Haunted Fuel Tank

[Dixie Life]

It has to be me. This was the first time I topped off one of my fuel tanks. I wanted to get an accurate 20 gal. marks and total capacity for each tank. Here’s the setup, I shut off all valves to isolate the tank. Made sure the tank vent was clear. Started pumping an called out every 20 gallons to wife who would mark the sight tube. All good so far. When she said it was a few inches from the top of the tank I shut off the fuel at 280 gals. Now get ready. When I shut off pumping the fuel she said fuel in the sight glass dropped about 3”. WHAT? I pumped another 20 gallons. When I stopped, the same thing happened again. ??? I know there has to be a simple explanation but I can’t find it. What am I missing?

 

[READY]

I assume you have multiple tanks? Did you check those levels? Perhaps there is a crossover you did not close or a broken valve? I once had a fuel manifold valve break internally. While it felt and looked like it operated normally it was not.

The only other thing I could think of was the vent was not letting air escape as fast as fuel flow so it settled. Not sure I believe that is even possible but there must be some logical explanation.

No chance your fuel overflow was dumping excess fuel? So that last 20 Gals or 3 inches was over filling and the excess as escaping via a tank overflow or somewhere in the bilge.

I cannot wait to hear what you find to be the true explanation.

 

[tiltrider1]

I am not fully understanding what you are saying.

If the fuel kept dropping to the same level then your tanks are tied together through the vent tubes.

If the sight line just dropped 3 inches when the pumping stopped but it was higher on the sight glass each time, this would be caused by too small a vent line or extreme pressure from a high speed fuel nozzel.

 

[Pete Meisinger]

There must be a crossover line you missed. If that is the case, all your measurements are off.

pete

 

[DDW]

The crossover line theory is easily checked by looking at the second tank sight glass. If it moves correspondingly, there is a connection between the two tanks. I think there are other possibilities though. A sight glass can be influenced by a few things, one of which is the air pressure inside the tank. Pumping fuel into a tank can raise the pressure (more likely) or lower it, the dynamics are pretty complex, especially when you suddenly start or stop pumping. 3" in a sight glass is only about 0.3 psi in air pressure. Sight glasses tend to have small holes connecting them to the tank so they can lag any changes by a few seconds. The flow of fuel into the tank (if the fill is anywhere near the sight glass) might also affect it. The easy solution is to stop pumping, wait for the sight glass to stabilize, and take your reading then.

 

[O C Diver]

Depending on the venting and a dip in the fuel hose, there can be pressure in the tank while filling. Until the pressure is equalized between the tank and outside through the vent line, the pressure in a site glass or Tank Tender will read slightly off. This happens with my water tanks.

Ted

 

[Island Cessna]

Most vent lines I have seen have a vertical loop in them to stop water flowing back into the fuel. If there happens to be liquid in the loop the tank pressure has exceed the depth of the liquid in the loop before actually starting to vent.
The fuel flowing down the fill pipe into the tank causes a pressure in the tank, in addition to the pressure caused by displacing air with fuel. The venting systems on our older boats were designed in an era where the flow rates of diesel sellers were much lower than today’s.
All of this points to the fact that there is a variable air pressure in the tank during active fuel filling. The sight gauge is a form of manometer, and depending on its design will respond in some measure to this variable pressure.
The only way to ensure a stable sight gauge measure is to stop filling, and let the pressure in the tank stabilize.
Note I said stable not accurate. The sight gauge reading is also affected by both lateral and fore/aft trim. Calibrating one tank while filling only that tank will be affected by the changing list. The reading is also affected by fore/aft trim (water tank level for instance). As pointed out above these readings are estimates, and getting too tied up in accurate calibration is not productive!