Rotax oil pressure pn 456180 definition wrong?

[Doug1]

Hi
Everyone

I have the newer Oil Pressure sensor installed, PN 456180 in a new aircraft built mid 2015 with a 912ULS.
I noticed the ems sensor chosen on the skyview system was "ROTAX OIL PRESSURE (HONEYWELL) not "ROTAX OIL PRESSURE (P/N 456180)"
On inspection of the latest sensor file "sensor_08_03_2015.sfg" I note that the definitions for the two sensors are identical, so it doesn't matter which was selected.
The Rotax document "SI-912-020-r8" specifies that the sensor P/N 456180 is a 4/20ma type with a linear output of pressure related to current, 0.75bar or 10.9PSI at 4ma and 10.75bar or 156PSI at 20ma. This current is fed into C37-P6 which is programmed to have a 200ohm pulldown resistor, creating a voltage of 0.8v at 4ma (ie10.9psi) and 4v at 20ma (ie 159psi).

But, with the low_coeff specified for the sensor, this will result in a reading of 144psi at 20ma instead of 156psi, an error of -6%. At about 60psi the error is about -20%!

i.e. the error in reading gets worse as the pressure reading falls.

It appears to me that the ems sensor definition is not correct for the rotax oil pressure sensor P/N 456180.

The correct low_coeff would be
{ c_x4 = 0
c_x3 = 0
c-x2 = 0
c_x1 = 45.324
c_x0 = -25.382
}

This will display 156psi at 20ma (correct) and 10.9psi at 4ma (correct)
At currents less than 4ma, the pressure displayed will fall to zero, then under 2.8ma a negative pressure would result in a red X which reflects a faulty sensor.

Also posted in ems general

Doug

 

[Dynon]

My off the cuff reaction is that we're usually pretty sure I about our sensor characterizations, though we've found bugs in them before. We'll have to research this one further.

 

[Doug1]

Hi Dynon Technical support

Wondering if you have "researched" this issue yet?
I would have thought this to be quite important as the error caused by the wrong sensor calibration (if it is wrong) is quite large, especially at low pressure.

The ramifications would be that the operator may be observing a low oil pressure when actually it is ok. For example 12psi, (the absolute minimum for a 912ULS) with the current calibration is actually 26psi with the calibration above.
A reading of 75psi with the current calibration (the maximum pressure for a 912ULS) is actually 89psi with the calibration above, far bit above the allowed maximum allowed.

Doug

 

[Dynon]

Sorry, I don't have an update from our engineering team on this yet.

 

[Dynon]

Very short version: SkyView's characteriztion is correct. We've tested it against an external gauge, and also did a bit of analysis. Here's the bulk of the analysis for those that want to nerd out on the details:

1) SkyView has a 200 ohm pull down on pin 6 / GP 11. But we also have a 10K pull up to 5V which cannot be turned off. Thus, when software chooses 200 ohm pull down - as this sensor does - you get both.

This means that if you put 4mA in, you get a voltage of 0.88V, not 0.80V like a straight 200 ohms would give. At 20ma, you get 4.02, not 4.00.

This causes the response to not be perfectly linear, which is why we have a more complex polynomial.

2) Our current polynomial gives -.25 PSI at 4ma and 144.49 PSI at 20ma, which is pretty accurate.

3) We based our model off the original Honeywell sensor that Rotax used. This is Honeywell part MLH010BSCDJ1231, and Rotax part 956413. Rotax started using this in mid-2008

The Honeywell datasheet shows this as a linear sensor from 4-20mA and 0-10 bar (145.04 PSI).

The datasheet doesn't cover this exact part number. It matches from MLH010BSC but the DJ1231 part is unknown. The pressure range is defined in the datasheet as the "010" part, so it's fully expected that this is a 0-10 bar sensor. Multiple references call this a 0-10bar or 0-145 PSI sensor on the web.

Datasheet: http://sensing.honeywell.com/honeywell-sensing-mlh-series-allmetal-pressure-sensors-product-sheet-008118-7-en.pdf?name=MLH010BAL03B

4) Rotax no longer uses the Honeywell sensor. They now use a Keller sensor, Rotax PN 456-180. This happened in 2012. All of our data is that these parts are electricaly interchangable. Rotax agrees since their officially-supported EMS (the Flydat) didn't change for the part change. So does this manufacturer of analog oil pressure gauges:

http://www.aircraftspruce.com/catalog/inpages/cps15-06649.php

Now, the issue is that Rotax has this new sensor in their rev guide for the engines. Page 39 of the following PDF:

http://www.dulfu.dk/CustomerData/Files/Folders/8-pdf/198_d05601.pdf

They show this sensor not as 0-10 bar, but 0.75 bar to 10.75 bar. This is about a 10 PSI offset, and is what is being reasonably assumed in the opening post here.

Rotax allows idle pressures as low as 0.8 bar. We we can't see why they would ask Keller to make a completely custom sender that deletes the 0-0.75 bar range. Standard Keller sensors go to zero:

http://www.kelleramerica.com/pdf-library/Piezoresistive%20transmitters%20for%20industrial%20applications-absolute%20and%20gauge%20references%2021%20Y.pdf

There is one reference online that claims it really is 0.75 to 10.75 bar, but that's one guy (last post on page):

http://www.rotaxowner.com/en/rotax-forum/3-4-stroke-technical-questions/5258-oil-pressure-sender-rotax-p-n-456-180?limitstart=0&start=20

Dynon's analysis (confirmed by physical re-testing, just to be sure):

Rotax's document SI-912-020 R8 doesn't depict the actual characterization of the sensor: the graph should be 0-10 bar, not 0.75 to 10.75 bar. Note the graph has other issues, showing a linear line on a non-linear horizontal scale, so it is suspect more generally.