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Figure 1 : A Circuit to illustrate a thermistor[1] Figure 2 : Relationship between Temperate and Resistance in
in thermistor[2]
From Figure 1 above, it can be noted that when the thermistor warms up as a result of heat from the hair dryer, resistance of the thermistor drops which result in current flowing from the 9v potential to the 0v, this current makes way to the base of transistor which lights up the led connected to it.[1]
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Basically any material above absolute zero temperature radiates in the infrared even ice is known to emit infrared radiation.Sensing is carried out by collecting a small amount of energy (usually 0.0001 watt) which is a radiation(black body) from the target, and generates electrical signal that is amplified transformed into voltage output.[5]
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This type of temperature sensor works by converting a temperature change into mechanical displacement.It contains two jointed strip which differ in expansivity.It works on two basic principle, one which is that a metal would expand when heated and contract when cooled, another is that the two different metals would expand and contract at different rates.[6]
Figure 5: A simple bimetallic strip when heated and when cooled[7]
It should be noted that the metal with high thermal expansion coefficient is usually placed on the outer part, when heated it expands hence causing the inner metal to contract, this in effect would cause the shape of the strip to bend in a "C" shape.
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The sensing element in most thermometers is the liquid placed in between a scaled capillary tube which exploits the expansivity of such elements, for example mercury or alcohol, Mercury is usually preferred because of its versatility, it is a metal in liquid form, it requires only one calibration temperature and has a very stable coefficient of expansivity.[8]
REFERENCES
- V. Ryan. THE THERMISTOR : http://www.technologystudent.com/elec1/therm1.htm Last accessed 15-4-2014
- Kitronik Ltd . HOW A THERMISTOR WORKS : http://www.kitronik.co.uk/blog/how-a-thermistor-works Last accessed 15-4-2014
- HowStuffWorks Inc. THERMOCOUPLE : http://science.howstuffworks.com/thermocouple-info.htm Last accessed 15-4-2014
- Analogue Dialogue. Matthew Duff and Joseph Towey
http://www.analog.com/library/analogdialogue/archives/44-10/thermocouple.pdf Last accessed 15-4-2014 - Metris Instruments. INFRARED TUTORIAL : http://www.metrisinst.com/faqs-and-tutorials Last accessed 15-4-2014
- Wikipedia : http://en.wikipedia.org/wiki/Bimetallic_strip . Last accessed 15-4-2014
- Wikimedia common : http://commons.wikimedia.org/wiki/File:Bimetal.gif .Last accessed 15-4-2014
- Answers : http://wiki.answers.com/Q/Why_is_mercury_used_as_a_thermometric_fluid?#slide=1 Last accessed 15-4-2014
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