[andrew phillips]

I was calibrating my manifold pressure sensor the other day and two different people gave me 2 different ways. Which is the right one?

In hangar, engine off.

1. set altimeter to ground elevation (420') and use the baro displayed as the current manifold pressure

2. set altimeter to zero feet and use the baro displayed as the current manifold pressure

 

[Kevin Horton]

I was calibrating my manifold pressure sensor the other day and two different people gave me 2 different ways. Which is the right one?

In hangar, engine off.

1. set altimeter to ground elevation (420') and use the baro displayed as the current manifold pressure

2. set altimeter to zero feet and use the baro displayed as the current manifold pressure

Option 2 should give the correct answer. I'm trying to puzzle out why it works, but it gave the correct answer in the two cases I ran the math for.

Of course this only works for those of us lucky enough to be close enough to sea level so the altimeter can be wound to zero before the altimeter setting hits its limit.

 

[Radomir]

I think this is similar/same to your option 2, but works around the problem Kevin mentioned -- Take current altimeter setting, then subtract 1" for every 1,000 ft of field elevation... that's your manifold pressure.

Sample math:

Current altimeter: 29.86
Field elevation: 1,480 ft

MAP: 29.86 - 1.48 = 28.38 (will show as 28.4 on an instrument with one decimal place)

 

[Kevin Horton]

I think this is similar/same to your option 2, but works around the problem Kevin mentioned -- Take current altimeter setting, then subtract 1" for every 1,000 ft of field elevation... that's your manifold pressure.

Sample math:

Current altimeter: 29.86
Field elevation: 1,480 ft

MAP: 29.86 - 1.48 = 28.38 (will show as 28.4 on an instrument with one decimal place)

Yeah, that is probably accurate within perhaps 0.15 inches over the range of values of interest.

Alternatively, I threw together an Excel spreadsheet to calculate ambient pressure given altimeter reading and altimeter setting (zipped version here, in case the excel file gets scrambled somehow).