Al,

Hmmmm, Further explanation would be helpful here.  I thought that the

altimeter was actually reading the air column weight or pressure at its

location.  If you set the altimeter to the field elevation, it seems it

should read the barometric pressure in the window

 

Gees; I was hoping you wouldn’t ask – then I wouldn’t have to think about itJ.

 

Well; it goes something like this; your altimeter setting is the barometric pressure of the station as if the station were at sea level. So the altimeter setting is barometric pressure with an altitude correction applied.  Your MAP is absolute ambient pressure.

Absolute pressure is measurement relative to a perfect vacuum, as in outer space. A zero reading occurs when the pressure is reduced to near perfect vacuum conditions. The ambient atmospheric pressure at sea level is approximately 29.92 inches of mercury, or 14.7 pounds per square inch, or 760 mm of mercury. As you go up the ambient atmospheric pressure decreases and your Absolute pressure gauge (MAP) reading also decreases.

 

I have a chart of the barometric vs ambient pressure, but I don’t know where I got it. Anybody know?  I’ll check the internet and see if I can find something; or I could scan it and send it.

 

Al G