Constant current thermal flow meter with temperature compensated bridge remote integrated structure and use thereof

Abstract

The application relates to a constant-current thermal flowmeter with a temperature compensation bridge far-end integrated structure and application thereof, and belongs to the technical field of thermal gas mass flow measurement. An upstream flow sensitive platinum winding, a downstream flow sensitive platinum winding, an inlet side far-end temperature compensation reference platinum winding and an outlet side far-end temperature compensation reference platinum winding are used to form a Wheatstone full bridge, common mode resistance changes caused by ambient temperature changes are offset or weakened in the bridge, and therefore sensor zero point drift is reduced.

Description

Technical Field

[0001] This invention relates to a constant current thermal flow meter with a temperature-compensated bridge circuit remote integrated structure and its application, belonging to the field of thermal gas mass flow measurement technology. Background Technology

[0002] Thermal flow meters are instruments that measure fluid flow based on thermodynamic principles. Their core working principle utilizes the heat exchange effect generated when fluid flows through a heating element, deducing the flow rate by detecting temperature changes. Thermal capillary flow sensors are typically installed on a bypass capillary in the main flow path. They heat the capillary using resistance wires or thermistors and sense temperature changes, then obtain the flow signal based on the heat transport changes caused by gas flow. A common dual-resistance wire structure typically involves two thermistor wires wound on the upstream and downstream sides of the capillary, respectively. Under constant current or bridge circuit excitation, the resistance difference or voltage difference between the two wires characterizes the gas mass flow rate.

[0003] In practical applications, especially in semiconductor process gas control, industrial gas metering, and mass flow controllers operating in high-temperature environments, platinum resistance thermometers are widely used as temperature sensing elements in thermal flow meters due to their stable resistance-temperature characteristics, good linearity, and high measurement accuracy. However, rising or fluctuating ambient temperatures can cause drift in the resistance wire value, bridge zero point, constant current source output, and subsequent amplifier circuit.

[0004] In existing technologies, some thermal flow meters use independent temperature sensors, software calibration models, or hermetically sealed structures to compensate for the influence of ambient temperature. However, these methods still have certain limitations. For patch, thin-film, or partially wound platinum resistance structures, the thermal contact between the platinum resistance and the outer surface of the capillary is usually limited to a finite area. It is difficult for the contact interface to maintain a continuous and uniform bonding state throughout the entire temperature measurement area, which can easily lead to the formation of microscopic air gaps or loosely bonded areas. This results in increased contact thermal resistance, unstable heat conduction paths, and decreased consistency of thermal response. Especially in low-flow capillary measurements, the output signal of the sensing element itself is relatively weak, and changes in interface thermal resistance and thermal inertia will directly affect the sensitivity and repeatability of upstream and downstream temperature difference detection.

[0005] Meanwhile, the non-effective contact parts, lead parts, and locally exposed areas of existing platinum resistance or resistance wire windings are easily exposed to the internal air environment of the sensor for extended periods. Factors such as ambient temperature fluctuations, heat transfer from the casing, natural air convection, and localized heating of the circuit board can all couple to the temperature sensing element through the exposed areas or lead wire heat conduction paths, causing temperature disturbances unrelated to the actual flow rate to be superimposed on the sensor output. For constant current thermal capillary flowmeters, the platinum resistance wire acts as both a heating element and a temperature-sensitive element; its resistance change is simultaneously affected by gas thermal transport and ambient thermal disturbances. Therefore, under high-temperature or variable-temperature conditions, it is prone to zero-point drift, increased output fluctuations, and increased flow conversion errors.

[0006] Furthermore, when relying solely on an external temperature sensor or backend software compensation, spatial differences and thermal response time differences often exist between the temperature sampling point and the flow-sensitive winding, making it difficult to directly suppress common-mode resistance drift caused by ambient temperature changes during the bridge output formation stage. Therefore, a constant-current thermal capillary flowmeter structure is needed that can balance stable heat exchange between the capillary and platinum resistance wire, isolation of the sensor packaging environment, and temperature compensation at the bridge level, in order to reduce output drift and signal fluctuations caused by ambient temperature changes and improve measurement stability under high-temperature or variable-temperature conditions. Summary of the Invention

[0007] To address the shortcomings of existing technologies, this invention provides a constant current thermal flow meter with a temperature-compensated bridge remote integrated structure and its application, achieving simultaneous flow differential detection and ambient temperature common-mode drift suppression under constant current excitation conditions, thereby improving the zero-point stability, response consistency, and measurement accuracy of the thermal flow meter under temperature variation environments.

[0008] The technical solution of the present invention is as follows: A constant current thermal flow meter with a temperature-compensated bridge circuit remote integrated structure includes a flow channel base, a capillary base, a capillary tube, and a PCB circuit assembly, wherein: A capillary base is installed on the flow channel base, and a capillary tube is installed on the upper side of the capillary base. The inlet and outlet ends of the capillary tube pass through the capillary base and are connected to the main flow channel inside the flow channel base. The area of ​​the capillary tube near the capillary base is the far-end temperature compensation reference area, and the area away from the capillary base is the flow-sensitive measurement area. At the far-end temperature compensation reference areas at both ends of the capillary tube, far-end temperature compensation reference platinum windings are respectively installed on the inlet side and the outlet side. At the capillary flow-sensitive measurement area, upstream flow-sensitive platinum windings and downstream flow-sensitive platinum windings are installed along the gas flow direction. The upstream flow-sensitive platinum windings, downstream flow-sensitive platinum windings, inlet-side far-end temperature compensation reference platinum windings, and outlet-side far-end temperature compensation reference platinum windings are connected to PCB circuit components.

