Instrument for detecting water in liquids with high resistivity. Encyclopedia of radio electronics and electrical engineering

Encyclopedia of radio electronics and electrical engineering / Measuring technology

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Quartz dissipative transducers described in the article by V. Savchenko and L. Gribova "A quartz resonator converts non-electric quantities into electrical ones" in "Radio", 2004, No. 2, on p. 34-36, have found application in devices for monitoring the humidity of gases and solids, in equipment for scientific research of new materials, etc. The problem of detecting water in liquids, in particular, in motor fuel, is no less important. One of the practical ways to solve this problem is described in the article below.

The quality of liquid fuel is determined by many factors, among which its water cut is of no small importance. Water in the fuel can be in different states of aggregation - dissolved, free and emulsion. At different fuel temperatures, from 0,002 to 0,007% of water dissolves in it, which cannot be visually controlled. As the temperature decreases, the solubility of water in the fuel decreases, and it settles in the form of droplets to the bottom of the tank.

Free water in the fuel increases the corrosion of metals in contact with the fuel several times, and in winter, freezing in the fuel line, it can cause the engine to stop. Therefore, the water content in the fuel is advisable, and in some cases it is simply necessary to control.

To increase the efficiency of visual control of the presence of free water, for example, potassium permanganate is added to the fuel sample, which, dissolving in water, paints it in a characteristic color that is clearly visible to the eye. Of course, such a control method is very inconvenient, therefore, indication of the presence of free water with the help of an automatic portable device is of great importance.

The complexity of control lies in the fact that the fuel, being a high-quality dielectric, has a very high specific resistance to electric current. A drop of water in the fuel, even placed between two electrodes, cannot be controlled with a simple DC megohmmeter, since the fuel film surrounding the drop does not allow it to come into close contact with the electrodes, due to which the electrical resistance in the circuit cannot decrease significantly.

To indicate free water in the fuel, we propose to use a dissipative quartz converter of electrical energy, which is highly sensitive to changes in large values ​​of electrical resistance. The device contains an electrical circuit of a vacuum quartz resonator and a capacitive sensor connected in series or in parallel. This circuit is called a quartz dissipative electrical energy converter, since its equivalent electrical resistance, being the output parameter of the converter, is determined by the energy loss in a sensor with a controlled dielectric, for example, in liquid hydrocarbon fuel.

On fig. 1a and XNUMXb show the design of the developed device for monitoring free water in fuel. The device is made in the form of a measuring cup made of organic glass with a lid and a handle. The handle contains batteries and a push-button switch located on its inner side. An LED is mounted in the upper part of the handle, by the glow of which the presence of water in liquid fuel is determined.

At the bottom of the mug there is a capacitive sensor, consisting of two cone-shaped coaxially placed electrodes, directed towards each other by their vertices, as shown schematically in Fig. 2. Both electrodes are stamped from sheet brass, and the upper (outer) one is truncated.

Fig. 2

The electrodes are fixed at the bottom of the mug so that an air annular gap approximately 0,25 mm wide is formed between them, which determines the electric capacitance of the sensor of about 0,8 pF without fuel. Under the bottom of the mug there is a board with details of the electronic part of the device.

About half a liter of fuel is poured into a mug. If there are drops of free water in it, then for some time they roll down the cone-shaped walls of the sensor into the gap and change the electrical resistance in the gap of the capacitive sensor. The lid of the mug, hinged on a hinge, is necessary to prevent atmospheric precipitation (rain, snow) from entering the working volume when working in the field.

On fig. 3 shows a schematic diagram of the device. The dissipative quartz converter contains a capacitive sensor Cd and an evacuated quartz resonator ZQ1 at a frequency of 300 kHz, having a dynamic (equivalent active) resistance Rd = 80 Ohm and a static capacitance Cst = 6,5 pF. The oscillator is made according to the capacitive three-point scheme on the VT1 transistor.

Fig. 3

The alternating voltage of the oscillator after detection by diodes VD1, VD2 with capacitor C5 enters the base of the transistor VT2 and closes it, which leads to a decrease in the collector current of the transistor; HL1 LED goes out.

In the absence of autogeneration, the collector current of the transistor UT2 is sufficient to light up the HL1 LED. The required collector current of this transistor is set by a selection of resistors of the voltage divider R4R5. By the brightness of the LED at the moment the device is turned on, one can judge the sufficiency of its supply voltage (3 V) received from two galvanic cells.

As the batteries age, the brightness of the LED decreases. The device remains operational up to a supply voltage of 2 V.

When the contacts of the SB1 button are closed, due to the large (over 500000) quality factor of the quartz resonator, self-generation cannot occur instantly. Within 1,5 ... 1,8 s, the nominal values ​​​​of the amplitude and frequency of the generator oscillations are smoothly established. While the generator has not reached normal mode, the HL1 LED is on. After the specified time, the generator turns on, and if there are no traces of water in the sensor of the device, the HL1 LED goes out, since the positive voltage at the base of the VT2 transistor will be compensated by the negative voltage from the detector.

The extinguishing of the LED indicates the readiness of the device for operation, i.e., for monitoring free water in the fuel. After pouring clean fuel into the measuring cup, the LED remains off. If there is at least one drop (0,023 ... 0,026 g or more) of water in the fuel, then the active losses in the converter will increase sharply, which will lead to the breakdown of self-generation and the switching on of the LED.

Note that a drop of free water in automotive fuel that has fallen into the gap between the sensor electrodes causes an increase in the active resistance of the converter by Ra = 400 Ohm. Theoretically, this corresponds to the inclusion of a loss resistance Rp = 1 GΩ in parallel with the capacitive sensor Cd. The calculation was carried out according to the formula:

Ra \u1d Rd / (2 + (omega * Cd * Rp) ^ XNUMX)

The sensitivity of the device is set by a trimmer capacitor C1. To check the sensitivity, a resistor with a resistance of 750 kOhm (MLT-0,25) is connected to the sensor electrodes. It is practically enough, holding the resistor by one terminal, to touch the central electrode of the sensor with the other. With normal sensitivity, after the resistor output touches the central electrode of the sensor, the LED turns on after 1 ... 2 s.

If we assume that the mass of fuel placed in the working volume of the device is 0,5 kg, and the mass of a drop of water is on average 0,025 g, then it turns out that the device reliably controls already five hundredths of a percent of free water.

Tests of the device with various types of liquid fuel were successful. It turned out to be suitable for monitoring the presence of free water in other dielectric liquids, for example, in acetone, benzene, etc.

Authors: V.Savchenko, L.Gribova, Ivanovo

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