Boil Off Question

[JDL]

Not sure if this belongs here exactly.

I am trying to dial in my system and have slowly gotten piece by piece dialed in and my boil off rate seems to be one of the last issues.

When dealing with wort, is there a formula that says:

If you start with 6 gallons at 1.051 and boil off one gallon it should equal 1.0XX?

My boil off is staying the same but the the changed in gravity is not. My last batch boiled off a gallon and it went from 1.051 to 1.057 but it seems to change each time.

I suspect the culprit is actually myself attempting to measure the volumes.

I measure the preboil volume at 150-170 degrees and then measure the post boil volume at 68 degrees. Hoping there is a system to even all this out and get better idea of what the final gravity will be.

Thanks in advance,
Jeff

 

[homebrewer_99]

There is probably some scientific-types out there who have a better answer concerning evaporation rates for specific pot diameters, etc., but IMHO, this is where taking good notes comes in handy.

Unfortunately, those notes only help you dial in your second and subsequent batches.

I take 1st and 2nd run temp/gravity readings from the mash tun as it goes into the boil pot. I also take PRE-BOIL gravity reading from the pot.

If the OG/FGs are met (match recipe) then I try to match subsequent batches to my results.

If the boil off is greater (resulting in a higher OG) then you can add water to the primary to bring it down to the recipe's OG.

If the boil off is less and the gravity is too low then you need to boil it down more OR let it sit and hope enough water evaporates off.

I'm sure others have their own comments and techniques to add.

 

[ajdelange]

When dealing with wort, is there a formula that says:

If you start with 6 gallons at 1.051 and boil off one gallon it should equal 1.0XX?

Yes, of course. Conservation of mass tells you what will happen. Assuming the 1.051 SG is at room temperature you are starting with 6*3.785*1.051*0.998203 = 23.8253 kg of wort which is 12.6249 Plato and thus contains 6*3.785*1.051*0.998203*Plato(1.051)/100 = 3.00792 kg of extract. You removed 1 gal of water which is 3.785 kg so you are left with 23.8253 - 3.785 = 20.0403 kg wort still containing 3.00792 kg extract so this new wort is now 100* 3.00792/20.0403 = 15.0094 °P and has SG = 1.06115.

Clearly the key to this is knowing how to convert SG to Plato and back. The ASBC formula

Plato = -616.868 + 1111.14*SG -630.272*SG*SG + 135.997*SG*SG*SG

does the first. Going back, that is finding the SG from the new Plato number, is a bit tougher. Use Excel's Solver or a root finder or get an approximation from the Lincoln equation.

I measure the preboil volume at 150-170 degrees and then measure the post boil volume at 68 degrees. Hoping there is a system to even all this out and get better idea of what the final gravity will be.

All volumes must be measured at (or at least corrected to) 20 °C. The ASBC polynomial wants 20/20 SG values.

 

[mirthfuldragon]

https://www.brewersfriend.com/dilution-and-boiloff-gravity-calculator/
Also important: https://www.brewersfriend.com/hydrometer-temp/

My notes reflect measured SG and temperature (and then corrected SG) during the process, and I take gravity readings of first runnings, sparge, pre-boil, pre-chill, and then finally from the fermentor itself after transfer.

 

[ajdelange]

Need to be a little thoughtful in using those calculators. The temptation in this particular problem is to say that OP started with 6 gal and boiled off 1 gal so he now has 5 gal and put that into the calculator. If you do that it returns 1.061 as the estimated new SG when the answer is actually 1.060. This is a small error, of course, but the reason for it is that if one starts with 6 gal and boils off a gallon of water the new volume is not 5 gal but 5.058 gal. Put this number into the calculator and it gives the right answer so the math behind it is correct.

An obvious question is as to why anyone would care about such nits. This is the Brew Science forum and while I think answers like "there's this calculator at..." are clearly valuable to many if not most there may still be at least a couple of readers here who are actually interested in how the problem is solved.

 

[JDL]

Thanks for the info.

Any compensation needed in measurements of volume readings based on the increase in temperature of runoff water compared to the volume at room temp?

Since it is fixed I am guessing that my volume reading is wrong? I cool all gravity samples to 70 degrees or so before testing so that should be stable.

 

[kh54s10]

I am sure the math will give you answers. I take the more simple route, at least now that I know my boil off rate. My system boils of just about 2 gallons per hour. So I have to collect a little over 7.25 gallons to end up with just a little over 5 gallons going into the fermenter. It varies a little with heat and humidity but not enough for me to worry about. I get very close to the volume I want and within a point or two (usually) of the predicted OG.

