Question about Gravity

Hey Guys,

Only my second post, but I have a burning question... I've had a potter around on the internet about Gravity, and I've ascertained that gravity is in relation to the amount of particles (?) in the water as compared to the original gravity of water (base mark?), and as such we can check the ABV of the beer by comparing the original gravity (OG) to the final gravity (FG).

Now... while it may (or may not) sound like I have a semi-solid understanding of gravity, I can assure you I do not! :lol:

I don't even really understand how to measure it.. the measurements I see on recipes is often a 10** number.. and my hydrometer doesn't have that reading. Does it assume the 10 and give the final 2 digits or something?

Also, and really my main reason for this questions is... what happens if you reach wort completion and you test your OG and you're not where the recipe says you should be? Are there a set number of things you've usually done wrong? Too high equals X or too low equals Y? And mainly, is there anything post mash that can be done to make final adjustments? Or is it simply a case of sticking as close to the recipe re; temps etc and you'll be fine?

I'm really at a beginner level, but I'm super keen to learn all parts as well as I can, so I'd really appreciate some guidance on this if you have the time and knowledge to share.

Thanks
Jon

EDIT: I mention (because I read somewhere), that you can gather an accurate ABV from comparing the OG to the FG, but I don't know how this is done. Is there a formulae?

Ticked off one question by using the search...

http://www.thehomebrewforum.co.uk/showthread.php?t=60895

This gives me some answers about Gravity, and how it works with regards to reading it.. but what about when it's wrong, or off recipe?

think of it this way.. a litre of pure water with a gravity reading of 1.000 at the temperature the hydrometer is calibrated for will weigh 1 kg.

a litre of beer wort with a gravity of 1.054 will weigh about 1.054kg there are about 54g of soluble sugars and salts dissolved in the liquid.

hope that helps..

edit - added words 'about' as of course if you could extract the salts and sugars dissolved in 1l of liquid you would have slightly less than 1l of pure water as the mass of sugars and salts will have some volume too.

but its a good enough yardstick to illustrate the point (and provide me with volume levels based on mass when i weigh my post mash liquor and take a gravity reading)

It's 1.0 not 10
No look at your hydrometer and it should make more sense.
Yes if you are over you have extracted more sugar out if the grain. You can add water to bring it down. If it's below then there could be loads of scenarios why but you can add dme or lme to bring it back up.
Yes there is a formalar but easier to use a online calculator

_jon_ said:

Ticked off one question by using the search...

http://www.thehomebrewforum.co.uk/showthread.php?t=60895

This gives me some answers about Gravity, and how it works with regards to reading it.. but what about when it's wrong, or off recipe?

Click to expand...

TBH it is easier to answer such questions in specific terms. So, if you think that your OG is 1.040 and your FG is 1.010 etc.

In the example above your ABV will be 4%.

This is the "correct" answer for most kits

If the result you get is not the one you like, then it may be possible to explore the reasons for that.

Fil said:

think of it this way.. a litre of pure water with a gravity reading of 1.000 at the temperature the hydrometer is calibrated for will weigh 1 kg.

a litre of beer wort with a gravity of 1.054 will weigh about 1.054kg there are about 54g of soluble sugars and salts dissolved in the liquid.

hope that helps..

edit - added words 'about' as of course if you could extract the salts and sugars dissolved in 1l of liquid you would have slightly less than 1l of pure water as the mass of sugars and salts will have some volume too.

but its a good enough yardstick to illustrate the point (and provide me with volume levels based on mass when i weigh my post mash liquor and take a gravity reading)

Click to expand...

That's great.. that makes so much more sense. So would it be true to say that higher gravity beers, mean higher ABV beers? Makes sense to be the case, since the starting point is 1.000 per litre, and the greater the difference from 1.000 the higher the ABV?

Leon103 said:

It's 1.0 not 10
No look at your hydrometer and it should make more sense.
Yes if you are over you have extracted more sugar out if the grain. You can add water to bring it down. If it's below then there could be loads of scenarios why but you can add dme or lme to bring it back up.
Yes there is a formalar but easier to use a online calculator

Click to expand...

