[Poobah58]

Brewed up 15 gallons of 1.103 Imperial Stout 5 weeks ago. Pitched a good amount of yeast but probably should have used more. Fermentation went well for a week, then died down. I racked after 2.5 weeks and it was only at 1.040. Added a pack of S-05 and it's still fermenting but it's only down to 1.35 at this point. My pH is 4.6. Is this OK? I plan on leaving it for another 2 weeks or so at the minimum but was wondering if the pH was slowing things down. I expected the SG to be much lower after 5 weeks.

[Kaiser]

That pH is fine. I would not worry.
Keep taking good notes and maybe you'll get enough data to establish a correlation. It's more difficult to control or predict beer pH than mash pH.

[euge]

You might try just letting it warm a few degrees. However, at that original gravity you may be done. You gotta have some sweetness to counter all that alcohol anyway.

[mabrungard]

Different yeasts can have different effects on beer pH.  Don't fret over beer pH.  If the mash and sparging pH's were in an appropriate range, there is little concern with beer pH.

[tom]

How does it taste?  I like my RIS f.g. in the low 30's.

Have you made this recipe before?  What f.g. were you expecting? 

[Poobah58]

Thanks guys, I'll leave it be. I have the ambient temp up to 72F. I started at 63F and let the ambient rise to about 68F. I bumped up the temp after racking. I only had 6.2% C-malts and mashed at 131 & 150 so I fully expect to get into the low 20's. I used lactic acid to get my water (for RIMS and sparge) down to a 5.4 pH range before mashing. Probably don't need to do that when I use a good amount of roasted malts. I'll check it a few weeks and pitch some CBC1 if I feel I need to get it any lower...

[euge]

This is where a carbonator-cap is of great assistance. It can reveal what the beer is like at that very moment.

[a10t2]

I only had 6.2% C-malts and mashed at 131 & 150 so I fully expect to get into the low 20's.

150°F is actually pretty low for a single saccharification rest. I wouldn't expect the attenuation you'd get from separate beta and alpha rests, for example.