Hitting Pre-boil gravity, missing Post-boil gravity

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jbags5

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Hello everyone:D

I've brewed about a dozen or so batches now. Gone from extract to BIAB to my current AG batch sparge setup in the past several months.

Looking back over my Beersmith recipe numbers for my beers, most of the time I'm missing my "post-boil" OG by a fair bit, but I seem to hit my preboil gravity targets (according to beersmith) almost every time. Usually the post-boil gravity number isn't more than a few points higher than the pre-boil readings.

Examples - the Saison I brewed last night - the estimated preboil gravity was 1.053 - I hit 1.050. The target OG was 1.066 - I hit 1.054.
The wheat beer I brewed a few months ago - preboil target was 1.042 - I hit 1.044; target OG was 1.052 - I hit 1.046.


Looking back over almost all my recipes, this seems to be the same story for most of my batches. However, sometimes I hit both numbers perfectly (this is the exception, not the rule), which confuses me further, since I haven't changed anything related to my boiling/cooling technique since day 1.

I can get into my brewing technique specifics if needed, but my question is what in general would cause this to happen? Is it something with my boil? I think I'm taking my hydrometer measurements properly (after letting my samples cool to room temp), but maybe I'm not?

My beer ends up tasting fine (great, IMO:mug:), so it's not a huge concern, but I'm curious as to the number discrepancy.

Thanks.
 
There is only one factor in the difference between pre-boil and post-boil gravity; boil-off. If you're not hitting your predicted post-boil volume you have to adjust the rate in your software.
 
^ exactly this I use beersmith 1 and 2 and you need to calculate your boil off here is the formula you want to use based upon your kettles diameter

Estimating Boil-Off from the Kettle Diameter
In metric, the estimate is...

Litres per hour boiled off = pi*(diameter of kettle in cms/2)*(diameter of kettle in cms/2)*0.00428.

So, for the 40 cm pot we used in our sample profile above, the calculation was...

Litres per hour boiled off = 3.14159*40/2*40/2*0.00428 = 5.38 L/Hr



If you need to you can use inches instead of metric.
 
You say you're cooling the samples to room temp, are you measuring it with a thermometer? 5-10 degrees could be the difference.

Other than that, I agree with the above, you're just dialing in your boiloff rate. Are you accurately measuring the volume before/after the boil? That's very important for these measurements. No offense to Aschecte, but I think it's pretty tough to accurately predict what your boiloff rate is going to be. Kettle diameter/geometry is only a small part of it, environmental conditions (humidity) come into play a little bit but your burner's heat output is the most significant IME. If you're using a kitchen stove you might see 0.5 gallons per hour, if you're outside on a monster propane burner you might see 2-3 times that.

As long as you know your volumes before and after, you can adjust your recipes accordingly.
 
"I can get into my brewing technique specifics if needed, but my question is what in general would cause this to happen?"

Bad measurements or improper calculations. Points sugar * volume before boil = points sugar * volume after boil. Sugar doesn't evaporate.

However, there is an error with every measurement. eg the scale on a hydrometer is only 1 point or about 2% error rate. The measurement on your volumes is a usually in 1/4 gallon increments at best. You put those together and being "off" by 5 or 6% is within the margin or error of your testing equipment.
 
You say you're cooling the samples to room temp, are you measuring it with a thermometer? 5-10 degrees could be the difference.

Other than that, I agree with the above, you're just dialing in your boiloff rate. Are you accurately measuring the volume before/after the boil? That's very important for these measurements. No offense to Aschecte, but I think it's pretty tough to accurately predict what your boiloff rate is going to be. Kettle diameter/geometry is only a small part of it, environmental conditions (humidity) come into play a little bit but your burner's heat output is the most significant IME. If you're using a kitchen stove you might see 0.5 gallons per hour, if you're outside on a monster propane burner you might see 2-3 times that.

As long as you know your volumes before and after, you can adjust your recipes accordingly.

No offense taken :mug: you are 100% correct different variables other besides diameter of the pot do come into play. I do this for a living and the formula I provided will get you pretty darn close. You need to remeber regardless of anything besides atomospheric pressure and elevation water boils at a constant appx. 212 degrees regardless of humidity. Boiling water obviously boils at 212 but flashes to steam and changes states at 213 degrees and cares less about any other variables ( for the most part ). Why the diameter of the pot becomes relevant is because of surface area for examle my pot is 40.67" diameter and boils off 1.47 gph. A kettle half the diameter would boil off appx 1/2 of that amount due to the reduced surface area. The one other area to look at is your btu output on the burner as that can effect boil of if not enough btu's are there to bring your volume to a rolling boil.
 
I have this problem as well. I think I have my boil off rate set too high to compensate for what are really equipment losses (I do use a pump both during the mash and to whirlpool/recirc at the end of the boil). My volumes are coming out fine, but like you jbags, my pre and post/boil gravities aren't telling the same story.
 
Usually stuff like this is just margin of error given less-than-precise volume (and hydrometer) measurements. Having a good proxy for your boiloff rate really helps.
 
"I can get into my brewing technique specifics if needed, but my question is what in general would cause this to happen?"

Bad measurements or improper calculations. Points sugar * volume before boil = points sugar * volume after boil. Sugar doesn't evaporate.