[0009] The upstream flow-sensitive platinum winding and the downstream flow-sensitive platinum winding are used to sense the difference in the thermal field between the upstream and downstream sides caused by gas flow; the inlet-side far-end temperature compensation reference platinum winding and the outlet-side far-end temperature compensation reference platinum winding are used to track the changes in ambient temperature and the overall common-mode temperature rise of the capillary, and participate in temperature compensation as a bridge reference arm.

[0010] According to a preferred embodiment of the present invention, the PCB circuit assembly includes a constant current excitation circuit, an instrumentation amplifier, a filter circuit, and an analog-to-digital converter. An upstream flow-sensitive platinum winding and an inlet-side distal temperature compensation reference platinum winding are connected in series to form a first voltage divider branch. A downstream flow-sensitive platinum winding and an outlet-side distal temperature compensation reference platinum winding are connected in series to form a second voltage divider branch. The first and second voltage divider branches are connected in parallel to form a Wheatstone bridge. The constant current excitation circuit is connected between the upper and lower excitation nodes of the Wheatstone bridge, providing a constant excitation current to the entire bridge circuit. I 0 The node between the upstream flow-sensitive platinum winding and the inlet-side far-end temperature compensation reference platinum winding constitutes the first bridge output node, and the node between the downstream flow-sensitive platinum winding and the outlet-side far-end temperature compensation reference platinum winding constitutes the second bridge output node. The first and second bridge output nodes are respectively connected to the non-inverting input and the inverting input of the instrumentation amplifier. The output of the instrumentation amplifier is connected in sequence to the filter circuit and the analog-to-digital converter unit.

[0011] According to a preferred embodiment of the present invention, a sealing shell is provided on the flow channel base to achieve overall sealing.

[0012] According to a preferred embodiment of the present invention, an insulating and thermally conductive protective layer is provided on the outer side of the upstream flow-sensitive platinum winding, the downstream flow-sensitive platinum winding, the inlet-side far-end temperature compensation reference platinum winding, and the outlet-side far-end temperature compensation reference platinum winding. The insulating and thermally conductive protective layer is made of ceramic insulating material, glass glaze insulating material, or high-temperature resistant insulating material.

[0013] Platinum winding refers to platinum resistance wire winding.

[0014] The application of the aforementioned constant current thermal flowmeter with a temperature-compensated bridge circuit remote integrated structure is described below: ① Machining of the flow channel base: Machining the main flow channel inside the flow channel base, setting the bypass inlet flow channel and bypass outlet flow channel on the main flow channel, and machining sealing grooves or sealing surfaces around the ports of the bypass inlet flow channel and bypass outlet flow channel. ② Capillary processing: Prepare capillary tubes according to the set dimensions, form winding positioning grooves in the flow-sensitive measurement area and the far-end temperature compensation reference area of ​​the capillary tube, and clean and insulate the outer surface of the capillary tube. ③ Winding preparation: Platinum wires of the same batch are wound separately in the flow-sensitive measurement area and the far-end temperature compensation reference area to form an upstream flow-sensitive platinum winding, a downstream flow-sensitive platinum winding, an inlet-side far-end temperature compensation reference platinum winding, and an outlet-side far-end temperature compensation reference platinum winding. Resistance matching and lead connection are performed on each platinum winding. ④ Insulating encapsulation, forming an insulating and thermally conductive protective layer on the outside of each platinum winding; ⑤ Module assembly: Install the capillary tube inside the sealed housing, connect the inlet and outlet ends of the capillary tube to the fluid interface of the sealed housing, and electrically connect each platinum winding lead to the PCB circuit component pad. ⑥ Tighten the seal, install the sealing shell on the flow channel base, align the fluid interface of the sealing shell with the bypass inlet flow channel and the bypass outlet flow channel respectively, and tighten the seal with fasteners to form a sealed connection; ⑦ Bridge circuit balancing and calibration: Perform zero-point balancing, constant current stability testing and flow calibration on the Wheatstone full bridge to establish the correspondence between the differential output voltage and flow rate of the bridge circuit.

[0015] The application of the aforementioned constant current thermal flowmeter with a temperature-compensated bridge-end integrated structure, and the measurement method are as follows: Step 1: Pass the fluid to be tested into the main channel of the flow channel base, and part of the fluid enters the capillary tube; Step 2: Start the constant current excitation circuit to make each platinum winding generate stable Joule heat under the action of constant current; Step 3: Real-time acquisition of the differential output voltage of the bridge circuit using the Wheatstone bridge acquisition circuit, and obtaining the digital flow detection signal through the instrumentation amplifier, filter circuit and analog-to-digital conversion circuit; Step 4: Perform primary hardware temperature compensation using a four-platinum-winding Wheatstone full bridge, and perform secondary digital compensation based on the bridge common-mode reference signal. Step 5: Calculate the total flow rate at the flow meter inlet based on the supplemented data.

[0016] According to a preferred embodiment of the present invention, in step 4, specifically: The analog-to-digital converters respectively collect the output node voltage of the first bridge circuit. V p Second bridge output node voltage V n The common-mode reference signal of the bridge circuit is calculated based on the two. V c : ; Then, based on the zero-point drift function obtained through pre-calibration... V z ( V c and relative sensitivity correction functionS ( V c Zero-point compensation and gain compensation are performed on the bridge voltage output by the instrumentation amplifier and filter circuit to obtain the compensated output voltage. V corr : ; in, V o The output voltage of the bridge circuit to be compensated; V z For the zero-point drift function V z ( V c The zero-point correction amount is calculated; S For relative sensitivity correction function S ( V c The calculated gain correction coefficient; When only linear zero-point compensation is performed, the zero-point drift function is expressed as: ; In the formula, V z0 To reference the bridge zero-point voltage under ambient temperature and zero flow conditions, V c0 As a common-mode reference signal at ambient temperature, λ This is the temperature compensation coefficient for zero-point drift relative to the common-mode reference signal; When there is a significant nonlinearity between the zero-point drift and the common-mode reference signal, a quadratic function is used for fitting: ; In the formula, b 0 , b 1 and b 2 The temperature compensation coefficient is obtained by fitting zero-flow-rate variable-temperature calibration data; Based on the capillary branch flow resistance Z c Flow resistance of branch paths from main flow channel Z m Calculate the capillary split ratio β Alternatively, it can be obtained through independent shunt calibration, in which the capillary bypass flow rate needs to be measured separately. q c and the total inlet standard flow Q std .

