RU2201580C2 - Device measuring small-scale flow rate of gas - Google Patents

Abstract

FIELD: measurement technology. SUBSTANCE: thermal flowmeter incorporates sealed heatinsulated metal case housing heat exchanger-heater and gas distribution chamber to supply flow coming into it into measurement and thermal compensation conduits which are identical. Temperature-sensitive elements in the form of semiconductor temperature-sensitive resistors of indirect heating are positioned in conduits of case, additional heaters are mounted on outer surfaces of conduits. Temperature-sensitive element of thermal compensation conduit is placed in electronic circuit of unit controlling power of heaters. EFFECT: increased accuracy, reliability and sensitivity of measurements in wide range of temperatures of supplied gas and environment. 1 dwg

Description

 The invention relates to measuring technique, namely to measuring the mass flow of gas and to the device of heat gas flow meters intended for use in monitoring and control of gas flow in the range of 0-100 mg / s with a wide variation of the inlet gas temperature and the ambient temperature.

 Known heat flow meters based on taking into account the effect of thermal effects on the environment [1].

 Such flowmeters contain a housing, a pipeline with heaters (heater) located (located) on it [2, 3]. The temperature sensors in the sections of the pipeline, which is the measuring element (IE), are either the heaters themselves [2], or the temperature sensor located at the output end of the IE [3].

 To eliminate the influence of ambient temperature, the body of the flowmeter [2] is thermostatically controlled using a separate heater. The body of the flow meter [3] is insulated, and the insulating section is equipped with a device for adjusting its temperature.

 To reduce the influence of the temperature of the gas entering the IE, the thermostatically controlled body of the flowmeter [2] is equipped with a heat exchanger, flowing through which the gas warms up to some uncontrolled temperature, not necessarily equal to the temperature of the body.

 The influence of the temperature of the gas entering the flow meter [4] is partially compensated by electronic devices that generate a compensating signal as a function of the temperature of the incoming gas.

 Known heat flow meter containing a housing, a measuring gas pipe with a heat-sensitive element (TEC) located in it with metal conductivity, automation equipment to maintain the temperature of the TEC constant. The voltage supplied to the HSE characterizes the gas flow rate [5].

 A common drawback of flow meters [2] - [5] is the insufficient sensitivity of the thermal stabilization systems of the gas pipeline body or TEC, due to the use of sensors with metal conductivity having low values of the temperature coefficient of resistance (TCR).

 Known flowmeters containing a gas pipeline body with a measuring and compensation channel (s), in which there are thermoelectric thermistors [6], pyroelectric temperature sensors [7]. Both flowmeters have very complex electronic devices that monitor the thermal regime of the system, which reduces the reliability of their operation. By the number of matching distinctive features, the heat flow meter [6] is adopted as a prototype.

The present invention is the proposal and implementation of the simplest device for measuring gas flow, providing, at the same time, the autonomy of the flow meter (i.e. the independence of its readings from the values of the temperature of the incoming gas T _I and the environment T _cf ), increasing its accuracy, sensitivity and reliability while expanding the range of measurements of mass flow, gas.

The proposed technical solution of the invention lies in the fact that in the known method of measuring gas flow rate, which consists in placing the heated TFE in the gas flow cooling it (see [5], [6], [7]), the gas flow is thermally stabilized to ensure the flowmeter is autonomous at a fixed temperature level T _p , independent of T _I and T _cf , with the help of heaters, the power of which is controlled by another TFC thermistor, identical to the measuring TEC, and located in the compensation channel identical to the working channel. Separation of the gas stream into two identical flow (G / 2) and temperature (T _p ) flow is carried out using a gas distribution chamber (GRK).

The drawing shows a General view of the proposed gas flow meter. It contains: heat-insulated (inside and outside, depending on operating conditions) sealed metal housing 1 with inlet and outlet openings for fittings; heater - heat exchanger (TO) 2 with a nichrome spiral 10 inside it; gas distribution chamber (GRK) 3, hermetically connected to and with two identical channels 4, 5; measuring TEC 6 (semiconductor thermistor) with indirect current heating I _c.p. included in the circuit of the converter 7 of the output signal to electric; thermal compensation TCHE 8 (semiconductor thermistor), included in the electronic circuit of the heater heater power control unit 9 (BOOM), the load of which is the coil 10 of the heat exchanger and the heaters 11 and 12 on the outer surfaces of the channels 4,5.

