I have been home brewing for more than 2 years and I always found my refractometer to be my best friend. Last week I took a job as a headbrewer in the local brewery/restaurant. Here comes the problem: the brewery has a very little boil off. It does not affect the beer (no DMS, no hazy beer) but the refractometer always reads a bit less at the end of the boil than before it. It happens with every beer, for example: pre-boil gravity of our bohemian pilsner reads 1.048 but the post-boil reads like 1.046. I did not observe this at home because I have a huge boil off. Is it possible that the refractometer is affected by some hop oils? It is an ATC refractometer.