I will admit to my total ignorance when it comes to pressurized storage containers and could use some help. I have had no luck trying to find answers to most of my questions on the net.
Here is an idea I have had for years and wondering if it can be safely pulled off. The goal would be to have a pressurized utility sprayer that I can carry around with me for spraying roundup etc. I am tired of pumping my 2-3 gallon sprayer every several minutes. I have a 5-gallon backpack model but once full I have a hard time getting it on/off by myself and it hurts my lower back as I walk around my 6 acres. I like the idea of a smaller, pressurized, hand held 2-3 gallon sprayer. I know they make battery powered units but I like to experiment & was wondering if an empty propane tank be repurposed (refilled with air) to provide the 10 PSI that I need? I do not think the high pressure paint-ball tanks is what I want as they are thousands of PSI and expensive.
I have long wanted to take an empty (small torch sized) 1-pound propane tank and refill it with air via my shop air compressor and try out my idea. I would attach a regulator to knock down the high pressure air down to ~10 PSI.
My 1st & most important question is what PSI can these small 1-pound propane tanks hold safely? The last thing I would want is to set my shop compressor too high & have a catastrophic failure due to too much pressure while refilling it. The next question would be where can I get a low-pressure regulator with a gauge so I can dial in the PSI into my 2-3 gallon sprayer.
Sounds a bit complex to set up for a backpack sprayer instead of using the rechargeable backpack ones. But, there used to be kits for sale to convert grill-type propane tanks to portable air tanks. You got a screw-in manifold with a hose, fill port for tire chuck, shutoff valve, and pressure relief valve. Relief was set for 125 psi.
The propane tank was ok as long as not rusted inside. You can find a kit on line. Those are probably what you see on You Tube. I have used them and they work, but the propane tank is a bulky shape to carry around.
Harbor freight makes an incredibly light weight aluminum air tank. Good to 140 psi. Better shape to carry. I got one to replace a steel inflator tank last year. No rust and unbelievably light to carry. You should look at it.
Probably an electric rechargeable 2-3 gallon sprayer would be simpler for you. Take a good look at what is available from the good brands.
Most of the grill size propane tank fires and explosions were caused when tank was heated by sunlight when carried in a car. The tank would heat, pressure would rise, the relief disc in the tank would release gas into the closed environment in the car, and something would ignite the gas, causing an explosion. Or, someone would store the tank to close to a heater or furnace. Tank would warm and release gas, and the appliance pilot would light it.
You don't have an explosive gas in a grill cylinder when using it as an air tank. Just be sure to ventilate first to get all gas out before using it for air.
To get a cheap 12v compressor which can put out some air, you could try to get a junkyard air shock/suspension air pump at a junk yard. Might even come with a small pressure tank.
If you have rechargeable tools, what kind of tire inflator is available for your brand of battery. Makita is excellent and might do what you want.
if you are talking about putting 100 psi air from your tank into a sprayer, all you need on it is a tire valve installed and an air pressure gauge on the inflator you are using to fill from 100 psi source. You can watch the gauge and stop at the right pressure.
Good luck with the project.