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General Category => All Grain Brewing => Topic started by: boapiu on May 07, 2013, 10:46:04 PM

Title: SG OG BG FG,... why is my post boil reading lower than pre boil?
Post by: boapiu on May 07, 2013, 10:46:04 PM
Twice now I have measured preboil gravity and then post boil and found they are close, but just a bit higher after the boil. All grain, 10 gallons, using a thermopen to check the temperature of my sample. I usually collect about 13 gallons of wort and have around 11+ after 60 minute boil. For example, today I got 1.016 at 180f pre boil (converts to 1.045?) and 1.042 at 70f post boil (1.043?). I am more than happy with my efficiency and all that, but I had thought the gravity would increase as volume decreased during the boil. Any thoughts? Maybe this is within the margin of error considering my instruments.
Title: SG OG BG FG,... why is my post boil reading lower than pre boil?
Post by: duboman on May 07, 2013, 10:51:01 PM
In reality, even with temp correction, IME readings taken over 100 degrees are worthless.

Also, when was the last time you calibrated your hydrometer?

Generally , assuming your boil off measurements are correct a 8-10 point rise would be expected between pre and post boil
Title: Re: SG OG BG FG,... why is my post boil reading lower than pre boil?
Post by: svejk on May 07, 2013, 10:57:32 PM
If I had to guess, I would also say it has something to do with the accuracy of the calculation used for adjusting for the temperature of the sample.  One trick I use is to put a large, shallow pyrex cake pan in the fridge on the morning of brew day.  When I put a hydrometer sample of wort into the pan, even at boiling temps, the thermal mass and surface area of the pan drops the temp of the sample very quickly into a reasonable range for checking the gravity.  I do that a lot less now that I have a refractometer, though...
Title: Re: SG OG BG FG,... why is my post boil reading lower than pre boil?
Post by: boapiu on May 07, 2013, 10:58:31 PM
Truly, I have never calibrated my hydrometer. But I do have a spare, next brew I will use them both and compare. I am thinking margin of error, and dont worry it too much. Thanks.
Title: Re: SG OG BG FG,... why is my post boil reading lower than pre boil?
Post by: boapiu on May 07, 2013, 11:01:44 PM
If measurements over 100f are worthless, how do you get a valid gravity pre boil?
Title: Re: SG OG BG FG,... why is my post boil reading lower than pre boil?
Post by: HoosierBrew on May 07, 2013, 11:16:10 PM
If measurements over 100f are worthless, how do you get a valid gravity pre boil?
I take a sample when my wort just comes to a boil, beacause you get better mixing of the wort that way IME, therefore more accurate readings.  But cool the sample to ~ room temp first. I put the sample on a saucer, into the freezer for several seconds. Refractometers (ATC or no) are supposedly not as accurate over 100df. Also , did you boil down to your final volume pretty closely?  That for sure would throw off your post boil OG.
Title: Re: SG OG BG FG,... why is my post boil reading lower than pre boil?
Post by: Jimmy K on May 07, 2013, 11:32:39 PM
I'm not certain, but I bet the relationship between density and temperature is not linear. So the corrections used near 60F would not be valid at higher temperatures.  It shouldn't take too long to cool a sample down either.
Title: Re: SG OG BG FG,... why is my post boil reading lower than pre boil?
Post by: HoosierBrew on May 07, 2013, 11:40:26 PM
FWIW, my samples are probably closer to 80df.  I normally aim for <100df.
Title: Re: SG OG BG FG,... why is my post boil reading lower than pre boil?
Post by: goschman on May 07, 2013, 11:59:35 PM
When taking preboil gravity, I put my sample in the freezer until it is below 80F
Title: SG OG BG FG,... why is my post boil reading lower than pre boil?
Post by: duboman on May 08, 2013, 12:00:03 AM

If measurements over 100f are worthless, how do you get a valid gravity pre boil?

You cool the sample down and then read it, it will take less than an hour to chill a sample and if you want to add DME you have plenty of time

Truly, I have never calibrated my hydrometer. But I do have a spare, next brew I will use them both and compare. I am thinking margin of error, and dont worry it too much. Thanks.

Take 60F distilled water and it should read 1.000, if not then add or subtract the difference for every reading you take with it to calibrate

All brewing tools should be calibrated to ensure your readings are correct, otherwise it's a crap shoot
Title: Re: SG OG BG FG,... why is my post boil reading lower than pre boil?
Post by: boapiu on May 08, 2013, 12:22:36 AM
I will try some calibration stuff via cooling etc,... And definitely check my readings with my spare hydrometer. Otherwise, the product is good, so okay. Thanks for the good ideas. Chilling the sample, I was relying on conversion software. Darn fine beer, either way.
Title: Re: SG OG BG FG,... why is my post boil reading lower than pre boil?
Post by: morticaixavier on May 08, 2013, 03:07:26 PM
I take about 8 oz of the wort from the kettle just after it comes to a boil and put it In a sealed mason jar in the freezer. Sealed because the evaporation from an open container can be significant. Sean Terrill mentioned something like 4 gravity points. so the sample on the saucer is likely higher than the bulk of the pre-boil wort.
Title: Re: SG OG BG FG,... why is my post boil reading lower than pre boil?
Post by: denny on May 08, 2013, 03:25:23 PM
If measurements over 100f are worthless, how do you get a valid gravity pre boil?

You cool it down.  I'm not near a fridge when I brew, so I take a sample and put it in a metal cocktail shaker.  I swirl that in a bowl of ice water for maybe a minute and it's down to the mid 60s and I can get a reading.
Title: Re: SG OG BG FG,... why is my post boil reading lower than pre boil?
Post by: weithman5 on May 08, 2013, 03:54:53 PM
doesn't the cocktail ruin the beer 8)
Title: Re: SG OG BG FG,... why is my post boil reading lower than pre boil?
Post by: rjharper on May 08, 2013, 04:00:55 PM
If measurements over 100f are worthless, how do you get a valid gravity pre boil?

I use my ATC Refractometer
Title: Re: SG OG BG FG,... why is my post boil reading lower than pre boil?
Post by: Wort-H.O.G. on May 08, 2013, 05:15:13 PM
If measurements over 100f are worthless, how do you get a valid gravity pre boil?

I use my ATC Refractometer

+1  quick and easy
Title: Re: SG OG BG FG,... why is my post boil reading lower than pre boil?
Post by: klickitat jim on May 09, 2013, 01:13:19 AM
Everything is brix until I get it I to the fermentor then hydrometer
Title: Re: SG OG BG FG,... why is my post boil reading lower than pre boil?
Post by: hubie on May 09, 2013, 01:51:36 PM
I'm not certain, but I bet the relationship between density and temperature is not linear.

It isn't.  I've coded up my own hydrometer correction and I've done a fit to the water density vs temperature data out of the CRC Handbook.  I'd put up a plot if I could figure out how to post it, but by eye it looks linear out around 100C, but it rolls off as you drop in temp.
Title: Re: SG OG BG FG,... why is my post boil reading lower than pre boil?
Post by: joe_feist on May 09, 2013, 04:41:16 PM
I do remember reaing (just don't have the sourse) that hydrometer temperature corrections are not linear and the further you get from the calibration temp, the more error is introuced.

+1 to several comments to cool the sample. It's quick, cheap, easy and more accurate. Never thought of the cocktail shaker, but I like it. I'll typically set my pyrex measuring cup in a pan with cold water/ice and swirl it around. That 3/4 cup or so of wort cools pretty fast.