Well that's confusing to. Just checked Mr Malty and he says 1.8 packs even if they are a month old. He also recomends >300 billion cells but says that 1.8 packs will deliver that.
I must be missing something here.
ahh well. I guess I will RDWHAHB and pitch the two packs as planned. There is something wonky in my understanding of this whole thing.
If you properly rehydrate the yeast, 2 11g packs will be plenty. If you just throw the yeast in the cool wort, you'll kill 50%+ of the yeast. BTW, the optimum rehydration temp varies a bit by strain, but most strains will be more like 100-105*, rather than 90*.
You also don't want to use RO water. Tap water would be better. It needs some minerals so the minerals inside the yeast don't get sucked out into the water.
I came across this that I thought was interesting:
http://www.fermentis.com/SHARED/Doc_52528.pdfViable cells per gram:
k-97 - 1.4*10^10
s-04 - 0.8*10^10
t-58 - 1.8*10^10
s-33 - 1.6*10^10
s-23 - 1.0*10^10
s-189 - 0.9*10^10