Pre-Boil Gravity Calculation

[paint_it_black]

Does anyone know a way of determining what my pre-boil gravity should be?
For instance, I know that my OG (after the boil and cooling) is supposed to be 1.059, and the batch size is 5 gallons. During my sparge I monitor the gravity so I can be sure to not go below 1.008. Once I hit 1.008 I stop the sparge. I'd like to be able to take a gravity reading of the body of sweet wort at that time to know whether I'll hit my OG after the boil. This will come in especially handy if I have to stop teh sparge, but have not reached my target pre-boil volume -- I may want to top it off with water prior to the boil. But in order to do that, I should know what gravity I should be bringing it down to. (This also segues into another question: Is there a way of figuring out what my gravity will be after adding x amount of water? (5 gallons of wort at x1 gravity + 1 gallon of water = 6 gallons of wort at x2 gravity).

Did any of that make sense? I'm in the process of brewing right now, so I haven't had tie to compose my thoughts properly. Apologies. If you understand what I'm getting at, please let me know your opinions.

 

[shortyjacobs]

It's simple math. Remember this formula. V1*C1 = V2*C2. V1 and V2 are your starting and ending volumes. C1 and C2 are your starting and ending sugar concentration in points, (1.050 = 50, 1.083 = 83, etc.).

Say preboil you get 1.050. You have 7 gallons. You want to boil down to 5. In this case, V1 = 7, V2 = 5, C1 = 50. So you get 7*50 = 5*C2. This leads to 7/5*50 = C2 = 70, meaning your OG will be 1.070.

For your other question, adding water. Say you have 4 gallons of wort at 1.063. You want to know what your gravity will be after adding, say, 1.25 gallons of water. Your final volume after the add will be 4+1.25 = 5.25. So V1 = 4, V2 = 5.25, C1 = 63. You get 4*63=5.25*C2, so C2 = 48, so your SG will be 1.048.

You can use this the other way. Say you have 4.5 gallons of 1.059 wort, and you want 1.050 wort. V1 = 4.5, C1 = 59, C2 = 50. You get 4.5*59 = V2*50. V2 = 5.31 gallons. Since you have 4.5 gallons now, you need 5.31 - 4.5 = 0.81 gallons of water extra.

 

[ajf]

It's simple math. Remember this formula. V1*C1 = V2*C2. V1 and V2 are your starting and ending volumes. C1 and C2 are your starting and ending sugar concentration in points, (1.050 = 50, 1.083 = 83, etc.).

Say preboil you get 1.050. You have 7 gallons. You want to boil down to 5. In this case, V1 = 7, V2 = 5, C1 = 50. So you get 7*50 = 5*C2. This leads to 7/5*50 = C2 = 70, meaning your OG will be 1.070.

For your other question, adding water. Say you have 4 gallons of wort at 1.063. You want to know what your gravity will be after adding, say, 1.25 gallons of water. Your final volume after the add will be 4+1.25 = 5.25. So V1 = 4, V2 = 5.25, C1 = 63. You get 4*63=5.25*C2, so C2 = 48, so your SG will be 1.048.

You can use this the other way. Say you have 4.5 gallons of 1.059 wort, and you want 1.050 wort. V1 = 4.5, C1 = 59, C2 = 50. You get 4.5*59 = V2*50. V2 = 5.31 gallons. Since you have 4.5 gallons now, you need 5.31 - 4.5 = 0.81 gallons of water needed

Nicely explained.

-a.

 

[paint_it_black]

It's simple math. Remember this formula. V1*C1 = V2*C2. V1 and V2 are your starting and ending volumes. C1 and C2 are your starting and ending sugar concentration in points, (1.050 = 50, 1.083 = 83, etc.).

Say preboil you get 1.050. You have 7 gallons. You want to boil down to 5. In this case, V1 = 7, V2 = 5, C1 = 50. So you get 7*50 = 5*C2. This leads to 7/5*50 = C2 = 70, meaning your OG will be 1.070.

For your other question, adding water. Say you have 4 gallons of wort at 1.063. You want to know what your gravity will be after adding, say, 1.25 gallons of water. Your final volume after the add will be 4+1.25 = 5.25. So V1 = 4, V2 = 5.25, C1 = 63. You get 4*63=5.25*C2, so C2 = 48, so your SG will be 1.048.

You can use this the other way. Say you have 4.5 gallons of 1.059 wort, and you want 1.050 wort. V1 = 4.5, C1 = 59, C2 = 50. You get 4.5*59 = V2*50. V2 = 5.31 gallons. Since you have 4.5 gallons now, you need 5.31 - 4.5 = 0.81 gallons of water needed

You. Are awesome.

