Physics Question in Making a Fuel Stick

[M20F-1968]

Perhaps  there is a MS'r  that knows both plastics, physics and hydrocarbons.

 Several years ago I bought some thick-walled, rigid, clear plastic  tubing.   It's outside diameter was a half-inch,  inside diameter 1/4 inch,  with a wall thickness of 1/8 inch.  

 my purpose was to use it as a fuel tank gauge.   I expect to be able to cut it to length,  polish the ends,  and use it essentially like the pipette with a an engraved scale on the outside,  calibrated in gallons of remaining fuel.  

 I dragged it out this past weekend, cut into length, sanded and finished the ends, and drop it vertically into the tank. I put my finger over the end as I would a pipette,  pulled  two out with my finger on the end of the tube,  and the fuel despite my finger basements,  promptly ran out of the tube.   I even went to the point of putting a bit of oil on the end of the tube to ensure good seal or even using duct tape and the same result occurred.

 whenever I've seen pipettes in a chemistry lab, they usually have a small outlet and small hole at the top over which one places one's finger.   Can someone quantify the physics going on here.   does this simply have something to do with the diameter of the tube itself or is this some sort of interplay between the hydrocarbon fuel and the plastic tube?

John Breda

[Hank]

It has to do with surface tension. Gravity pulls the fuel inside the tube down, resisted by the surface tension at the bottom combined with the vacuum formed at the top where your thumb seals it. If the column of liquid is too large, surface tension at the bottom can't hold, the surface breaks and it all comes down rapidly. 

P.S.-- my dip tube is about 1/8" ID, with about thecsame wall thickness. Surface tension is a pretty weak force.

Edited September 24, 2018 by Hank

[neilpilot]

John, I don't think that there's any plastic-fuel interaction.  Pipettes are inherently inaccurate and require technique.  I recommend a free paint stirrer from your favorite hardware'c PMN department.

[Yetti]

I believe Sherwin Williams has a better quality paint stirrer.     A piece of white oak would work well.   The straw in the glass trick always seemed to be inaccurate. YMMV

[Hank]

31 minutes ago, Yetti said:

I believe Sherwin Williams has a better quality paint stirrer.     A piece of white oak would work well.   The straw in the glass trick always seemed to be inaccurate. YMMV

My dip tube last week read 16 gal. Filled that tank up with 10 gal, to rated 26 gal capacity. At reseal, it took 52.3 to fill both from empty.

I dip approximately in the center of the cap opening. 

[jaylw314]

Two problems--the surface tension issue @Hank mentioned, and fuel evaporates quickly enough it can "fill up" the vacuum above the fluid level.  Come to think of it, both are actually the same problem in physics...

Also, it's pretty tough to get a good finger seal on 1/4" tubing, smaller might be easier.  You could lick your thumb to make I easier to seal, but that seems like a bad idea around leaded avgas

[Yetti]

39 minutes ago, Hank said:

 

I dip approximately in the center of the cap opening. 

Mine is center of edge on the wing root side.

[Hank]

Found pictures of mine

Works great.

[Andy95W]

Hank- how did you mark the tube, and do the marks appear to be long lasting?  Thanks!

[HRM]

18 hours ago, M20F-1968 said:

Perhaps  there is a MS'r  that knows both plastics, physics and hydrocarbons.

Save yourself the analysis and just buy a "Universal" Fuel Hawk (0.012 AMU). Works great and they worked all the physics out so that it pipettes AVGAS perfectly.

[Hank]

6 hours ago, Andy95W said:

Hank- how did you mark the tube, and do the marks appear to be long lasting?  Thanks!

Mark with a Sharpie, then use a Dremel engraver. The same one you clean your plugs with, but with a smaller tip. Once it's in the plastic, it will never come off . . . . . even if you want it to . . . .

Disclosure:  the dipstick came with the plane, which I bought in 2007. Never done anything to the stick except use it and put it back on the hatrack. Is that long-lasting enough? 

Edited September 25, 2018 by Hank

[carusoam]

One part tube diameter, ID

One part surface tension

One part viscosity 

When the ID is too large, and the viscosity too low, a bubble forms and starts to rise up the center.... the liquid flows downwards at the edges....

When close, multiple bubbles will rise emptying the tube... too big or too little viscosity, one long bubble forms....

As for surface tension, and hydrogen bonding... the water likes to stay together, fuel doesn’t seem to have the same inter molecular forces... easier to form a mist....

For more interesting ideas look up capillary effect... how trees feed themselves without any mechanical water pumps....

 

If it didn’t work this way we could carry a bucket upside down and the water would stay in it...  

 

PP thoughts only, not a biologist or fluid mechanic...

Best regards,

-a-

[Nokomis449]

Mine did the same thing when I made it.  I solved it by putting about 1/2" of silicone inside the very top of the tube, and placing a toothpick-sized hole in the silicone.  That slowed the fuel exit enough that an accurate measurement could be seen before the fuel started dripping out.

I found it necessary to give the tube a few seconds for the air to displace thru the tiny hole, else the reading would be shy of actual.  I marked the tube with a Sharpie and it's been in use for about 6 years and still readable.  My tube was not rigid, so I safety wired it to a small rod slightly larger than a coat hanger.  This also allowed me to raise the bottom of the tube slightly above the bottom of the rod so the bottom of the wing wouldn't prevent fuel from entering the tube.  

[Skates97]

12 hours ago, HRM said:

Save yourself the analysis and just buy a "Universal" Fuel Hawk (0.012 AMU). Works great and they worked all the physics out so that it pipettes AVGAS perfectly.

