Don't know anyone routinely using a bucket for secondary...in fact, don't know anyone using a secondary for anything except long-term aging of sours and the like. If you want to purge a secondary with CO2, a keg is the best solution...distant second best... a carboy.
What ARE you into? Others have suggested that the perfect way to purge a container is to fill it with water and purge it out with CO2. It's impractical, and so is running CO2 for several minutes. CO2 is heavier than Oxygen. There's probably no better way to purge air out of a head space than to fill it with BEER.
wut?? secondary used for: lagers tripels quads belgian strongs wood-aging sours (as you said) fruit additions
Yes...which is why I said "and the like"...I actually use a secondary (keg) for everything...because I keg and don't want to primary ferment in the keg Cheers
I secondary for wood aging, dry hopping or aging with Brett or souring yeast. I purge both kegs and secondary fermentors with CO2. Kegs before filling I will put a liquid side connector on the gas line and purge it through the dip tube, like inchrisin said, CO2 is heavier than air so in theory it should push the air out the top. Carboys, I just put the gas line down in the carboy with no connector on it and open the valve for 20 seconds or so. It's never a bad idea but not sure that it is necessary. For what it's worth Societe brewing in San Diego purges growlers before they fill them.
I only purge anything unless I'm trying to keep certain conditions at bay. Purge with Co2 when I sour my wort to keep the nasties at bay. Purge my kegs with Co2 anytime I fill them, and I also dry hop in the keg, so I'm not needing to purge a secondary or anything.
My recommendation is for you to dry hop in your primary. There is no need to dry hop in a secondary and eliminating a transfer has the benefit of mitigating potential infections. Feel free to dry hop in the secondary if you are so inclined. A plastic bucket is not ideal for a secondary since it is oxygen permeable and the beer will get slightly oxidized because of this oxygen permeability issue. Cheers!
i guess i am the odd one here... since my brewery is haphazard at best, yes i do use a secondary if only to free up a yeast cake for the next brew. and i just blast a lot of CO2 through a vinyl hose into the head space of each vessel. CO2 is heavier than air. so blow CO2 into the bucket for as long as you think it takes to push the ambient air out. then you are ready to transfer. no tips or tricks involved. if you want to blow a few grand on an optical oxygen probe go ahead. you will be able to tell when the oxygen levels are reduced to zero. or spend 30 cents on CO2. Cheers.
Problem is that this will take roughly forever, especially if you can't pressurize the bucket. An analogy is filling a bucket with cherry coolaid. Next, squirt water into the bucket through a hose, displacing some of the coolaid (which will overflow the bucket, along with some of the new water) at whatever rate you like. How long until there's no red left?
When you get two gasses moving around freely in a container, they both fill it evenly, i.e. one doesn't push the other in a particular direction. Think about the CO2 and O2 in your living room. Is it any easier to breath near the ceiling than near the floor? Gravity does affect CO2 slightly more than O2, but it's not strong enough to overcome wind/air currents, convection, diffusion (due to constant movement of gas molecules), and other random forces. Trying to displace a gas with another gas in a non-pressurized container just isn't efficient. If you could pressurize, you could calculate how long it would take (i.e. how many pressurize/release cycles at a given pressure) to purge whatever percentage of the original gas you'd like. Like purging a keg. IIRC, @hopfenunmaltz has done the math on that.
I like the idea of filling a vessel with water, then pushing all of it out with CO2. Then you know its as close to 100% as you can get.
To expand on this point, this is the concept of Kinetic Molecular Theory. The molecules in a gas are in constant random motion, they are bouncing off of other molecules and tend to move from regions of higher concentrations to lower. This is what makes air homogeneous. While CO2 is heavier than air, it will diffuse equally throughout its surroundings. If this were not the case then all the CO2 in our atomosphere would sink to the bottom. (Actually Xenon would be on the floor, followed by Krypton, then Carbon Dioxide). I see this regularly when mixing diving gases. Adding Oxygen to air (Nitrox) is done routinely. Even though O2 is heavier than air it diffuses completely. Similarly, when Helium (extremely light) is added to air (Trimix) it distributes itself throughout the mixture. It can be added to the top of a cylinder of air and will sink (diffuse) to the bottom, even though it is lighter. The process isn't instantaneous, but it occurs, and these type mixtures are used to sustain human life. That said, there is still a benefit (IMO) to purging a carboy with CO2. Even if it doesn't completely displace air, it will reduce some of air which is a benefit (every red coolaid molecule displaced is an advantage). When bottling with a beer gun I purge the bottle then purge the headspace with a squirt of CO2. Does it completely remove the air? Nope, but it kicks some of it out. An open bucket is tougher to purge, but a carboy can have some of the air eliminated and the cost of the CO2 is pretty minor. I believe most all keggers use this technique and the math is pretty straightforward. If you purge the headspace of a keg to 15psi (1 additional atmosphere) you have introduced CO2 in a equal amount of the air . . . so air now occupies 1/2 of the headspace where the pressure is now ~30psia (absolute). Diffusion is near immediate due to the force of the entry gas. Venting to one atmosphere allows 1/2 of the headspace gas to escape with the remaining mixture having 1/2 of the air it had originally. If the next burb is at the same pressure you will lose 1/2 again (1/4 air remaining), the third burb will have 1/8 of the air remaining. Of course the oxygen content is 21% of this figure. Raising the "burbing" pressure obviously speeds up the removal of air. The math is less important than remembering to always burb a keg after it has been opened.
Foundation brewing in Portland Maine also purges growlers. If you don't have a counter pressure filling system, its a great way to ensure you're sending off a legit growler
