Temperature Sensing
Conduction, convection and radiation are all involved in temperature sensing, for temperature to be measured or sensed, Temperature sensing however differs among different types of devices, some require close proximity to the measurement or detection point while some necessarily doesn't require proximity. Below is an illustration of how individual sensing device works to detect temperature change:
The Thermistor
This works basically by exploiting the a variation in resistance to detect temperature. The circuit below illustrates how a thermistor circuitry works
Figure 1 : A Circuit to illustrate a thermistor[1] Figure 2 : Relationship between Temperate and Resistance
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]
The Thermocouple:
The thermocouple senses temperature as a result of the combination of two dissimilar metals or alloy connected via a joint. Temperature sensing occurs at the point called the joint, the ends of both metals or alloys are connected to a voltmeter.
Figure 3: A Thermocouple Temperature measurement system (Iron constantan)[4]
The two junctions exhibits a difference in potential, which is indicated by the voltmeter, which is used to find the temperature of the heated junction, usually from a table in a handbook.The thermocouple can be used as a heat sensor in a gas appliance by connecting it junction to the pilot light, the current generated is sent through an electromagnet which opens a valve that allows gas passage to the appliance if current flow, and closes the valve when current doesn't flow hence blocking the passage of gas to the appliance.[3]
Infrared Temperature Sensor
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]
Figure 4: The Infrared Field of View[5]
When measuring temperature of small objects for instance ant, the temperature we get an average temperature ant and the adjacent surface because of the field of view (FOV). This indicated the field of view been an important factor when using infrared temperature device. Conclusively, it should be noted that temperature measurement can take place as long as the FOV is filled by the target.[5]
Bimetallic Strip
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.
Liquid in Glass Thermometer
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
