[goudron]

From How To Brew section 6.1:

Lag time:

refers to the amount of time that passes from when the yeast is pitched to when the airlock really starts bubbling on the fermenter. A long lagtime (more than 24 hours)...

How much of lag time is actually time that passes before fermentation starts rolling, and how much time is spent filling up the head space of the fermentor?  How much pressure needs to build up for a regular airlock to bubble?  I guess not too much.

What this really got me thinking about is how much head speace should be in the fermentor relative to the amount of wort.  How much is too much space?  I'm wondering because I plan on making 5 gallons to start, and I'll use the 6.5 gallon fermentor, but I'm guessing I will later gravitate to smaller batches to compare different variables head to head whenever possible.  Is 20-25% of the fermenter being head space ideal?  That would make 3.85 gal a good batch size to fill a 5 gallon carboy, right?  I'm guessing the headspace could be smaller, but that too much headspace is bad.

Does the exposed surface of wort as a proportion of wort volume make a difference?  ie, if I had a wider 6.5 gallon fermentor that's half the height of the standard plastic pails will that produce a noticeably different beer?

[Hokerer]

Does the exposed surface of wort as a proportion of wort volume make a difference?  ie, if I had a wider 6.5 gallon fermentor that's half the height of the standard plastic pails will that produce a noticeably different beer?

At brewery scale, fermenter geometry can make a difference but on at homebrew scales, no, it's not going to make any difference.

[a10t2]

How much of lag time is actually time that passes before fermentation starts rolling, and how much time is spent filling up the head space of the fermentor?  How much pressure needs to build up for a regular airlock to bubble?

The pressure required to move a 1" column of water is ~250 Pa, or 0.036 psi. So filling the headspace is a non-issue. Some respiration would have to occur for the wort to become saturated with CO₂, but that's also negligible - about 0.2°P of fermentation, or an SG drop of a little less than one "point" - give or take a bit depending on temperature.

Does the exposed surface of wort as a proportion of wort volume make a difference?  ie, if I had a wider 6.5 gallon fermentor that's half the height of the standard plastic pails will that produce a noticeably different beer?

The amount of headspace makes no difference, because the pressure of the air column is the same whether it's inside or outside the fermenter. The height of the wort column does matter, because the hydrostatic pressure will affect the flavor compounds created during reproduction and fermentation. All other things being equal, a taller wort column will result in suppressed ester formation, for example. But like hokerer said, the effect won't be noticeable at homebrew scales. IME, it isn't even noticeable in 7 bbl fermenters.

[goudron]

Excellent news, and good to know, thanks to you both!

[euge]

I have found on the homebrew scale that having plenty of headspace is helpful but not necessary. I like my yeast to stay in the fermenter so give plenty of room- usually just under three gallons. The krausen still hits the lid.