RU27339U1 - DEVICE FOR THERMAL FLOW MEASUREMENT - Google Patents

Abstract

A device for thermal flow measurement containing a power source, a thermally sensitive element (thermistor) with direct cyclic heating, located in the pipeline flow, and a comparator, characterized in that, in order to improve the measurement accuracy and achieve independence of the measurement result from changes in the thermal conductivity of the medium, temperature and pressure, the device additionally contains a second thermosensitive element (thermistor) with direct cyclic heating from a given temperature and a fixed duration a heating pulse, located in the pipeline, but in a place protected from flow, a second comparator, a microprocessor element, a medium temperature sensor, two key elements, a serial interface unit and six resistors that form two identical bridge circuits with thermistors, connected to the source with the terminal leads power supply, medium ones to the comparators, while the first terminal of each thermistor is connected to the plus of the power source, between the second terminal of the thermistor and the minus of the power source, the key an element connected to the output of the microprocessor element, the outputs of the comparators and the medium temperature sensor are respectively connected to its inputs, and the other two inputs and outputs are connected to the serial interface unit, which is the digital output of the device, while the duration of the heating pulses of the first heat-sensitive element is proportional to the frequency of cyclic heating of the second a heat-sensitive element, and the frequency of the heating-cooling cycles of the first heat-sensitive element is the determining pair

Description

2002111127

{SHRRRRRShShRSH | gMKI 5 GO 1F I / 68

Device for thermal flow measurement.

The utility model relates to instrumentation and can be used to measure the speed or flow rate of a gas or liquid stream.

Thermal flow meters are known containing thermosensitive elements operating in an unsteady mode, containing power sources and a measuring circuit. In these flowmeters, during cyclic operation of a thermosensitive element, the parameters of its impulse (transient) characteristics are measured: the amplitude of the output signal, the transition time of the thermosensitive element from one temperature state to another, or the rate of the regular cooling mode of the thermocouple, by which the flow rate is judged (A.S. USSR N 556329, M. Cl. 2 G 01 F 1/68, 04/30/77, Bull. M16).

A device is also known for thermal measurement of flow (as USSR N 1126818, MKI 3 G 01 F 1/68, 30.11.84, Bull. N 44), in which according to the method of thermal measurement of the flow of liquid or gas by cyclic heating cooled controlled by the flow of a quartz thermosensitive element (CTEC) between its upper and lower boundary temperature values, at which the flow rate is judged by the transition time of the CTEC from one temperature state to another, changing the lower value of the heating temperature in time, keeping the difference t mperatur heating-cooling.

This device contains a power source, a quartz thermosensitive element, a heater, a subtractor, an indication device RS-trigger, three comparators, a frequency-code converter, two adders, an arithmetic device, four storage devices and a time-code converter.

The disadvantages of this device is that it is focused on the use of only quartz heat-sensitive elements, which necessarily require the use of an indirect support, which increases the size of the sensor and limits the scope. In addition, the measurement process with the claimed method does not occur continuously, but at quantized time intervals, which reduces the accuracy of the measurement. The device itself also has a complex structure and a large number of various discrete elements (local devices), which in general reduces reliability.

The disadvantage of these devices is also that they have a dependence of the measurement result on the thermal conductivity of the medium, which in turn depends not only on temperature, but also on the composition of the gas (liquid) and pressure in the pipeline, which is especially important for main pipelines with high gas pressure .

The purpose of the proposed utility model is to increase the measurement accuracy, simplify the device and calculations by a controlled parameter, achieve independence of the measurement result from changes in the thermal conductivity of the medium, temperature and pressure.

This goal is achieved by the fact that according to the method of thermal measurement of gas or liquid flow by cyclically heating a temperature-sensitive element cooled by a controlled flow to a predetermined temperature, the flow rate is judged by the frequency of cyclic heating, while the duration of the heating pulses is changed by the calibration characteristic depending on the frequency of cyclic heating the second heat-sensitive element, protected from the stream, the duration of the heating pulses of which is constant. The calibration characteristic is constructed in such a way that at zero flow rate the frequency of cyclic heating-cooling of the first heat-sensitive element remains constant over the entire range of changes in the thermal conductivity of the medium caused by changes in temperature, composition and pressure. The second heat-sensitive element, protected from flow, is actually a meter of static parameters of the medium. At the same time, in a device for thermal flow measurement containing a power source, a direct cyclic heat-sensitive element (thermistor) located in the pipeline flow and a comparator, an additional second heat-sensitive element (thermistor) with direct cyclic heating from a given temperature and a fixed pulse duration is additionally contained heating located in the pipeline, but in a place protected from the flow, the second comparator, microprocessor element, medium temperature sensor, two keys elements, a serial interface unit and six resistors, forming two identical bridge circuits with thermistors, the extreme leads connected to the power source, the middle leads to the comparators, the first terminal of each thermistor connected to the plus of the power source, between the second terminal of the thermistor and the minus of the power source includes a key element connected to the output of the microprocessor element, the inputs of which are connected respectively to the outputs of the comparators and the medium temperature sensor, and two others in odavyhoda connected to the serial interface unit which is a digital output device, wherein, the duration of the heating pulses of the first temperature sensing element proportional to the frequency of cyclic heating the second temperature sensing element, and the frequency of cycles of the first heating-cooling temperature sensing element is an output device parameter.

