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Very weak fermentation

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To each his own. It sounds like you're quite a serious brewer making many many beers. I'll probably never make more than a couple per year and enjoy seeing visible signs of the process, and my fermenter is in the basement so no UV issues. Cheers.
This is where the Tilt hydrometer is useful. A bit expensive and I’ve had issues getting mine calibrated but its really nice to be able to pull up your hydrometer on your phone and monitor fermentation activity without opening your fermenter and risking contamination.
 
Just to wrap up this thread. I ended up brewing another beer (an extract KOA coconut porter kit) using Imperial Yeast A07 Flagship using a starter with stir plate and it worked great! Powerful fermentation. After a week I moved the porter to a secondary and then transferred the beer discussed in this thread (imperial stout extract kit) onto the yeast cake from the porter hoping it would nudge the attenuation further. Let it sit for a month but it only dropped from 1.030 to 1.028. Bottled a few days ago and it tasted ok, although a bit sweet. We'll see how it tastes when aged, carbonated, and cooled at a later date, might be pretty good by then.

Now I've decided to try the same imperial stout recipe again and hopefully fix the stuck fermentation issue this time around. Brew day was yesterday and It's going like crazy right now, 18 hours after pitching yeast, in fact now I'd like advice on what to do when CO2 and krausen pops the lid and overflows the top of the bucket! The video was shot shortly before the lid popped off. This is 5 gallons in a 6.5 gallon bucket so there should have been enough headroom haha.

 
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The blow-off setup uses a hose connected to the tube inside a three piece airlock. The airlock tube is fairly small diameter and the end inside the bucket tapers to a point with three vertical slots in it. I think those slots plugged up with junk from the krausen. Next time I think I should create a system where the full width of the blow-off hose sticks into the bucket and maybe then the bucket wouldn't have pressurized enough to push the lid off.
 
The blow-off setup uses a hose connected to the tube inside a three piece airlock. The airlock tube is fairly small diameter and the end inside the bucket tapers to a point with three vertical slots in it. I think those slots plugged up with junk from the krausen. Next time I think I should create a system where the full width of the blow-off hose sticks into the bucket and maybe then the bucket wouldn't have pressurized enough to push the lid off.
I think you're right. Need to be able to get the full blow off tube through the lid. If it won't fit through the existing hole, you can drill another the right diameter and put a grommet in it for a good seal. Right now you might could leave the lid loose until high krausen is done. Set the fermenter in a container just it case!
 
I have done the tube to 3-piece airlock as well and yeah, the narrow airlock neck isn't always enough. I now jam a bent 1/2" SS pipe (doesn't kink) into a rubber stopper, and then the rest of the hose to that.
 
If I'm making an Imperial stout or other high gravity beer, I make a 2.5 gallon batch of regular strength beer and then pitch the imperial stout on to the existing yeast cake (or re-pitch the entire yeast cake). Sometimes I'll make a starter for the small batch, sometimes not. I don't have a clue how many billion cells I have, but it always works pretty good. Pitching a large amount of healthy and active yeast is just something I have gotten used to doing and it really makes a difference in high gravity beers and lower gravity lagers.
 
Thanks for the ideas everyone. I think when I try my next big beer, I'll have most of the kinks worked out, three times a charm.

Things I did this time to fix a very weak fermentation on the previous attempt:
1. used a yeast that was one month old rather than five - Imperial A41 Tartan yeast with 200 billion cells per package
2. used a pitch rate calculator
3. made a 2 liter starter on a stir plate with one pack, added a second pack straight at pitch,
4. pitched into 75 degree wort rather than 85 degrees
5. agitated wort vigorously and aerated with an aquarium pump and air stone for 15 minutes
 
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