I second this, very accurate and easy to use. I did add a remove before flight flag to the second one so it would stand out after I left the first one in the wing at the pump once... Easy to miss a clear plastic tube sitting on the wing next to the fuselage. 

[0TreeLemur]

On 9/24/2018 at 4:26 PM, M20F-1968 said:

Perhaps  there is a MS'r  that knows both plastics, physics and hydrocarbons.

 Several years ago I bought some thick-walled, rigid, clear plastic  tubing.   It's outside diameter was a half-inch,  inside diameter 1/4 inch,  with a wall thickness of 1/8 inch.  

 my purpose was to use it as a fuel tank gauge.   I expect to be able to cut it to length,  polish the ends,  and use it essentially like the pipette with a an engraved scale on the outside,  calibrated in gallons of remaining fuel.  

 I dragged it out this past weekend, cut into length, sanded and finished the ends, and drop it vertically into the tank. I put my finger over the end as I would a pipette,  pulled  two out with my finger on the end of the tube,  and the fuel despite my finger basements,  promptly ran out of the tube.   I even went to the point of putting a bit of oil on the end of the tube to ensure good seal or even using duct tape and the same result occurred.

 whenever I've seen pipettes in a chemistry lab, they usually have a small outlet and small hole at the top over which one places one's finger.   Can someone quantify the physics going on here.   does this simply have something to do with the diameter of the tube itself or is this some sort of interplay between the hydrocarbon fuel and the plastic tube?

John Breda

Wetting contact angle plays an important role in this behavior.  It might be that the plastic tubing you bought has a wetting contact angle near or greater than 90 degrees, which will negate the surface tension effect, and allow air to freely enter the tube, which lets the fuel run out.  It will be a property of the tubing and the liquid.  If the wetting contact angle is >90, then the liquid is considered a "non-wetting" liquid for that solid.   No molecular affinity.   That probably explains your problem.  The plastic has a greater affinity for contact with air than for gasoline at an interface between the three.  I'm a fluids nerd.

[M20F-1968]

Fred_20:

The tube I bought is polycarbonate and has a 1/4" ID.

I can get one that is 1/8" ID, made from either polycarbonate or acrylic.  Which should I try next?

John Breda

[M20F-1968]

 

11 minutes ago, Fred_2O said:

Wetting contact angle plays an important role in this behavior.  It might be that the plastic tubing you bought has a wetting contact angle near or greater than 90 degrees, which will negate the surface tension effect, and allow air to freely enter the tube, which lets the fuel run out.  It will be a property of the tubing and the liquid.  If the wetting contact angle is >90, then the liquid is considered a "non-wetting" liquid for that solid.   No molecular affinity.   That probably explains your problem.  The plastic has a greater affinity for contact with air than for gasoline at an interface between the three.  I'm a fluids nerd.

The tube I bought is polycarbonate and has a 1/4" ID.

I can get one that is 1/8" ID, made from either polycarbonate or acrylic.  Which should I try next?

John Breda

[0TreeLemur]

18 minutes ago, M20F-1968 said:

 

The tube I bought is polycarbonate and has a 1/4" ID.

I can get one that is 1/8" ID, made from either polycarbonate or acrylic.  Which should I try next?

John Breda

Hi John- I've searched for data on wetting contact angle data for gasoline- haven't been able to find it.   I'd suggest go with the 1/8" ID of either.   That should help.

[0TreeLemur]

The fuel stick in the photo by @Hank looks like acrylic to me.  I'd try 1/8" ID acrylic.

 BTW- @M20F-1968 I really like the paint job on your aircraft.

[bob865]

On ‎9‎/‎26‎/‎2018 at 12:26 AM, Skates97 said:

I second this, very accurate and easy to use. I did add a remove before flight flag to the second one so it would stand out after I left the first one in the wing at the pump once... Easy to miss a clear plastic tube sitting on the wing next to the fuselage. 

Mine is missing now.  The flag is a good idea.  When I get a new one I'll have add the flag too.  I'm pretty sure I did miss the clear tube on the wing.

[jaylw314]

1 hour ago, bob865 said:

Mine is missing now.  The flag is a good idea.  When I get a new one I'll have add the flag too.  I'm pretty sure I did miss the clear tube on the wing.

When I used one previously, I put the calibration data on a orange 4x6" card and laminated it with the thickest plastic available, then chained it to the fuel stick.  That way, the card was visible, and because it was laminated it was stiff enough to keep someone from dropping the whole stick into the fuel tank (it was for a flight club Cherokee).

I have to admit, I've never bothered with one in the Mooney.  I've always fueled to the tabs or full before any trip longer than a couple hours anyways, and the endurance with 50 gal is more than 5 hours.

[Skates97]

10 hours ago, bob865 said:

Mine is missing now.  The flag is a good idea.  When I get a new one I'll have add the flag too.  I'm pretty sure I did miss the clear tube on the wing.

Here's what I did, I took some duct tape and folded it to go through the grommet and then a couple strips to hold it to the end. 

[M20F-1968]

So I bought new tubing, both acrylic and polycarbonate.  They are in long lengths.   1/4" wall thickness and 1/8" ID.  I tried a 4 foot length of polycarbonate and it held the fuel level fine.  Once I shorten it it should work even better.  The acrylic tubing is 6 feet in length and have not tried it.

Now to find time to drain the tanks and do the work.

John Breda