Nifty fuel/water test
I was given a tube of this gear to try out, it is specifically for testing tanks for water residue. Smear some on a dipstick and just dunk it in the tank, the presence of water turns the paste red . Available from here https://www.oilybits.com/water-removal/kolor-kut-water-finding-paste/prod_149.html
Smear a bit on a stick.
test bottle with water/diesel
It shows the exact amount of water so you can tell how much is at the bottom.
The paste turns red on contact with water only and not affected by diesel so just need a smear on the stick for a few inches and push to the bottom of the tank or barrel. Its does not creep either , I measured the water in the bottle and it was exactly the same as the red mark on the stick so its pretty accurate.
Bob