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Temperature

Temperature. Measuring devices. Types of devices. 4 basic types of temperature measuring devicesare used Each uses a different principle: Mechanical (liquid-in-glass thermometers, bimetallic strips, etc.). Thermojunctive (thermocouples).

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Temperature

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  1. Temperature Measuring devices

  2. Types of devices • 4 basic types of temperature measuring devicesare used • Each uses a different principle: • Mechanical (liquid-in-glass thermometers, bimetallic strips, etc.). • Thermojunctive (thermocouples). • Thermoresistive (RTDs and thermistors). • Radiative (infrared and optical pyrometers).

  3. Mechanical temperature measuring devices • Working Principle : • Temperature change causes some kind of mechanical motion as materials expand with a rise in temperature. • Mechanical thermometers make use liquids, solids, or even gases as the temperature-sensitive material. • The mechanical motion is read on a physical scale .

  4. Liquid-in-glass thermometer • The most common is liquid-in-glass thermometer. • Working Principle : Liquid expands as the temperature rises and the scale is calibrated to read temperature directly. • Make use of mercury or some kind of alcohol is used for the liquid

  5. Bimetallic strip thermometer • Two dissimilar metals are bonded together and is termed as bimetallic strip, as sketched. • If A metal has a smaller coefficient of thermal expansion than metal B , with temperature increase ; metal B expands more than does metal A . • It causes the bimetallic strip to curl upwards.

  6. Pressure thermometer • Mechanical thermometer that uses gas expansion instead of a liquid or solid. • Suppose the gas inside the bulb and tube can be considered an ideal gas. The ideal gas law isPV = mRT • R is a constant. The bulb and tube are of constant volume, so V is a constant. Also, the mass, m, of gas in the sealed bulb and tube must be constant. Hence, the above equation reduces to P = constant times T. • A pressure thermometer therefore measures temperature indirectly by measuring pressure.

  7. Typical Thermocouple Circuit

  8. Thermistors • A thermistor is similar to an RTD, but a semiconductor material is used instead of a metal. A thermistor is a solid state device. • A thermistor has larger sensitivity than does an RTD, but the resistance change with temperature is nonlinear. • Resistance of a thermistordecreases with increase in temperature. • Thermistors cannot be used to measure high temperatures either. The maximum temperature of operation is sometimes only 100 or 200 oC.

  9. Radiativetemp.measuring devices (radiativepyrometry) • Working Principle: • Radiative properties of an object change with temperature. • Advantages : • There is no physical contact with the object whose temperature is being measured. • Very high temperatures can be measured.

  10. The fundamental equation for radiation from a body is the Stefan-Boltzmann equation, where • E = emissive power radiated per unit area (units of W/m2). • It is defined as the fraction of blackbody radiation emitted by an actual surface and must lie between 0 and 1. • E is dimensionless. Its value depends greatly on the type of surface. A blackbody has an emissivity of exactly 1.

  11. Thank you Any Queries ?

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