Author Topic: Gravity almost the same after Boil?  (Read 874 times)

Offline Alex Spain

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Gravity almost the same after Boil?
« on: September 20, 2021, 08:22:17 am »
Hi folks,

Brewed my 4th ever batch on Saturday and got some concerns. I was brewing Juicy NEIPA recipe from Clawhammer supply blog.
It was a hazy NEIpa. Grain bill was 70% Pilsner malt and 30% of flaked malts (oak, barley, wheat).

My pre-boil gravity was around 1.058 which is fine (glass hydrometer used). Brewfather app predicted pre-boil gravity of 1.057 and FG to be 1.072 after the boil.

I was boiling for 75 minutes and was quite surprised to see final gravity at only 1.060

Not really sure if it's possible to boil wrong? I mean boil is boil, how can you do it wrong way? (I boiled with the lid off, of course)

Float digital hydrometer showed 1.051 in the fermenter later on. I think I'd need to re-calibrate it, cuz I assume a glass hydrometer would be more precise.

Any advice appreciated :)

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Re: Gravity almost the same after Boil?
« Reply #1 on: September 20, 2021, 08:26:33 am »
Nope, not possible. Must be a measurement error somewhere
Life begins at 60.....1.060, that is!

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Offline Alex Spain

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Re: Gravity almost the same after Boil?
« Reply #2 on: September 20, 2021, 08:58:41 am »
Thanks Denny!

I agree there must be a measurement error, but how could glass hydrometer show wrong numbers? I was cooling down the wort to 19-20C before measuring.

And my Float is really letting me down. I was expecting Float to be precise, but it is showing even lower numbers than my glass hydrometer.

Online BrewBama

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Gravity almost the same after Boil?
« Reply #3 on: September 20, 2021, 09:01:06 am »
I agree that there has to be a measurement error. You are removing water by boiling therefore concentrating the wort. Gravity has to increase.

To predict post boil gravity, multiply pre boil gravity point by pre boil volume and then divide by post boil volume. For example 1.040 at 7.5 gal and you boil off 1 gal: (40 * 7.5) / 6.5 = 1.046

To test your hydrometer, use distilled water at the calibration temp for the hydrometer.  It should read zero. If it doesn’t you found your culprit. If it does check your volume.

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« Last Edit: September 20, 2021, 09:26:35 am by BrewBama »

Offline RC

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Re: Gravity almost the same after Boil?
« Reply #4 on: September 20, 2021, 09:27:37 am »
I lean toward equipment error but could also be that your hydrometer sample wasn't mixed well enough. Wort stratifies by density pretty rapidly. It should be well mixed immediately before taking the sample in order to homogenize the density. If you shut off the burner and let the wort sit a few minutes, and then took the sample without mixing, you'd be taking from hotter, lower-density wort at the top of the kettle.

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Re: Gravity almost the same after Boil?
« Reply #5 on: September 20, 2021, 09:29:07 am »
I lean toward equipment error but could also be that your hydrometer sample wasn't mixed well enough. Wort stratifies by density pretty rapidly. It should be well mixed immediately before taking the sample in order to homogenize the density. If you shut off the burner and let the wort sit a few minutes, and then took the sample without mixing, you'd be taking from hotter, lower-density wort at the top of the kettle.

I've heard that for 25 years, but I've never seen it happen...even when I tried to make it happen to test the theory.  Have you personally experienced it?
Life begins at 60.....1.060, that is!

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Offline RC

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Re: Gravity almost the same after Boil?
« Reply #6 on: September 20, 2021, 09:48:38 am »
I lean toward equipment error but could also be that your hydrometer sample wasn't mixed well enough. Wort stratifies by density pretty rapidly. It should be well mixed immediately before taking the sample in order to homogenize the density. If you shut off the burner and let the wort sit a few minutes, and then took the sample without mixing, you'd be taking from hotter, lower-density wort at the top of the kettle.

I've heard that for 25 years, but I've never seen it happen...even when I tried to make it happen to test the theory.  Have you personally experienced it?

It most definitely happened to me once when I was brewing commercially. Well-mixing the wort made a 7 or 8 pt gravity difference, it was eye-opening. But that was a 40-bbl kettle. I haven't experienced this with a 10-gal one, and admittedly it doesn't seem very plausible that on such a small scale it would cause that large a discrepancy. Which is why I lean toward equipment/measurement error.

