Moeller Float Gage Accuracy
 

My newly purchased 12 has the fuselage fuel tank with the Moeller gage. As delivered, it shows about 3/4 full, but it is actually near 1/2 full. I hear that they are so far off as to be useless, but I want to re-label it or something to be more accurate. Has anyone else had this trouble and made a solution?

I, frankly, pretty much ignore it. Although it goes against every "Thou shalt never rely on your gas gauge" instruction I've ever had, My Skyview (measured) fuel quantity is VERY accurate and I've come to believe. No so with the Moeller.

Don
When I first got my SLSA the Moeller gauge was pretty accurate as compared to expected fuel burn and fuel loaded. After three years I've gotten my k-factor down so as the fuel burn I've experienced is reflected on the SkyView EMS and backed up by the Moeller gauge.

That said over this past year my Moeller gauge began to stick or not move. Possibly due to my mechanic removing the tank a couple of times for annual and installing mods.
Lately the Moeller will stick after adding fuel until I repeatedly tap the gauge to make it move to where it should be. In flight the gauge won't move until a full tank has burned down to about 15 gallons then it will move down accurately reflecting tank level.

The RV-12 fuel system has been frustrating in my experience. I'd much prefer to have fixed wings and a fuel tank in each say 12 to 15 gallons with no cabin tank.
Ive managed the fuel with a combination of eyeball, fuel loaded v fuel used , tapping the Moeller, and now fine tuning the k-factor.

I have three fuel gauges...

1. The float gauge. Displays on the D-180. Very accurate, but only when the fuel level drops down to 14 gallons or less, otherwise it's pegged at full. I use it as my primary indication when I'm flying, after the first hour or two.
2. The D180 fuel totalizer. Fairly accurate, but the more fuel you burn the further off you geet. Fortunately its little white lies tell me I have a little less fuel than I really do. It's close enough that really, if I have to worry about HOW accurate it is I know I'm way too close to empty. And, I don't think it's just the K factor. Sometimes the cruise fuel burn shows what I expect, sometimes it's one or more GPH higher than I know I'm probably burning. I don't know what's up with that.
3. The Moeller gauge. I think it's about right at the bottom end, but all I really use it for is a rough idea of how far I have to go when dumping gas into the tank. It's a nice little double check during pre-flight, but I don't really count on it for any sort of accuracy.

Point is, all three tell me the same thing under all but completely full tank conditions. If I see one out of whack, I know it's time to investigate.

Mine originally had a sight glass too. It's gone. It leaked like a sieve, and was opaque enough to be useless anyway.

I've not had any frustrations with the fuel system - it's worked as advertised without any problems. Having said that, like you, I'd prefer to have the fuel in the wings rather than behind my passenger ...

I have four ways to determine fuel quantity before flight: Moeller, sight-gage on side of tank, Dynon D-180, and dip stick.

The only one I completely trust is the dip stick – very accurate / never lies…

Since the RV-12 fuel tank by design slopes upward as it moves aft, neither the float sensor for the EFIS fuel gauge or the float on the Moeller gauge are at the highest fuel level point when the tank is totally full (the fuel filling the aft portion of the tank is above the highest float levels), so neither will show any change until you have burned off 3-4 gallons of fuel (The Moeller is further aft so it will begin to show a change a little sooner than the EFIS gauge).

Thanks a lot guys, that is very helpful. I have wing tanks in my main RV12 largely for those reasons. The ferry pilot told me the K factor for the red cube is way off, needs a much larger number put in.

This is not a problem with the RV-12 (and not exclusive to the RV-12 either... all of the other RV models have gauges that can't read the upper 4 gallons or so because of wing dihedral). When is it that important to have a absolute gauge indication of the first 3 or 4 gallons that get used. The important thing is for accuracy at that last 3 or 4 gallons. The RV-12 tank does well with this.

A long / full span skinny tank in the wing with the dihedral tilt will be effected even worse than any of the other RV's are.

Just like the wing tanks in most RVs the first 5 gallons or so must be burned before any gauge will be effective. I found the Moeller to be pretty accurate from about ten gallons down. Once you start experimenting with K Factor the fuel burned indication will start to become more accurate but the gauge itself is no different from the Moeller. I always kept track of climb time and cruise time and never took off in the 12 with less than 10 gallons. My 7A with duel Skyviews will only show 15 gallons when full and will not move until it is down to that level but the fuel flow is pretty accurate.

