Author Topic: newbie refractometer question  (Read 873 times)

Offline foxbrew

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newbie refractometer question
« on: September 24, 2013, 01:50:18 PM »
hey all ...

just brewed an IPA and tested the preboil gravity with my refractometer (temp??) and hit my target (1.044) dead on.  i did a 90 minute boil and then tested again and had a reading of 1.056 (it was supposed to be 1.062).  after sitting down at my computer later that night, i realized that i made a total rookie mistake and tested the sample only seconds after pulling it from the boil.  in other words, i'm very sure that the wort sample was well over 140-150 degrees when i took a reading.

all of that being said, could this be the reason for missing my OG?  from what i've read, i now know that taking a refractometer reading too hot can cause inaccurate numbers ... i'm just not sure by how much or in what direction.  any input would be greatly appreciated.  still trying to figure this all grain thing out!!!

thanks in advance!

Offline theDarkSide

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Re: newbie refractometer question
« Reply #1 on: September 24, 2013, 01:56:44 PM »
Does your refractometer have ATC (Automatic Temperature Compensating)?

A few drops from the boil and then put on the glass will cool almost immediately.  .006 difference in what you "expect" to get is no big deal and could be attributed to the vigorousness of your boil, humidity levels, outside temp, etc.
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Offline duncan

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Re: newbie refractometer question
« Reply #2 on: September 24, 2013, 01:57:56 PM »
Also be sure to calibrate the refractometer with distilled water for max accuracy.

Offline morticaixavier

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Re: newbie refractometer question
« Reply #3 on: September 24, 2013, 02:24:27 PM »
I would check my volumes before looking to the refractometer.

If you have .5 extra gallons of wort that accounts for your difference almost exactly
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Offline svejk

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Re: newbie refractometer question
« Reply #4 on: September 24, 2013, 03:14:17 PM »
I have noticed with my refractometer if I set it aside for a few minutes and check it, the gravity will usually rise by a bit.  It has ATC, but it just isn't as quick as I would have expected.  My usual practice is to use my refractometer for checking gravities throughout the brew day.  Once the wort is chilled and in the fermenter, I check the gravity with a hydrometer because my standard practice is to aim a bit high on gravity and a bit low on volume in the fermenter so I can dilute with water to hit the targeted OG.  My hydrometer is more accurate than my refractometer, so I like the accuracy for when I'm doing the dilution.

Offline morticaixavier

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Re: newbie refractometer question
« Reply #5 on: September 24, 2013, 03:23:27 PM »
I have noticed with my refractometer if I set it aside for a few minutes and check it, the gravity will usually rise by a bit.  It has ATC, but it just isn't as quick as I would have expected.  My usual practice is to use my refractometer for checking gravities throughout the brew day.  Once the wort is chilled and in the fermenter, I check the gravity with a hydrometer because my standard practice is to aim a bit high on gravity and a bit low on volume in the fermenter so I can dilute with water to hit the targeted OG.  My hydrometer is more accurate than my refractometer, so I like the accuracy for when I'm doing the dilution.

that could also be from evaporation. The sugar doesn't and the water does
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Offline foxbrew

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Re: newbie refractometer question
« Reply #6 on: September 24, 2013, 04:01:54 PM »
i should have included my volumes in the original post ... sorry.

- recipe called for 9 gallons.  after fly sparging, i collected about 7.5 gallons.  added approx 1.5 gallons.

- boiled for 90 mins.

- final volume was at approx 6.5 gallons (recipe estimated 6.75 gallons)

again, thanks for all of your replies.  very much appreciated.  all of my brewing knowledge has been pulled from the internet and books.  i think it may be time to join a club and have someone who knows what they're doing look over my shoulder for a brew session or two!

Online garc_mall

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Re: newbie refractometer question
« Reply #7 on: September 24, 2013, 04:41:16 PM »
So, running some math through, and I don't see how that recipe goes from 9gal at 1.044 to 6.75 gal at 1.062. I calculate 1.059 @ 6.75 gal, and 1.061 @ 6.5 gal.

I do believe that the refractometer underestimates the gravity at higher temperatures, but can't be sure.

When I do it, I just note the numbers, and don't worry too much if my gravity is off by a point or two.

If you want to know for sure, you need to be really exact with your volumes. If you measured the volume (6.5gal) at 150*, you would end up with about a 2-2.5% difference in volume (due to contraction).

I think you are doing fine, but joining a club will certainly allow you to find someone who can critique your processes, and it is also great to see how others brew their beer.
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Offline Slowbrew

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Re: newbie refractometer question
« Reply #8 on: September 25, 2013, 06:56:15 AM »
I don't know if it makes any real difference, but when I take my high temp samples I cover the opening on the plastic pipette with my thumb and dip the bulb in a bucket of cool water for a couple of seconds.  Then I take my readings.

I've never done it for accuracy though.  I just don't like dropping boiling liquids on glass if I don't have to.  I may have to try it both ways next time I brew just to see if there is any difference.

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Offline denny

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Re: newbie refractometer question
« Reply #9 on: September 25, 2013, 08:18:13 AM »
Does your refractometer have ATC (Automatic Temperature Compensating)?

A few drops from the boil and then put on the glass will cool almost immediately.  .006 difference in what you "expect" to get is no big deal and could be attributed to the vigorousness of your boil, humidity levels, outside temp, etc.

ATC corrects for the temp of the refractometer, not the sample.  Maybe your refractometer is different.

A sample put on a refractometer is so small and cools so quickly that sample temp really isn't an issue.
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Offline bluesman

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Re: newbie refractometer question
« Reply #10 on: September 25, 2013, 09:38:46 AM »
If I get an unusual reading, I'll retake the measurement. If it repeats then I know that I have a degree of repeatability, but the calibration with distilled water will give you an indication of how accurate the measurement will be. Evaporation of the sample during sampling is also another variable. Try rapidly sampling to mitigate this factor. Close the daylight plate immediately upon dropping the sample on the measuring prism.
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