German Brewing

I'd second the mash profile. It is possible to step up too quickly and denature enzymes.

Hmm. What can be done with the mash schedule? I use a Herms which bumps temp in the mash with about 1.25C/min. I don't ever overshoot the herms to get quicker temp-raises, as I don't want to expose the enzymes to to much heat. The enzymes never see a higher temperature than the set-temp.

Im running one pack of yeast on a stir plate, I don't have a microscope so I target (with a yeast calculator) about 2.5-3bn cells/ml.

Don't have a DO meter either, but running a 0.5 micron stone for about 90-120 seconds at about 0.5L/min.

They have both been fermenting at a stable 9C (temp controlled fridge) since 12th of may.

I'm not having any troubles wih ale yeasts, so I guess maybe it's a fermentation/yeast issue.

Abbey nails fermentation regularly. I'd heed his advice any day

What I didn't mention is that I propagate in 3 steps -- that may well have a substantially different effect on the vitality of the yeast. Mine goes pretty fast after pitch. That 50/50 suggestion is indeed something worth trying befor yous splash out on a DO meter. In my case, the DO meter made a huge difference when I got it about 4-5 years ago. But that's specific to me and my process. Your mileage may vary.

I gotta wonder if your fridge is colder than you think it is. Have you verified that your temp controller probe is calibrated? If you're actually fermenting at say 7c, I can see your yeast stalling at 1.018.

For what it's worth, nowadays I pitch at 7-8c, ferment at 9c, and let rise to 12c when I rack to spund. Primary takes about 6 days, spunding another 4-5, then I drop to 2.5c to clear it. Ready to drink at about 20-25 days old.

Ah, see, this is where we still differ. With calibrated thermometer (they are calibrated every year), pitch 5, ferment 7-8, transfer 5, then it dissapears for 8 weeks. Everyone will eventually settle on their own method. A stopped fermentation at 1.018 SG would make a pretty authentic Czech brew though... for what it's worth

I checked the temp-probe against a thermapen and its still calibrated. I've had better luck with doing a 68c instead of a 72c, so maybe I'll just go back to that. But its weird since everybody are able to pull off the 72 step and still make it ferment to target sg.