D120 and Princeton cap probe fails calib in RV-10

[CheckerBird]

We're trying to calibrate the D120 with Princeton capacitive sending units in an RV-10. The Princeton sending units are the 2-setpoint type, and the sending unit successfully went thru the initialization routine (empty + full)  as described in its documentation just fine. We then emptied the tank and set up the D120 for calibration, telling it that the tank was 30 gallons, and it began the procedure prompting us to fill the tank 2 gallons at a time.  Once we got to the 20 gallon mark, the sensor value readings pretty much stopped changing, the D120 kept asking us if we'd actually added fuel before pressing the ADD button.  For the remaining 10 gallons, only once did it actually register a change. After pressing the ADD button, the sensor value displayed would start changing around, sometimes going up by a couple points, and sometimes going down. For the final 2 gallons, before pressing the final button to signify the tank was full, the sensor value on the screen dropped about 16 points from where it's peak value was.  (peak was reported at 466, the final value was 450). Needless to say, the D120 reported that calibration had failed.  We only did one tank thus far, and the resultant failed calibration says the Left tank with 30 gallons of gas in it displayed on the D120 screen as "Full" with 12.9 gallons in it.  After power cycling the D120, then it showed 10.3 gallons and a full gauge graphic.

Any ideas what we should do to try to troubleshoot this calibration problem?

 

[dynonsupport]

Re: D120 and Princeton cap probe fails calib in RV

It sounds like it can only be one of two things. Either the calibration in the Priceton is wrong, or the probe is installed in such a way that it can't get the top of the tank. Since we could see the output over the first gallons, the wiring is right and the probe is outputting voltage. There's really nothing we can do in the EMS- if the probe stops sending us different voltages, we can't calibrate further.

Are you 100% sure that given the shape of the tank and where the probe is that it can see those last gallons?

 

[CheckerBird]

Re: D120 and Princeton cap probe fails calib in RV

The dihedral of the wing/tanks is indeed very likely putting the entire length of the sender probe tube completely under fuel when the tank is filled past 20 gallons. The sending unit only fits within the inboard section of the tank up to the 1st baffle anyway (which is likely completely filled long before the remainder of the tank), and Van's own resistive/float type mechanical senders and analog gauges only scale up to 24 gallons in RV-10 tanks.

If the D120 is told that the tank holds 30 gallons, and the sensor (and calibration routine) has maxed out at 20 gallons, shouldn't it at least show 20 gallons (or perhaps a ">20" or "20+" character string indicated on the display would be desirable), instead of the 10 to 13 gallons, but with a full green arc, that it's presently doing?

Should we just run the calibration procedure, perhaps telling the D120 that the tanks are only 20 or 24 gallons each instead?

 

[dynonsupport]

Re: D120 and Princeton cap probe fails calib in RV

Telling the EMS how many gallons you have is only used to decide how many gallons we want you to add at a time. If you tell us a 100 gallon tank, we won't make you do it 2 at a time, and if you tell us a 8 gallon tank, we'll do .5 gallons at once.

We always top out where the sender does. If you tell us a 8 gallon tank but keep going (and the sender keeps going) to 20 gallons, we'll calibrate all the way up there.

The simple fact in this case is that the sender stopped increasing once you got to 13 gallons, so we only show you 13. We will in many cases show you a 13+, but this assumes the sender is putting out a bit more voltage than is indicated at 13. If you think that it originally stopped at 20 gallons, but now shows 10-13, I'd really look into the princeton and it's calibration. We won't show you any numbers on the screen if we don't have a valid calibration.

You can measure the output of the princeton with a voltmeter, so you can see for yourself that it's not changing. You'll need to work on that first, and then re-do the calibration on the EMS. Also, make sure you are using and calibrating with the fuel you will use long term. Capacitive senders are highly sensitive to fuel changes (especially any alcohol in the fuel)