[0017] According to a further preferred embodiment of the present invention, the temperature compensation coefficient λThe zero-flow rate is obtained by maintaining zero flow in the capillary tube, placing the flow meter at N preset ambient temperature points, and recording the zero-flow output voltage after thermal stability is achieved at each temperature point. V z and common-mode reference signal V c ;by V c For independent variable, V z For linear least squares fitting of the dependent variable, the temperature compensation coefficient λ is calculated according to the following formula: ; In the formula, C cz Common-mode reference signal V c With zero flow output voltage V z The sample covariance, Common-mode reference signal V c The sample variances are both calculated from N zero-flow temperature-varying calibration points.

[0018] According to a preferred embodiment of the present invention, in step 5, the compensated output voltage is... V corr Substitute into the capillary bypass flow calibration function F c The capillary bypass flow rate was calculated. q c : ; Then, based on the predetermined capillary split ratio β Convert the capillary bypass flow rate to the total inlet flow rate of the flow meter. Q : ; The capillary branch is connected in parallel with the main flow branch equipped with a laminar flow splitter element, and both are subjected to the same pressure difference. Z c For the capillary branch flow resistance, The flow conversion function obtained after calibration. Z m For the main flow branch flow resistance, q m For the main flow branches other than the capillary bypass, the flow splitting relationship satisfies: ; ; Define capillary split ratio β Capillary bypass flowq c Total inbound traffic Q The ratio is: ; Finally, the compensated output voltage is substituted into the preset flow calibration relationship to calculate the capillary bypass flow rate, and the main flow rate is obtained by converting it according to the split ratio.

[0019] According to a further preferred embodiment of the present invention, the flow calibration function Fc The flow rate is obtained as follows: Multiple known flow points are sequentially input into the flow meter under test using a standard flow meter or standard mass flow controller. After each flow point reaches a steady state, the capillary bypass flow rate q is recorded. c and the corresponding compensated output voltage V corr ,by V corr For independent variable, q c As the dependent variable, a calibration function is established using the least squares method, polynomial fitting, or piecewise interpolation. When using m-order polynomial fitting, the calibration function is expressed as: ; In the formula, a 0 、a 1 、…、a m is the flow calibration coefficient obtained by fitting calibration data, and m is the polynomial order determined based on the degree of nonlinearity of the sensor output.

[0020] According to a preferred embodiment of the present invention, in the measurement method, when the ambient temperature changes while there is no flow or the flow rate remains stable in the capillary, the four platinum windings generate similar common-mode resistance changes. When there is flow in the capillary, the upstream flow-sensitive platinum winding and the downstream flow-sensitive platinum winding generate differential resistance changes due to the gas thermal transport effect. The inlet-side far-end temperature compensation reference platinum winding and the outlet-side far-end temperature compensation reference platinum winding track the changes in ambient temperature and the overall temperature rise of the capillary. Their resistance changes are expressed as follows: ； ； ； In the formula, R is the reference resistance of a single platinum winding under the conditions of ambient temperature, zero flow, and preset constant current operation. R 1 , R 2 , R3 , R 4 These are the operating resistance values ​​of the upstream flow-sensitive platinum winding, the downstream flow-sensitive platinum winding, the inlet-side far-end temperature compensation reference platinum winding, and the outlet-side far-end temperature compensation reference platinum winding, respectively; Δ RT This is the common resistance increment caused by changes in ambient temperature and the overall common-mode temperature rise of the capillary tube. The change in differential resistance caused by gas flow; Therefore, the change in common-mode resistance Δ caused by ambient temperature R t The flow rate is suppressed in the bridge circuit, and the bridge circuit output mainly retains the differential resistance change related to the flow rate, i.e.: ; In the formula, K is a proportionality coefficient determined by the bridge circuit excitation, bridge arm resistance, and subsequent stage amplification factor. It is jointly determined by the bridge circuit excitation and the total gain of the subsequent stage circuit, assuming the four bridge arms are approximately balanced and a constant current is used. I 0 When excited, K is approximately expressed as: ; Where G is the total voltage gain of the instrumentation amplifier and filter circuit in the flow signal frequency band; The operating current of the constant current excitation circuit is determined based on the allowable heat generation and heat transfer capacity of the platinum winding, satisfying the following:

[0021] In the formula, h i Let be the equivalent heat transfer coefficient between the i-th platinum winding and the capillary tube and the surrounding medium. A i To achieve effective contact heat exchange area, Δ T max To allow the maximum temperature rise, Rᵢ ( T max ) is the resistance value of the i-th platinum winding at the highest operating temperature. This limitation can prevent the platinum winding from overheating and keep the constant current heating and bridge temperature compensation stable.

[0022] The beneficial effects of this invention are as follows: 1. Common-mode temperature drift is suppressed and zero-point stability is improved. This invention constructs a Wheatstone full bridge by combining the upstream flow-sensitive platinum winding, the downstream flow-sensitive platinum winding, the inlet-side far-end temperature compensation reference platinum winding, and the outlet-side far-end temperature compensation reference platinum winding. This allows the common-mode resistance change caused by ambient temperature changes to be canceled or weakened in the bridge circuit, thereby reducing sensor zero-point drift.

