Suitable plastic for sight guage?

[sailorman]

The sight tubes have to be open to atmosphere to work, but I can't see why there couldn't be a valve on the top of the tube. I noticed your oak spar which sounds a good idea.

boat tanks have a breather so will work

 

[Corribee Boy]

Not if the top of the tube's closed, Sailorman, but I should have thought of connecting it back to the breather.

I've got an internal float connected to a sort of gauge on the top of the tank, but it's almost unreadable due to being hidden in the locker so I still use a dipstick myself.

 

[gordmac]

The plastic tube is cheap, just replace it when required. If you don't have a normally closed valve on the bottom of the tube please fit one, not worth the risk.