Do a diacetyl test. It's easy. Take two samples of beer. Heat one sample to about 140 or so for an hour. Don't use a microwave. You have to cover the sample with foil or something, when heating. Keep the other sample at room temp or cool it down a little. After the sample is heated, cool it down to the same temp as the other sample. Then smell and taste it. If it smells like butter, you got diacetyl. Then there's no need of the other sample. If it doesn't smell like butter, taste it and compare the taste to the other sample. If they taste different. The yeast isn't done working. I brew mainly tri-decoctioned Pils. My diacetyl rest can be as short as 3 days and as long as four weeks. It all depends on the yeast and a few other things. Anyone saying that a 3-5 day diacetyl rest is long enough, isn't always correct. Especially, when using certain German or Slovak yeast strains. One reason that the beer in a keg doesn't taste like diacetyl as often. Is because the beer was racked into it too early. The hydrometer reading remained stable. So, you figured it was time to keg. However, the yeast became dormant. The yeast, being dormant, didn't knock out the alpha stuff, that it breaks down into diacetyl. Then, the keg is chilled down. Ensuring, that the yeast won't do anything. The yeast, never was in a condition to break down the alpha crap, into diacetyl. Then, when you bottle the beer. The yeast gets roused up, the residual yeast warms up a little and begins working on the alpha thing, while in the bottle. After aging in the bottle, Bingo, diacetyl. Sometimes doing the secondary thing isn't a bad idea. Transfering to a secondary can rouse up the yeast. That's the time when it can break down the alpha jiggers to diacetyl. If you have real good yeast and leave it in the primary for a few weeks, hopefully the yeast will break down the alpha stuff into diacetyl. Then, a diacetyl rest can remove the diacetyl. There's only one time I use an hydrometer during the brewing process, thats for knowing the Grav after cooling. I'm not big in the ABV wonder world. After that, I use a sugar test kit and a pH meter. If I discover a high level of fermentable sugar in the beer after a few weeks in the fermenter and if the pH isn't right for the yeast I use. Then the yeast needs rousing. If that doesn't work. I add more yeast when the starter is in high krausen. The high ferment temps of an ale should eliminate diacetyl. If it doesn't, I'd think that the yeast is crappy or the ferment temps weren't right. I'm assuming your making Lager or Pils.
