Can refractometers be damaged if they get too hot? I accidentally left mine in the sun for ~30 minutes and now I'm wondering if it's the source of my problems. It's one of the those cheap ones you can get on eBay shipped directly from China. I brought it inside to let it cool back down to room temperature, and then re-calibrated it to room temperature... but then my problems began.
Back story: I've used a refractometer for my last 5-6 all-grain BIAB batches and always check my pre-boil OG with it. My mash efficiency was consistently in the 75% range until my second-to-last batch, which all of sudden registered around 55% based on my refractometer OG reading! I added a bunch of DME to compensate and wondered if I got a bad crush at my LHBS...
My last batch I had the same problem, super low pre-boil OG. Again I added a bunch of DME. I had the grains crushed at a different shop, so I'm now looking at the refractometer as the source of my efficiency woes.
In this latest batch, a bit of strangeness is that my OG post-boil actually decreased (based on my refractometer) relative to pre-boil. This was based on a sample of leftover wort in the bottom of the kettle. Is it possible that my refractometer is at issue, or am I doing something completely wrong?