ive had a local mobile man quote me 45 quid but since my cars in jp boards next week for service im gonna have them do it (£60 + vat). just as a point of note there is no way (that i know of) to measure the level of gas remaining until you empty the system and weigh the gas that you have alreadytaken out!
In liquid form maybe but never in a month of sundays in gas form.....surelydont mock junior, its actually true...
weigh an empty pop bottle, then put a few psi of pressure in it and weigh it again. It weighs more! We did it in science when I was a kid.
Also all the different gases weigh different amounts, compare helium to oxygen etc.
For this you would need very accurate scalesquite, each particle of the element will have a certain weight...different pressures means different number of particles per set volume of space and different weights!