How do I know if the CO2 tank is empty?

[ILMSTMF]

I know, kind of dumb question.

A wise friend explained to me that the capacity gauge on the regulator is usually not a perfect indicator of tank fullness. He told me that once the needle goes into the red zone that the needle will move much faster towards the completely empty indicator then how quickly the needle moved before entering that range. And he was right.

The 20# tank has lasted well over a year and has recently entered the red zone on my Taprite regulator. I have one keg that is "naturally carbonating" at 12psi (I didn't do a 30psi force carb on this one). My concern is that the gas will run out before the beer is fully carbonated. I don't want to waste gas if I don't have to but, would pressing the poppet of a gas QD then listening for a hiss be enough to determine if there's still gas in the tank?

All thoughts / advice appreciated, thank you.

 

[grampamark]

The empty weight ("tare weight”) should be stamped around the neck of the bottle, where the certification date is stamped. Disconnect the tank, set it on your bathroom scale, and the difference between the weight of the tank and the empty weight is the amount of Co2, in pounds of liquid, remaining.

Yeah, I know a bathroom scale isn't a lab grade instrument, but it will give you a better number than picking up the tank and guessing.

 

[TechFanMD]

If the beer is carbonating, it is 'using' gas through absorption under pressure. If the gauge on the low pressure side of the regulator is keeping up with that usage (staying at a reading of 12psi) then it must still be getting gas from the tank -OR- you've reached the carbonation level of that beer at that temperature and reached equilibrium (no longer 'using' gas). I'd leave it alone as long as it's holding your set pressure on the low pressure gauge or until you're too nervous and need to get a fill.

 

[balrog]

Do you have two gauges? One being tank side (usually reading somewhere around 600psi) and one on the regulated side to read the 8psi or 12psi. If the downstream, or low, gauge is still reading pressure then you still have gas.

Now, my regulator reads pressure and is not a volume gauge, and I've not seen volume gauge on CO2. On propane, yes, but not on CO2. @day_trippr posted a chart "the other day" somewhere about pressures in tanks as they emptied, and that was useful-ish to know that the high side gauge will drop as the tank empties.

My humble opinion is that weight is the only way to know, as per @grampamark . If you cannot find the tare weight and you did not weigh it empty before filling there's not much chance you'll know how close you are.

the chart looked like this:

 

[Dland]

Get another tank of CO2? The scale idea should work, but if you are concerned about not being able to finish carbing a beer over the $1.23 worth of CO2 left in tank, getting another one should alleviate the worry. Turn up the regulator and see how far the secondary gauge goes up, that will give you an idea how close to empty you are.

If you have more than one tank, you never have to worry and you can use residual CO2 for transfers and kegs purging.

 

[grampamark]

Here's what the weight stamp looks like.

Here's how much I have left

Time to visit the welding shop.

 

[ILMSTMF]

Wow, thank you all very much!

Yeah, I know a bathroom scale isn't a lab grade instrument, but it will give you a better number than picking up the tank and guessing.

Yup!

If the beer is carbonating, it is 'using' gas through absorption under pressure. If the gauge on the low pressure side of the regulator is keeping up with that usage (staying at a reading of 12psi) then it must still be getting gas from the tank -OR- you've reached the carbonation level of that beer at that temperature and reached equilibrium (no longer 'using' gas).

I didn't know that! Yeah, the "set pressure" gauge is still reading as 12psi. I don't think it's fully carbonated, FWIW. Just put gas on 3 1/2 days ago.

Do you have two gauges? One being tank side (usually reading somewhere around 600psi) and one on the regulated side to read the 8psi or 12psi. If the downstream, or low, gauge is still reading pressure then you still have gas.


My humble opinion is that weight is the only way to know,

If I didn't already clarify, there's 3 gauges on my dual regulator. The two "set pressure" gauges and the fill level gauge. I think the top end of the green has a value of 1000.

Anyway, I think I'll be taking the tank off and weighing it just to be sure.

Meanwhile, have to play devil's advocate. I can't get another fill for a week. Let's assume the gas runs out and the beer doesn't fully reach my desired level of carbonation. Safe? Just put gas back on it whenever I possibly can and let it finish carbonating? Or is this big no-no? Thank you!

 

[balrog]

Safe.
No problem.
Just flatter than you'd like it to be.

 

[ILMSTMF]

@balrog Dig it, thanks. I can continue carbonating it once I get CO2 back in stock though, yeah? I know, dumb. Keg pressure is still a new concept for me. Bottling was (is) as simple as add a little sugar to the beer, cap it, carbs up over time.

 

[balrog]

Yup. It's a chemical reaction; you're dissolving something into the beer. You fill the headspace with pressurized CO2 and it dissolves into the beer. Remove the constant pressure of a full tank continuously filling the headspace and the keg just comes to equilibrium. It'll dissolve only what the headspace pressure and beer temp allow. So if you have it under 12psi at 40° and the tank goes empty, the headspace will likely drop from 12 to 8-10 adn the amount dissolved in the beer will be < 2.2volumesCO2. Just hook up new tank and headspace goes back up to 12psi and more CO2 dissolves into the beer until equilibrium is reached again with 12psi headspace and beer temp.

 

[TechFanMD]

@balrog Dig it, thanks. I can continue carbonating it once I get CO2 back in stock though, yeah? I know, dumb. Keg pressure is still a new concept for me. Bottling was (is) as simple as add a little sugar to the beer, cap it, carbs up over time.

You're fine to pause and start recarbonating whenever.

Money doesn't grow on trees, and we all pick and choose what to spend it on....but it can be worth keeping an eye on Facebook Marketplace, Craiglist, local Facebook yard sale sites, etc for used CO2 tanks and even tank/regulator sets. I got a tank and regulator for $50 and have been using the tank for 6 months (so I assume it was pretty full, I've put 6 kegs through so far). I also got two additional tanks.....one had a regulator and was bundled with a bunch of other homebrewing stuff, so I paid about $25 for it once I sold the stuff I didn't want. If you're patient and keep an eye out, it's amazing what you can get for a reasonable price.

 

[mongoose33]

As long as you have liquid CO2 in the tank, your high-pressure gauge shouldn't move (unless temp changes, as @balrog shows above). As you draw the gas off the headspace of the tank, some of the liquid CO2 turns to gas, repressurizing the headspace. So as long as there's liquid in the tank, the pressure is the same.

Once that liquid runs out, all you have at that point is high-pressure CO2. It'll continue to feed your keg until it drops below the head pressure in the keg.

So, imagine your high-pressure gauge is in the red but reads 200psi. This is what you have remaining in gas (not liquid) in the tank. That's enough to feed the keg for a while, and presuming you want something like 2.5 volumes of CO2 in your beer, that's plenty.

 

[ILMSTMF]

Update (for anyone who cares haha)

Tare weight is 28.3# and my loose scale is measuring the tank at 31#. I didn't take the regulator off. Even if I took 2 pounds off to be super cautious, still liquid in there so, all good! Gonna ride that out until my CO2 connection comes through... happy holidays! lol