This is interesting. Would it be 'common' to use water or dme to adjust the gravity? Or would most people be okay with just accepting what they've got and trying to work out where they went wrong?

Slid said:

TBH it is easier to answer such questions in specific terms. So, if you think that your OG is 1.040 and your FG is 1.010 etc.

In the example above your ABV will be 4%.

This is the "correct" answer for most kits

If the result you get is not the one you like, then it may be possible to explore the reasons for that.

Click to expand...

So you want to end up with a gravity lower than that of the original? Is this because the more sugar that had been consumed by the yeast the higher the ABV? I'm a little confused by this concept?

Apologies if it's taking me a while to understand it, but once I get it I'll never need to ask again :)

if brewing kits then as long as you measure correctly both volume of water you use and the mass of any sugar additions you make you should be pretty close to targets if not spot on.. a easy error is to rely on volume markings on brewbuckets and FVs, Always best to calibrate 1l of water in a jug by weighing it and adding 1 x at a time to verify/regrade your bucket or FV.. (Be mindfull adding liquor over 80-85c? into an empty FV made from hdpe, it may soften it enough to change shape and screw up volume markings..)

If brewing AG well its a case of brew by brew refinement for brew #1 your BHE (brew house efficiency), and biol off rate, the two major factors which influence your OG (original gravity) will be estimates, brew #2 you will have the results of brew#1 to refine those estimates etc... a graded dipstick is a useful tool to monitor the boil and with some control of the heat input you can turn up the heat or turn it down a tad depending on progress durring the boil. But as long as your OG is over 1.032 it should ferment out with enough alcohol to maintain shelf life. And imho unless way off target its generally best to live with the result and adjust the volume of sparge liquor or grain used for the following brews..

Yes..the yeast eats the sugar and produces CO2 and alcohol. The CO2 bubbles through the airlock and the alcohol stays in the beer. The hydrometer sits lower after fermentation as the liquid is less dense. Ideally you reach the suggested fg or final gravity. It can vary with various yeast types but for various reasons it can stick.

_jon_ said:

That's great.. that makes so much more sense. So would it be true to say that higher gravity beers, mean higher ABV beers? Makes sense to be the case, since the starting point is 1.000 per litre, and the greater the difference from 1.000 the higher the ABV?

Click to expand...

Yes.. generally speaking the higher the OG the higher the expected FG too, as the complex mix of sugars derived from a mash will contain a proportion of complex sugars that the yeast can not break down and consume which remain to provide the body and mouthfeel of a beer.

Hi Fil,

Thanks for getting back to me. I'll be brewing AG only from now on, I've brewed a bunch of kits and they've all come out okay. But this coming batch of AG will be my first. And the first time I've used any of the equipment I've recently purchased.

Your second paragraph was interesting but confusing. I've watched probably 100 hours of YouTube videos on AG brewing, and I have downloaded a spreadsheet which I can fill out with my actual measurements... but I suppose I should understand what I should be making notes about.

I've seen people on youtube use a measuring stick during the boil, but embarrassingly I thought that was just simply to measure that they poured enough water into the boil kettle.. They were actually measuring the evaporation during the boil?

.........I've got so much to learn lol

_jon_ said:

This is interesting. Would it be 'common' to use water or dme to adjust the gravity? Or would most people be okay with just accepting what they've got and trying to work out where they went wrong?

Click to expand...

I just accept but have quickly worked out my setup and use brewing software to assist. I have produced beer well above my target but I just live with it

_jon_ said:

Hi Fil,

Thanks for getting back to me. I'll be brewing AG only from now on, I've brewed a bunch of kits and they've all come out okay. But this coming batch of AG will be my first. And the first time I've used any of the equipment I've recently purchased.

Your second paragraph was interesting but confusing. I've watched probably 100 hours of YouTube videos on AG brewing, and I have downloaded a spreadsheet which I can fill out with my actual measurements... but I suppose I should understand what I should be making notes about.

I've seen people on youtube use a measuring stick during the boil, but embarrassingly I thought that was just simply to measure that they poured enough water into the boil kettle.. They were actually measuring the evaporation during the boil?

