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 Figure 2 : Relationship between Temperate and Resistance in thermistor
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.
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
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.
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.
Figure 4: The Infrared Field of View
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
to be continued.....
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http://www.metrisinst.com/faqs-and-tutorials
http://www.analog.com/library/analogdialogue/archives/44-10/thermocouple.pdf
http://science.howstuffworks.com/thermocouple-info.htm