Refractometer Temperature

[siwelwerd]

A couple weeks ago when I was brewing, I noticed something odd. After I started the mash, I carried the rest of my equipment, including my refractometer outside. I figure this would be a good time to calibrate it, so I do so. I then set it back on the table in the sun. A couple hours later, as my wort is coming to a boil, I think "Maybe I'll double check that calibration before I measure the pre boil gravity". It turns out, it had managed to lose its calibration over the course of two hours. The only thing I can think of is that it was sitting on a table in the sun, and was noticeably warm to touch when I picked it up the second time. Now, my refractometer supposedly automatically corrects for the temperature of the sample, but can the temperature of the refractometer itself actually skew things?

On another note, I calibrate with tap water, as DI is not handy; will this make any measurable difference?

[Ozwald]

First the second part, yes and no. It really depends on your water. Being in the southeast I really wouldn't trust it. Up in the mountains I did it all the time since I was drawing off from snow melt that was only picking up a little bit of 'stuff' from the reservoir to the tap. You could always grab some distilled & compare them side by side at the exact same temperature (preferably at the calibration temp) but since distilled typically doesn't come in bottles less than a gallon I would just suggest keeping the gallon around for testing. Your great great grandkids will probably have some left considering how much you use it.

As for the ATC & temperature... bullshit. The ATC is practically non-existent (at least in the ones sold by homebrew shops, perhaps there are nicer, lab grade ones out there). I've tested mine & get different readings at 60, 64 & 68 from the same sample. They're within a few points but if you're looking for an exact reading you'll have to calibrate it to a known hydrometer at a specific temp. Laying it out in the sun is essentially the same thing, only the internal temperature of the unit has changed instead of the sample. Same concept (to a lesser degree) as the mirage that snipers deal with.

They can be a nice toy to have around & convenient, but just like every other convenience, you're sacrificing something for it. In the case of a refract that would be accuracy.

Since I brew the same recipes over and over I use mine to know that I'm on track, but I don't completely believe the readings for the number. For example if I know a recipe is supposed to be 1.042 post boil, I'm happy as long as I'm reading within 4 or 5 points since I'll know it's more than likely on target. The other great thing they're good for is to monitor fermentation. Again, not for exact numbers, but to watch the gravity drop.

[siwelwerd]

As for the ATC & temperature... bullshit. The ATC is practically non-existent (at least in the ones sold by homebrew shops, perhaps there are nicer, lab grade ones out there). I've tested mine & get different readings at 60, 64 & 68 from the same sample. They're within a few points but if you're looking for an exact reading you'll have to calibrate it to a known hydrometer at a specific temp.

Well, I've always figured that ATC or no, those two drops don't have much thermal mass and should equillibrate to the ambient temperature fairly quickly.

[jeffjm]

Agreed about the thermal mass.

Regarding multiple samples providing different readings, I've run into that as well. More than once, I've calibrated with tap water, pulled a sample, let it cool a moment, placed it on the glass, recorded the measurement, and then checked calibration again, only to find my calibration is off. As a result, I don't consider that I've got an accurate measurement until I get the same thing twice in a row.

Could it be that the refractometer is delicate enough that it's very easy to knock something inside out of alignment?

[Ozwald]

Agreed about the thermal mass.

Regarding multiple samples providing different readings, I've run into that as well. More than once, I've calibrated with tap water, pulled a sample, let it cool a moment, placed it on the glass, recorded the measurement, and then checked calibration again, only to find my calibration is off. As a result, I don't consider that I've got an accurate measurement until I get the same thing twice in a row.

Could it be that the refractometer is delicate enough that it's very easy to knock something inside out of alignment?

The majority of the inside is just air. Like I related it to mirage, it's going to affect how light travels through there. If you place a hot(ter) drop on the lens, agreed it has little thermal mass, but where does that heat transfer to? Unlike mirage we're measuring very specific waves of light so it doesn't take that much to throw it off... like leaving it in the sun. The other thing, besides the internal air, that's going to be affected is the bi-metal strip that basically works like a bi-metal thermometer only this one is for aiming the lenses for the accuracy of the reading. If your temp is off, or you subject it to temp swings, it's not going to give an accurate reading.

[hoodie]

One thing i always check if I get a crazy reading, after the calibration of course, is whether the sample is mixed well. Especially pre and post boil. I would try comparing samples between a refractometer and a hydrometer. Beersmith has a tool for further calibrating your refractometer based on those results. DO use distilled water though.

[Bugeater]

The so called ATC function is designed to work when the refractometer is at "normal" room temperature. It corrects for minor variations caused by the sample. When you stick it in the hot sun or take it outside for a winter time brew, it cannot make enough of a correction and your reading will be off.

I primarily use this gadget during the boil to see if my gravity is getting close to target so I can adjust the boil time.