Boyles law. Henrys law, Charles law. Effects on the body at pressure. Signs and symptoms of pressure related problems.
Get proper training.
BSAC 1st class diver
Club instructor
Services Air Diving Supervisor.
Some research will reveal what they fill theirs with.
Mine was an old brass one,
Fragrance free baby oil was used here:-
Suunto marine compass repair – Njål Brekke
There will be a fill hole on it. Get some compass fluid and a syringe.
Turn it until bubble is at the fill hole and add fluid.
I filled mine with vodka - works fine.