EXPANSION THERMOMETERS • EXPANSION OF SOLIDS • (BIMETAL STRIPS) • Bimetal strips consist of two different metal strips having different coefficient of expansion are bonding together. When the strip is subjected to a temperature higher than the bonding temperature, it will bend in one direction. When the strip is subjected to a temperature lower than the bonding temperature, it will bend in the other direction
This has the advantage of compact size while providing the long length of the strip, Bimetal thermometers are inexpensive, compact, and robust. Its working range is from –30 to 550 Cº. Metal used in manufacturing these thermometers are Invar, and Stainless steel, or yellow brass
EXPANSION OF LIQUIDS Liquid in Glass • Increase in temperature causes liquid to expand and rise up in the capillary tube, upon which are etched scale marks, Mercury is commonly used in these thermometers to expand in a glass capillary tube. The range of these thermometers is expanded from –37.8 (mercury freezing point ) to 315 Cº ±0.25 Cº The maximum limit may be extended to 510 Cº by filling the capillary tube with Nitrogen to increase the pressure on the Mercury and thereby increase its boiling temperature. Alcohol may be used instead of Mercury. It has a higher coefficient of expansion, but its boiling temperature is low. The range of the thermometers that use Alcohol is from –70 to 65 Cº ±0.5 Cº
This type of thermometers is not expensive, and it could be easily read portable. However, it is fragile, and its response is poor. Also it is not suitable for distance reading, and surface measurements.
Liquid in Metal • A metal bulb containing a liquid or gas or vapor (commonly mercury in stainless steel bulb) is immersed in the measured environment. The bulb is connected by means of capillary tube to a pressure-measuring device. An increase in the temperature causes the fluid inside the bulb to expand, thereby increasing the pressure indicating by the pressure gauge. This type of thermometers is inexpensive, robust, and could be arranged for remote reading (up to 65 m). The rang of these thermometers is –100 to 550 Cº ±1 Cº
TEMPERATURE ELECTRICAL TRANSDUCERS • RESISTANCE CHANGE THERMOMETRS • Electric resistance of materials changes as temperature changes. The resistance change thermometer consists of a sensor that exhibits a change in its resistance as the temperature change. This sensor is then interfaced with a whetstone bridge. Thereby the change of the resistance is converted to an output voltage, which may be then magnified and indicated on any display device. The output voltage is proportional to the temperature change and could be correlated to the measured temperature degree.
Resistance Thermometer Detector • (RTD) • The sensor of the RTD is a platinum or Nickel or Nickel-Copper alloy wire wounded on a ceramic bobbin and protected by a glass coating or by Stainless steel tube,
Platinum, Nickel, and Nickel-Copper alloy have a positive coefficient of resistively, and are stable. • The sensor is then interfaced to Whetstone bridge and the output voltage is taken as an indication of the measuring temperature.
Thermistor • Thermistors are fabricated from semi conducting materials, which are formed in the shape of a small bead, disc, or rod, Fig. 8.8. Thermistors exhibits a high negative coefficient of temperature, which is non linear but typically ten times that of platinum or Nickel. The applicable range of the thermistors is –70 to 250 Cº ±0.01 C. The thermistors is interfaced to Whetstone bridge and the output voltage is taken as an indication of the measuring temperature.
THERMO-COUPLEThermo-Electric Effect • If two dissimilar wires are connected together to form a closed circuit, and if the two junctions are at different temperature, small e.m.f. is generated between the two junction and causes a current to circulate, Fig.8.11.
For good repeatability of temperature measurement, the cold junction must be monitored at a constant temperature (reference junction). This may be done by immersing the junction in an ice-water mixture., Fig. 8.11. However this is not practical in industrial field. Instead the ambient temperature may be added or the thermocouple may be connected to a compensating circuit, Fig. 8. 13. The thermistor will produce a volt correspond to the ambient temperature. This volt is added to that produced by the thermocouple. Table 8.1 lists the different types of thermocouples and its applicable range.
TEMPERATURE MEASURED BY RADIATION. (PYROMETER) • OPTICAL PYROMETER • The radiation from the source is viewed through the lenses and the filter arrangements. The lamp is placed in the optical path of the incoming radiation. By adjusting the lamp current, the color of the filament may be made to match the color of the incoming radiation,Fig.8.15. The intensity of the current is taken as indication of the measuring temperature. The range of this pyrometer is from 650 to up
TEMPERATURE MEASURED BY CHANGE OF COLOR • Paints or crayons change color or appearance at a fixed temperature. It is used for welding, stress relief, heat treatment, heating of steel parts.