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Fuel Dipstick Accuracy?


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Can't believe we are the first to encounter this... we recently drained the tanks for fuel line work, measuring what we took out into the jerry cans we use for refueling. According to the dipsticks we should have had around 16 gallons in the tanks, but we measured out closer to 13 gallons. We scratched our heads over this discrepancy, setting it aside until we were ready to refuel. We then measured 19.5 gallons in the cans and poured them into the dry tanks. The dipstick came up with approximately 22 gallons. So it appears to us that the dipstick is reading between 2-3 gallons high. Has anyone else tested this? We have the L/R style dipstick, if that makes any difference.

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Mitch, you definitely aren't the first to notice this and you will find discussion on the topic elsewhere here. Some CT's have L/R stick but others have a stick that shows gallons on one side and liter on the other so just one sided. Personally, I've found that my L/R dipstick reads fairly accurately if I use just the Right side of the stick for both tanks on my CTSW. Best to calibrate the dipstick by the method you used, which is, fill a known amount in the tanks and mark the stick. I marked the fuel level on my fuel sight tubes with paint marking pen to show 3 gallons remaining on each side to insure I know when I get to my 6 gallon minimum.

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Mitch, you definitely aren't the first to notice this and you will find discussion on the topic elsewhere here. Some CT's have L/R stick but others have a stick that shows gallons on one side and liter on the other so just one sided. Personally, I've found that my L/R dipstick reads fairly accurately if I use just the Right side of the stick for both tanks on my CTSW. Best to calibrate the dipstick by the method you used, which is, fill a known amount in the tanks and mark the stick. I marked the fuel level on my fuel sight tubes with paint marking pen to show 3 gallons remaining on each side to insure I know when I get to my 6 gallon minimum.

 

I searched but did not find any discussions of this specific issue, but I'll try again. Another thing we noticed when filling the tanks with a measured amount is that reading both tanks with the dipstick "backwards" results in the same total fuel load. Perhaps this explains why later CTs are shipped with a non-directional stick. I have a Fuel Hawk from a 172 that I am tempted to try and recalibrate but at this point I'm not sure I understand the asymmetrically of the CT tanks well enough to have confidence in the results. It also seems like the method would be painstaking, as in, a real pain.

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How much fast does the fuel cross level? I'm guessing the fuel valve must be turned off to calibrate each tank accurately. I guess that is two separate observations. Cross level speed, anyone know?

 

I don't believe that closing the fuel valve would have much if any impact on the syphoning effect since the valve is downstream of the Y-connection between the tanks. So you'd have to block the flow where the lines come down from the tanks and through the firewall. This seems like an inherently messy operation, and one you'd think FD would have done when it calibrated the dipstick, so we don't have to. It's annoying to have it indicate wrong (especially since it measures 2-3 gallons high). At this point we've filed this information under things the pilot should know. The solution really needs to come from FD.

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Your right that the fuel shut off is down stream of the fuel tank lines that interconnect the fuel tanks. The fuel tank to tank is an open circuit. That's why we can transfer fuel in flight by flying a ball out of center.

The fuel gravity transfer on the ground is extremely slow and it shouldn't be a factor unless you dip each tank on different days and even then the transfer doesn't happen unless there is a wing tilt by a low tire, a low spot on the hangar floor or a very out of balance fuel level in each wing.

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I'll attest to the extra 2 gallons/side on the dipstick reading. After hundreds of hours comparing what my sight tubes, Dynon, and the dipsticks say, 2 gallons high is about right in the middle range on the dipstick. Full is still full, and low readings (<2gal) are pretty close, but if the dipstick says 10 gal, it's probably 8 or less.

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Is it possible that FD thinks there is 2 gal usuable fuel downstream from the bottom of the tanks measurement and they compensated for that "outside the tanks fuel"?

 

This dosen't seem reasonable but there must be some explanation folks think the dipsticks read 2 gal high.

 

A known stick calibration into a known tank capacity should be simple technology.

 

I think we have been using that since the day of the first fuel tank.

 

I will keep a log and pay particular attention to readings and quantities during the Alaska trip this summer.

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We have a right-left dipstick which has been shown to be very accurate across the full range of the tanks. Note that the right and left scales differ by more than a gallon.

 

Obviously if the fuel is sloshing around at all, which it does, the dip stick will read high. I wait for the fuel to settle before taking a reading, and then do it quickly to eliminate the high fuel reading bias.

 

Finally the bottom of our tank is not flat right under the fuel port. It seems some excess epoxy or sealant dripped in from the front of the opening during manufacture. I slide the dip stick down along the back edge.

 

Mike Koerner

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  • 2 months later...

Dick was right,

 

Since I had to drain the fuel to do the wing check I decided to check actual vs. dipstick reading. Adding fuel in 2 gal increments a made a chart of actual vs. reading. The left reading was worthless, no, less than worthless because it read HIGH! The right however was generally pretty close for both tanks. This finally explains some very odd fuel flow rate anomalies I've experienced. I now have my 5 gal. mark on both sight tubes and only use the right indicator dip stick.

