Capacitive Convertor Question--Getting a sore forehead

Anyone have any experience with Dynons capacitive converters hooked with MGL's RDAC? I have been trying for two days to get my fuel reading up, and have run into a huge snag as nothing is working but everything is reading 5 Volts. Should the RDAC have five volts at the Fuel level input if nothing is connected to it? Also the converter is sending 5 voilts down the signal wire back to the RDAC. I think the converter may be faulty bacause it will not change voltage if the tank is full or empty. Hence resistance is changing but the voltage is not. This was verified with an ohm meter. I am out of ideas but again and am tired of banging my head on the wall. Someone PLEASE help. I know there are configurations like this out there flying.

Yes but why?

The resistance sensors work perfectly well and survive all kinds of banging around during negative G aerobatics. The caps just seem to be a lot of bother from where I stand.

I think they are also only accurate for what ever fuel you calibrated them with..Although i'm not sure if it makes any difference in real life.

Frank

Caps is what is installed in the tank. No floats. Maybe this should be in the -- If i could do it all over again thread.

Maybe

There are a few that seem to get them to work though...have you called Dynon?..They are normally very helpful

Frank

For anyone interested the problem has been diagnosed. My capacitve senders are actually to blame as they are not giving infinitive resistance. Talking to vans there was a SB on the washers installed with the caps that was sent out a few years back. The proper washers are white, the faulty insulating washers that are bad are black. Well I have the black ones installed. i don't know if this is entirely to blame but I would like to think so. Both tanks are reading the same resistance, and they should be reading none. Anyways thanks to Dynon support for help in diagnosing the problem, the converters and the instrument work just fine. Now to pull the tanks back off of the wing and get some floats put in there. OMG that is a lot of bolts and screws.

SB Number?

I wasn't aware of this SB. Do you have an SB number or a link to it?? I can't find it on Vans Website with their other Service Letters:
http://vansaircraft.com/public/notices.htm

It was a letter that Vans sent out a few years back to customers that ordered the cap convertors when they had there bad run of BLACK washers, I didn't see it on the site either. I talked to Gus at Vans last week and he confirmed this and said that the letter was sent out with the new WHITE washers. I vaguely remember the letter. however not the washers. I remember that I already had the tanks all sealed up when i got the letter, so it was rather pointless at that point.

Probe question

jrvssgl said:

For anyone interested the problem has been diagnosed. My capacitve senders are actually to blame as they are not giving infinitive resistance.

Click to expand...

Forgive me, I'm not following. The RDAC install guide suggests probes that offer resistance in the 100 to 500 ohm range, so why would one want infinite resistance (as in an open circuit)?

Also, the instructions reference attaching capacitance probes directly to the RDAC. Was that a none starter?

nomocom said:

Forgive me, I'm not following. The RDAC install guide suggests probes that offer resistance in the 100 to 500 ohm range, so why would one want infinite resistance (as in an open circuit)?

Also, the instructions reference attaching capacitance probes directly to the RDAC. Was that a none starter?

Click to expand...

Most of the RDAC analog inputs include a fairly low impedance pull-up resistor to an internal 5V source.
The resistor is either 1500 ohms or 1000 ohms depending on the type of input. Fuel level sender inputs have the 1000 ohm resistor.
Typical resistive senders create a resistance from 0 to 100 ohms to ground depending on fuel level. This translates into a voltage from 0 to 0.5V approximately depending on fuel level.

However, the RDAC will accept any voltage in the range of 0 to 2.5V as valid, anything above that will result in the ?? display on the fuel level readout (indicative of a faulty sender).

Capacitive level senders can be connected as long as the sensor electronics is designed to either emulate a normal resistive sender or if it is able to produce a voltage output in the range of 0-2.5V considering that it has to drive a 1000 ohm resistor to 5V.

Capacitive senders need to be calibrated to a particular fuel type, normally 100LL. Automotive fuels may give widely inaccurate results as the composition of these fuels may not be well controlled (in particular in some countries). Capacitive senders work on the bases that the fluid must be non-conductive and have a fixed, known dialectric constant. As soon as the fuel composition is changed (for example contains detergents, ethanol or methanol), this constant changes and it can change by a large amount rendering any previous calibration useless.

Normal resistive senders, as long as you fit one of good quality, tend to work very well and rarely give a problem. Of course the capacitive senders may have advantages when it comes to installation in odd sized or restricted tanks.

BTW, there is a clever way of making a capacitive sender using just two self adhesive stips of metal tape OUTSIDE of the tank, next to each other. This works as long as the tank is plastic or fibre-glass (does not work with metal tanks). The change in capacitance is less than the submerged version but that may not be a huge problem in many cases. Can make for simple installations...

Rainier
CEO MGL Avionics

Prior to ETOH being put in our auto fuel, my cap units in the tank worked very well. Even switching interchangably between Avgas and Mogas.


10 percent alcohol will nearly double the range of the fuel tanks. Or so the fuel gage indicates. LOL


I would stay away from capacitance senders, if given a choice.

If your switching fuels all the time...Forget capacitive senders.

But if all you use is avgas or stick with the same alternative fuel all the time they work great!