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Note added at 6 minutes (2007-12-27 09:25:50 GMT)
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http://en.wikipedia.org/wiki/Bar_(unit)

In some countries, pressure is measured with reference to atmospheric pressure. This is gauge pressure and denoted by barg, often written with no spaces, spoken "bar gauge", and sometimes using symbols such as 'bar(g)'. For example, if someone says that their car tyres are pressurised to 2.3 bar they actually mean bar gauge: the pressure in the tyre is really 3.3 bar, but only 2.3 bar above atmospheric, which is the scale a tyre gauge would read. When absolute pressure is desired, it is sometimes denoted 'bara' or 'bar(a)' for "bar absolute". The alteration of units of measure for this purpose is now deprecated, with qualification of the physical property being preferred, e.g., "The gauge pressure is 2.3 bar; the absolute pressure is 3.3 bar".

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Note added at 2 heures (2007-12-27 12:13:44 GMT)
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Pardon c'est bar Jauge :
La pression en Bar Absolu se convertit d‘abord en BarG (”Bar jauge‘ œ pression effective) en
soustrayant 1,00 Bar Absolu de la lecture du capteur.
http://209.85.129.104/search?q=cache:S02WU2Wjg9kJ:www.analox...

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Note added at 3 heures (2007-12-27 13:12:46 GMT)
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également appelé bar effectif par opposition à bar absolu : http://www.capcomespace.net/dossiers/espace_europeen/ariane/...