Originally Posted by
India Four Two
Xray4277,
You make a valid point, except that boost pressure gauges are not "gauge pressure" devices. They are reading absolute pressure,
Sorry, still labouring my point I know, but boost indicator does NOT show absolute pressure. Not in the strict engineering sense.
Most pressure displays will have bar (g) or bar (a) (or psi, mm, inches, whatever system of units you are using) to differentiate between gauge pressure and absolute pressure, as defined in my earlier post. Actually, if you look at the two displays in Megan's post, the manifold pressure display is labelled with inches of mercury ABSOLUTE, which is why it is reading 29 inches or thereabouts at rest at sea level. The Spitfire boost gauge shows zero under the same conditions because it shows (engineering wise) GAUGE pressure, which knocks off the sea level pressure of around 14.7 psi (equivalent to 29 inched of mercury).