How much gas left in tank?

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trustyrusty

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Hi

I have a 2.6 kg tank, that has very little gas in it, but I think enough to take to a party. I don’t have scale. If I put regulator on and if there is some pressure there will be something? Ie if full is showing 900 psi or what ever it shows, but shows 200 psi… is that something or just residue tank pressure? I have a 6 kg tank, I don’t need to refill for a while and it’s a pain to fill. I have drive 40ks. I was thinking of using a soda stream connection.
 

duncbrewer

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Normally the CO2 is liquid in the tank so sometimes you can hear it slosh around. I weigh my empty cylinder with reg and tube on when empty and also weigh it after a fill. Useful guide to CO2 usage.
I find that when the regulator pressure on the regulator gauge that is in KPa starts to drop, mine is at 500 normally it means the cylinder will run out imminently, it's because that liquid CO2 is virtually gone and you are running on vapour.
So if it's dropped you ain't got a lot.
Sodastream bottles are more expensive per gram of CO2 but if you get the refill kit you will be able to refill them cheaply from your 6kg tank. Should easily serve a lot of beer with a sodastream, but not good choice to carb up a beer and general use.
I have the sodastream to mini reg adapter as well but haven't had a chance to use that in anger yet.
 

trustyrusty

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Thanks - interesting you can fill soda stream bulbs from tank? Save chucking them out. Its just for a party.. cheers
 

JDW81

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Some cylinder basics/gas to give you some insight.

The critical temp of CO2 (i.e. the temperature above which it can't be pressurised into a liquid) is around 31C, the critical pressure is ~74 bar/7400 kPa (the pressure required to turn it into a liquid at it's critical temperature)

If your ambient temp is below 31C the CO2 will by and large be in liquid form (which has a mass, but doesn't exert and pressure in the cylinder). The CO2 vapour/gas above the liquid is what exerts the pressure in the tank and gives you a reading on you regulator. The pressure above the liquid always remains the same while ever there is liquid in the cylinder (it will change with temperature), and doesn't start to fall until the liquid is all gone and all that is remaining is the residual gas. Once your pressure starts to fall you are literally running on fumes and you won't have much CO2 left (how long you'll get out of it depends on how many kegs you're running, your serving pressure, how many beers you pull or if there are any leaks).

From some basic maths (P1V1 = P2V2 with a little bit or rearrangement and using kPa for for ease), assuming your 2.6kg cylinder is 2.6L volume at 200 PSI (1379 kPa) and at atmospheric pressure of 101kPa you're volume (V2) will = P1xV1/P2 = 2.6x1379/101= 35.5L of gas left, which isn't much. There are obviously a few assumptions in here (i.e. your regulator is accurate, you live at sea level etc).

Long story short, if it were me, I'd refill the cylinder before the party, take the big one (or wear the extra cost of gas using the soda stream attachment). You definitely don't want your gas running out mid party. That would be a crime against humanity.

JD
 

mynameisrodney

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Thanks - interesting you can fill soda stream bulbs from tank? Save chucking them out. Its just for a party.. cheers
Pretty sure Dunc meant the 400g refillable sodastream bottles you normally swap at the supermarket. As far as I'm aware the little bulbs are not able to be refilled but I could be wrong.

It works out to be about $4 per fill for me (filling off my 2.6kg) instead of $19 to swap at coles/woolies.
 

duncbrewer

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Just back from a pub / brewery lunch. @mynameisrodney is correct I meant the Sodastream bottles, hadn't crossed my mind that the bulbs were called sodastream. In UK I remember them being called sparkler bulbs or CO2 bulbs. It's daylight robbery using the sparkler bulbs or swaps of Sodastream bottles refill is the way to go. If you do go for refilling sodastream bottle freeze it first it allows a lot more gas / liquid in.
@JDW81 Haven't put my physics hat on for a few years and now I remember why. But pleased to see I had the gist of the capacity in simple terms.
 

trustyrusty

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the soda stream option might be good, if I can refill from my 6 kg, good party option.

is there a tank to tank connection to transfer option, if you can do to sodastream, why not tank? 6 kg to 2.6 kg
 

duncbrewer

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Filling tanks relies on a really good pressure differential and for big tanks they use a compressor to help force the CO2 in.

