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Missing pre-boil gravity, hitting post-boil gravity

denimglen

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Hey all,

Just trying to figure something out here.

For my past 10ish batches I've noticed that my preboil gravity is significantly higher than what Beersmith calculates, yet at the end of the boil I'm getting the correct gravity.

Here's an example batch and what I've entered into Beersmith:

Batch Size - 20L

Boil Volume - 29.5L

Boil Time - 65 mins

Brewhouse efficiency - 66%

Lauter tun deadspace - 2.0L

Evap rate - 19% (Boil off 5.8%)

Lost to boil trub/chiller - 1.5L

Cooling loss - 4% (0.83L)

Preboil gravity - 1.031

OG est - 1.042

___________


So I mash, get 29.0L into the kettle, at 1.037.  Boil is done and dusted, transfer 20.0L to fermenter at an OG of 1.042.


At the end of the day it's not a big deal because I'm almost always +-2 gravity points of the estimate for the usual gravity beers I brew.  But I'd like to brew some bigger beers and have a better estimate of my gravities.  Interesting thing is that when I first started brewing all grain and using Beersmith the pre boil gravities were fine, but after a couple years of equipment changes and adjustments in BS the measurements are way out.


Any ideas how I can fix this up?


Cheers :-D
 
It sounds like equipment settings to me. Duplicate an equipment setup (Not the current because you may take the problem with you). Take all the measurements again and then compare. Basically you are double checking your equipment numbers.

Here is a refresher on equipment setup
http://www.beersmith.com/equipment_setup.htm

Cheers
Preston
 
Thanks mate but I've recently changed kettle and mash tun and recalibrated and re-entered for that and seen no difference.

I had a look over that guide you posted to double check and everything seems ok.

I don't think it would be the volume measurements on my HLT or Kettle because the volume measurements as I'm getting what beersmith tells me there.

I wouldn't think it's my refractometer as I've checked it multiple times against my hydrometer and it seems fine.  I'll double check it tonight anyway.

I'm quite positive I'm getting the correct volume into the fermenter, I measured my Better-Bottle out 2L at a time and the 20L mark coincides with their diagram on their website.

It could be the 'loss to trub' value, as I discovered after this batch that it was higher (closer to 3.2L rather than 1.5L like originally though), but I'd have to increase this to like 6.0L to change the gravity on the sheet to get what I actually got on the brew day.

It's gotta be something simple I'm missing, I'm sure if it was as complicated as it seems I would have figured it out by now haha.

 
Can you post your recipe.bsm file for us to look at? Maybe something will jump out and we can nail it down for you.  ;D

Cheers
Preston
 
denimglen said:
Evap rate - 19% (Boil off 5.8%)
Preboil gravity - 1.031
OG est - 1.042

Interesting thing is that when I first started brewing all grain and using Beersmith the pre boil gravities were fine, but after a couple years of equipment changes and adjustments in BS the measurements are way out.

That evap rate is quite high.  My hunch, based on these comments, is that with equipment changes and more practice, your extraction EE% has gotten better, so pre-boils are rising, and your evap rate is really not 19% and the lower actual evap rate is getting it from the pre-boil to the final OG fairly closely. 

Next brew, measure your volumes closely, use a slightly higher EE%, and a lower evap rate to see if that tightens it a bit.  If so, continue tweaking until nailed down. 
 
Thanks MaltLicker

I changed kettle last brew for like the 4th time (I love equipment!) so I had pretty good notes of pre boil, post boil, evap rate etc, will re-enter my equipment and give it another shot this time.

Like noted in the post above my evap rate was actually lower but my loss to trub was higher, switching the values to the correct ones doesn't seem to make a difference though.

I'm recalibrating my refractometer in a minute, I'll check over all my old recipes to see if there's any sort of consistency then I'll give it another brew and see what happens.  Damn guess I'll have to brew again this weekend :p

I'll come back and post up my recipes if I don't get it sorted.  Thanks for the help guys.
 
The info. and calculations below come from the AWESOME book "Designing Great Beers" by Ray Daniels.


Either your preboil or post boil readings (volumes or hydrometer readings) are off and this is the only possibility, so you just need to figure which it is.  Here is why:

Your pre boil total gravity units in the kettle will always equal your post boil total gravity units in the kettle.