 

Good call Dick,

 

al meyer

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Hi Al. Good information. Knowing when I am to a comfortable reserve remaining is key information. For me, this is when either sight tube shows 3 gallons. Before getting to this level of fuel, I'm looking for the nearest airport to land and refill my tanks. Completely draining the tanks and filling with a known amount and then marking the sight tubes leaves no doubt how much fuel is on board. This is easy to do when the tanks are drained at the required wing removal inspection.

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Another consideration here is that fuel quantity measurement is not precise under the best of circumstances. Fuel is denser in cold weather. If you filled your tanks in hot weather and measured the quantity again the following morning you'd have less by volume. Not sure how much of a factor this could be, but two gallons (the measuring difference some of us have found on the dipsticks) into 30 gallons is about 6%.

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According to the engineeringtoolbox.com, the volumetric expansion coefficient of gasoline is 0.00056 per degree F. So, a 100 degree change in fuel temperature results in a 5.6% change in gasoline volume. The relationship is linear, so a 10 degree change in fuel temperature gives a 0.56% change in fuel volume, etc. Real world maximum temperature changes are around 40 degrees F, resulting in a change of 0.67 gallons per wing with 30 gallons of fuel on board.

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According to the engineeringtoolbox.com, the volumetric expansion coefficient of gasoline is 0.00056 per degree F. So, a 100 degree change in fuel temperature results in a 5.6% change in gasoline volume. The relationship is linear, so a 10 degree change in fuel temperature gives a 0.56% change in fuel volume, etc. Real world maximum temperature changes are around 40 degrees F, resulting in a change of 0.67 gallons per wing with 30 gallons of fuel on board.

 

So I take it from your calculations that temperature might account for about a gallon or so?

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Actually, I made an error. Its 0.67 gallons change total (with a starting volume of 30 gallons). The per wing change would be half of that. See formulas, below.

 

Change in fuel volume = 0.00056/deg F x temperature change in deg F x starting volume in gallons

 

Change in fuel volume = 0.00056 x 40 degrees x 30 gallons

 

Change in fuel volume = 0.67 gallons (note, that is total, not per wing).

 

For a change of one gallon in 30, temperature has to change by 60 degrees F. Still, that's only 0.5 gallons per wing, which is hard to measure with any precision at all. A one gallon change per wing would require a 120 degree F change in temperature (and 15 gallons to start, per wing).

 

Overall, except for very uncommon circumstances, given that the precision of the dipstick is about one gallon, such temperature related changes in fuel volume are unmeasurable unless many repeated measures are obtained and averaged every time the dipstick is used.

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All of this talk about the fuel level makes me think about the different levels in the two tanks. I know it has been discussed before but I still don't understand how the levels can become so different in the two tanks after filling up. It has been said that flying with one wing slightly lower than the other one is the cause. Well then why doesn't it even out when back in the hangar. I have checked the fuel level after sitting in the hangar for a week and one side will have 3,4,or even 5 gallons more than the other side and the fuel shutoff was open. Why doesn't the fuel seek a level that is the same in both tanks? Given the way it is plumbed I would think it would even out.

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Several factors are at work here. One is when you are flying. Fuel will follow the ball. If the ball is out to the left fuel will transfer from the right wing to the left.

 

Back on the ground there are a few other factors. I have found floors not to be as level as many think, low tire on one side, slightly sprung gear leg on one side. Anything that may cause a wing to be just slightly down from the other and it may not be noticeable. Then there is the fuel itself where one level wants to seek the level of the other. The fuel valve does nothing as it only stops the fuel to the engine. It is an open circuit between the tanks.

 

And yes sometimes the fuel in a sitting plane doesn't seem to transfer because of a short sit time or the other factors listed above.

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Roger summed it up. Given enough time to equalize, the level of fuel in the two tanks will be identical. But, that is identical fuel level with respect to the horizon (or the center of the earth), not necessarily with respect to the wings (or fuel tanks). If the fuel levels in the two tanks are unequal after sufficient time for equilibration (don't know exact time, but a day should be more than enough), then the plane (the wings) is (are) not level. Other explanations are obscure and hypothetical, such as a defect in the bottom surface of the tank on which the bottom of the dipstick rests.

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I would be curious if mine didn't transfer sitting for a day or so. Why would it not? One wing higher (could level the plane to verify). Vent restricted? Hmmm, not sure. Could check them. One of the course wing filters gooped up? Naww.

 

Could you prop a wheel up a couple of inches over night to force a transfer (be careful to not overflow the down wing) and see if it works?

 

Just thinking out loud. If the plane would not transfer in an expected manner, I'd like to know before I took off on a long trip or one where I thought I'd get down to the last hour of fuel.

 

 

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