How to Choose Resistance Strain Gauges for Load Cells

How to Choose Resistance Strain Gauges for Load Cells

【Summary】In order to produce a strain gauge load cell that consistently meets the requirements of national standards, in addition to designing a reasonable elastic element structure, using excellent metal materials, and implementing a stable and repeatable manufacturing process, the selection of the elastic element has the best compatibility and work Characteristic resistance strain gauges, strain adhesives, and protective agents are essential. Among them, the resistance strain gauge manufacturing process level, various working characteristics and self-compensation functions have a great influence on the accuracy and stability of the corresponding variant load cell, which is the source of product quality control. For this reason, this paper analyzes the working principle of resistance strain gauge; sensitive grid and substrate material; geometric shape and surface quality; manufacturing process equipment and process conditions; mechanical and electrical performance; main parameter indicators, etc., and puts forward a strain gauge load cell. The selection principle of using resistance strain gauge and strain adhesive.

【Key Words】Load cell, Resistance strain gauge, Work characteristics, Resistance alloy, Sensitive grid, Base

1:Overview

The working principle and manufacturing process of the strain gauge load cell (hereinafter referred to as the load cell) are determined. It is a resistance strain gauge with four pieces or multiples of the four pieces, and the strain adhesive is pasted on the surface of the strain zone of the elastic element. After high-temperature curing and post-curing treatment, it is firmly combined into one. Complete all manufacturing work by assembling and welding Wheatstone bridge circuits, implementing various circuit compensation and adjustment, protection and sealing, experimental testing and calibration and other processes. It is not difficult to conclude from this that the deformation of the elastic element of the load cell is transferred to the sensitive grid of the resistance strain gauge through the adhesive layer of the strain adhesive, completing the task of converting mechanical quantity to electric quantity. Therefore, the technical performance of resistance strain gauges, strain adhesives, and product quality directly affect the accuracy and stability of the load cell, which is an important part of the quality control of the load cell. This requires that when selecting resistance strain gauges, load cell manufacturers should have a comprehensive understanding of the manufacturer's resistance strain gauge design and manufacturing process level. Focus on the heat treatment and stability treatment process equipment of strain resistance alloy foil; the hot press forming process equipment of the resistance strain gauge substrate and the sensitive grid; the intelligent etching process equipment controlled by the electronic computer; the process equipment for adjusting the resistance value without prestress and the fully sealed grid ; Graphic recognition and automatic detection equipment for working characteristics of resistance strain gauge quality inspection.Specific consideration should also be given to the sensitive grid and base material used in the resistance strain gauge; geometry and surface quality; electrical and mechanical properties; main parameters, etc., in order to achieve the best match between the resistance strain gauge structure, the sensitive grid used and the base material and elastic components Sex. Because the excellent base material can maintain the fixed geometry of the sensitive grid, and maintain a high insulation resistance value between it and the elastic element. If the strain adhesive is not properly selected, the thickness of the adhesive film between the resistance strain gauge base and the elastic element will be uneven, resulting in increased sensitivity coefficient dispersion, and even non-linear output of the load cell.

Since the load cell mostly uses a Wheatstone full-bridge circuit, as long as the temperature of the elastic element is uniform and the resistance strain gauge has consistent heat output characteristics, the influence of temperature on the resistance strain gauge can be offset at this time, and it is not necessary to use a temperature self-compensating resistor Strain gauges can also meet the accuracy requirements of load cells. However, from the analysis of the working characteristics of the products of resistance strain gauge manufacturers at home and abroad, it is not difficult to conclude that due to the difference in the quality of the strain resistance alloy foil selected by the resistance strain gauge manufacturers, the differences in the manufacturing process equipment and conditions, the resistance produced The temperature characteristics of strain gauges vary greatly, and their dispersion is also greater. For resistance strain gauges attached to elastic elements, the most obvious interference is temperature. On the one hand, the environmental temperature change causes the specific resistance of the sensitive grid material of the resistance strain gauge to change, which causes the resistance value to change; on the other hand, the different linear expansion coefficients of the elastic element material and the sensitive grid material cause thermal deformation of the resistance strain gauge , The additional strain produced by the two is the main source of measurement error.This requires load cell manufacturers to choose resistance strain gauges, but also require resistance strain gauges with small deviations in thermal output characteristics or with temperature self-compensation functions. The above-mentioned reasons determine that load cell manufacturing companies, especially those companies that do not have a resistance strain gauge manufacturing department in the company, must scientifically and rationally choose resistance strain gauges for various structural elastic components.

