From a technical article at superstreetonline.com:
A turbocharged engine revolves around one central idea, and that is boost. Boost is defined as the increase in manifold pressure above atmospheric pressure. So what the hell does that mean?
Basically, boost is a gauge of the amount of charged air pressure produced by the turbocharger. As boost increases, more charged air is being force-fed into the motor and more horsepower is being made. Boost levels are usually measured in pounds per square inch (psi).
Full text of the article can be found
here.
Automotive boost gauges typically read 0 at atmospheric pressure, and boost is always measured
above atmospheric. This is even verified by the article you linked to above: "The BAR gauge in the Audi is an absolute pressure gauge and it takes into account the atmospheric pressure. It is an international unit of pressure that is used in most countries. It is equal to 14.5 PSIA or approximately 1 atmosphere. This is why our gauge read about 1 BAR when the engine is off at sea level.
...
Atmospheric pressure can also be ignored when it comes to BAR. Sometimes a european tuner will say the engine is boosted to... or is running 1.1 BAR of
boost. In this case he is referring to what we would call 15.9 PSIG boost. He could also say the engine runs at 2.11 BAR."
Notice that they are talking about Audi. Mercedes-Benz ignores atmospheric pressure, and starts the gauge at 0, not 1. Perhaps you will believe the Mercedes-Benz manual for the 300D. The procedure for checking boost pressure can be found
here.