Thermal mass flow meter
By using rectangular, annular, or folded heating elements in thermal mass flow meters, the problem of insufficient measurement accuracy at low flow rates is solved. By increasing the contact area and designing the control circuit, the measurement accuracy of fluid mass flow rate is improved.
Patent Information
- Application Number
- CN202520235466.3
- Authority / Receiving Office
- CN · China
- Patent Type
- Utility models(China)
- Current Assignee / Owner
- Filing Date
- 2025-02-14
- Publication Date
- 2026-01-30
- Estimated Expiration
- 2035-02-14
AI Technical Summary
Existing thermal mass flow meters have low measurement accuracy at low fluid flow rates, mainly because the heating element has a small heat capacity, which leads to a reduced heat exchange rate between the fluid and the heating element, resulting in insufficient temperature change and affecting measurement accuracy.
Rectangular, ring-shaped, or faceted heating elements are deployed downstream of the fluid flow to increase the contact area between the fluid and the heating elements. The control circuit enables the acquisition and control of the fluid temperature, thereby improving the heat exchange rate and amplifying the temperature change.
It improves the measurement accuracy of fluid mass flow rate in low flow rate scenarios by increasing the contact area between the fluid and the heating element, enhancing the heat exchange rate, improving the temperature change, and thus improving the measurement accuracy.
Smart Images
Figure CN223856534U_ABST
Abstract
Description
TECHNICAL FIELD
[0001] The present application relates to the technical field of industrial measurement, and in particular, to a thermal mass flowmeter. BACKGROUND
[0002] Thermal mass flowmeters (TMF) are commonly used measuring devices for measuring the mass flow of a fluid. Existing thermal mass flowmeters are mostly composed of a heating element, a temperature sensor and a data processor. The temperature sensor is used to measure the actual temperature of the fluid, and the heating element is heated to a set temperature to obtain the temperature difference between the temperature sensor and the heating element. When the fluid starts to flow, the heating element will be affected by heat exchange and will carry away part of the heat by the fluid flowing through, thereby causing the temperature difference to change. At this time, the data processor determines the current fluid flow rate according to the pre-stored corresponding relationship between the fluid flow rate and the temperature difference, and determines the product of the fluid flow rate, the cross-sectional area of the measuring pipe and the fluid density as the mass flow of the fluid.
[0003] However, the existing thermal mass flowmeter has the problem of low precision at low fluid flow rate. Specifically, compared with a heating element with a large heat capacity, a heating element with a small heat capacity will have a larger temperature change when releasing the same amount of heat. Therefore, the existing heating element mostly uses a small-diameter metal wire to reduce the heat capacity of the heating element by reducing the diameter of the metal wire. However, the small diameter of the metal wire reduces the contact area between the fluid and the heating element, which further reduces the heat exchange rate between the fluid and the heating element at low fluid flow rate, resulting in a decrease in the amount of heat released by the heating element, and a decrease or even no change in the temperature change of the heating element, thereby causing a small or even no change in the temperature difference, which affects the measurement precision of the mass flow. Therefore, how to improve the measurement precision of the mass flow of the fluid at low flow rate has become a problem to be solved. CONTENT OF THE INVENTION
[0004] In view of the above problems, the present application provides a thermal mass flowmeter to achieve the purpose of improving the measurement precision of the mass flow of the fluid at low flow rate. The specific scheme is as follows:
[0005] The first aspect of the present application provides a thermal mass flowmeter, comprising:
[0006] A temperature sensor, a heating element, a control circuit and a display, a temperature output end of the temperature sensor is electrically connected with a signal input end of the control circuit, an electrode end of the heating element is electrically connected with a measurement end of the control circuit, a mass flow output end of the control circuit is electrically connected with an input end of the display, wherein the temperature sensor is arranged at an upstream position of a fluid flow direction, the heating element is arranged at a downstream position of the fluid flow direction, and the heating element is in any one of a rectangular shape, a ring shape and a folded surface shape.
[0007] In a possible implementation, the control circuit comprises:
[0008] A current sensor, a heating power supply and a central processing unit, a positive pole of the heating power supply is electrically connected with a first electrode end of the electrode end of the heating element, a second electrode end of the electrode end of the heating element is electrically connected with a first end of the current sensor, a second end of the current sensor is electrically connected with a negative pole of the heating power supply, a signal output end of the current sensor is electrically connected with a first input end of the central processing unit, wherein the positive pole of the heating power supply and the first end of the current sensor constitute a measurement end of the control circuit.
