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Old 05-23-2006, 06:46 PM   #1
Brewno
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Default Hydrometer reading

Hi,

I've been reading "The New Complete Joy of Home Brewing by Charlie Papazian" .

In it he mentions that hydrometers give accurate readings at 60 degrees F.
He says for every 10 degrees above that you have to add .004-.006 to the reading (my decimal points may be wrong but you get the idea).

I haven't read this anywhere else and when I questioned another homebrewer he said: " I just drop my hydrometer in the liquid take a reading and don't worry about it, then I take my final gravity and that's that. All I worry about is making it then drinking it" He never heard of the 60 degree thing.

Anyone?

Tommy
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Old 05-23-2006, 06:51 PM   #2
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Hydrometers are definitely calibrated for a particular temperature. Liquids are denser at lower temperatures so the hydrometer will read differently. However, you can't really say that all are calibrated for 60F...I've seen some for 68F. Also, most of them come with a little slip of paper in the tube which tells what the correction factor at a given temperature is.

If you're going to the trouble to take a reading, might as well get an accurate one IMO.
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Old 05-23-2006, 06:54 PM   #3
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The gravity readings are affected by temperature and charlie's chart should give you the correct reading..

that being said.. (i'm guessing here, somebody correct me if i'm wrong)
If you take the OG and FG readings at the same temperature then your ABV calculation should still be correct..

if the OG and FG readings are at different temp then it will be off.

and after all is said and done..

you'll still drink the beer

ws
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Old 05-23-2006, 07:04 PM   #4
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yeah funny thing about specific gravity, it has no units. Its just a ratio between water and whatever your reading. apparently Scientific hydro's are calibrated at 4 degrees C (39.2F) but most beer ones are at 60F, (water should read 1.000 @ 60 degrees)

you could calibrate it yourself by sampling water at 60-70 degrees. You should be around about 1.000 I did this recently and found mine to be 'off' by .002 degrees. (i actually just remembered this as i was writing and now realize my last 2 batches have different readings then what i wrote down, oops!)Once its calibrated it is a really convenient tool.
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Old 05-23-2006, 07:07 PM   #5
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Quote:
Originally Posted by waskelton4
The gravity readings are affected by temperature and charlie's chart should give you the correct reading..

that being said.. (i'm guessing here, somebody correct me if i'm wrong)
If you take the OG and FG readings at the same temperature then your ABV calculation should still be correct..

if the OG and FG readings are at different temp then it will be off.

and after all is said and done..

you'll still drink the beer

ws
I was kind of thinking along these same lines. The problem is my math skills aren't good enough to know if that would work out or just make me look stupid
But, if what's being measured is liquid density and this needs to be done at a certain temp, then couldn't the readings be wrong? In other words, maybe the OG and the FG (if both measured at the same temp) will give you a correct reading as far as getting a ballpark calculation or to show that your brew is good to go but not necessarily a "true" ABV reading.

I don't know how or why but it kind of reminds me of the old "what weights more a hundred lbs of bricks or a hundred lbs of feathers?"

Don't mind me, I have no clue what I'm talking about

Tommy
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Old 05-23-2006, 07:15 PM   #6
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Quote:
Originally Posted by Brewno

"what weights more a hundred lbs of bricks or a hundred lbs of feathers?"

Tommy
Definitley bricks weigh more. Ive done masonry and believe me its much harder on your back to move 100 lbs of bricks then 100 lbs of feathers. (1 brick vs. 1 feather at a time)
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Old 05-23-2006, 07:22 PM   #7
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Quote:
Originally Posted by Brewno
In other words, maybe the OG and the FG (if both measured at the same temp) will give you a correct reading as far as getting a ballpark calculation or to show that your brew is good to go but not necessarily a "true" ABV reading.
I think (i've only done it once) that the ABV forumla uses the difference of OG and FG.

(OG - FG) * 131 = ABV

or something like that
so if OG and FG are both wrong .. but are wrong by the same amount in the same direction.. then ABV will be correct.

wow.. that was the most math i've done since freshman year college..

and i could be equally as wrong as most of that freshman year college math too

ws
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Old 05-23-2006, 07:31 PM   #8
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Quote:
Originally Posted by Bjorn Borg
Definitley bricks weigh more. Ive done masonry and believe me its much harder on your back to move 100 lbs of bricks then 100 lbs of feathers. (1 brick vs. 1 feather at a time)
So much for "100 lbs is 100 lbs"


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Old 05-23-2006, 07:32 PM   #9
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Quote:
Originally Posted by Bjorn Borg
yeah funny thing about specific gravity, it has no units.
The unit of specific gravity is Kg/L. It just happens that 1 L of water weighs 1 Kg at 4*C (40F). I usually care about taking an accurate reading. But mostly I just note the temp and the SG reading and let Beersmith do the calculation when I have it around and care for the actual SG value.

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Old 05-23-2006, 07:35 PM   #10
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Here's a link to a hydrometer correction utility:

http://hbd.org/cgi-bin/recipator/rec...ydrometer.html

It's especially helpful in doing AG because need to take the SG of your collected wort which is well above 60 degrees F.

(ProMash and Beersmith do this as well, I'm sure)
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