“Pressure” and “vacuum” are terms in everyday use in the automotive industry. Manufacturers recommend a pressure to which tires need to be inflated. And manifold vacuum is used to operate a brake booster.
Gases exert pressure on all bodies they make contact with. This applies also to the air in earth’s atmosphere. Air has mass, and as a result it exerts pressure, called atmospheric pressure, not only on the earth’s surface, but also on all objects on the earth’s surface.
Atmospheric pressure varies with altitude, but at sea level, it is calculated as 101.3 kilo-Newtons per square meter, or, 101.3 kilo-Pascals; or 14.7 pounds per square inch or PSI.
But if this is so, why does a pressure gauge read zero when its not in use? This is because the gauge indicates pressure above atmospheric pressure only. This reading is called gauge pressure.
If absolute pressure was needed, an Absolute Pressure Gauge would be required, and it would read 101.3 kilo-Pascals or 14.7 PSI when not in use.
Absolute pressure equals gauge pressure plus atmospheric pressure.
Readings on an oil pressure gauge, or a tire gauge, should really have 101.3 Kilo-Pascals or 14.7 PSI added to them. It is normal practice however, for the gauge reading alone to be taken as the accepted value.
If pressure being measured is below atmospheric, a pressure gauge with its zero reading is of no value. This is why, for pressures below atmospheric, as in a manifold vacuum or depression, a vacuum gauge is normally used.
In a gasoline engine, the position of the throttle-plate controls the volume of air, or air-fuel mixture, entering the manifold. At idle speed, the pistons draw air away from the manifold at a faster rate than it can pass the throttle-plate into the manifold. A high vacuum, or a depression, exists.
At wide-open throttle, depending on load, the vacuum is much less, and pressure in the manifold rises, closer to atmospheric.
A vacuum gauge can be calibrated in millimeters of mercury, and the scale reads from zero to 760, or inches of mercury in a scale reading from '0' to '30'.
The scale is derived from the fact that atmospheric pressure supports a column of mercury 760 millimeters or 30 inches high. This glass tube, closed at one end, is filled with mercury, then inverted in a bowl of mercury. The column of mercury in the tube falls to about 760 millimeters or 30 inches. The space above the mercury is a vacuum, and atmospheric pressure on the exposed surface of the mercury supports the column.
At sea level, atmospheric pressure supports a 30 inches or 760 millimeter column of mercury but at higher altitude, its height falls, which indicates that atmospheric pressure at that altitude is less.
A scale of zero to 760 millimeters or 0 to 30 inches can thus be used to indicate the degree of vacuum, or depression that exists, below atmospheric pressure.
At idle speeds, a vacuum gauge connected to the intake manifold will indicate a reading of approximately 450 to 500 millimeters or 17.5 to 19.5 inches of mercury, depending on altitude.
Some engine management systems signal changes in atmospheric pressure by using a barometric pressure sensor in the ECU. This is because, above sea level, air pressure is reduced. So to maintain correct air-fuel ratio, a vehicle has to reduce the amount of fuel delivered to the engine.