Why does anyone use a hydrometer?

[jasea3632]

I use a hydrometer because i like to watch something bobbing up and down in a tube it soothes the inner demons
BTW can we get a facepalm smiley

 

[ScottM]

Not quite on the same level but I use a fairly complex FG calculation for when the sugar's are known. Obivously with grain brewing this is never going to be the case so it's fairly useless there, but when you know what you are putting into the brew... you can be fairly accurate with what will come out.
...
The assumptions of my calculation are:

You know (or can work out) exactly how much fermentables are being added to your brew.
You know (or can work out) the exact yield of what you are adding.
You know the attenuation of the yeast (Higher attenuation effectively means the yeast is less fussy with it's sugar varieties).
...
FG = OG + 0.624 - sqrt((((100/(1000*volume)*(total fermentability*0.645))*0.789*(yeast attenuation/100*1.33))/100.3)-(OG-(125.65/200.6))^2+0.7817+OG^2-(OG*125.65/100.3));

Interesting stuff, I like a bit of maths and I'm constantly finding ways to make my brewing spreadsheet more complicated :)

I've probably misunderstood, but the above assumptions sound a little bit circular to me? You're calculating the FG, but how do you know the attenuation of the yeast unless you measure the FG? I keep a record of attenuation for each brew, and it's rarely the median of the stated range.

The attenuation is actually the least important factor believe it or not. Most yeasts are around the 75% mark but it's a bit of a fake figure.

If you took the lowest attenuating yeast, lets say 65%, and put it in a volume of water with nutrient and brewing sugar, the apparent attentuation will be closer to 100% than it will 65%. The attenuation figure is based on the assumption that a % of the brew will be easily fermentable (lets say a guaranteed 60% is always easy). An attenuation lower than 75% (on the yeast packet) simply means that the yeast isn't as aggressive to non-simple sugars and prefers the easily fermentable stuff, higher than 75% means it is more aggressive to non-simple sugars.

A funny analogy popped into my head. 3 guys locked in 3 rooms with 5 day old pizza, a medium well t-bone and some crisps. One guy is a fat greedy bloater, another is a normal guy and the last one is a vegetarian. The fatty will probably scoff the lot, the normal guy will probably do in the t-bone and the crisps and the veggy will probably just scoff the crisps. High attenuation, normal attenuation, low attenuation


The actual attenuation figures come from the yeast information itself. They are all tested when sold, I got loads of information just searching around. The WLP and WYeast attenuations are all very easy to get. As you will know, this is the best they have gotten out of the yeast.... but my calculation gives the potential FG that can be achieved, so it's all good.

I often hit the potential or just below it so the calculation seems pretty accurate. Of course it's only going to be as accurate as the information you put into it so you need to be sure about the weights, yields, fermentability and attenuation of everything you are using :)

 

[anthonyUK]

So Scott, from a theory perspective it is great to get data to back it up and to also to show that your process is as expected but still no more accurate that reading a glass float?

I don't want that to sound like it is a pointless exercise as that is not intended but an exercise in proving that a hydrometer is correct too?

 

[Duxuk]

BTW can we get a facepalm smiley

You can copy any smiley you want into Photobucket, facebook etc. and just link it here.

Here's one controlled under the 1971 misuse of drugs act.
Though I often look like that after a few bottles of good homebrew.

 

[ScottM]

So Scott, from a theory perspective it is great to get data to back it up and to also to show that your process is as expected but still no more accurate that reading a glass float?

I don't want that to sound like it is a pointless exercise as that is not intended but an exercise in proving that a hydrometer is correct too?

Oh, that's not the reason for my calculation. I only posted about what I was doing with the FG as it was in a similar vain to what the OP was attempting to calculate. Ie, we both took into consideration the gravity of ethanol vs water etc.

I would never not use a hydrometer, it's really the only way to tell if it's finished and if you have reached the potential FG or not.

The reason for my calculations is to get a reasonably accurate guess on potential ABV when making a recipe. I use them in my homebrewbuddy site.

A simple example is when brewing a light lager. I would use 3kg of extra light DME to make the lager. Extra light DME is approx 80% fermentable with a yield of approx 83%. Bunging that into any homebrew calculator will show an approx OG of around 1.045 @ 23L. If I were to use the traditional attenuation rule (generally around 75%) on that recipe the estimated FG would be 1.011 (all calculators I have tried use this method). In reality the potential FG would be more like 1.005... as is found with most lager kits/recipes.

This may not seem like a big problem but if you are aiming for a 5% lager then 3kg of extra light DME wouldn't appear to be enough (when trusting the 1.011 figure) so you would end up adding more DME till you hit an OG of 1.052... where the potential FG would appear to be 1.013. This all seems great until it comes to the measuring.

Your beer would ferment away quite happily but would end up way past 1.013, sitting somewhere around 1.005-1.006 and giving you an ABV of over 6% which is way too high.