If you measure the specific gravity correctly (by stirring the wort first and cooling the wort sample down to get an accurate reading) you will know before you even start your boil exactly what your finishing gravity will be.

This is because total gravity (beg. of boil) = Total gravity (end of boil).

For example:
You have 8 gallons of runoff from the mash. The specific gravity of this wort is 1.038. You plan to boil long enough to yield 5.5 gallons of finished beer. Because the total gravity will not change during the boil, you can calculate the finishing gravity of the boil:

(38 GU x 8 gallons) divided by 5.5 gallons = 55.3 GU.

This calculation tells you that this wort will have a gravity of 1.055 when boiled down to 5.5 gallons.

If you look at the equation above, the GU (gravity units) are the #'s after the "0" when reading your gravity. 38 is 1.038 and 55.3 would be 1.0553 (rounded down to 1.055). Do the math from the equation and you will see that 38 x 8 = 304 total gravity units and 55.3 x 5.5 = 304 gravity units.










denimglen said:
Hey all,

Just trying to figure something out here.

For my past 10ish batches I've noticed that my preboil gravity is significantly higher than what Beersmith calculates, yet at the end of the boil I'm getting the correct gravity.

Here's an example batch and what I've entered into Beersmith:

Batch Size - 20L

Boil Volume - 29.5L

Boil Time - 65 mins

Brewhouse efficiency - 66%

Lauter tun deadspace - 2.0L

Evap rate - 19% (Boil off 5.8%)

Lost to boil trub/chiller - 1.5L

Cooling loss - 4% (0.83L)

Preboil gravity - 1.031

OG est - 1.042

___________


So I mash, get 29.0L into the kettle, at 1.037.  Boil is done and dusted, transfer 20.0L to fermenter at an OG of 1.042.


At the end of the day it's not a big deal because I'm almost always +-2 gravity points of the estimate for the usual gravity beers I brew.  But I'd like to brew some bigger beers and have a better estimate of my gravities.  Interesting thing is that when I first started brewing all grain and using Beersmith the pre boil gravities were fine, but after a couple years of equipment changes and adjustments in BS the measurements are way out.


Any ideas how I can fix this up?


Cheers :-D
 
Thanks.

I know the whole buzz about the TGU, which is why I was confused that the pre and post boil gravities don't match up.

I just wanted to also rule out software error - which is why I posted up here.

But it looks like it's operator error.

I thought I was stirring the wort enough before taking my pre-boil gravity.  I stirred the nuts off it and even made sure to move my spoon up and down through the wort to stop any layering - that coupled with filling from the bottom of the pot through my valve I thought I was doing enough to avoid any layering.

I've only done one batch since I started this topic but it looks like it confirmed that stirring doesn't do the trick.  I took one gravity reading before the boil after stirring - it was short.  Took another one after the boil had been going for about 2 minutes and it was correct.

I'm mashing another batch now so hopefully I'll be able to confirm.

Thanks for the help everyone.
 
I'm glad you figured out when your problem was.

I can verify that the Beersmith Software is "dead on."

The batch I brewed on Saturday was 13.60 gallons @ 1.063 SG preboil.  Post boil was 11.28 gallons @ 1.076.  The #'s are corrected for temp.  I always put the hydrometer sample in the fridge and take the reading when it's cooled to 60 degrees.

The math from above to verify Beersmith computes it correctly:
13.6 x 63 = 856.8
11.25 x 76 = 855  (if the hyrdro reading was 76.1 the #'s would match perfectly, but who can read a hydro. that accurately?).

I'm pretty new to AG brewing, so I hadn't "brushed up" on this info. prior to brew day.  My SG was supposed to be 1.086.  Knowing what I know now, I could have just boiled it down to 9.96 gallons and hit my SG spot on.  Of course this book also tells me how much Dry Malt Extract I could add to still hit my final desired batch size of 11.50 gallons and my desired SG of 1.086.  The math is so simple.  I can't wait to brew again and pull this off.  Of course I know have my efficiency and other Beersmith #'s down, so it shouldn't be a problem. 
Anyway, I think everyone should read the chapter "Hitting your specific Gravity" in that book (Designing Great Beers by Ray Daniels). 
 
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