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Old 01-03-2009, 09:47 PM   #1
High_Desert_Brew
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Default Altitude and Specific Gravity

Hi all, I'm a newbie brewing just my second batch. I'm wondering if anybody knows any correlation between altitude and specific gravity. I'm brewing at ~6300 feet and my two beers so far have lower OGs than the recipes indicate they should have. I just put an Oatmeal Stout (from extract kit) into the fermenter that was supposed to be ~1.053 and it came out 1.031 with temperature correction. Since this peaked my curiosity, I measured the gravity of my tap water and it came out at 0.994 (temperature corrected). Is there something to this altitude thing or is my hydrometer screwed up?



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Old 01-03-2009, 10:37 PM   #2
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It certainly can make a difference. Specifically with the floating hydrometers. Firstly, specific gravity is a relative measurement. Normally, because the density of water varies with temperature and pressure, the relative point of 1.0000 is usually referred to the density of water at 4°C (39.2°F) and a normal pressure of 1 atm.

As your air pressure decreases, the density of water decreases (as you've shown by measurement). The volume of your floaty hyrdrometer will also change as it is a sealed instrument and there is less gas "crushing" it (I am not sure if they are vacuums or some gas or just air, but it should be reading as if it were *very* slightly larger like maybe not perceptible).
As the density decreases, the solubility is also decreased (as there are less molecules to shear apart the others, therefore creating less ionic bonds). The rate of dissolution is also affected by solvent density, and believe it or not, solutes are less "attractive" at lower solvent densities because of surface area of the ion exchange is also lower. So there's like 3 things going against you. Not to mention the ion bonds that need to be destroyed requires energy, and you'll have less molecular kinetic energy at lower densities... so maybe there's like four factors working against you at higher altitudes.

Buy some distilled or (even better) RO water, measure it with your hydromter and compare it to your tap water. see if you can get it to 39.2 degrees instead of using a table to make the correction. Check your measurement against your altitude (maybe if you want to get *really* crazy, check your barometer). Then calculate what it *should* be.

Also, do your beers taste good?



References:
Van der Waals equation - Wikipedia, the free encyclopedia



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Old 01-03-2009, 10:49 PM   #3
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Thanks, I suspected the lower atmospheric pressure would play a role in this (it's ~ 610 Torr here). Heck, my water boils at ~195°F.... I'll check the hydrometer against RO water sometime and see what I get. This should be a fun experiment!

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Old 01-04-2009, 11:58 AM   #4
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Huh? A hydrometer works by comparing the buoyancy of a solution with that of air. Air is 1000 times less dense than water so altitude differences are simply WAY too small to be detected (less than .1% at a mile ASL).

Also, I've never seen a brewing hydrometer calibrated for 39F. Most I've seen are calibrated to 60F.

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Old 06-03-2009, 03:05 AM   #5
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I've been intrigued by this principle and I went into full turbo geek mode to figure out what the effect of altitude might be on Specific gravity.

I live at 4500 feet above sea level and have routinely obtained significantly lower spec gravs on my brews.

Despite jkarps comments, a hydrometer is an instrument used to measure the specific gravity (or relative density) of liquids; that is, the ratio of the density of the liquid to the density of water.

The formula:

Specific Gravity = density of substance/density of water

"density of substance" would be considered to be the wort contents (sugars etc) and we'd consider that constant

Density of fluids change with both temperature and pressure. Assuming the temperature is constant, as atmospheric pressure decrease so does density as demonstrated by this diagram which shows the density of water in relationship to temperature and pressure.



In theory, higher altitudes would render lower specific gravities. How much they'd effect is hard to say. I want to calculate raw numbers, but I can't figure out how to calculate water density at different altitudes. I couldn't find the formula. And as you can see here, the relationship is somewhat curvilinear.

Ok enough of that. Time to drink some beer.

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Old 06-03-2009, 04:31 AM   #6
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It sounds like your hydrometer might be a little funky...I would adjust your measurements based on your .994 test.

It would also make sense that during the boil, you'll lose less vapor since it is boiling at a slightly cooler temperature...or am I way off on this one?

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Old 06-03-2009, 05:27 AM   #7
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Quote:
Originally Posted by Sediment View Post
In theory, higher altitudes would render lower specific gravities. How much they'd effect is hard to say. I want to calculate raw numbers, but I can't figure out how to calculate water density at different altitudes. I couldn't find the formula. And as you can see here, the relationship is somewhat curvilinear.

Ok enough of that. Time to drink some beer.
I'm not sure such a beast exists, at least not in a form that makes sense under homebrewing conditions. Looking at your plot above, an increase in pressure from 1 bar to 25 bar (that's 25 atmospheres, folks) results in a change in indicated gravity of around one point.

Given that we're talking about differences of less than 1/4 bar (typical atm pressure at my home in Denver is 0.83 bar), the difference on specific gravity measurement is within the range of instrument and operator error.

OP, as a newbie brewer, I think you're experiencing the fact that when you mix a high-gravity extract boil, which I'm assuming you're doing, with low-gravity water to dilute, it's really hard to mix it completely, so the less dense water floats on top of the dense wort and throws your gravity readings off. Don't worry about it. With extract, as long as you get the extract amount and water volume right, you'll be in the right range for specific gravity.
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Old 06-03-2009, 11:36 AM   #8
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Everything you needed was right on the site you pulled that graphic from. Did you do the math? The Bulk Modulus of water is 3.12 10^5 psi, lbf/in^2. 10^5 psi?!?!?!?!?! How big is your altitude pressure difference? I'll leave that as your homework.

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Old 06-03-2009, 11:45 AM   #9
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I think my head 'sploded with all the uber geek speak. Imma grab a homebrew. May I suggest you guys do the same?

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Old 06-03-2009, 07:29 PM   #10
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I think many people are saying the right thing, but it's all quite confusing;

jds is correct: the difference in air pressure isn't nearly enough to affect the measurable density of any water-based solution (using a homebrew hydrometer).

jkarp seems correct but isn't explaining it enough. I'll do the math for everyone:

Bulk Modulus: "It is defined as the pressure increase needed to cause a given relative decrease in volume."(wikipedia)

Bulk Modulus (K) for water is 3.12 x 10^5 psi. The difference in air pressure between sea level and High Desert Brew's home is 760-610 torr = 150 torr = 0.20 atm = 2.90 psi.

2.90 psi / 3.12 x 10^5 psi = 9.3 x 10^-6 or ~0.001%

Assuming everything else is equal and that the density of water at sea level is 1.0000, then the density of water in High Desert Brew's house is 0.9991, a negligible difference.

Keep in mind here I am saying "density" but specific gravity can be substituted at will since we are talking about water. Along the same lines, if the water is less dense at higher altitude, then the water acting as the solvent in wort is also less dense to the same degree, so any differences would be the same for both.

I would listen to jds--I have made the same mistake of measuring the gravity of wort that I had just diluted and it was way low. I shook the fermenter around to make a more homogenous solution and took the measurement again and it was right on.

And of course I would also listen to SmugMug and RDWHAHB



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