Factors affecting attenuation

I brewed an oatmeal stout OG was 1.067 and finished at 1.022.
My brown ale started at 1.052 and finished at 1.020
Stout was mashed at 154F
Brown ale was mashed at 152F
I used S-04 in both.
Im a bit dissappointed that the brown ale didn't get a bit lower.
All other factors (ferm. temp etc) were constant.
What factors influence attenuation abd why didn't the brown ale go lower ?

Attenuation doesn't just depend on mash temperature. Yes, to some point you can control the fermentability of your wort with the mash temp, but it also depends on things such as how much yeast you pitch, how well you oxygenate, fermentation temperature, and the overall health of the yeast. In addition, the amount or percentage of specialty malts can also play a role in your attenuation. If you post the recipes we might be able to help you a little more. How long did both ferment for?

Just saw that ferm. temp was constant. Do you have a temp controller?

Mash thermometer could be reading lower than actual. Check it for accuracy.

I don't have a temp controller - I meant that both the stout and brown ale were fermented next to each other more or less at the same time - two weeks apart - same weather.
Ive calibrated my thermometer.
I don't have the recipe with me but it was pale 2 row, toasted 2 row, toasted oats and some vienna.
I over cooked the oats - got a slight burned taste - although SWMBO can't taste burned at least.
Stout was down to 1.022 within a few days. The brown ale the same. I left them in primary for three weeks.

All sounds pretty normal with your methods, and the recipes don't look like you used much specialty grain so I would check your thermometer again. Are you sure your hydrometer is accurate?

In addition to mash temp, pitching rates, temp control, etc S-04 is not a highly attenuative yeast. Use Nottingham or S-05 if you are looking for higher attenuation..

Hydrometer is accurate. Possibly the packet of yeast was older than the others.

Mr Berta has the right of it. S-04 is, compared to other yeasts, not highly attenuative. Had you used Nottingham or S-05, you would have seen higher apparent attenuation.

Your stout fermented quite well for S-04, giving ~68% apparent attenuation. Your brown ale shows ~63%. Both are perfectly in range for S-04, which in my experience yields an average of 65%.

Chalk it up to learning the characteristics of yeast strains.

Cheers,

Bob

Cool. I made a stout last night and I split the batch in two - one with S-04 and one with US-05 - just to see....