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Ships machinery- Electrical and mechanical Tachometers

A number of speed measuring devices are in use utilising either mechanical or electrical principles in their operation.
Mechanical Tachometers
A simple portable device uses the governor principle to obtain a measurement of speed.



Two masses are fixed on leaf springs which are fastened to the driven shaft at one end and a sliding collar at the other (Figure below). The sliding collar, through a link mechanism, moves a pointer over a scale. As the driven shaft increases in speed the weights move out under centrifugal force, causing an axial movement of the sliding collar. This in turn moves the pointer to give a reading of speed.

Electrical Tachometers

The drag cup generator device uses an aluminium cup which is rotated in a laminated iron electromagnet stator (Figure below). The stator has two separate windings at right angles to eaeh other. An a.c. supply is provided to one winding and eddy currents are set up in the rotating aluminium cup. This results in an induced e.m.f. in the other stator winding which is proportional to the speed of rotation. The output voltage is measured on a voltmeter calibrated to read in units of speed.

mechnical tachometer
Fig: Drag cup generator type tachometer

Drag cup generator type tachometer
Fig: Drag cup generator type tachometer

Tachogenerators provide a voltage value which is proportional to the speed and may be a.c. or d.c. instruments. The d.c. tachogenerator is a small d.c, generator with a permanent field. The output voltage is proportional to speed and may be measured on a voltmeter calibrated in units of speed. The a.c. tachogenerator is a small brushless alternator with a rotating multi-pole permanent magnet. The output voltage is again measured by a voltmeter although the varying frequency will affect the accuracy of this instrument.

pickup tachometer
Fig: Pickup tachometer

Various pick-up devices can be used in conjunction with a digital counter to give a direct reading of speed. An inductive pick-up tachometer is shown in Figure (a). As the individual teeth pass the coil they induce an e.m.f. pulse which is appropriately modified and then fed to a digital counter. A capacitive pick-up tachometer is shown in Figure (b). As the rotating vane passes between the plates a capacitance change occurs in the form of a pulse. This is modified and then fed to the digital counter.



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 4-70C. 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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