Thanks!!

 

[SDBrewer]

Say preboil you get 1.050. You have 7 gallons. You want to boil down to 5. In this case, V1 = 7, V2 = 5, C1 = 50. So you get 7*50 = 5*C2. This leads to 7/5*50 = C2 = 70, meaning your OG will be 1.070.

Looks like I'm re-opening this thread after quite a while, but I have a similar question, that I can't seem to find the answer to anywhere online. I'm hoping someone here can help me out.

I began taking pre-boil gravity readings to make my brewing as consistent as possible. I've taken readings on my last (4) AG batches. I've come up with some odd inconsistencies. I'll preface with the following: I am using the formula quoted above for determining my target pre-boil gravity; brewing 6 gallon batches, assuming a 20% boil off rate. I'm also adding excess sparge runnings during the boil because my kettle isn't large enough to hold all the mash and sparge runnings, so I have to add it in as water boils off . I'm boiling the excess sparge runnings (about 1 1/2 gallons) on another burner to keep the boil off rates/condensing of the wort the same.

1. IIPA: target pre-boil G: 1.071, target OG: 1.089; actual pre-boil G: 1.079, actual OG: 1.09
2. Imperial Oatmeal Stout: target pre-boil G: 1.053, target OG: 1.078; actual pre-boil G: 1.070, actual OG: 1.076
3. IPA: target pre-boil G: 1.052, target OG: 1.068; actual pre-boil G: 1.050, actual OG: 1.052
4. IIPA: target pre-boil G: 1.070, target OG: 1.093; actual pre-boil G: 1.072, actual OG: 1.081

When I take the pre-boil G, I am mixing both the mash and sparge together 1:1 or even 1:2 to err on the light side (sparge). As you can see, I'm coming very close to my target pre-boil G or even way over in the case of the Stout, then barely gaining any points, and falling short of my target OG.

What are your thoughts?

 

[Yooper]

When you take the preboil gravity, are you chilling the sample down to under 100 degrees before temperature correcting the reading?

 

[SDBrewer]

When you take the preboil gravity, are you chilling the sample down to under 100 degrees before temperature correcting the reading?

Yes - I get it down between 60 and 80

 

[paint_it_black]

Looks like I'm re-opening this thread after quite a while, but I have a similar question, that I can't seem to find the answer to anywhere online. I'm hoping someone here can help me out.

I began taking pre-boil gravity readings to make my brewing as consistent as possible. I've taken readings on my last (4) AG batches. I've come up with some odd inconsistencies. I'll preface with the following: I am using the formula quoted above for determining my target pre-boil gravity; brewing 6 gallon batches, assuming a 20% boil off rate. I'm also adding excess sparge runnings during the boil because my kettle isn't large enough to hold all the mash and sparge runnings, so I have to add it in as water boils off . I'm boiling the excess sparge runnings (about 1 1/2 gallons) on another burner to keep the boil off rates/condensing of the wort the same.

1. IIPA: target pre-boil G: 1.071, target OG: 1.089; actual pre-boil G: 1.079, actual OG: 1.09
2. Imperial Oatmeal Stout: target pre-boil G: 1.053, target OG: 1.078; actual pre-boil G: 1.070, actual OG: 1.076
3. IPA: target pre-boil G: 1.052, target OG: 1.068; actual pre-boil G: 1.050, actual OG: 1.052
4. IIPA: target pre-boil G: 1.070, target OG: 1.093; actual pre-boil G: 1.072, actual OG: 1.081

When I take the pre-boil G, I am mixing both the mash and sparge together 1:1 or even 1:2 to err on the light side (sparge). As you can see, I'm coming very close to my target pre-boil G or even way over in the case of the Stout, then barely gaining any points, and falling short of my target OG.

What are your thoughts?

I haven't gone back and read all of the thread and studied the suggested formula for calculation, but it would appear that either something isn't being accounted for/something's being messed up by your multiple-pot brewing method, or the formula itself isn't right.

As you can you see from your results, when your actual FG matches your target FG, your calculated pre-boil gravity is wrong, and similarly, when your calculated pre-boile gravity matches the actual pre-boil gravity, you don't meet your target FG.

Specific gravity is simply a measurement of how much sugar is in the water/wort. Whenwe boil the only thing that's removed is more water, so theoretically, the calculation should be something like

x (pre-boil gravity) / [pre-boil volume] = target FG / [final volume]


And then cross-multiply to solve for x. I'm pretty sure that's another way of saying the previously mentioned formula.


