Department of Mechanical Engineering

MEMS1049

MechatronicsChapter 9

Temperature Sensors 9-4

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Temperature Temperature scales

Celsius (oC) Kelvin (K) Fahrenheit (oF) Rankine (oR)

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Temperature sensor

Liquid in glass temperature sensor

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Temperature sensor

Bimetallic strip

A bimetallic strip is used to convert a temperature change into mechanical displacement. The strip consists of two strips of different metals which expand at different rates as they are heated, usually steel and copper, or in some cases brass instead of copper.

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Temperature sensor

Bimetallic strip

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Temperature sensor

Electrical Resistance Thermometer

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Temperature sensor

Electrical Resistance Thermometer Resistance thermometer elements can be supplied which

function up to 1000 °C. The relation between temperature and resistance is given by the Callendar-Van Dusenequation

Here, RT is the resistance at temperature T, R0 is the resistance at 0 °C, and the constants (for an alpha=0.00385 platinum RTD) are

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Temperature sensor

Electrical Resistance Thermometer Resistance thermometers, also called resistance temperature

detectors or resistive thermal devices (RTDs), are temperature sensors that exploit the predictable change in electrical resistance of some materials with changing temperature.

Film thermometers have a layer of platinum on a substrate; the layer may be extremely thin, perhaps one micrometer.

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Temperature sensor

Electrical Resistance Thermometer

Wire-wound thermometers can have greater accuracy, especially for wide temperature ranges. The coil diameter provides a compromise between mechanical stability and allowing expansion of the wire to minimize strain and consequential drift.

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Temperature sensor

Electrical Resistance Thermometer

Coil elements have largely replaced wire-wound elements in industry. This design has a wire coil which can expand freely over temperature, held in place by some mechanical support which lets the coil keep its shape.

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Temperature sensor

Thermistor A thermistor is a type of resistor whose resistance

varies significantly with temperature, more so than in standard resistors.

NTC beadTemperature coefficient of resistance (TCR)

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Thermoelectricity Three thermoelectric effects

– The Seebeck, Peltier, and Thomson

The Seebeck Effect:– Temperature gradient sets up an electric field

that acts in opposite direction to the temperature gradient

– is the most frequently used, for example, in thermocouples for measurement of temperature difference∆V=α1 ·(Tcold-Thot) + α2 ·(Thot -Tcold)

= (α2 - α1)·(Thot -Tcold )α2 , α1 are the seeback coefficients of materials 1 and 2

– Thermocouples can be readily implemented on Si substrates using combinations of thin metal films or Poly-Si

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The Peltier Effect– A current flow across a junction of two dissimilar

materials causes a heat flux, thus cooling one side and heating the other

– is used to make thermoelectric coolers and refrigerators

Thermoelectricity The Seebeck Effect

– The effect is that a voltage, the thermoelectric EMF, is created in the presence of a temperature difference between two different metals or semiconductors. This causes a continuous current in the conductors if they form a complete loop. The voltage created is of the order of several microvolts per kelvin difference. One such combination, copper-constantan, has a Seebeckcoefficient of 41 microvolts per kelvin at room temperature

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Thermoelectricity

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Temperature sensor

Thermal couples Law of lead wire temperature

Law of intermediate lead wires

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Temperature sensor

Thermal couples Law of intermediate junction metal

Law of intermediate temperature

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Temperature sensor

Thermal couples Law of intermediate metal

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Temperature sensor

Thermal couples

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Temperature sensor

Thermal couples Attaching leadwires of selected metal

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Temperature sensor

Thermopiles: A thermopile is an electronic device that converts thermal energy into electrical energy. It is composed of several thermocouples connected usually in series or less commonly in parallel. Thermopiles do not respond to absolute temperature, but generate an output voltage proportional to a local temperature difference or temperature gradient.