[0009] A second input end of the central processing unit is a signal input end of the control circuit.
[0010] An output end of the central processing unit is a mass flow output end of the control circuit.
[0011] In a possible implementation, the material of the heating element is a carbon nanotube film.
[0012] In a possible implementation, the thickness of the rectangular heating element is not greater than 1 micrometer, the width is less than 1 centimeter, and the length is not greater than the inner wall diameter of the measuring tube.
[0013] In a possible implementation, the side wall thickness of the ring-shaped heating element is not greater than 1 micrometer, the outer circle radius is less than the inner wall diameter of the measuring tube, and the length is less than 1 centimeter.
[0014] In a possible implementation, the rectangular heating element is generated by superimposing a preset number of layers of the rectangular carbon nanotube film.
[0015] In a possible implementation, a pointing line of the fluid flow direction is perpendicular to the circular cross section of the ring-shaped heating element.
[0016] In a possible implementation, the folded surface-shaped heating element is composed of two rectangular heating elements cut along the short side at a preset angle.
[0017] In a possible implementation, the control circuit further includes: a current amplifier, a first end of the current amplifier is electrically connected with a second electrode end of the electrode ends of the heating element, and a second end of the current amplifier is electrically connected with the first end of the current sensor.
[0018] In a possible implementation, the control circuit further includes: a triode amplification circuit, the triode amplification circuit includes:
[0019] a base bias resistor, a collector bias resistor and a triode, a positive pole of a first power supply end of the control circuit is electrically connected with a first end of the base bias resistor, a second end of the base bias resistor is electrically connected with a base of the triode, an emitter of the triode, a negative pole of the first power supply end and a negative pole of a second power supply end of the control circuit are electrically connected and grounded, a positive pole of the second power supply end of the control circuit is electrically connected with a first end of the collector bias resistor, and a second end of the collector bias resistor is electrically connected with a collector of the triode.
[0020] a base of the triode is electrically connected with a second electrode end of the electrode ends of the heating element, and a collector of the triode is electrically connected with the first end of the current sensor.
[0021] By the technical scheme, the application provides a thermal mass flowmeter, which realizes the collection of the actual temperature of the fluid by electrically connecting the temperature output end of the temperature sensor arranged at the upstream position of the fluid flow direction with the signal input end of the control circuit, realizes the heating control and temperature collection of the heating element by electrically connecting the electrode end of the heating element arranged at the downstream position of the fluid flow direction with the measurement end of the control circuit, and realizes the measurement and display of the mass flow of the fluid by electrically connecting the mass flow output end of the control circuit with the input end of the display. In addition, compared with the existing small-diameter metal wire, the heating element in any one of the shapes of the rectangle, the ring and the folded surface increases the contact area of the fluid and the heating element, thereby improving the heat exchange rate of the fluid and the heating element in the low flow rate scene, amplifying the temperature change amount of the heating element, and improving the measurement precision of the mass flow. BRIEF DESCRIPTION OF DRAWINGS
[0022] The above and other features, advantages, and aspects of the present disclosure will become more apparent by describing in detail the following specific embodiments with reference to the attached drawings. Throughout the drawings, the same or similar reference numerals refer to the same or similar elements. It should be understood that the drawings are schematic, and the sizes of the elements and the like are not necessarily drawn to scale.
[0023] Figure 1 a structural schematic diagram of the thermal mass flowmeter provided by the application;
[0024] Figure 2A structure diagram of a thermal mass flow meter provided for a possible implementation of the present application;
[0025] Figure 3 A three-view diagram of a prior art wire-shaped heating element provided for the present application in a measuring pipe;
[0026] Figure 4 A three-view diagram of a rectangular heating element provided for the present application in a measuring pipe;
[0027] Figure 5 A three-view diagram of a ring-shaped heating element provided for the present application in a measuring pipe;
[0028] Figure 6 A three-view diagram of a folded surface-shaped heating element provided for the present application in a measuring pipe;
[0029] Figure 7 A circuit structure diagram of a thermal mass flow meter provided for the present application;
[0030] Figure 8 A structure diagram of a rectangular heating element provided for the present application;
[0031] Figure 9 A structure diagram of a rectangular heating element provided for a possible implementation of the present application;
[0032] Figure 10 A structure diagram of a ring-shaped heating element provided for a possible implementation of the present application;
[0033] Figure 11 A structure diagram of a three-terminal amplifier circuit provided for the present application. DETAILED DESCRIPTION
[0034] The embodiments of the present application are described below in conjunction with the accompanying drawings. The terms used in the embodiment part of the present application are only used to explain the specific embodiments of the present application, and are not intended to limit the present application.
