Refractometer question

I have been getting very different results with my refractometer depending on when the sample is taken during the brew day. My pre boil and end of boil gravities always seem to be on target with what I expect out of my system but then after I cool the wort and I take a reading the gravity can drop as much as 15 points!

Today towards the end of the boil I took a reading and it was 1.080 then 20 minutes later as I was racking into my carboys it read 1.065. What is going on here? I always blow on the sample which is only a few drops to cool it off and yes my refractometer is calibrated. Can't figure it out.

Any thoughts?

well, just to test, I would take a sample during the boil, measure it, and then set some more aside to cool completely to room temperature. Maybe your blowing method isn't cooling it sufficiently.

If you're leaving the sample on the refractometer while it cools, its possible that some of the water is evaporating, leaving a more concentrated solution. I don't suppose you have a hydrometer to test a reading at all? Wouldn't really help this time, but might explain stuff in the future. Also, does your refractometer have ATC, or are you using it in wildly different temperature areas (ie outside at 100 versus inside at 65)? I can't think of any other way for you to drop that much gravity, unless your chiller is pumping water into your beer, which I feel like you would notice. You might have varying levels of particulate in your samples at different times, but I always thought they just made the line fuzzier, didn't actually move it up and down.

I've noticed readings all over the place with my refractometer. So though it may be overkill, I calibrate over and over until I get the same reading 3 times in a row. I also calibrate before each time I take a sample.
Then I take my sample and do the same with taking multiple readings until I get three steady readings.
Since I started doing that, I've been having much less problems with the fluctuation in the readings I get from it and my gravity readings are much closer to where I'm expecting them.

Blowing on it doesn't really affect much since the thermal mass of the tool is much larger than the drop or two of wort you put on it and the temps equalize almost instantly even if the wort is straight out of the boil.

How many drops are you putting on it?

I put only 1-2 drops on my refractometer and it's plenty for a reading. As mentioned above, the thermal mass of 4 drops is significantly greater than that of 1-2 drops.

A refractometer with ATC compensates for the ambient temperature not the sample temperature. So if you don't have ATC with your refractometer even the change in temperature from inside to outside would make a difference. Do you use distilled water to calibrate your refractometer? Tap water has enough dissolved solids in it to throw off your calibration.