How low of a PSI do you need?

[Nonsense]

Just started getting into kegging. My regulator has a range of "Order gas" at or below 500 psi. Assuming a 5 lb co2 tank and brewing a moderately carbonated ale. How low of PSI do you need be able to complete carbonation? Would there be any left to serve with?

 

[AJinJacksonville]

I may be misreading your question...but I've got a Taprite regulator that shows me on the lower end of red (which I would assume would mean time to refill aka "order") and I'm still pouring just fine. My 5lb tank has run through about 6-7 kegs worth of beer and is just now in the red to where I need to refill/swap before the next kegging. I've never paid too much attention to the numbers. I've been force-carbonating each one and with no leaks the 5lb'er lasted way longer than I expected.

 

[Qhrumphf]

A CO2 cylinder, when full, is full of liquid CO2 at very high pressure. As the CO2 is used, the liquid will evaporate into CO2 gas, filling the headspace of the cylinder and keeping the same pressure.

That pressure is also directly tied to the temperature. A full CO2 will read lower at fridge temps than it will at room temp. The volume of gas isn't any different, just the pressure of the tank.

When the liquid CO2 (the actual fuel source if you will) is entirely evaporated and your cylinder is literally running on fumes, *then* you'll start seeing the high pressure gauge drop. It's basically imminent failure. May be enough to *serve* a keg. I wouldn't trust it to carb.

The direct answer to your question would depend on the temperature of the CO2 tank, the amount of beer you're carbonating, and the level you're carbing to.

It could be mathed (don't know it off the top of my head) given enough information.

 

[camonick]

I’m not sure I’m fully understanding your question either, but your regulator should have 2 gauges. The one you mention is reading the vapor pressure in your tank. The other gauge is your line/ keg pressure. It is the one you’d be concerned with while carbonating. Use a carbonation table to determine your desired carbonation level: https://www.kegerators.com/articles/carbonation-table-pressure-chart/

As mentioned above, a 5 lb tank should have sufficient gas to carbonate and serve quite a few kegs.

 

[grampamark]

If you want to know how much Co2 you have left, weigh your tank. On the neck of the bottle, along with the test date and some other info, you will see a stamped marking “T. W. XXlbs”. That indicates the tare, or empty, weight of your tank. I currently have a steel, 20lb bottle. The tare weight is 31.6 lbs. I weighed it today as the tank pressure was getting close to the red. It weighed 36.5 lbs, so I have just under 5 lbs remaining.