actually it's not metric (sorry) \:\(

There are two scales for pressure in the metric system: small - pascals as Ed pointed out (newtons per square meter) and very small - dynes (centimeter gram seconds per square centimeter).

Bar and Torr are used in meteorology to measure barometric pressure. One bar is roughly equivalent to one atmosphere.

The slight differences we see in how the readings from these bar(g)/atm graduated gauges are interpretted have to do with the way users of the English System round off (14.6956 psi is called 14.7) versus the way Metric System users round off (101.325 kiloPascals is called 100).

In the end such 'bar-gauge' readings are basically measuring system neutral and represent the pressures 'above' one atmosphere at sea level - since the gauge reads 0 in the open air.

I agree that the metric system is alot easier for calcuations - but having grown up and used the English system all my life it's a more comfortable and familiar perspective for us yanks.

If what you are saying is that a 'bar' graduated gauge is 'rounded' for the benefit of a metric system user - whereas an 'atm' graduated gauge is 'rounded' for the benefit of an english system user then I agree. But this distinction just determines which 'tick' marks get a round number on the face of the dial. The basic scale is the pressure of the atmosphere at sea level which is neither english or metric - hence the easy translation to compression ratio.

[Linked Image from 49fastback.com]