Pre- & Post-Boil Gravity Reading question

I'm about 5 batches into all grain brewing and still working out the kinks. I'm finally remembering to pull samples to measure gravity pre- and post-boil.

In my batch today, I pulled my pre-boil sample off the top of my kettle when I was done lautering. Cooled it down, measured and it was 10 pts lower than the meager effeciency I've been getting. Expected 1.050; got 1.040

So I said 'oh well' and continued my brew day. When I finished and pulled my post-boil sample from my cooled kettle, it measured 1.060 - right on where I was expecting the batch to be...however, this was of course with hops (4 oz in recipe) & break stuff in it.

I've searched through the topics here and found a reference to 'mixing well' before taking your pre-boil sample. I'm surprised that there could be a 10-pt difference from the top of the kettle to the bottom?

So, anyhow, just looking for confirmation on technique here. I'm assuming that had I mixed the pre-boil kettle before taking a sample, I would've gotten a higher reading, yeah? Also, is my post-boil sample giving me a true reading, considering the suspending trub?

Thanks in advance

mlutha said:

I'm assuming that had I mixed the pre-boil kettle before taking a sample, I would've gotten a higher reading, yeah? Also, is my post-boil sample giving me a true reading, considering the suspending trub?

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On the mixing - it's possible. You should definitely stir it up gently to get a homogenous wort before taking a sample. Also, make sure you are correcting for temp correctly based on you hydrometer's calibration temp.

As for the post-boil reading, I have been told that trub will not interfere with the reading because that material is not part of the solution, but simply suspended in it. I have not personally performed any tests to confirm this though. If you are concerned, let the carboy rest for an hour before taking the reading and pitching yeast.

I actually take my brix reading into the boiler after it's started to boil and mix itself a bit.

it'll be down to the mixing,try measuring the gravity of the first runnings next time you brew and measure aain towards the end of the sparge-there is a huge difference.all that heavier stuff naturally just wants to sit on the bottom

post-boil should always be higher because you'll have less volume. did you check what the volume was pre- and post-boil?