Instrument measuring temperature may be divided into two general groups.
1.Thermometer - Instruments inserted in or attached to the substances or body that is to be measured.Thermometer depend on heat conduction or convection from the body of instrument.
2.Pyrometer - Instrument located at a distance from the substance or body that is to be measured.
Pyrometers measure radiated heat and infer temperature from measurement.
Temperature is indicated by the changes it causes in certain instruments.
Changes in density - Liquid in Glass thermometer
Changes in length or volume - Bimetallic element,Filled system
Voltage generated at the junction of two dissimilar metals - Thermocouple
Resonant Frequency of Crystals - Quartz thermometer
Changes in Electrical Resistance - RTD
Two Kinds of Pyrometers
1.Radiation Pyrometer - which senses the intensity of radiation emitted
2.Optical Pyrometer - which senses the brightness of radiation emitted