| Paul,
not sure what is going on there. I also don't know much about the geometry of Legacy tanks. If they were perfectly rectilinear, you would expect a linear change in frequency as you add fuel. Obviously tanks aren't linear or they wouldn't bother with a 10-point system. However, I would record the frequencies when you do your next calibration. If it doesn't look reasonable (especially if freq is jumping around erratically ) then you probably have a problem with the probe.
You might consider, though, plugging a frequency generator into your system instead of the probe just to make sure the problem isn't somewhere else. I had problems with mine going to "empty" when it wasn't and tracked it down to grounding problems in that path. You would have to talk to EI or Princeton about how to plug in a freq generator. In the extreme case you could order a new probe but hook it up before you put it in the airplane and see if you can bench test it like Adam did. On Oct 4, 2012, at 8:43 AM, Paul Miller wrote: Adam, Colyn: This thread has been helpful for me. I have a problematic right probe in the Legacy that I'm dreading pulling the wing to replace. If I adjust the wires I can get a good signal but it is not consistent. We fixed the wiring in place with some RTV to prevent movement and I then swapped the old Princetons for 10-point models and tried to re-calibrate. Oh, it was very painful. I would fill one tank while draining the other, I would miss a setpoint and have to start over. Finally, got it done but the results are not good on either tank. The right stays full now full time and left won't come off full for at least 5 gallons.
BTW, I did use the frequency meter on the Princetons and saw the 2khz and 4khz ranges but never understood those results until Adam's testing. I am going to re-calibrate at least one more time next month so do you have any thoughts on how best to do this with the 10-point units? I do put the wings and tail on jacks so there is no movement. I drained about 3.3g each time (or filled) and I waited about 2 minutes between fills for the units to settle down. I think it might need more time to settle down though. I will put the portable scope on the wires to make sure I'm getting the square wave too. Thoughts appreciated.
Paul Calgary
Hi Colyn – I’m measuring the
signal before it goes into the Princeton
converter. The square wave looks fairly clean and is the correct amplitude
(5v). When I originally bench tested the probe, I taped over the holes near the
head, stood it on end, and filled it with fuel. Several aluminum chips came out
when I poured out the fuel after the test, presumably left over from the
manufacturing process. I’m wondering if any contamination could be
causing a complete or partial short between the center conductor and the outer
tube. The only other possibility would appear to be some sort of failure of the
electronics within the probe head. Short of pulling the wing and replacing the
probe, I’m not sure what else I can do at this point.
Adam, are you measuring that right at the probe or is there something
in the middle?
I have not had anything like this occur with my EI probes. I did
however have a lot of problems with the previous (old) princeton converters.
If memory serves, these convert from freq to 0-5V. The new
10-point Princeton converters have been great.
Posted for Adam Molny <Adam@ValidationPartners.com>:
My EI fuel probes output a square wave whose frequency is proportional to
fuel level. Before installation, bench testing showed that the probes put
out 4kHz when dry and 2kHz when full. I calibrated my Dynon SkyView
accordingly. Now, at 70 hours the left probe has become unreliable. A few
weeks ago the frequencies changed to 3kHz empty and 2kHz full. I calibrated
the Dynon, but now the probe is putting out 3kHz when full.
Has anyone had similar problems?
I'll have to remove the left wing to get
the probe out, so I'd like to have a plan of action first.
Thanks,
Adam Molny
Legacy N181AM
72 hours hobbs
time [We have an EI capacitive probe on our test stand and
noticed that it is subject to large swings relative to temperature. Not
the probe itself, but rather the oscillator module whose frequency is driven by
the probe. The easiest way for me to fill our 280 gallon tank is to pull
the truck out into the Florida sunshine and expose the oscillator to full
sunlight... soon as it heats up the tank tops itself right off... damnedest
thing you ever saw. Only problem is it won't stay full.
<Marv> ]
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