Bob, this is something we need to verify in your 2015 RV-12. Very valuable thread, thanks to all that have contributed.

Jim,
Did you make your dip stick? If so, any specs you can share?

Thanks and Happy New Year!
Doug

Scott,
I didn?t build my 2012 RV-12. Was wondering if the occasional strong fuel pump gas stream can neg impact fuel tank floats/innards/accuracy?

Thanks and Happy Flying New Year!
Doug

I don't know if I understand your question correctly, but if you mean a high velocity stream from a fuel nozzle having a neg. effect on the fuel tank and sender? No, I am not aware of any problem caused by that. I don't see how it would be possible since the filler tube makes the fuel enter the tank pretty much straight down at the back of the tank but both of the fuel level floats are well fwd in the tank.
A change in fuel gauge accuracy is usually caused by an increase in resistance between airframe ground and the mounting flange (ground path) of the fuel level sender.
The RV-12iS redesign included adding a ground wire between the airframe and one of the sender mounting screws, to help mitigate this problem.

The stick is wood approx 1/8" x 3/8" x 36". The end that gets inserted into the tank is pointed so it can sneak around the fuel return tube at the base of the filler neck. Start with empty tank and fill with 2 gallon increments each time inserting the stick and marking the level with a graphite pencil. Dead nuts... :D

I did what Jim did and used flexible translucent tubing. Although the shape of the tank isn?t a uniform cube I found that once you?re past the sloped tank floor it was pretty much one gallon to the inch. I got 20 gal into my tank, but that filled most of the filler neck so as soon as the fuel started filling the neck the linearity shifted. I?ll check my dimensions on the marks when I?m at the hangar on Wednesday and post them.

Hmmm that,s interesting Scott, I have just calibrated the fuel tank sender in my 12 with Skyview and a standard 19.8 US Gal tank and had trouble getting good figures with nice voltage "steps". Next time I turned the power on the fuel quantity was blank. Turn out to be a poorly grounded sender. Made a 12" jumper with a couple of ring lugs, grounded to sender mounting screw with an MS35338-38 star washer and plenty of goop and the other end to the conduit P Clamp on the spar carry thru... Perfect!

Some observations here in Southern California, where we fly the RV-12 year round.

Moeller doesn't start registering off of full until 4 to 5 gallons are burned off. That leaved the Moeller having a range of your last 15 or 16 gallons. Last 2 fillups, the Moeller was showing smack dab a half a tank left. Fillups were 12.2 and 11.7 gallons, brimmed to maybe 2 inches short of the top of the fillup neck. So ABOUT 8 gallons left, when the Moeller shows half a tank.

The prop on this plane is tuned not for climbing but for cruising. At 1300 # with both of us 5000 ASL, throttle to the wall, we see 5250 to 5300 rpm, level flight, max. LL100, will burn just over 5 gallons an hour at 112 kt IAS. For some reason, Mogas seems to give another 60 to 80 more rpms at full throttle? Just observations.

Any ideas why more rpm on the Mogas, greatly appreciated. Perhaps the Rotax, being designed for Mogas and the lower octane, makes more torque due to the burn characteristics of the flame front of Mogas?

Just a FYI.... as always YMMV, trust, but verify, this is but one data point of many.

Fuel
 

Jim,

Is there any reason a dowel rod could not be used to measure and mark the fuel quantity in the tank?

Your prop isn't pitched for cruising.... it is over pitched.
This will have a negative effect on oil temp in hot weather and can cause other problems (besides giving up climb performance that you don't have to).

I have no comment on the RPM difference. I have never seen any indication of a change with fuel type. My guess is that you are seeing differences with different weather conditions (density altitude).

Round dowel rod will work fine. Needs to sneak around the fuel return line that points up at the base of the filler neck.

You should be careful when using the dowel rod. Make sure it misses the fuel return line that is at the bottom of the filler neck.

What "other problems" are you suggesting?

This is not my plane, the owner set the plane up with the prop pitch he has because he mostly flies solo. That puts his gross weight with a full tank at about 1060 lbs on a full tank, if that makes any difference.

