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weirdboy · 08-01-2011, 02:00 AM

The first time I used my refractometer, I used the difference between hydrometer reading and refractometer reading to calculate a correction factor. After applying this correction factor the two have agreed for every other reading I've done pre fermentation.

Obviously for mid or post ferment you will have to use some of the calculators or spreadsheets which are around that let you correct for the alcohol content, which affects the index of refraction.

**iaefebs** · 08-01-2011, 02:23 AM

Quote:

Originally Posted by ajdelange

It must be said that the agreement between hydrometer and refractometer with wort is usually better than what you observed. In fact the agreement is often quite good .

I agree... quite good in my experience.

jmf143 · 08-01-2011, 01:45 PM

Quote:

Originally Posted by Sithdad

Get some distilled water and test both your hydrometer and your refractometer. Your refractometer may need adjusting. Also, some refractometers need to have their results adjusted based on a comparable hydrometer result. For example, after adjusting your refractometer you test a wort sample with your hydrometer and your refractometer. If, your temperature adjusted, hydrometer reads 1.040 and your refractometer reads 1.044 your refractometer has and adjusted wort value of 0.004. This would mean that all of your readings, from your refractometer, would require an additional 0.004 being added.

Shouldn't he be subtracting .004 since the refractometer is reading "high" by that amount compared to the hydro?

ArcaneXor · 08-01-2011, 02:51 PM

Readings can be thrown off a lot because the sample size is so small. You need to make sure the wort is well-mixed (stir it just before drawing the sample), that there is no break material or hop matter on the refractometer slide, and that there are no air bubbles. I always take three samples and average the readings, and usually end up within 2 or so SG points of my hydro reading.

At one point I thought my refractometer was a waste of money. Now that I know how to sample and use Sean Terril's correction formulas for FG calculations (vs. the rather terrible ones from BeerSmith, MoreBeer, etc) , I am very happy with it.

**Homercidal** · 08-01-2011, 03:22 PM

You may need to adjust the conversion for that particular refractometer. I use a phone app called brewzor, usually, and you can tweak the correction factor.

spook · 08-14-2011, 09:50 PM

I recently bought a refractometer and noticed similar discrepancies when checking samples during fermentation. From what I've found so far it seems that the alcohol present after fermentation begins changes the refractive index and skews the reading. There are correction calculations that take into account OG I assume to infer the alcohol content and correct for it.

I'm getting ready to go all grain, and so I think I will keep the refractometer to monitor gravity when mashing but I'm not sure its worth the hassle of the conversions for monitoring the gravity during fermentation.

**ajdelange** · 08-15-2011, 04:28 PM

Quote:

Originally Posted by spook

From what I've found so far it seems that the alcohol present after fermentation begins changes the refractive index and skews the reading. There are correction calculations that take into account OG I assume to infer the alcohol content and correct for it.

Ethanol has a higher refractive index, 1.361, than water 1.330, (589.3nm) so adding alcohol to pure water increases its refractive index. Adding sugar to water also increases its refractive index. So as beer ferments the reduction in extract (sugar) lowers the refractive index but the increase in ethanol raises it thus partially offsetting the decrease.

There are formulas that try to correct for this and they tend to work well for families of beers but not very well when taken over the universe of beer types.

**Mpavlik22** · 08-15-2011, 08:01 PM

Quote:

Originally Posted by spook

I recently bought a refractometer and noticed similar discrepancies when checking samples during fermentation. From what I've found so far it seems that the alcohol present after fermentation begins changes the refractive index and skews the reading. There are correction calculations that take into account OG I assume to infer the alcohol content and correct for it.

I'm getting ready to go all grain, and so I think I will keep the refractometer to monitor gravity when mashing but I'm not sure its worth the hassle of the conversions for monitoring the gravity during fermentation.

The discrepancies I experienced are pre boil & post boil as explained in my first post. They are not from during fermentation as already stated alcohol throws the refractometers reading off.

**ajf** · 08-16-2011, 01:06 AM

Download a copy of Promash
Run the program, and click on Options/System Settings/Instrument Calibration
Then read the description of the Brix Correction Factor, and click on help.
The help screen gives you an excellent description of what the correction factor is, and how to calculate it.

For pre-fermentation readings, I have found that my refractometer (using the correction factor) is very accurate for my pale ales, but it may need a different correction factor for other types of beers. I'm just too lazy to find out.

-a.