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Figure: Working principle of bi-metallic Strip, Source:http://www.electronics-tutorials.ws/io/io_3.html [4]
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Figure: Expansivity values for some metals in units of K*-1 X 0*-5, Source: http://sensorsandtransducers.wordpress.com/2012/02/06/bimetallic-strip/[6]
There are two main types of bi-metallic strips, which change their expansion according to temperature changes of the object. These are “snap-action” and “creep-action”. The “snap-action” types give instantaneous action “ON/OFF” or “OFF/ON” on the electrical contacts at a set temperature point, and the slower “creep-action” types that gradually change their position as the temperature changes.
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Types of Thermocouple
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Source: http://www.thomasnet.com/articles/automation-electronics/Thermocouples-Types[3]
Advantage
- It is simple and rugged in construction
- It can measure wide range of temperatures up to 2600°C
- Fast Response
- Inexpensive
- Calibration can be checked easily
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The relative change in resistance (temperature coefficient of resistance) varies only slightly over the useful range of the sensor. The R vs T relationship is defined as the amount of resistance change of the sensor per degree of temperature change. Then estimate of RTD sensitivity can be noted from typical values of the linear fractional change in resistance with temperature. For platinum, this number is typically on the order of 0.004/°C, and for nickel a typical value is 0.005/°C. Thus, with platinum, for example, a change of only 0.4 W would be expected for a 100-W RTD if the temperature is changed by 1°C. Usually, a specification will provide calibration information either as a graph of resistance versus temperature or as a table of values from which the sensitivity can be determined. For the same materials, however, this number is relatively constant because it is a function of resistivity.
References
Curtis [1]Curtis D. Johnson, Process Control Instrumentation Technology, Resistance Temperature Detectors, 2006. http://zone.ni.com/devzone/cda/ph/p/id/208#toc3
The [2]The Michigan Chemical Process Dynamics&Control, 2006. https://controls.engin.umich.edu/wiki/index.php/TemperatureSensors
[3]Thomasnet, Types of Thermocouples, 2014. http://www.thomasnet.com/articles/automation-electronics/Thermocouples-Types
[4]Wayne Storr, Basic Electronics Tutorials. http://www.electronics-tutorials.ws/io/io_3.html
[5] Web material, Sensors and Transducers. http://sensorsandtransducers.wordpress.com/2012/02/06/bimetallic-strip/
[6]Web material, Temperature Sensor- The Thermistor. http://www.facstaff.bucknell.edu/mastascu/elessonshtml/Sensors/TempR.html