He usually cruises at 21-22" of manifold pressure and 105-110kts IAS 2500 - 7500 ft ASL, a little faster and in the yellow, if the air is glassy and smooth.

As Scott said, the prop is over-pitched. Full throttle in level flight should easily red line at 5500+ RPM. When pitched correctly ~ 115 kts at 5400 PRM is the norm with throttle pulled about about 1".

Over-pitch can cause serious detriment to the engine - detonation, high heat/wear, not to mention < TBO.

Search forum for setting prop pitch...

And slower cruise speed, and higher fuel consumption...

We will be flying to Copper State Fly In, this weekend. I should have a GoPro Hero 7 Black set up to video what the gauges are showing, so that I can more closely know and speak factually about observations on fuel usage, rpm and IAS readings.

Bob seems to think when he set the props that he would see 5500 rpm at level flight, but doesn't remember the IAS obtained in level flight, or the elevation of flight ASL. About 130 hours now on the Hobbs. He does remember a longer trip from So CA to Oregon and back, and average fuel consumption worked out to 4.9 gallons per hour for the whole trip. No idea how fast he was flying, then.

I guess I don’t understand why so much concern about exact fuel gage readings, or for that matter, differences in fuel burn rates. The gages we have… Moeller, digital EMS, and old sight gage on the side of the tank – all are very accurate at low fuel levels where it counts most. So if you’re burning 4.5, or 5, or 5.5 GPH you still need to plan and maintain sufficient fuel reserve.

I count my blessings that our fuel gages are far more accurate than wing root float gages in and old Cessna or electric analog gages in a Piper.

Jim,

My experience is that the sensor float travel does not span the entire height of the tank, so if you bend the float wire to bottom out at the tank bottom I agree with your statement of accuracy at the low end, but if a builder does not do that the indicated level will not be accurate at low level. My float reaches the upper limit of travel at 17 gallons, but is very accurate below 6 gallons. I usually have to fly about an hour on a full tank before the indicated level drops to 16 gallons.

Rich

My resistive float gauge reads 14 gallons until the level dips below that. It's accurate down to very low levels. The Moeller is useful while filling the tank and double-checking during preflight. I can't see it while flying. The D180 generally knows how much I have left, but it's a little pessimistic -- which I like.

What is the old saying? A man with two watches is never sure what time it is, right? I have three fuel gauges. But if I'm worried about whether I have X gallons or X.5 gallons, I've done something really wrong.

Dale

Why can't you see the Moeller gage in flight? A glance over your right shoulder should be all that's necessary.

I can't get twisted around far enough. I mean, I can see it, but not well enough to get more than a rough approximation of what half or quarter of the range it might be indicating. And with two much more accurate indicators in front of me, I haven't really tried that hard.

Calibrated at same time of Dynon calibration. The float arm was adjusted to provide accurate low level indication. I can quickly cross check Dynon and time in flight.

[quote=skydiverlv;1322643]Calibrated at same time of Dynon calibration. The float arm was adjusted to provide accurate low level indication. I can quickly cross check Dynon and time in flight.


Excellent... How did you make stencil?

[quote= Excellent... How did you make stencil?[/QUOTE]

I cut a paperboard blank to mark indicators while filling tank then sent to AircraftEngravers.net to match the panel overlays. .010 lexan reverse engraved. Any color combination you desire.

Can you reference job or invoice # so I can contact them and not have to re-invent the wheel... so to speak?

ditto!

[I just learned something about this bulletin board system. it will not allow me to publish anything that has less than 10 chars. Hence this meaningless addendum.]

Same here.

Unable to locate specific invoice for this particular order. I do have two other invoices that indicate customer# of V537-XX with job number -01 & -02. I would assume the fuel tank placard would be either -03 or -04. Be aware that if ordered, your placement of gallon indicators may need adjustment for accuracy related to float arm angle. Also, the invoices I have show .020 lexan, not .010 as stated in previous post.

Thanks Larry. I have contacted Engravers.net and asked them if they can search their job files for this fuel placard.

I'll post price when I hear back from them...

Would they have the fuel gage indicator on file I could order? For gross indications!