[0035] The embodiments of the present application are described below in conjunction with the accompanying drawings. It is known to those of ordinary skill in the art that, as technology develops and new scenarios appear, the technical solutions provided by the embodiments of the present application are also applicable to similar technical problems.
[0036] The terms "first", "second", and the like in the description and in the claims of the present application and above drawings are used for distinguishing between similar objects and not necessarily for describing a specific sequential or chronological order. It is to be understood that the terms so used are interchangeable under appropriate circumstances and are merely employed in the descriptions for clarity. Additionally, the terms "comprise", "comprising", "have", "has", "including", "including", "contain", "containing", or any other variation thereof, are intended to cover a non-exclusive inclusion, such that a process, method, article, or apparatus that comprises, has, includes, contains a list of elements does not include only those elements but can include other elements not expressly listed or inherent to such process, method, article, or apparatus.
[0037] The first aspect of the present application provides a thermal mass flow meter, as shown in the figure, which comprises: Figure 1
[0038] A temperature sensor 11, a heating element 12, a control circuit 13 and a display 14, the temperature output end of the temperature sensor 11 is electrically connected to the signal input end of the control circuit 13, the electrode end of the heating element 12 is electrically connected to the measurement end of the control circuit 13, and the mass flow output end of the control circuit 13 is electrically connected to the input end of the display 14, wherein the temperature sensor 11 is disposed at an upstream position of the fluid flow direction, the heating element 12 is disposed at a downstream position of the fluid flow direction, and the shape of the heating element 12 is any one of rectangular, annular and folded surface.
[0039] It should be noted that in actual application scenarios, the type of the above-mentioned temperature transducer 11 can be various, including but not limited to: thermistor, thermocouple, resistance temperature, etc.
[0040] It should be noted that in actual application scenarios, the material of the above-mentioned heating element 12 is thermistor material.
[0041] It should be noted that in actual application scenarios, the above-mentioned control circuit 13 is a digital circuit for collecting the temperature of the heating element 12, and calculating the mass flow of the fluid based on the collected temperature of the heating element 12 and the temperature collected by the temperature sensor 11.
[0042] It should be noted that in actual application scenarios, the above-mentioned display 14 is a display device for displaying the mass flow of the fluid sent by the control circuit 13 through the mass flow output end.
[0043] It should be noted that in actual application scenarios, the above-mentioned temperature sensor 11, heating element 12 and display 14 can be powered by the control circuit 13.
[0044] It should be noted that the present application realizes the collection of the actual temperature of the fluid by electrically connecting the temperature output end of the temperature sensor 11 arranged at the upstream position of the fluid flow direction with the signal input end of the control circuit 13, realizes the heating control and temperature collection of the heating element 12 by electrically connecting the electrode end of the heating element 12 arranged at the downstream position of the fluid flow direction with the measurement end of the control circuit 13, and realizes the measurement and display of the mass flow of the fluid by electrically connecting the mass flow output end of the control circuit 13 with the input end of the display 14.
[0045] In a possible implementation, in order to improve the utilization rate of the device, the above-mentioned thermal mass flow meter can also integrate multiple groups of temperature sensors 11 and heating elements 12 by using a multiplexer (MUX), so as to measure the mass flow of the fluid in multiple test pipes at the same time by using one control circuit. The structural schematic diagram of the thermal mass flow meter added with the multiplexer is as shown in Figure 2 The temperature output end of the temperature sensor 11 is electrically connected with the input terminal of the multiplexer 15, the electrode end of the heating element is electrically connected with the input terminal of the multiplexer 15, the signal output end of the multiplexer 15 is electrically connected with the signal input end of the control circuit 13, and the mass flow output end of the control circuit 13 is electrically connected with the input end of the display 14.
[0046] It should be noted that, in the actual application scenario, the above-mentioned rectangle, ring and folded surface refer to the projection shape of the heating element on the projection plane perpendicular to the fluid flow direction.