[0023] 2. The flow detection and temperature compensation functions are clearly divided into zones. This invention sets the upstream flow-sensitive platinum winding and the downstream flow-sensitive platinum winding in the flow-sensitive measurement zone to sense the difference in the upstream and downstream thermal fields caused by gas flow; and sets the far-end temperature compensation reference platinum windings on the inlet side and the outlet side in the far-end temperature compensation reference zone to track the changes in ambient temperature and capillary common mode temperature rise, thus realizing the zoned arrangement of flow differential detection and temperature compensation from a structural perspective.

[0024] 3. The constant current excitation structure is simple and the signal output is stable. This invention uses a constant current excitation method to drive the platinum resistance wire winding, so that the platinum winding can simultaneously serve as a heating element and a temperature sensing element. Under a constant operating current, the gas thermal transport change is converted into resistance change and differential voltage signal, which facilitates the stable detection of small flow rates of gas.

[0025] 4. Enhanced thermal response consistency and improved dynamic measurement performance: Due to the more stable thermal contact state between the platinum winding and the capillary, the temperature change of the capillary wall caused by gas flow can be transmitted to the platinum winding more promptly, reducing the response lag caused by contact thermal resistance and thermal inertia, and improving the response consistency of the thermal flow meter under dynamic flow changes.

[0026] 5. Improved bridge arm matching and more reliable differential signal detection: The four platinum windings in this invention are made of the same material or materials with matching temperature coefficients of resistance, and are matched in nominal resistance at a preset operating temperature. This helps to reduce bridge arm imbalance error and improve the Wheatstone full bridge's ability to detect effective flow differential signals.

[0027] 6. Adaptable to miniaturized integrated applications, improving system reliability. This invention integrates the capillary tube, four platinum windings, constant current excitation circuit and bridge acquisition circuit into a compact design, which is easy to apply to thermal mass flow meters and mass flow controllers. It is beneficial to improve the measurement reliability and control stability in scenarios such as small flow gas measurement and semiconductor process gas control. Attached Figure Description

[0028] Figure 1 This is a schematic diagram of the capillary arrangement structure of the present invention; Figure 2 This is a schematic diagram of the PCB circuit assembly connection of the present invention; Figure 3 This is a schematic diagram of the overall structure of the present invention; The components include: 1. Sealed housing; 2. Capillary base; 3. Capillary; 4. PCB circuit assembly; 5. Upstream flow-sensitive platinum winding; 6. Downstream flow-sensitive platinum winding; 7. Inlet-side distal temperature compensation reference platinum winding; 8. Outlet-side distal temperature compensation reference platinum winding; 9. Flow channel base; 10. Constant current excitation circuit; 11. Instrumentation amplifier; 12. Filter circuit; 13. Flow-sensitive measurement area; 14. Distant temperature compensation reference area; 15. Capillary inlet end; 16. Capillary outlet end; and 17. Analog-to-digital converter unit. Detailed Implementation

[0029] The present invention will be further described below with reference to the embodiments and accompanying drawings, but is not limited thereto.

[0030] Example 1: like Figure 1-3 As shown, this embodiment provides a constant current thermal flow meter with a temperature-compensated bridge remote integrated structure, including a flow channel base 9, a capillary base 2, a capillary 3, and a PCB circuit assembly 4, wherein: A capillary base 2 is mounted on the flow channel base 9, and a capillary 3 is mounted on the upper side of the capillary base 2. The capillary inlet end 15 and the capillary outlet end 16 both penetrate the capillary base and connect to the main flow channel within the flow channel base. Therefore, the flow channel base 9 contains a main flow channel, a laminar flow splitting element, a bypass inlet flow channel, and a bypass outlet flow channel. The bypass inlet flow channel and bypass outlet flow channel are sealed and connected to the inlet and outlet ends of the capillary, respectively, allowing a portion of the fluid to be measured to flow through the capillary 3 after laminar flow splitting. The capillary 3 adopts a U-shaped capillary structure, including a transverse section at the top and vertical sections on both sides. The upper transverse section of the capillary 3 is designated as the flow-sensitive measurement area 13. An upstream flow-sensitive platinum winding 5 and a downstream flow-sensitive platinum winding 6 are sequentially arranged along the gas flow direction within the flow measurement zone 13. Gas enters the capillary 3 through the capillary inlet 15, ascends along the left vertical pipe section, descends along the right vertical pipe section after passing through the flow-sensitive measurement zone 13, and exits through the capillary outlet 16. Both the upstream flow-sensitive platinum winding 5 and the downstream flow-sensitive platinum winding 6 form thermal contact with the outer surface of the capillary 3 and generate Joule heat under constant current excitation. When the gas flows through the capillary 3, the heat near the upstream flow-sensitive platinum winding 5 is carried away by the flowing gas, while the heat-carrying gas near the downstream flow-sensitive platinum winding 6 affects the gas flow rate. The two form a temperature difference and resistance difference related to the gas mass flow rate.

[0031] The vertical sections on both sides of the capillary tube 3 are set as remote temperature compensation reference areas 14. The remote temperature compensation reference platinum winding 7 on the inlet side and the remote temperature compensation reference platinum winding 8 on the outlet side are respectively set in the corresponding remote temperature compensation reference areas 14. The remote temperature compensation reference areas 14 and the flow-sensitive measurement area 13 are arranged at intervals along the length of the capillary tube 3, so that the remote temperature compensation reference platinum winding 7 on the inlet side and the remote temperature compensation reference platinum winding 8 on the outlet side are subject to a smaller local flow thermal asymmetry effect relative to the upstream flow-sensitive platinum winding 5 and the downstream flow-sensitive platinum winding 6. They are mainly used to track the ambient temperature and the overall common mode temperature rise of the capillary tube 3. The upstream flow-sensitive platinum winding 5, the downstream flow-sensitive platinum winding 6, the remote temperature compensation reference platinum winding 7 on the inlet side and the remote temperature compensation reference platinum winding 8 on the outlet side are connected to PCB circuit components 4.

