In our instructions, we direct brewers to use a blow off tube for the first few days of fermentation and then switch to a 3-piece airlock - why?
Depending on the beer you are brewing and the temperature of the room you are fermenting in, there is a good chance you will have a very active fermentation for the first three days from when you add yeast. When a fermentation is active, the beer will generate tons of co2 and Krausen - the foamy head that sits at the top of actively fermenting beer. Whether we like it or not, Krausen tends to leave the fermenter. When it does, it can get quite messy and clog devices of small diameters such as airlocks quickly. When this happens, Krausen may end up on your ceiling. For this reason, a blow-off tube is quite important.
But fermentation is done, why switch to a 3-piece airlock?
The reality is that once an active fermentation is done, your sanitizer water holding the tubing is likely filthy. Swapping it out with new sanitizer is a good idea and a 3-piece airlock will use less of it. Additionally, 3-piece airlocks take up less of a footprint which is important to those with space constraints and people brewing several batches at once. Lastly, we feel that there is less likely to be a leak with an airlock, since it is easy to see when it is working (the internal piece bubbles or remains in suspension when co2 is travelling through it). For this reason, we feel you are less likely to oxidize your beer with a 3-piece airlock rather than a blow off tube that will rarely bubble past the first few days of fermentation.