2:Structure and working principle of resistance strain gauge

2.1. Resistance strain gauge structure

Resistance strain gauges, also called resistance strain gauges, are resistance strain sensitive components used to measure unidirectional or multidirectional strain values on the surface of various components. It is composed of sensitive grid, substrate, covering layer, adhesive and flat leads, etc. Its basic structure diagram is shown in Figure 1.

Figure 1 Schematic diagram of basic structure of resistance strain gauge

1-Lead, 2-Covering layer, 3-Base, 4-Sensitive grid

(1) Sensitive grid

It is the most important component of the resistance strain gauge. It is wound with metal filaments or etched into a grid shape with metal foil. Its function is to convert the strain of the elastic element into the amount of resistance change.

(2) Base

It supports and protects the sensitive grid to maintain the specified geometric shape and relative position, and to ensure a high insulation performance between the sensitive grid and the elastic element. It is mostly made of various adhesives and organic resin films.

(3) Covering layer

Protect the sensitive grid from mechanical damage and enhance the adhesion between the sensitive grid and the substrate.

(4) Flat lead

The flat metal wire drawn from the sensitive grid is used to connect the resistance strain gauge with the measuring bridge circuit.

From the structure of the resistance strain gauge and the role of each part, it is not difficult to conclude that only the material of each component is excellent, and the manufacturing process is scientific and precise, can it be firmly integrated and become a complete strain-resistance conversion element.

2.2 Working principle of resistance strain gauge

The resistance strain gauge is pasted on the surface of the strain zone of the elastic element, and is firmly combined into a whole after high temperature curing and post curing treatment. Both the resistance strain gauge substrate and the strain adhesive are in surface contact, which facilitates the effective transmission of deformation of the elastic element. The experimental research shows that the deformation of the elastic element is transmitted to the resistance strain gage substrate and the sensitive grid in the form of shear. The displacement of the two ends of the resistance strain gage substrate is the largest during deformation, so the shear stress is the largest, which produces a certain end effect. The displacement at the middle section is zero, so the shear stress is zero, which is a stable strain transfer zone. It is not difficult to conclude from this: the deformation of the elastic element is through the transfer of shear stress, and the strain transfer is realized by the sensitive grid of the tensile or compressive resistance strain gauge, that is, the central shear stress of the sensitive grid is zero, and most areas are in a unidirectional stress state. The transmitted strain is basically equal to the strain of the elastic element.

In order to reduce the influence of the end effect, the selection principle of resistance strain gauges and adhesives is: try to choose resistance strain gauges with frame-shaped ends; there must be enough distance from the edge of the substrate to the ends to increase the area of the shear stress transmission zone; The sensitive grid or sensitive wire should be thinner or finer, and the substrate thickness is required to be 25~30μm to ensure effective strain transmission; the thinner the adhesive layer, the better, and the greater the adhesive elastic modulus and shear elastic modulus, the better. It is beneficial to reduce the hysteresis and creep errors of the load cell.