[0032] PCB circuit assembly 4 includes a constant current excitation circuit 10, an instrumentation amplifier 11, a filter circuit 12, and an analog-to-digital converter 17. The upstream flow-sensitive platinum winding 5 and the inlet-side far-end temperature compensation reference platinum winding 7 are connected in series to form a first voltage divider branch. The downstream flow-sensitive platinum winding 6 and the outlet-side far-end temperature compensation reference platinum winding 8 are connected in series to form a second voltage divider branch. The first and second voltage divider branches are connected in parallel to form a Wheatstone bridge. The constant current excitation circuit 10 is connected between the upper and lower excitation nodes of the Wheatstone bridge to provide a constant excitation current to the entire bridge circuit. I 0 The node between the upstream flow-sensitive platinum winding 5 and the inlet-side far-end temperature compensation reference platinum winding 7 constitutes the first bridge output node, and the node between the downstream flow-sensitive platinum winding 6 and the outlet-side far-end temperature compensation reference platinum winding 8 constitutes the second bridge output node. The first bridge output node and the second bridge output node are respectively connected to the non-inverting input terminal and the inverting input terminal of the instrumentation amplifier 11. The output terminal of the instrumentation amplifier 11 is connected in sequence to the filter circuit 12 and the analog-to-digital conversion unit 17.

[0033] The constant current excitation circuit 10 is used to provide a constant excitation current to the Wheatstone full bridge composed of four platinum windings. The instrumentation amplifier 11, the filter circuit 12 and the analog-to-digital conversion unit 17 are used for amplification, filtering and digital acquisition of the bridge differential signal. The control unit is used to perform bridge zero-point compensation, sensitivity correction and flow conversion.

[0034] A sealing shell 1 is provided on the flow channel base 9 to achieve overall sealing.

[0035] The upstream flow-sensitive platinum winding 5, the downstream flow-sensitive platinum winding 6, the inlet-side far-end temperature compensation reference platinum winding 7, and the outlet-side far-end temperature compensation reference platinum winding 8 are all provided with an insulating and thermally conductive protective layer. The insulating and thermally conductive protective layer is made of ceramic insulating material, glass glaze insulating material, or high-temperature resistant insulating material.

[0036] The upstream flow-sensitive platinum winding 5, the downstream flow-sensitive platinum winding 6, the inlet-side far-end temperature compensation reference platinum winding 7, and the outlet-side far-end temperature compensation reference platinum winding 8 are made of the same material or platinum wire with a matching temperature coefficient of resistance, and have matching nominal resistance values ​​at the preset operating temperature. Each platinum winding can be fixed to the outer surface of the capillary 3 by winding, embedding, bonding, sintering, or encapsulating with an insulating and thermally conductive layer, as long as its axial position on the capillary 3 can be limited and a stable thermal contact can be formed.

[0037] The application of the aforementioned constant current thermal flowmeter with a temperature-compensated bridge circuit remote integrated structure is described below: ① Machining of flow channel base 9: Machining of main flow channel within flow channel base 9; setting of bypass inlet flow channel and bypass outlet flow channel on main flow channel; and machining of sealing grooves or sealing surfaces around the ports of bypass inlet flow channel and bypass outlet flow channel. ② Capillary tube 3 processing: prepare capillary tube 3 according to the set size, form winding positioning groove in the flow-sensitive measurement area 13 and the far-end temperature compensation reference area 14 of the capillary tube, and clean and insulate the outer surface of the capillary tube. ③ Winding preparation: Platinum wires of the same batch are wound into the flow-sensitive measurement area 13 and the far-end temperature compensation reference area 14 respectively to form the upstream flow-sensitive platinum winding 5, the downstream flow-sensitive platinum winding 6, the inlet-side far-end temperature compensation reference platinum winding 7 and the outlet-side far-end temperature compensation reference platinum winding 8, and resistance matching and lead connection are performed on each platinum winding. ④ Insulating encapsulation, forming an insulating and thermally conductive protective layer on the outside of each platinum winding; ⑤ Module assembly: Install the capillary tube 3 inside the sealed housing 1, so that the inlet and outlet ends of the capillary tube are connected to the fluid interface of the sealed housing respectively, and electrically connect each platinum winding lead to the solder pad of the PCB circuit assembly 4. ⑥ Tighten the seal, install the sealing housing 1 on the flow channel base 9, align the fluid interface of the sealing housing with the bypass inlet flow channel and the bypass outlet flow channel respectively, and tighten the seal with fasteners to form a sealed connection; ⑦ Bridge circuit balancing and calibration: Perform zero-point balancing, constant current stability testing and flow calibration on the Wheatstone full bridge to establish the correspondence between the differential output voltage and flow rate of the bridge circuit.

[0038] The application of the aforementioned constant current thermal flowmeter with a temperature-compensated bridge-end integrated structure, and the measurement method are as follows: Step 1: Pass the fluid to be tested into the main channel of the flow channel base, and part of the fluid enters the capillary tube; Step 2: Start the constant current excitation circuit to make each platinum winding generate stable Joule heat under the action of constant current; Step 3: Real-time acquisition of the differential output voltage of the bridge circuit using the Wheatstone bridge acquisition circuit, and obtaining the digital flow detection signal through the instrumentation amplifier, filter circuit and analog-to-digital conversion circuit; Step 4: Perform primary hardware temperature compensation using a four-platinum-winding Wheatstone full bridge, and perform secondary digital compensation based on the bridge common-mode reference signal. Specifically: The analog-to-digital converters respectively collect the output node voltage of the first bridge circuit. V p Second bridge output node voltage V n The common-mode reference signal of the bridge circuit is calculated based on the two. V c : ; Then, based on the zero-point drift function obtained through pre-calibration... V z ( V c and relative sensitivity correction function S ( V c Zero-point compensation and gain compensation are performed on the bridge voltage output by the instrumentation amplifier and filter circuit to obtain the compensated output voltage. V corr : ; in, V o The output voltage of the bridge circuit to be compensated; V z For the zero-point drift function V z ( V c The zero-point correction amount is calculated; S For relative sensitivity correction function S ( V c The calculated gain correction coefficient; When only linear zero-point compensation is performed, the zero-point drift function is expressed as: ; In the formula, V z0 To reference the bridge zero-point voltage under ambient temperature and zero flow conditions, V c0 As a common-mode reference signal at ambient temperature, λ This is the temperature compensation coefficient for zero-point drift relative to the common-mode reference signal; When there is a significant nonlinearity between the zero-point drift and the common-mode reference signal, a quadratic function is used for fitting: ; In the formula, b 0 , b 1 and b 2 The temperature compensation coefficient is obtained by fitting zero-flow-rate variable-temperature calibration data; Based on the capillary branch flow resistance Z c Flow resistance of branch paths from main flow channel Z m Calculate the capillary split ratio β Alternatively, it can be obtained through independent shunt calibration, in which the capillary bypass flow rate needs to be measured separately. q c and the total inlet standard flow Q std .

