Author Topic: Can't hit correct OG  (Read 5060 times)

Browny

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Can't hit correct OG
« on: May 21, 2006, 01:16:53 AM »
Hi Folks,
The board seems a little quiet so I better give everyone something to think about. I am a new AG Brewer and I've done 14 AG Brews to date. The first 1/2 dozen or so brews I worked out my system for lauter tun dead space, boil off, loss to trub etc and now have my volumes spot on. I know I need 30L + 1kg per kilo of grain to end up with 21L in the fermenter after a 60 min boil. 32L if it is 90min.
Now I come to preboil gravity and OG. In nearly all cases I hit my preboil gravity or exceed it but I never make my OG after boiling. You are probably thinking that I'm not getting the boil off I think I am, but if that were the case my volumes would be out and they are not. I'll give you an example of the latest brew I put down, a Sierra Navada Porter clone. The preboil gravity by Brewsmith is 1.040 and after a 90 min boil is 1.061. I achieved 1.046 on my preboil gravity and only 1.052 for the post boil. I am using a refractometer and allowing ample time for the wort to cool to 20C. Is there a logical explanation to this ?

cheers

Browny

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Re: Can't hit correct OG
« Reply #1 on: May 21, 2006, 09:50:43 AM »
Browny,
  There is a difference in the way the pre-boil and post-boil gravities are calculated.  The final gravity relies primarily on your grain bill, volume and overall brewhouse efficiency.  If you are consistently coming in low, then your overall brewhouse efficiency might be set too high.  Try reducing it a few points and see if you get a bit closer.

  There is no shame in losing a few points on your brewhouse efficiency - it will improve as you get more experience under your belt.  At worst, a lower efficiency might drive you to add a few more cents in grain to your mash.

Cheers!
Brad
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Browny

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Re: Can't hit correct OG
« Reply #2 on: May 22, 2006, 01:48:53 AM »
So are you saying there is no relationship between the preboil SG and the boil time ? I would have thought these two factors would be the most important followed by volume. I don't see how adding more grain will help, the disparity will still be there.

cheers

Browny

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Re: Can't hit correct OG
« Reply #3 on: May 22, 2006, 07:20:15 PM »
Browny,
  Unfortunately the way the brewhouse efficiency number is defined - it takes into account ALL losses in the system including boil losses, trub losses, etc...  When I created BeerSmith I used this definition (consistent with other programs and common brewing definitions) to determine the OG estimate.  Had I not done it this way we would have been using it as the mash efficiency and not the overall brewhouse efficiency.

  Therefore boill time does not factor into the estimated OG of the overall brew.  I know this seems a little odd, but if you accept the fact that brewhouse efficiency accounts for all losses (including the boil), it does make some sense.

Cheers!
Brad
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Browny

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Re: Can't hit correct OG
« Reply #4 on: May 24, 2006, 01:33:50 AM »
My brain hurts  :(

cheers

Browny