When I used a refractometer I always felt the need to take 2-4 readings to have confidence in the number. With two hydrometers calibrated at different temperatures - one reading and I'm done. The brewers set I bought doesn't require much liquid to take a reading either.I'm using a refractometer. And I'm not too concerned on that because the post-boil reading from the refractometer and hydrometer are very close. And I obviously let the sample cool before checking with the refractometer.