Thought the magnetic ones were a bit cheap.
Think i will go down the gaslite bottle route ,seems to be the best option.
The theory behind them is sound, and when fitted and used correctly they can provide an easy to see temperature differential between the vapour and liquid layers when the bottle is supplying gas. No batteries required.
However, if you can tolerate the inconvenience of running out of gas from time to time, why have an indicator? Apart from weighing the bottle, none of the methods I have seen can be totally relied upon to give a consistently precise indication of when all the liquefied gas has been evaporated, and even to obtain the accuracy needed to check by weighing requires the bottle to be disconnected from the pigtail or regulator.
Consequently, if when using a single bottle system, if you rely on any type of indicator, there is a good chance you'll change the bottle before it's really empty.
If you can accommodate the weight, you would be best served by having an automatic change over valve which should indicate which bottle it's emptied.