in. hg?

Inches of mercury - a unit of measurement for pressure.
Does anyone know why in the SC the vacuum half of the gauge is in inches Hg and the boost half is in psi? Convention?
 
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It's just convention.

You can use either unit of measure for air pressure, inches of mercury or pounds per square inch. It's handy for our gauges that a range of 0 to 30 in of Hg (left side) is equivalent to a range of 0 to 15 psi (right side) so the gauge responds the same going either direction. You could label the vaccuum side from 15 psi (top) to 0 psi (far left) if you want to measure vaccuum in psi meaning absolute pressure because normal atmospheric pressure is about 14.7 psi at sea level. You can see this could be messy because then straight up would be 0 psi boost and 15 psi vaccuum or I guess you could label the left side as 0 to negative 15 (-15) which is equally confusing. It's much more conventional to use in of Hg for vaccuum.

For those who are interested, in of Hg means the height of a column of pure Mercury in inches in an evacuated tube caused by atmospheric pressure pushing the Mercury up the tube from an open cistern. I guess my Meteorology background is showing.

Jon
 
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