In looking at your data, it looks like when your target FG and actual FG allign, your target pre-boil gravity is *lower* than it should be -- this means your calculation tells you you need more water than you actually do. So I can only guess that yoru current boiling method is not boiling off as much water as you expect it to. ...Maybe? I don't really know what I'm talking about here, just taking a stab at it.

I'd say that you're consistently hitting your target FGs when you don't try to make your pre-boil gravity right, so maybe trying to think too in-depth is actually harming you? Maybe just stick with what you're doing and don't worry about calculating the pre-boil gravity?

 

[erikpete18]

When I take the pre-boil G, I am mixing both the mash and sparge together 1:1 or even 1:2 to err on the light side (sparge).

So you're taking a sample of your first runnings (+ a little sparge) and a sample of your leftover sparge and mixing them together to get a reading? If that's the case, its likely that you just can't hit the right ratio of one to the other to get it right. What you may want to do is calculate out gravity points (gravity x volume) for both the main pot and your left over sparge. That way you can calculate out the total gravity points you have, then you can divide by either your pre-boil volume or post-boil volume to get your numbers.

Otherwise your first two sets of final gravity numbers look pretty good. Your numbers for batch 3 can't be right with your pre-boil and post-boil unless you hardly boiled off any volume, in which case it might just be that you had a lower efficiency that batch and weren't able to get an accurate pre-boil measurement.

Also, is the 20% boil-off rate what you've measured, or is that an estimation? I never liked percentages, because boil off rate is going to be dependent on temperature and surface area, not volume. If you had a tall, skinny pot and boiled 2 gal or 10 gal of water, you could get 1 gal of boil off in both cases, but the percentages would be completely different. Seeing as how you are also boiling in a second pot, I could see how it would be tough to estimate without running through a few batches and figuring out your pre-boil and post-boil volumes and inputting those numbers.

 

[SDBrewer]

So you're taking a sample of your first runnings (+ a little sparge) and a sample of your leftover sparge and mixing them together to get a reading?

That is correct.

If that's the case, its likely that you just can't hit the right ratio of one to the other to get it right. What you may want to do is calculate out gravity points (gravity x volume) for both the main pot and your left over sparge. That way you can calculate out the total gravity points you have, then you can divide by either your pre-boil volume or post-boil volume to get your numbers.

You're probably right that I'm not hitting the ratios, but that's why I've tried to go heavy on the sparge side of the ratio to give me a lower pre-boil reading. I'll try to take separate readings and combine the numbers using gravity points next time. The difficulty would be separating out the two runnings into different pots to take the gravity readings, then combining them without risking hot side aeration. I guess I risk it a little by adding in about 1.5 gallons during the boil. I'll give it some thought.

Otherwise your first two sets of final gravity numbers look pretty good. Your numbers for batch 3 can't be right with your pre-boil and post-boil unless you hardly boiled off any volume, in which case it might just be that you had a lower efficiency that batch and weren't able to get an accurate pre-boil measurement.

You're very astute - my efficiency dropped way off on batch #3. I was hitting 71 to 73% efficiency with a fine grind, but when I ground batch #3 grains at the LHBS, they had adjusted their grind to a courser grind, and I was in a rush, so I couldn't adjust the grinder. My efficiency dropped off to 55% for that batch. I got a real fine grind on batch #4, but my efficiency only improved to 63%. (I thought I was on target though, because my pre-boil reading looked good).

Also, is the 20% boil-off rate what you've measured, or is that an estimation? I never liked percentages, because boil off rate is going to be dependent on temperature and surface area, not volume. If you had a tall, skinny pot and boiled 2 gal or 10 gal of water, you could get 1 gal of boil off in both cases, but the percentages would be completely different. Seeing as how you are also boiling in a second pot, I could see how it would be tough to estimate without running through a few batches and figuring out your pre-boil and post-boil volumes and inputting those numbers.

It is an estimate, though a fairly accurate one. I don't have markings on my kettle, but I know what level on the kettle equal 6 gallons, so I get real close to my targeted pre-boil volumes. I know how much I have post-boil since my carboy is marked and I calculate it based off that. So I might be off by a couple percentage points, but that probably wouldn't make a big difference.

 

[SDBrewer]

In looking at your data, it looks like when your target FG and actual FG allign, your target pre-boil gravity is *lower* than it should be -- this means your calculation tells you you need more water than you actually do. So I can only guess that yoru current boiling method is not boiling off as much water as you expect it to. ...Maybe? I don't really know what I'm talking about here, just taking a stab at it.

My boil off rate seems to be fairly accurate (see description given in response to erikpete18). It might be off slightly, but I doubt that's enough to make much of a difference.

