How it works
An interesting feature of constantan is that its resistivity not only varies extremely little with temperature, but is capable of not only increasing but also decreasing with heating, depending on the composition of a particular batch of the alloy, its heat treatment and the temperature value. Typical temperature characteristics of constantan resistance and some others alloys, that are used for strain gage manufacturing, are shown on the figure:
- Constant alloy
- FeCrAl alloy
- Karma alloy
- Platinum-tungsten alloy
When a part with strain gages is heated, two processes occur simultaneously: thermal expansion of the part, causing a corresponding change in the length of the strain gage, which is glued to it, and temperature change in the resistivity of the alloy from which the sensing element is made. By selecting such alloy composition and its heat treatment (annealing) it is possible to minimize the temperature change of resistance of strain gages when they are glued to a certain material. This is called temperature compensation of strain gages. Temperature compensated strain gages are produced for different materials:
| Material | Coefficient of linear thermal expansion, 10⁻⁶°C⁻¹ |
|---|---|
| Titanium | 8.6 |
| Iron, forged | 11.3 |
| Steel | 13.0 |
| Austenitic stainless steel | 16.0 |
| Aluminum | 22.2 |
| Magnesium | 25.0 |
For more accurate compensation of the temperature effect of strain gage resistance changes on the readings of strain gage equipment in a wider temperature range, the so-called circuit compensation is used. For this purpose, two or four strain gages are glued on the part, which are in the same temperature conditions and connect them to the bridge circuit so that the effect on the output signal of the bridge due to the effect of deformation on different strain gages is summed up, and the temperature change in resistance, which is the same in all strain gages, is subtracted (compensated).
