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

Thermal cutoff switch

Thermal cutoff switch 

       Thermal switch is an electromechanical device which opens and closes the contacts to control the flow of electrical current in response to temperature change. The term “Thermal Cutoff Switch” generally refers to how the switch is used, ie. It cuts off the current to critical machinery when a temperature limit is exceeded preventing potential burn out or failure.  The applications of, and need for electromechanical thermal switch devices are broad and cover a huge diversity of industrial applications. In their most basic form, they can be found on home appliances such as dryers for over temp protection.  Thermal cutoff switches are also used widely in sophisticated industrial equipment as well as commercial jetliners. There are a number of different technologies used to implement a thermal cutoff switch that rely on expanding elements to provide the movement to open or close contacts, including vapor filled, rod and tube, Bimetal, and  bimetallic disc to name a few.

Applications

        Thermal switches are specified as cutoff switches because they represent a straightforward approach to shutting down a system if a critical temperature is reached. The simplicity of an electromechanical thermal switch is what makes this approach so desirable to designers, as they are passive devices which require no power, and will reliably change state at the specified set point. 

Applications include:

a) Plastics extruder barrel overtemp detection.
b) Brake overtemp indication
c) Engine cooling fan control d) Clutch overtemp in escalators
e) Bleed air overtemp indication on aircraft environmental control systems f) Window defrost overtemp on military vehicles
g) Overtemp in refinery process
h) Avionics overtemp on aircraft avionics
i) Gas shut-off  flame detection on railroad switch de-icing
j) Flame detection in aircraft engines.