Manifold Pressure Indication

[clardner]

First a question and secondly a comment...

I just finished installing an EMS-D10 and noticed a 0.5" to 1.0" differential between the manifold pressure indication on the unit and ambient pressure. For example, today at 925 MSL and 30.7" ambient pressure, the EMS inicated 29.8" when the engine was off. Why is there such a difference and should there be a difference? Shouldn't the indication be equal to the local barometric pressure?

Secondly, I would like to commend you on a great product with an easy to follow installation manual. The manual was very easy to follow with no difficulties with the installation.

Thank you,
Chris Lardner

 

[dynonsupport]

Was 30.7" true ambient pressure, or was it the pressure you put in for "baro" on the EFIS?

The number ATIS reads out is the ambient pressure at sea level. You put this in the EFIS so that if the EFIS reads 30.7" on the static port, it shows 0" altitude. When the pressure is lower than what is set in the baro window, that means you are higher than sea level. Since you are at 925 feet, your pressure is lower than it is at sea level.

Pressure drops at about 0.1" per 100 feet. So you should be about 0.9" lower on your MAP than your baro setting. Sounds like you are dead on.

This is true of all MAP gauges, it's just hard to see 0.9" on most mechanical gauges