Princeton capacitive probe, and ethanol?

[13brv3]

Greetings,

I've used only non-ethanol fuel so far, but it would be nice to move to the much cheaper pump gas that can contain up to 10% ethanol around here. I'm running a Rotax 912UL, and Rotax is fine with it. I'm assuming the fuel tanks Sonex sells are safe for 10% ethanol, since people do seem to be using it. I haven't seen an official Sonex statement that says it's OK, but it seems common, and the chemical resistance charts I've seen show no issue with ethanol and polyethelyne.

In searches, I did see a number of posts that said the Princeton capacitive fuel probe will be killed by the ethanol. If you go to the current manufacturers site for the probes, it says right in the description "- Does not work with fuels containing ethanol.".

I could see corrosion issues if you let ethanol fuel collect water, and sit for a long time, but I'm not sure if that would be an issue with normal usage. Are people actually using ethanol laced fuel with the capacitive probe? Is there another capacitive probe that claims to be ethanol safe?

The combination of the Dynon cal, and the capacitive probe is the absolute best fuel level gauge I've ever had in a plane. It's the first time I've ever felt like I actually could trust a fuel reading, so I hate to screw that up.

Thanks,
Rusty

[T41pilot]

I have used a diy capacitive fuel probe in my ultralight for the past 9 years using auto fuel and it has been reliable. The thing with auto fuel is that it is a mixture that can change somewhat from one load to the next or as the seasons change or a change in supplier. Fuel is the dielectric between the plates of the capacitor that comprises your probe. Capacitance changes with changes in dielectric value or distance between the plates. The distance between the plates on the probe is fixed so it is all about the fuel. The capacitance values/range typical in fuel probes is a very small number. Therefore very small changes in dielectric value can lead to a large change in capacitance within the range that your probe is looking at. That results in errors when using auto gas due to differences in the mixtures of alcohol and other components used in the fuel. Throw in a weather change as well and the density of the fuel will change inducing a little more error. That being said, the error on my probe seems to usually be within a half gallon on my 5 gallon tank. A 10% error which for the ultralight is OK but maybe not for your application. Typically Avgas is more tightly refined and errors are small. In conclusion, your probe should work with auto fuel but accuracy could vary somewhat. It's just a matter of try and see. If there is any water present in the fuel than all bets are off. Water conducts well and can short the plates of the probe yielding a worthless reading or even damaging the probe circuitry.

[karmarepair]

I've asked the current marketer for the probes to clarify, response pends. I too, would like to run MoGas, and it's unavailable in California without Ethanol.

[13brv3]

Great answer Gregg. I've read comments about recalibrating the probe when switching between ethanol and non-ethanol, but that seems pretty unrealistic.

I'll be interested to hear what Red (bought the business from Princeton) says. Their description clearly states that it won't work, but it would be nice to know if that means accuracy, or probe damage.

Rusty

[13brv3]

After more research, it seems that ethanol in the fuel makes the capacitive probe an unacceptable option. The dielectric constant of gasoline is 2, where ethanol is 24.3, so the difference is huge. Most estimates I read for the inaccuracy are much worse than what Gregg is seeing. The typical estimate is that you'll read double the amount of fuel if you add 10% ethanol. If you used it all the time, and could calibrate the probe for it, then maybe that would be OK? Switching back and forth would be hopelessly inaccurate it seems.

I also read that it's possible to make a dual probe unit where one probe is always submerged. That probe would measure the dielectric constant and use it to adjust the readings from the main probe. I didn't find any such probe for sale.

I'm not sure where this leaves me other than continuing to avoid gas that has ethanol. I wouldn't consider installing a sight tube from a safety standpoint. It would be possible to install a float sensor, but it's not something I'd pull the tank out to install.

Rusty

[T41pilot]

The probe in my ultralight was always calibrated with 89 octane auto fuel and oil mixed in since I'm using a Rotax 447 2 stroke in that aircraft. I have never changed the type or octane of the fuel used so hence my result. If I went to a different octane or even a different brand of fuel then certainly the calibration would no longer be what it was before. I do a new calibration in the spring of every year and for the most part it holds up all summer. The trick is to stay with the same type of fuel including octane for a good result. There again since Avgas refining requirements are tighter, you should get pretty much the same stuff no matter where you buy it short of a crap storage tank at a given facility so the probe calibration would end up better. The circuit I use to look at my probe is rather rudimentary but seems to work. I would assume that Prince has a more elaborate way to do it, so I'm at a loss why it can't be calibrated to a particular fuel as long as you stuck to that fuel without change.

[13brv3]

I'm assuming the Princeton unit can be calibrated to whatever fuel you have, but there may be limits to how far you can get from the expected avgas standard. The problem is that I couldn't really stick with one particular fuel. I've done my calibrations and flying with non-ethanol 93 octane, but I have used a tank of 100LL. I didn't notice any difference in fuel level accuracy, but I also didn't really look too closely at it.

Ideally, I'd want to be able to switch between avgas and autofuel with and without ethanol. Even if I could stick with one particular auto fuel type, you'd have to be able to switch between that and avgas when you do any sort of cross country.

I'm starting to wonder if there's a non-contact level sensor that could be attached to the flat rear of the tank that would sense through the plastic, yet not be capacitive. I haven't seen one that would give a continuous reading, but there are some that sense when the liquid is at their level. Maybe an array of those?

Rusty

[bvolcko38]

I calibrated my probe with 100ll. As long as I use 100ll its accurate. When I switch to 90 non ethanol the gauge reads 2 gallons high when not full.

[13brv3]

Sounds like the capacitive fuel probe is far from ideal when you're not running avgas full time. Had I known about this when I was installing the tank, I would have installed a float sensor.
Rusty

[DCASonex]

I see in AOPA's notice this morning that the FAA has just approved the GAMI 100 fuel for all piston engine aircraft and that the transition should begin. (it will take a few years) but wonder how this will affect the Princeton probes. I would assume that the corrosion that was due to ethanol will not be a problem, hope we can simply re-calibrate. Our Jabiru and Rotax engines should should be happier on the new lead free stuff.

David A.