[HoosierBrew]

If I took a reading sample right at flameout, no. But after cooling started,  readings usually seemed more erratic. I attributed it to wort stratification. Nowadays I just take a preboil refractometer reading (actually a couple minutes into the boil to get good mixing) and a hydrometer FG reading, and call it good. It's by no means the only way to get there. Just my routine.

Since I've heard this for years, I've tested it several times and never seen it happen.  Readings immediately post boil from various depths were the same as reading 15 min. and 1/2 hour later.  Now, that's iust me, which is why I'm looking for evidence that might contradict what I've seen.

And I didn't do any proper experiments, Denny. Just mostly observations. Like for example, taking a flameout reading (hydrometer sometimes, refractometer sometimes) that lined up with where I should be with my preboil reading, and then took another after cooling that was several points off (with accurate volumes). So honestly I just switched to the preboil and FG thing, and it's habit/routine now.

Edit - And of couse evaporation after cooling would change the reading a little. But I'm talking 10 or 12 points difference sometimes.

[a10t2]

I'm confused.  The gentleman I quoted a10t2, said it could have affected the measurement.  You yourself said the problem was maybe the mixing.  Doesn't getting a correct reading from a refractometer depend on a homogeneous sample?

Two different things being discussed in this thread:

• Refractometer vs. hydrometer: For hot-side readings, the small volume of a refractometer sample makes it easier to obtain readings quickly. The flip side is that the small sample is more susceptible to evaporation effects, so it should be drawn using a pipette/syringe/etc.
• Full boil vs. partial boil: After mixing wort and water, there's no practical way to obtain a homogenous sample. Regardless of what you're using the take the readings, the way to get an accurate OG is to measure the wort gravity and then calculate the effect of dilution.

I use a hydrometer not a refractometer. I am guessing everybody that was referring to mixing was talking about samples for a refractometer and not a hydrometer sample which is much larger and presumably of much more uniform gravity.  I did not initially understand.

No, trying to homogenize wort and water is independent of measurement technique. It would take, quite literally, several hours of stirring to obtain a homogenous sample with either instrument.