As i have said in other threads, I am new to kegging. I bought my setup brand new, all virgin parts. When I got my CO2 cylinder it read 800 psi. I did all the checks to make sure there are no leaks. I have kegged 2 batches and now my guage reads in the “order more gas” range which is still around 500 psi. Is this like the gas guage in my car where it is full today and then drops down to 3/4 very fast and then the rest of the way is very slow to drop? Or is it possible that I am low on gas? How can that be?
Co2 is measured by weight, not pressure. The pressure will change with temperature. The only important pressure reading is what you’re setting to pressurize your kegs. Personally. I remove the high pressure gauge on my co2 regulators and plug them. I find that they get in the way and have no importance. On my Nitro set up, that’s another issue.
So I guess I should weigh my cylinder when it’s full and then determine the amount by subsequent weighings.
Like another poster said, the high side measurement gauge can be deceptive. Temperature shifts in the day can change the reading on that gauge causing unnecessary worry. I’ve even had one read 900psi on a completely empty tank. It’s best to thoroughly test your connections with soapy water/water submersion to ensure you don’t have a leak anywhere down the chain. After that, ignoring the high pressure gauge becomes a breeze 
[quote=“in_the_basement”]…Is this like the gas guage in my car where it is full today and then drops down to 3/4 very fast and then the rest of the way is very slow to drop? Or is it possible that I am low on gas? How can that be?[/quote] No, it is not like the gas guage in your car. The high pressure guage measures pressure not volume. As long as the temp stays constant the needle won’t move until just before you’re out of CO2.
Is your tank in the keezer with your kegs?
