel. If you set the measurement limit of the multimeter to 2 V, the readings will be in liters.

It is of interest to connect the output of the meter's rectifier to the inputs of the multimeter when powered from the same source. In a multimeter, none of the inputs are directly connected to the power supply, but the DC output of the rectifier does not have to be connected to the power supply either. This made it possible to connect the rectifier output directly to the "COM" and "VΩmA" inputs of the multimeter, and to connect the rectifier's AC output to the power supply, capacitor C3 had to be installed.

All parts of the meter are installed on a single-sided printed circuit board made of foil fiberglass 1,5 mm thick, the drawing of which is shown in Fig. 2. Fixed resistors are used - MLT, tuning - imported CA9V or domestic SP3-19a, capacitor C2 - K10-17, KM-5. Diodes KD510A can be replaced by any low-power pulse silicon.

Rice. 2. Board drawing

To connect to the power supply, multimeter circuits and to the sensor, pins with a diameter of 1 mm from the RP series connector are soldered to the board, and a block from a used nine-volt battery is soldered to two pins to connect to the multimeter power connector. The option of installing the meter board in the battery compartment of the M83x multimeter is shown in fig. 3. A power supply (adapter) from a television antenna amplifier was used as a power source. In it, the microcircuit stabilizer for a voltage of 12 V is replaced by a nine-volt one (78L09).

Rice. 3. Board installation option

If you use a microammeter instead of a multimeter for indication, you can use a 5 V charger from a cell phone as a power source. In this case, the generation frequency should be increased by selecting the capacitor C2, and the resistor R5 should be excluded. However, it should be borne in mind that often the voltage stability of chargers is low, and the output voltage itself slightly exceeds 5 V. Therefore, it is necessary to first check that the voltage practically does not change when a resistor with a resistance of 100 ... 200 Ohm is connected to the output of the charger and when the mains voltage changes. If this is not the case, an integrated regulator with a low voltage drop, the so-called Low drop for a voltage of 5 V, should be installed on its output, for example, LM2931Z-5.0 or KR1158EN5 with any letter index.

The sensor is made of two strips of baking aluminum foil 120 mm wide and length corresponding to the height of the barrel. For connection, narrow strips of copper foil are used, which are wrapped at the ends of the foil strips, and wires leading to the board are soldered to these strips. The strips are attached to the surface of the barrel with a sticky packaging film. If the distance between the barrel and the converter board is more than half a meter, to eliminate the effect of capacitive coupling between the conductors, the sensor should be connected to the output of the DD1.4 element with a shielded wire, the screen of which must be connected to the plus or minus of the power source, and the resistance of the resistor R4 should be halved. If the distance between the barrel and the indicator is more than three meters, it is better to separate the board and the indicator by installing the board no further than half a meter from the barrel.

Setting up the device is not difficult. If a pointer microammeter is used as an indicator, it is necessary, without soldering resistor R5, by selecting capacitor C2 and resistor R3, to achieve readings (with an almost full barrel) corresponding to 150 l, or 100%. When using a multimeter, this is achieved by selecting capacitor C2 and resistor R5, as well as smoothly by resistor R3. The design of the sensor is illustrated in Fig. 4. For indication, the author used a multimeter manufactured many years ago with LED indicators [2].

Rice. 4. Sensor design

The device can be supplemented with a second sensor in the form of two horizontal half-rings fixed on the top of the barrel, and an automatic machine can be made that turns off the pump when the barrel is filled. One half ring is connected to the output of the generator, the other - to the input of the rectifier. Capacitor C3 is not installed, the lower output of the rectifier according to the circuit is connected to the minus of the source, and the other output is connected to a threshold device that turns off the pump. A hose with a non-return valve [1] must be installed to avoid the consequences of an emergency overflow!

Literature

1. Biryukov S. Water supply in the country. - CAM, 2015, No. 6, p. 12-14.
2. Biryukov S. Portable digital multimeter. - Collection "To help the radio amateur", vol. 100. - M.: DOSAAF, 1988.

Author: S. Biryukov

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