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MAP Sensor Calibration and % Power

Canadian_JOY

Well Known Member
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.
 
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.
 
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.
 
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:


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.
 
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.
 
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.
 
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.
 
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.
 
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".
 
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.
 
I really don’t know, but I suspect the GRT calculation of power does not reference OAT, e.g., it just assumes standard temperature. I note that a 20 F temperature difference from standard results in about the same power error as 0.3” of Hg error in MP. I do know that the calculation does not look at mixture, either. Small error if you run rich of peak, but a substantial over-statement of power if you run LOP.
 
Fuel flow

I really don’t know, but I suspect the GRT calculation of power does not reference OAT, e.g., it just assumes standard temperature. I note that a 20 F temperature difference from standard results in about the same power error as 0.3” of Hg error in MP. I do know that the calculation does not look at mixture, either. Small error if you run rich of peak, but a substantial over-statement of power if you run LOP.
Seems like something they could fix easily, since they know OAT and Fuel Flow. Also seems with all the information they have, they could help you lean perfectly just by telling you to move the red lever forward or back.
 
Also seems with all the information they have, they could help you lean perfectly just by telling you to move the red lever forward or back.

Only if they can read your mind. Like most efis units the box can find peak egt, then tell you the temperature drop if you move rich or lean. You get to choose if you want to burn gas at best power, or save gas (and how much) LOP.
 
Only if they can read your mind. Like most efis units the box can find peak egt, then tell you the temperature drop if you move rich or lean. You get to choose if you want to burn gas at best power, or save gas (and how much) LOP.
True. I was thinking more of help when on the too rich side of the curve. What I've seen going from full rich to lean is that I not only save fuel, but get better performance, up to a point.

I am still flying racetracks during my testing phase and have not been on an actual cruise with all these amazing screens, so perhaps during a cruise you have plenty of time to play with this and find it manually.
 
What I've seen going from full rich to lean is that I not only save fuel, but get better performance, up to a point.
y.

This is as it should be, by design. At sea level full power takeoff I burn 24 g/hr, but best power mixture is closer to 19 g/hr. That extra gas is for cooling and detonation resistance.
 
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