I've always brewed 5 gallon batches, which typically take 3 - 5 days to ferment out. I brewed a 2.5 gallon batch last night - typical West Coast IPA with a 1.056 O.G. I pitched a packet of SafAle US05. No starter. Less than twelve hours later, the fermentation was chugging, seemingly at full steam. Now, about 18 hours later, things seem to have really slowed down. My question is - is there any expectation that a 2.5 gallon beer would ferment appreciably faster than a 5 gallon batch at the same O.G.? I'm fermenting in a bucket, so don't have a great way to check for terminal gravity without oxygen exposure.
Did your 5 gallon batches also use a single pack of US-05? All other things being equal, if you pitch more yeast cells (per beer volume) you can expect fermentation to happen faster. But there's no reason to rush, especially if you can't/don't want to measure gravity.
yes, I would also typically use 1 pack of SafAle yeast. Makes sense that a greater yeast to sugar ratio would speed things along.
I would just dip a santized glass into the bucket and take some out, Pour it into your hydrometer tube and measure the beer, O2 pickup is impossible. Drink the result, Smaller batches take the same time my best estimate is 12 days in the primary any sooner and your pushing it.
Air is a rather nonviscous fluid. The simple act of removing the lid will no doubt produce turbulence that will mix the CO2 and air to some extent. Cheers!
As @utahbeerdude says, O2 will get in. Once it's in, it will reach the beer via diffusion, where it will dissolve until it reaches equilibrium (per its own partial pressure) between the beer and the headspace. Here's a video demonstrating diffusion. The video shows Bromine gas, which is even heavier than the CO2 "blanket" in a fermenter. It also shows NO2 (later in the video), which is closer to the mass of CO2.
In addition to diffusion, placing a wine thief in your fermenter will displace the headspace CO2 in an amount equal to the volume of the tube. Then when you withdraw the Thief, air will replace the space vacated. For the OP, taking a grav sample with a wine thief is not ideal, but rarely fatal to the beer. The idea is to work fast: have the Thief sanitized in advance, open the lid carefully, take the sample and re-lid quickly. I tend to give the process some extra time and boost the temp a mite to minimize the number of samples . . . ideally only twice per brew.
[QUOTE="placing a wine thief in your fermenter will displace the headspace CO2 in an amount equal to the volume of the tube. Then when you withdraw the Thief, air will replace the space vacated" That's why I use the dip glass almost zero liquid disturbance. The thief is is to bulky. I'm more concerned with sanitation then getting overly scientific
