Checking your MAP gauge

I have found several MAP (manifold absolute pressure) gauges that had errors, both the mechanical style as well as on avionics. Here's a way to check your MAP. The formula for pressure at a given altitude is PSL(1-6.88E-6 h)^5.256. Now don't get scared by raising to a power. Most math calculators can do this with the X^Y key. So take your field elevation h in feet, multiply it by 0.000,006,88, subtract the result from 1, then raise this value to the 5.256 power. Now obtain the surface barometric pressure and multiply it by this result. You now have the field barometric pressure.
EX: 1579' X 6.88E-6 = 0.01086. 1 - 0.01086 = 0.98914. 0.98914^5.256 = 0.94421. 30.03" X 0.94421 = 28.35".
When you look at your mechanical MAP gauge or turn on your avionics and let it warm up, this is the number you should get. Any other value is your gauge error. Now use your un-corrected MAP to determine your engine's induction pressure drop. Start your engine and warm it up. After it's warmed up, give it WOT and note the MAP. Subtract the MAP you saw with the engine off from this and you have your total induction pressure drop. If you have an MA3 or 4 carb, 1.3" to 1.5" is due to the carb drop. Anything else is due to duct, filter, and with FI, throttle-body loss. So now you have the MAP correction to your MAP indication, at least at the field baro, and your plumbing loss. (Plumb, from the Latin word for lead).

Another quicker way is to adjust your baro setting such that you have 0 (Zero) feet displayed as the altitude. Then read the baro setting in the window and it should be real close to your MAP setting with the engine off.

Vic

Also..

I did not trust the accuracy of the gauge even when it started from the correct number so when I had my airplane IFR certified I had the MAP checked, too.

I recently had a little issue with a UMA manifold pressure gauge. It's the electronic, pressure transducer version. When I powered the gauge it would wind around past the maximum 35" shown on the dial.

I called UMA tech support. At first they suggested that my wiring was wrong. Easy enough to imagine... After tracing wires in my harness and probing for continuity with a multimeter, I finally removed the transducer and gauge and bench-tested it. Same results.

I finally started reading off the voltage readings I was getting from the various wires to the UMA tech support person... At this point, he kinda gave me an "uh oh, I think I know what the problem is..." response. Apparently the gauge left the factory with a needle calibrated to a different scale than what was shown on the face of the instrument. He sheepishly offered to walk me through the process of taking the gauge apart and adjusting the potentiometer. He also gave me the option of sending it back to UMA for adjustment.

I'm pretty comfortable taking things apart, so I adjusted the gauge with his direction by using a 1.5 volt battery and some jeweler's screwdrivers.

I've been very happy with my UMA gauges and don't mean to suggest that there's anything chronically wrong with them. I only share this story to make others aware that there's always a possibility of a defective or miscalibrated instrument.

vic syracuse said:

Another quicker way is to adjust your baro setting such that you have 0 (Zero) feet displayed as the altitude. Then read the baro setting in the window and it should be real close to your MAP setting with the engine off.

Vic

Click to expand...

Hey Vic, I often get confused about this. Doesn't this method only apply if you're at sea level? Let's say I'm at 2,500 ft., field elevation (I really am)...shouldn't my altimeter be set to field elevation to get the actual barometric pressure, then I can make sure my MP pressure reads the same with the engine off?

lostpilot28 said:

Hey Vic, I often get confused about this. Doesn't this method only apply if you're at sea level? Let's say I'm at 2,500 ft., field elevation (I really am)...shouldn't my altimeter be set to field elevation to get the actual barometric pressure, then I can make sure my MP pressure reads the same with the engine off?

Click to expand...

Yes, one setting must be fact. If you know ALT, then you can get BARO. If you know BARO, you can get ALT. If you set it at 29.92, then you get pressure ALT. If you set it at "0" and you are not at sea level, then you get nothing useful.

I remember a co-worker who had just purchased a 3 gauge weather station and knowing that I was a pilot, asked me "what was the barometric pressure in this area?". I told him I would phone it in to him when I get to my hangar.....

Guys, this works every time. And I just verified it using the fancy formula in the earlier post.
Again, here are the steps:

Adjust your Altimeter so it reads ZERO feet.
Read the number in your Kollsman window (the baro number)
That number is what your MAP gauge should read with the engine off.

As an example, before I sent this I went and verified it. Field elevation here is 840 feet. The barometer today is 30.00. I set my altimeter to zero and the Kollsman (barometer) read 29.08. I looked at my MAP and it read 29.1. Close enough for me, since it only reads to one decimal place. Using the formula above on my HP 14B calculator, I came up with 29.09.

I worked avionics in the USAF many years ago, and this was how we quickly checked them on the flight line. Of course, with the old mechanical altimeters you couldn't be at too high of a field elevation because there wasn't enough room to wind the altimeter back down to zero. The digital altimeters have much wider adjustment range to them.

By the way, don't forget to reset your altimeter to filed elevation or the correct baro setting prior to takeoff.

Vic

Vic

Yes, I think I get it now. If you are going to make a pressure gauge that will read manifold pressure, it must have a base reading. So, I guess that they set it to "0" in a controlled environment so it is always reading from "0". If that is the case, then reading your altimeter kollsman window with your altimeter set to "0" would read the same as your MAP gauge.

Thanks Vic for the great tip.

Field Zero

I'm happy that you verified the formula. What you previously said about setting the altimeter to read zero works, as long as you're at a field that isn't too high. The highest you can be and use that method is a field at 2691' when the sea-level pressure is 31.0", which results in a setting of 28.10" on the Kollsman! See, that's the problem. A mechanical altimeter can only be corrected, at least mine, from 28.10" to 31.00". 'Don't know about the electronic variety. So for those people whose field is higher to where you can't zero your altimeter, use the formula!