[0047] In order to facilitate the understanding of the shape of the above-mentioned heating element, the following tricks and a possible implementation of the present application are described:
[0048] It should be noted that the following Figure 3 , Figure 4 , Figure 5 and Figure 6 all include the measuring cylinder 31, the temperature sensor 11 and the heating element 12, and the fluid flow direction is from the temperature sensor 11 to the heating element 12. As Figure 3 shows three views of the existing wire-shaped heating element in the measuring pipe. As Figure 4 shows three views of the rectangular heating element in the measuring pipe. As Figure 5 shows three views of the ring-shaped heating element in the measuring pipe. As Figure 6 shows three views of the folded surface-shaped heating element in the measuring pipe. From the above Figure 3 , Figure 4 , Figure 5 and Figure 6It is known that the contact area between the heating element and the fluid is greater than that between the heating element and the fluid in any of the above-mentioned rectangular, annular, and folded shapes. Therefore, this application improves the heat exchange rate between the fluid and the heating element in low flow rate scenarios by configuring the heating element in any of the rectangular, annular, and folded shapes, taking advantage of the fact that the contact area between the heating element and the fluid in the above-mentioned shapes is larger than that of existing small-diameter metal wires. This amplifies the temperature change of the heating element and improves the measurement accuracy of mass flow rate.
[0049] It should be noted that, in practical applications, in order to reduce the heat capacity of heating elements of any of the above-mentioned rectangular, ring, and folded shapes, this can be achieved by compressing the thickness of the heating elements of these shapes.
[0050] This application achieves the acquisition of the actual fluid temperature by electrically connecting the temperature output terminal of a temperature sensor deployed upstream of the fluid flow direction to the signal input terminal of the control circuit. It also achieves heating control and temperature acquisition by electrically connecting the electrode of a heating element deployed downstream of the fluid flow direction to the measurement terminal of the control circuit. Furthermore, by electrically connecting the mass flow rate output terminal of the control circuit to the input terminal of the display, it achieves the measurement and display of the fluid mass flow rate. Moreover, since heating elements of any shape—rectangular, annular, or planar—increase the contact area between the fluid and the heating element compared to existing small-diameter metal wires, the heat exchange rate between the fluid and the heating element is improved in low-flow-rate scenarios, thereby amplifying the temperature change of the heating element and improving the measurement accuracy of the mass flow rate.
[0051] In one possible implementation, the above is as follows: Figure 1 The control circuit 13 shown includes:
[0052] The system includes a current sensor, a heating power supply, and a central processing unit. The positive terminal of the heating power supply is electrically connected to the first terminal of the heating element, the second terminal of the heating element is electrically connected to the first terminal of the current sensor, the second terminal of the current sensor is electrically connected to the negative terminal of the heating power supply, and the signal output terminal of the current sensor is electrically connected to the first input terminal of the central processing unit. The positive terminal of the heating power supply and the first terminal of the current sensor constitute the measurement terminal of the control circuit.
[0053] The second input terminal of the central processing unit is the signal input terminal of the control circuit;
[0054] The output of the central processing unit is the mass flow output of the control circuit.
[0055] To facilitate the discussion of the above... Figure 1 The connection relationships of the various devices in the thermal mass flow meter shown are explained here in conjunction with one possible implementation of this application: For example... Figure 7As shown, it is a circuit structure schematic diagram of a thermal mass flow meter. The temperature output end of the temperature sensor (THERM) 11 is electrically connected with the second input end of the central processing unit 73. The positive pole of the heating power supply 72 is electrically connected with the first electrode end of the heating element 12, the second electrode end of the heating element 12 is electrically connected with the first end of the current sensor 71, and the second end of the current sensor 71 is electrically connected with the negative pole of the heating power supply 72. The signal output end of the current sensor 71 is electrically connected with the first input end of the central processing unit 73. The output end of the central processing unit 73 is electrically connected with the input end of the display 14.
