How to make resistance strain sensor in low temper

How to make resistance strain sensors in low temperature environment

using resistance strain gauges as sensitive elements to make various resistance strain sensors (such as force measurement, weighing, displacement, acceleration and torque sensors), which have the characteristics of high precision, good stability, simple fabrication, low price, and easy matching of electrical signals with subsequent measurement and control instruments, are widely used in various departments of industry, including mechanical quantity sensors, Resistance strain sensors still dominate today., Generally, resistance strain sensors are suitable for room temperature (normal temperature) environment, and the use temperature range is -20~+60 ° C or -40~+80 ° C. There are a lot of research and public data at home and abroad on temperature range sensors above or below this temperature range, while there are less data on low-temperature resistance strain sensors

according to the relevant resistance strain gauge standards, the so-called low temperature environment refers to -30~-160 ° C, and the extremely low temperature (or deep low temperature) refers to the temperature that can be reached from -162 ° C (liquefied natural gas LNG) to liquid helium (lhe). The low temperature involved in this paper is the above extremely low temperature or deep low temperature range. There are the following four aspects. The emerging product sectors within this scope include:

(1) superconducting application technology: power generation, transmission system, maglev train, etc

(2) liquid hydrogen (LH 2, -253 ° C) related technologies: hydrogen energy system, liquid hydrogen engine, etc

(3) Atomic Energy: Tokamak device

(4) LNG application technology: new energy system, refrigeration sector application, etc

the practicality of low temperature technology will inevitably attract people's attention to structural safety, reliability and economy under environmental conditions. In addition to measuring the stress of structures and components at low temperature, various resistance strain sensors (such as low-temperature extensometer and low-temperature stress sensor) suitable for low-temperature environmental conditions are also required to measure the mechanical properties of various structural component materials at low temperature, and monitor various pressure changes in the application process, so as to provide a reliable guarantee for the quality of product structure and the safety of equipment operation

due to the particularity of low-temperature environment, generally the commercially available resistance strain sensors are not suitable for low-temperature environment. In addition, the amount of low-temperature sensors is particularly small, so we must actually need the environment and conditions in work, and develop and design various resistance strain sensors by ourselves. This paper mainly introduces several problems that should be paid attention to when developing and designing various low temperature resistance strain sensors for reference

II. Several problems should be paid attention to in the production of low-temperature resistance strain sensors

the basic structure of low-temperature resistance strain sensors is basically the same as that of similar resistance strain sensors used at room temperature. The selection of materials and application technology should be based on the particularity of low-temperature application environment. Generally, the following aspects should be paid attention to

1. Sensor elastomer design and material selection

the elastomer design criteria of low-temperature resistance strain sensor are basically the same as those of various high-temperature resistance strain sensors. Sensor accuracy requirements, service life and output sensitivity, etc. elastomer strain is generally controlled within the range of 800~1500 M/m. The calculation formulas of various typical elastomer structures are listed in Table 1

elastomer materials are required to have good elasticity, high tensile strength, high fatigue life and no brittle fracture at low temperature., Generally speaking, most elastomeric materials used at room temperature can be selected. However, at present, stainless steel, aluminum alloy, beryllium bronze and invar are often used as pressure sensors, while beryllium bronze, titanium alloy and other materials can be used as sensors such as extensometers

2. Low temperature resistance strain gauge

low temperature strain gauge is the key sensitive element of low temperature resistance strain sensor, and its performance affects the performance indicators of the sensor. Low temperature strain gauge is usually composed of substrate, sensitive grid, adhesive and coating. The material properties of each component directly affect the basic properties of low temperature strain gauge

Table 1 calculation formula of structural strain of various typical elastomers

① heat output caused by temperature change

resistance strain sensor is pasted on the sensor elastomer by adhesive. When the elastomer deforms under the action of external force, the elastomer deforms, and the adhesive layer is transferred to the sensitive grid, causing the resistance of the sensitive grid material to change. The resistance change value is linear with the external force on the elastomer, The external force on the elastomer can be known by measuring the resistance change value

in the actual working state of the sensor, the elastomer is subjected to external forces and is often affected by the change of ambient temperature. The false output of the strain gauge caused by temperature change is usually called thermal output (E T). Its value is related to the resistance temperature coefficient (a R), sensitivity coefficient (k), linear expansion coefficient (a g), linear expansion coefficient of elastomer material (a m) and temperature change (d t) of strain gauge sensitive grid material, that is: (1)

for sensors, generally, the thermal output value of strain gauge used is required to be small, so as to ensure accuracy and stability. When selecting strain gauge sensitive grid material, It must be matched with the linear expansion coefficient of elastomer material, that is: (2)

the above formula is also the basic formula for making various temperature self compensating strain gauges

kanfman Research Report [6] points out the heat output of various resistance alloys at low temperature, and the typical structure is shown in Figure 1. The figure shows the characteristics of advance (Cu Ni alloy), karma, Budd alloy, nickel V (Ni Cr and Ni Cr modified alloy) and stabilized armour D (Fe Cr Al alloy). From room temperature to 4.2K (lhe), various sensitive elements have different temperature characteristics. The strain gauge of Cu Ni alloy (Constantan, advance) element has a large heat output at low temperature. When the substrate is the same, the heat output of strain gauge made of karma and nichrome V is smaller than that of C rigid polyurethane foam material, which is different from U-Ni alloy in foreign standards and domestic standards. It can be seen from the figure that the thermal output of various strain gauges has a minimum value in the temperature range of 10~20k. The research shows that the resistance temperature coefficient of Ni Cr modified alloys (such as karma, etc.) can be adjusted by alloy composition and heat treatment process, which is convenient to make self compensating strain gauges suitable for various elastomer materials and temperature. At present, most of the low-temperature self compensating strain gauges use karma and other alloys as sensitive grids

