fuel tank vent

[NWade]

I am a little disturbed about reading in this thread the notion that liquid gasoline does not expand much when its temperature increases

I am sorry you are disturbed, but it doesn’t change the fact that it is true - liquids do not change volume very much* when the temperature increases. Gasses change volume quite a lot by comparison, and it is those vapors that you want to allow to escape.

I understand that you had a bad experience with a car once leaking fuel in a hot day, but many possible things could have been wrong with that fuel system.

*Notice that no one says it has zero change; just that it is a very small change. But it’s also not a purely isolated change - as you increase the temperature you’re also increasing the rate of evaporation of the gasoline, which will then flow out the vent.

—Noel
Sonex #1339

[Area 51%]

Gasoline changes shape to the tune of .000528 per degree F.

Someone please check my math, but I get the 16.5 gallons of gas in the Legacy tank expanding by a little over 22oz between a 70deg morning and a90deg afternoon. That seems like more than enough to get a siphon started if you're not careful about leaving a little airspace in the tank.

[SNX1508]

I am a little disturbed about reading in this thread the notion that liquid gasoline does not expand much when its temperature increases

I am sorry you are disturbed, but it doesn’t change the fact that it is true - liquids do not change volume very much* when the temperature increases.

—Noel
Sonex #1339

Perhaps the laws of physics cease to apply in your garage, but yes, the volume of liquids does in fact noticeably change with changes in temperature, as stated by Area 51%.

Local gas stations even have notices posted on the pumps stating that the volume of fuel pumped will vary with temperature.
Liquid ammonia has such a great change in volume with an increase in temperature that containers are typically only filled to 85% to avoid a catastrophic explosion.

[GraemeSmith]

If you are REALLY worried about Weight and Balance - you can use 6.15lbs per US gallon in the dead of winter here in New England and 5.85 lbs per gallon for the weight of Avgas in the current heat of summer!

When fuel is delivered at the FBO - they might have ordered "10,000 gallons" - but what they pay for is an amount of energy density. The gallonage delivered is temperature measured and compensated for to arrive at an amount of energy delivered - and so paid for.

Yes - liquid volume changes with temperature!

--

Meantime - that which started all this - I spent part of yesterday fitting a small check valve to the vent line at the tank end and extending the vent line along the bottom of the aircraft to clear the exhausts.

[Area 51%]

Funny thing...water increases in volume both when it gets hot, and cold (below 32deg F).

[Bacon8tor]

I'm going to buy Peter Anson's fuel vent check valve. Like anything it has to be used properly. If you did fill the tank to the brim then cap it the natural expansion of the fuel will put some amount of stress on the tank. But who does that? Filling to the brim might bring on other problems like, um, overflow? Just don't fill your tank to the brim. The magic formula to prevent fuel expansion overflow might be say one inch below the brim. And if you do happen to accidentally fill to the brim either drain out some fuel or fly some off right away.

[DCASonex]

Don't forget that the plastic tanks also expand with heat.

David A.