Gauge resistance in ohms expresses electrical resistance under free conditions at room temperature, unbonded as supplied.
The amount shown in the following equation is called the gauge factor. In this equation, 
indicates the strain generated due to uniaxial stress in the direction of the strain gauge axis.
R/R shows ratio of resistance change due to strain . This is generally indicated by specifying the Poisson's ratio of the test specimen used.
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k: gauge factor
R: gauge resistance
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This dimension represents the actual grid length in the sensitive direction.
The gauge also exhibits sensitivity in the direction perpendicular to the axial direction. The amount shown in the following equation due to the uniaxial strain (t) in the direction perpendicular to the gauge axis, and the resistance variation generated thereby, is called transverse sensitivity (kt).
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kt: gauge factor
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Generally, the ratio between transverse sensitivity and longitudinal sensitivity is called the transverse sensitivity ratio.
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The strain limit or allowable elongation percent depends on the properties of the wire or foil material, backing, and adhesive used. In general, the strain limit for a gauge with a short gauge length is slightly lower than that for one with a longer gauge length in the same series.
Fatigue life
When strain is repeatedly applied to the gauge, it causes increased resistance under zero strain, peeling-off the gauge, or disconnection, resulting in failure. The number of repeated cycles that the gauge can endure is called fatigue life. It is generally indicated by the repetition number under the specified conditions of strain amount and repetition speed as apparent strain drifts to 100 µm/m from the beginning. The fatigue life of TML gauges depends mainly on the properties of the backing material and adhesive used. This varies somewhat with the size and configuration of the grid. In general, larger gauges exhibit better fatigue performance. It is advisable to use foil gauges where maximum resistance to fatigue is required.
Self-temperature compensated gauges
The ambient temperature change may cause a variation of the strain gauge resistance. The amount of variation is subject to the thermal expansion of both the strain gauge material and the specimen, together with the thermal coefficient of resistance of the gauge material. Self-temperature compensated gauges are commonly used to minimize the apparent strain produced by changes in temperature. For optimum efficacy, such gauges should be used for the specified material.
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Temperature compensation range
This refers to the applicable temperature range for self-temperature compensated strain gauges. Compensation is accurate within approximately ±1.8µm/m/K. For greater accuracy, corrections can be made using the curves for apparent strain vs. temperature which are supplied with each package of gauges.
The temperature range listed in the "Normal" column of the selection is for stable static measurement. The "Short-Term or Special" column indicates the range for dynamic measurement, short term measurement or measurement without temperature change.
While the measured object is generating dynamic strain, the strain is transmitted to the sensing part via the bonding material and carrier backing. This transmission is done very quickly so as to avoid problems with usual mechanical vibration. Generally, in the quick strain phenomenon, the wavelength of the strain is short and the strain distribution in the gauge is not uniform, while the gauge measures the average strain in its active length. That my cause a problem. Thus, gauges with a shorter gauge length have higher response.
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For bonding on concrete surfaces, the gauge length should exaggregate approximately 3 times aggregate.
If the lead wire is extended with the quarter or half bridge method, an error will result due to apparent deterioration of the gauge factor because the resistance of the lead wire is in series with the gauge. Conversely, with the full bridge method, apparent deterioration of the gauge factor occurs since the resistance of the lead wires lowers the exciting voltage.
Typical connection and applicable lead wires
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Strain gauge and lead wire
| Strain gauge |
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| Parallel 2-wire preattached to quarter bridge |  |
| Parallel 3-wire preattached to quarter bridge with 3-wire |  |
Gauge factor correction due to lead wire
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R: nominal resistance of gauge r: total resistance per meter of lead wire (ohms/m) L: length of lead wire (m) |
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