[0039] Temperature compensation coefficient λ The zero-flow rate is obtained by maintaining zero flow in the capillary tube, placing the flow meter at N preset ambient temperature points, and recording the zero-flow output voltage after thermal stability is achieved at each temperature point. V z and common-mode reference signal V c ;by V c For independent variable, V z For linear least squares fitting of the dependent variable, the temperature compensation coefficient λ is calculated according to the following formula: ; In the formula, C cz Common-mode reference signal V c With zero flow output voltage V z The sample covariance, Common-mode reference signal V c The sample variances are both calculated from N zero-flow temperature-varying calibration points.

[0040] Step 5: Calculate the total inlet flow rate of the flow meter based on the supplemented data; Compensate output voltage V corr Substitute into the capillary bypass flow calibration function F c The capillary bypass flow rate was calculated. qc : ; Then, based on the predetermined capillary split ratio β Convert the capillary bypass flow rate to the total inlet flow rate of the flow meter. Q : ; The capillary branch is connected in parallel with the main flow branch equipped with a laminar flow splitter element, and both are subjected to the same pressure difference. Z c For the capillary branch flow resistance, The flow conversion function obtained after calibration. Z m For the main flow branch flow resistance, q m For the main flow branches other than the capillary bypass, the flow splitting relationship satisfies: ; ; Define capillary split ratio β Capillary bypass flow q c Total inbound traffic Q The ratio is: ; Finally, the compensated output voltage is substituted into the preset flow calibration relationship to calculate the capillary bypass flow rate, and the main flow rate is obtained by converting it according to the split ratio.

[0041] Flow calibration function Fc The flow rate is obtained as follows: Multiple known flow points are sequentially input into the flow meter under test using a standard flow meter or standard mass flow controller. After each flow point reaches a steady state, the capillary bypass flow rate q is recorded. c and the corresponding compensated output voltage V corr ,by V corr For independent variable, q c As the dependent variable, a calibration function is established using the least squares method, polynomial fitting, or piecewise interpolation. When using m-order polynomial fitting, the calibration function is expressed as: ; In the formula, a 0 、a 1 、…、a mis the flow calibration coefficient obtained by fitting calibration data, and m is the polynomial order determined based on the degree of nonlinearity of the sensor output.

[0042] In the measurement method, when the ambient temperature changes while there is no flow or the flow rate remains stable in the capillary, the four platinum windings produce similar common-mode resistance changes. When there is flow in the capillary, the upstream flow-sensitive platinum winding and the downstream flow-sensitive platinum winding produce differential resistance changes due to the gas thermal transport effect. The inlet-side far-end temperature compensation reference platinum winding and the outlet-side far-end temperature compensation reference platinum winding track the changes in ambient temperature and the overall temperature rise of the capillary. Their resistance changes are expressed as follows: ； ； ； In the formula, R is the reference resistance of a single platinum winding under the conditions of ambient temperature, zero flow, and preset constant current operation. R 1 , R 2 , R 3 , R 4 These are the operating resistance values ​​of the upstream flow-sensitive platinum winding, the downstream flow-sensitive platinum winding, the inlet-side far-end temperature compensation reference platinum winding, and the outlet-side far-end temperature compensation reference platinum winding, respectively; Δ RT This is the common resistance increment caused by changes in ambient temperature and the overall common-mode temperature rise of the capillary tube. The change in differential resistance caused by gas flow; Therefore, the change in common-mode resistance Δ caused by ambient temperature R t The flow rate is suppressed in the bridge circuit, and the bridge circuit output mainly retains the differential resistance change related to the flow rate, i.e.: ; In the formula, K is a proportionality coefficient determined by the bridge circuit excitation, bridge arm resistance, and subsequent stage amplification factor. It is jointly determined by the bridge circuit excitation and the total gain of the subsequent stage circuit, assuming the four bridge arms are approximately balanced and a constant current is used. I 0 When excited, K is approximately expressed as: ; Where G is the total voltage gain of the instrumentation amplifier and filter circuit in the flow signal frequency band; The operating current of the constant current excitation circuit is determined based on the allowable heat generation and heat transfer capacity of the platinum winding, satisfying the following:

[0043] In the formula, h i Let be the equivalent heat transfer coefficient between the i-th platinum winding and the capillary tube and the surrounding medium. A i To achieve effective contact heat exchange area, Δ T max To allow the maximum temperature rise, Rᵢ ( T max ) is the resistance value of the i-th platinum winding at the highest operating temperature. This limitation can prevent the platinum winding from overheating and keep the constant current heating and bridge temperature compensation stable.

