Boil off volume and SG change not making sense

str1p3s

I was wondering if anyone else has dealt with this or has any ideas what happened.
My friend brewed yesterday, collected 8 gallons of wort with a pre boil gravity of 1.050, taken with a refractometer. He boiled for an hour down to roughly 6 gallons. His expected OG was 1.067 but it was 1.054 measured with a hydrometer. Temperature was close to 60F. Every calculator I found has said that concentrating 1.050 wort from 8 gallons to 6 should result in 1.067.
We checked his refractometer and hydrometer with distilled water to make sure they were accurate. We also hooked the chiller back up to the hose to make sure it wasn't leaking into the wort.
We're pretty baffled. Concentrating wort shouldn't be effected by some sort of process error, right? It's a simple formula. We had to have measured wrong either pre boil or post but I can't figure out how. Any ideas?

DBhomebrew

Stratification. Was the full wort mixed up before pulling a sample?

day_trippr

Ayup. That's the typical root cause. There was another thread clearly sporting the same syndrome here last week...
Cheers!

str1p3s

He said he gave it a good stir first, but I think that is where I'm leaning as the cause also.

seilenos

Were the "8" and "6" gallons from a scale on the boil kettle or were they estimated from starting water volume and fermenter volume or somewhere else?

bracconiere

if it was a PRE-boil gravity of 1.050, then boiling should have pretty thoroughly mixed it? i know i beat this trick over the head as of late, but double check when it's done fermenting. Compare a refrac reading & hydro reading....or just a refrac, assuming a FG of ~1.010, a refrac would read around 8 BRIX if it was 1.067 OG, and 6.5 if actually 1.054...
and you can get 'closer' if you drop a hydro in it too...(i love that trick! only reason i have a refractometer!  

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DBhomebrew

True. Perhaps the pre-boil wort wasn't thoroughly mixed. Perhaps the post-boil was taken after the wort was left to chill and settle.
+1 on questioning volumetric accuracy

str1p3s

The volumes were based on markings on the boil kettle and on the fermenter. I was referring to the pre boil gravity that was stirred first. I assume the post boil is accurate because it was thoroughly mixed by the boil and we whirlpool the wort with a pump during chilling, mixing it further.

DBhomebrew

Those might be worth confirmation.