A pressure gauge with a scale of 0-20 PSI is actually reading 14.7 PSI(absolute) when at zero because its measuring the atmospheric pressure. The 0-20 scale is the guage pressure in PSIG.
Less than 14.7 PSI, 760mm HG , 1 BAR is a vacuum.
A guage showing 4.7 negative PSI is showing 10PSI absolute pressure which is equal to 524.13mm Hg.
Hg is a measurement of a column of mercury in a positive or negative pressure.
Basically what I am trying to say, rather badly, is that absolte pressure is a reference to absolute zero pressure...outerspace...no pressure at all. Gauge pressures are referenced to the zero point on the gauge which is 14.7 PSI above absolute pressure.