In FIG. 1 is a functional diagram of the proposed device, where 1 is a power source, 2 is a microprocessor element, 3.4 are key elements, 5.6 are comparators, 7 is a medium temperature sensor, 8 is a serial interface unit, 9.10 are the first and second heat-sensitive elements , 11 ... 16 - bridge resistors.

The device operates as follows. In the initial state, before turning on the power, the thermistors have a medium temperature Tc. After turning on the power, the microprocessor element (2) under the control of the program laid down in it, through the key elements (3,4), it begins to periodically turn on the heating of the thermistors (9,10), connecting them to the power source (1) for a fixed time. After the end of each heating pulse, the microprocessor element controls the state of the comparator (4), determining the temperature transition of the thermistors through the lower boundary value, which is set by resistors 11-16 of the bridge circuits. After the temperature of the thermistors passes through the set value, the measurement process begins. From this moment, the duration of the heating pulse of the first thermistor is determined by the microprocessor element according to the frequency code of the cyclic nafew of the second thermistor, the duration of the heating pulse of which is always constant. The value of the heating duration of the first thermistor is set by the microprocessor element by direct recalculation of the cyclic heating frequency code of the second thermistor or from the table stored in its memory.

After the selected heating time has elapsed, the microprocessor transfers the key element to a high-impedance state, the process of cooling the thermistor to a given temperature begins, which is fixed by the comparator. Next, the heating-cooling process is repeated. The microprocessor calculates the number of heating-cooling cycles for a certain period of time, thus measuring the frequency of the process - a parameter that determines the flow rate or flow rate of gas or liquid.

Calculations for gases and gas mixtures are made on the basis of the expression arising from the basic thermodynamic relations for natural and forced convective heat transfer as applied to the heating-cooling cycle with the assumption that the parameters change linearly with small temperature changes.

PRT,. (TH - TC) (TI + tz) s + MT „- Tc) (2 yaaru / ri) - (t, + tg) S;

where is the mass velocity (ij% where (Ti + T2)

and for zero flow rate () we get FO ko Х (Тн - TC) / TI (1).

In this case, the formula for calculating the mass flow rate is reduced to the expression:

(- -1): kl (- -1) (2)

or volumetric flow rate, in normalized (reduced to normal

conditions) units, for the time interval AT:

 -S -AT / pH k, (/ p „) (- 1) -S -AT (3).

77/7 RT, F 1 1 / „t 1 // I l

2g / / 1 (7, -G) 5

We accept T const, Foi const, TH is the same for both heat-sensitive elements, therefore:

Fo2 k2; I (TH - Te) or X kj ROSE / (Т „- Тс); (4)

TI 1 ki2 Fo2 - the duration of the heating of the first heat-sensitive element. Taking into account (4), and that for gases and their mixtures, we can take ri kcmcX ,, and mf / Pn const, we obtain the flow formula (3) in the form:

VH - k4 (Fo2 / (Т „- TC)) (ksF-1) -S -AT (5).

Thus, formula (5) contains only the constants and variables measured by the proposed device: the cyclic nafew frequencies of the first and second heat-sensitive elements and the temperature of the medium. Designations adopted in the formulas:

 TH, heating duration;

T2 - cooling duration;

F - cycle frequency heating-cooling

FO, FOI, Fo2 - frequency at zero speed (flow rate);

TH is the average heating temperature;

Tc is the temperature of the medium;

A, - thermal conductivity;

t | - viscosity;

pv is the flow rate (mass);

S is the cross-sectional area of the stream;

S is the surface area of the sensor (thermistor);

PH is the density of the medium under normal conditions;

Шс - average molecular weight of the gas mixture;

VH - flow rate for the time interval AT;

kiky - proportionality coefficients (constants);

n - a measure of the degree of Reynolds number in the coefficient of forced heat transfer (n 0.5).

In the practical application of the device, the following advantages are manifested:

 when using the device on natural gas pipelines, there is no need to periodically enter into the computer (as in similar devices) the percentage of gas and its density; there is the possibility of reliable calibration of the device in air blowing installations with subsequent use of natural gas in pipelines;

 the presence of a microprocessor element allows calculations to be made both by direct calculation and tabularly by calibration characteristics, and the latter allow taking into account both the design and individual features of the sensors and inaccuracies in the formulas, for example, a change in the exponent n with a change in the flow rate;

the digital output is convenient both for displaying and for entering into automated systems (telemetry, process control, etc.).

Claims (1)

A device for thermal flow measurement, containing a power source, a thermally sensitive element (thermistor) with direct cyclic heating, located in the pipeline flow, and a comparator, characterized in that, in order to improve the measurement accuracy and achieve independence of the measurement result from changes in the thermal conductivity of the medium, temperature and pressure, the device additionally contains a second thermosensitive element (thermistor) with direct cyclic heating from a given temperature and a fixed duration a heating pulse, located in the pipeline, but in a place protected from flow, a second comparator, a microprocessor element, a medium temperature sensor, two key elements, a serial interface unit and six resistors that form two identical bridge circuits with thermistors, connected to the source with the terminal leads power supply, medium ones to the comparators, while the first terminal of each thermistor is connected to the plus of the power source, between the second terminal of the thermistor and the minus of the power source, the key an element connected to the output of the microprocessor element, the outputs of the comparators and the medium temperature sensor are respectively connected to its inputs, and the other two inputs and outputs are connected to the serial interface unit, which is the digital output of the device, while the duration of the heating pulses of the first heat-sensitive element is proportional to the frequency of cyclic heating of the second a heat-sensitive element, and the frequency of the heating-cooling cycles of the first heat-sensitive element is the determining pair meter flow.