Originally Posted by Mash
So am I correct in assuming that if anything other than plain
water is used to be carbonated, a huge mess is likely to be
Well, you start by carbonating a sodastream bottle of chilled water that you filled up to a line about 85% high; there is an injector that just reaches the water. Then you add syrup and cap and give it one somersault to mix.
If you don't use their crap syrup, there is about 1/3 chance it will foam like a bomb so start unknowns over the sink. Stuff like grape concentrate and dry sugar are dangerous, especially if you unwisely carbonated to a high level.
There is one third chance that something new will start to foam, but be slow enough that you can cap it quickly and will calm down for opening if you rechill it a bit. Especially if you chilled the flavoring and poured it in really slow.
There is one third chance that it will behave nicely, then you can learn how much you can max the carbonation without trouble or with a quick grab for the cap. I find thawed concentrate pink lemonade is an well behaved and tasty example.
I think it may be intentionally rigged to be difficult if you don't use their flavors. There is barely enough room for conventional sweet flavors in that 15% headroom... no room for foam anyway. They get around this by putting half splenda (yuk) even in the non-diet sodastream flavors because it takes less room to be equally sweet. I expect it would be harder to carbonate if you kept the water level lower where the injector doesn't reach. I find switching to their optional half liter smaller bottles is easier to avoid foam... there may be more headroom percentage wise or something, but it just works better and easier for me.