Digital refractometer, measurement swings

[jdudek]

Hi All,
I used a digital refractometer for the first time while brewing last weekend. It's a Milwaukee NA871. I had issues with repeatable readings. Reading for a given sample varied across 1 Brix unit. My final sample read anywhere between 13.8 to 15.5. Most readings would start low and start creeping up the more readings i took over time.

I read that any solids in the wort samples will greatly skew the reading, which makes good sense. I am pretty certain this could have been an issue as i have not filtered the samples in any way.

so a couple of questions:

1- any general recommendations about using a digital refractometer? is variability in the readings pretty common or should it be manageable?

2- are samples that are not clear likely to read higher, lower or could go either way?

thanks,

 

[Robert65]

I've never experienced this. If you took a few drops from the mash and let it sit in the well, and over several minutes the readings went up, do you think it's possible this was explainable by evaporation of the sample?

 

[jdudek]

I'd get variable readings of the sort i described with in a minute or 2, i would imagine that's not enough time for any significant evaporation.

 

[Qhrumphf]

If you think of what Plato or Brix actually are (percentage of extract/sugar by weight), it's not difficult to fathom why a small amount of evaporation can have a significant impact on gravity. Especially if you do what many do and drop hot sample into to refract and rely on that to cool it.

I haven't used that particular digital refractometer, but the one I use regularly doesn't do too well with both ATC and registering the temp of the sample properly. As a result, the further the sample temp is away from the ambient temp the meter is at (in either direction), the wonkier and less consistent the readings get.

I also rinse the lense thoroughly before filling it using the sample I'm about to test. And it's also calibrated with DI water at least once every day I use it.

 

[SEndorf]

I have nothing concrete to back this up, but I find holding my hand over the sample provides no drift. I have bright spots over my work table.

 

[Robert65]

I have nothing concrete to back this up, but I find holding my hand over the sample provides no drift. I have bright spots over my work table.

Cupping my hand over it is my SOP. I frequently get the external light error message if I dont, and I realize this is the best way to ensure consistent illumination by the light the instrument assumes is doing the job. (I don't have the OP's unit either FWIW, I have a Hanna specifically made for brewing.) I get very consistent and accurate readings.

 

[SEndorf]

Cupping my hand over it is my SOP. I frequently get the external light error message if I dont, and I realize this is the best way to ensure consistent illumination by the light the instrument assumes is doing the job. (I don't have the OP's unit either FWIW, I have a Hanna specifically made for brewing.) I get very consistent and accurate readings.

Good to know. I also have a Hanna (HI98611). I find it very reliable.

 

[jdudek]

Thanks for the response. I just gave it a try with a sample of fermenting wort. Got anything between 6.6 and 7.6, seconds apart in no particular order. Calibrated first with distilled water, rinsed, clean, cupped turned of the light, turned on the light...

 

[Qhrumphf]

Fermenting wort is probably not the best test of a refractometer. If you want to test the accuracy making some known gravity worts using DME. You could verify further alongside a known hydrometer.

 

[Vale71]

If you're talking about hot wort then I would expect gravity to increase as the sample quickly cools to room temperature with possibly a little evaporation helping along. Contrary to popular opinion temperature compensation means the instrument compensates for its own current temperature and not for the temperature of the sample which the instrument, even an electronic one, has no way of measuring. So if you drop hot wort on a cold lens you can expect it to cool down rather quickly and the reading will go up, because the density of the sample actually is increasing with decreasing temperature.
If you're measuring fermenting wort then you can expect turbidity to mess with the measurement. Whilst suspended solids will certainly not change the refractive index of the liquid they will definitely cause light to be randomly scattered making the reading fuzzy. How an electronic instrument deals with that depends on its own firmware. My refractomenter actually has a tendency to creep towards lower readings if I let a fermenting wort saple rest on the lens. My theory is that yeast will drop on the lens thus altering the image in a way that my instrument will read a progressively lower value as the lens becomes ever dirtier and dirtier. I just assume the first reading is the better one and go with that. Alternatively you could just filter the sample.

 

[matt_m]

I have the MA871. I just calibrate every measurement. Probably overkill but a small lab bottle of distilled water makes it easy plus it makes sure the lens is clean. I have some small stainless ramekins like restaurants use for sauces that I put my samples in and set them in a water bath for a couple minutes to bring down the temp before reading. After dropping the sample I wait for the temp to stabilize, maybe up to a minute but usually less. And I always cup my hand over the sample for any calibration or reading. Following those steps I've been able to get consistent readings that match random tests against a standard floating hydrometer.

 

[jdudek]

i had left my sample to sit overnight in a small vessel. This morning most solids have fallen to the bottom and the top part liquid is very clear. I measured this and got 10 7.4 readings in a row, ie very stable. So i am tending towards the suspended solids problem.

Thanks @Vale71 about the temperature explanation, that makes a lot of sense. I always wondered how in the world would the refractometer be able to measure the temperature of a sample just sitting there... calibrating against its own internal temperature makes a lot more sense.

So next brew i will let samples cool down, and strain them before measurement... see if i get more consistent results.

thanks for all the responses, appreciate the support!

 

[matt_m]

That's a good point. I've also started grabbing my OG sample from my fermenter's sample port. I've been doing it because its cooler, but also typically has much fewer solids.