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Encyclopedia of radio electronics and electrical engineering / Indicators, sensors, detectors

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The level meters (DUT) used in industry are mostly inconvenient to set up, their readings are time dependent. The pressure transducers used for these purposes contain several devices in the measuring "chain", therefore, they require careful adjustment.

The change in the density of solutions (due to changes in temperature) also contributes to the violation of the level reading. The connecting tubes, which supply the differential pressure to the differential pressure gauges, do not have a liquid flow during measurements, therefore, even with hot water in the tank, the tubes easily freeze. The situation is the same with the "clogging" of the tube: frequent maintenance is required.

Electronic level meters (EDMs) manufactured commercially often contain a large number of parts, while lacking linearity and stability of readings. "Handicraft" EMDs made by cooperatives often have circuits with oscillating circuits, and if not properly tuned, their readings may decrease as the liquid level increases.

At the ENZIM plant (Ladyzhin) in 1990, several EIUs were installed according to the schemes below and the following repairs were carried out: the power supply chip (PSU) was discarded; BP was made according to our scheme; the electrolytic capacitor was changed a couple of times; sensor - an insulated cable "soaked" with shampoo - replaced by a cable in fluoroplastic insulation.

Figure 1 shows a diagram of a simple capacitance meter with a linear scale. Of course, in terms of accuracy of reading, it is inferior to digital ones, but when selecting parts by a radio amateur, it is very convenient, since it is noticeable on the scale in which direction the capacitance of the capacitors under test differs.

If a radio amateur makes a circuit for several capacitance measurement ranges (pins 2 and 6 of the DA1 timer should be connected to the connection point of the frequency-setting RC circuits, and all trimmers are permanently connected to pin 3 of the timer), then one exemplary capacitor will be required to adjust each capacitance measurement range.

The complex internal circuitry of the timer works simply. The two comparators (inputs 2 and 6) and the flip-flop with output 3 have two steady states:

1) zero output when the input voltage is higher than 1/3 of the supply voltage;

2) high output voltage when the input voltage is lower than 2/3 of the supply voltage.

With this in mind, the voltage across the capacitor C1 fluctuates all the time between 1/3 and 2/3 of the supply voltage, and a sequence of rectangular pulses is generated at the output of the timer.

The KR1006VI1 chip is good because by changing the resistance of the resistor R1 from 200 ohms to 10 MΩ and the capacitance of the capacitor C1 from 10 pF to the maximum, you can get an oscillation period from fractions of a microsecond to hundreds of seconds.

The zener diode VD1 is always installed at the input of the timer, so that when setting up, it does not “break through” the timer inputs with a network pickup on the soldering iron and wires.

On the transistor VT1, a node for linear conversion of the input frequency signals (from the timer) and the test capacitance into electric current is assembled.

Due to the unusual inclusion of VT1 and VD2, they recharge the tested capacitor in turn at the moments of increasing and decreasing the voltage of the output pulses. If the capacitor is charged through the diode VD2 and the resistor R4 (as well as the "common" resistor R7 with the transistor), then the discharge is determined by the potential of the base of the transistor and, due to the high amplifying properties of this transistor, occurs through the collector circuit and further into the measuring circuit! Only two hundredth of the discharge current goes to the base of the transistor!

To preserve the collector voltage (so that the transistor can work as an amplifier), the base potential is "shifted" towards the "plus" supply using the divider R4 and R5. In order to ensure the "survivability" of the circuit, the resistance of the resistors R2, R4, R7, R14 should not be reduced. The numbering of parts is such that the description of this scheme is also suitable for subsequent ones (the same part numbers perform the same function).

The output current pulses from the converter capacitance and frequency - current are integrated by capacitor C5. Using resistor R6, you can adjust the output according to the exemplary capacitor. Capacitors C3 and C4 smooth out the supply voltage ripples, C2 maintains a constant voltage at the comparison nodes of the timer comparators.

A short circuit in the circuit of the tested capacitor closes the transistor VT1 and does not lead to an accident.

If the PA1 measuring head is large, the mounting plate can be mounted directly on the measuring head terminals. A stabilized power supply can be made in a separate case (Fig. 2).

The circuit is designed so that one end of the measured capacitor is attached to the body, unlike simpler circuits, therefore, such a circuit allows you to measure the level of conductive liquids in tanks (Fig. 3).

