CN111024269B - A planar heat flow sensor for measuring heat flow along a wall and its calibration method - Google Patents

Abstract

The invention discloses a plane heat flow sensor for measuring heat flow along a wall and a calibration method thereof. The thin film heat flow sensor consists of 3 platinum resistors and a thermopile, and the thermopile is placed between the two outermost platinum resistors. When calibrating the static characteristics of the sensor, a calibration idea is proposed to measure the Seebeck coefficient based on the temperature difference and thermoelectric potential of the hot and cold ends of the thermopile under the condition of small heat flow. According to the calibration idea, a calibration device was built, and the ambient temperature was set between 40°C and 100°C. At the same time, a low-power laser was used to load the center of the sensor, and the heat flow was transmitted along the radial direction of the sensor. When the system is stable, a small temperature difference is formed at the hot and cold ends of the thermopile, and the Seebeck coefficient is obtained through the temperature difference and the output of the corresponding thermoelectric potential, and the relationship between the Seebeck coefficient and temperature is obtained according to different ambient temperatures, and the calibration of the Seebeck coefficient is realized. .

Description

A planar heat flow sensor for measuring heat flow along a wall and its calibration method

technical field

The invention belongs to the technical field of thin film heat flow sensors, and relates to a planar heat flow sensor for measuring heat flow along a wall surface and a calibration method thereof.

Background technique

With the increasing demand for thermal measurement and thermal management, the monitoring of the heat transfer process has become increasingly important in many heat transfer systems. The development of thin-film technology has provided good working conditions for micro-scale heat transfer measurement, and thin-film heat flow sensors have also emerged. Thermopile heat flow sensor is the most common type of thin film heat flow sensor. It was first composed of a thermopile wound on a thermal resistance layer to measure the heat flux vertically passing through the thermal resistance layer. Because of its fast response, small size, and high precision, it is widely used in heat flow measurement in aerospace, building energy conservation, medical, agriculture, fuel cells, and fire experiments.

At present, most heat flow sensors measure the heat flow perpendicular to the surface of the sensor, and there are few studies on the sensor that directly measures the heat flow along the wall. The planar thermopile thin film heat flow sensor refers to the sensor in which the hot and cold ends of the thermopile are located on the same plane. In order to reduce the size as much as possible, it is generally designed to be circular. In the past ten years, NASA has developed a thermopile planar heat flow sensor for the measurement of heat flow on the surface of engine turbine blades. At the same time, in the field of thermal analysis, planar heat flow sensors are also used in the measurement of sample heat flow in differential scanning calorimeters.

Although some planar heat flow sensors have been developed and applied, there are few studies on their calibration methods. The precise calibration of thermopile heat flow sensors has always been one of the difficulties in the thermal field. Thermopile-type heat flow sensors commonly used in the aerospace field are calibrated using arc lamps or black body furnaces. These methods are complex, expensive, and require known incident heat flux density, which are not suitable for general field or system calibration, but are applied to planar heat flux sensors of differential scanning calorimeters, which are usually based on the phase transition temperature and phase transition temperature of standard materials. The enthalpy is used for calibration. Due to the limited types of standard materials, this calibration method cannot achieve continuous calibration.

Aiming at the problem of difficult calibration of the thermopile thin film heat flow sensor, the present invention designs a new type of flat thin film heat flow sensor, which realizes the static characteristic calibration of the sensor by simulating and analyzing the heat conduction law of the sensor and combining laser heating and temperature extrapolation.

SUMMARY OF THE INVENTION

The invention discloses a planar heat flow sensor capable of measuring heat flow along the wall, and proposes a sensor connection structure for fixing small-sized pins through metal probes and realizing stable signal output. Calibration of the sensor Seebeck coefficient.

In order to achieve the above object, the technical scheme adopted in the present invention is:

The utility model relates to a planar thermopile type thin film heat flow sensor whose cold and hot junctions are located on the same plane. Three thin film platinum resistors and a thermopile composed of multiple groups of gold-platinum thermocouples in series are plated on an insulating base.