However, there is an error with every measurement. eg the scale on a hydrometer is only 1 point or about 2% error rate. The measurement on your volumes is a usually in 1/4 gallon increments at best. You put those together and being "off" by 5 or 6% is within the margin or error of your testing equipment.

But more happens during the boil than just evaporation.
You will lose some gravity points to the hot break which precipitates at the start of the boil. If you chill the wort with a chiiler before taking the OG measurement, you will lose some more to the cold break. If you are like me, and measure the OG in the fermenter, you will lose more to hop absorption, equipment losses, and trub.

I looks like the OP is boiling off less than 0.5 gallons, which seems very low to me. I boil off about 1.4 gallons per hour. Does he have the correct boil off rate entered in Beersmith?

-a.
 
But more happens during the boil than just evaporation.
You will lose some gravity points to the hot break which precipitates at the start of the boil. If you chill the wort with a chiiler before taking the OG measurement, you will lose some more to the cold break. If you are like me, and measure the OG in the fermenter, you will lose more to hop absorption, equipment losses, and trub.

I looks like the OP is boiling off less than 0.5 gallons, which seems very low to me. I boil off about 1.4 gallons per hour. Does he have the correct boil off rate entered in Beersmith?

-a.

How do you lose gravity points to hot break? Does this mean that the more aggressive the hotbreak the more gravity points you'll lose? Because I do 5G batches (7G pre boil) in a 10G kettle and I let the hot break go as aggressively as it wants. On rare occasions it even boils over, but the 10G pot gives it plenty of headspace.

For those who assume this is measurement error, while that would usually be my assumption, measurement error doesn't explain the consistency of his results. I can relate to him. Like me, it happens every brew, and it always happens the same way: hit the numbers on the mash, miss the OG to the downside.
 
Nobody has asked so I will....

Are you hitting your volume into primary or are you more or less and topping off?

Topping off might be lowering your OG and being over means you are not boiling off enough.

I agree that .5 gallons is awfully low for boil off over an hour.
 
How do you lose gravity points to hot break? Does this mean that the more aggressive the hotbreak the more gravity points you'll lose? Because I do 5G batches (7G pre boil) in a 10G kettle and I let the hot break go as aggressively as it wants. On rare occasions it even boils over, but the 10G pot gives it plenty of headspace.

For those who assume this is measurement error, while that would usually be my assumption, measurement error doesn't explain the consistency of his results. I can relate to him. Like me, it happens every brew, and it always happens the same way: hit the numbers on the mash, miss the OG to the downside.

Prior to boiling, the proteins that cause the hot break are dissolved in the wort, and anything dissolved in the wort will affect the S.G.

During the boil, those proteins get precipitated (i.e. leave the solution.) Once they are precipitated, they will not affect the S.G. As they sink, they are obviously heavier than water, thus the S.G. must drop after they have precipitated, even if you take account of the evaporation that will occur while the break is being generated (which will cause the S.G. to increase).

I have no idea how many gravity points will be lost during the break, or even if it is measurable with typical home brewer equipment.

-a
 
2 row is only about 5% soluble protein and only a small fraction falls out in the hotbreak.

If you are always off a particular amount, that is probably an error in how you measure or calculate. eg not accounting for lost wort in any trub you leave behind.
 
Prior to boiling, the proteins that cause the hot break are dissolved in the wort, and anything dissolved in the wort will affect the S.G.

During the boil, those proteins get precipitated (i.e. leave the solution.) Once they are precipitated, they will not affect the S.G. As they sink, they are obviously heavier than water, thus the S.G. must drop after they have precipitated, even if you take account of the evaporation that will occur while the break is being generated (which will cause the S.G. to increase).

I have no idea how many gravity points will be lost during the break, or even if it is measurable with typical home brewer equipment.

-a

Well that's damn informative, thanks! I love your sig line btw.

I adjusted my boil off rate and loss rates on beersmith, I'll be interested to see if this helps solve my problem. I wonder if OP is even reading this thread anymore LOL!
 
Aschecte said:
^

Litres per hour boiled off = pi*(diameter of kettle in cms/2)*(diameter of kettle in cms/2)*0.00428.

Very informative, and thank you.

But that's the strangest way I've ever seen (pi)r^2
 
Very informative, and thank you.

But that's the strangest way I've ever seen (pi)r^2

No problem it's a really good ball park as to where you need to be for your boil off and should put you in a good place. Just remember that rate is determined by a rolling steady boil not "crab eyes" or a volcano so keep in mind not to under or over fire your burner as btu's out will alter the boil off.
 
Besides not boiling off as much as beersmith thinks you are. I found there is some stratification of gravity in the kettle before the boil. I usually let it boil for 10 minutes before taking a reading, or making sure it's well stirred.
 
For those who assume this is measurement error, while that would usually be my assumption, measurement error doesn't explain the consistency of his results. I can relate to him. Like me, it happens every brew, and it always happens the same way: hit the numbers on the mash, miss the OG to the downside.

Very good point. I agree. My "error" is very consistent. Seems more like systematic error, could be explained by ajf's hot break thing. Good stuff guys.
 
I can confirm this uses the same formula I provided as I've cross checked it for accuracy. Biab, extract, or all grain has no impact on boil off as long as you don't over fire the burner.
 
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