2.3. Sensitive grid and base material of resistance strain gauge

1. Resistance strain gauge sensitive grid alloy material

Different types of resistance strain gauges have different requirements for the performance of sensitive grid materials. In summary, their main performance requirements are as follows:

(1) The strain sensitivity coefficient is relatively large, which is constant within the measured strain range;

(2) High resistivity, good uniformity and stability;

(3) The resistance temperature coefficient is small, the dispersion degree is also small, it is linear with the temperature change, and the repeated temperature rise and fall is good;

(4) Strong anti-oxidation and corrosion resistance;

(5) Maintain sufficient tensile strength within the working range;

(6) High fatigue life and high elongation;

(7) It is easy to weld, and the thermoelectric potential to the lead material is small;

(8) Good manufacturability, easy to wire drawing, rolling and welding.

The most commonly used resistance strain gauge sensitive grid alloy materials include constantan, nickel-chromium alloy, nickel-chromium modified alloy, iron-chromium aluminum alloy, etc.

Constantan (Cu 55%, Ni 45%) maintains stable alloy performance at -50°C to 250°C; good process performance, easy to roll and weld; change the temperature coefficient of resistance by changing the alloy composition, the degree of processing deformation, and the heat treatment specification It can be changed in a wide range from negative to positive, and can be used to manufacture temperature self-compensating resistance strain gauges suitable for various materials; the sensitivity coefficient KS is almost constant within the range of elastic and plastic deformation, and can be used to manufacture resistances for measuring large strains Strain gauge. The disadvantages are: the sensitivity coefficient is small, KS=2.0; the resistivity is low, ρ =0.45～0.52 Ω mm2/M.

Nickel-chromium alloy (Ni 80%, Cr 20%) is characterized by a large sensitivity coefficient, KS=2.1～2.3; a higher resistivity, twice that of Constantan, ρ =1.0～1.1 Ω mm2/M. Because the resistance temperature coefficient is large and difficult to adjust, it must be improved.

Nickel-chromium modified alloy, that is, adding a small amount of other elements to the nickel-chromium alloy to improve the properties of the alloy. Add aluminum and iron to form Karma alloy (Ni 74, Cr 20, Al 3, Fe 3); add aluminum and copper to form Evan alloy (Ni 75, Cr 20, Al 3, Cu 2). The common features are: high sensitivity coefficient KS=2.4～2.6, and negative temperature coefficient; high resistivity ρ=1.24～1.42 Ω mm2/M; low temperature coefficient of resistance α=±20×10-6/℃; The thermoelectric potential to copper is 3μV/℃; stable performance, corrosion resistance, long fatigue life; can adjust and control the temperature coefficient of resistance like constantan; use temperature range -269～4000℃.

Fe-chromium aluminum alloy (Cr 20-50%, Al 5-10%) is a sensitive grid material with good performance. It has high resistivity, large sensitivity coefficient (KS≈2.8), and low temperature coefficient of resistance. When changing the content of chromium and aluminum, adding trace elements such as vanadium and molybdenum, or changing the heat treatment specifications, the temperature coefficient of resistance can be changed in a large range. It is especially suitable for high-temperature self-compensating resistance strain gauges. The maximum working temperature can be Up to 8000℃;

2. Base material for resistance strain gauge

The base materials of resistance strain gauges mainly include: phenolic resin, epoxy resin, phenolic-epoxy resin, polyimide, alkali-free glass fiber cloth and new base material polyether ether ketone, etc. Phenolic resin has excellent heat resistance, moisture resistance, oil resistance, and chemical resistance, but it is brittle after film formation, and toughening agents must be added to improve its brittleness.

Epoxy resin has good bonding properties and high strength; it has excellent water resistance, oil resistance and chemical resistance; there is no solvent volatilization during curing, and the volume shrinkage is small. Simple epoxy is more brittle after curing, and toughening agent must be added to improve heat resistance and impact resistance, and phenolic resin is used to modify it.

Phenolic-epoxy resin has the characteristics of high rigidity and good heat resistance of phenolic resin. It also has the advantages of high epoxy resin bonding strength, small curing shrinkage, and good electrical insulation performance.

