Digital Refractometer Mystery: Questions / Best Practices

[Killshakes]

I picked up a Milwaukee MA884 digital refractometer a few months back, which I use for taking pre-boil wort gravity and OG readings. I still use my glass hydrometer for FG readings. The MA884 has worked well so far, but I've noticed one quirk that has me wondering if I'm using optimally: If I let a test sample sit on the reader / lens for a while--maybe ten minutes--and then retest it, the gravity reading is always higher.

Case in point: Last night I brewed a Saison. I took a small sample of the boiled wort that had been chilled to approximately 70 degrees and allowed it to sit for a minute or two before testing it on the recently-calibrated MA884. Result: 12.9 Brix, or 1.053 SG. I then left the sample sitting on the MA884 for about ten minutes and retested it by hitting the "read" button again. Result: 14.8 Brix, or 1.063 SG.

That's a pretty big difference in OG, and by extension, efficiency--something like 65% to 77%, or a difference of about 12%. It has me wondering: Which is the more reliable reading? 1.063 is much closer to my expected OG (1.060), and that sample did have more of a chance to come to the same temperature as the MA884 / the room. However, the MA884 supposedly provides temperature compensated readings, and I wonder if some chemical change might have occurred in the wort during its ~10 minute rest on the lens that could influence the results of the machine.

What do you think? Do any of you have experience with digital refractometers, and if so, have you encountered something like this yourself? Let me know.

 

[fourfarthing]

Sub'd as I have the same questions

 

[flars]

The problem may have been due to evaporation of the sample on the lens. Evaporation will usually cause an upward drift in SG.

 

[Killshakes]

The problem may have been due to evaporation of the sample on the lens. Evaporation will usually cause an upward drift in SG.

The sample sizes are typically small, but still, I wonder if significant evaporation would likely occur over the course of 10 minutes @ room temperature.

Evaporation may be another complicating factor, though. I could see the second reading being the result of a combined (1) reduction in temperature and (2) concentration of the sample due to evaporation. Perhaps there is also something happening with the suspended break proteins, or other matter (hops, etc.), in the sample while it sits.

 

[Killshakes]

Does anyone know how much temperature skews refractometer readings? Could a reduction of, say, 5 degrees Fahrenheit cause so dramatic an uptick in SG? What about 10 degrees?

I just messed around with this:

https://www.brewersfriend.com/hydrometer-temp/

Using a calibration temp of 70 degrees, it took a change of 50 degrees (!) for a reading of 12.9 Brix (~1.053) to shift to 14.8 Brix (~1.063).

 

[raysmithtx]

I have the same meter and I've experienced the same thing. I think it might the the solids in the sample settling to the bottom of the sample but I haven't strained them out to test that theory.

I'll try using a paper towel as a filter or a piece of coffee filter to see if it helps stabilize the readings on my next batch (about 2 weeks).

Unless someone knows the answer or is brewing before then and can test the samples filtered and unfiltered.

 

[Bigdaddyale]

I have noticed if you raise the plexiglass lid and set it back down on the sample you will get a different reading everytime

 

[Killshakes]

I have noticed if you raise the plexiglass lid and set it back down on the sample you will get a different reading everytime

I think you're thinking of a manual refractometer. Digital models like the Milwaukee we've been discussing don't have a plexiglass lid.

 

[schematix]

For a while i was having a similar issue on my manual refract and believe the issue came down to evaporation.

I was taking a sample of near boiling wort directly with a pippette for pre-boil gravity check. It almost always read 10% high (which actually is a good estimate for post boil gravity).

In the hunt for this oddity (among others), I purchased a set of 3 lab grade precision hydrometers. They are accurate to within 0.1 Plato (about half an SG). They are amazing tools that i know i can always trust. They also overlap a little bit so if you happen to be in the overlap range you can get 2 independent measurements to increase confidence. I'd highly suggest anyone who has concerns about gravity measurements buy a set of these for establishing a known reference point. It's a bit over $100 for the 3 so not cheap, but good tools aren't....

They showed that the refract was always dead on accurate as long as the wort had been chilled down to a reasonable temp (usually about 25C, which corresponds to the range you want to be in for pH measurement). Now i sample into a cup and cap it... give it a bit in the freezer to chill, then go for it.

After these experiments i actually regained trust in my refract. I had basically relegated it to permanent storage until i figured out it was operator error.

I've also found that the refractometer correction calculators for post-fermentation are extremely accurate, so much so that i stopped using my hydrometers for this task. It does require that the sample be degassed. This can be accomplished by lightly squishing the lid and lifting it up and down a few times. Sometimes it takes a few repeats and the last time it helps to be gentle. YMMV. You know it's good when you have a sharp line and not a blurry one.

 

[Killshakes]

For a while i was having a similar issue on my manual refract and believe the issue came down to evaporation.

I was taking a sample of near boiling wort directly with a pippette for pre-boil gravity check. It almost always read 10% high (which actually is a good estimate for post boil gravity).

Were you running into this problem when you took OG readings too, or just with your preboil gravity readings? The phenomenon I encountered was that the same exact sample of boiled, chilled (approx. 75 F, which is a little cooler than 25 C) wort produced two very different readings on the same, recently calibrated digital refractometer at room temperature, even though the readings were only taken 10 minutes apart. I wasn't testing hot wort.