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AFS3400/Dynon Capacative senders/Fuel Readings

Steve Sampson

Well Known Member
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?
 
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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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.
 
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!
 
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.
 
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.
 
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....
 
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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Brian, thanks for that clarification. I did not see the converters mentioned, but I will look again.

Randle, your comments are practical and helpful. I am flying home on Friday but think the idea of tapping on the converters with the system powered up is a good one. With no bad joints in the wiring system I struggle to believe I made two in the fuel system, so do keep returning to the converters as the probable cause. However no one else is saying they have had them as a problem. Being in the UK a chat with Dynon is expensive also, but perhaps I can Skype them. In the end cheapest might be a new converter to see what introducing that does. The problem that is really hard to prove is a loose connection inside the tank. Perhaps next time I fly I should do lots of uncoordinated flying and see if that has an effect.
Thanks for the interest....its helpful. Steve.
 
Go to the Appendix 7.1 of the EMS-D120 Installation guide and you will see that the Dynon Freq to Voltage convertor needs to be supplied with 12 volts. You said 5v in an earlier post. Make sure it is 12.

(10 to 15v per document) Over 15 causes damage. Standard aircraft bus 12v is probably not good enough to keep these things stable.

I have mine powered by instrument quality regulated 12v out of my GRT EIS 4000. No Problems.

It outputs 0 to 5 volts back to the EIS input for gage reading.
 
They really do need to have 12V, not 5V. I just looked at the schematic and they are regulated to 5V inside, so if you feed them 5V they will misbehave. Regulators need more input voltage than their output to regulate properly.

They can take "noisy" aircraft "12V" just fine. They are fully regulated inside as long as you give them at least 7V and less than 15 they should work. Like all Dynon equipment, they are designed to handle the >15V spikes that an aircraft bus sometimes has.

Otherwise I'm a bit stumped as well as how something could affect both units at once.

--Ian Jordan
Dynon Avionics
 
They really do need to have 12V, not 5V. I just looked at the schematic and they are regulated to 5V inside, so if you feed them 5V they will misbehave. Regulators need more input voltage than their output to regulate properly.

They can take "noisy" aircraft "12V" just fine. They are fully regulated inside as long as you give them at least 7V and less than 15 they should work. Like all Dynon equipment, they are designed to handle the >15V spikes that an aircraft bus sometimes has.

Otherwise I'm a bit stumped as well as how something could affect both units at once.

--Ian Jordan
Dynon Avionics

Ian, they are certainly getting the supply from the aircraft bus, a very steady 14.6V. I just remembered they operated at 5V and could not remember where the voltage was dropped. So the supply voltage is a red herring.

So, have you not had other instances of symptoms as I described earlier. It happens on both tanks randomly. For a while it was only one tank, now both. No cross correlation. As I said before, another member here pm'd mme to say he had the same problem. Any simple tests I can do? Thanks.
 
What i did

Hi All
In 2008, after I came back from Oshkosh I had the same issue with my capacitive sender?s .The repair was to cut large access holes in the rear baffles of both fuel tanks. Rewire the capacitive plates and replace the BNC connector, be sure to seal the wire ends with proseal and the BNC connector at the tank bulkhead. This procedure was omitted in the builder manual. The culprit for the inconsistent readings was fuel wicking up the wire and changing the capacitance. After enduring this repair procedure, I had 5 years of trouble free fuel readings, until now. My new system is exhibiting the same faults.
 
Dayton, I am pleased to hear I am not alone.

I just flew two legs, about 800 miles in total. The first leg both tanks behaved pretty well flawlessly. The second leg suddenly one tank dropped to about 27 litres, and then just before landing came back up to a correct reading of about 40. The other tank switched occasionally between zero and a correct reading.

I really would be pleased to hear something from Dynon.

Anyone else had the same problem?
 
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