Help me understand hydrometers

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r4dyce

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Hey there,
I'm hoping someone can clarify how hydrometers work. I understand how to use them and how to read them but I don't quite understand why they work.
My understanding (please correct me if I'm wrong) is that it's basically a weighted density meter. The more sugars present in the wort the denser the fluid , the higher the meter floats. After fermentation there are less sugars, the meter sinks further and the difference is essentially how much sugar was consumed.
I guess what I'm not grasping is why it's assumed that only sugars change the density of the fluid. Wouldn't dissolved salts, gases, other by products, flavor and aroma molecules, hop particles etc all effect the density? Aren't there other changes occuring outside of sugar consumption?
Example: Beer 1 is a single malt light color low hopped wort at og 1.060.
Beer 2 is a dark complex stout at 1.060.
Wouldn't beer 2 og be artificially inflated by all the other "junk" dissolved in the wort in comparison to beer 1?

I know everyone uses it so it must work, I'm just trying to understand the science behind it. Thanks ahead of time, Please feel free to get technical , I'm happy to learn.
 
Hops will displace liquid, not change its density. My guess (leaving the scientist to confirm or debunk) is that most dissolved items in wort are negligible from a practical sense and the real reason for OG & FG is to determine the conversion of sugars and the residual sugars. Hydrometers do not discern between fermentable and un-fermentable sugars.

Back to hops. While they won't affect specific gravity, clinging a hydrometer will give false readings - a will gas bubbles etc.
 
It measures the density of what is dissolved in solution, so yes, salts too, but hopefully there isn't enough salt in solution to change your hydrometer reading, because that would be some salty beer

Gasses, yes... but that's why you spin the hydrometer, so gas doesn’t affect the reading

Floating crap shouldn't affect it
 
People used to use hydrometers to check whether they had enough antifreeze in their car radiator or you can use an hydrometer to measure the salinity of water - so yes, an hydrometer measures the density of a liquid - but in wine making or brewing the critical material that you are measuring is the amount of sugar dissolved in the water (or fruit juice = water + sugars) .
 
Yes, hydrometers are affected by the density of the liquid by whatever means alters the density.

Which is also why temperature of the sample is important. And is also why the hydrometer is calibrated using clean water. If your water has enough "stuff" in it to alter the reading, the amount of alteration will be evident in a water only reading and can be deducted from subsequent readings.
 
So I guess what I'm gathering is that despite everything else going on the only appreciable reaction that we're concerned with is the conversion of sugars.
 
So I guess what I'm gathering is that despite everything else going on the only appreciable reaction that we're concerned with is the conversion of sugars.

Conversion of sugars into alcohol, yes, that would be the entire point behind This Thing of Ours™

;)

And please take that winky face as a sign I am joking with you
 
We're concerned with only the change in density from OG to FG. The only reaction that should have occurred (fingers crossed) is yeast converting sugar to CO2 and alcohol. The original wort and the fermented beer should still have all the same salts, dissolved hops, etc. so they shouldn't skew your difference between OG and FG readings.

I hope that makes sense.
 
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