Bonded, electrical resistance strain gage sensors require an excitation voltage for operation. The voltage drop across the gage resistance is measured before and after deformation to determine the strain value associated with that voltage change. Many bonded strain sensor grids have a small surface area [<4.5 mm2 (0.01 in2)]. The I2R heating caused by the excitation voltage can result in a resistance drift, which the instrument interprets as a change in strain level on the strain gage sensor. This self-heating is particularly evident on poor heat-sink materials, like plastics. Higher resistance values, like 350 and 1000 ohm, reduce self-heating and are especially helpful when testing on materials with low thermal conductivity. For stable, accurate, static strain measurements, it is best to keep the grid power density near 2-3 kW/m2 (1-2 W/in2). An often overlooked useful feature of strain gage sensor instrumentation is the ability to turn-off the excitation. This is helpful when troubleshooting noise in the strain gage circuit. If the instrument continues to indicate a reading other than zero when the excitation is removed, then the circuit is picking up noise. This noise can be caused by any number of common sources, like 60 Hz electrical hum, or high-power motors, transformers, and generators.