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  #1  
Old 10-15-2020, 06:43 PM
Canadian_JOY Canadian_JOY is offline
 
Join Date: Aug 2007
Location: Ontario, Canada
Posts: 2,369
Default MAP Sensor Calibration and % Power

Reaching out to the brain trust to hopefully provoke a good discussion on this topic. I've just installed a GRT EIS-66R engine monitor; it features an internal manifold pressure (MAP) sensor. I've discovered this sensor cannot be calibrated in software as can be done with the stand-alone MAP sensor used with the EIS4000 product.

With this being the case I would like to touch off a discussion around how one should determine the magnitude of MAP sensor error which exists. We will have some "known" values such as airfield elevation and prevailing altimeter setting as well as reliable and recently-calibrated altimeter information.

Once the sensor error is known, how should one use this information to adjust engine settings to achieve a particular power setting?

The second question arises from my belief that MAP sensor error will ultimately result in incorrect % power readings. For those of us who like to do "apples to apples" performance comparisons it's important that we achieve a consistent baseline in power measurement.

Thanks in advance for sharing your expertise in this arena.
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  #2  
Old 10-15-2020, 07:09 PM
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rjcthree rjcthree is offline
 
Join Date: Jan 2007
Location: Bay Village, OH
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Default Use your altimeter

Dial your altimeter to zero feet of altitude. What’s read in the window for baro should match with the MAP reading from the EMS (non-draft hanger, engine not running). The delta is your calibration difference, assuming you trust your altimeter.
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  #3  
Old 10-16-2020, 02:55 PM
Canadian_JOY Canadian_JOY is offline
 
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Location: Ontario, Canada
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Default

Quote:
Originally Posted by rjcthree View Post
Dial your altimeter to zero feet of altitude. What’s read in the window for baro should match with the MAP reading from the EMS (non-draft hanger, engine not running). The delta is your calibration difference, assuming you trust your altimeter.
Thanks for your comment. I just completed full calibrations of all three altimeters (that was an expensive trip to the local avionics shop!) so I have very good faith in them and have used them to calibrate my pitot-static tester as well. At this point I'm pretty certain I have a solid static pressure reference in the aircraft. The GRT MAP sensor clearly is exhibiting an error - I will be moving forward with characterizing that error to see if it remains constant across time and temperature.
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  #4  
Old 10-16-2020, 02:57 PM
Canadian_JOY Canadian_JOY is offline
 
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Location: Ontario, Canada
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Default

Quote:
Originally Posted by RV8JD View Post
Adjust for airport elevation with this approximation: Set Field Elevation, note the value in the Kollsman window, and subtract 1" HG per 1000 ft above Sea Level from the Kollsman window value.

Or use this Station Pressure calculator, if you have a current Altimeter setting:
http://www.csgnetwork.com/stationpressurecalc.html
Then compare with the MAP reading from your EIS.
Thanks for the link to that station pressure calculator, Carl. That will provide an excellent cross-reference against which to sanity check my test data.
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  #5  
Old 10-16-2020, 03:25 PM
BobTurner BobTurner is offline
 
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Location: Livermore, CA
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Default

1. Disconnect MAP sensor, leave open end of hose inside the cabin. Plug the hole in the intake manifold.
2. Use alt air for altimeter (if not available disconnect static input so altimeter sees cabin pressure).
3. Set altimeter to 29.92”
4. Go fly. Record altimeter readings and MP readings at various altitudes.
5. Land. Look up chart of pressure alt vs “Hg. Convert pressure alt data to in of Hg, compare to MAP data.