The thin film sensor is printed on an alumina ceramic substrate with a thermal conductivity of 36 W/m·K, and the substrate is a circular sheet with a diameter of 22 mm and a thickness of 0.36 mm.

The thin-film platinum resistors on the thin-film heat flow sensor are arc-shaped and are arranged on both sides of the hot and cold ends of the thermopile respectively, with radii of 1.55mm, 2.55mm and 7.05mm respectively, each platinum resistor is 0.1mm wide and has a film thickness of 10μm .

The thermopile on the heat flow sensor is formed by 44 gold-platinum thermocouples connected in series, which are arranged on the surface of the substrate in an arc shape. The width of each gold-platinum thermocouple is 0.1 mm, and the overlapping area at both ends of the electrodes constitutes the cold end and the hot end of the thermopile respectively. Among them, the radius of the arc where the hot end is located is 3.5mm, and the radius of the arc where the cold end is located is 6.5mm.

The static characteristic calibration method of the thin film heat flow sensor is as follows:

Laser heating and temperature extrapolation were used. In order to simulate the actual heat transfer situation, a laser is loaded at the center of the sensor as a heat source, and the heat flows along the radial direction of the sensor. The main calibration steps are as follows: First, load the laser in the center of the sensor. After the laser is absorbed by the center of the sensor surface, the heat is transferred from the center to the edge of the sensor. After the system is thermally balanced, according to a certain temperature distribution law, the temperature of the hot and cold ends of the thermopile is derived from the temperature of the thin-film platinum resistance to obtain the temperature difference. Finally, the relationship between the temperature difference and the output of the thermopile is established to realize the static calibration of the sensor.

The experimental system corresponding to the above method is mainly divided into four modules, namely a laser generating module, a sensor receiving module, an ambient temperature measurement control module and a data acquisition module. The laser generating module includes a laser generator, a laser controller and an optical bracket. The main function of this module is to realize the vertical laser light output by fixing the laser, and the laser power can be adjusted, and the adjustable range is 0-3000mW; the sensor receiving module realizes the sensor signal extraction, and the sensor is placed on the hollow copper bracket in the center , Due to the small size of the sensor pins, it is difficult to lead out the signal line by soldering, so a thinner probe is used to place it on the pin and fix it through an adapter board. The specific implementation method is as follows: drilling through holes at the positions corresponding to the sensor pins on the connection board, fixing the probe cover, purchasing the needle cover with wire to make the signal output through the leads, and finally fixing the adapter board with screws; the furnace temperature control system is composed of Ambient temperature measurement and control module, which realizes the internal temperature control of the furnace body through PID algorithm and measures it through thermocouple, and provides a constant temperature field of 40℃～100℃ for sensor calibration; the sensor output signal is collected by the data acquisition system and fed back to the host computer show. Among them, the thermoelectric potential of thermopile is measured by Fluke1586A, and the resistance value of thin film platinum resistance is obtained by Fluke1529 with four-wire measurement principle.

The invention integrates three arc-shaped platinum resistances and one thermopile on the sensor, two platinum resistances are located inside the thermopile hot junction, and the other platinum resistance is located outside the thermopile cold junction, Mark the three platinum resistors as R1, R2 and R3 in order of radius from large to small. R1 and R2 are used for temperature extrapolation because they are relatively close to the cold and hot ends respectively to obtain the temperature of the hot and cold junction.

During the calibration process of the present invention, different ambient temperature changes are realized through furnace temperature control, and heat flow is loaded at the center of the sensor at the same time, and Seebeck coefficients corresponding to different ambient temperatures are obtained when the heat flow is small.

