Hello how does one know when the last 1/3 of fermentation. Is it measure in day’s or by hydrometer?

Well to do it accurately you would know your desired FG (and this assumes you are good at estimating based on the variables like pitch count, mash temp, gravity, apparent attenuation of the yeast, or you’ve brewed the recipe before). Then you you would just do (OG-FG)/3+FG.

So say your brew was 1.075 OG and your expected FG was 1.015 that would be (1.075-1.015)/3 + 1.015 = 1.035

For the record, I have never done this. If all you need to know is when to do a diacetyl rest, then I usually just wing it. Usually 8-10 days for a lager and 4-5 for an ale. It all depends on what the yeast is doing and based on past experiences with that strain. Or you could just do it right around when you see the krausen start to fall.

Hydrometer, always hydrometer. There are probably dozens of factors that can play into fermentability, but in general for most beer styles and most yeast strains, the final gravity is usually approximately 1/4 as much as the original gravity. Therefore, if you want to know how far your fermentation is, it will always be contingent on the original gravity.

As an example, say you started with a wort of 1.060 original gravity. The final gravity, then, on the average, would be expected to hit approximately 1/4 as much or 1.015 (you should only use the digits to the right of the decimal point). This is a difference of 60-15=45 gravity units. So then if you want to know when the fermentation is down to the last 1/3, you need to divide this by 3, so 45/3=15. But then you still need to add that back to the bottom gravity of 1.015, so the final answer is 15+15=30 or 1.030. Therefore, at a gravity of 1.030, your 1.060 wort is down to the last 1/3 of fermentation.

In other words, when the gravity is about half of what you started with, the fermentation is about 2/3 done. Man oh man, now that math is easy. I guess I myself learned something today.

:cheers:

[quote=“dmtaylo2”]Hydrometer, always hydrometer. There are probably dozens of factors that can play into fermentability, but in general for most beer styles and most yeast strains, the final gravity is usually approximately 1/4 as much as the original gravity. Therefore, if you want to know how far your fermentation is, it will always be contingent on the original gravity.

As an example, say you started with a wort of 1.060 original gravity. The final gravity, then, on the average, would be expected to hit approximately 1/4 as much or 1.015 (you should only use the digits to the right of the decimal point). This is a difference of 60-15=45 gravity units. So then if you want to know when the fermentation is down to the last 1/3, you need to divide this by 3, so 45/3=15. But then you still need to add that back to the bottom gravity of 1.015, so the final answer is 15+15=30 or 1.030. Therefore, at a gravity of 1.030, your 1.060 wort is down to the last 1/3 of fermentation.

In other words, when the gravity is about half of what you started with, the fermentation is about 2/3 done. Man oh man, now that math is easy. I guess I myself learned something today.

:cheers: [/quote]

I gotta agree… But I will add this… I f you take a hydrometer reading near the end of fermentation for three days straight and have the same number it,s for the most part ready to bottle or keg…Tank :cheers:

You guys always check gravity before doing a diacetyl rest?

I didn’t say that… if you do a taste test and cannot taste any diacetyl, then a D rest is optional. It will never hurt to do one but it is not always required. Also if fermentation is clearly slowed to a crawl after a vigorous big krausen period you can reasonably D rest without a gravity check. However for a newbie a gravity check is the best suggestion.