Weird Gravity Measurements

[dachbach]

I need some help diagnosing a strange gravity measurement.

My pre-boil gravity reading today was 1.052 for 7 gallons. My post boil, right-before-pitching-yeast gravity was 1.050 for 5.5 gallons. I know This can't possibly be right because gravity readings should go up as you boil off water.

Does anyone have ideas on what's going on here? My post boil gravity today was exactly what Beersmith said it was going to be so my pre-boil reading must have been off. Could the much hotter wort temperature be messing with my refractometer? I thought those things were not affected by temps... I checked both measurements multiple times to make sure.

I'm pretty sure this has happened to me before but I don't have the best notes on some of my previous batches.

 

[Facinerous]

Pretty sure refractometers are affected by temp. Though I don't know by how much. It also depends on when you took that reading. If it was your first runnings, and not the collected/mixed pre-boil volume then that would easily throw off your reading.

 

[dachbach]

The pre-boil reading was after first runnings and sparging was completed and mixed.

My first runnings out of the mash was 1.078

 

[pdxal]

Many refractometers have automatic temperature correction. Plus, a drop of fluid cools very quickly.
My guess would be that the pre boil sample wasn't mixed well, and you measured a layer of higher gravity stratified wort.
Regardless, be happy your post boil measurement was spot on, because that is what matters.