how do I know if I got a full tank of co2?

If you just want to see what I am asking and skip all the drama just scroll to the last sentence.

Sorry about the long winded story here, I just gotta' vent though because I think I got shafted. I just got my first co2 tank, keg, etc... from a new lhbs that is considerably closer to me. So I purchased everything I needed from them and bought 5# of co2 as well for $15. They filled the 5# tank for me and sent me on my way (not the most friendly or even well educated people, I ended up showing them the proper way to set up a keg which is a rather scary since there is no way a rookie first timer should know more than the "experts" but I digress).

Anyway, I took the tank home, hooked it up to the regulator, and opened up the valve to discover the pressure was only about 740psi or so. I did a little research online where I discovered that a fully charged 5# tank should read about 846psi at 71 degrees. However the best way to find out how much was in there was to weigh it and subtract the tare weight of the tank and a little bit extra for the valve. It should be close to 5#. Well duh, this made perfect sense to me so I set off to weigh my tank. It came back at 8.69# total. The tank weighs 7.55# so it appeared to me that I only got about a pound of co2... Upon finding this out I quickly left for the shop to explain this to them. When I got to the store, I explained to them that I thought it was short. They promptly hooked it up to their tanks and gave it a charge. Then they hooked it up to a regulator (which they struggled to figure out how to get on) and said it was reading 850psi. I asked if they could weigh it and they told me that the weight never came out right anyway so there was no point. So I just took my tank and left. Lesson learned I will continue driving the 1.5 hours to the next closest lhbs and find a welding supplier for my gas. An I completly off base here?

So I guess my question is what is the most reliable way to determine how much co2 is in a tank? Do I look up the temp/pressure on a chart, or - after subtracting the 7.55# canister - should I be somewhere in the 5# range?


Thank you very much for your time.

You got screwed. The only way to know how much CO2 is in the tank is by weight. The tank pressure will be near constant at a given temp as long as there's some liquid CO2 in there.

Well crap... I knew it, but it still stings to get it confirmed. I guess I won't be going there anymore. Perhaps they are rather knowledgable about homebrewing and are just clueless about kegging, but I'll never know now.

Thanks for the help.

Now that I'm going to need more gas sooner than anticipated, does anyone know of a good welding supply place near Katy, TX?

Yeah you got screwed and those guys sound like morons. I hope they gave you a refund at least. You are correct, the only way to properly fill is to weigh it. Were they filling the tank with liquid CO2? Was it cold afterwards? If they're just topping it off with gas they're definitely doing it wrong, and if they're filling with liquid and not weighing it they could potentially be overfilling tanks. Which is quite dangerous.

Check for a fire extinguisher shop as well, that's where I get mine filled. A quick Google search tells me there's one in Katy (Certified Fire Protection & Fire Extinguishers) and a few right outside.

I'm not sure if you got hosed or not.

Just something I wanted to pass to others on this forum. I take my CO2 tank to a guy who deals only in gasses. He supplies gas to bars and industrial sites. He delivers propane to homes and all of that. So he is a gas guy.

Anyway, when he fills my tank, he always puts gas in the tank and then opens the valve all the way so the gas he just put in blows completely out. Then he fills the tank completely and I'm on my way.

So I asked him - "Why do you open the valve and let all the gas blow out and then refill it again?"

He said "it cools the tank."

I undertsand that gas moving from high pressure to low preassure at a high rate will cause extreme cooling. Likewise, compressing gas at a high rate will have a heating effect. He went onto explain that the tank needs cooled so that he can fill the tank to the proper level. He said- If gas is put into the tank without cooling first, the tank and gas would heat up, and less gas would be put into the tank. He said anyone who takes their tanks to a dealer who does not cool the tank first is getting less gas and may be paying for more gas than they are getting.

If my gas dealer is accurate... This leaves me to wonder... how many people are getting less gas than they are paying for?

You're absolutely correct. Any proper gas supplier will do what you described, and it's why most places are happy when you bring them a nice cold CO2 tank to fill (from a cold car or from your kegerator).

If you don't cool down the tank first as you described, it might not be possible to fit all 5# in there. But regardless, the tank should be filled by weight, and a 5# tank should be filled to 5#.

I guess what I'm saying is that a supplier that doesn't chill the tank first is a supplier that isn't filling by weight (or else it would be obvious that they're charging for 5# but only selling 4#, or whatever). And not filling by weight is a very dangerous way to fill CO2.

As an overfilled tank warms up, the safety disc in the stem can burst. If this happens as you're driving home you could damage your hearing (it's loud) or suffocate (if the windows/vents are closed), but more realistically you'll probably crap your pants and crash your car.

So avoid those suppliers. And maybe even report them.

zachattack said:

But regardless, the tank should be filled by weight, and a 5# tank should be filled to 5#.

Click to expand...

The tank sits on a scale as it is filled. It is weighed to ensure that it is full.

After reviewing my post, I realized I left that part out.

You didn't need to include that part, the way you described your supplier (as a legit, knowledgeable and safe operation) I just assumed that's how they would fill.

The weight of the empty tank is usually stamped on the side of it somewhere. Looks like this

TW 4.7#

Then you can weight the "full" tank to confirm you got what you paid for.

Wow thanks for all the information. Definitely more educated now and will not be taken to the cleaners again.

As far as getting a refund, I think I have wasted enough time with these morons and will just file it under stupid tax.

I never even thought about fire extinguisher places. Makes perfect sense though, thanks for the tip.

So if the colder the better is the idea, is it safe to move the tank from the refrigerator to the freezer for an hour before I take it to get filled and then transport it in a cooler?

I'm not sure if that's necessary. If you call the fire extinguisher shop, I'd just ask them what they'd prefer. Some shops seem to like it. The colder it is when you bring it in, the less CO2 they have to use to chill it.

In many places when something is sold by weight, a certified scale must be used. If that LHBS is selling a 5lb fill without using a scale that has been inspected and certified, you may be able to report them to the local enforcement folks.

I would start by trying to educate them on the proper procedure. If that is met with resistance, then I would report them.