I've noticed that my correctly calibrated refractometer gives different readings, with nearly every drop of the slide cover.
I have had the same sample give me more than 3 different readings, when I'm testing my boil.
In fact, a friend of mine insists on opening & closing his cover, until he likes what he sees.
I think this is possibly a bad practice...
While I understand that he's not changing sugar content, I imagine that something has to be skewing the results, if the reading is changing...My first guess is definitely temperature, but I hypothesize that temperature is not the only culprit.
A correctly made hydrometer is the more accurate beer-making tool, IMHO.
But, it's very nice to be able to get readings during the sparge or boil.
I'm just very skeptical of how folks use the refractometer.
I have recently resorted to allowing my sample-filled refractometer sit for several minutes before I read it.
This seems to alleviate any questions regarding whether or not I can trust the measurement.
He could very well be skewing the results. By opening the cover, you allow a little evaporation to occur. that could give you a higher than normal reading. When you place that drop on the lens it should be closed immediately and I allow mine to set for about 30 seconds before taking a reading.