Originally Posted by el_caro
Many say these calculators are too conservative but MrMalty suggests you need about 750Ml for a 10g batch based on 6 week old repitch from slurry if I an reading it correctly.
Apart from having to guess at the yeast concentration, and dial in non-yeast percentage, Mr. Malty is really conservative with it's viability numbers. It claims a yeast harvested today has 94% viability, and one harvested 6 weeks ago has 27% viability. That means if the slurry is 6 weeks old, you need more than 3 times the amount to pitch. I just don't believe it.
At 8 weeks it is down to 10 percent ........ and guess what, it is still 10% at 11 months. So you loose 90% viability in the first 8 weeks and then it is unchanged for up to a year. The viability numbers just are not right.
I always recommend calculating as though you just harvested the yeast, and maybe bumping it up a little if you want.
So this is a direct pitch yeast pack (not slurry) from Wyeast.
1 BBL is about 31 gallons. 2 BBL = 62 gallons. You are brewing 1/6th of this, so you will need roughly 1/6th of the yeast, or about 130 ml (make it 150 ml for good measure).