SU1264003A1 - Thermal flowmeter - Google Patents

Abstract

The invention can be used to measure the flow rate of a liquid and a gas and to reduce measurement errors. The pulse sequence at the output of the master oscillator 19 determines the time of the measurement cycle and is a reference for forcing the signals of the control unit 1. Multivibrators 20 and 21 control the gopshe pulses to the flow switch 2 and switch 9, which causes the gas flow to stop the thermistor 6, which is switched by the flow switch 2 from the measuring branch of the pipeline 3 to the additional branch 4, and the multivibrators 20 and 21 form an installation signal initial conditions of the integrator 14. Schemes 22 and 23 of the delay of debt; SS bake the required time shift of the pulse from the output of the standby multi (L vibrator 21 and thereby generate control signals for the reversible counter 17 and the key 13, respectively. 2 Il.

Description

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The invention relates to instrument engineering and can be used to measure the flow rate of a liquid and a gas flow.

The purpose of the invention is to reduce measurement errors.

Figure 1 shows the flowchart; FIG. 2 - time diagrams of efo work

The flow meter contains a software control unit 1, a flow switch 2, an effusive branch of pipeline 3, an additional branch of pipeline 4, a pavement measuring circuit 5 with a temperature-sensitive element 6 in one of the arms of the unbalance amplifier of bridge 7, a voltage-current converter 8, an electronic switch 9, an analog memory device 105 comparator llj reference voltage source 12 electronic key ISs integrator 14, frequency controlled oscillator 15, square pulse shaper 16 reversible counter 17 and the display unit 18, wherein the codes of the software control unit 1 are connected respectively to the zeroing input of the integrator 145 with the control inputs of the flow switch 2, the electronic switch 9, the reversing counter 17 and the electronic switch 13, the temperature sensor 6j bridge 5 amplifier 7 and the converter 8 form an autobalance a bridge circuit, the code of which is connected via an electronic switch 9 to the first input of the comparator 11, the second input of which is connected to the second code of the electronic switch 9 via an analogue the storage device 10 s of the comparator 11 is connected to the control input of the electronic key 13, the input of which is connected to the output of the source of the reference voltage 12, and the output of the electronic key 13 through a serial connection of the integrator 14, the controlled frequency generator 15. and the driver of the rectangular impulses 16 is connected with the diagonal of the power supply of bridge 5 and with the input of the reversible counter 17j whose output is connected to the input of the display unit 18 "

The device operates in two stages as follows.

In the first stage, from the output of the standby multivibrator 20 of the control unit 1 to the flow switch 2 comes

control voltage Uupn, unlocking the flow switch 2, t, e, providing the switch for the gas flow} to the inlet fitting,

a flow switch 2p with a measuring branch of the pipeline 3, thereby providing the effect of the measured flow on the thermistor R - 6 (Fig.26)

Flow switch 2 is designed for alternately switching the signal from the program control unit 1 of the gas flow 5 supplied to its input socket with the meter 5 by the branch of pipeline 3 and the additional branch 4, which are connected to its outlet connectors. In this case, in the second cycle of the measuring cycle, when the measuring branch is closed

0 pipeline 3 with a temperature-sensitive element 6, the flow switch 2 re-switches the flow to an additional branch 4 having the same hydrodynamic resistance as iz-.

5 the measuring branch of the pipe line 3. Due to this, continuity and the absence of fluctuations of the measured flow in the flow conduit is ensured.

Elements Rear 6, 1, 8 form an auto-balance bridge circuit, which ensures the constancy of the operating point of the thermistor (i.e., the constant of its temperature). In this case, it is possible to significantly improve the flow rate of the flow meter, since for a feedback system which is self-balancing, and the bridge circuit increases the response almost K times S where K is the gain of the unbalance amplifier 5 compared to the speed of the open-loop system.

Therefore, for thermistor 6, you can write the powerful-balance equation. taiz acting on him

P p

+ P

. p p i-Q g F e

ABOUT J- POS1imp

where RD is the power corresponding to the selected operating point of the thermistor b (i.e., providing resistance equal to the resistance of the bridge shoulders) at a normal temperature ();

PQ power5 taken from R

blown by its flow and is proportional to the flow rate;

Claims (1)