Claims

1. A constant current thermal flow meter with a temperature-compensated bridge circuit remote integrated structure, characterized in that, Includes a flow channel base, a capillary base, a capillary tube, and a PCB circuit assembly, wherein: A capillary base is installed on the flow channel base, and a capillary tube is installed on the upper side of the capillary base. The inlet and outlet ends of the capillary tube pass through the capillary base and are connected to the main flow channel inside the flow channel base. The area of ​​the capillary tube near the capillary base is the far-end temperature compensation reference area, and the area away from the capillary base is the flow-sensitive measurement area. At the far-end temperature compensation reference areas at both ends of the capillary tube, far-end temperature compensation reference platinum windings are respectively installed on the inlet side and the outlet side. At the capillary flow-sensitive measurement area, upstream flow-sensitive platinum windings and downstream flow-sensitive platinum windings are installed along the gas flow direction. The upstream flow-sensitive platinum windings, downstream flow-sensitive platinum windings, inlet-side far-end temperature compensation reference platinum windings, and outlet-side far-end temperature compensation reference platinum windings are connected to PCB circuit components.

2. The constant current thermal flow meter with a temperature-compensated bridge remote integrated structure as described in claim 1, characterized in that, The PCB circuit assembly includes a constant current excitation circuit, an instrumentation amplifier, a filter circuit, and an analog-to-digital converter. The upstream flow-sensitive platinum winding and the inlet-side distal temperature compensation reference platinum winding are connected in series to form the first voltage divider branch. The downstream flow-sensitive platinum winding and the outlet-side distal temperature compensation reference platinum winding are connected in series to form the second voltage divider branch. The first and second voltage divider branches are connected in parallel to form a Wheatstone bridge. The constant current excitation circuit is connected between the upper and lower excitation nodes of the Wheatstone bridge, providing a constant excitation current to the entire bridge circuit. I 0 The node between the upstream flow-sensitive platinum winding and the inlet-side far-end temperature compensation reference platinum winding constitutes the first bridge output node, and the node between the downstream flow-sensitive platinum winding and the outlet-side far-end temperature compensation reference platinum winding constitutes the second bridge output node. The first and second bridge output nodes are respectively connected to the non-inverting input and the inverting input of the instrumentation amplifier. The output of the instrumentation amplifier is connected in sequence to the filter circuit and the analog-to-digital converter unit.

3. The constant current thermal flow meter with a temperature-compensated bridge remote integrated structure as described in claim 2, characterized in that, A sealed outer shell is provided on the flow channel base.

4. The constant current thermal flow meter with a temperature-compensated bridge remote integrated structure as described in claim 3, characterized in that, An insulating and thermally conductive protective layer is provided on the outer side of the upstream flow-sensitive platinum winding, the downstream flow-sensitive platinum winding, the inlet-side far-end temperature compensation reference platinum winding, and the outlet-side far-end temperature compensation reference platinum winding. The insulating and thermally conductive protective layer is made of ceramic insulating material, glass glaze insulating material, or high-temperature resistant insulating material.

5. The application of the constant current thermal flow meter with temperature-compensated bridge remote integrated structure as described in claim 4, characterized in that, The preparation method is as follows: ① Processing of the flow channel base: The main flow channel is processed inside the flow channel base, and the bypass inlet flow channel and bypass outlet flow channel are set on the main flow channel; ② Capillary processing: Prepare capillary tubes according to the set dimensions, form winding positioning grooves in the flow-sensitive measurement area and the far-end temperature compensation reference area of ​​the capillary tube, and clean and insulate the outer surface of the capillary tube. ③ Winding preparation: Platinum wires of the same batch are wound separately in the flow-sensitive measurement area and the far-end temperature compensation reference area to form an upstream flow-sensitive platinum winding, a downstream flow-sensitive platinum winding, an inlet-side far-end temperature compensation reference platinum winding, and an outlet-side far-end temperature compensation reference platinum winding. Resistance matching and lead connection are performed on each platinum winding. ④ Insulating encapsulation, forming an insulating and thermally conductive protective layer on the outside of each platinum winding; ⑤ Module assembly: Install the capillary tube inside the sealed housing, connect the inlet and outlet ends of the capillary tube to the fluid interface of the sealed housing, and electrically connect each platinum winding lead to the PCB circuit component pad. ⑥ Tighten the seal and install the sealing shell on the flow channel base, aligning the fluid interface of the sealing shell with the bypass inlet flow channel and the bypass outlet flow channel respectively to form a sealed connection; ⑦ Bridge circuit balancing and calibration: Perform zero-point balancing, constant current stability testing and flow calibration on the Wheatstone full bridge to establish the correspondence between the differential output voltage and flow rate of the bridge circuit.

6. The application of the constant current thermal flow meter with a temperature-compensated bridge remote integrated structure as described in claim 4, characterized in that, The measurement method is as follows: Step 1: Pass the fluid to be tested into the main channel of the flow channel base, and part of the fluid enters the capillary tube; Step 2: Start the constant current excitation circuit to make each platinum winding generate stable Joule heat under the action of constant current; Step 3: Real-time acquisition of the differential output voltage of the bridge circuit using the Wheatstone bridge acquisition circuit, and obtaining the digital flow detection signal through the instrumentation amplifier, filter circuit and analog-to-digital conversion circuit; Step 4: Perform primary hardware temperature compensation using a four-platinum-winding Wheatstone full bridge, and perform secondary digital compensation based on the bridge common-mode reference signal. Step 5: Calculate the total flow rate at the flow meter inlet based on the supplemented data.