Polyimide was successfully developed in the 1960s. It is a chemical reaction of aromatic diamine and dianhydride to form a macromolecule containing an amide group and a carboxylic acid group, and then form an imine ring through polymerization. The name comes from this. Its characteristics are: large elastic modulus, high peeling strength; good film formation, transparent and flexible film; excellent moisture resistance, heat resistance, oil resistance, acid and alkali resistance; strong nuclear radiation resistance; good electrical insulation performance; creeping The deformation and mechanical hysteresis is small, and the strain limit is high; the operating temperature is -600C～2500C. The disadvantage is that the curing temperature is higher and the adhesive performance is poor.

The alkali-free glass fiber cloth is a reinforcement for other base materials. Its characteristics are: high rigidity, high strength; small curing shrinkage, good stability; good moisture resistance and heat resistance; high electrical insulation performance; low creep and hysteresis ; The working temperature is high, generally up to 4000C～4500C. The disadvantage is that the flexibility after coating is poor, and the appropriate curing temperature should be selected to make it dry and not brittle.

The new base material—polyether ether ketone (PEEK) is a polymer composed of repeating units containing one ketone bond and two ether bonds in the main chain structure, and belongs to a special polymer material. It is characterized by good physical and chemical properties such as high temperature resistance and chemical corrosion resistance; it can be used as high temperature structural materials and electrical insulation materials; it can be combined with glass fiber or carbon fiber to prepare reinforced materials; it has high melting point, softening point, and high tensile strength (melting point 334 ℃, softening point 168℃, tensile strength 132~148MPa); it has good rigidity and flexibility, and has outstanding fatigue resistance under alternating stress, which can be beautiful compared with alloy materials; it has good manufacturability and can be made into 25μ m, 12.5μm film.

4:Main parameters of resistance strain gauge

1. Resistance value R

The resistance value measured at room temperature without pasting or external force. The resistivity change of the sensitive grid material and the manufacturing process determine that the resistance value of the resistance strain gauge has a large dispersion, which is expressed by deviation and tolerance. Class A resistance strain gauge has a deviation of ±0.5% from the nominal value and a tolerance of ±0.1% from the average value.

2. Sensitivity coefficient K

The ratio of the relative change of resistance △R/R to the axial strain △L/L=εL is called the sensitivity coefficient K. K is a constant within a certain strain range, and is linearly related to. Due to the influence of the sensitive grid geometry, adhesive and substrate, the sensitivity coefficient K of the strain gauge is always smaller than the sensitivity coefficient KS of the foil.

The sensitivity coefficient of a single resistance strain gage Ki=K meter ε meter/ε machine or Ki=(△R/R)/ε machine, the average sensitivity coefficient of resistance strain gage K flat=∑Ki/n.

3. Mechanical lag Zi

The resistance strain gauge is pasted on the test piece. When the temperature is constant, in the process of increasing or decreasing the strain, for the same strain ε i, the resistance strain gauge indicates the strain has a difference △εi, which is the mechanical hysteresis. The main reason is that after the resistance strain gauge is subjected to mechanical strain, there is residual deformation inside, which causes a small amount of irreversible change in the resistance of the sensitive grid; the sensitive grid is deformed improperly; and the strain adhesive is not sufficiently cured.

4. Zero drift P

The resistance strain gauge is pasted on the test piece. When the temperature is constant, the indicated strain changes with time when it is not under stress. This is called the zero point drift of the resistance strain gauge, which has nothing to do with mechanical strain. The main influencing factors are the temperature effect of the sensitive grid after the working current is passed; the internal stress generated when the resistance strain gauge is manufactured and pasted gradually changes; the strain adhesive is not cured sufficiently; the insulation resistance of the resistance strain gauge is reduced by moisture; and the distribution capacitance of the wire changes. In fact, drift and creep exist at the same time, and the creep value includes the drift value at the same time.