This is the reason for my calculations. When brewing extract, kits, wine or cider the 75% rule just doesn't apply. Most people don't notice this because they are brewing with low fermentability ingredients, as soon as you go over 75% you aren't going to be accurate at all as all standard calculators make this assumption.

Another simple example is a wine. Starting gravity of 1.090, according to standard calculators the FG would be around 1.022 lol.

I made an example earlier with water. I tried it out on the hopville beer calculus recipe maker. 23L of water, 3kg of sugar and some nutrient.... FG 1.010 (or thereabouts). In reality it would finish way past 1.000.

 

[Unclejona]

I guess its because I am vain and I like to be able to sound knowledable to my beer friends (non-brewers) :hmm:

 

[Leeds Chimp]

use one as I cant be assed to work out all the letters and numbers ......
two reading...put into online calc and gives me booze rating.....

and it sounds good to use one :rofl:

 

[IPA]

Scheelings said

"When life hands you lemons, throw them in life's face and shout I want hops, not lemons!"

I say When life hands you lemons shout " I want gin as well"

 

[jarenault]

A few months back at work I knocked up a simple fermentation in a 2L erlenmeyer flask. Just sucrose, yeast and water. Not for drinking, just to test an idea.
Using precision scales, I weighed the set up at the beginning and end and calculated the alcohol produced based on the mass lost in CO2. I also took hydrometer readings and calculated the alcohol content using that, more traditional, method.
There was remarkable consistency between results. I don't have the numbers to hand but it was less than a percentage point. I guess evaporation is somewhat negligible in a closed system with an airlock.

Caveat: n=1

 

[rpt]

A few months back at work I knocked up a simple fermentation in a 2L erlenmeyer flask. Just sucrose, yeast and water. Not for drinking, just to test an idea.
Using precision scales, I weighed the set up at the beginning and end and calculated the alcohol produced based on the mass lost in CO2. I also took hydrometer readings and calculated the alcohol content using that, more traditional, method.
There was remarkable consistency between results. I don't have the numbers to hand but it was less than a percentage point. I guess evaporation is somewhat negligible in a closed system with an airlock.

Caveat: n=1

What percentage attenuation did you get?

 

[jarenault]

A few months back at work I knocked up a simple fermentation in a 2L erlenmeyer flask. Just sucrose, yeast and water. Not for drinking, just to test an idea.
Using precision scales, I weighed the set up at the beginning and end and calculated the alcohol produced based on the mass lost in CO2. I also took hydrometer readings and calculated the alcohol content using that, more traditional, method.
There was remarkable consistency between results. I don't have the numbers to hand but it was less than a percentage point. I guess evaporation is somewhat negligible in a closed system with an airlock.

Caveat: n=1

What percentage attenuation did you get?

Not sure. I didn't record the amount of sugar I put in. I really just cobbled it together to test the idea, in principle, of measuring alcohol content from loss of mass, so I only weighed the complete set up. I should have tested the attenuation, actually, as it would have been trivial to do so.
Might do it again if I get a quiet day at work this week.

 

[ScottM]

You should be able to work out the attenuation based on your OG and FG.

Did you keep a hold of the recordings?

 

[rpt]

You should be able to work out the attenuation based on your OG and FG.

Yes, I was asking about the apparent attenuation.

 

[ScottM]

You should be able to work out the attenuation based on your OG and FG.

Yes, I was asking about the apparent attenuation.

Unfortunately the apparent attenuation will probably be around 110%.

 

[RobWalker]

my answer - because you can read it and it will tell you things and it costs about £2. :

 

[jarenault]

You should be able to work out the attenuation based on your OG and FG.

Did you keep a hold of the recordings?

I'll have them somewhere. I'll have a look tomorrow.

I don't brew for a living by the way. I work in a biological lab. This was just a wee personal experiment.

As a semi-relevant piece of trivia - the same thing happens when people lose weight. The majority of the carbon - and therefore the majority of the mass - lost in metabolism is removed from the body as CO2 in your breath. You probably already knew that but people seem to find it surprising.

 

[jonnymorris]

...removed from the body as CO2 in your breath.

I did not know that. Cool.

 

[rpt]

Unfortunately the apparent attenuation will probably be around 110%.

Why's that unfortunate?

 

[ScottM]

Unfortunately the apparent attenuation will probably be around 110%.

Why's that unfortunate?

Because attenuation won't really relate to anything when fermenting pure sugar. Any alcohol yeast will convert virtually all the sugar to alcohol so the actual attenuation will be 100%.

Attenuation is only really valid when there is a mix of sugars, ie complex through to simple, as the attenuation is based on how far down the road to complex the yeast will go. When a brew is 100% simple sugar, the yeast will convert all the sugar.

 

[rpt]

Attenuation is only really valid when there is a mix of sugars, ie complex through to simple, as the attenuation is based on how far down the road to complex the yeast will go. When a brew is 100% simple sugar, the yeast will convert all the sugar.

That's why I was asking. I want to know whether that is really true.