Thermometers, Thermocouple, Radiation pyrometer & Thermistor

Temperature measurement instruments Shipboard machinery must operate within certain desired parameters. Instrumentation enables the parameters—pressure, temperature, and so on—to be measured or displayed against a scale. Temperature measurement by instruments will give a value in degrees Celsius (°C). This scale of measurement is normally used for all readings and temperature values required except when dealing with theoretical calculations involving the gas laws, when absolute values are required Liquid-in-glass thermometer Various liquids are used in this type of instrument, depending upon the temperature range, e.g. mercury -35°C to +350°C, alcohol -80°C to 4-70°C. An increase in temperature causes the liquid to rise up the narrow glass stem and the reading is taken from a scale on the glass. High-temperature-measuring mercury liquid thermometers will have the space above the mercury filled with nitrogen under pressure. Liquid in metal thermometer The use of a metal bulb and capillary bourdon tube filled with liquid offers advantages of robustness and a wide temperature range. The use of mercury, for instance, provides a range from —39°C to +650°C. The bourdon tube may be spiral or helical and on increasing

temperature it tends to straighten. The free end movement is transmitted through linkages to a pointer moving over a scale.

Bimetallic strip thermometers A bimetallic strip is made up of two different metals firmly bonded together. When a temperature change occurs different amounts of expansion occur in the two metals, causing a bending or twisting of the strip. A helical coil of bimetallic material with one end fixed is used in one form of thermometer. The coiling or uncoiling of the helix with temperature change will cause movement of a pointer fitted to the free end of the bimetallic strip. The choice of metals for the strip will determine the range, which can be from — 30°C to +550°C.

Fig: Bimetallic strip

Thermocouple The thermocouple is a type of electrical thermometer. When two different metals are joined to form a closed circuit and exposed to different temperatures at their junction a current will flow which can be used to measure temperature. The arrangement used is shown in Figure, where extra wires or compensating leads are introduced to complete the circuit and include the indicator. As long as the two ends A and B are at the same temperature the thermoelectric effect is not influenced. The appropriate choice of metals will enable temperature ranges from ~200°C to +1400°C.

Fig: Thermocouple

Radiation pyrometer A pyrometer is generally considered to be a high-temperature measuring thermometer. In the optical, or disappearing filament, type shown in Figure, radiation from the heat source is directed into the unit. The current through a heated filament lamp is adjusted until, when viewed through the telescope, it seems to disappear. The radiation from the lamp and from the heat source are therefore the same. The current through the lamp is a measure of the temperature of the heat source, and the ammeter is calibrated in units of temperature. The absorption screen is used to absorb some of the radiant energy from the heat source and thus extend the measuring range of the instrument. The monochromatic filter produces single-color, usually red, light to simplify filament radiation matching.

Fig: Optical pyrometer

Thermistor This is a type of electrical thermometer which uses resistance change to measure temperature. The thermistor is a semi-conducting material made up of finely divided copper to which is added cobalt, nickel and manganese oxides. The mixture is formed under pressure into various shapes, such as beads or rods, depending upon the application. They are usually glass coated or placed under a thin metal cap. A change in temperature causes a fall in the thermistor resistance which can be measured in an electric circuit and a reading relating to temperature can be given. Their small size and high sensitivity are particular advantages. A range of measurement from — 250°G to + 1500°C is possible  

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