In order to solve the problem that the pin size of the thin film heat flow sensor is small and the spacing is dense, which leads to the difficulty of drawing out the lead, the invention proposes a high temperature resistant metal probe and a corresponding needle sleeve with a wire to extract the sensor signal, and is fixed by an adapter plate. solution. The high temperature metal probe is a test needle with a spring inside. When building the sensor connection device, first, in order to transfer heat faster and more evenly, pure copper with a thermal conductivity of 381W/m·K is selected as the bracket to support the sensor, and a hole with a diameter of 21mm is dug in the middle for placing the sensor; secondly , Choose a high temperature resistant PCB, punch corresponding holes on the PCB according to the corresponding positions of the pins on the sensor and the size of the pins, so that the probes of suitable specifications can pass through and fix the PCB board and the high temperature probes by welding. Since the pins of the sensor are all concentrated on one side, in order to maintain balance, the same hole is punched in the symmetrical position of the pin hole of the PCB board about the origin and fixed with the same probe. A hole with a diameter of 5mm is drilled in the center of the PCB to ensure that the laser passes through; finally, the adapter board is fixed with the copper bracket by screws and forms a pressing force on the sensor to ensure good contact of the sensor pins.

The beneficial effects that the present invention has are:

1. The planar heat flow sensor of the present invention is composed of three thin-film platinum resistors and one thermopile. According to the temperature difference between the hot and cold ends of the thermopile, the measurement of the heat flow along the radial direction of the sensor is realized.

2. The present invention proposes a lead-out method suitable for small-sized pins, so as to realize stable output of measurement signals.

3. During calibration, the present invention proposes a method to measure the Seebeck coefficient of the sensor corresponding to the temperature by changing the ambient temperature under the condition of passing a small heat flow, so as to realize the sensor calibration.

Description of drawings

FIG. 1 is a top view of the overall structure of a planar thin film heat flow sensor for measuring heat flow along the wall.

Figure 2 is a schematic structural diagram of a calibration experimental platform for a planar thin-film heat flow sensor.

FIG. 3 is a schematic diagram of a signal extraction device of a planar thin film heat flow sensor.

Figure 4 is a fitting diagram of the Seebeck coefficient measured at different ambient temperatures.

Detailed ways

Below in conjunction with accompanying drawing, the present invention is described in further detail:

As shown in FIG. 1 , the planar heat flow sensor of the present invention is printed on an alumina ceramic substrate 1-1, and is composed of three platinum resistors and one thermopile. The three platinum resistors 1-2, 1-3, and 1-4 are arc-shaped and are arranged on the surface of the base according to their radii. The thermopile 1-5 is composed of 44 thermocouples connected in series. The positive electrode of the thermocouple is gold 1-6, and the negative electrode is platinum 1-7. The overlapping parts of gold and platinum constitute the cold end 1-8 and the hot end 1- of the thermopile. 9. During calibration, the resistance value of each thin-film platinum resistor is measured by a four-wire system, and two pins for measuring the thermopile potential are added to form a total of 12 pins 1-10, and the area of each pin is 0.3mm×1mm.

As shown in FIG. 2 , the calibration device system of the planar thin film heat flow sensor is divided into four modules, a laser generating module 1 , a sensor receiving module 2 , an ambient temperature measurement and control module 3 and a data acquisition module 4 . When the sensor device is built, first fix the laser 2-1 with the laser bracket 2-3 to ensure that it emits light vertically, and the laser controller 2-2 is connected to the laser to control the output power of the laser. In the sensor receiving module, select the furnace 2-4 with a suitable furnace cavity size, place the sensor in the middle position and connect the probe to make the signal output normally. The ambient temperature measurement and control system 2-5 realizes the internal temperature control of the furnace body through PID algorithm and measures it through thermocouples, providing a constant temperature field of 40°C to 100°C for sensor calibration, and the temperature control accuracy is ±0.2°C. The data generated by the sensor is collected by the data collection device 2-6 and transmitted to the upper computer 2-7 for data processing and display.

