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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:
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.
Picture 1: Thermal output curve of the strain gages
Picture 2:Thermal output curve of the Constantan and
Karma self temperature compensation strain gages
Notes for using STC gages：
(1)At present, ZEMIC 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.