# Copper Strain Guage

My understanding is that strain gauges are constructed using Constantan etched onto a flex PCB. According to wikipedia Constantan is highly suitable strain gauge material due to its high gauge factor. I recently came across this document which shows that copper has a higher guage factor (2.6 compared to 2.1) than Constantan. If this is true why are strain gauges constructed with constantan and not copper??

What issues would arise from using copper to construct strain gauges?

• Compare the tempco of both materials – PlasmaHH Feb 17 '17 at 16:16
• Yes its appears constantan will be less effected by temperature fluctuation. With that said I would have assumed that temperature compensation would could simply implemented by having an unloaded strain gauge to bench mark against. – user3095420 Feb 17 '17 at 16:22
• why would you pay twice as much then (for not only the gauge material but the surrounding circuit too)? – PlasmaHH Feb 17 '17 at 16:23
• My idea was to potentially etch hundreds of strain guages straight onto a PCB panel to avoid many of the complexities associated with mounting strain gauges onto surfaces. However my PCB manufacturer does not support constantan on FR4. – user3095420 Feb 17 '17 at 16:30

The main reason for using Constantan (made from copper and nickel) is its temperature coefficient. Look at the following graph (it is in german, sorry) - taken from [1]. It shows specific resistivity and TempCo over nickel percentage.

Constantan has fairly high resistivity, which is good because you do not need so many squares to get to reasonable resistances like 120 Ohm. It also has a very low TempCo which you can even fine-tune during production to adapt to the thermal expansion of different material.

I did not look at Wikipedia, but if it says one wants high gauge factor metals for good strain gages, it is wrong. In fact you want a gage factor of approximately 2 because this means, that geometry effects dominate over electron mobility effects as the physical reason of the gage factor [2]. Electron mobility is heavily temperature dependent, therefore you don't want it. So a gage factor of two means low temperature dependancy, this holds for Constantan and NiCr (Karma and variants).

Beware: This is only true for metals. Semiconductors are a totally different story.

[2] G. Arlt, The sensitivity of strain gauges J. Appl. Phys., vol. 49, no. 7, p. 4273, 1978.

As @plasmahh mentions, temperature coefficient of the resistivity of Constantan (as the name of the alloy implies) is much less so errors due to temperature gradients would be much higher for copper.

Also consider the much higher resistivity which makes it easier to make a practical total resistance such as the standard 350$\Omega$. Lower resistance would have negative consequences for the system because you would need a higher current (more heating) and/or much higher gain (so more noise and drift in the electronics and from thermoelectric EMFs in the interconnections).