Gas tank getting empty?

[Agrajag]

Does anyone have a good solution to knowing when their tank is nearly empty? I'm pretty sure I'm only going to realise it's empty when I'm in the middle of a roast and the stove goes out which obviously isn't ideal. I read you can pour hot water on the side of the tank to feel where it's up to, but I tried that and it doesn't seem very accurate.

Also in Australia we have "swap'n'go" where you just take your empty tank and swap it for a full one, so you might not get exactly the same kind of tank, so weighing it doesn't seem to be an option. Any other ideas?

[SeanSheep]

Each individual gas cylinder should have a "tare" weight stamped somewhere on or near the collar. If you weigh the tank (when disconnected) then subtract the tare you should get how much actual gas is left within a fairly reasonable margin of error.
 
When you boil the kettle and pour it over the side of the gas cylinder you should see a faint line of condensation and a difference in temperature somewhere on the tank and that will tell you the level that the gas is at. This process can be made easier by special magnets that are designed for the task, see http://www.amazon.com/Removable-Accu-Level-Propane-Gauge-Magnetic/dp/B0010P3OU2 , http://www.raysoutdoors.com.au/Product/Hammerhead-Accu-Level-Removable-Propane-Tank-Gauge/131440.
 
There are also pressure gauges you can put in-line to give you an indication of when you're getting low. See https://www.bunnings.com.au/gasfuse-safety-gauge_p3231002
 
Hope this helps.

[Agrajag]

Yeah thanks that's very helpful, I hadn't noticed the weight stamped on it, I think that will be the easiest. Cheers!

[easygene]

I happen to have a propane Weber grill.  It sits on back deck by my roast cart.  The grill has a 16 position vertical indicator with a physical pointer that indicates degree of weight remaining in the tank.
As I heft the tank occasionally when in doubt I can keep track of what is left almost to the tenth as I can swiftly change them around.
Swiftly?  Well, 2 minutes.

[archgetty]

I've found that the only really accurate way to tell is the weight of the propane:

https://www.amazon.com/Grill-Gauge-GG-1100-Original/dp/B0012GTU3O?ie=UTF8&*Version*=1&*entries*=0

Magnets and gauges give only vague indications.