For me I am not that concerned with exacts I just want good beer and I have been successful.

 

[beermanpete]

How are you measuring volume? And when?
The boil-off rate can be determined using only water if you want to go through the exercise.
What type of heat source do you use and how precisely can you control it?

 

[homebrewer_99]

My system boils of just about 2 gallons per hour. So I have to collect a little over 7.25 gallons to end up with just a little over 5 gallons going into the fermenter. It varies a little with heat and humidity but not enough for me to worry about. I get very close to the volume I want and within a point or two (usually) of the predicted OG.

That's a lot. Is your boil pot a tub?

My boil off about 1/2 gal per 1/2 hour. Once it gets to a rolling boil you can turn down the heat? It doesn't have to be rip roaring rolling strong the entire time.

 

[kh54s10]

That's a lot. Is your boil pot a tub?

My boil off about 1/2 gal per 1/2 hour. Once it gets to a rolling boil you can turn down the heat? It doesn't have to be rip roaring rolling strong the entire time.

10 gallon pot. It's pretty wide so there is a fair amount of evaporation surface. I have a Bayou Classic SP10 burner and it is difficult to get it to run steady at a low boil so it is a little on the vigorous side, but not exceedingly so.

 

[corax]

You removed 1 gal of water which is 3.785 kg so you are left with 24.052 - 3.785 = 20.267 kg wort still containing 3.00792 kg extract

I was mostly following along until this. Where does 24.052 come from? I assume it's the mass of the pre-boil wort, but how is it calculated?

 

[homebrewer_99]

10 gallon pot. It's pretty wide so there is a fair amount of evaporation surface. I have a Bayou Classic SP10 burner and it is difficult to get it to run steady at a low boil so it is a little on the vigorous side, but not exceedingly so.

I use a Bayou Classic also. My pot is about 9 gallons and heat the wort up on high, but when it starts rolling boil I cut back on the gas a bit just to keep it rolling.

 

[kh54s10]

I use a Bayou Classic also. My pot is about 9 gallons and heat the wort up on high, but when it starts rolling boil I cut back on the gas a bit just to keep it rolling.

I cut it back as low as it will stay going and still get 2 gallons per hour boil off. I want to go electric in the future so am reluctant to buy a Blichmann burner. But maybe I should.

 

[homebrewer_99]

I cut it back as low as it will stay going and still get 2 gallons per hour boil off. I want to go electric in the future so am reluctant to buy a Blichmann burner. But maybe I should.

If I'm reading you right...are you having problems keeping it lit? Is it the wind?

After I purchased the Bayou I bought a section of that shiny silver corrugated stove pipe and made a wind shield. I cut it down so it would slide in under the grate section and surround the burner section. Basically creating a chimney.

I also saved the large cardboard box my Weber Smoky Mountain (22.5") smoker came in and I place it around the cooker and pot as a wind blocker (adding a small cut out for the gas line). I've been using it for years without any problems. It's lightweight, stores flat and easily replacable.

If that's your issue then I'd hold off on buying a new piece of equipment you're not sure about.

 

[ajdelange]

I was mostly following along until this. Where does 24.052 come from? I assume it's the mass of the pre-boil wort, but how is it calculated?

No, I'm afraid it is what I calculated when I first wrote the post and fat-fingered 1.061 instead of 1.051 for the OG. When I saw the mistake and edited I missed this part. The post has been edited again and, I hope, is now error free. I apologize for the confusion.

 

[corax]

No, I'm afraid it is what I calculated when I first wrote the post and fat-fingered 1.061 instead of 1.051 for the OG. When I saw the mistake and edited I missed this part. The post has been edited again and, I hope, is now error free. I apologize for the confusion.

Thanks! Good information, I'm going to update my mashing spreadsheet.

 

[kh54s10]

I cut it back as low as it will stay going and still get 2 gallons per hour boil off. I want to go electric in the future so am reluctant to buy a Blichmann burner. But maybe I should.

If I'm reading you right...are you having problems keeping it lit? Is it the wind?

No, it is just that the adjustment is not very good at low levels. I might be able to get it a bit lower but I have a spot that works.

 

[homebrewer_99]

No, it is just that the adjustment is not very good at low levels. I might be able to get it a bit lower but I have a spot that works.

If it works...

 

[doug293cz]

The simple equation relating pre- and post-boil volumes and SG's is this:

Pre-Boil Volume * (Pre-Boil SG - 1) = Post Boil Volume * (Post-Boil SG - 1)
​

This is not quite as accurate as the full mass balance method given by @ajdelange above, but your volume and SG measurement errors are going to be larger than the error from using the above simplified equation.

Brew on