Avionics: Oil quantity sensor installed.

Following my experience with the failed Airflow Systems 2007X cooler, I wanted to have an indication of how much oil was in the sump during flight.  Aircraft Extras makes just such a product.  The sensor, contained in a tube welded to a fitting, is placed in an unused drain plug.  The sensor wires are routed through the firewall into the electronics box which, following calibration by the user (filling an empty sump with a series of known increments of oil), in my case connects to my EFIS for display of the oil quantity.  I also got the optional temperature sensor since...well...why not?  

Below is an image of the sensor as delivered for my engine (Lycoming IO-390-EXP119) and the electronics box (pic from Aircraft Extras).  The design of this sensor is exceptional and seemingly all possible use cases were thought of by Aircraft Extras.  E.g., separate indicator lights (if not used in conjunction with an EFIS) can be accommodated and a remote programming button is supported should the electronics box be mounted in an inaccessible location.

To order the sensor, one needs to make a measurement to determine what length sensor is appropriate for the engine.  Below shows the oil plugs for my engine (IO-390-C/EXP119) looking up at the sump where the top of the figure is the aft side (taken from the Lycoming maintenance manual, MM-IO-390-C Series, page 72-20).  The oil drain plug is circled in red whereas the right forward plug is circled in green.  For reference, my sump is Lycoming part 56B28511.

 

 

Below left is an image of my sump before it was installed (from back when I changed my engine to the -EXP119 variant) and the right shows the engine without the sump.  Based on these two images, it seemed reasonable that the right forward drain plug would capture a significant volume of oil via the sensor (the plug next to the oil screen, the lowest spot on the sump, is used for draining and is thus unavailable).  I measured 4 and 5/16" from the bottom of the sump to the ceiling.  Thus my sensor length was 4.065"  and I thought 60" length cables were appropriate for my installation. This yields Aircraft Extras part S4.06-1/2NPT-C60.  

Below shows the sensor installed.  I added heat shields along the two nearest exhaust tubes (only one of which is visible) to help deflect heat away from the coax.

I calibrated the sensor using 0.5 quart increments up to the max of 7 quarts.  Following calibration, my sensor output about 4.2 V (out of 5 V possible) at 7 quarts.  Obviously, that's not the full 5 V output.  So using a 7/5 scaling, that translates to about a maximum of 5.9 quarts readable (i.e., the remaining 1.1 quarts were above the top of the sensor and not available for measurement).  That's great since I don't usually fill beyond 6 quarts so I would be able to see a meaningful reduction in oil in flight (however, read the last paragraph below to understand how to interpret the sensor's indication in flight).  On my next oil change, I'll recalibrate the sensor up to a 6 quart max so the sensor output has more resolution.

My panel just doesn't have space to mount the electronics box anywhere other than in the map box cutout.  It wasn't practical to use screws, so I cinched it down with wire ties to the SkyRadar mount (which is now a Stratux mount).   After this image was taken, I used a pair of right-angle SMA adapters to better route the coaxes.  I didn't have the right tool to cleanly crimp the sleeves on the connectors, so the former look a little flat.  If you're curious why there's a USB power socket adjacent to the electronics box, go here.

Wiring map for the electronics box is below.  It shares its power by connecting to my Aircraft Extras Fuel Guardian and thus is protected by that 1 A fuse (which also powers my CO Guardian).  The sensor is configured to have 0-5 V outputs for both outputs with 0-300 °F scaling for the temperature output.

Here is an image from my EFIS in flight showing the sump temperature and oil quantity (bottom right two meters).  It's interesting that the sump temperature (where oil collects before the oil cooler) shows only a few degrees higher than at the oil cooler exit.  I would have expected a larger temperature difference between the two locations.  However, these sensors may not have similar accuracies.  An explanation for the oil quantity indicating lower than actual follows.

Here is a graph from a flight showing the oil pressure, oil quantity and sump temperature.  The x-axis is the time in minutes since start.  It's interesting any time the OilP goes up, the oil quantity goes down.  I think this is because at high OilP there is a high volume of oil moving out of the sump.  Thus, the oil may not have enough time to settle and collect at the front of the sump (where the sensor is) before the oil pump pulls it through the screen (I presume high OilP translates to a high volume of oil moving).  

Thus, in my case, the sensor always gives an artificially low number when the engine is running and as power increases, the indicated oil quantity is reduced accordingly.  I have the EFIS "low oil" alarm set to 3 quarts for now (which translates to maybe 5.6 quarts of actual volume in flight).  I will experiment with that setting over time to get me an active alarm around 5 quarts of actual volume in the engine in flight.