I respect the fact that you’ve given this a lot of thought and are dedicated to improving the accuracy of your measurements. I merely disagree with the premise that your refractometer readings are off because of evaporation
. Instead, I think it is due to sample stratification
. After all, don’t we wait 15 minutes or so for the hot and cold break to settle before transferring? Why wouldn’t sugars and proteins of various shapes, sizes, and weights be any different?Long version:
Three key parts to evaporation are heat, humidity and air movement (source: http://en.wikipedia.org/wiki/Evaporation)
. As I see it, plunging a dropperette beneath the surface of the liquid to draw a sample of wort (regardless of temp), promptly placing a very small sample mass of just one or two drops on the cooler refractometer surface (which has a much larger mass and serves as a heat sink), and placing the cover plate over it pretty much removes all three parts necessary for evaporation. There’s even less kinetic energy to drive evaporation as the liquid cools, the humidity probably does decrease (but you’re reading within a few seconds, so it should be negligible), and the air movement is nil (as the cover plate prevents exposure to air movement and may limit humidity changes since there’s practically no exposed surface area to permit it).
The refractive index is dependent on
1. The material/media in which it is traveling
2. The frequency of light
3. The temperature
As you mentioned, I think it is quite possible that you may be noticing temperature and specific gravity stratification of your pulled samples. This sample inhomogeneity may be yielding slightly lower readings if you’re pulling smaller samples from the upper and/or outer portions of the larger sample.
Assuming you’re doing the readings in the same place regardless of your sampling method, the frequency of light isn’t changing (whether you’re consistenly brewing outside or in the kitchen). If you’re doing one reading outside with one method and another reading inside with a different method and using a different light source—give it some thought and eliminate that variable.
Temperature gradients can cause fluctuation in refractive index within a single medium to generate a refractive effect, e.g., mirages and looming artifacts (source: http://micro.magnet.fsu.edu/primer/lightandcolor/refractionintro.html
). I don’t know if there’s enough of a temperature gradient with a refractometer to create this refractive effect to a significant degree that we’d notice it. My bias is that a temperature gradient is not playing a role with the refractometer readings. IMO, even the chilled specimens are more likely to be causing specific gravity stratification in the time it takes to chill the sample, which I perceive as having a greater effect on one’s refractometer readings than temperature gradient effects would.
And, like Kai said earlier, it is best to acclimatize your refractometer to ideal conditions. Thanks for letting me participate in such an interesting discussion.