[0056] It should be noted that in the actual application scenario, since the material of the above-mentioned heating element is a thermistor material, that is, the internal resistance of the heating element will fluctuate in the case of temperature drop, and since the change of the internal resistance will cause the change of the current value flowing out of the heating element. Therefore, the positive pole of the heating power supply is electrically connected with the first electrode end of the electrode end of the heating element, the second electrode end of the electrode end of the heating element is electrically connected with the first end of the current sensor, and the second end of the current sensor is electrically connected with the negative pole of the heating power supply. The current sensor realizes the collection of the current value flowing out of the heating element, and the signal output end of the current sensor is electrically connected with the first input end of the central processing unit, so that the central processing unit calculates the internal resistance of the heating element according to the pre-stored voltage of the heating power supply and the current value sent by the current sensor, and determines the current fluid flow rate based on the pre-stored corresponding relationship between the internal resistance and the flow rate. The calculation formula of the above-mentioned central processing unit for calculating the internal resistance R can be: R=U / I, U is the pre-stored voltage of the heating power supply, and I is the current value sent by the current sensor.
[0057] It should be noted that in the actual application scenario, the calculation formula of the central processing unit for calculating the mass flow of the fluid is: m=v×S×ρ, to obtain the mass flow m, wherein v is the flow rate, S is the pre-stored cross-sectional area of the measuring pipe, and p is the density of the fluid.
[0058] In a possible implementation, the material of the heating element is carbon nanotube film.
[0059] It should be noted that in the actual application scenario, the above-mentioned carbon nanotube film (Carbon nanotube films) is a two-dimensional carbon nanotube network structure obtained by physical or chemical methods from a carbon nanotube array. Since the thickness of the single-layer carbon nanotube film is only a few tens of nanometers, the heat capacity of the heating element made of carbon nanotube film material is much smaller than that of the existing metal wire (with a diameter of at least 10 microns), so that the thermosensitive characteristics of the heating element made of carbon nanotube film material are superior to those of the existing metal wire, thereby improving the measurement accuracy of the fluid mass flow in the low flow rate scenario.
[0060] In one possible implementation, after obtaining the carbon nanotube film, the heating element can be obtained simply by adding electrodes to both ends of the carbon nanotube film. For example... Figure 8 The diagram shows a rectangular heating element. The two ends are electrodes, and the middle part is a carbon nanotube film.
[0061] In one possible implementation, the rectangular heating element has a thickness of no more than 1 micrometer, a width of less than 1 centimeter, and a length of no more than the inner diameter of the measuring tube.
[0062] In one possible implementation, a schematic diagram of the rectangular heating element described above is shown below. Figure 9 As shown. Wherein, the length L, width W, and thickness H are as follows: Figure 9 As shown.
[0063] In one possible implementation, the sidewall thickness of the annular heating element is no more than 1 micrometer, the outer radius is smaller than the inner diameter of the measuring tube, and the length is less than 1 centimeter.
[0064] In one possible implementation, a schematic diagram of the aforementioned annular heating element is shown below. Figure 10 As shown. The length L, sidewall thickness W, and outer radius R are as follows: Figure 10 As shown.
[0065] In one possible implementation, the rectangular heating element is generated by stacking a predetermined number of rectangular carbon nanotube films.
[0066] It should be noted that in practical applications, since the thickness of a single-layer carbon nanotube film is constant, its heat capacity is also constant. Therefore, for different applications, heating elements with different heat capacities can be obtained by stacking a predetermined number of rectangular carbon nanotube films. The same method can be used for heating elements of other shapes.
[0067] In one possible implementation, the direction of fluid flow is perpendicular to the circular cross-section of the annular heating element.
[0068] In one possible implementation, the faceted heating element is composed of two rectangular heating elements cut together along their short sides, and the two rectangular heating elements are at a predetermined angle.
[0069] It should be noted that in practical applications, when two rectangular heating elements are joined along their short sides, there is a gap facing the fluid flow direction, which can affect the fluid velocity. Therefore, this application configures the two rectangular heating elements to be joined along their short sides, so that the resulting faceted heating element does not have too much impact on the fluid velocity, thereby improving the measurement accuracy of mass flow rate.
[0070] In a possible implementation, the control circuit 13 further includes: a current amplifier, a first end of the current amplifier is electrically connected with a second electrode end of the electrode ends of the heating element, and a second end of the current amplifier is electrically connected with the first end of the current sensor.
[0071] It should be noted that, in an actual application scenario, the current amplifier is a device for amplifying current. Since the current flowing out of the heating element is small, there is even a risk of exceeding the collection range of the current sensor. Therefore, in order to avoid the above risk, the first end of the current amplifier is electrically connected with the second electrode end of the electrode ends of the heating element, and the second end of the current amplifier is electrically connected with the first end of the current sensor, so as to amplify the current signal input into the current sensor, thereby improving the measurement accuracy of the mass flow.