The flow meter operates as follows. Through an inlet fitting (not shown) a gas of temperature T _in enters a heat exchanger 2, in which it is heated to a temperature T _r and enters a gas distribution chamber (GRK) 3, dividing the gas stream into two identical in flow rate (G / 2) and flow temperature, then entering the measuring 4 and thermocompensating 5 channels, respectively.

Located in the channel 5, the heat-sensitive TCE 8 takes the temperature of the incoming gas flow, and its ohmic resistance becomes equal to R (T _r ). If T _r ≠ T _p is the maximum possible value of T _I and T _cf , which is possible under operating conditions, then under the influence of a mismatch signal ΔR (T _r , T _p ) from the control unit 9, a power reducing ΔR is supplied to the heating spirals 10, 11, 12 to zero. This leads to thermal stabilization of the gas flow at a fixed level T _p , which ensures the independence of the flow meter readings from the values of T _I and T _cf , i.e. its autonomy. Additional spirals 11, 12 on the walls of channels 4, 5 compensate for the cooling of the gas as it flows along the GDS and contribute to the more rapid achievement of a fixed temperature level T _{n of the} coolant. An additional heating of the walls of the channels 4, 5 improves the protection of the HSE from the temperature T _cf and at the same time reduces the correction for the radiative heat exchange of the HSE with the walls of these channels.

The heat-sensitive element TCE 6 in the measuring channel 4 is overheated by an indirect heating current I _c.p. by ΔT - 20 K relative to a fixed level T _p washing the gas stream with a flow rate of G / 2. When interacting with the cooling gas stream, the temperature of the thermoelectric thermistor changes, which causes a change in its ohmic resistance and, therefore, the value of the output electrical signal converted by circuit 7, which is a measure of the gas flow through the measuring channel (i.e., G / 2).

 From channels 4, 5, gas flows, with a flow rate of G / 2 each, enter the internal volume of the sealed housing 1 of the flowmeter and the gas flows with the flow rate G into the gas network through an outlet fitting (not shown).

Thus, the thermal stabilization of the gas flow at a fixed level allows us to distinguish the flow rate as the only and unambiguous function of the influence on the enthalpy of TEC, regardless of the values of the temperature of the incoming gas T _in and the external environment T _cf. In contrast to the prototype, the constant temperature is not maintained at TCE, but at the temperature of the gas stream at the level of T _p with the help of heaters, the power of which is controlled by the thermal stabilization circuit.

 Improving the accuracy and sensitivity of the inventive flowmeter is provided, as in the prototype, using semiconductor thermistors as temperature sensors.

 Unlike the prototype, the proposed flow meter has two channels made identical, due to which any of them can be considered as a bypass line with a flow rate of G / 2. The prototype of the uneven channel cross-section requires knowledge of the coefficient of division of the flow between the channels. The same drawback is inherent in the flow meter [7].

 The reliability of the device depends on the number and complexity of control systems. In the proposed flow meter, such a system - the system of thermal stabilization of the gas stream - is one, while the prototype has three with positive and negative feedbacks controlled by software. The use of a flow meter [7] with its most complicated serving electronic-thermal system is justified only in special cases of fast pulsations of the temperature and velocity (flow) of the gas stream.

Literature
1.Korotkov P.A. in other. Thermal flow meters. - L.: Mechanical Engineering, 1969.

 2. Copyright certificate SU 1223065, cl. G 01 F 1/68, 1986.

 3. Copyright certificate SU 586330, G 01 F 1/68, 1977.

 4. The patent of Germany 2929427, MKI G 01 F 1/68.

 5. Patent FR 2459962, MKI G 01 F 1/68.

 6. Patent US 4653321, CL 73-197, 1987.

 7. Copyright certificate SU 870945, M. Cl. 4 G 01 F 1/68, 1981.

Claims (1)

 A thermal micro-flow meter, comprising a housing with a gas distribution chamber and channels located therein for accommodating the corresponding heat-sensitive elements - a measuring one made in the form of a semiconductor thermistor with indirect heating, and a temperature-compensating one - in the form of a semiconductor thermistor, characterized in that a controlled-power heater-heat exchanger is introduced into it , heaters placed on the walls of the channels of the housing, made identical, and the control unit power of the heater-heat exchange Rennik and channel heaters, the electronic circuit of which includes a heat-compensating heat-sensitive element.