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My family was very nice to me this Christmas and I got a bayou classic burner and an 32qt. pot with 2 ports and a bazooka screen as well as an immersion chiller. I had built a 10 gallon mash tun over the summer (I was doing partial mashes for the last couple batches) so now I am ready for all grain full boil 5 gallon batches. Only thing is I have never done this before (can't wait though) so I don't know the efficiency of the system. I picked my first recipe, my fiance loves the rogue hazelnut nectar so I am going to attempt to duplicate that as best I can.

My question is this, when do you measure the OG to determine if you have missed your mark? I purchased my ingredients along with 1 lb. of Light DME. I only need about 1 cup of that for my starter so I figure if I miss my OG I could use the rest of that to hopefully get to where I need to be. So, when would I measure the gravity of the wort to determine if I need to add the DME? Do I check it immediately after the I have sparged (pre-boil) or do I check after the full boil and hop additions? If the later is the case obviously I would need to boil a small 'wort' for the dme so how would I calculate how much I would need to use?

BTW: here is the recipe I think I am going to be using (any suggestions are also welcome): http://brewershub.com/recipes/rogue-hazelnut-brown-nectar-clone

Totally psyched for all grain brewing (or 95% grain brewing if I need this DME after missing my mark)!

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Measuring the OG post boil isn't the right place because if its off, how do you fix it? You'll have to calculate how much you are off and what amount of DME to add, all the while the wort is sitting hot, but cooling. The hop profile will be changing slightly as it sits hot. If you add the DME at this point you'd have to stir it to get the DME well mixed, this encourages some hot oxidation potentially. And you may be tempted to boil some more because of sanitiation concerns with your spoon and the unboiled DME going in.

The way to adjust for a missed gravity is to do it after all the wort is collected and you are waiting for things to come to a boil. This is where knowing your collected volume to within a quart or so is important. Take a hydrometer reading and figure out the total gravity points you have collected. (For example you collect 6.5 gallons of 1.050 wort that's 325 gravity points, 6.5*50) IF your recipe was formulated for a 5.5 gallon final kettle volume at 1.063 you needed 350 points (5.5*63) Subtracting 325 from 350 means you came up 25 points short. DME is ~45 ppg so 25/45=is ~0.55lbs. You would add the 0.55lbs of DME to the kettle and you should be in good shape towards hitting the original target of 1.063.

On a side not hitting that target OG is also dependent on dialing in your boil efficiency. In my example I am assuming a one gallon boil off. But that might not be your evap rate due to elevation, relative humidity and pot geometry...blah blah blah. I suppose that adding DME post boil is a work around not knowing your boil off rate, but I think the other issues doing it post boil are more nerve racking to me.

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So it sounds like the first few times I use my new setup I'll be brewing in the dark kind of as I don't know the boil off rate and I'm not positive what the efficiency of the tun truly is. I've only used the tun 1 time for a partial mash. Also, it sounds like I should go figure out a way to know exactly how much water I have in my kettle at all times, otherwise I will have no idea how to tell what my boil off rate is. Any suggestions there? I was thinking about getting a wooden dowel (to dip) and marking it in 1/2 gallon increments by pouring water into the kettle and marking it, thoughts? –  tomcocca Dec 30 '11 at 13:30
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You need to measure your gravity at least twice -- once at the start of the boil, and again at the end.

Measuring your pre-boil gravity will let you know if you've extracted too little (or too much) sugar from your mash. Then you can adjust with DME or water to get your pre-boil gravity right.

Adjusting your gravity at the start of the boil is nice because then your hop extraction should be the same. At least one IBU formula uses the gravity of the wort as a factor in determining hop extraction (although some people now think that wort gravity is not a very big factor in hop extraction rates).

Measuring your post-boil gravity will give you the wort's OG, so that's obviously useful.

To figure out what your pre-boil gravity should be, use this formula:

Expected OG * (1 - percent of wort lost to evaporation) = pre boil gravity.

For example, on my burner I evaporate about 10% per hour. So if I boil for 1 hour, I expect to lose 10% of my wort to evaporation. If I'm making a beer with an expected OG of 1.060 then:

60 * (1 - .10) = 54

So my pre-boil gravity should be 1.054.

If I boil for 90 minutes, my evaporation rate will be 15%, so:

60 * (1 - .15) = 51

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The starting OG is what goes into the fermentor - what the yeast get their hands on. It's simplest to take a hydrometer/refractometer sample after the boil and the wort chilled, either from the kettle or directly from the fermentor before pitching the yeast.

If you discover you missed your target OG, then pull a slightly larger sample of wort (say a pint), and use that to boil up the DME needed to reach target OG. Add this back to the kettle or fermentor, and stir with a stanitized utensil and take a gravity measurement again. The stirring is necessary to ensure that the newly introduced sugars from the DME are distributed uniformly throughout the wort - without stirring, the boiled DME will collect at the bottom.

You could use water rather than wort to boil the DME, but then you are diluting the wort slightly, so you have to add more DME than just what you need to hit target when boiling it in wort.

As always when working with post-boil wort (taking samples and stirring), use sanitary procedures.

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