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Old 11-17-2012, 03:26 AM   #1
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Default Innacurate Hydrometer

I was skeptical of a FG reading on my Irish Stout at bottling so I took a reading of water at 60F and it read 1.005. It is the basic triple scale hydrometer that came with my first brewing kit. Can I just subtract 5 points from all readings, or do I need a new hydrometer? Is there a more accurate hydrometer if I get a new one?
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Old 11-17-2012, 03:34 AM   #2
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Look into getting a refractometer...you wont regret it. I would probably just get a new hydrometer if not. Also, use distilled water to check your current one before you write it off as being bad. Dissolved salts and other compounds are measured by the hydrometer as well and may be noticable in your tap water.
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Old 11-17-2012, 03:38 AM   #3
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Quote:
Originally Posted by wobrien
I was skeptical of a FG reading on my Irish Stout at bottling so I took a reading of water at 60F and it read 1.005. It is the basic triple scale hydrometer that came with my first brewing kit. Can I just subtract 5 points from all readings, or do I need a new hydrometer? Is there a more accurate hydrometer if I get a new one?
Toss it and get a new one. Refractometers are great, though I still use a hydro to verify FG (I like tasting it&#128521.
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Old 11-17-2012, 03:44 AM   #4
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Quote:
Originally Posted by wobrien View Post
I was skeptical of a FG reading on my Irish Stout at bottling so I took a reading of water at 60F and it read 1.005. It is the basic triple scale hydrometer that came with my first brewing kit. Can I just subtract 5 points from all readings, or do I need a new hydrometer? Is there a more accurate hydrometer if I get a new one?
the scale is linear, so yes, you can just subtract 0.005 from your readings
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Old 11-17-2012, 03:45 AM   #5
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I'll try a reading in distilled water when I get a chance. Not ready to spend the money for a refractometer, but there has to be a more accurate hydrometer, no? I feel like the paper glued inside the tube could easily shift.
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Old 11-17-2012, 04:21 AM   #6
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Originally Posted by mb2696 View Post
the scale is linear, so yes, you can just subtract 0.005 from your readings
Nope. It is not linear. The error can become greater as the gravity increases. I've done some signficant testing on this. I'll find the results and post, but I have (had) 3 hydrometers and one of them was bad.

Throw the hydrometer away.

[edit]

Conclusion: Hydro #2 got thrown away. It had error that was definitely NOT linear. Measured same samples with a refractometer and 3 hydrometers. The data:
Refractometer: Water: 1.000 Test Solution: 1.100 (as much as I could discern the fine lines)
Hydro 1: Water: 1.000 Test Solution: 1.101
Hydro 2: Water: 1.002 Test Solution: 1.111
Hydro 3: Water: 1.000 Test Solution: 1.100
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Old 11-17-2012, 04:42 AM   #7
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Whether it's linear or not will depend on why it's inaccurate. If it's because the paper with the scale on it has shifted inside the glass tube, then it'll just be an offset. If it's because the tube is the wrong diameter (or is not cylindrical), then it will be more than an offset (though not necessarily nonlinear---if it were conical instead of cylindrical, it'd be linear, but not just an offset).
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Old 11-17-2012, 11:48 AM   #8
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Quote:
Originally Posted by zeg View Post
Whether it's linear or not will depend on why it's inaccurate. If it's because the paper with the scale on it has shifted inside the glass tube, then it'll just be an offset. If it's because the tube is the wrong diameter (or is not cylindrical), then it will be more than an offset (though not necessarily nonlinear---if it were conical instead of cylindrical, it'd be linear, but not just an offset).
Great point zeg -- thank you for pointing that out. 1) I was misapplying the term linear and 2) I hadn't even thought of the shape of the glass!
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Old 11-17-2012, 12:04 PM   #9
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You're only off by 5 points. My bet is that it is due to dissolved stuff in your water. Distilled water should read spot on, but not tap water. Get a second hydrometer and see if the readings match.

You could standardize it by dissolving sugar in water at different concentrations and then seeing what the specific gravity reads. I suppose that's a lot of work for a $5 item.

If all you are interested in is determining %abv, that's calculated by the difference of two readings. If the hydrometer is off in the same direction (systematic error), the errors should cancel when subtracting. I'd be more concerned if I had 2 hydrometers that read differently than each other than if my single hydrometer read off what I expected.
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Old 11-17-2012, 12:04 PM   #10
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Quote:
Originally Posted by passedpawn View Post
Nope. It is not linear. The error can become greater as the gravity increases. I've done some signficant testing on this. I'll find the results and post, but I have (had) 3 hydrometers and one of them was bad.

Throw the hydrometer away.

[edit]

Conclusion: Hydro #2 got thrown away. It had error that was definitely NOT linear. Measured same samples with a refractometer and 3 hydrometers. The data:
Refractometer: Water: 1.000 Test Solution: 1.100 (as much as I could discern the fine lines)
Hydro 1: Water: 1.000 Test Solution: 1.101
Hydro 2: Water: 1.002 Test Solution: 1.111
Hydro 3: Water: 1.000 Test Solution: 1.100
I did misspeak by saying that since it's linear it's just an offset.

However, for the reasons zeg pointed out, just because "the error can become greater as the gravity increases" does not mean it is not linear. It could simply be that the glass is the wrong diameter in which case the correction would be a scaling AND offset, which is still linear.

In your experiment I would agree that it looks like the correction for hydro 2 it not simply an offset. However, this does not mean that a hydro should be thrown out if it doesn't read 1.000 in water. You need a second high-ish gravity verification solution (like what you've done). On the other hand, it's $5 for a new one, so maybe it's not worth the time?
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