A strain gauge works by changing its resistance when it is stretched, but exactly how much resistance change you get for a given amount of stretch is set by one number: the gauge factor. It is the sensitivity spec that tells you how a raw strain translates into a resistance change, and therefore into the voltage a SCADA system eventually reads. This page is about that number rather than the sensor device: what gauge factor means, why metal foil gauges sit around a value of two while semiconductor gauges are far higher, and how the figure lets you scale a raw microstrain signal into engineering units. It is aimed at anyone who needs to turn a strain gauge's output into a real pressure or load reading.
Gauge factor in one line: Gauge factor is the ratio of a strain gauge's fractional change in resistance to the strain that caused it - in other words, how much the resistance changes per unit of stretch. A metal foil gauge has a gauge factor of roughly two, meaning its fractional resistance change is about twice the strain, while semiconductor gauges have much higher gauge factors and therefore much greater sensitivity.
Gauge factor, often written GF, is defined as the fractional change in resistance divided by the strain. Strain is itself a fractional change - the change in length divided by the original length - and it is dimensionless, usually reported in microstrain, where one microstrain is a length change of one part per million. Gauge factor is likewise dimensionless because it is one fraction divided by another. A gauge factor of two says that if you strain the gauge by some small amount, its resistance changes fractionally by twice that amount.
The number matters because a strain gauge does not report strain directly; it reports resistance. To recover the strain that a structure actually experienced, you have to invert the relationship, and gauge factor is the constant that lets you do it. Measure the fractional resistance change, divide by the gauge factor, and you have the strain. Every conversion from a gauge's electrical output back to a mechanical quantity runs through this constant, which is why it appears prominently on any strain gauge's specification.
Because the resistance changes involved are tiny - a foil gauge might change its resistance by a fraction of a percent at working strains - a strain gauge is almost always wired into a Wheatstone bridge that turns that small resistance change into a measurable voltage. The gauge factor sets how large the resistance change is for a given strain, and the bridge and its excitation set how that resistance change becomes a voltage. The two work together, but the gauge factor is the part that belongs to the gauge itself.
In a metal foil gauge, the resistance change comes mostly from geometry. When you stretch a fine metal conductor, it gets longer and thinner, and both changes raise its resistance - a longer, narrower wire resists current more. That geometric effect alone accounts for a gauge factor of roughly two in common foil gauges, with the exact value depending on the alloy. There is a small additional contribution from the metal's resistivity changing under strain, but for foil gauges the geometry dominates, which is why their gauge factor clusters near two across many products.
Semiconductor gauges work on a fundamentally different mechanism and reach far higher gauge factors as a result. In silicon, straining the crystal changes its resistivity strongly through the piezoresistive effect - the material's intrinsic resistance to current shifts with stress, quite apart from any change in shape. That resistivity change dwarfs the geometric contribution, so a semiconductor gauge can have a gauge factor many tens of times that of foil. The payoff is much greater sensitivity: the same strain produces a far larger resistance change and thus a far larger signal.
That extra sensitivity comes with tradeoffs. Semiconductor gauges are more sensitive to temperature and their response is more nonlinear than foil, so they demand more careful compensation. The choice between foil and semiconductor is therefore a choice about how much sensitivity you need versus how much compensation you are willing to do. It is also why the gauge factor is never quoted alone on a serious datasheet - it comes with a temperature coefficient and often a tolerance, because the sensitivity it describes is only meaningful alongside the conditions under which it holds.
For a SCADA installation, gauge factor is the link between the electrical signal that arrives and the physical quantity an operator wants to see. A strain-gauge pressure sensor or load cell delivers a bridge output that traces back through the gauge factor to the strain in a sensing element, and that strain in turn corresponds to the pressure or force applied. The instrument or transmitter uses the gauge factor - baked into its calibration - to convert the raw resistance change into a strain, and then a mechanical relationship converts strain into the engineering unit. Get the gauge factor wrong and every reading is scaled wrong by the same proportion.
In practice the raw conversion is handled close to the sensor, in a strain indicator or a smart transmitter that already knows the gauge factor and outputs a scaled signal such as 4-20 mA or a digital value. A cloud SCADA platform such as Merobix then applies the sensor's overall calibration to present pressure, weight, or load in the units the operator needs, without anyone having to see the microstrain underneath. Still, knowing that the gauge factor sits inside that chain makes calibration and troubleshooting far clearer, because a reading that is proportionally off across the whole range points to a sensitivity or gauge-factor error rather than a zero problem.
The distinction between a scaling error and a zero error is exactly the kind of thing gauge factor helps diagnose. A wrong zero shifts every reading up or down by a constant, while a wrong gauge factor or sensitivity multiplies every reading by a constant, so a sensor that reads correctly at zero load but drifts progressively further off as load increases is showing a sensitivity problem tied to the gauge's scaling. Recognizing that the gauge factor governs the slope of the response, not its offset, turns a confusing calibration into a targeted fix.
A metal foil strain gauge typically has a gauge factor of about two, meaning its fractional resistance change is roughly twice the applied strain. Semiconductor strain gauges have much higher gauge factors - many tens - because their resistance changes through the piezoresistive effect rather than mainly through geometry. The exact value is listed on the gauge's datasheet along with a tolerance and a temperature coefficient.
Because they change resistance through a different mechanism. A foil gauge's resistance changes mostly because stretching makes the metal longer and thinner, a geometric effect that yields a gauge factor near two. A silicon semiconductor gauge changes resistivity strongly under stress through the piezoresistive effect, which dwarfs the geometric contribution and produces a far larger resistance change for the same strain, giving much higher sensitivity at the cost of more temperature sensitivity and nonlinearity.
You use the gauge factor to turn the measured fractional resistance change back into strain - fractional resistance change divided by gauge factor equals strain - and then a mechanical relationship converts that strain into pressure, force, or weight. In practice a strain indicator or smart transmitter does this conversion using the calibrated gauge factor and outputs a scaled signal, which a SCADA platform then presents in engineering units.
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