Gas level indicators

Gnash58

Hi all. I'm thinking of buying a gas level indicator. They seem to range in price from £60+ down to £1.99 - so a lot of choice. Can anyone recommend 1 or are they a waste of money? I do have two bottles so can change over when 1 runs out and replace the
empty 1 asap. But I'd like to have an indicator (if they work) to give me an idea of the level. Cheers, Gary