Yesterday I brewed a strong porter, and had problems with the refractometer: I took several samples during the boil, cooled them, and read them with the refractometer, and had the impression that the boiling-off had little effect on the gravity. At the end of the boil I took another sample, and bang, spot on. I don't understand why that was so.
Is there a good technique for taking these samples? Would a sample taken from the top during the boil be less dense, and therefore give an inaccurate result?