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  #1  
Old 07-02-2013, 03:07 AM
Steve Sampson Steve Sampson is offline
 
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Location: N. Yorkshire, England
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Default AFS3400/Dynon Capacative senders/Fuel Readings

I have a very disconcerting fuel gauge reading problem.

I have an AFS3400 connected to Dynon capacitive senders at the tanks. Every few minutes in flight a tank will, on the gauge drain to zero, then some minutes later refill. It can be either gauge, rarely both together. This generates many disconcerting warnings.

Clearly an intermittent connection is the obvious thing to look for, and I have done this many times. Just occasionally the tank will only partially refill to the actual level for a limited period, but usually it reads correctly or nothing.

Is this problem familiar to anyone else? I begin to wonder if the problem is NOT my external wiring, but either inside the tanks, the Dynon senders or the AFS.

Thoughts?
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  #2  
Old 07-02-2013, 11:10 AM
Randle Randle is offline
 
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Location: Atascosa, TX
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Default

First question, do both tanks perform the same discrepancy just at different times and does one side do it more than the other? I am not familiar exactly how the convertor for the Dynon probes work or are constructed as my search has yielded zilch, but in the aircraft I work with that demonstrate the same discrepancies, it sounds like;
1. An intermittent excitation feed short to the probe from the convertor that is varying the energy available for storage in between the probe plates. This will be caused by a bad electrical connection between the convertor and probe, most commonly at a splice or BNC. The delayed reaction or “slow fluxuations” come from the Wheatstone Bridge in the convertor attempting to create the balance needed to create the output signal for your gauge.
2. The bridge in the convertor is breaking down. It usually is the bridge rheostat (if you have access to it to check), or the internal A/C convertor if those probes run off of the common 5-6 Hz excitation.

If both tanks are affected simultaneously, it usually is an internal A/C convertor problem when one convertor is used for both tank units, but since the tanks don’t go down together, it sounds more like an excitation issue.
How are your convertors wired, on the same breaker, the same power feed wire?
Do the fluctuations coincide with turbulence or a certain altitude?
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Last edited by Randle : 07-02-2013 at 11:17 AM.
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  #3  
Old 07-02-2013, 01:45 PM
Steve Sampson Steve Sampson is offline
 
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Randle, thanks for the interest.

There is no pattern I can discern. At any time, one or both tanks can suddenly go to zero and then a random time later refill. It has only happened a couple of times with both, but it has happened. Yes, one side does do it more than the other but I think this has switched over. (I will have to start observed behaviour immediately after each flight.)

The way they are fed is one, from memory 5V supply, which then divides off to each tank. I have obviously been over the joints several times but dont think its that. The rest of my wiring has been very reliable.

I have just looked up the Dynon Fuel senders and while they used to sell them I see they now say: " Dynon Avionics does not supply fuel quantity senders. " I wonder if they had lots of problems. Perhaps someone will join in.
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  #4  
Old 07-02-2013, 02:58 PM
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Noah Noah is offline
 
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I had a similar problem with Princeton converters and AFS EM box. The AFS box filters (time averages) the input so when the input signal to the AFS drops out, it shows the tank slowly emptying (like over 2-3 seconds). When the intermittent connection is re-established, the tank slowly fills up again (over another 2-3 seconds).

It sounds like you have either a power problem to the capactive converters being intermittent or the signal between the converter and the AFS box is intermittent. In my case the signal was intermittent due to a bad coax connection at the wing root. The converter has a wire that runs to the wing root. That connection was intermittent. Being that that is out in the weather somewhat, it may need to be cleaned.

Good Luck!
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Old 07-02-2013, 05:36 PM
Randle Randle is offline
 
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The 5v supply to the 2 converters, does it also supply any other electonics ?
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  #6  
Old 07-03-2013, 10:18 AM
Steve Sampson Steve Sampson is offline
 
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Quote:
Originally Posted by Randle View Post
The 5v supply to the 2 converters, does it also supply any other electonics ?
No, just a canopy warning diode, but the canopy stays shut in flight!
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  #7  
Old 07-03-2013, 10:27 AM
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Noah Noah is offline
 
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Jumper your canopy open switch so the Canopy Open LED is always lit, and observe the LED when the tanks show empty. If the LED goes off, it is a 5V supply problem. If not, it is a signal problem. Doesn't seem like it would be a 5V supply problem, since both tanks would simultaneously be showing empty and refilling when power was restored to the converters.
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  #8  
Old 07-09-2013, 12:44 AM
Steve Sampson Steve Sampson is offline
 
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Quote:
Originally Posted by Noah View Post
Doesn't seem like it would be a 5V supply problem, since both tanks would simultaneously be showing empty and refilling when power was restored to the converters.
Cant see the point of jumpering for the reason you give. I wonder if its a fault right at the sender? I was hoping others would chime in re Dynon senders, but since none have I guess this is unique to me.
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  #9  
Old 07-09-2013, 05:38 AM
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Brantel Brantel is offline
 
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Quote:
Originally Posted by Steve Sampson View Post
I have just looked up the Dynon Fuel senders and while they used to sell them I see they now say: " Dynon Avionics does not supply fuel quantity senders. " I wonder if they had lots of problems. Perhaps someone will join in.
They still make and sell the capacitive to voltage converters. Not the sender itself. They never did make a sender. Just the converters....
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  #10  
Old 07-09-2013, 08:49 AM
Randle Randle is offline
 
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Default another option

From my experience with capacitance type fuel indicating (the only type I work with at the good ole workplace), the control units (converters) get flaky and shell out substantially more often than the tank units do. Since both sides are doing it, swapping them from one side to the other does not sound like a beneficial move for you. You may end up looking at buying yourself a pair of convertors. The disadvantage that is highlighted in this scenario is the lack of maintenance and discrepancy history with this system. Before you bite the bullet on the converters, since you aren’t getting any help from anyone on here who has had a similar problem, you next best option is to call Dynon Support or Princeton. If there is a chance that it is the probes, the way I check them is to connect a capacitance meter (Goodrich PSD90-1C) directly to the probe, and monitor “wet cap” readings while tapping on the fueled tank around the mounting points of the probe and throughout the tank exterior with a small dead blow (~16oz) to look for spikes or drops in capacitance. I continue the check while transferring fuel out of the tank and shaking the aircraft (attempting to check for conductive FOD in the tank that could cause the plates to short). Seeing how these meters, even the el cheapos, will cost you more than 2 new converters, you can try doing this test with the system powered up and monitor the EMS. Tapping on the converter with your fingers may not be a bad idea as well. Have you talked with Dynon yet to see if there is a history of this issue?
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If you are 90% efficient, and your buddy who checks the 10% you missed is also 90% efficient, together you're 99% efficient.

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wings nearing completion

Last edited by Randle : 07-09-2013 at 08:54 AM.
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