5. Creep θ

When the resistance strain gauge is subjected to mechanical strain, shear stress is generated inside it, and its end shear stress is the largest, causing "slip" between the adhesive layers, and the absolute value of the transmitted strain gradually decreases to cause creep. The greater the mechanical strain, the greater the creep of the resistance strain gauge. How to improve creep performance is mainly to select adhesives and substrate materials with large elastic modulus; appropriately reduce the thickness of the adhesive layer and substrate and make them fully cured; select strain gauges with frame-shaped ends. Generally, the direction of creep caused by tensile strain and compressive strain is opposite. The compressive strain gauge is positive and the tensile strain gauge is negative, and its curve shape is basically symmetrical.

6. Insulation resistance Rm

Insulation resistance Rm refers to the resistance value between the sensitive grid and lead of the pasted resistance strain gauge and the test piece. The insulation resistance Rm of the high-accuracy load cell should be greater than 5000MΩ. The main effect of the decrease of insulation resistance on the measurement system is the decrease of sensitivity, indicating errors in the strain; zero drift, and even failure of the measurement system. Most of the instruments for detecting the insulation resistance Rm of resistance strain gauges use low-voltage high-resistance meters or insulation resistance testers. The specified measurement voltage is 10～100V (50V is the best), and the measurement accuracy is not less than 5%.

7. Heat output Ct

It is the indicated strain caused by temperature changes. The temperature coefficient of resistance of the sensitive grid material of the resistance strain gauge and the difference between the coefficient of linear expansion of the material of the test piece are the main internal cause of the heat output of the strain gauge, and the temperature change of the working environment is the external cause of the heat output. The actual heat output of the resistance strain gauge cannot be calculated by the formula. It can only be calibrated on various component materials, measured by the average heat output coefficient. The calculation formula is:

8. Fatigue life N

The number of cycles from continuous work to fatigue failure of the strain gauge attached to the specimen under the action of alternating stress of constant amplitude is defined as the fatigue life. It is related to factors such as the material of the resistance strain gage, the manufacturing process, the lead wire welding, and the quality of the paste. Now, the strain gauge fatigue life is 107～108. In most cases, the main factors that affect the working life of the resistance strain gage pasted on the elastic element are: the performance of the strain adhesive and the base material to transmit strain; the bonding quality of the resistance strain gage. Select strain adhesive and base material with high shear strength, require base film thickness of 25~30μm, strain adhesive layer thickness of 5μm, and after proper curing and post curing treatment, higher strain limit can be obtained .

5. How to choose resistance strain gauges for load cells

5.1. Selection of resistance strain gauge manufacturers

The resistance strain gauge manufacturer should meet the following conditions:

(1) Have a high level of theoretical analysis and structural design (such as strain gauge structure finite element analysis, frame-shaped head and elliptical ring optimized design, etc.);

(2) Have advanced and complete process equipment and testing methods, especially advanced foil heat treatment equipment;

(3) A new process of bonding the foil and the film together by using a hot press;

(4) Using stress-free or automatic resistance adjustment technology;

(5) Adopt advanced sealing gate plane technology;

(6) It has a complete working characteristic test and detection device;

(7) Adopt scientific quality control and management methods.

5.2.The resistance strain gauge should have excellent characteristics and the best match with the elastic element

Since the resistance strain gauge cannot be used twice after the test, it can only measure the working characteristics and fatigue life under laboratory environmental conditions, which brings great difficulties to research and selection. Currently, resistance strain gauges are screened mostly around sensitive gate materials and structural forms, substrate and covering layer materials, and manufacturing process levels.

Under the action of external load, the elastic element transfers extremely small deformation to the sensitive grid through the strain adhesive and the resistance strain gauge substrate. Because the transferred deformation is very small, the working characteristics of the resistance strain gauge and the strain adhesive are very demanding, mainly : Good strain transmission linearity; small hysteresis and creep; small temperature and time drift; good electrical insulation performance and no moisture absorption.

