Oil pressure gauge

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Is the mechanical type of oil pressure gauge as used in 84 and earlier Mercedes of the Boudon Tube type? I suspect it is, just because that is a pretty common and reliable type of gauge. If it is, then is the gauge actually measuring air pressure? I have been in the throes of replacing the gauge on my '84 190E (it was leaky at the gauge) and the p.o. had solved the problem by plugging the tube at the oil filter housing. In this process, I note that the gauge is well above the level of the oil filter housing and believe that the oil pressure pushes into the tube, pressurizing the air in the tube which is what the Boudon tube measures. That holds true until the gauge fractures and pushes oil all the way out through the gauge into the instrument cluster. Anyway, can anyone explain what is going on if the above is not correct? Thanks! John in ID