Brief Introduction to Strain Gages Compensation and Selection

Ⅰ. Self-Temperature Compensation Strain Gages
Introduction:
The strain gages that are installed on surface of a tested object without any outside force, when environmental temperature changes, the resistance value will be changed accordingly. This phenomenon is called strain gages thermal output. It is result from interactions and cumulation of resistance temperature coefficient of grid materials, sensitive grid materials and linear dilatability coefficient of the tested objects. It is shown as the formula below:
εt=[(αg/K)*(βS-βg)]△t
In the above formula, αg and βg refer to resistance temperature coefficient of the grid materials and linear dilatability of strain gages respectively; K refers to gage factor; βS refers to linear dilatability.
coefficient of the tested object; △t refers to relative temperature changes of reference departure temperature.
Thermal output is the largest error resource of strain measurement in static state as shown in picture 1 . With increasing of the temperature effect, the decentralization of thermal output will also be increased. If there are temperature grads or instant changes during test, the difference will become larger . Therefore, the ideal circumstance is that strain gages thermal output value is close to zero. The strain gages that fulfill this requirement are called self-temperature compensation strain gages.
By adjusting alloy elements' ratio of the strain gages grid material or changing foil's cold rolled reduction and proper heat treatment, the crystal configuration of the sensitive grid would be recombined and its temperature coefficient of the resistance would be changed. In order to make strain gages' thermal output close to zero and to realize self-temperature compensation for spring element or tested object materials, to meet the requirement of the high precision strain analysis and transducer production. Picture 2 is the typical thermal output curve of the Constantan, Karma self-temperature compensation strain gages. In the range of +20~~+250℃, their thermal output value is very small.

Notes for using STC gages：
(1)At present, EXACT offers self-temperature compensation strain gages with codes of: 9，11，16、23，27，Among them, “ 9”is used for alloy titanium materials (the typical value of the linear coefficient expansion is 8.8×10(-6) /℃)；“11”used for alloy steel, Martensite stainless steel and deposit scleroses stainless steel materials( the typical value is 16×10(-6) /℃); “23” used for alloy aluminum materials (the typical value is 23.2×10(-6) /℃ );“27”used for alloy magnesium materials (the typical value is 26.1×10(-6) /℃).
(2) When the self-temperature compensation gages matches the material of tested object, it is not necessary to compensate thermal output within the range of compensation temperature.
(3) In case that the material of the tested object required by self-temperature compensation gages do not match the material of the tested object that is used, we should utilize two or four gages to form a half bridge or full bridge to minimize the temperature effect.
(4) When measuring with Quarter Bridge, we should install a strain gage on“compensated object” which is the same material as the tested object. The strain gage should be from the same lot as the one installed on the tested object. The two gages should be under the same temperature environment and located next to each other in the Wheatstone bridge.
Ⅱ. Self-Creep Compensation Strain Gages
Introduction:
The creep characteristics exist in spring element because of an elasticity of its materials, which makes the transducer output increasing with the addition of time (positive creep), and depends on several variables such as spring element material, structure, strain field, span, heat treatment and test temperature, etc. The backing of gages and adhesive for bonding have high viscoelasticity, results in the output decreasing with the addition of time; but grid material of gages has an elasticity which makes the output increasing with the addition of time. The result of accumulation is that the strain gages have positive or negative creep under fixed load; its direction and value could be adjusted by modifying the design of grid structure, backing material ratio and key technology parameter. For example, changing the dimension of the end grid and fixing the other parameters, we can get the curve of creep characteristic After selecting materials of spring element, if gage creep is equal to spring element creep in value but the direction is opposite, then we can compensate the creep of spring element. In the same way, during making transducers, the creep error caused by other factors could be adjusted this way, and the combined creep value could be limited in minimum range (as shown in picture 3). EXACT offers many models of gages which standard creep grads to be selected by transducer manufacturers. (The N※, T※ in strain gages designation refer to creep code, different codes represent different creep value. The rule is: creep difference between any two-neighbor codes is 0.01-0.015%FS/ 30min)

Notes for using self-Creep Compensation strain gages：
(1)For the first time using, please select one or two models of gages which have great different creep values (different creep codes) and bond them onto the spring element. The matched creep codes will be determined according to actual test value of comprehensive creep and direction.
(2) For transducers with the same spring materials and structure, the smaller the capacity is, the more positive creep it would be, therefore a more negative creep code should be selected.
(3) Different element material exhibits different creep characteristics. Therefore, different creep code should be selected for steel and aluminum transducers with the same capacity and structure.
(4) Transducer creep depends on many variables such as spring elements, strain gages, adhesive as well as the sealing form, protective coating, technique parameters, etc. The direction and magnitude of such error can be predicted, and shall be considered when selecting creep code.
Ⅲ. Self-Elastic Modulus Compensation Strain Gages Introduction:

Introduction:
With rising of the ambient temperature, the elastic modulus materials will go down. According to the theory of Hooke, as environmental temperature increases the deformation of this structure will be bigger even if the load is not changed. Therefore, the tested strain will be increased along with it. At that time, if the gage factor can be reduced properly with temperature, the output of gages will not be changed as temperature changes. Therefore, the compensation of elastic modulus will be realized. This kind of strain gages is also called self-elastic modulus compensation strain gages.
The self-elastic modulus compensation strain gages perform the function of common gages and elastic modulus compensation resistor. It can also provide good correction of the sensitivity error of transducer that is caused by material of elastic modulus changes with temperature. If self-elastic modulus compensation strain gages are matched well spring materials, the temperature drift of transducer sensitivity will be less than 0.002%FS/℃ . Compared to common used methods, the selfelastic modulus compensation strain gages take the advantages of high accuracy in compensation, good stability, higher sensitivity, easier usage, lower cost and so on. However, the thermal output of strain gages only with self-elastic modulus compensation is a little bit larger, so zero temperature drift of transducers will be larger, which limits to further improve the precision of transducers. After many years research, we have developed and produced strain gages with self-temperature compensation and self-elastic modulus compensation that solve these problems. Especially for strain gages with half and full bridge. They have become very popular because of their good temperature capability.
Notes for using EXACT gages：
(1) In order to get satisfied compensation result, the elastic modulus compensation gages must be matched transducer spring materials. Generally, we should choose proper strain gages by testing at least five transducers.
(2)The gages have no functions of self-temperature compensation for most materials; their thermal output is larger than those of ordinary self-temperature compensation gages, therefore they are recommended to use for transducers with smaller temperature grads. It is better to adopt half-bridge or full-bridge gages to gain less zero-temperature drift.
(3)Soldering of elastic modulus compensation gages is more difficult than those of common gages. A special flux can be available from our factory. Carefully solder and clean them completely.