Which Connection Cables to Use for Electrical Foil Strain Gauge Applications Structural Tests/Durability Measurements

There are a lot of cables available in the market which may be used for strain gauge applications.

The success of a measurement depends on the right connection cables. Not only do they have to transfer the measurement signals from the sensor to the DAQ system, but they also have to avoid interference signals and resist stress during their use.

Ideally, the cable should not have influence on the strain measurement. In reality, however, cables/wires could have an influence on the measurement signal. The effects of the wires can be minimized to an acceptable level. HBM offers a wide choice of different measurement cables and small-scaled stranded wires for a wide range of applications.  There are some important points to consider when selecting the right cable for your application:

Multi-stranded wires with tin coatings are mostly used for strain gauge applications. Usually, copper conductors are used as wires (most common standard because of good price-to-conduction ratio).

The strain measurement signal in a quarter-bridge configuration is very sensitive:

• A typical excitation voltage for a strain gauge quarter bridge is 2.5 V
• Strain applied to the strain gauge creates a bridge voltage output that is relatively low!

(0.000125V for 100µm/m strain respectively 0.0025V for 2000µm/m strain). This is visualized in the graphs below for a typical quarter-bridge application.

This measurement voltage signal must not be interfered with by external signals. This is the reason why the right measurement cable is absolutely necessary!

Bridge voltage output signal with 100µm/m strain signal. The bridge excitation voltage (red) is very small in comparison to the excitation voltage (blue):

4 Environmental requirements on cables/wires:

The coating and the wire insulation influence the temperature range to which they can be exposed. The following diagram shows the typical temperature range of cables depending on their coating.

• For most applications, PVC-insulated cables are suitable; they offer an excellent price-to-performance ratio (up to 80 °C)

• For medium temperatures, TPE cables are a good choice (up to 150 °C)

• For higher temperatures, we recommend using PFA cables (up to 250 °C) or polyimide-coated cables (>300 °C)

• At low temperatures, standard cables can get brittle. This is especially critical when they are used in dynamic test environments. Using cables with a PTFE, PI, or glass fiber sheath is recommended in this case.

There are many more requirements on cables such as resistance against different fluids and flammability. The following table provides an overview of typical cable jackets/insulation materials and their temperature range:

5 Diameter of conductor:

• The diameter of the wire has a huge influence on the resistance. The excitation voltage of a strain gauge bridge generates a current which heats up the conductor. The smaller the diameter the higher the temperature increase on the wire.

• To minimize errors, the largest possible wire diameter shall be used to minimize the effect of lead-wire resistance in strain gauge applications and the thermal effects of the cable.

• In some applications, thin cables are necessary to reduce inertia/weight or allow for a small bending radius.

• Long sensor cables generally require bigger wire diameters.

A small-diameter cable shall be used at the strain gauge to reduce the solder amount and parasitic stress on the gauge.However, it must be taken into account that these thin wires affect the circuit stability and sensitivity.

• A solder terminal can be used as the point of intersection between the measurement cable and the connection wire of the strain gauge. This method enables a transition from a small-diameter cable to a cable with a thicker diameter:

Strain gauge connected with HBM's patented 4-wire technology:

• Generally, keep the length as short as possible to minimize thermal and electromagnetic interference.
• For long distances, choose bigger conductor diameters to keep the influence of resistance low.
• If signals are transmitted with high frequency and DC also a low-capacitance wire is recommended.

8 Cable protection around the strain gauge for harsh conditions

• Moisture-proof connection between the wire and the protective coating.  Therefore, maximum adhesion between the covering agent and the connection cable and the material surface is required
• Fluoropolymer cables shall be etched previously to allow proper sealing of the measurement cable
• Special cables with water-blocking tape are recommended for submersion under water (please contact the HBM Service Team)
• Ensure a minimum length of the protective agent around the cable access to maximize the creep distance and ensure that critical points are sealed