Leadwire Selection Considerations
Different strain gage installation conditions and test specifications often necessitate the use of different types or sizes of leadwires. For accurate, reliable strain measurements, it is important to use an appropriate type of leadwire for each installation.
Solid Copper: Copper, because of its low resistivity, is the most widely used conductor material. In a Wheatstone bridge circuit, it is desirable to reduce leadwire resistance in order to minimize leadwire desensitization. The downside of copper is that it has a large temperature coefficient of resistance, is strain sensitive (gage factor is nominally 2.0), and has poor fatigue life and corrosion resistance. When a solid copper wire or ribbon is used as an intermediate lead connection between a bondable terminal and strain gage solder pad, a strain relief loop must be provided to avoid premature failure of the jumper lead.
Stranded Copper: Stranded copper wires are preferred as the main leadwires in systems that may be subjected to mechanical loads, cable movement, or other possible sources of damage. Care must be exercised when removing insulation from stranded conductors to avoid damage to individual strands, which will cause premature fatigue failure and / or increased and unstable resistance. Exposed strands should be fused together with solder before attachment to any terminal so that loose strands will not result in unstable connection resistance or create a short or path to ground.
Clad Copper: The use of tinned, plated, or metal-clad solid and stranded copper conductors is recommended to improve corrosion resistance and elevated temperature capability. Clad conductors are preferred to plated materials because their mechanical and electrical integrity is superior. Nickel-clad copper is considered stable to 698°F (370°C) for strain gage applications and for limited service to 995°F (535°C). Stainless steel clad copper wire is stable to 800°F (427°C) and serviceable to 1300°F (704°C) for dynamic readings.
NickelClad Silver: Nickel-clad silver wire has been employed for stable service to 1000°F (538°C) and limited service to 1500°F (816°C).
*Nichrome or Karma: Nickel-chromium alloys are relatively stable as leadwire or ribbon to 900°F (482°C), and serviceable up a maximum of 1800°F to 2000°F (982°C to 1093°C) for dynamic applications.
*Alumel: Alumel conductors — typically used in mineral-insulated, metal-jacketed cables — are stable to 2300°F (1260°C).
*NOTE: The high resistivity of conductors such as Nichrome and Alumel limits their use to short length in the extreme environment. Connection to clad-copper conductors is recommended as soon as the mechanical and/or thermal environment will allow.
This information was compiled from several sources including: The Strain Gage User’s Handbook; Richard L. Hannah paper on Strain Gage Connection; ARI Industries; Personal experience