Data Acquisition

28 Practical Data Acquisition for Instrumentation and Control SystemsIn addition to the wiring suggestions made above, it is important to consider isolationand over-voltage protection in the measurement circuitry, especially as a safeguard fromcharge buildup and other transient over-voltages on long thermocouple cables.2.7 Strain gaugesStrain gauges are the most widely used devices for the measurement of force, or moreparticularly strain resulting from force. The most common type of strain gauge is thebonded resistance strain gauge, which consists of a resistive material, usually metal film afew micrometers thick, bonded to a polyester backing plate. A typical strain gauge isshown in Figure 2.15.Figure 2.15Typical bonded resistance strain gaugeThe strain gauge operates on the principle that when strained, the length, cross-sectionalarea and resistivity of the metal film changes, thus changing the resistance of theconductor. When attached to a unit under test by an adhesive of some kind, the straingauge experiences the same strain as the unit. The amount of strain can be measured bydetecting changes in the resistance. Provided the change in length of the strain gauge issmall, the relationship between resistance and strain is linear.The ratio of the percentage change in resistance to the percentage change in length isknown as the ‘gauge factor’ (GF) and is a measure of the sensitivity of the gauge.GF∆R/ R0∆ρ/ ρ= = 1 + 2σ+∆L/ L0∆L/ L0Where:R 0 = resistance in ohmsρ = resistivity in ohms per meterL 0 = length in meters∆R/R 0 = fractional resistance changeσ = Poisson’s ratio∆L/L 0 = fractional change in length∆ρ/ρ = fractional change in resistivity