Bodgineering - DIY carbonation

[Oldgreybeard]

My trusty Sodastream has given up the ghost, been repaired a few times, but the damage now is beyond repair. I've long been unhappy with the rip-off business model that Sodastream use, for what is a very simple bit of kit. So, rather than buy another one, and stay locked-in to having to keep spending money on their (very, very expensive) CO2, I've decided to build my own replacement.

I have a stock of Sodastream bottles (even they use a unique, proprietary, thread and cap), so will initially re-use those, but will probably switch to using recycled PET pop bottles (they all seem to use the same thread). This is all a bit experimental, so rather than fork out for a big CO2 cylinder deposit, I'm starting out with a cheap MIG welding bottle of gas. If the set up works, I can very easily get a thread adapter to convert the regulator to fit a standard cylinder. Bits bought so far are a CO2 welding gas bottle, a fairly cheap (£18) regulator with output pressure gauge, plus a stainless steel hose tail that I'm going to bolt through a drilled Sodastream bottle cap. I have various bits of pneumatic hose lying around, which should do fine for connecting it up.

From what I've read this should work fine, all that's needed is a way to set the CO2 pressure to between 30psi and 50psi (depending on how fizzy you want stuff to get).

More to follow once I get the bits put together.

[Oliver90owner]

(even they use a unique, proprietary, thread and cap)

There is good reason for that - avoids litigation when the thing explodes.

[Oldgreybeard]

(even they use a unique, proprietary, thread and cap)

There is good reason for that - avoids litigation when the thing explodes.

Maybe, but they have an absolutely massive safety factor, plus the cap never sees the gas pressure during carbonation, as that's done with the lip of the bottle held directly against the seal in the unit. The cap only ever has to hold the pressure from post-carbonated water, probably no more than 30psi. Also, unlike disposable PET pop bottles, the Sodastream bottles don't have the safety pressure relief slots in the thread, the slots that are there to stop the cap from flying off like a projectile by releasing pressure gracefully around the threads.

Ordinary (non-reusable) PET pop bottles are a great deal more flimsy and yet are fine for pressures in excess of 150psi. The standard pressure relief valve in a Sodastream operates at about 40psi and Sodastream PET bottles are around 5 times thicker. If I had to guess, then I would think that the Sodastream bottles are probably OK for several hundred psi.

[Stinsy]

Sounds like a great plan to me! I hate getting stuck in a “system” for consumables!

[Oliver90owner]

Bodger,

I wasn’t considering the cap. I would be more worried about the muppets substituting glass bottles not designed for resisting any pressure!

Remember the winnebago driver that successfully sued - after the vehicle crashed when he left the driver’s seat to brew a drink? ‘Auto-pilot’ was only controlling the speed.

[Oldgreybeard]

Bodger,

I wasn’t considering the cap. I would be more worried about the muppets substituting glass bottles not designed for resisting any pressure!

Remember the winnebago driver that successfully sued - after the vehicle crashed when he left the driver’s seat to brew a drink? ‘Auto-pilot’ was only controlling the speed.

Glass bottles are OK, Sodastream themselves sell a range that fit the machines, plus there are third party manufacturers of Sodastream compatible bottles (glass, stainless steel and PET). The type of bottle has no real bearing on the oddball thread size the caps use, as that isn't a part of the way the machine operates (it just clamps to the top of whatever bottle is in it).

The caps are only used after the water has been carbonated. You clip the bottle into the machine (not using the screw thread) activate it, wait for the pressure relief valve to "burp", remove the bottle and then fit the screw cap. You could carbonate a bottle of water that has no thread, as long as you could get it to clip into the machine OK and seal.

As for the apocryphal (and fake) Winnebago story, always worth checking Snopes to see if there is any grain of truth in stories that seem to be beyond belief, like this one: https://www.snopes.com/fact-check/cruise-uncontrol/

[Oliver90owner]

Bodger,

I wasn’t considering the cap. I would be more worried about the muppets substituting glass bottles not designed for resisting any pressure!

Remember the winnebago driver that successfully sued - after the vehicle crashed when he left the driver’s seat to brew a drink? ‘Auto-pilot’ was only controlling the speed.

Glass bottles are OK, Sodastream themselves sell a range that fit the machines, plus there are third party manufacturers of Sodastream compatible bottles (glass, stainless steel and PET). The type of bottle has no real bearing on the oddball thread size the caps use, as that isn't a part of the way the machine operates (it just clamps to the top of whatever bottle is in it).

The caps are only used after the water has been carbonated. You clip the bottle into the machine (not using the screw thread) activate it, wait for the pressure relief valve to "burp", remove the bottle and then fit the screw cap. You could carbonate a bottle of water that has no thread, as long as you could get it to clip into the machine OK and seal.

As for the apocryphal (and fake) Winnebago story, always worth checking Snopes to see if there is any grain of truth in stories that seem to be beyond belief, like this one: https://www.snopes.com/fact-check/cruise-uncontrol/

You, too, seem to have missed the safety point. NOT ALL glass bottles are designed to resist pressure. Muppits, out there, will try their hardest to be better idiots than Soda Stream’s efforts to avoid them injuring themselves.

