Post Boil Gravity...why?

[chrisluki]

Hey guys

I have a brewers friend checklist that I use on brew days, you fill in all the relevant bits of info about your brew as you go.

Anyway, one of the things it asks for is a Post Boil Gravity reading. Why take this reading? What is it used for? My reading was above my OG reading, but that may have been due to some sediment in the sample and it being 25 degrees, not 20.

Cheers

Chris

 

[Bribie G]

I would have thought that post boil gravity was the same as original gravity, but used in two different contexts.

"Post boil gravity" would be used in the context of the hot side of the brewing process, "pre boil gravity vs post boil gravity" to indicate what boiloff you would require to hit the number.

Whereas "original gravity" is in the context of the cold side, i.e. fermenting, with the "final gravity" at the end of the process.

Maybe a commercial brewer on the forum could elaborate.

Edit: it also occurs to me that in real world brewing, there could be a difference if something is to be added before fermentation, such as sugar or maltose syrup. So a wort with a post boil gravity of 1040 might have sugaz etc added, resulting in an OG of 1048.. as an example.

 

[Camo6]

I imagine if you add top up water to the fermentor due to limited kettle size, for example, then this field might bear some relevance.

 

[indica86]

Post boil gravity IMO is the same as OG and used to see how you performed regarding the numbers predicted by beersmith or whatever you use. Used with post boil volume it allows you to adjust your hopping rates to achieve the desired IBU and also the malt bill to achieve your desired colour.

 

[Adr_0]

You sure it's not meant to be the same thing? Otherwise, it might be the 3-4% change in volume/density that you get from between boiling (low density/high volume) to pre-pitch temp (15-20°C, high density/low volume).

 

[Yob]

Wouldn't it be for working out you efficiency?

 

[droid]

sugar does not evaporate during the boil therefore post boil gravity is always higher

<edit>
so how would you get your desired end abv if you don't take an after boil?

pre boil measures efficiency and gives a start point gravity to work with

 

[Camo6]

I suppose a large starter will also make the OG different to the post boil gravity.

 

[Camo6]

sugar does not evaporate during the boil therefore post boil gravity is always higher

<edit>
so how would you get your desired end abv if you don't take an after boil?

pre boil measures efficiency and gives a start point gravity to work with

I think the OP's comparing post boil gravity with original gravity, which are pretty much the same thing give or take a few variables.

 

[droid]

i don't have software here so being new to AG things get a bit crazy

in my own example;

i wanted a mid strength for my last brew so i decided to go for an og of 1.045ish thinking if i can get it down to 1008 or 1010

extracted 1.040

once boiled off i had a whopping 1.062

added water till i hit 1.046

no idea of fg no crystal ball here

but if i didn't have post boil which was way different to pre boil how would i get close to my desired abv?

how does one know what adjustment to make?

moronically yours
jon

 

[droid]

**** am i mad?

 

[Ducatiboy stu]

**** am i mad?

You will have to ask your psychiatrist that one

 

[droid]

he's dead

 

[michaeld16]

i don't have software here so being new to AG things get a bit crazy

in my own example;

i wanted a mid strength for my last brew so i decided to go for an og of 1.045ish thinking if i can get it down to 1008 or 1010

extracted 1.040

once boiled off i had a whopping 1.062

added water till i hit 1.046

no idea of fg no crystal ball here

but if i didn't have post boil which was way different to pre boil how would i get close to my desired abv?

how does one know what adjustment to make?

moronically yours
jon

How long are you boiling for? What is your boil off percantage? If you know your boil off% that can help you work out your numbers

 

[Sambrew]

It's not really that hard to take that reading is it? It would be the same as your O.G. unless you had adjustments to make. And how else would you know what adjustments you need to make?

 

[Blind Dog]

t

You sure it's not meant to be the same thing? Otherwise, it might be the 3-4% change in volume/density that you get from between boiling (low density/high volume) to pre-pitch temp (15-20°C, high density/low volume).

All gravity readings should be taken at the same temp to counter this, usually 20C but the calibration temp should be noted somewhere on your hydrometer

Post boil gravity measures the specific gravity as the wort exits the kettle, OG measures the specific gravity just before you add yeast. The 2 will be the same unless you added fermentables, diluted the wort, wild yeast / infection got in post boil, or you somehow stuffed up. Pre boil gravity helps you work out mash efficiency and, by comparing to post boil gravity, boil off. Personally, I generally only bother measuring OG and FG nowadays unless I'm brewing something out of the ordinary, but used to measure pretty much everything

 

[scon]

Try beersmith for free for 21 days and it'll do all of the calculations for you. The full version is $20 or so and is one of the best brewing related investments you can make.

 

[Adr_0]

t

All gravity readings should be taken at the same temp to counter this, usually 20C but the calibration temp should be noted somewhere on your hydrometer
Post boil gravity measures the specific gravity as the wort exits the kettle, OG measures the specific gravity just before you add yeast. The 2 will be the same unless you added fermentables, diluted the wort, wild yeast / infection got in post boil, or you somehow stuffed up. Pre boil gravity helps you work out mash efficiency and, by comparing to post boil gravity, boil off. Personally, I generally only bother measuring OG and FG nowadays unless I'm brewing something out of the ordinary, but used to measure pretty much everything

Good point, my mistake. But yes as you said they should be the same unless you add water or sugars before you pitch.

 

[droid]

How long are you boiling for? What is your boil off percantage? If you know your boil off% that can help you work out your numbers

guess i'm still working it out, I can only boil 20ltrs which ends up being 16 with trub loss and evap
if I keep doing the same batch size I guess I will know more

are peeps here getting exactly the og they wanted after boil? wouldn't it change a bit from day to day? bit more evap here bit less there

if I take one reading after boil and it is where I want it then that's "hitting my numbers" right?

if I take a reading and it's not what I want so I add some water for example then do another reading or even keep adding water and taking readings till I hit the exact gravity I wanted. the last reading is my OG and the first is my post boil

i'm just thinking that if you are getting loose og numbers which will mean variable fg's and abv's then taking a post boil reading will allow you to get the og exact and the resulting fg abv very close to exact depending on ferment

I think I shall move on...

 

[michaeld16]

Dont worry mate it all comes together with experience ive only been ag forr about 12 months now took a few goes to get used to my system brew as often as ya can i wasnt to worried about hitting numbers for about the first 4 brews just getting know what my losses and such were then it comes together