Saw this mentioned a few times in another thread so I decided to see what happens in my kitchen. I use a refractometer and collect a few tablespoons in a bowl then use spoon to stir and dose. These are the results:
8:50 pulled 3 tablespoons from kettle
8:54 reading 12b at 73 degrees
9:08 12.2b at 67
9:40 12.4b at 67
9:56 12.4 b at 67
10:15 12.6b at 67
point 2b is about .85 gravity points so over the course of 1 hour I would have seen about a 1.7 increase in the gravity of the solution due to evaporation. About 1/3 of a tablespoon was used per reading. Since I was able to obtain a suitable temperature in less than 5 minutes, I will continue be ignorant of evaporation. While writing this I realize I should have put an open hydrometer size glass in the freezer to compare, next time.
I wish i could say i use my refractometer but it just collects dust. It was just to inconsistent for me. On the evaporation observation mine varies batch to batch i believe its related to relative Humidity in my region ( or at least i think thats what it is). I havent charted it per say in that relation but it sure seems that way. I just take a cooled hydrometer reading every 15 min and adjust my boil as needed to get my FG. Works pretty much every time.
Did you chill a sample in a sealed container in order to have a control? If you’re cooling it in the open air for those 4 min, 12°Bx probably wasn’t the baseline reading.
FWIW, I use a small syringe to pull ~0.5 mL for refractometer samples. It cools to room temperature in less than a minute and I can then cover with my finger and shake to homogenize the sample.
I did not do that. It was a spur of the moment thing. I will certainly do it next brew to compare vs my initial reading. I was interested in seeing how much evaporation would change the reading over time, as I take such a small sample it seemed it would be more susceptible to evaporation vs a hydrometer sized sample in a freezer-which I used to do. I believe the evaporation will occur faster during rapid cooling and I will need that control to determine that. While my wort decreased in size after each sample the temp was essentially stationary. How big a role humidity plays is something else to consider.
I spoon a small sample into an old white labs vial then put the cap on. Once it cools I give it a shake then take my refractometer sample. That takes care of the evaporation concern.
Was able to brew again today, so I took some readings.
3 tsp sample from top of kettle cooled in 3 minutes to 68 reading 13.6b
3 tsp sample from ball valve at bottom of kettle cooled to 68 in 3 minutes 13.6b
3 oz sample sealed and chiiled then shaken and sampled about 10 mins at 67 13.4b (was really close to 13.5)
.2 brix is less than one gravity point. Evaporation is a non issue for me, stratification as well.