Thingy said:
Its a shame that there doesnt appear to be any sort of accurate guage save for taking the bottle off and weighing it.
I was looking at one of these
https://chorleybottlegas.co.uk/shop/lpg-equipment/propane-cylinder-contents-gauge/
I am always dubious about adding something else to the line, the more connections, the more chance of a leak. I am also led to believe the guages are wildly inaccurate.
The gauge you are looking at is just a pressure gauge, and it designed to monitor the pressure inside the bottle.
Your right, such gauges are a poor solution to monitoring bottle content, because as long as there is any liquified gas remaining in the bottle, the pressure inside the bottle will be defined by the temperature of the bottle not how much liquefied gas remains. It does give you a quick visual indication of when the bottle is nearing empty.
The pressure will only start to drop significantly when all the liquefied gas has evaporated , and there is only compressed vapour remaining, and as it is used up the pressure will start to drop. Temperature will still be a major factor though.
You can get liquid crystal strips to stick to the outside of the bottle. These change colour depending on the temperature of the surface they are stuck to. They will detect the temperature difference of the bottle wall if it's in contact with vapour or liquified gas. These will only work if gas is being drawn off the bottle. The principle relies on the fact that the liquefied gas at the bottom of a bottle has to boil to release vapour. The boiling requires heat energy, and it gets its heat by drawing heat through the bottle wall from the ambient air around the bottle. This cools the bottle wall more quickly than the bottle wall against the vapour at the top of the bottle. The LCD strip will show the temperature change at the surface of the liquified gas. It doesn't work when no gas is being used, and the bottle temperature has evened out.
There are some systems that use ultrasonics to detect the level of the liquid in the bottle. With this system you can have a remote display inside the caravan for convenience.
So far all these system will indicate when you are running low on gas, that may be sufficient for many users, but they can't tell you how much gas you have remaining. The only reliable way I am aware of knowing how much gas you have is to weigh the bottle and to subtract the tare weight of the bottle. That is why gas in bottles is sold by weight and not any other method.