Production practice has proved that for the load cell of the same structure, the resistance strain gauges of different structures are pasted, and the creep index, the pass rate and the fatigue life of the same accuracy level are significantly different. This is because the structure of the sensitive grid, Due to improper selection of base length and base width, load cell manufacturers should pay great attention to them and choose resistance strain gauges scientifically and reasonably.

5.3. Resistance strain gauge structure and size selection

(1). Selection of sensitive grid geometry

According to the structural form of the elastic element and the stress characteristics of tension, compression, bending, and shear used in the strain zone, uniaxial type, 45⁰ biaxial type, 45⁰ uniaxial type, half bridge type and full bridge type are selected

And other geometric shapes. For example, cylindrical and cylindrical elastic elements mostly choose uniaxial and half-bridge resistance strain gauges; shear beam elastic elements choose 45⁰ biaxial resistance strain gauges; spoke elastic elements

Choose 45⁰ uniaxial resistance strain gages for more sexual components; full-bridge resistance strain gages can be selected for elastic components that produce positive and negative strains (such as small-scale parallel beam structures).

(2) Selection of sensitive grid materials

The sensitive grid material and the heat treatment process determine the working characteristics of the resistance strain gauge, and its main performance requirements are: a large sensitivity coefficient; a high and stable resistivity; and a small temperature coefficient of resistance. Load cells of different accuracy levels, different uses, and different manufacturing processes have different requirements for the sensitive grid materials of resistance strain gauges. Constantan (copper-nickel alloy) is often chosen as the sensitive grid material for resistance strain gauge load cells with accuracy levels above C3; the load cells used in body scales are relatively simple in manufacturing process and do not perform elastic modulus compensation, so choose more Karma and Evan alloys suitable for manufacturing elastic modulus self-compensating resistance strain gauges are sensitive grid materials.

(3) Base length selection

The base length refers to the effective working length of the sensitive grid of the resistance strain gauge. It is known from the principle of strain transfer that the strain gauge reflects the average strain in the strain area of the elastic element. The base length should be selected according to the structure of the elastic element and the stress distribution, and the utilization coefficient C of the resistance strain gauge should be as close as possible to 1, and C is the ratio of the average strain to the maximum strain.

which is

When the strain is distributed exponentially, the best choice is that the strain change in the sensitive grid area is not more than 10-15% of the maximum absolute strain, so as to obtain the largest and uniform strain as much as possible, while improving the fatigue life and reducing the creep. The base length of resistance strain gauge is usually 1.5～3.2mm. In order to increase the amount of strain and increase the output sensitivity, the elastic element of the double-hole parallel beam can choose a resistance strain gauge with a smaller base length.

(4) Base width selection

When the position of the resistance strain gauge in the strain area of the elastic element allows, the base width of the resistance strain gauge is as wide as possible, so that the sensitive grid area is large, the heat dissipation is good, the allowable current is large, and the manufacturability is good.

(5) Horizontal grid selection

Traditional resistance strain gauges have two major defects: one is that the difference between the elastic modulus of the base part and the metal part is about 30 times, and the grid wire near the arc end ring cannot correctly transmit the strain under the base; the second is the shear strain in the base. Too high near the arc end ring leads to increased creep. Therefore, try to choose a new strain gauge with a frame-shaped end to reduce the shear strain effect of the end on the substrate.

(6) Base selection

Resistance strain gauge substrate is an important strain transmission medium, which requires good high temperature stability, high rigidity, small creep hysteresis and high shear modulus. In order to ensure the effective transmission of the strain generated in the strain zone of the elastic element and to have a higher insulation resistance, the thickness of the resistance strain gauge substrate is required to be 20-30 μm. In recent years, domestic load cell manufacturing companies have chosen more resistance strain gauges of various specifications made of a new type of base material—polyetheretherketone (PEEK), whose base thickness basically meets the above requirements.

