Low OG Reading and Evaporation During the Boil

I recently posted a short article on how to install a thermometer on the side of your mash tun.  I mentioned in that article that I had been struggling to hit my original gravity (OG) for my last three batches.  At first I considered it an aberration, however, after another two batches I started to become concerned.  My OG readings were consistently low by about 10 points (or 0.01).  This was concerning to me because I had a pretty successful track record of being within a few points (plus or minus) of my target OG.

Before the first batch with the low OG reading I had added a Chugger brew pump and a counterflow wort chiller.  These two items replaced my old immersion style wort chiller and racking cane for both cooling and transferring wort after the boil.  Both items worked fantastic and my brew chilled to groundwater temp quickly and efficiently.  Only problem was that my OG reading was low.

My gut told me that the pump and counterflow chiller addition was somehow related to the low OG reading.  At the time this made no sense since I couldn't figure out how the chiller could impact the OG.

I went about two more batches with the same results.  The beer was turning out fine all along so I wasn't hugely concerned, but I did want to know why I couldn't reach the expected OG readings with any consistency.

I turned to the internet and posted a thread on Homebrew Talk on the topic.  I got back some great, technical responses (thanks RM-MN and IslandLizard).  Their theory was that I wasn't getting a quality milling on the grain.  I always mill my grain at the homebrew shop and that hadn't changed in the last few batches so I doubted this could be the problem.

Finally, I was preparing to brew another batch and consulted the folks at MoreBeer!.  After discussing the problem and my brew process we concluded that I was collecting too much wort during the sparge.

When I had my immersion chiller I would let the sparge run dry and I would always end up with 7.5-8 gallons of wort to boil.  With the immersion chiller it used to take 30-45 minutes for my wort to cool to the point where it was no longer steaming, but with the new counterflow chiller that entire process happened in 5 minutes or less.  With the new chiller I wasn't allowing the wort to continue to evaporate during the cooling process.  In other words the same volume of wort cooled through the counterflow chiller would results in a lower OG!

I realized I needed to monitor my sparge more closely and only collect as much wort as I needed.  After doing some research online and reviewing beer apps like BeerSmith and iBrewMaster, I concluded that for a 5.50 gallon batch, 7 gallons of wort would be appropriate.  I've brewed two batches now with the idea of collecting 7 gallons of wort and my OG readings have been corrected.

These figures work well for my system, but may not work well for others.  I would suggest testing your system to figure out what will work well for you.  You can perform this testing using the following process:

1.  Place 7 gallons of water into your brew pot and bring to a boil.

2.  Boil for 1 hour.

3.  Cool water as you would normally cool your wort after boiling.

4.  Measure the amount of water remaining to determine how much boiled off.

5.  Adjust the initial volume of water (or wort, more importantly) you start with to reached your desired final volume.

Pretty simple process.  Good luck!