Offline Wilbur

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Re: Gravity almost the same after Boil?
« Reply #7 on: September 20, 2021, 10:07:12 am »
Do you happen to know the volumes? There might be a measurement error, but maybe your boil off rate is not correct in brew father. If Brewfather has a boiloff rate of 1 gallon/hr and your actual boil off rate is only 1/2 gallon/hr then post boil gravity is definitely not going to match your calculated.

Offline RC

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Re: Gravity almost the same after Boil?
« Reply #8 on: September 20, 2021, 11:26:16 am »
I lean toward equipment error but could also be that your hydrometer sample wasn't mixed well enough. Wort stratifies by density pretty rapidly. It should be well mixed immediately before taking the sample in order to homogenize the density. If you shut off the burner and let the wort sit a few minutes, and then took the sample without mixing, you'd be taking from hotter, lower-density wort at the top of the kettle.

I've heard that for 25 years, but I've never seen it happen...even when I tried to make it happen to test the theory.  Have you personally experienced it?

I happen to be brewing at the moment and tested this. 10 gal kettle, using a refractometer. Collected all the wort, kettle full vol = 8.75 gal. Let it sit a few minutes, then took a sample from the very top. Grav = 1.035. Then mixed well, immediately took another sample, grav = 1.037. Repeated three times for each. So there's a difference, albeit it's very small, not enough to matter to me. The delta probably would have increased if I let the wort sit longer, so I'm curious about that, but had to get on with the brew day...

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Re: Gravity almost the same after Boil?
« Reply #9 on: September 20, 2021, 11:47:53 am »
I lean toward equipment error but could also be that your hydrometer sample wasn't mixed well enough. Wort stratifies by density pretty rapidly. It should be well mixed immediately before taking the sample in order to homogenize the density. If you shut off the burner and let the wort sit a few minutes, and then took the sample without mixing, you'd be taking from hotter, lower-density wort at the top of the kettle.

I've heard that for 25 years, but I've never seen it happen...even when I tried to make it happen to test the theory.  Have you personally experienced it?


I happen to be brewing at the moment and tested this. 10 gal kettle, using a refractometer. Collected all the wort, kettle full vol = 8.75 gal. Let it sit a few minutes, then took a sample from the very top. Grav = 1.035. Then mixed well, immediately took another sample, grav = 1.037. Repeated three times for each. So there's a difference, albeit it's very small, not enough to matter to me. The delta probably would have increased if I let the wort sit longer, so I'm curious about that, but had to get on with the brew day...

To me that's within the margin of error for measurement
Life begins at 60.....1.060, that is!

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Offline Drewch

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Re: Gravity almost the same after Boil?
« Reply #10 on: September 21, 2021, 02:53:26 pm »
Nope, not possible. Must be a measurement error somewhere

If you had anything close to a boil for 75 minutes, there should have been significant concentration of the wort.  That's just physics ... you're removing water from the solution while the other stuff stays in the kettle.  Even if the boil off estimate was wrong, it should be more than 2 points different.  One measurement or the other has to be off. 

Did you cool both samples before measuring SG (or otherwise correct for temperature)?  Hydrometers are calibrated at specific temperature usually near room temperature ish.
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Offline Alex Spain

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Re: Gravity almost the same after Boil?
« Reply #11 on: September 21, 2021, 04:04:24 pm »
Did you cool both samples before measuring SG (or otherwise correct for temperature)?  Hydrometers are calibrated at specific temperature usually near room temperature ish.
Yes, I did cool it to 20C first time and to 19C the second time. The wort had a lot of sediment though, probably due to 30% of flaked malts. It was clearly visible that the sediment was getting down to the bottom and the upper part was clear wort.

I bought some distilled water today to test my glass hydrometer as per BrewBama's advice. It is concerning me though that both glass hydrometer and Float were showing strange numbers. I'll try to re-calibrate my Float as well, once I get it out of the fermenter.

Float is showing 1.007 gravity at the moment, but the foam in the fermenter is around 1.18 inches thick still and CO2 is there like crazy, hence I doubt that fermentation has finished.

I sent an email to Float support a month ago about strange gravity numbers and they replied that foam can be an issue though.

Just curious now what was the original gravity of my batch actually :) I reckon I'll have 6% ABV for sure

Offline Drewch

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Re: Gravity almost the same after Boil?
« Reply #12 on: September 21, 2021, 10:29:11 pm »
Just curious now what was the original gravity of my batch actually :) I reckon I'll have 6% ABV for sure

If you have a refractometer, you can estimate OG based on FG and °Br.   Or at least, so says the internet: http://www.woodlandbrew.com/2013/02/abv-without-og.html
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