Thank you Pelagic for the link. I am familiar with the difference between ullage and a sounding, I didn't come to use the term by accident
A sounding tells you how much fluid is in a tank, and ullage tells you how much fluid isn't in a tank.
I agree, ullage is useful for petro chemical cargos, because it does allow you to calculate for expansion. Ultimately they are two terms used for measuring how much fluid is or isn't in a tank though.
I used the term ullage because I have a large funny
shaped fuel tank
on my boat, and I tend to measure from the tank top down (because I can see the top of the fuel
level with a flash light). You may have noticed I said "check" your bilge
and take an "ullage" of your fuel tank
About 17 or 18 years ago I did a stint on a chemical tanker as a third officer and learned the distinction then.
We can call it a sounding if you are in the mood for splitting hairs though.