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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.

Liquid in glass thermometer

Fig:Liquid in glass thermometer

Bimetalic strip thermometerFig:Bimetalic strip thermometer

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.


ThermocoupleFig:Thermocouple

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.


Optical pyrometerFig:Optical pyrometer

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-colour, usually red, light to simplify filament radiation matching.


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.



Ships instruments:

  1. Oxygen analyser working principle
    The measuring of oxygen content in an atmosphere is important, particularly when entering enclosed spaces. Also inert gas systems use exhaust gases which must be monitored to ensure that their oxygen content is below 5%. One type of instrument used to measure oxygen content utilises the fact that oxygen is attracted by a magnetic field, that is, it is paramagnetic....

  2. Temperature measurement instruments - use of Thermometers, Thermocouple, Radiation pyrometer & Thermistor

    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.....

  3. Pressure measurement instruments - U-tube Manometer, Mercury Barometer & Aneroid Barometer
    Various liquids are used in this type of instrument, depending upon the temperature range, e.g. mercury -35 deg C to +350 deg C, alcohol -80 degC 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.....

  4. Level measurement by Pneumatic gauge
    This is a device which uses a mercury manometer in conjunction with a hemispherical bell and piping to measure tank level. The arrangement is shown in Figure. A hemispherical bell is fitted near the bottom of the tank and connected by small bore piping to the mercury manometer. A selector cock enables one manometer to be connected to a number of tanks, usually a pair....

  5. Salinometer -How to determine water purity ?
    Water purity, in terms of the absence of salts, is essential where it is to be used as boiler feed. Pure water has a high resistance to the flow of electricity whereas salt water has a high electrical conductivity. A measure of conductivity, in Siemens, is a measure of purity.....

  6. Oil in water monitor
    Current regulations with respect to the discharge of oily water from ships set limits of concentration 15 parts per million. A monitor is required in order to measure these values and provide both continuous records and an alarm where the permitted level is exceeded.....

  7. Viscosity control of marine fuels
    Viscosity control of fuels is essential if correct atomisation and combustion is to take place. Increasing the temperature of a fuel will reduce its viscosity, and vice-versa. As a result of the varying properties of marine fuels, often within one tank, actual viscosity must be continuously measured and then corrected by temperature adjustment. ....

  8. Flow measurement
    Flow measurement can be quantity measurement, where the amount of liquid which has passed in a particular time is given, or a flow velocity which, when multiplied by the pipe area, will give a rate of flow.....

  9. Moving coil meter
    Electrical measurements of current or voltage are usually made by a moving coil meter. The meter construction is the same for each but its arrangement in the circuit is different.....

  10. Electrical and mechanical Tachometers
    A number of speed measuring devices are in use utilising either mechanical or electrical principles in their operation.....

  11. The measurement of torsion -Torsionmeters
    The measurement of torsion is usually made by electrical means. The twisting or torsion of a rotating shaft can be measured in a number of different ways to give a value of applied torque. Shaft power can then be calculated by multiplying the torque by the rotational speed of the shaft....

  12. Viscosity control of marine fuels
    Viscosity control of fuels is essential if correct atomisation and combustion is to take place. Increasing the temperature of a fuel will reduce its viscosity, and vice-versa. As a result of the varying properties of marine fuels, often within one tank, actual viscosity must be continuously measured and then corrected by temperature adjustment.....




Marine machineries - Useful tags

Marine diesel engines ||Steam generating plant ||Air conditioning system ||Compressed air ||Marine batteries ||Cargo refrigeration ||Centrifugal pump ||Various coolers ||Emergency power supply ||Exhaust gas heat exchangers ||Feed system ||Feed extraction pump || Flow measurement || Four stroke engines || Fuel injector || Fuel oil system || Fuel oil treatment ||Gearboxes || Governor || Marine incinerator || Lub oil filters || MAN B&W engine || Marine condensers || Oily water separator || Overspeed protection devices || Piston & piston rings || Crankshaft deflection || Marine pumps || Various refrigerants || Sewage treatment plant || Propellers || Power Plants || Starting air system || Steam turbines || Steering gear || Sulzer engine || Turbine gearing || Turbochargers || Two stroke engines || UMS operations || Drydocking & major repairs || Critical machinery || Deck machineries & cargo gears || Control and instrumentation ||Fire protection ||Engine room safety ||






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