Remaining CO2 in Cylinder

[NBBrewer]

So I am using the ideal gas law PV=nRT to determine the remaining CO2 in my 20 lb tank.

I measured the tank pressure as 20 bar converted to 19.74 atmosphere
I also looked up the volume of the tank as 13.4 litres
I then measured the temperature as 10.4 degrees celcius converted to 283 Kelvin and plugged all the numbers in to give me about a pound of CO2 remaining.

Are there any wrong assumptions with my calculations?

 

[VikeMan]

The CO2 in the tank is normally a liquid, with gas in the headspace at a constant pressure (for any given temperature). That's why (for a given temp) the high pressure gauge doesn't move until the CO2 level is very low, i.e. there's no more liquid. Once there is no liquid CO2 remaining, your calculation should work, though I think you need use absolute pressure rather than gauge pressure.

 

[NBBrewer]

Thanks Vikeman. So plugging numbers in while taking approximate atmospheric pressure into consideration gives me only a slightly higher mass of CO2 by a fraction of a pound. My weight of the cylinder is about 26 lbs and I can't see a tare weight on the cylinder unless it's something other than lb or kg.

 

[Punx Clever]

You are looking for a marking that starts with a "T". My current 20# CO2 cylinder has a rare weight of 11.4 kg, which comes out to just about 25 lbs.

 

[IslandLizard]

AFAIK, every tank has a T (Tare Weight) stamped into the neck area. Perhaps it's underneath one of those labels?

Gas law doesn't apply to the liquid CO2 itself, and that's where the bulk of your CO2 reserve is. So you do need to subtract the TW from gross weight to determine that. Don't forget to remove the regulator and whatever hangs off it, or subtract that weight too.

 

[NBBrewer]

I could find anything with a T or TW but these numbers are stamped. I thought they were inspection dates but maybe its tare in kilograms

 

[NBBrewer]

So I am using the ideal gas law PV=nRT to determine the remaining CO2 in my 20 lb tank.

I measured the tank pressure as 20 bar converted to 19.74 atmosphere
I also looked up the volume of the tank as 13.4 litres
I then measured the temperature as 10.4 degrees celcius converted to 283 Kelvin and plugged all the numbers in to give me about a pound of CO2 remaining.

Are there any wrong assumptions with my calculations?

As a side question maybe a new thread... does anyone know at what temp/pressure does a 20 lb cylinder become gas only? Or a modification to the ideal gas equation to account for this.There must be a curve or calculator for that.

It also seems when I am getting a 20 lb tank I am getting about 17 or 18 lbs of CO2.

 

[IslandLizard]

It also seems when I am getting a 20 lb tank I am getting about 17 or 18 lbs of CO2.

That's on the outfit that fills them, they're not filling them to their (official) capacity. So you get 10-15% less.
If the pricing is accordingly, or just great wherever you stand, then there's little to complain about.

Right before filling they should be freshly purged so the tank is as cold as can be. That's how you get to the (official) maximum fill level.

 

[Vale71]

At that pressure you have no liquid CO2 left so your assumptions and your calculations are correct.

As a side question maybe a new thread... does anyone know at what temp/pressure does a 20 lb cylinder become gas only? Or a modification to the ideal gas equation to account for this.There must be a curve or calculator for that.

It also seems when I am getting a 20 lb tank I am getting about 17 or 18 lbs of CO2.

Here you go:

https://www.agas.com/media/2426/r744-pt-chart.pdf
At 20 bar you're already running on fumes, unless you actually live in a walk-in freezer...