SELECTING LEADWIRE FOR STRAIN GAGE INSTALLATION
Because strain gages can be employed for data acquisition in a wide range of industries, the methods engineers use to install strain gages also vary widely. A strain gage used to determine the stress on an aircraft during flight will have different installation requirements than a strain gage used to measure torque in drag racing. As engineers, it is essential to understand which materials to use for the most effective strain gage installation.
What is a strain gage leadwire?
In order to connect the strain gage to the data acquisition device, engineers must use an extension leadwire. 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.
So, how do I know which leadwire to use with my strain gage?
Knowing which leadwire to use for your strain gage installation can be tricky. But, it is essential knowledge for accurate data acquisition. Here, our engineers break down the types of leadwires and how to use them.
Because of its low resistivity, solid copper is the most widely used conductor material for stain gage installation. 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 wires are preferred as the main leadwires in data acquisition systems that may be subjected to mechanical loads, cable movement, or other possible sources of damage. Strain gage engineers must be careful when removing insulation from stranded conductors in order to avoid damage to individual strands, which can cause premature fatigue failure and/or increased and unstable resistance. Exposed strands should be fused together with solder before being attached to any terminal. This ensures that loose strands will not result in unstable connection resistance.
We recommend the use of tinned, plated, or metal-clad solid and stranded copper conductors to improve corrosion resistance and elevated temperature capability. Clad copper conductors are superior to plated materials because of their mechanical and electrical integrity. 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 is serviceable to 1300°F (704°C) for dynamic readings.
Nickel Clad Silver
In more extreme or harsh environments for strain gage use, 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 Alloys
Nickel-chromium alloys are relatively stable as strain gage leadwire or ribbon to 900°F (482°C) and are serviceable up a maximum of 1800°F to 2000°F (982°C to 1093°C) for dynamic applications.
Alumel conductor leadwires — typically used in mineral-insulated, metal-jacketed cables — are stable to 2300°F (1260°C).
It is important to note, when choosing the proper leadwire for your strain gage installation, that the high resistivity of conductors such as Nichrome and Alumel can limit their use to short lengths in extreme environments. Connection to clad-copper conductors is recommended as soon as the mechanical and/or thermal environment will allow.
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