Temperature is defined as, “the degree of hotness and coldness of a body or an environment measured on definite scale”. Also, Temperature can be defined as, “the driving force or potential causing flow of energy in the form of heat”. Temperature can be measured in various units, such as, Degree Fahrenheit, Degree Celsius,
CLASSIFICATION OF TEMPERATURE MEASURING INSTRUMENTS
Temperature Measuring Instruments may be classified,
1. According to the range of temperature measurement, or
2. According to the nature of change produced in the temperature sensing element.
Classification of Temperature Measuring Instruments
Mechanical Thermometers
1. Metallic expansion thermometers
(a) Expansion rod thermometer
(b) Bimetallic thermometer
2. Filled System thermometers
(a) Liquid filled thermometer
(b) Mercury filled thermometer
(c) Vapour pressure thermometer
Electrical Thermometers
- Metallic resistance thermometer (RTD)
- Thermistor
- ThermocoupleRadiation Pyrometers
- Total radiation pyrometer
- Optical pyrometer
NON-ELECTRICAL METHODS OF TEMPERATURE MEASUREMENT
Following are the instruments working on Non-electrical methods of measurement:
1. Bimetallic thermometers.
2. Liquid in glass thermometer (Pressure thermometer).
3. Liquid in metal thermometer (Pressure thermometer).
4. Vapour pressure thermometer.
ELECTRICAL METHODS OF TEMPERATURE MEASUREMENT
Temperature measuring devices working on electrical methods of temperature measurement are called as Electrical transducers. These Electrical transducers convert the measured temperature into an equivalent electrical signal, such as, resistance in RTD and thermistor, whereas, voltage in thermocouple. These methods are very convenient, as they furnish an electrical signal. Electrical signal has the advantage that it can be easily detected, amplified or used for control purposes. If temperature measuring devices working on electrical methods are properly calibrated, they give rise to highly accurate measurements. Methods used are of two types.
(A) Using conductors (metals), and (B) Using semiconductors.
Almost all metals used are of PTC type, whereas, semiconductors may be of PTC or NTC type. There are three types of temperature measuring instruments based on electrical concept.
(a) Resistance Thermometer (RTD), (b) Thermistor, (c) Thermocouple