P - power5 is equivalent to the impact on K of the deviation of the ambient temperature from the normal value (+ P, - s, & T) in case the ambient temperature exceeds dT of its nominal value and -RT s, 4T, if the ambient temperature less by a nominal value; . c, is the coefficient of proportionality; - the power of the pulse signal coming from the rectangular pulse generator 16 per posg is the power dissipated on R under the action of a direct current of the converter 8, which ensures the constancy of the operating point of the thermistor Ry - 6, i.e. supporting bridge 5 in a balanced state; Moreover, Р „„ „, where с, is the proportionality coefficient i; U, c; - respectively, the amplitudes and duration of the pulse shaper 16; F is the frequency of the pulses, equal to the output frequency of the generator 15. When the values of U are constant. The power P. is linear and is related to the value of the output voltage of the integrator 14. I.y is the coefficient proportional to where. onality; UBU ,. The output voltage of the unbalanced bridge unbalance power. The power of the pulse signal Vn is designed to compensate for the influence of the gaseous flow on the thermistor 6. At the first stage of the measurement cycle (when R flows through the flow), the control unit 1 sets the initial conditions for the integrator 14, providing an initial voltage at its output value at which the power is equal to or slightly higher than the power that is taken by the flow from the thermistor in the case of maximum: gtnogo flow rate (i.e. at the maximum gas flow). In this case, the power balance equation has the form + P - P. P - imp on the post. At the same time, the output of the unbalance amplifier 7 sets the voltage (1) 034 eu U S providing the balance of bridge 5 (Figure 2e). This voltage through the switch II of the switch 9 enters the analog control unit 10. Once installed, the frequency F at the output of the generator of the controlled frequency 15 is entered into the reversible counter 17. In the second measurement cycle at time t (switch 9 is transferred to state I, and simultaneously the control signal to flow switch 2 is stopped, which causes the thermistor 6 to stop blowing gas flow, which is switched by flow switch 2 from the measuring branch of pipeline 3 to additional branch 4 (Fig. 26). As a result, power takeoff P from thermistor 6 stops, which causes imbalance of bridge 5 and a decrease in voltage at the output of the unbalance amplifier of bridge 7 to the value of The bridge 5 is again in a balanced state (Fig. 2e). At the end of the transition process at time t, the key 13 is closed using the control unit 1 and the input of the integrator 14 is energized from the voltage source 12. Under the action of the input voltage the output voltage of the integrator 14 begins to decrease, providing a decrease in the output frequency of the generator of the controlled frequency 15, and consequently, RIMP. A decrease in the power JL f causes an increase in the output unbalance amplifier of the bridge 7, keeping bridge balance 5. At time t, when the output voltage of the amplifier unbalance of the bridge 7 reaches the value U, 5th and earlier lO, the device is compressed by the device 1 1, measuring 1k key 13 and stopping the supply voltage to the input of the integrator 14. This results to stop the change in the output voltage of the integrator 14, and therefore to stop the change in the frequency F of the generator of the controlled frequency 15 and the power P) ,,,. In this case, the output of the controlled frequency generator 15 will set the frequency F (Fig. 2g) and the power of the pulse signal in this case is equal to Р ,, - г 112 1 1 (, 1. The power balance equation looks like no flow since, for gas P (j 0. Considering that the output voltage of the unbalance amplifier of the bridge 7 for the time interval tt, -t, (2) corresponds to the expression equal to the value of the output voltage for the time interval of this amplifier tt, O, which corresponds to (1), the conclusion follows that the power of direct current P in equations (2) It also takes into account that the interval, the time t During which the operation is performed by the impulse capacity of the Noh power of the signal P ,, t, is small and during this time the ambient temperature remains almost unchanged, that the value of P in equations (1) and (2) has the same value o Subtracting from expression (1) expression (1), we obtain: (FF) a (FF P p p Q imp them Since the power value is PQ is a function of the gas flow rate Q, then the differential frequency AF F - F uniquely determines the flow rate of the gas medium in the inlet. In order to obtain a number proportional to the value of uF, the reversible counter 17 is transferred to the subtraction mode by the signal from the control unit 1 and the frequency F is subtracted from the previously recorded frequency F (Figure 2d) The resulting number accumulated in the reversible counter 17 is fed to the display unit 18 At the end of the measurement process at the moment t, a gas flow is supplied to the thermistor 6, the initial conditions are brought to the integrator 14 and the devices are ready to perform repeated measurements. The control unit 1 works as follows. The output of master oscillator 19 (Fig. 2a) determines the measurement cycle time and is the reference for generating 036 signals, control unit 1, cartoon: vibrators 20 and 21 form control pulses for flow switch 2 and switch 9, and also the signal sets the initial conditions of the integrator 14, respectively (figb) in. The delay circuits 22 and 23 provide the necessary time shift of the pulse from the output of the standby multivibrator 21 and those. The most control signals are generated by the reversible counter 17 and the key 13, respectively (Fig. 2d and 2d). Claims of the invention A heat flow meter comprising a pipeline with a temperature-sensitive element included in the electric bridge arm, a bridge imbalance amplifier and a display unit, characterized in that, in order to reduce measurement errors, a software control unit, a flow switch, an electronic switch storing device, comparator, reference voltage source, electronic key, integrator, voltage-frequency converter and reversible counter, with a switch The outflow is switched on by its entrance to the pipeline, with the pipeline downstream of the flow switch made of two identical cross-section branches connected respectively to the flow switch outputs, and the temperature-sensitive element is installed in one of the branches, the outputs of the program control unit are connected respectively to the integrator zeroing input the inputs of the flow switch, electronic switch, reversible counter and electronic. the key, the temperature-sensitive element, the bridge and the amplifier form an auto-balance bridge circuit, the output of which is connected via an electronic switch to the first input of the comparator, the second input of which is connected to the output of a storage device, the input of which is connected to the second output of the electronic switch, the output of the comparator is connected to the control input of the electronic the key, the input of which is connected to the output of the source of the reference voltage, and the output of the electronic key in series 112640038 A connected integrator and a conversion counter, the output voltage-frequency coaster is connected to the input power of the bridge and to the input terminal. 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