Here's a crush from a brew I did:
You can see intact husks from the barley, some larger pieces of the internal seed, and some but not a ton of flour. This is a good crush.
Here's a crush a friend of mine got at a LHBS. Some of the barley is broken, but most is not affected. This is a terrible crush, and needs to be run through again, and perhaps the mill gap shunk.
How long and what kind of conversion efficiency you achieve is dependent on a lot of things: mash temp, thickness of grist, whether you stirred or not (I do at 15- and 30-minutes), fineness of the crush, the water composition (pH, e.g.).
I used to crush w/ my barley crusher at a .020 gap; very fine flour, mostly, some larger chunks. Typically I'd have up to 90 percent conversion after about 15 minutes. I was doing BIAB.
Now I use a different mill, gap at .035. The particles are larger, and it takes longer for all the starch to gelatinize in the mash--but it still gets there, just takes longer. I'm somewhat a believer that in addition to converting starch we're also extracting flavors in the mash, so going a full hour or even 75 minutes is a standard practice for me.
The Low Oxygen folks argue that one doesn't want flour in the grist, or as little as possible. The theory, as I understand it, is those smaller particles are more subject to oxidation and thus flavor degradation. The argument makes sense, though back when I was doing BIAB and a very fine crush, I made some pretty great beers. I've been employing low-oxygen techniques myself, trying to see if those methods will produce a noticeable great-leap-forward. I've had some success, but I'm not ready to endorse the approach generally. It's a fiddly process, one that lengthens the brew day, and until I see a noticeable improvement in the beer over what I was doing.......that said, I've seen some improvement, but also struggled with some elements of it. Jury? Still out for me.