If you have a modest vacuum pump available you can do this on the ground, by plumbing the static line and MAP line into a T, then to the pump.
You could also call out the pitot static test guy again, have him calibrate the MAP.
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  #6  
Old 10-16-2020, 08:56 PM
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Foghorn Foghorn is offline
 
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Default

Just curious but did you reach out to GRT and did they have a solution? I have the same setup with the EIS-66R but haven’t done any avionics testing yet.
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  #7  
Old 10-16-2020, 11:04 PM
Canadian_JOY Canadian_JOY is offline
 
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Quote:
Originally Posted by BobTurner View Post
1. Disconnect MAP sensor, leave open end of hose inside the cabin. Plug the hole in the intake manifold.
2. Use alt air for altimeter (if not available disconnect static input so altimeter sees cabin pressure).
3. Set altimeter to 29.92”
4. Go fly. Record altimeter readings and MP readings at various altitudes.
5. Land. Look up chart of pressure alt vs “Hg. Convert pressure alt data to in of Hg, compare to MAP data.

If you have a modest vacuum pump available you can do this on the ground, by plumbing the static line and MAP line into a T, then to the pump.
You could also call out the pitot static test guy again, have him calibrate the MAP.
Bob - since I have a pitot-static tester which I have calibrated against my recent triple altimeter calibration, which in turn was done by an avionics shop with a pitot-static tester calibrated to a traceable source, I think I can rely on the accuracy of both the altimeters in the aircraft as well as my pitot-static test set. As I have been reading through responses here I'm coming back to my original thought to use the pitot-static tester to create a calibration card for the MAP sensor. Thanks for your suggestions as they help point me in the right direction.
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  #8  
Old 10-16-2020, 11:08 PM
Canadian_JOY Canadian_JOY is offline
 
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Quote:
Originally Posted by Foghorn View Post
Just curious but did you reach out to GRT and did they have a solution? I have the same setup with the EIS-66R but haven’t done any avionics testing yet.
In fact it was Jeff at GRT who came back to me with the rather astonishing answer that the MAP sensor in the EIS-66R cannot be calibrated by the user. It seemed he was as surprised by this as I was. Their solution seems to be... "live with it." When I asked a subsequent question and provided Jeff my estimate of MAP error he indicated the individual who was responsible for the EIS-66R design thought my error was fairly normal for that sensor. If my estimations are correct I'm thinking the MAP error may corelate to something on the order of 3-5% error in computed %HP. That's certainly a bunch more than I'm comfortable accepting.
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  #9  
Old 10-17-2020, 12:25 AM
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rv8ch rv8ch is offline
 
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Default MAP sensor calibration

Duh - I didn't even know this was a thing. I have the EIS4000 with external sensor so will have a look at this. Seems surprising that they would feel that 3-5% error is "good enough".
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  #10  
Old 10-17-2020, 06:42 PM
Canadian_JOY Canadian_JOY is offline
 
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My estimate of error was mis-stated in a previous post. I thought the error would have contributed to a mis-reading of % Power by 3-5%. If I made it sound like the MAP error was 3-5% I apologize for my lack of clarity.

After calibrating the MAP sensor today using my pitot-static test set I discovered the absolute error in inches of mercury is approximately 0.3"Hg. It's not a flat offset error but rather it's a bit bumpy, sometimes more, sometimes less than the 0.3", depending on pressure level. I went up for a test flight after testing the MAP sensor and discovered that at 55%, 65% and 75% power I can pretty much apply a good "rule of thumb" calibration factor. If I want 55%, set power to 58% indicated. Same for 65% and 75% - set indicated power 2-3% higher than desired actual engine power level.

Now that I know this offset value for both MAP and % Power I will be able to set power to a known value so any testing I do with respect to airspeeds, fuel consumption etc can use a common reference. (I should mention this "common reference" implies commonality with test data taken while my previous AF2500 engine monitor was in place.)

Mickey - with the EIS4000 and its external MAP sensor, calibration of the sensor is possible through the usual Y=MX+B slope and offset calibration that you will have applied to other sensors. The EIS-66R, because its MAP sensor is internal to the box, does not have the capacity to provide for user calibration of the MAP sensor.

Before anybody asks, yes, I have asked GRT to add user calibration of the MAP sensor to EIS-66R software.
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