(7) Resistance value selection

The high-accuracy load cell hopes that the working current of the resistance strain gauge does not exceed 20mA, which is beneficial to improve the accuracy and stability. A high resistance value can increase the Wheatstone bridge voltage and improve the output sensitivity of the load cell, but the change of the insulation resistance has a greater impact on it. The bridge circuit's ability to resist common mode interference decreases and affects the stability. Generally, 350Ω is appropriate. In special cases, you can choose a resistance strain gauge with a large resistance value (eg 1000Ω).

(8) Selection of various compensation functions

The selection principle of the temperature self-compensating resistance strain gauge is to match the linear expansion coefficient of the elastic element material; the selection principle of the creep self-compensating resistance strain gauge is to determine the limit compensation of the load cell according to the inherent creep magnitude and direction The resistance strain gauge with the corresponding creep number; the modulus self-compensating resistance strain gauge is selected according to the accuracy level of the load cell and the metal material used in the elastic element, and is mostly used for load cells with low accuracy requirements.

6.Selection of strain adhesive for load cell

Any non-metallic substance that can form a thin film by which the resistance strain gauge is tightly bonded to the surface of the elastic element of the load cell, and which transmits strain is called strain adhesive. The requirements for strain adhesives are: high bonding strength, high shear strength; high elastic modulus, good temperature characteristics; small curing shrinkage, good stability performance; low hysteresis and creep, high electrical insulation performance, more domestic and foreign applications Solvent-free adhesives based on epoxy resin and epoxy phenolic resin as adhesives, for example, two strain adhesives developed and produced by the American V-MM company:

M-Bond 43-B high-accuracy load cell is a one-component, solvent-diluted, epoxy resin adhesive, which can be used as an adhesive or as a protective coating. Operating temperature -269～＋120℃. (30 ml per bottle).

M-Bond 610 high-accuracy load cell uses two-component, solvent-diluted, high-performance epoxy phenolic resin adhesive. It can form an ultra-thin, strong and gapless glue layer less than 0.005mm. Operating temperature -269～230℃.

The common features of M-Bond610 and 43B are: no solvent volatilization during curing, small volume shrinkage, thin adhesive layer, small creep and hysteresis, high insulation performance, and good three-proof performance. Curing agent: 11 g/bottle; resin: 14 g/bottle.

High temperature strain adhesive for high temperature load cell:

The special epoxy-phenolic resin adhesive M-Bond450, developed by V-MM in the United States, can be used to attach high-temperature resistance strain gauges. It also has the characteristics of no solvent volatilization during curing, small volume shrinkage, thin adhesive layer, small creep and hysteresis, and high insulation performance. The working temperature is: -269℃～260℃. Curing agent: 12.5 g/bottle; resin: 12.5 g/bottle.

7:Concluding remarks

The resistance strain measurement method based on the resistance strain gauge is mainly used to measure the strain on the surface of various structural parts and various types of strain sensors. Although it has the characteristics of simple measurement method; compact size, light weight, easy to paste; resistance strain gage; high strain measurement sensitivity, fast frequency response; large measurement range and high accuracy, but its working characteristics are not omnipotent. Even the Class A products specified in the national standards for resistance strain gauges cannot fully meet the requirements of load cells. Therefore, it is necessary to select resistance strain gauges and strain adhesives scientifically and reasonably according to the above analysis and requirements according to the requirements of the load cell's use environment, the structural characteristics of the elastic elements, the sealing conditions, and the accuracy level. The main selection content is resistance strain gauge structure, sensitive grid material, substrate material, substrate and protective surface adhesive thickness, lead material, resistance nominal value and deviation value, sensitivity coefficient, mechanical hysteresis, creep, room temperature lateral effect, temperature range, Temperature compensation range, matching temperature coefficient, fatigue life, etc. Once the resistance strain gauge is selected and it has been tested and proved to fully meet the technical requirements of the load cell, it should not be replaced at will in order to ensure a stable and repeatable manufacturing process.