Filling a sodastream bottle from another CO2 tank works best the bigger and higher the pressure is in the donor tank.

You can do 6 to 2.6 it but you won't get as good a fill as the pros get. It's a little hairy I'd say to diy it.
 

Ballaratguy

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I refill soda stream bottles all the time. You need the liquid to transfer so if transferring from your gas bottle you need to have the bottle inside down so the liquid is transferred not the gas
If you fill from the gas part of the tank you will only get about 2-300 grams into the soda stream bottle
 

peteru

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To maximise the pressure difference and get the recipient vessel to "suck up" as much CO2 as possible, submerge the empty recipient gas vessel in hot water and release all the residual pressure. After that, seal the recipient vessel and refrigerate it. The combination of low pressure (after cooling) and cold vessel should allow for a better fill ratio. It won't double the amount of CO2 you can fit into the recipient bottle, but it will make enough difference to notice.

Having said that, I prefer to just book a next day bottle swap through Gas Guyz (links to 10% discount), as they are available in my area. They do 2.6kg and 6kg cylinders as well as soda stream. I was very impressed by their service, ordered a swap at 3 or 4 in the morning and had the delivery before 11am the same day. 👍
 

mynameisrodney

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I hadn't heard that tip about hot water before, I'll have to give it a go.

I've been leaving overnight in the freezer before refilling, my last one I got 450g in so was happy with that.
 

chrisfromperth

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Haven't put my physics hat on for a few years and now I remember why.
The physics is super simple. By freezing the Sodastream bottle you're cooling the gas inside and therefore lowering the pressure. If it's just residual gas in the bottle, as it usually is when you want to fill it, it follows a proportion of 1/272nd drop in pressure per degree. Room temp 25, freezer temp -18, gas pressure in the bottle drops about 16%. When you connect a room temperature CO2 cylinder containing liquid (inverted so the liquid flows) the pressure in the donor cylinder is at least 16% higher. The liquid will flow until pressure is equalised. You want that to happen as quickly as possible to get the maximum liquid CO2 into the Sodastream before the Sodastream warms back up. After disconnection you can stick the Sodastream back in the freezer again and do this 2 or 3 times. Sodastream fills to 400g. I sometimes get 380g but I'm generally happy (lazy) with 200-250. I see that Rodney managed 450g (good going).

Why can't we get as much of a fill as the factory? They fill them a bit differently. See the nut on the side of the valve? The one with the little hole? That gets loosened to allow gas out and liquid CO2 is forced through the main valve. When liquid CO2 squirts out of the little hole the nut is screwed up tight. Don't try this at home, I have a feeling that without the appropriate automation it would be dangerous (frostbite, anyone?) and wasteful.
 

perc63

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So by me having my co2 bottle inside my keezer or fridge the gas inside the bottle would be lower??? so when I take the bottle out there should be pressure in the bottle slightly higher than in the keezer... yes or no.
 

JDW81

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So by me having my co2 bottle inside my keezer or fridge the gas inside the bottle would be lower??? so when I take the bottle out there should be pressure in the bottle slightly higher than in the keezer... yes or no.
Kind of (a bit of yes and no at the same time)

A full cylinder of CO2 is mostly liquid (see my post above), with some gas above the liquid which exerts a pressure. There's an equilibrium that forms based on temp/pressure etc, however the number of CO2 molecules available for you to use for your beer remains the same.

For the nerds in the room, it's basically Gay-Lussac's law in action: I.e for a given mass and constant volume, the pressure exerted by a gas on the sides of its container is directly proportional to its temperature.

Your cylinder won't last longer if you keep it cold, however the pressure inside will be lower based on the above principle.
 

Sleepy Weasel

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I refill Sodastream bottles for family and friends all the time, and I found this Kegland Youtube video very helpful. Featuring the much maligned Kee...

Things to note:
Weigh the Sodastream cylinder before and after, so you know how good a fill you're getting.
Freeze the recipient cylinder first.
Invert the donor cylinder so you're delivering liquefied CO2 and not gas.
Crack the cylinder slowly and don't open it too far. Take your time, or the Sodastream bottle's valve will shut off.
 

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