Aiming at the problem that the planar thin film heat flow sensor has many pins and small size, and it is difficult to lead out the test lead, a method is proposed that the test signal is led out by a high temperature resistant probe with a small size. As shown in Figure 3, when the sensor receiving module is assembled, in addition to the ambient temperature required by the furnace body control, a set of devices for drawing out sensor signal lines is installed in the furnace body. In order to ensure good heat transfer performance, copper with high thermal conductivity is selected as the material of the sensor support frame 3-1, the center is hollowed out, and the sensor 3-2 is placed on the surface at the center of the furnace cavity. Select high temperature resistant PCB material to design and make a suitable size PCB adapter board, punch holes at the positions corresponding to the sensor pins and where the laser passes through, and fix the probe cover 3-4 and the adapter board 3-5 by welding. Then install the high temperature resistant probes 3-3 into the needle sleeve to realize the one-to-one correspondence between the sensor pins and the probes. The high temperature probe is equipped with a spring. The adapter plate is fixed with the copper bracket by four screws 3-6, and realizes the extrusion of the probe to ensure good contact between the probe and the pins.

In the static characteristic calibration experiment of the thin film heat flow sensor, the thermoelectric potential E of the thermocouple has the following relationship with the temperature T:

E=a ₀ +a ₁ T+a ₂ T ² +...+a ₉ T ⁹ (1)

where a _i is the fitting coefficient, i=0, 1...9.

When the temperature of the hot end and the cold end _of the _thermopile are T1 and T2, respectively, according to the relationship between the output potential and temperature of the thermopile, we get:

V _out = n·[a ₁ (T ₁ -T ₂ )+a ₂ (T ₁ ² -T ₂ ² )+...+a ₉ (T ₁ ⁹ -T ₂ ⁹ )] (2)

where n is the number of thermocouples connected in series with the thermopile. Assuming T ₁ =ΔT+T ₂ , ΔT→0, then the above formula can be simplified as:

V _out ≈n(a ₁ +2a ₂ T ₂ +3a ₃ T ₂ ² …9a ₉ T ₂ ⁸ )DT (3)

By definition, the content in brackets in the above formula is the Seebeck coefficient. It can be seen that when the temperature difference ΔT is small, the Seebeck coefficient can be measured by formula (3). In the case of small temperature difference, the Seebeck coefficients corresponding to different temperatures can be obtained by providing different ambient temperatures.

The temperature of the hot and cold ends of the thermopile is obtained according to the temperature extrapolation method. The temperature extrapolation method is that when the laser is loaded at the center of the sensor, according to the known temperature of the two thin film platinum resistors R1 and R2 combined with the cylinder wall temperature extrapolation model, the corresponding temperature value is obtained by substituting the radius of the hot and cold ends of the thermopile. The calculation model of the cylinder wall is:

T(r)=C ₁ ln r+C ₂ (4)

According to the calibration results, in the range of 40℃-100℃, when the temperature difference is about 0.31℃, as the ambient temperature increases, the Seebeck coefficient increases. Fitting the Seebeck coefficient measured at each ambient temperature, the result As shown in Figure 4.

The advantages of the planar heat flow sensor of the present invention include:

(1) A thin film heat flow sensor for measuring in-plane heat flow transfer is realized. Structurally, the hot and cold ends of the thermopile used to measure the temperature difference are printed on the same plane to measure the heat flow transfer measurement along the plane.

(2) Aiming at the problem of difficult wiring of small-sized pins, a method of using probes to lead out test signal lines is proposed, which can be applied in the field of precision measurement and can output test signals stably.

(3) In the calibration device, the purpose of measuring the Seebeck coefficient at different temperatures is achieved by controlling the furnace temperature.

(4) A method to accurately measure the Seebeck coefficient under the condition of small heat flow is proposed.