Instead of the tested capacitor, the capacitance of the level sensor is connected to the input of the circuit - an insulated conductor vertically fixed inside the capacitance. If a pin insulated with PTFE is not available, a cable in PTFE insulation can be used. In order not to make "titanic" efforts to isolate the lower output of the cable, which will still close, you need to bring both ends of the cable up through the sealing and insulating bosses. The converter unit should be fixed near the output of the capacitance sensor from the vessel, so that the "extra" capacitance of the connecting cable is not connected to the converter input. The power supply and the indicating head are installed in the electrical panel.

The power supply and the output signal pass through a 4-wire cable (if two vessels with measured levels are located side by side, four wires are enough to power and receive the output signal from both transducers).

Consider the differences between the circuit in Fig. 3 and the circuit in Fig. 1. Resistor R2 has a higher value to reduce the tuning range. The capacitance of the capacitor C1, which determines the frequency of the generator "roughly", is set in relation to the object. The scheme is wide-range, it allows you to measure the capacitance in the range of tens of picofarads and tens of microfarads, which corresponds to level measurement in the range "from a glass to the ocean." The linear capacitance of the sensor is very different (the fluoroplastic insulation of the cable has a thickness of about 1 mm, and the cable, which can be used in places with low temperatures as a sensor, can have an insulation thickness of several millimeters), industrial tanks with liquids have a height of from decimeters to tens of meters, Therefore, we provide indicative data.

Due to the linear nature of the change in the output signal from the input capacitance and the frequency of the generator on DA1, setting up the circuit on the object is simple: if the output signal at full capacity is small, you should reduce the capacitance C1 so that the generator frequency increases and the output signal increases (and vice versa), and such a "rough "Adjustment is easily done within thousands of times!

Transistor VT1 of the conversion unit is turned on "vice versa" so that its output signal is connected to the storage capacitor C5 and resistor R6 connected to the "plus" of the power source. Transistors VT2 and VT3 convert the voltage drop across R6 into an output current of 0 ... 5 mA coming from the "plus" to the case in order to connect the PA1 measuring head with the second pin to the case. The output signal is current - when the resistance of the measuring head changes (even when the second one is connected in series), the value of the readings does not change. This is determined by comparing the input voltage drop across resistor R6 and the "current" voltage across R8. The comparison transistor VT2 has a good gain, and the second of the composite transistors (VT3) is included as a current amplifier. To compensate for the voltage drop at the B-E junction of the input transistor of the VT2 pair, a silicon diode VD6 is connected in series with the input resistor R3.

The output transistor is relatively powerful, since with a short circuit of the capacitive sensor, the output current increases.

When measuring the level by the capacitive method, it is essential to have the initial (zero) capacitance of the sensor when there is still no water in the tank.

To reduce the readings of the output device, we "select" part of the current through R8 from the transistors to the resistor R9. Thus, some current, determined by the tuning resistor R9, goes through the emitter resistor of the comparing transistor VT2, and this part of the current does not go to the output device!

Thus, the complete setup of the device includes:

• "rough" adjustment of the measurement range by capacitor C1;
• 100% setting at full capacity by resistor R1;
• adjustment of "zero" with an empty capacity by resistor R9.

The reserve retuning body is the resistor R6, a change in the resistance of which without changing the frequency of the generator on DA1 also leads to a change in the amplitude of the output signal.

Is it necessary to solder parts of other denominations when setting up the device at the facility? No! Unlike industrial (and even imported) devices, we use simulators of the capacitive signal of the level sensor (Fig. 4).

After installing the level sensor, it is necessary to measure the capacitance of the sensor when the tank is empty C0 and after filling with liquid by 100% - C100.

After that, you can make a phone call to another city and solder and set up the EIM according to our scheme. Indeed, the output signal is proportional to the capacitance of the sensor, and the character of the signal change depending on the capacitance is also linear. If you "bind" the beginning and end of the scale, then everything turns out simply! It is not necessary to fill 60-cc containers with water many times in order to consistently adjust the 0 and 100% scales of an industrial device. It is necessary to switch S1 to the "Setting" position, and "click" the S2 toggle switch at least a hundred times, successively adjusting the scale of the device.

After that, you need to fill the container with water once through the water meter and record the meter readings corresponding to integer divisions of the scale.

