Converts one pressure to another, type a value in any field and hit tab. Compensation is made for altitude, PSIg, inches of mercury and PR are calculated at the given altitude (use 0 for sea level).
* - My Audi has a BAR gauge in the dash that displays this, read below.
** - These are the two common values you see on a mechanical boost gauge.
You have two common units (BAR and psig) for measuring boost pressure (I've tossed in a couple more just for fun).
An absolute scale, where 0.0 ATM is a total vacuum and 1.0 ATM is nominal sea-level atmospheric pressure. This measure is pretty useless and not widely used.
An absolute scale, where 0.0 BAR is a vacuum and 1.01325 BAR is sea-level. The BAR scale is very common, especially on European cars. A BAR gauge is almost always an electronic instrument driven by the MAP (Manifold Absolute Pressure) sensor in your ECU. The MAP sensor has an absolute reference, usually an aneroid capsule, which allows it to determine the actual ambient pressure just as a barometer does.
If you drive well above sea level and turn an electronic BAR gauge on without starting the engine, you should see a value well below 1 (say about 0.84 in Denver).
The Pascal is the standard metric unit of pressure, where 1 Pascal = 1 Newton/meter2. Since a Pascal is so puny (big area, small force), kiloPascals are the most commonly used metric scale, again using an absolute scale, with 0.0 being a vacuum and 101.325 being the standardized sea-level pressure.
Pounds per Square Inch (absolute), just English zero-based with 14.7 being nominal sea-level.
Pounds per Square Inch (gauge), the standard English pressure measurement, which is NOT absolute, but is relative to ambient atmospheric pressure. Most aftermarket mechanical boost gauges measure psig, so if you take this gauge to the moon and it will still read 0 when disconnected.
Inches of mercury are used in the US to measure barometric pressure and values below atmospheric pressure (i.e., when the psig value goes negative). The value shown here is relative to atmospheric, and is equivalent to what you see on your boost or vacuum gauge.
Pressure Ratio is a ratio of the given pressure relative to the ambient atmospheric pressure. This is important when you look at a turbocharger's compressor efficiency map. See the glossary entry.