I'd say that you're consistently hitting your target FGs when you don't try to make your pre-boil gravity right, so maybe trying to think too in-depth is actually harming you? Maybe just stick with what you're doing and don't worry about calculating the pre-boil gravity?

Only batch #1 & #2 really hit the numbers, batches #3 & 4 have been off on the final OG by 16 and 12 points respectively. I wasn't really concerned on batches #1 & 2, when my pre-boil readings didn't directly translate into my OG calculations since I hit the OG's.
On batch #3, I thought it was odd, but partially chalked it up to my efficiency dropping because of a course grind. I decided to really look into it once Batch #4 came out low on the OG after partially correcting the efficiency problem with a finer grind.

 

[erikpete18]

The difficulty would be separating out the two runnings into different pots to take the gravity readings, then combining them without risking hot side aeration.

I was just thinking of taking measurements with the two pots you've already got going on. No need to worry about getting a true "first running" reading, just need to figure out the gravities of your BK and your secondary pot since it sounds like you've got a pretty good handle on volumes. If you really want to get into determining volumes I've seen people mark a spoon/dowel that they can put into their pot rather than marking up their kettle, just a thought.

Just thought of something too now that you mentioned HSA. How well are you mixing your wort in your BK after you do your runnings? If you are trying to avoid HSA and not stirring well before taking your pre-boil reading, the heavier runnings might be sitting on the bottom of the kettle and the lighter on top, so depending on where you drew a sample from you might not get a well-mixed reading.

Otherwise it sounds like you've got a good reason for why batch #3 didn't work (damn LHBS mills ). I'm assuming batch 4 you went back to a tighter crush, but it might be some other little thing that made your efficiency worse than expected. Could be a lot of things that would be pretty easy to come across, especially on beers that big.

 

[SDBrewer]

How well are you mixing your wort in your BK after you do your runnings? If you are trying to avoid HSA and not stirring well before taking your pre-boil reading, the heavier runnings might be sitting on the bottom of the kettle and the lighter on top, so depending on where you drew a sample from you might not get a well-mixed reading.

I don't think I'm stirring very much if any before I take the pre-boil reading. I'll make sure I do that.

Otherwise it sounds like you've got a good reason for why batch #3 didn't work (damn LHBS mills ). I'm assuming batch 4 you went back to a tighter crush, but it might be some other little thing that made your efficiency worse than expected. Could be a lot of things that would be pretty easy to come across, especially on beers that big.

Yep - I had probably my finest crush of all the AG batches, but my efficiency was still low. I'm thinking it was probably the design of my MT. The drain hole is a couple inches off the bottom, so I usually tip the cooler back before I start mashing out/sparging, but I forgot on Batch #4, so I probably lost a lot of efficiency because of the mash/sparge sitting on the bottom of the cooler - that actually could have been the problem for Batch #4 more than just some quirk in the pre-boil reading. I'll make sure to tip it back before I mash out/sparge this time! Perhaps if I can get the ratios of mash/sparge runnings in the pre-boil sample and get my efficiency back to 73% or so, my problems will be solved! I'm planning on brewing an Imperial Oatmeal Stout again this weekend, so I'll let you know what happens.

 

[erikpete18]

Good luck with the next batch!

 

[SDBrewer]

I don't think I'm stirring very much if any before I take the pre-boil reading. I'll make sure I do that.
I'm planning on brewing an Imperial Oatmeal Stout again this weekend, so I'll let you know what happens.

I brewed up the Oatmeal Stout this last weekend (regular OS, not Imperial) with much better success as far as hitting my numbers. My efficiency got back to where it should be (77%), so I was actually above my target gravities, but the numbers made sense this time.
Target Pre-boil G: 1.041; Actual G:1.050
Target OG: 1.053; Actual OG: 1.060
I think technically, with a pre-boil gravity of 1.050, my OG should have been 1.063, but 1.060 is pretty darn close, so I'm happy with that.

I had about 6 gallons of mash/sparge runnings in my main kettle and about 1 3/4 gallons in my second kettle. I mixed the runnings in the main kettle real well before I took a sample. I mixed it at a ratio of 2:1 (1/2 cup of main kettle mash/sparge runnings and 1/4 cup of the sparge runnings from the second kettle).

I'm still not entirely sure why I had my original problems, but if I keep doing this method every time, I should be able to hit my numbers. Thanks for all your help!

On a side note, I was able to revamp my chilling methodology a bit and go from cooling 6 gallons of wort in 40 minutes down to about 5 minutes. Here is my blog post about it: http://chismbrewing.blogspot.com/2011/04/super-efficient-wort-chilling-method.html