[Oldgreybeard]

I'm not missing any points at all, just looking at whether the pressure of stored carbonated water presents a significant risk, as it clearly doesn't when you crunch the numbers. The stresses are tiny, so tiny that flimsy PET bottles from the cheapest supermarket, along with budget beer bottles and very flimsy aluminium cans, can easily take the pressure with no risk that the things will explode.

That's not surprising at all, 10 seconds with a calculator shows that the force on a plastic cap is pretty small in reality. Take a standard pop bottle cap. They have a 25mm thread and an internal diameter exposed to pressure of just under 20mm, so an area of about 314mm². At the pressure relief valve pressure (which the cap will never ever see anyway) the force on the cap trying to push it off is only about 87N, next to bugger all in the scheme of things.

The flimsy pop bottles rupture at around 190psi and under test they always fail by splitting longitudinally, usually the rupture starts around the mid-point. Safe working pressure for these flimsy bottles is around 100psi, about three times the maximum they will see in use. Glass beer and pop bottles are a bit weaker, their burst pressure is around 150psi and their safe working pressure is about 90psi, still way more than they would ever see during carbonation.

The common sense test needs to be applied, just as it does to fake stories about Winnebago's on autopilot. Hundreds of thousands of bottles of all sorts of carbonated drinks are sold every day. They get left in the sun, in hot cars, they get bashed around in transit and in use, yet we never hear of them exploding. Same goes for old glass pop bottles from decades ago. Corona bottles were re-used dozens of times, for example, as they were all sold with a deposit for the bottle. The bottles often looked a bit tired , but just like modern bottles they didn't ever explode from the modest pressure of carbonated drinks.

Finally, if you want a potentially risky bottle then look at champagne. Champagne bottles sit at a much higher pressure than carbonated drinks, typically around 70psi to 90psi, around three times the pressure in a carbonated soft drink bottle. They don't explode regularly either.

[AE-NMidlands]

I think that a lot of modern containers are a lot more highly engineered and specified than people realise (including glass bottles for pressure applications.) "Simple" pressurised beer cans are made from astonishingly thin alloys. I tried to push one blade of a pair of scissors between the plastic sleeve and a can once... it went straight through the "tin!"

I would trust PET (and have, making water-propelled kids rockets with them, also leaving fermenting syrup in them with no ruptures - to date!) I also have a stock of old beer bottles: pint crown-cork - which I used with plastic caps for safety - and brown quart ones with screw-in stoppers. No way would I try to use a milk bottle for a pressurised application.
A

[Oldgreybeard]

I think that a lot of modern containers are a lot more highly engineered and specified than people realise (including glass bottles for pressure applications.) "Simple" pressurised beer cans are made from astonishingly thin alloys. I tried to push one blade of a pair of scissors between the plastic sleeve and a can once... it went straight through the "tin!"

I would trust PET (and have, making water-propelled kids rockets with them, also leaving fermenting syrup in them with no ruptures - to date!) I also have a stock of old beer bottles: pint crown-cork - which I used with plastic caps for safety - and brown quart ones with screw-in stoppers. No way would I try to use a milk bottle for a pressurised application.
A

Indeed they are. I've measured up the standard re-usable Sodastream PET bottles and they are absolutely massively over-engineered when compared to ordinary pop bottles. PET pop bottles are only about 0.3mm thick, whereas the Sodastream PET bottles are about 1.6mm thick.

I know that the burst pressure of a thin wall 2 litre PET pop bottle is around 190psi, and they've been proven to work safely when re-used many times at around half that pressure by tens of thousands of kids making rockets with them and pumping them up to maybe 100psi. They also have a clever pressure relief mechanism, the slots in the threads, which causes the bottle to vent safely if it is over-pressured (as long as the standard cap is used).

My guess is that the re-usable Sodastream bottles are probably OK for pressures of a few hundred psi, and as the highest pressure they can see in use is the 40psi pressure relief valve setting on the machine they have a massive safety factor, way more than normal pop bottles. Just a pity that they don't have the standard threads that pop bottles have. PET fizzy drink bottles all seem to share the same design at the top, at least for sizes up to about 2 litres, and this design is the same (except for the dimensions) as the Sodastream bottles.

There's a flange that is used to hold the bottle against the filler during manufacture (which is the same as the way Sodastream bottles are held against the seal) with a coarse thread ( 3.18mm pitch) to take the cap after the bottle has been filled. This thread is a standard from the International Society of Beverage Technologists, believe it or not. There's no good reason for Sodastream to choose to use an oddball cap size and thread, other than to lock people in to having to buy their (expensive) bottles. This is their business model, forcing customers to buy their expensive gas and consumables.

The gas price comparison is pretty shocking. I can get a standard 6.35kg (liquid gas mass) bottle refill (no bottle rental fee) locally for £36. The special Sodastream proprietary bottle refill costs £13 and that special bottle holds 425g of CO2. The price per kg of CO2 is:

Sodastream bottle = £30.59/kg

6.35kg cylinder = £5.67/kg

A 6.35kg cylinder isn't very large, either. It's enough to carbonate around 900 litres of water, though, so would easily last me for a couple of years before I'd need to get it refilled.