Readings a refractometer

Ok I had something interesting happen today. I always take a pbg reading with my refractometer to see if I'm close to my expected pbg. So I took my dropper sample let it cool like five min then dropped it onto the sight glass, it read x so it put the refractometer down, never cleaned the sample off as I like to come back to it as I know as it cools the reading due to the higher temp becomes a little more accurate. So I let this one sit and I checked it three times over say 15 min and my reading jumped about two points from the 5 mini reading. So is the last reading, the coolest temp wise of the three readings more accurate one.

So my pbg reading at five minutes was 11.4 now five more min later it's at 12.4 it finally stopped moving at 13. This is crazy. My goal was 11.4.

That's a good question. One thing I found out is that my refractometer is so sensitive that I have to be very careful when I put it down or it seems to go out of calibration. Mine was $25, but is the exact same one in my LHBS.

I brewed in Saturday and I did the same as you, but it went from .037 to .044 in a 15 min time period. I left it a bit longer and it sat at .045. I went with the .045 because I cleaned it off and retested a sample, the remaining wort in my dropper, and it read .045. I was thinking it was just the temp needing to drop, and it's more sensitive to temp than we are.

I've noticed mine does the same thing.

So I guess the better reading is the cooler when the wort is at its most concentrated as it cools?

I don't know about that. I would think that some of the water works its way out from under the glass and evaporates while leaving the sugar behind. It's just a theory. That's why I always take a big enough sample with my dropper to double check once it's cooled more. Basically I get my first reading, let it sit, go back and check it, if the number has changed I test the remaining liquid in my dropper. Then I take that reading, or split the difference depending on what I get.

JosephN said:

I don't know about that. I would think that some of the water works its way out from under the glass and evaporates while leaving the sugar behind. It's just a theory. That's why I always take a big enough sample with my dropper to double check once it's cooled more. Basically I get my first reading, let it sit, go back and check it, if the number has changed I test the remaining liquid in my dropper. Then I take that reading, or split the difference depending on what I get.

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That's what I was wondering because I noticed as the sample cooled it became noticeably thicker so like u said could have been just the more dense sugary wort left behind minus the water which isn't neccessrily a true reading.

I always go w/the first reading. The way I understand it is that that one drop you put on it cools so fast that you don't need to let it cool more.

I always read immediately. I have noticed that if the wort is cloudy, the indicator line in the refractometer is less distinct.

C-Rider said:

I always go w/the first reading. The way I understand it is that that one drop you put on it cools so fast that you don't need to let it cool more.

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This right here. The small amount you put on the lens is cool enough. Take the first reading.

JosephN said:

I don't know about that. I would think that some of the water works its way out from under the glass and evaporates while leaving the sugar behind. It's just a theory. That's why I always take a big enough sample with my dropper to double check once it's cooled more. Basically I get my first reading, let it sit, go back and check it, if the number has changed I test the remaining liquid in my dropper. Then I take that reading, or split the difference depending on what I get.

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I do the same and I've also heard evaporation can change the reading over time.

What is important for accuracy (because of the way the instrument works) is that the sample is the same temperature as the instrument. Letting the dropper cool, then applying drops to the instrument doesn't achieve anything. Apply the drops right away (regardless of temperature) and wait a few seconds for them to acclimate to the instrument temperature. That reading will be correct (to the limits of the instrument calibration).

Yup, I believe the instructions that came with my refractometer said to let the sample rest on the refractometer for 30 seconds to equalize temperature before taking your reading.

dsaavedra said:

Yup, I believe the instructions that came with my refractometer said to let the sample rest on the refractometer for 30 seconds to equalize temperature before taking your reading.

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Mine said something similar, but it didn't talk about temp. It just said let it set for 30sec to adjust. I wasn't sure what that meant, but it makes sense.

The refractometer I purchased stated it was ATC, Automatic Temp Correcting. I suspect its simply that the sample is so small that temperatures quickly equalize, what with that large hunk of glass and aluminum receiving a tiny (I do 3 drops) sample of wort. I think next time I brew I will leave my eye to the eyepiece and see if it shifts over 60-90 seconds or so. Never thought to check.

The ATC thing can be confusing. That doesn't apply to the sample temperature; it applies to the instrument temperature. It means that the instrument will operate accurately within a range of maybe 60 degrees F to about 80 degrees F - so lang as the sample is at the same temp as the device. Without ATC, or outside the ATC range, you'd have to apply a correction factor depending on instrument temperature. Again, the sample would need to be the same temp as the device. There is no easy way to calculate a correction factor for a difference between sample and device temp.

oh, OK. thanks for the clarificatin.

I know it doesn't take long to get the sample to get to the same temp. I took a sample yesterday of a pumpkin porter in a ferm tank and I put it in the freezer for a few mins. I put a digital temp gauge in the test tube and it read 47F and I could just watch the temp raise from just setting out in the ambient temp. It raised at about 1 degree per min, even though I didn't time it this is just an approximate. I would think that a tiny dropper would cool quickly of you set it in a small glass with room temp water in it, just deep enough to cover the bubble full of liquid. I hope that made sense. Again that's why I set out my refractometer with my sample and just let it cool for 15 mins or so. It just kind of seemed logical to me.

What about the cleanliness of the sample? I noticed today when I took a sample right at flame out my reading was 1.050, then I chilled the wort down to 60, let the trub settle and the reading increased to 1.053. Do particles and trub mess with accurate readings?

see post 8

I have always read mine quickly. It takes only a minute or so to stabilize. After that it has never changed to my knowledge. Though I have never waited longer than a couple of minutes. Even when I do see any change it is only one point, maybe two.

I noticed the difference in cloudy wort vs clearer wort. The line was more hazzy/ less defined with cloudy wort. Also I made sure to pull about 5-6 different samples throughout the whole process yesterday. I had to let my dropper rest in a cup of room temp water for a bit to cool down the 210F wort. I waited about 5 mins and it was cool enough to get an accurate reading. I also didn't have issues with it changing more than .001-.002 even with it sitting around for a while.

My fermenting wort readings are constantly off even with crystal clear beers. I measured 6.8 Brix on my last IPA, used Sean Terril and Beersmith's corrector and was expecting 1.008 (which seemed low). Sure enough the hydrometer reading was 1.012 (spot on FG).

I only use the refractometer as an indicator that gravity isn't moving now and also to calcultate my pre-boil gravity

lolcats said:

My fermenting wort readings are constantly off even with crystal clear beers. I measured 6.8 Brix on my last IPA, used Sean Terril and Beersmith's corrector and was expecting 1.008 (which seemed low). Sure enough the hydrometer reading was 1.012 (spot on FG).

I only use the refractometer as an indicator that gravity isn't moving now and also to calcultate my pre-boil gravity

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If I understand you correctly, you have used you refractometer to measure "fermented beer" or "wort that has started fermenting"? Unless I'm mistaken the refractometer is only good for "unfermented" wort. If you use it on fermented stuff, even being crystal clear, my understanding is, you would get an incorrect reading.

JosephN said:

If I understand you correctly, you have used you refractometer to measure "fermented beer" or "wort that has started fermenting"? Unless I'm mistaken the refractometer is only good for "unfermented" wort. If you use it on fermented stuff, even being crystal clear, my understanding is, you would get an incorrect reading.

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I agree, but there are a few calculators out there (sean terril, beersmith..etc). That claim being able to determine fermented beer gravity using a correction factor

http://seanterrill.com/2012/01/06/refractometer-calculator/

You do what you like, but once the yeast chew the first molecule of wort, I put the refractometer away.