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Floats vs. Capacitors

zsadecki

Active Member
Since there's no proper place for this question in the 'never ending debates' section, I figured I'd post it here.. I'm getting to the point where I need to order my wings since they have 8-10 weeks lead time. So the question is, what are everyone's thoughts on the float level sensors vs. capacitive? Obviously there's the moving parts vs. no moving parts question, a plus for the capacitors... and I know with capactive level sensors they are influenced by medium in the tank (avgas, mogas, additives), so that's a definite minus for them... Any other tradeoffs here? And how big of a deal are the two I just mentioned?
 
I don't think anyone would argue that (accurate) awareness of fuel level remaining ranks right up there at the top of things to take seriously.
For that reason, I'd say that either way doesn't make any difference, and simply chose what will work best with whatever gauge or monitor you want to use.

The reason I say it doesn't matter that much is because I wouldn't trust either way to be as accurate as I'd like, so while I'll certainly have fuel gauges, I will be more dependant on a fuel flow meter.
That way, you trust the fuel flow readings most, and then just use the gauges as a crosscheck to make sure that they agree with the fuel flow because they won't if you are leaking or venting fuel directly overboard.
In that case, if the gauges show 1/4 tank but the fuel flow meter says 3/4, it's time to land and see what's going on.
 
go with float and flow

I went with the capacitive tank gauges and a flow scan. Next time I'll save the money from the capacitive senders and use floats and the flow scan. The capacitive senders are very easy to calibrate but it's an extra 200 just for the convertors, I understand there are some engine monitors out there that don't need the convertors but I'm not sure which ones those are.
 
Fuel Senders

I'm going with capacitance senders for 2 reasons. First is the no moving parts issue and second, I am putting in flop tubes and I did not want to have to add another access panel in the tank baffle. For accuracy I don't think it matters muc. I'll be putting in some kind of monitoring systems with fuel flow and will probably use that more than the guages. The guages will provide a cross check to the fuel flow monitor.
 
I have the capacitance type senders but am thinking about trading them for floats. I haven't put the back panel on the tanks yet so I can still change my mind. I like the old glass tube check before I take off and fly more by time than what the gages say. With a low wing you can see if you left a cap loose much easier than a high wing too.
Jim RV-9A wings 90919 Arkansas
 
My 2cents

I am using the floats. I was told by Vans, (1) the cost difference, and (2) if one goes bad, you have to disassemble the tanks to fix.
 
next time it's floats for me

I used the capacitance senders on my RV-7 tanks. I can't get mine to work reliably. They calibrate perfectly on the ground, but once the engine is running something goes awry. I don't know if it's field(s), or static build-up in flight, or what. Still working on it.

Floats for me next time, I'm pretty sure.

)_( Dan
RV-7 N714D
http://www.rvproject.com
 
dan said:
I used the capacitance senders on my RV-7 tanks. I can't get mine to work reliably. They calibrate perfectly on the ground, but once the engine is running something goes awry. I don't know if it's field(s), or static build-up in flight, or what. Still working on it.

Floats for me next time, I'm pretty sure.

)_( Dan
RV-7 N714D
http://www.rvproject.com

Dan, do you think that aeration of the fuel is a possibility? Vibration causing the fuel to become filled with bubbles and causing a poor/inaccurate reading?


:confused: CJ
 
Turn Coordinator is the culprit

No, I don't think aeration is a factor. My latest theory is that the turn coordinator and its wiring is the culprit. Sounds ridiculous, but I believe it's da truth.

The turn coordinator wiring runs right by a non-shielded section of the left tank quantity sender wiring (where there's a connector). The other day I started playing with it, and by moving that wiring in flight, and by disconnecting the turn coordinator in flight (reaching up under the panel and unscrewing the connector), it did affect the left tank quantity reading.

My plan is to get rid of the Mid-Continent turn coordinator and upgrade my Digitrak to a Pictorial Pilot. I'll kill lots of birds with one stone by doing that. Get rid of the mechanical gyro, get rid of about 1/2 amp of current consumption, get rid of the field right by my panel-mounted compass, and I will gain a solid-state fed "horizon" as a more reliable (than a turn coord.) backup to my Dynon.

I'll try to remember to post my results here after doing that swapout. As wacky as it sounds, the turn coordinator and its wiring is, I believe, the enemy of accurate fuel quantity readings in my case. I know, it sounds ridiculous.