② the change of sensitivity coefficient caused by temperature

the ratio of the change rate of the resistance of the strain gauge to the strain, which is usually called the sensitivity coefficient (k) of the strain gauge. Its value is related to the geometry and material characteristics of the strain gauge sensor. For general metal resistance materials, the sensitivity coefficient is mostly about 2.0 at room temperature. In low temperature environment, the sensitivity coefficient increases as the temperature decreases. The typical properties of various resistance alloys are shown in Figure 2. It can be seen from the figure that when most resistance alloys are subjected to tension or compression, their sensitivity coefficients are somewhat different. The difference between stabilized Armourd (Fe Cr AI alloy) is greater. During tension and compression, the sensitivity coefficient is inconsistent, which will reduce the output sensitivity of the sensor and increase the nonlinear error of the sensor., In the field of strain measurement, when making resistance strain sensors for low temperature, it is advisable to choose materials with small difference in sensitivity coefficient during tensile or compressive strain. It can be seen from the figure that the difference in sensitivity coefficient between tensile and compressive deformation is relatively small for karma, nichrome V and advance materials

the sensitivity coefficient of resistance alloy elements changes. If the room temperature is also used as the base electricity, the changes in the high-temperature zone and low-temperature zone are shown in Figure 3. It can be seen from the figure that the sensitivity coefficient of strain gauges made of Ni Cr Alloys (such as karma, SK, etc.) and Cu Ni Alloys (such as constantan/advance) changes with temperature in exactly the opposite way. The sensitivity coefficient of Ni Cr alloy increases with the decrease of temperature, while the sensitivity coefficient of Cu Ni alloy decreases with the decrease of temperature at low temperature. The difference between the sensitivity coefficients of the two during tension and compression is larger than that of Ni Cr alloy

the sensitivity coefficient characteristics of strain gauges made of Ni Cr alloy are shown in Figure 4 in the temperature range from room temperature to 4.2K. It can be seen from the figure that the change rate of Ni Cr Alloys (kfl-, SK) increases almost linearly from room temperature to 200K, then slowly, and almost does not increase at temperatures below 100k

③ the effect of magnetic field on the performance of strain gauge

to sum up, low-temperature resistance strain sensor should be made of Ni Cr alloy. Its origin:

(1) Ni Cr alloy (such as karma, etc.) is made of strain gauge, and its temperature change causes heat output. The alloy composition and heat treatment process can be adjusted to change its resistance temperature coefficient (a R) and match the linear expansion coefficient (a m) of sensor elastomer material, The temperature self compensating strain gauge is made. When the strain gauge is deformed by tension and compression, the sensitivity coefficient difference between the two is small, and the sensor is also small affected by temperature changes, which is conducive to improving the stability of the sensor in low temperature environment

(2) the sensitivity coefficient of Cu Ni Alloys decreases with the decrease of temperature, while the sensitivity coefficient of Ni Cr Alloys increases with the decrease of temperature. This tendency is consistent with the tendency that the elastic modulus (E) of sensor elastomer material increases with the decrease of temperature, which is conducive to the compensation of sensor sensitivity (range). In addition, the sensitivity coefficient of strain gauge made of Ni Cr alloy increases linearly at room temperature to 200K, then slowly increases, and its change is quite small below 100k. Thus, the sensor is made, and its output sensitivity changes in this way. When the calibration temperature affects the sensitivity change, the calibration temperature changes within the range of 100k, while below 100k can be considered as unchanged, that is, the sensitivity at 100k indicates the sensitivity of the sensor below 100k

(3) the influence of magnetic field on Pt-W alloy is much smaller than that of Ni Cr alloy, but it is expensive and has poor temperature characteristics, so it is not suitable to be used as a general sensor. The karma alloy strain gauge shows negative output when the magnetic field strength is less than 1t, and then it changes linearly with the increase of the magnetic field. Sensor calibration can ensure a certain accuracy range

3. Low temperature adhesive and protective agent

adhesive and protective agent have a direct relationship with the performance of low temperature resistance strain sensor. Especially adhesive, which is particularly important for sensors. A large number of tests have proved that:

(1) polyimide film is generally used as the base material of low-temperature strain gauge, and it is appropriate to use glass fiber reinforced epoxy phenolic adhesive film. These materials have small shrinkage at low temperature, good flexibility, and good bonding effect with adhesive

(2) thermosetting adhesives such as polyimide and modified epoxy phenolic adhesives are generally used for pasting strain gauges. After this kind of adhesive is pasted, after pressure regenerative curing and post curing treatment, the adhesive layer has high bonding strength, good electrical insulation performance, small creep, hysteresis and zero drift of the sensor, and good stability of the sensor

(3) protective agent plays a very important role in improving the stability of the sensor at low temperature. In sensor applications, the temperature of the strain gauge on the elastomer surface is gradually reduced by the injection of refrigerant at room temperature. During the cooling process of elastomer, trace moisture in the air is adsorbed on the elastomer surface to form white frost. When the sensor is subjected to periodic temperature changes, the white frost on the surface of the strain gauge melts into water, so that the strain gauge and adhesive absorb water and change (reduce) the insulation resistance, resulting in zero drift, creep and hysteresis increase of the sensor, and even failure of the sensor in serious cases

in addition, when the strain gauge on the surface of the pasted sensor elastomer is immersed in the low-temperature medium, the current flowing through the strain gauge generates Joule heat in the strain gauge. When it contacts the low-temperature medium, there is excitation between the strain gauge and the medium