I had a visit from an air conditioner repairman who brought with him a tank of refrigerant. He measured the system and determined that the pressure was about 20 psi too low. So he added refrigerant to the system until it was right.
When it came time to pay him, he said, "well I had to raise your system by 20 psi so it's going to be [20 x price for 1 psi]."
The price was fine, but I am still wondering: would it have taken the same amount of gas to raise my system from 0 to 20 psi as it would to go from 80 to 100, say?
Or, in other words, is it absurd to charge by pressure delta instead of some more reliable measure of what was dispensed from the tank?