LT3856B - Device for measuring liquid level - Google Patents

Abstract

Invention applies to the field of measuring engineering and may be used for the measuring of liquid levels of normal and low electrical transmission capacities. The invention aims to increase the precision of measurement controlling the liquid level of the low electrical transmission capacity. The aim is achieved by the following arrangement: the device with a Nichrome string put in the middle of the metal case of insulators also has the second Nichrome string put in parallel to the first string and symmetrically towards the case. The top end of the first string is connected to the output of the permanent alternating-current generator, and the lower end of the second string is connected to the second output of an opposite phase of the permanent current generator. Both strings top and lower ends are connected to outputs of the relevant differential amplifier, the outputs of which are connected to a two input summing device.

Description

The invention relates to a measuring technique and can be used to measure the level of electrically permeable and poorly permeable fluids.

A known device with a float (FR Application No. 2485726, 1980) for measuring the level of a fluid having a mechanical actuator between a float immersed in a control fluid and a meter (counting device). The measurement error of this device is determined by the precision of the mechanical drive manufacture and the absence of elastic deformation of the float suspension. Another disadvantage of this type of equipment is the complexity of their construction as the drive and float are clumsy.

Also known is a device (USSR a.l. No. 8014377,1981) for controlling the fluid level in tanks having a nichromic string housed in a metal housing on the insulators. The operating principle of this unit is based on the passage of a stable alternating current through the string and the voltage drop across the string as measured by the fluid level. As the fluid level changes, the length of the part of the string that flows at a steady current changes, so that the voltage drop across the string is functionally related to the fluid level.

The disadvantage of this unit is the additional errors that occur when controlling the low electrical permeability fluid level, which is related to the final value of the output impedance of the stable current generator supplying the sensor string.

SUMMARY OF THE INVENTION An object of the invention is to increase the accuracy of measurement by controlling the level of a fluid with low electrical conductivity.

This goal is achieved by having two stable AC generators with the same amplitude but operating in different phases, and the outputs of the generators are connected to two nichrome strings located inside or outside the insulators in a metal housing vertically contained in the controlled fluid, the output of the first oscillator is connected to the upper end of the first string, and the output of the second oscillator is connected to the lower end of the second string. The alternating current corresponding to the level of the liquid to be measured is fed to each of the strings into a separate differential amplifier, and the outputs of the differential amplifiers are connected to a two-input sumer. Due to the fact that the strings are connected to the outputs of the opposite phase oscillators, the current drop between each of the strings and the casing has the same amplitude but different marks and therefore compensates when summed. This eliminates the error of variation in the electrical conductivity of the controlled fluid. Enabling the second sensitive element string reduced the precision requirements for the element base used in the transducer because of string parameters, poor geometry, non-parallelism, and so on. does not matter the accuracy of the measurement.

FIG. 1 is a flow chart of a device for measuring fluid levels.

FIG. 2 - equivalent measuring scheme.

The device for measuring the fluid level has a clock pulse generator 1 whose output is coupled to a phase rotor 2, and the outputs and phase rotor input are connected to the inputs 3 and 4 of the constant current generator respectively. The outputs of the generators are connected to two parallel nichrome strings 5 and 6 on the insulator metal housing 7 in the middle or outside. The upper and lower ends of the strings are connected to the inputs of the differential amplifiers 8, 9, and their outputs are connected to the dual-sumator 10. The output of the sumator is connected to the input of the high-pass filter 11 and its output. The output of the comparator 12 is connected to the input of the subwoofer filter 13 and its output to the input of the opposite function isolation unit 14. The output of unit 14 is connected to a digital voltmeter 15.

The device for measuring the fluid level works as follows.

When filling the reservoir with fluid to be controlled, strings 5 and 6 are immersed to the same depth. The stroke pulse generator 1 generates rectangular pulses at 500 Hz which are fed to the phase rotor 2. The phase shift is 180 ° thanks to the phase rotator. The two constant current generators 3 and 4 are started by opposite-phase voltages, furthermore, at the output of the generator 3, stable current pulses are formed, which are displaced by a phase 180 ° at the output of the current pulse generator 4, respectively. The output of the generator 3 is connected to the upper end of the string 5 and the output of the generator 4 to the lower end of the string 6.

In the equivalent measuring scheme shown in FIG. 2, the following notations are included:

R _c i is the resistance of the top end of the string 5 to the surface layer of the liquid,

R _C 2 is the resistance of the strap 6 from the lower end to the surface layer of the liquid,

Rz is the complex liquid impedance between the string and the case.

In the case of strings 5 and 6 arranged symmetrically with respect to housing 7, the complex fluid impedance between string 5 and housing and between string 6 and housing is equal.

Ι _{+ φ} - stable alternating current with zero phase shift,

Ι_ _φ - Stable AC with 180 ° phase shift.

Flowing a current to the chord 5 section from the top end to a fluid surface layer and on through a complex fluid resistance of the casing 7, the differential amplifier 8 the output from the signal proportional to the strings 5 sections unsubmerged the liquid, the size of a current I _{+ (p} amplifier 8 enhancement coefficient. flowing current _{Ι_} φ strings 6 section from its lower end to a fluid surface layer and on through a complex fluid resistance of the body 7 and the differential amplifier 9 output forming a signal which is proportional to the chord six sections are immersed in the liquid, the size of the current _{Ι_} φ and amplifier factor 9. The output signals of amplifiers 8 and 9 are summed by estimating the displacements by the sum of the two inputs 10, in which case the error caused by possible changes in the stable current Ι + φ and Ι_ _φ due to the low electrical conductivity of some fluids is eliminated. The output signal 10 goes up From the output of the exact rectifier 12, a single-polarized pulsing signal is transmitted to the low-pass filter 13, where a constant component is emitted. Due to the inverse dependence of the output voltage on the fluid level, the output signal of the low-pass filter 13 is converted by the inverse function unit 14 to a normal state. The output voltage of unit 14 is simply proportional to the measured fluid level by a digital voltmeter 15.

The device for measuring the liquid level can be implemented by means of two opposite-phase stable current generators with an output impedance of at least 20 ohms. The sensor uses a stainless steel tube with two nichrome strings placed on the outside of the insulator, with lead leads from the lower ends of the string in the middle of the tube. The measuring part of the device uses K1408, K544, K140, K572 series chips.

The device provided significantly increases the accuracy of the liquid level measurement compared to the prototype due to the elimination of the error due to possible changes in the electrical conductivity of the controlled liquid to the downside.

Claims (3)

DEFINITION OF INVENTION A device for measuring the level of a liquid, having a nichrome string placed 5 on the insulators inside the metal housing, characterized in that it has a second nichrome string placed on the insulators parallel to the first string and symmetrical to the housing, the upper end of the first string being connected to the output of a stable alternator and the lower end of the second string the opposite of a stable current generator With a 10-phase output, the upper and lower ends of both strings are connected to the inputs of the respective differential amplifiers, the outputs of which are connected to a two-input sumer.