Blow-off big enough?

I recently bought a couple 3 gallon glass carboys for some 2.5 gallon batches I want to make, and a 6.5 gallon glass carboy for 5 gallon batches.

When I bought the carboys, I asked the LHBS fellow what I needed for blow-off tubes, and what he gave me was small gauge (3/8"?)tubing that fit inside the drilled holes in the carboy bungs/stoppers.

Looking through some pictures today on this website, I see that lots of people use blow-off tubes that are big enough (1") to fill the entire neck diameter of the carboy...without the need for a stopper/bung.

Now I'm confused, will that little small gauge tubing be enough for a vigorous blow-off, or will I need to get some 1" tubing?

I have used tubing that fits snuggly inside the opening of the carboy. I can't remember the diameter. But, a larger opening is typically better to ensure that the tubing doesn't clog.

I have only needed to add a blow off tube for one batch, and it was a batch that fermented at too high of a temp. All of my other batches were fine with a three-piece airlock.

You should be OK with a 3/8" tubing. That was what I used to use for blow off tubes.

You may also want to look at Fermcap-S drops if you find that your batches are getting too full. I use 2 drops per gallon in my carboy just before I add my yeast and I have not needed a blow-off tube since then.

If you are doing 5 gals in 6.5 gal carboy you are giving the beer quite a bit of headspace to krausen. You could probably get away with the 3/8", but a 1" would be the safest way to go.

Thanks, that's good to know about the 6.5 gal carboy. How about the 3 gallon carboy with just 0.5 gallon headspace?

Naked_Eskimo said:

Thanks, that's good to know about the 6.5 gal carboy. How about the 3 gallon carboy with just 0.5 gallon headspace?

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I'm not 100% certain about a 2.5 gal batch. For the few small batches I've done I just used a 5 gal carboy. I use a bucket for fermentation, but the bucket is only 6.5 gals. I do 5 gal batches and never use a blowoff. I think I recall one time that I had krausen in my airlock, and it really wasn't that big of an issue. My opinion would be, if you are concerned enough that you want to use a blow off, then go all the way. Get the largest tube that will fit tightly down the neck of the carboy, regardless of the size of the carboy. The opening on the 6.5 gal and the 3 gal should be the same size, but it can vary when you buy the smaller carboys.

Look at this way, if the krausen comes up through the airlock with no blow off attached, it's just going to get some beer on your counter or floor or wherever unless it gets clogged. Then you could have some type of explosion. However, if you just attach a 3/8" tube to your airlock, the only problem you've solved is keeping beer off the floor. The airlock can still get clogged and you can still have an explosion. With the large tube in the neck of the carboy, you make a larger escape tunnel for the krausen. This reduces the chance of clogging. Also, there is a chance that if it does clog, the pressure may force the tube out of the carboy before you have an explosion, which would then just result in having some beer on the floor.