Getting to finish the barbecue

Those who depend on bottled gas for their cooking can be relied on to always have a spare, filled bottle. As soon as the bottle attached to the cooker is empty, it is replaced with the filled bottle and a replacement obtained for the empty one. The problem comes with occasional users of bottled propane or butane, whether this is for a building work or road maintenance, a gas barbecue, a boat, a caravan or to take for camping. Who wants to have to cart two gas bottles up a mountain for a camp? The only way to be sure how much gas is left in a bottle is to weigh it. Since gas bottles are usually made of steel and quite heavy, it is not easy to tell by picking one up, whether the bottle is full, half full, or almost empty so to do this, the user would need to to be in possession of a proper weighing machine. The Challenge Our challenge this month, therefore is to come up with the simplest and most cost-effective way of establishing how much gas remains inside a cylinder of propane or butane gas. The first idea one might try is tapping it, but, personally we can't tell any difference in note between a full, half full and nearly empty gas cylinder. There seem to be greater acoustic differences between cylinders than are afforded by their being full or empty. Or one could try shaking it (hopefully not with the cooker connected) to detect the inertia of the liquid inside, slopping about. It would be possible to find the internal level of the remaining liquid gas using ultrasound, but such systems are neither low cost, quick or simple to use, or very compact. One could purchase several filled gas cylinders for the price of a single system and it would be simpler to have a spare gas cylinder than a pair of electronic transducers, cables, and an instrument in a carrying case. In any case, if used only occasionally, the batteries will almost certainly be found to be flat when the instrument is taken out and deployed. What is needed is something simple. The solution that we offer in next month's edition of 'Eureka' does the job most simply and inexpensively. Once you see it, may consider it obvious, except that nobody seems to have thought of it before, allowing the inventor to patent it. It is commercially available, but not in the UK, despite being a British invention. See if you can come up with anything better.