The only accurate way of establishing the amount of gas left in a cylinder is to weigh. I.e on the side of the bottle will be the tare weight (net weight) on a gaslow system I would expect the weight to include the weight of the regulator too. subtract tare from actual weight will give an indication of what is left.
Gauges are at best, a poor indication for the following reasons.
Gas Bottles are filled with liquid gas, 80% at full, the remaining 20% is vapour. (which is where the water trick comes in )..The gauges measure vapour pressure, which explains why the gauges dont drop. when the gas becomes close to empty there is a slight Vapour pressure drop which triggers the gauge. This still doesnt indicate what is left only a VP drop. So don't expect it to tell you 1 kg or 2 etc.
There is another factor to consider too, and that is the Vapour pressure will change with atmospheric conditions i.e atmospheric pressure and temperature will vary. there is no contingency built into the gauges to compensate for this, result inaccurate readings. An extreme example is butane in winter, There can be plenty of gas in the bottle, but low temperatures stop it from gassing so guess what bottle shows empty on the gauge!
Spring balance scales and the technique described first paragraph is the only accurate indicator.
Channa
P.s the reason the water trick works, is that lpg gasses through drawing heat through the wall of the bottle ( wetted area) it is the temperature difference between the wetted area and remaining gas which shows visually on the side of the bottle when hot water is poured down it.
To go completely off topic seeing as I am on one, my explanation of how LPG gasses drawing heat is why we dont wrap bottles in insulation in winter it has the opposite to desired effect.