[0072] In a possible implementation, the control circuit further includes: a triode amplification circuit, the triode amplification circuit includes:
[0073] a base bias resistor, a collector bias resistor and a triode, a positive electrode of a first power supply end of the control circuit is electrically connected with a first end of the base bias resistor, a second end of the base bias resistor is electrically connected with a base of the triode, an emitter of the triode, a negative electrode of the first power supply end and a negative electrode of a second power supply end of the control circuit are electrically connected and grounded, a positive electrode of the second power supply end of the control circuit is electrically connected with a first end of the collector bias resistor, and a second end of the collector bias resistor is electrically connected with a collector of the triode.
[0074] a base of the triode is electrically connected with a second electrode end of the electrode ends of the heating element, and a collector of the triode is electrically connected with the first end of the current sensor.
[0075] It should be noted that, in an actual application scenario, a structure diagram of the triode amplification circuit is as shown in Figure 11 The triode amplification circuit has the same effect as the current amplifier. The base of the triode is electrically connected with the second electrode end of the electrode ends of the heating element, and the collector of the triode is electrically connected with the first end of the current sensor, so as to amplify the current signal input into the current sensor, thereby improving the measurement accuracy of the mass flow.
Claims
1. A thermal mass flow meter characterized by, The application relates to a heating element for a gas flow meter, which comprises a temperature sensor, a heating element, a control circuit and a display, wherein the temperature output end of the temperature sensor is electrically connected with the signal input end of the control circuit, the electrode end of the heating element is electrically connected with the measuring end of the control circuit, and the mass flow output end of the control circuit is electrically connected with the input end of the display. The control circuit comprises a current sensor, a heating power supply and a central processing unit, wherein the positive pole of the heating power supply is electrically connected with the first electrode end of the heating element, the second electrode end of the heating element is electrically connected with the first end of the current sensor, the second end of the current sensor is electrically connected with the negative pole of the heating power supply, the signal output end of the current sensor is electrically connected with the first input end of the central processing unit, and the positive pole of the heating power supply and the first end of the current sensor constitute the measuring end of the control circuit.
2. The thermal mass flow meter of claim 1, wherein, The second input end of the central processing unit is the signal input end of the control circuit. The output end of the central processing unit is the mass flow output end of the control circuit. The material of the heating element is carbon nanotube film. The thickness of the rectangular heating element is not more than 1 micrometer, the width is less than 1 centimeter, and the length is not more than the inner wall diameter of the measuring tube.
3. The thermal mass flow meter of claim 1, wherein, The side wall thickness of the annular heating element is not more than 1 micrometer, the outer circle radius is less than the inner wall diameter of the measuring tube, and the length is less than 1 centimeter.
4. The thermal mass flow meter of claim 3, wherein, The rectangular heating element is generated by superimposing a preset number of layers of the rectangular carbon nanotube film.
5. The thermal mass flow meter of claim 3, wherein, The heating element is generated by cutting two rectangular heating elements along the short edges and connecting the two rectangular heating elements at a preset angle.
6. The thermal mass flow meter of claim 3, wherein, The control circuit further comprises a current amplifier, wherein the first end of the current amplifier is electrically connected with the second electrode end of the heating element, and the second end of the current amplifier is electrically connected with the first end of the current sensor.
7. The thermal mass flow meter of claim 1, wherein, The control circuit further comprises a triode amplification circuit, which comprises a base bias resistor, a collector bias resistor and a triode, wherein the positive pole of the first power supply end of the control circuit is electrically connected with the first end of the base bias resistor, the second end of the base bias resistor is electrically connected with the base of the triode, the emitter of the triode, the negative pole of the first power supply end and the negative pole of the second power supply end of the control circuit are electrically connected and grounded, the positive pole of the second power supply end of the control circuit is electrically connected with the first end of the collector bias resistor, and the second end of the collector bias resistor is electrically connected with the collector of the triode.
8. The thermal mass flow meter of claim 1, wherein, The base of the triode is electrically connected with the second electrode end of the heating element, and the collector of the triode is electrically connected with the first end of the current sensor.
9. The thermal mass flow meter of claim 2, wherein, 10. The thermal mass flow meter of claim 2, wherein,