7. The application of the constant current thermal flow meter with temperature-compensated bridge remote integrated structure as described in claim 6, characterized in that, In step 4, specifically: The analog-to-digital converters respectively collect the output node voltage of the first bridge circuit. V p Second bridge output node voltage V n The common-mode reference signal of the bridge circuit is calculated based on the two. V c : ； Then, based on the zero-point drift function obtained through pre-calibration... V z ( V c ) and relative sensitivity correction function S ( V c Zero-point compensation and gain compensation are performed on the bridge voltage output by the instrumentation amplifier and filter circuit to obtain the compensated output voltage. V corr : ； in, V o The output voltage of the bridge circuit to be compensated; V z For the zero-point drift function V z ( V c The zero-point correction amount is calculated; S For relative sensitivity correction function S ( V c The calculated gain correction coefficient; When only linear zero-point compensation is performed, the zero-point drift function is expressed as: ； In the formula, V z0 To reference the bridge zero-point voltage under ambient temperature and zero flow conditions, V c0 As a common-mode reference signal at ambient temperature, λ This is the temperature compensation coefficient for zero-point drift relative to the common-mode reference signal; When there is a significant nonlinearity between the zero-point drift and the common-mode reference signal, a quadratic function is used for fitting: ； In the formula, b 0 , b 1 and b 2 The temperature compensation coefficient is obtained by fitting zero-flow-rate variable-temperature calibration data; Based on the capillary branch flow resistance Z c Flow resistance of branch paths from main flow channel Z m Calculate the capillary split ratio β Alternatively, it can be obtained through independent shunt calibration, in which the capillary bypass flow rate needs to be measured separately. q c and the total inlet standard flow Q std .

8. The application of the constant current thermal flow meter with a temperature-compensated bridge remote integrated structure as described in claim 7, characterized in that, Temperature compensation coefficient λ The zero-flow rate is obtained by maintaining zero flow in the capillary tube, placing the flow meter at N preset ambient temperature points, and recording the zero-flow output voltage after thermal stability is achieved at each temperature point. V z and common-mode reference signal V c ;by V c For independent variable, V z For linear least squares fitting of the dependent variable, the temperature compensation coefficient λ is calculated according to the following formula: ； In the formula, C cz Common-mode reference signal V c With zero flow output voltage V z The sample covariance, Common-mode reference signal V c The sample variance.

9. The application of the constant current thermal flow meter with a temperature-compensated bridge remote integrated structure as described in claim 8, characterized in that, In step 5, the compensated output voltage will be... V corr Substitute into the capillary bypass flow calibration function F c The capillary bypass flow rate was calculated. q c : ； Then, based on the predetermined capillary split ratio β Convert the capillary bypass flow rate to the total inlet flow rate of the flow meter. Q : ； The capillary branch is connected in parallel with the main flow branch, and both are subjected to the same pressure difference. Z c For the capillary branch flow resistance, The flow conversion function obtained after calibration. Z m For the main flow branch flow resistance, q m For the main flow branches other than the capillary bypass, the flow splitting relationship satisfies: ； ； Define capillary split ratio β Capillary bypass flow q c Total inbound traffic Q The ratio is: ； Finally, the compensated output voltage is substituted into the preset flow calibration relationship to calculate the capillary bypass flow rate, and the main flow rate is calculated based on the shunt ratio. Flow calibration function Fc The flow rate is obtained as follows: Multiple known flow points are sequentially input into the flow meter under test using a standard flow meter or standard mass flow controller. After each flow point reaches a steady state, the capillary bypass flow rate q is recorded. c and the corresponding compensated output voltage V corr ,by V corr For independent variable, q c As the dependent variable, a calibration function is established using the least squares method, polynomial fitting, or piecewise interpolation. When using m-order polynomial fitting, the calibration function is expressed as: ； In the formula, a 0 、a 1 、…、a m is the flow calibration coefficient obtained by fitting calibration data, and m is the polynomial order determined based on the degree of nonlinearity of the sensor output.

10. The application of the constant current thermal flowmeter with a temperature-compensated bridge remote integrated structure as described in claim 9, characterized in that, In the measurement method, when the ambient temperature changes while there is no flow or the flow rate remains stable in the capillary, the four platinum windings produce similar common-mode resistance changes. When there is flow in the capillary, the upstream flow-sensitive platinum winding and the downstream flow-sensitive platinum winding produce differential resistance changes due to the gas thermal transport effect. The inlet-side far-end temperature compensation reference platinum winding and the outlet-side far-end temperature compensation reference platinum winding track the changes in ambient temperature and the overall temperature rise of the capillary. Their resistance changes are expressed as follows: ； ； ； In the formula, R is the reference resistance of a single platinum winding under the conditions of ambient temperature, zero flow, and preset constant current operation. R 1 , R 2 , R 3 , R 4 These are the operating resistance values ​​of the upstream flow-sensitive platinum winding, the downstream flow-sensitive platinum winding, the inlet-side far-end temperature compensation reference platinum winding, and the outlet-side far-end temperature compensation reference platinum winding, respectively; Δ RT This is the common resistance increment caused by changes in ambient temperature and the overall common-mode temperature rise of the capillary tube. The change in differential resistance caused by gas flow; Therefore, the change in common-mode resistance Δ caused by ambient temperature R t The flow rate is suppressed in the bridge circuit, and the bridge circuit output mainly retains the differential resistance change related to the flow rate, i.e.: ； In the formula, K is a proportionality coefficient determined by the bridge circuit excitation, bridge arm resistance, and subsequent stage amplification factor. It is jointly determined by the bridge circuit excitation and the total gain of the subsequent stage circuit, assuming the four bridge arms are approximately balanced and a constant current is used. I 0 When excited, K is approximately expressed as: ； Where G is the total voltage gain of the instrumentation amplifier and filter circuit in the flow signal frequency band; The operating current of the constant current excitation circuit is determined based on the allowable heat generation and heat transfer capacity of the platinum winding, satisfying the following: ； In the formula, h i Let be the equivalent heat transfer coefficient between the i-th platinum winding and the capillary tube and the surrounding medium. A i To achieve effective contact heat exchange area, Δ T max To allow the maximum temperature rise, Rᵢ ( T max ) represents the resistance value of the i-th platinum winding at the highest operating temperature.

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