.........I've got so much to learn lol

Click to expand...

There are as many twists on a brew day as there are brewers, we all have our little ways of doing things. the two main factors that influence your OG are your brew house efficiency BHE, how efficient your mash is in converting the starches to sugars. and how much sparge liquor you employ.

iirc most recipies will assume something like a 75% BHE, but 80-85% BHE isnt unheard of,, you can google the calculation or check out the forum calculators (ive not visited them in a while..)

the other major factor is the boil off rate you achieve, with a 5 gallon brew you could boil off anything between 2-8l depending on heat input and circumstance.

a dipstick marked with your target volume is a ready reckoner for boil progress. if boiling for 90 minutes you can check very easily after 30 minutes how much liquor has boiled off and if significantly less than 1/3rd of your estimate it is an opportunity to apply more heat, or less if your boiling off too much and can dial back on the heat input..
(this is why many diy boiler builds use 2 x kettle elements 1 is sufficient to boil but 2 lets u blast a bit if needed as well as getting upto boil a bit quicker)

But if at the end of the brew your off target, well as long as its strong enough to sit on the shelf, so what? next brew will be closer due to lessons learnt.

But if important to you saving some hlt/sparge liquor can let you liquor back (dilute) the brew to increase volume or drop gravity.

If under gravity you can boil longer But with brew #1 your losses to deadspace and hop absorbtion are only estimates and longer boiling with hops loose in the kettle can result in less aroma qualities from your hops and more bitterness.

BTW BHE isnt a measure of how good a brewer you are, as equipment and grain both play a huge part in it. old grain sat on a shelf for ages wont be as good as grain milled a day or two in advance of brewing.. judge your brewing on the beer out of the FV, its the cold side (post boil) that all the real skills come in

This is great stuff, I'm learning loads.. and I thank you :)

FV? Fermentation Vessel?

I've just checked a future recipe which I'd like to make, and I see that there is a pre boil volume of 6.9l Gallons, and a Post Boil Volume of 5.81 Gallons... So it now makes sense. I need to evaporate 1.1 gallons of wort during the boil. So I need to find a good time/temp ratio with my equipment to be able to do that. Correct?

People use dipstick and the like but to find my boil of rate I did the following. Put 5L of water in my pot. Boil it for 30 mins. Then minus what I had left from five. Double it and this gave me for an hour boil (or triple for 90 min boil). I found it to be very accurate and has worked for both the BIAB pots I've had

Ahh that makes sense.. of course I suppose temperature makes no difference as the gravity is not different enough to change the boil temperature of 100c?

So really we're really testing for the time it takes to boil off / evaporation rate?

_jon_ said:

Ahh that makes sense.. of course I suppose temperature makes no difference as the gravity is not different enough to change the boil temperature of 100c?

So really we're really testing for the time it takes to boil off / evaporation rate?

Click to expand...

oh yes it is.. it only takes a teaspoon of salt to raise the boiling temp of a 1/2 -1 gallon of water to boil pasta, and your adding a few kilos of sugars into 5 gallons of beer, Check yourself when your boiling .. and as you boil off water the gravity increases as will the temp needed to achieve a rolling boil, tho you may need a very precise thermometer to detect that.

bear in mind your hop additions are generally timed from the end of the boil.
So if you extend the boil period you can loose aroma qualities and increase bitterness a few ibus. you can of course hold back adding the last additions of hops until you know your close to target volume.. but thats complicating things and on an initial brewday avoid any over complications imho..
as long as its got enough gravity to ferment into beer with a shelf life its a success and the results of your brew (bhe and boil off and the effect on targets) can be taken into account for brew #2 which should then be close to spot on

There is an equation to work out ABV from OG and FG readings, and it's dead simple. For the purposes of the equation, take only the last two digits of the gravity reading and treat them as a whole number. Subtract the FG from the OG, and divide the result by 7.36.

For example, let's say your OG is 1.040 and your FG is 1.010.
40 -10=30
30÷7.36=4.076
ABV=4.076%