When and how to take Post-mash and Pre-boil Gravities

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csantoni

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I'm curious what the methods are for taking a Post-mash and/or Pre-boil gravity reading? I fly sparge so the runnings at the top of the kettle are lower gravity than the first runnings at the bottom.

I usually stir it all up as it's heating and then pull a sample but I'm wondering if there's another way to get a reading that's homogenized.
 
Do you use a hydrometer? Then not really another way to get an accurate one.
I do like to take readings of first and second runnings, but then, I batch sparge, so one sample does it for each reading.
 
Yeah, I use a hydrometer. I also use a refractometer but I haven't entirely dialed in the correction on it so I always take a hydrometer reading for the important steps.
 
Post mash gravity must be taken before any sparge water is added. It you do a mash-out, then the measurement should be taken after mash-out. All the wort must be homogenized prior to taking the SG sample. If you do BIAB, then aggressive stirring is all you need. If you have a false bottom, then you need to stir, then vorlauf well, and stir some more. If you have a malt pipe, then you need to lift, drain, and resubmerge the pipe multiple times to mix in the wort below and between the pipe and vessel wall.

Pre-boil should be measured after all sparging is complete, and the wort has been aggressively stirred to homogenize all the different runnings. If you are doing LoDO, and don't want to aggressively stir, then let the wort boil for a couple of minutes before taking the sample, or just use the post-boil sample and correct for boil off volume.

Brew on :mug:
 
Post mash gravity must be taken before any sparge water is added. It you do a mash-out, then the measurement should be taken after mash-out. All the wort must be homogenized prior to taking the SG sample. If you do BIAB, then aggressive stirring is all you need. If you have a false bottom, then you need to stir, then vorlauf well, and stir some more. If you have a malt pipe, then you need to lift, drain, and resubmerge the pipe multiple times to mix in the wort below and between the pipe and vessel wall.

Pre-boil should be measured after all sparging is complete, and the wort has been aggressively stirred to homogenize all the different runnings. If you are doing LoDO, and don't want to aggressively stir, then let the wort boil for a couple of minutes before taking the sample, or just use the post-boil sample and correct for boil off volume.

Brew on :mug:

That's not the way I understand it or have measured it. Post mash gravity is the gravity of all the wort after mashing is complete. That includes after any sparge is complete.

Pre boil gravity would be the same as post mash gravity unless you are adding top up water or additional fermentables.
 
That's not the way I understand it or have measured it. Post mash gravity is the gravity of all the wort after mashing is complete. That includes after any sparge is complete.

Pre boil gravity would be the same as post mash gravity unless you are adding top up water or additional fermentables.
You can't measure conversion efficiency (separately from mash efficiency) unless you measure the SG before any sparge water is added. Once you do any run off and add sparge water things get confounded with lauter efficiency. If you want to diagnose where your low efficiency is originating, you need to separately measure conversion efficiency, and then calculate lauter efficiency as:

Lauter Efficiency = Mash Efficiency / Conversion Efficiency​

Brew on :mug:
 
I think it's possible that the beersmith field for post-mash only accounts for top-up water to the kettle and not the way @doug293cz is describing it. His explanation makes more sense because all my recipes show the predicted values for post-mash and pre-boil as the same in beersmith and I don't top-up the kettle.
 
I think it's possible that the beersmith field for post-mash only accounts for top-up water to the kettle and not the way @doug293cz is describing it. His explanation makes more sense because all my recipes show the predicted values for post-mash and pre-boil as the same in beersmith and I don't top-up the kettle.
My problem with a lot of brewing software, including BeerSmith, is they don't use predictive models for mash efficiency based on grain bill size vs. pre-boil volume. The methods for doing so are well known among beer math nerds, so they are easy to implement. In BeerSmith, you tell it what you think your efficiency is going to be.

The only efficiency that you can't predict a priori is conversion efficiency. But, it is easy to measure if you take the needed measurements, and then it shouldn't vary with different recipes or grain bill sizes (if you have good process consistency.) Mash efficiency, and therefore brewhouse efficiency, do vary with varying grain bill sizes.

Brew on :mug:
 
Mash efficiency, and therefore brewhouse efficiency, do vary with varying grain bill sizes.
This explains a lot and if I had really thought about the calculations before using the software it would have been obvious. Time to start doing some supplemental math in addition to the tool.
 
Learn something new every day - still after all these years. I think?
I haven't found how "post mash" and "pre-boil" gravities matter in Beersmith, and thus have always treated them the same. On brew day I target the pre-boil value, however, because if I hit that number everything else all the way to the tap is pretty much pre-ordained as long as I don't do anything out of scope (<= read "stupid" :D).

Anyway, when the fly sparge is done I set the bk to recirculate at full speed while I'm cleaning/drying out the hlt and then the mlt. After a few minutes I'll draw a pre-boil gravity & pH sample, and by the time I've cleaned the two kettles it's cooled down enough to take a reading. I plug that same value into both Beersmith "post mash" and "pre-boil" entries...

Cheers!
 
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