Hydrometer usage and a peculiar issue - Home Brew Forums
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Old 09-08-2009, 02:13 PM   #1
Sep 2009
Posts: 1

Hey all,
Long time reader, first time poster (i know, cliche).

Anyway, I brewed my first home made all-grain recipe this weekend (meaning we didnt purchase an AG recipie from Autin Home Brew, and came up with it myself). Everything went extremely well, including the use of my new march pump, but thats not what my question is concerning.

We use this program called brew-pal for the iphone, it will convert gravity readings based on the temperature of the wort being measured. I find it very useful for that along with all the recipe creation and brew sheets you can print out, etc.

Anyway my question is in regards to gravity and the measuring of gravity. I use a hydrometer. But, I'll give you my recipe first so you can know what I'm working with:

18lbs Pilsen
1lbs special B
1lbs crystal 120L

2oz Perle (7.5 AA) at 60 minutes (begining of boil)
2 tablets of Whirlfloc at 15 minutes left
1oz Perle (7.5 AA) at 0 minutes (end of boil)

Pretty simple recipe, but should be good.

Anyway, I take gravity throughout the process. About every 10 minutes during the mash and every 15 during the boil.

So I mashed out and pumped into the boil kettle. At that point I stired well and took a gravity reading, I was at 1.052 (using brewpal to correct the gravity reading based on the temperature) and thought "great, once I boil down I should be on target. Well, when the boil was done I took another gravity from the output of the CFC which output the wort at around 68 degrees. That gave me a reading of 1.045!! I was VERY confused. I was thinking, wait, boiling off should INCREASE gravity! not decrease it! what happened?!

So now I'm convinced that I'm not taking gravity readings properly or something. Has anyone experienced their gravity actually decreasing from begining of the boil to the end?

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Old 09-08-2009, 02:27 PM   #2
dontman's Avatar
Oct 2008
Philly, PA
Posts: 2,402
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What happens is that as you get up in the higher temperatures the hydrometer can become very inaccurate.

I've tested this and seen it for myself. I measured gravity at 150, corrected for temp, then cooled it down to 90, corrected again, and the reading was always off. Sometimes in differing directions.

Best advice is to just cool it down at least to 100 or so. Around there I find I can get pretty accurate readings.

So to repeat, even though there is a formula that is supposed to correct for any temp to adjust for the differing densities of liquid as it changes temps, it is very unreliable.
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Old 09-08-2009, 02:45 PM   #3
freddyb's Avatar
Apr 2008
Posts: 184
Liked 7 Times on 7 Posts

With as many gravity readings as you're taking on your brewday, a refractometer would be a good investment. Because it uses such a small sample size (one or two drops), your wort cools to ambient temp within a few seconds, negating errors due to temperature adjustments done by calculation or software. I bought one and absolutely love it. I still use my hydrometer for OG and FG readings, but everything else is done with the refractometer now.

Just don't point the refractometer out the window at your neighbor's house when you take the readings...it looks quite pervy from their perspective

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Old 09-08-2009, 04:37 PM   #4
Edcculus's Avatar
Jun 2007
Greenville, SC
Posts: 4,546
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+1 to it being inaccurate at high temps. But why are you taking gravity readings during the mash? That seems like an incredible waste of time. You aren't looking to reach a certain gravity in the mash. The point is to convert all of the starches to sugars. You should be doing a starch conversion test using iodine instead of gravity readings.

Its kind of redundant to take multiple readings during a boil too. Its not like you are going to shorten the boil. Most of the time, a hop schedule is based on 60 min boil. Since you have to stick to that range, taking the gravity at 30 min is a useless reading. Also, if you use a program like Beersmith to approximate boil off rates, all you need to do is take a preboil gravity reading, then another one when you are getting ready to flame out.

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Old 09-08-2009, 04:52 PM   #5
Jan 2009
Posts: 748
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no thats it's going to make a difference in the flucuations,but are you sure that iphone app is for a hydrometor that is calibrated to the same temp as yours?
For example is it for hydro calibrated to 70 and yours is 60.

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