The invention describes a flat thin film heat flow sensor for measuring the heat flow along the wall surface, which is prepared on the surface of alumina ceramics by screen printing technology. The sensor consists of 3 platinum resistance resistors and 1 thermopile. The platinum resistance is arranged in an arc shape according to the radius from large to small, and is represented as R1, R2 and R3 respectively. A thermopile is distributed between R1 and R2. The thermopile is made up of 44 gold-platinum thermocouples in series, and the hot end and the cold end are separated by 3mm. When the thin film heat flow sensor is calibrated, the furnace body provides a constant ambient temperature of 40°C to 100°C, the laser loads different heat fluxes, and the static calibration of the heat flux sensor is realized through the output of the sensor under different ambient temperatures and different laser powers.

Claims (7)

1. A planar film heat flow sensor for realizing heat flow measurement along a wall surface is characterized in that a thermopile for realizing temperature difference measurement is printed on the surface of a substrate, and three circular arc-shaped film platinum resistors are integrated on the substrate to jointly form the planar film heat flow sensor; the thermopile is formed by connecting a plurality of gold-platinum thermocouples in series, and the platinum resistors are three concentric arcs with different radiuses and are distributed on two sides of the cold end and the hot end of the thermopile.

2. The planar thin film heat flow sensor of claim 1, wherein the substrate is an alumina ceramic substrate with a diameter of 22mm, a thickness of 0.36mm, and a thermal conductivity of 36W/m-K.

3. The planar thin film heat flow sensor of claim 1, wherein the three circular arc-shaped thin film platinum resistors have arc radii of 1.55mm, 2.55mm and 7.05mm, each platinum resistor has a width of 0.1mm and a film thickness of 10 μm.

4. The planar thin-film heat flow sensor of claim 3, wherein the thermopile is formed by forty-four gold platinum thermocouples connected in series and arranged on the surface of the substrate in an arc shape; the width of each gold-platinum thermocouple is 0.1mm, and the overlapping areas at the two ends of the electrode respectively form the cold end and the hot end of the thermopile; wherein, the radius of the arc where the hot end is located is 3.5mm, and the radius of the arc where the cold end is located is 6.5 mm.

5. The method of calibrating the planar thin film heat flow sensor of claim 1, wherein:

loading laser to the center of the sensor in a constant temperature field with adjustable temperature, and transferring heat to the edge along the center of the sensor after the laser is absorbed by the center of the surface of the sensor; after heat balance, according to the temperature distribution rule, the temperature of the cold end and the hot end of the thermopile is deduced from the temperature of the thin film platinum resistor, so that the temperature difference is obtained, and the relationship between the temperature difference and the output of the thermopile is established to realize the static calibration of the sensor.

6. The calibration method according to claim 5, characterized in that: the high-temperature-resistant probe and the corresponding needle sleeve are matched together and fixed on the high-temperature-resistant PCB, and the position of the high-temperature-resistant probe corresponds to the position of the pin of the sensor, so that the stable output of the test signal of the sensor is realized.

7. The calibration method according to claim 5, characterized in that:

in the static calibration of the thin film heat flow sensor, the thermoelectric potential E of the thermocouple has the following relationship with the temperature T:

E＝a₀+a₁T+a₂T²+···+a₉T⁹ (1)

wherein a is_iAs a fitting coefficient, i ═ 0,1.. 9;

when the temperatures of the hot end and the cold end of the thermopile are respectively T₁And T₂According to the relationship between the output potential and the temperature of the thermopile, the following results are obtained:

V_out＝n·[a₁(T₁-T₂)+a₂(T₁ ²-T₂ ²)+...+a₉(T₁ ⁹-T₂ ⁹)] (2)

wherein n is a thermoelectric even number of the thermopiles connected in series; let T be₁＝ΔT+T₂Δ T → 0, then the above equation is simplified to:

V_out≈n(a₁+2a₂T₂+3a₃T₂ ²···9a₉T₂ ⁸)ΔT (3)

according to the definition, the content in parentheses in the formula (3) is the seebeck coefficient; when the temperature difference delta T is small, the Seebeck coefficient is measured by the formula (3); under the condition of small temperature difference, different environmental temperatures are provided, and corresponding Seebeck coefficients at different temperatures can be obtained.

Images