SU916996A1 - Liquid level converter - Google Patents

Description

(5t) FLUID LEVEL CONVERTER

I

The invention relates to a technique for measuring and controlling the level of a liquid or an interphase level using thermal sensing elements heated by an electric current, and can be used in control and measuring systems of various branches of the national economy for collecting and converting information on process parameters, pre-. property in the most difficult climatic and production conditions.

Known devices with two thermocouples in the form of thermistors located at a given level.

The device contains a working thermoelement heated by current and an unheated compensatory one, which are included in a differential bridge circuit with an amplifier, a threshold element and an output indicator til.

The disadvantage of such schemes is that when measured over a wide range of temperatures of controlled media, measurement accuracy is not ensured.

The closest in technical terms to the present invention is a device containing a worker with a heater loaded in a controlled environment and a compensation cable included in the differential bridge circuit are the other two. the arms of which comprise constant resistors, an amplifier, a threshold element, a key element in the heater circuit, whose control input is connected to the output of the threshold element 121.

The drawback of the device is the conversion errors of the bridge circuit due to the instability of the power source of the bridge, the change in the transmission coefficient of the bridge circuit in a wide range of temperature controlled environments, the nonidentity of the static characteristics of the operating and compensation thermistors, and the time and temperature instability of the voltage regulator. These factors worsen the resulting conversion accuracy. The purpose of the invention is to increase the level conversion accuracy of the output signal. This goal is achieved by the fact that, in the converter, an additional output of the shoulder resistor and the measuring diagonal of the bridge circuit are connected to the input of the amplifier via different inputs of the additional switch, the control input of which is connected to the output of the threshold element. Figure 1 shows the structural scheme of the working and compensatory thermoelements for analogue control devices; FIG. 2 is a structural diagram for positional control devices; FIG. FIG. 3 shows a circuit diagram of the time of a liquid level pulse converter; in fig. - waveform of the converter at different points. The device contains a worker 1 and compensatory 2 thermistor, immersed in a controlled environment. Worker 1 with heater J and compensatory 2 thermistors with identical temperature characteristics are connected to constants. shoulder resistors i. and 5. form a differential bridge circuit. An additional shoulder resistor 6 is included in the shoulder of the compensating thermistor 2, the value of which is proportional to the temperature overheating of the working thermistor 1 - "g, at which the heater 3 is turned off. The measuring diagonal and additional output of the shoulder resistor 6 of the prototype circuit are connected via switch 7 to the inputs of amplifier 8, the output of which is connected to threshold element 9. Successively, a key element 10 is connected to the heater 3 circuit. The control inputs of the switch 7 and key element 10 are connected to the output of the threshold element 9. The device operates as follows. When the device is initially turned on, the working thermoelement 1 is not heated, the voltage in the measuring diagonal of the bridge (Fig3) and at the additional output of the braking resistor 6 are 1) negative. The voltage through the switch 7 is fed to the input of the amplifier 8 and, amplified in power by means of the threshold element 9, connects the heater 3 to the power source and. This steady state is maintained throughout the entire time that the operating thermistor 1 is heated to a temperature of -d-j ,. At temperature-Si, the voltage UJL (O, and the voltage at the auxiliary output (from a part of the braking resistor 6 Uuj. () Dd with heating of the operating thermistor resist at zero-voltage, the heater 3 disconnects from the power source Ui and the connection voltage Uu., equal to {i amplifier 8 with the aid of switch 1 of amplifier 7. The switching process is regenerative and stable, since the voltage at the input of amplifier B instantaneously increases from 0 to + UjUjo. Now in the process of cooling the working thermal cutter source 1 from the voltage from a part of resistor 6 decreases from + Uu. to O / in the measuring diagonal (T) to O)) - When going to UjUj through O, the key element 10 and switch 7 return to the initial state. The switching process is also of a regenerative nature, since at the input of the amplifier g the current instantaneously increases to ~ –1), l. The working thermistor begins to heat up and then repeat. The output, - formative parameter, transforms the cooling time T ,,. “It is completely insensitive to the instability of the source of pi.tani heating3. Thus, in the proposed scheme, switching at temperatures -I occurs at the moment of equilibrium of the bridge measuring circuit. Consequently, the accuracy of the conversion is no longer affected by the instability of the power supply driver of the bridge circuit. The change in the transmission coefficient of the bridge circuit with the change

The heuru of the controlled environment does not affect the accuracy of the transformation either, since it is proportional to the part of the shoulder resistor 6 and not to the voltage at the output of the bridge, as in the prototype. In this case, the shoulder resistor 6 can be made of highly stable wire.

Claims (2)

1. The patent of Germany No. 1293 61; cl. iZ e 3, pub. 19bE. 2. Authors certificate of the USSR 70 763689, c. C 01 F 23/22, 1978 (prototype). g / ag.2