How can the change in Thermal output be kept very close to zero?
Strain gauges are usually installed on a surface whether it is for stress analyses or sensor production, without any external forces applied to the surface. When environmental temperature changes, the resistance of the strain gauge changes accordingly. This phenomenon is called the strain gauges thermal output. This thermal output is the result of interactions and superposition of the resistance
temperature coefficient of grid materials, the sensitive grid materials and the linear expansion coefficient. The effects of these factors is described in the formula below:
In this formula αg and βg refer to the resistance temperature coefficient of the grid material and the linear expansion coefficient of the strain gauge. K refers to the strain gauges gauge-factor and
βs refers to the linear expansion coefficient of the tested object . Δt refers to the relative change in temperature of the tested subject and environment. Common strain gauges often have a large thermal output as shown in figure 1. Thermal output is the biggest source of errors in static strain measurements. When temperature increases the dispersion and therefore the thermal output value increases. Ideally the thermal output of strain gauges should be zero. To meet this requirement a self-temperature compensating strain gauge is used. In figure 2 the typical thermal output of a constantan and a self-temperature compensating karma strain gauge are displayed.
Figure 1: Thermal output curve of strain gauges on different steel materials
Figure 2: Thermal output for Self-Temperature compensated Karma and Constantan alloy strain gauges
By adjusting the composition ratio of the alloys of the strain gauge sensitivity grid material, and make use of cold rolling and proper heat treatment, the internal crystalline structure of the gauge material can be altered in a way it will compensate the changes of the material due to temperature change. This way the change in Thermal output can be kept very close to zero and the standards for highprecision sensors and stress analysis can be met. Note that the self-temperature compensation is only in a small temperature range from approximately + 20℃ up to +250℃ .