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I am attempting to get a Sonex Stratomaster resistive fuel probe working and am experiencing some strange behaviour.

If I look inside the probe, the green wire is connected to an output labelled Vout on the probe's circuit board. I would not expect to be using a voltage output with a probe that is configured to deliver a resistive load. I am wondering if the probe has been supplied with the green wire connected to the wrong output from the board. Can anyone confirm if the green wire ought to be soldered to the unused pad labelled 'MGL' on the PCB.

Picture of the inside of the black box:

An ohm meter works by injecting a constant current and then measuring voltage. The documentation says the probe "simulates" a resistive probe. A voltage output makes sense in this context. I assume it is a capacitive probe.

This is correct. I had a heck of a time calibrating the probe. It looks like you have an older model? Is the hole in the bottom of the probe a small "pin" hole or a slot? Did you expose the probe to water? Are you using MGL avionics? I have the new instruction sheet in the hanger and can send it to you as a PDF later in the week. I started with the older probe and purchased a new one from Sonex while waiting for Princeton. I am a little impatient I didn't even wait for a reply from them. The new probe from Sonex had a bad box but the probe was good. Princeton sent me a replacement on the first old probe which I got on the same day I tried to calibrate the Sonex probe. Todd from Princeton talked me through troubleshooting ad found the problem. He told me that the green wire cannot be tested for resistance.

Here is the email for Princeton todds@princeton-electronics.com and the phone number 616-243-8800. It takes a little time for them to get back to you but they are very helpful.

Thanks for the replies. This probe (purchased in Feb 2014) is the "Sonex Fuel Level Probe for Stratomaster Instruments (Resistance-type sender)" but it sounds like we have been making an incorrect assumption that resistance can be measured. We are attempting to make a homemade Arduino based fuel gauge so that's part of the confusion.

Anyway we'll contact Princeton to confirm things. Thanks for the email address.

Craig,

I'm curious about the DIY resistive fuel probe. I've been able to put together a capacitive v.1 DIY probe and am reworking to v.2. My v.1 was soldered and had some epoxy. I'm concerned the heat may have damaged the epoxy, but it seems to work. I'm curious how you determine resistance with a probe.

Can you shoot a few pics this way and maybe a schematic.

Thanks.

Hi Craig
The Princeton measures capacitance not resistance it only outputs resistance, from memory the probe itself read from around 120 pf up but the output from green wire is definitely resistive so you have to use the Princeton with the green wire not the unit with the yellow wire as its output is voltage
Regards Bish