Are you measuring with a Hydrometer or a Refrac? I have noticed that until dissolved oxygen gasses off of my sample my Hydrometer will read low-> possibly an issue with different DO levels on the top and bottom of you FV. Trub could potentially throw off a hydrometer reading too.
I usually take my sample off the zwickel just before the KO is finished. My theory is that the wort is still mixing at that point and should be a homogenous mixture. I have noticed that the gravity changes (+ .2-.3°P) from the beginning of the KO to the end because the hot wort is still gassing off water vapor-> that could account for a bit of a lower reading on top; if the wort is not really homogenous and in fact "layers" in the FV.
Have you tried retesting you samples 30 min, 60 min, 90 min after KO allowing some time for mixture? Or testing straight from the kettle or transfer line during KO?
Last edited by Jephro; 07-01-2010 at 12:26 PM.
Jeff Byrne (aka jephro)
11 year pro craft brewer available for hire...
Puyallup, Wa - for now