Calibrate refractometer to hydrometer?

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Shalenkur

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I usually use only a hydrometer, but I did my first brew last night with new refractometer. Calibrated it first with distilled water per instructions. Long story short I thought I came up short on gravity until I was done and decided to check with hydrometer. The refractometer measures 1.045, hydrometer measures 1.050, same sample at 60 degrees F.

Wouldn't it make sense to calibrate refractometer so it reads the same as hydrometer on a given sample in a common target range? I was brewing a stout fwiw.
 

Birrofilo

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Refractometers give good results with a pure water-glucose mixture. When other substances are present in the liquid, such as complex sugars, proteins, not to mention yeast, refractometers don't give reliable results because the relation between the refractive index of the liquid and its density cannot anymore correctly established.

Calibration can be achieved by setting a correction factor which will vary with each different wort.

By way of example, you can establish that a certain correction factor works well for a stout before fermentation, but the same correction factor might be off with a stout brewed differently, or with another wort altoghether, because the liquid will contain different substances which will result in a different refraction index at the same density.

Yesterday I prepared some sugar wash for my distilling needs. The hydrometer says 1,068-1,069, the manual density calculations say 1,06821 so the hydrometer is right, but the refractometer says 16,0 °B, corresponding to 1,065. The liquid also contains some tiny lemon juice, some tomato purée and some yeast. More importantly, the mixture contains sucrose rather than glucose, and that probably has a slight different refraction index. For that mixture, my correction factor is 0,97. Next time that I prepare a sugar wash, applying a correction factor of 0,97 to the refractometer reading should give me the correct density.

E.g. next time I use the refractometer with this kind of wash, I obtain 16,0 °B, I divide by 0,97 and I obtain 16,5 °B which corresponds to a relative density of 1,068. For the same kind of liquid and for a certain span of density that will work satisfactorily.
 

Kickass

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I recently used a refractometer for the first time and a little disappointed at its consistency. I calibrated mine with distilled water, used it and went one. Four days later I went to use it and noticed it was about .005 SG points off so I calibrated it again. Over the course of the next hour, I put some distilled water on it and check it periodically. Every time it gave me a reading +/- .004 SG points from 0. I really don’t know how much to trust this thing.
 

Birrofilo

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@Kickass

If you have a digital refractometer it is important that you clean the sample from solids in suspension, e.g. with a paper coffee filter.

If you have an analogic refractometer, the reading is dependent from the quality of the light. Ideally you should use it with natural light (day light), if you use a different light source (a LED lamp for instance) you should at least calibrate again with the new light source.

Also, in four days your sample might have undergone some evaporation and its density might have increased.

Yes it's a bit frustrating that the results are not always so "spot on" as with a hydrometer, but on the other hand it's more practical, you don't have to make your eyes bleed trying to read the hydrometer ;-) you waste less wort and it's much more difficult to break a refractometer than a hydrometer.

My hydrometer is so stupid that it hasn't the ,005 values marked with a different line. I have to count whether I see 8 or 9 little lines from the first round value above, which isn't so easy, especially through a plastic cylinder which is not perfectly transparent not uniform. Plus, I must take a reading on the upper part of the meniscus, and often I cannot see the meniscus. For easiest reading one must fill the cylinder so that the liquid line on the hydrometer is seen directly and not through the plastic.
 

Brooothru

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@Kickass

If you have a digital refractometer it is important that you clean the sample from solids in suspension, e.g. with a paper coffee filter.

If you have an analogic refractometer, the reading is dependent from the quality of the light. Ideally you should use it with natural light (day light), if you use a different light source (a LED lamp for instance) you should at least calibrate again with the new light source.

Also, in four days your sample might have undergone some evaporation and its density might have increased.

Yes it's a bit frustrating that the results are not always so "spot on" as with a hydrometer, but on the other hand it's more practical, you don't have to make your eyes bleed trying to read the hydrometer ;-) you waste less wort and it's much more difficult to break a refractometer than a hydrometer.

My hydrometer is so stupid that it hasn't the ,005 values marked with a different line. I have to count whether I see 8 or 9 little lines from the first round value above, which isn't so easy, especially through a plastic cylinder which is not perfectly transparent not uniform. Plus, I must take a reading on the upper part of the meniscus, and often I cannot see the meniscus. For easiest reading one must fill the cylinder so that the liquid line on the hydrometer is seen directly and not through the plastic.

All correct.

After years of using a variety of hydrometers, I started using a hand held analog refractometer about five years ago for both beer and wine. The motivations were bifocals (difficulty discerning a meniscus), and wastage (1-2 ml samples vs 6-8 oz). Since refractometers measure "sugar", and since different sugars have different refractive indices, you have to make mathematical corrections for the specific sugar you are measuring. Different mashes/worts are comprised of different sugars in different proportions, but the primary sugar in wort is maltose.

If you had a wort containing only the maltose sugar, you could multiply the refractive index by 1.04 to correct for this variance. Since maltose predominates, the result will be pretty close unless your grain bill has a large amount of other sugars.

When I first started using a refractometer I would run parallel samples with narrow range hydrometers. This validated the readings I was getting from the refractometer readings. I would calibrate each method with ~60F distilled water. Almost none of my beer recipes use honey, corn sugar, fruit juices, etc. By applying a 1.04 WRI (wort refractive index) to beer samples, the derived specific gravities were consistently the same with hydrometer readings for SG. The sugars in wine must are near enough to not require a brix correction factor. In both cases I feel comfortable that the derived results I get are well within acceptable tolerance for homebrew use. In fact I trust the refractometer more than the hydrometers.

A few months ago I dropped my analog refractometer and apparently knocked a combining/focusing lens out of alignment. I replaced it with an electronic/digital refractometer and initially ran parallel measurements between hydrometers and the damaged analog and digital refractometers. All three rendered nearly identical results, certainly within a reasonable error or interpolation.

The refractometer derived RIs do need formulaic calculations during and after fermentation, but those are quite simple using online calculators. Another factor is suspended material in the sample. If you want accuracy you'll have to use a filtered or settled sample (degassed as well). Once again, not a big deal, but necessary for accuracy.

One last item I'll mention in passing. I've noticed that (at least with the digital electronic refractometer) the optical light source can have a very slight effect on readings. I have noticed that if I measure under the fluorescent light in my work area, the RI can vary from one performed under incandescent light or natural sunlight, but you'd have to be measuring SG to 4 or 5 decimal places. I know I couldn't accurately read a hydrometer to that level of fidelity, nor could I observe that resolution with a manual analog refractometer.

That said, if you require that level of accuracy, you'll have to surrender your "I'm a homebrewer" membership card.😉

Brooo Brother
 
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