I may not be so anti-capacitance after all...we shall see...

)_( Dan
RV-7 N714D
http://www.rvproject.com
 
Yes, please let me know your findings. You almost have me sending back my caps!

I hope you weren't doing all this fiddling around in IMC!

:p CJ
 
Initially I wanted to us the cap. senders but had nothing but trouble in trying to calibrate them. They were very inconsistant and the ultimate problem turned out being the method where the sender head attached to the probe. The head was a plastic/nylon material and it attached to the alum. probe tube with a small pop rivet... this joint was very prone to cause vibration variances in the reading accuracy... I switched back to floats and glad I did - beware! :(
 
Bad timing but, CAP SENDERS FOR SALE

This discussion would come up as I changed my mind to float senders...I am offering a set of capacitence senders for an RV9 wing for sale or trade for float types. New $60, Asking $40.

Will
 
fuel senders

The ACS 2002 accepts either resistive or capacitance info according to them. It also has the two calibration curves which is neat if you're building a tail dragger. Steve Raddatz had nothing but good to say about the ACS and he's built two RV-8's with 'em. Also, there's new software coming out (soon?) that adds neat tricks like percent power. Bill
 
Most (if not all) of the engine monitors that state they can use capacitance fuel senders require an external device to convert the capacitance reading to a voltage output. The engine monitors expect 0-5vdc input for their fuel sensing. There are a couple of converters available such as Princeton Electronics or Blue Mountain, there may be others. I think that the biggest issue with capacitance senders is that while the plates themselves can create a fairly stable and consistant capacitor, at least in 100LL fuel, there are other things that can affect the capacitance and may vary from plane to plane. Basically, you are creating a capacitor between the plates and ribs to which they are attached, but there is also a capacitance between the wire that is run between the plates to the BNC connector and the tank. This capacitance could vary depending on the installation. Also there is also a capacitance between the wire running from the BNC jack to the converter and the airframe. This capacitance should, hopefully, be a small percentage of the capacitance of the plates, therefore having little effect, however if it is not, there may be difficulty getting the total capacitance into the range expected by the converter. I just installed my capacitance senders and the capacitance of the plates is about 235pf (empty). Hopefully this is about the right value. As fuel is added, the capacitance will increase, I believe to about 400pf. It should be noted that the capacitance values expected by the converters are based on using 100LL. The capacitance using mogas could be significantly different, and the converters may not be able to handle it properly as the maximum capacitiane with mogas may be higher than the converter's expected range.

I'm still along way from testing my tanks with fuel in them, so I hope I won't have any problems.
 
Floats Installed

I installed floats in all four tanks and expected no trouble because I flight plan 10 GPH fuel burn with the Lycoming O-360 (24 years of running this engine at normal cruise high power settings has locked this number in - full throttle power increases the fuel burn rate by several GPH - a flow gauge would be nice in that situation), almost always takeoff with full tanks, add a 1 hour reserve, fly by the clock and use the gauges to warn me if some anomaly is causing a low fuel indication that should be resolved at the nearest airport. During the gauge calibration process with two really nice and expensive EI gauges I was very disappointed with the nonlinearity and inconsistency of the indicator readouts (floats stick and jump and respond differently in different physical fuel level situations - how much fuel does it take to float off of the bottom and with the float at the upper end of its range at its root location you are still adding fuel at the higher outboard fill location for example) but I went through the process and repeated it in tanks that just didn't track reasonably on the gauge. It was a big disappointment to have spent hundreds of dollars for these excellent gauges and see the results when tied to the float sender inaccuracies. At that point I was wishing I would have invested in the capacitive senders even though I do not know how they work - necessity forces me to to learn new things in times like these. I went ahead with the project and flew with the float senders in place and found that they work just fine for my purposes - I could have done without the EI gauges perhaps but the two tank levels separate and collective indications in one gauge helped my panel space allocation by requiring only two gauges for the four tanks. After FINALLY getting the access plates sealed there is no way I'm going back in there unless a sender fails and then I will install a new float. My conclusion - floats are a simple solution for generally INDICATING the the fuel remaining - that's all I ask so it works fine for me.
 
Great discussion on this guys! Looks like both have their own set of issues.

For better or worse, I sent in my wing kit order with capacitive senders selected yesterday! Now if it just wasn't for that 12 week lead time....
 
