Page 114 - CITS - WCS - Mechanical

WORKSHOP SCIENCE - CITS

Absolute Pressure = External Air Pressure + Over Pressure

P = 1015 mbar + 100 mbar x 615 mm/750 mm
a
= 1835 mbar
P = 1.835 bar = 1.835 x 10 Pascal
5
a
If the barometer reading is in mm, it is always necessary to convert into m bar.

Pressure and Vacuum gauges
Bourdon tube pressure gauges (Fig 5)

Fig 5

In I.C. Engines, Bourdon tube pressure gauges are widely employed for measurement of pressure, temperature
and vacuum. In these gauges, a Bourdon tube which is a tempered, one is used and it attempts to straighten out
under pressure and temperature and contact under vacuum.
The working is briefly described here refer to figure. A phosphor bronze tube or elliptical cross section is used and
bent to an arc of a circle. The free end of the tube is sealed under internal pressure (or temperature), it attempts to
straighten out. During this process, it operates sector, pinion and needle which indicates pressure or temperature
over a calibrated dial. The entire unit is mounted on a casing and covered with glass and frame and around it.
Vacuum gauges
These are also of Bourdon tube type gauges where the tube attempts to contract under vacuum thus recording
vacuum of the engine in mm Hg (millimeters of mercury)
A reading of 760 mm Hg is prefect vacuum (zero absolute pressure)

A zero of say 300 mm Hg means to say that 300 mm of vacuum is equivalent to (760-300) 460 milliHg absolute
pressure.

• Vacuum gauges are often used by service mechanics to find out the mechanical condition of the engine and
whether valves, ignition timing and carburetor setting are correct and carry out fine adjustments to obtain the
best performance of the engine.
• Vacuum in Diesel Engine governors: This is measured by water column methods in fuel injection test bench

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CITS : WCS - Mechanical - Exercise 10 CITS : WCS - Mechanical - Exercise 10