Floater type LI for liquid ethylene

There are several different senarios for what is happening, one is that you have a device that measures level by bouyancy (it does not need to float on the top of the liquid) but measures the pushing up or pulling up with a solid rod arrangement. This is due to the deeper in the liquid the bouy is submerged the greater the effect.

Another is that the liquid ethylene (I don't know what temperature it is at, so this is a guess) is at so low a temperature that it penetrates into your bouy/float and causes it to sink as it is now saturated. (Liquid nitrogen will flow through solid objects like steel plate because it stops the moleules moving and flows between..

that would be my 2 cents, I hope it helps.