In practice, we do more prosaically. Since capacitance meters in different places can be configured differently (even a different piece of wire at the input!), We try to select capacitors on the spot that simulate the initial and final electrical capacitances of the vessel. With some skill, the selection of capacity can be carried out from 3 ... 5 denominations.

On the scale (this is a trick from practice), we try to "set" the initial capacity not to 0, but to the first division, so that the disconnection of the circuit or the breakage of the sensor "strike" the operator. Damage to the insulation of the sensor, leading to a short circuit of the input of the circuit, causes the pointer indicating device to go off scale.

The circuit in Fig. 3 is suitable for installation by beginners, but to ensure ease of setup and linearity of the scale, it is better to make a circuit according to Fig. 5, especially if a series of instruments is required for the same measurement conditions.

(click to enlarge)

Let's consider this diagram in more detail than the previous ones, and since the numbering of parts in the diagrams is the same, this description will also explain the previous diagrams.

Details that smooth out voltage ripples:

• C3, C4 - nutrition;
• C2 - timer reference voltage;
• C5 - storage capacitance voltage at the output of the converter.

Active (non-linear) elements:

• DA1 - semiconductor microcircuit - timer - rectangular pulse generator for operation of the capacitance - voltage converter;
• VT1 - transistor of the capacitance-voltage converter, which, with each pulse of the generator, recharges the measured capacitance and gives a current pulse to R6 and C5;
• VD2 - silicon diode that "reverse" recharges the input capacitance (works in tandem with VT1);
• VT2 - field effect transistor converter voltage - output current;
• VT3 - a bipolar transistor, a more powerful "helper" of VT2 (they act as one field-effect transistor with a large steepness);
• VT4 - output transistor, connected with a common base, stabilizes the supply voltage to VT2, VT3, allowing them to work the same when the load resistance changes;
• VD1 - protective zener diode in the timer input circuit;
• VD3 - a zener diode that maintains the required base potential of the output transistor;
• VD4, VD5 - elements of protection against the reverse supply of supply voltage to the circuit and the penetration of high voltage from the output (measuring instrument circuit) to the circuit elements, this is possible in case of accidents.

Current limiters:

• R7 - in the capacity sensor circuit;
• R13 - in the circuit of the measuring device (the resistor should burn out when high voltage enters the circuit of the measuring device).

Adjustable elements:

• R1 and C1 - oscillation frequencies of the generator;
• R6 (additional adjustment) - voltage level at the input of the converter voltage - current;
• R9 - adjusting the "zero" output.

Adjustment limit (in place):

• R2 (not less than 200 Ohm) - minimum resistance (maximum frequency);
• R3 - maximum resistance (minimum frequency);
• R10 (not less than 250 Ohm) current selection from transistors
• VT2 and VT3: current reduces the readings of the pointer device;
• R11 - minimum current draw (without this resistor, the "zero" adjustment range to the right will be too large).

The limitation of adjustments is necessary so that when manufacturing a series of devices with the same input signal limits, do not look for variable resistors with a rating that is not included in a series of standard resistances and at the same time ensure that the device is tuned within a narrow range around the norms, i.e. facilitate adjustment.

If the industry made devices, such limiters would be made using switches or jumpers, but it is much easier for a radio amateur to solder a resistor of the desired rating.

Details that support the required mode of operation of the cascades:

• R4, R5 - "shift" the potential of the pulse voltage based on the transistor-converter VT1 to "zero" in order to provide a voltage margin on the collector (otherwise the amplifying properties of the transistor will deteriorate);
• R6 - matches the average current coming from the VT1 collector with the maximum voltage at the input of the voltage-to-current converter (this resistor can also "roughly" adjust the maximum output signal);
• R8 - resistor at the source of transistor VT2 of the voltage-current conversion stage, this resistor sets the conversion scale;
• R12 - provides power to the zener diode with the necessary current.

Similar to the previous ones, this circuit contains constant capacitors that simulate the capacitance of the sensor when the tank is empty and filled with liquid.

Compared to commercially available capacitive level transmitters, the circuit has the following advantages:

• less complicated circuit (much); linearity of readings depending on the level; wide range of tuning;
• high reliability; ease and speed of finding out the cause of incorrect readings;
• incredible, only 28 parts, of which four blocks (cascades) are mounted!

Author: N.P. Goreiko

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