Strain guage

 

Strain guages

Strain gauges consist of a metal foil strip, flat length of metal wire, or a strip of semiconductor material which can be stuck onto surfaces like a postage stamp. When the wire, foil, strip, or semiconductor is stretched, its resistance R changes. The fractional change in resistance ΔR/R is proportional to the strain ε, i.e.: Δ R/R =G

where G, the constant of proportionality, is termed the gauge factor. Metal strain gauges typically have gauge factors of the order of 2.0. When such a strain gauge is stretched its resistance increases, and when compressed its resistance decreases. Strain is ‘change in length/original length’ and so the resistance change of a strain gauge is a measurement of the change in length of the gauge and hence the surface to which the strain gauge is attached. Thus a displacement sensor might be constructed by attaching strain gauges to a cantilever, the free end of the cantilever being moved as a result of the linear displacement being monitored. When the cantilever is bent, the electrical resistance strain gauges mounted on the element are strained and so give a resistance change which can be monitored and which is a measure of the displacement. 

With strain gauges mounted as shown in above picture, when the cantilever is deflected downwards the gauge on the upper surface is stretched and the gauge on the lower surface is compressed. Thus the gauge on the upper surface increases in resistance while that on the lower surface decreases. Typically, this type of sensor is used for linear displacements of the order of 1 mm to 30 mm, having a non-linearity error of about 61% of full range. A commercially available displacement sensor, based on the arrangement shown in, has the following in its specification: 

Range 0 to 100 mm

Non-linearity error 60.1% of full range

Temperature sensitivity 60.01% of full range/°C

 A problem that has to be overcome with strain gauges is that the resistance of the gauge changes when the temperature changes and so methods have to be used to compensate for such changes in order that the effects of temperature can be eliminated. 

General Purpose Precision strain gauges

General purpose precision strain gauges are encapsulated constantan foil strain gauges offered in a wide variety of patterns for scientific, industrial and experimental stress analysis. These precision strain gauges can be used for experimental stress analysis monitoring industrial equipment or various scientific applications. In the General purpose strain gauge section you will find the strain gauge patterns next to the part numbers so that you will be able to see the geometry of the strain gauge. The gauge dimensions are also provided in and SI (Metric, mm) and US Customary (English, inches) units. General purpose precision strain gauges are offered in linear patterns, dual parallel- grid patterns, Tee rosettes (0/90°), rectangular or delta (45° or 60°), stacked or planar rosettes, and shear patterns.

Transducer Quality strain gauges

Transducer-quality strain gauges are for customers who are manufacturing transducers or similar sensing devices. Transducer-quality strain gauges feature a tighter tolerance on the carrier trim dimensions which allows the carrier edge to be used for strain gauge alignment if required. They also feature tighter tolerances on nominal resistance values. These gauges can be creep adjusted to meet a transducer manufacturer’s specifications and they can be customized to the unique requirements of a transducer. They are also excellent gauges off-the- shelf for experimental stress analysis and/or strain verification projects.

STRAIN GAUGE SELECTION CONSIDERATIONS

• Gauges Length
• Number of Gauges in Gauge Pattern
• Arrangement of Gauges in Gauge Pattern
• Grid Resistance
• Strain-Sensitive Alloy
• Carrier Material
• Gauge Width
• Solder Tab Type
• Configuration of Solder Tab
• Availability