Altimeter vs manifold pressure gauge

[moontownMooney]

I recently had to redo the settings on my JPI due to an unexplained factory reset (likely during the annual). To get the current local pressure at field elevation, in order to set the JPI's manifold pressure offset/calibration, I simply set my altimeter to zero altitude and read the pressure from the window. However, now my JPI is reading about an inch higher than my engine MP gauge. Thoughts? I would think my IFR certified altimeter would be more reliable than an MP gauge, but does this mean my MP gauge needs adjusted or am I missing something.

 

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[bdash]

You don't mention what model JPI engine monitor you have, but the manual for the EDM 830 says in Section 8 Programming Manifold Pressure (MAP) to set the MAP to the airport's current altimeter setting when you're doing the adjustment at a sea level airport. If you're not at sea level, they provide a table in which you look up the value to use based on airport altimeter setting and field elevation.

[carusoam]

Sounds like you may be missing something easy regarding proper calibration of your JPI...

What is your MP reading while the engine is off? (Ship’s MP gauge?)

does it match the nearest barometer? 

-My FBO has a barometer I can check against...

-plenty of weather stations nearby for current barometer readings...

- when you set the altimeter to the actual field elevation... what does the khoelsman’s window report? Does it match the barometer setting?  (I think barometers get calibrated based on the instrument’s elevation ASL...)

Setting the altimeter to zero seems to be adding some strangeness to the process... this will generate an offset that you will need to carry through every step of the process. Unless your plane is at Sea level.

All three should be pretty close. The FARs define how close the altimeter should be...

If the JPI MP is off, check your procedure again.

The old ship’s MP is probably the least calibrated device in the plane. Or on the planet...

Anything in prior flights not make sense compared to the POH?

Like Max MP at Sea Level?

Expect the old ships gauge to need help, or the calibration...

Expect the calibration steps of the JPI to have missed something...  (most likely)

Expect the altimeter went out of calibration... unlikely, but possible...

Or the local mechanics barometer is off... and he is using it to calibrate all other things...

Let me guess... your field elevation is about 1k’... the offset you are using is 2” but should be about 1”... to make sense? Something like that?

PP thoughts only not a mechanic...

Best regards,

-a-

[N231BN]

You need to set field elevation to get the sea level pressure reading and then subtract 1" for every 1000 ft of elevation to get actual pressure.

[Awful_Charlie]

RTFM!!! The JPI one! There's a table in there as it is *not* directly readable from the altimeter