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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]
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Advantages
- 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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A thermistor is type of resistor whose resistance varies significantly with the temperature. it is constructed of semi conductor material with a resistivity that is especially sensitive to temperature.
Working principle of
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Thermistor:
A thermistor is a type of resistor whose resistance varies significantly with temperature. Thermistors use internal electrodes that sense surrounding heat and measure it through electrical impulses. They also help to control heat to some extent, usually causing the device to which they are attached to heat up much more slowly than it normally would. They are made with semi-conductor materials using temperature-sensitive resistanceThe material used in thermistor is generally a ceramic or polymer. There are two kinds of thermistors: NTC (Negative Thermal Coefficient) and PTC (Positive Thermal Coefficient). With the former, the resistance of the thermistor decreases as the temperature increases, and with the latter, the resistance increases as the temperature rises.
The basic mathematical model used for thermistors is the Steinhart-Hart equation, discovered by oceanographers I.S. Steinhart and S.R. Hart. In its simplest form it is:
1/T = a + b(ln R) +c(ln R)3
where T is the = temperature, a,b , and c are = coefficients that are measured, ln is the natural log, and R is the R= resistance in ohms.Ohms.
Advantages:
1) They are cheap.
2) Stable
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RTD stands for Resistance Temperature Detector. RTDs are sometimes referred to generally as resistance thermometers. An RTD is a temperature sensor that measures temperature using the principle of the resistance of a metal changes with temperature.In practice a small electric current transmitted through a piece of a metal which are platinum ,copper or nickel(the rtd element or resisitor ) located in the area where temperature is to be measured.The resistance value of the RTD elements then measured by an instrument. This resistance value is then correlated to temperature.They are used in many industrial applications below 600 °C, due to higher accuracy and repeatability. Platinum is the most stable resistance-temperature relationship over the largest temperature range and best metal for RTDs because it follows a very linear resistance-temperature relationship and it follows the R vs T relationship in a highly repeatable manner over a wide temperature range and the .The unique properties of platinum make it the material of choice for temperature standards over the range of -272.5 °C to 961.78 °C, and is used in the sensors that define the International Temperature Standard. Platinum is chosen also because of its chemical inertness
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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