I made up my mind and received my new floats today in the mail. Now I can finish the tank and start on the other one. I will put some kind of flow meter in my plane too.
Jim Wings 90919 Arkansas
 
Excellant thread. I held off on getting sending units when I ordered my -8 wings. I thought for sure I would go with captive, but have decided to go with floats.
 
Capacitance fuel level rules

I can't follow all the post on which BRAND or TYPE of capacitance fuel level device is being talked about. I read Dan's T&B causing problems, BNC connectors and "converters". Anyone have Princeton Brand probe? Most of the discussion seems to be in regards to Van's plate type probe.

I have the Princeton units and they look great and work well on the bench. No converters are required; the probe (aluminum tube) and sealed electronics are all in one. The probe uses 0-5V output and sends a calibrated voltage to the EIS for empty thru full. I can't see the output being that sensitive since the probe is sending a calibrated voltage. It uses plan wires to connect it. There are no BNC connectors, only low volt wires. It looks like a high quality measuring device. http://www.aircraftspruce.com/catalog/inpages/princefuelprobes.php (Buy the ones pre-bent for the RV tank from GRT)

The Princeton Brand of capacitance fuel level sender has two models, 3 set points and 5 set points and comes pre-bent for RV tanks. The 3 level version is about $95 each. The "set points" allow you to set a linear output for non-linear tank shapes. I have not flown with it yet. By the way all Boeing jets use capacitance senders. I want the same technology as a B777 :D
Cheers George
Mustang said:
(From below) George,Yes, the Boeings do use them. You should see what happens when they get a little water in between the plates! Yikes! It's not pretty. Cheers, Pete
Pete, how does that happen? In 10 years of flying 737/757/767 I have never seen the "water between the plates" problem. With say 34,000 lbs of fuel sloshing around a dozen probes, a little water does not seem likely to do much. As you already know the temp compensated fuel quantity computer accounts for single probe errors. G
 
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George,
Yes, the Boeings do use them. You should see what happens when they get a little water in between the plates! Yikes! It's not pretty.
Cheers, Pete
 
Recent Float Experience

Flew out to California last Wednesday 3/23/05 and returned last night 3/27/05 with some long weather circumnavigation legs (4.7 hour longest - 55 gallons onboard - probably 51 usable) and the float/EI instruments worked just fine. When the red light comes on (the last one in the quantity remaining sequence) the tank is still delivering fuel to the engine but it is a real attention getter. I am sorry that some of my carefully recorded data appear to be lost, especially the long leg into the wind from Silver City, New Mexico to El Monte, California but some survived. With changes in attitude such as a climb followed by a level off the tip tank lights would change from red back to green briefly because of the long tank configuration and the float position; downdrafts caused a similar response. The engine is O-360-A1A, 2450 rpm, leaned 50 degrees rich of peak EGT, tank system is LT (left tip) = 8.5 gal, LM (left main) = 19 gallons, RM = 19, RT = 8.5. Santa Ana, CA to El Paso, Texas fuel burn (time-hours/refill quantity) LT(0.7/6.1), LM(1.7/11.9 - a lot of idle time at SNA), RM(1.7/14), RT(0.6/5.2) - so the 10gph flight planning assumption gives a little margin. The RM was yellow - occassionally went red and showed 3 gallons remaining and the LT was indicating a similar status. On the leg from El Paso to Oklahoma City the numbers were LT (0.6/5.1), LM(1.4/12.3), RM(1.05/~10.4), RT(.5/~4) - the fill numbers are inaccurate on the right side as I did not catch the precise number after the RT was filled before the RM was started - close but not exact. Conclusion - for me this supports my earlier assumption the the indications are sufficiently accurate and the quantity remaining sufficiently conservative to permit safe operations in the 10 GPH fuel burn/5 hours out of gas flight planning and direct gauge indications of low fuel anomalies.

P.S. I found the Silver City to El Monte Data:
RT (0.6/5.9), RM (1.2/11.5), LM (1.6/14.2), LT (0.5/4.3)
No change in conclusion.
 
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princeton senders

GMC jetpilot

Is there any chance of you posting a picture of the pre-bent Princeton senders. I need to fit capacitance senders as I'm planning a diesel installation and the PFA here in England are worried about resistive gauges causing an explosion in the tanks.

I've seen the pictures of Princeton gauge pictures on Spruce - just a little unclear about how they would fit in an RV tank...

Thanks

Dave
 
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