Fuel bladders - do they make fuel gauges inaccurate?

[rwabdu]

Trying to figure out how accurate my fuel gauges might be for long cross country flights. Do fuel bladders shrink as fuel is burned and does that alter the accuracy of the fuel gauges? I have a JPI EDM 700 and it usually reads that there’s a few more gallons burned than I actually find when I fill the tanks as well. The difference between 4 and 5 hours between fuel stops makes a significant difference in my fight planing but that’s less than 11 gal of fuel in my M20C.

[RobertGary1]

No expert but I thjnk they’re pinned to avoid that. Otherwise you may think you filled it when it was actually collapsed. 

[M20F-1968]

I would not be looking to burn the last 11 gallons on a long distance flight.  The best way to tell what you have burned is to have a fuel totalizer.  

I would not rely on the tank gauges if you are looking to dip into the last 11 gallons.

John Breda

[rwabdu]

7 minutes ago, philip_g said:

You have the new capacity stickers from the stc, yeah?

That makes sense, airplane is new to me, trying to figure it all out. Will look into the STC. thank you. 

[RobertGary1]

18 minutes ago, philip_g said:

You have the new capacity stickers from the stc, yeah?

I think he’s asking if the tank capacity could change in flight. It’s a logical question and it why the installation requires the bladders to be pinned to avoid collapsing  when empty. 

[rwabdu]

11 minutes ago, RobertGary1 said:

I think he’s asking if the tank capacity could change in flight. It’s a logical question and it why the installation requires the bladders to be pinned to avoid collapsing  when empty. 

Yeah, the bladders seem to shrink a bit when empty like a deflating balloon. When I fill up, I will fill the tanks to the top, wait a minute and then the level will drop slowly and I can add more fuel as if the bladder is stretching out a bit. I also notice the gauges seem to hang at 3/4 full for a long time. Makes me think that the gauges won’t show that fuel is being used because the badder might just shrink keeping the fuel level in the gauge at the same height. 
 

24 minutes ago, M20F-1968 said:

I would not be looking to burn the last 11 gallons on a long distance flight.  The best way to tell what you have burned is to have a fuel totalizer.  

I would not rely on the tank gauges if you are looking to dip into the last 11 gallons.

John Breda

I agree, won’t be pushing the last hour of fuel but I’m uncomfortable with the tanks much below half unless I know exactly how much fuel is left. Don’t know the airplane well enough yet to really trust calculations for fuel burn on long flights, maybe in a year or so. Planing 4h flights crossing some mountains and Canada. If I’m not really sure how much fuel is in there, probably will re-fuel quite often. 

 

[pirate]

I have never experienced my bladders shrink or collapse from lack of fuel. 
 When filling my tanks I generally wait a couple minutes for the fuel to go down and top off again, this is because the first bladder takes the fuel quickly then fills the other bladders through smaller interconnecting tubes. 

[Alan Fox]

Couple of things , 1 the bladders dont shrink , They are installed so that they dont collapse , they are impregnated and vulcanised cloth they dont stretch either , When filling , the vent tubes between the bladders are 3/8 pipes , they take a few minutes to equalize , that is normal , The picture of the stickers under the gages is not legal , nor is it per the STC , The gages are supposed to be removed , and the stickers are REQUIRED to be installed on the gage.....

[takair]

One thing I noticed in my own aircraft, the bladders create a more non-linear indication, even with the provided stickers.  Basically, it does not adequately account for the added outboard bladder.  As a result, the first quarter tank seems to take a long time to burn off and each subsequent quarter tank appears to burn off faster.  Ideally, the markings would account for this.  As others have said, when filling, it takes some time for the fuel to migrate through the connecting tubes…

[moosebreath]

My factory gauges read perfectly with the bladders.  The tanks do fill more slowly as mentioned above.

[Warren Dunes]

On 11/29/2021 at 6:25 PM, M20F-1968 said:

I would not rely on the tank gauges...

FTFY

In answer to the OP: Fuel Bladders, do they make fuel gauges inaccurate?

No. Fuel Gauges are intrensically inaccurate.

 

[as350]

I've been wonder about this as well. However, after many long flights using the data from EDM-700, a stopwatch, and detailed refuel records, I've come to the conclusion that the OEM fuel gauges are garbage and completely unreliable. I'm contemplating whether spending the money on the CiES sensors would improve the fuel quantity indications. 

[BDPetersen]

I dutifully sent my 2 sending units off for overhaul as I installed my bladders. Back in service, they are as irrelevant as ever. Presumably attributable to the factory gauge itself. Also on initial fill up I attempted to create an accurate dipstick for the new system. The readings were bizarre. Haven’t revisited that effort yet. It seems time and fuel flow are the only tools.

[rbridges]

My factory gauges have never worked great, but I have noticed this.  When I fill up tanks, I usually tell them to stop a 1/4" from the top.  Otherwise, it will sometimes blow fuel out the vents.  I don't know if that's a bladder thing or if sealed tanks do the same thing.  

[ArtVandelay]

My factory gauges have never worked great, but I have noticed this.  When I fill up tanks, I usually tell them to stop a 1/4" from the top.  Otherwise, it will sometimes blow fuel out the vents.  I don't know if that's a bladder thing or if sealed tanks do the same thing.  

I tell them stop at 1/2”, wet tanks, I’ve never seen it blown out, but I hate seeing them overflow the tanks.