Bottle carbonisation - not that straightforward

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bobtheUKbrewer2

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Hi All, I bottle mainly in 2 litre PET bottles these days. If I fill them to just 3mm from the top, the space volume above the beer is close to 1 ml. However my normal bottling free space is about 7 cm which is a 10 ml head space. So the amount of CO2 from bottle fermentation to pressurise each of these two bottles is very different. Is there a table anywhere that takes account of head space volume ? Thanks.
 

VikeMan

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There's no table that I'm aware of. It's possible to do the calculations, but they are tedious. And, I doubt you'd really notice any difference. The dirty little secret about the bottle carbonation calculators is that they assume no headspace at all, and they work just fine.
 
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bobtheUKbrewer2

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I think I am going towards much reduced headspace but always with a few PET bottles to act as pressure gauges - thanks VikeMan.
 

TheBluePhantom

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That might be a rather bad idea. Remember the headspace is there for expansion. If you bottle cool and later transport or store warm, you may have the liquid expand to fill the bottle, and then stretch or break the bottle.

Really the carbonation wouldn't be affected much. At 2 liter volume, a 10 ml space is only 0.5% of your total volume. removing that from your carbonation volume you would take a 2.0 volumes beer up to a whopping 2.01 volumes. Not worth writing home about. Overfilling on the other hand, you can take a 2.0 liter volume and reduce it to a sticky mess on the floor. Not worth the risk in my opinion.
 

ncbrewer

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I don't remember where, but I saw a video where someone filled PET bottles leaving some reasonable head space. Then he squeezed the bottle to push the beer right up to the top, and capped it. That leaves room for expansion while leaving no air in the bottle.
 

InspectorJon

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There are several other threads here where folks discuss leaving very little head space with no ill effects. I have been bottling with 1/4 to 1/8 inch of headroom for a couple years now. I think the idea of bottles exploding because of too little head space is a result of logical thinking run wild. There is a lot of that here on HBT. It sounds true so it must be... I don't recommend leaving no head space.
I'll add my experience to this discussion. This has yielded good results for me several times. I don't take any particular LODO precautions while brewing. I ferment in a glass carboy (I know...) and when fermentation is near done I seal it up with a mylar balloon so I can cold crash without suck back issues. I bottle directly from the fermenter to bottles that are individually primed using an auto siphon and bottling wand. I fill to very near the top and cap with oxygen absorbing caps. No purging or bubbling. I use my gravity testing graduated cylinder to pour enough beer into each bottle to get it right up near the top. This beer was bottled 60 days ago and is actually better now than it was a month ago. The bottle in the picture is one of the lower fills. I usually get a bit more in but at this point we are only talking about a few ml.

View attachment 709961
 

neale

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I've been filling my glass bottles to around 5mm from the top for my last 5 or so brews and only had one explode that I think I'd filled a bit too full. I'm pretty sure that the majority of CO2 that is produced get dissolved into the liquid.

The blowing of bottles is probably more down to expansion of the liquid not the gas, so as long as there is enough space for liquid expansion your good to go. I'll be sticking to my 5mm from the top.

I've done experiments leaving a large headspace and a 5mm headspace and there is definitely less oxidation with the reduced headspace as well.
 
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