Hydrometer vs. Refractometer

[shawnduthie]

I have both a Refractometer and a Hydrometer. I have found that I am getting different readings from the two. Both were calibrated with water and read 1.000. But when I measure my pre boil gravity, the Refractometer reads 10 points above the Hydrometer.

Which one do I believe? How can I check to see which one is correct?

 

[GilSwillBasementBrews]

Are you adjusting the hydrometer reading for temperature? It should have its calibration temp on the paper slip inside the hydrometer.

 

[Oginme]

The reading on a hydrometer of wort or fermenting/fermented beer at or near the calibration temperature of the hydrometer should be pretty accurate.

When using a refractometer, you should be reading the Brix scale. There are calculators on the web to help with converting the Brix reading to specific gravity. This works for pre-fermenting wort only. Even with an automatic temperature correcting model, I recommend allowing the refractometer to sit for a few minutes before checking the reading.

If you are using the refractometer for fermenting or fermented beer, you will need to make a correction for the alcohol content. Again, there are calculators on the web to help you make the conversion. You will need to know the original gravity (usually in Brix) to make the conversion.

 

[ajdelange]

Believe the hydrometer but be sure you are using it correctly (sample at the correct temperature or reading corrected for temperature). Also be sure that the refractometer sample is at the correct temperature or is corrected for temperature. Even if the refractometer is being correctly used it is not that unusual to find it's reading off by as much as 10 points but usually they are closer than that (say within 4 points).

Both instruments are designed to read correctly in binary solutions of sucrose and water. Hydrometers are much less sensitive to sugar spectra that differ from pure sucrose than refractometers.

 

[orangehero]

AJ in your experience does that apply to digital refractometers also and are there any tips for using it correctly?

My precision hydrometer and ±0.1° Brix digital refractometer never get the same O.G. reading, even though I believe both to be accurate (refractometer tested with ±2 mg sucrose solutions, hydrometer with pure water).

The digital also can measure refractive index. Would that be a better measurement to use and is there a formula to convert it to wort S.G.?

 

[WoodlandBrew]

That sounds like a nice refractometer. What is the brand and model?

In case you want to use refraction index here is a conversion table:
http://homepages.gac.edu/~cellab/chpts/chpt3/table3-2.html

 

[TheRussMeister]

I have been having the same issue with my tools too, but not as extreme. I have a relatively new, regularly calibrated hydrometer, and a digital refractometer. I always let the sample cool to between 60-70 before using either of them. I experience difference more like what AJ had mentioned, after conversion to gravity, they can be off by 5 points max (this is for wort, not beer). I have also found that the range of how far it is off varies with OG and color, but I need more data points to make any statements about how. As AJ mentioned, the types of sugars meant to be measured by the instrument is important. When I had purchased my digital refractometer, while browsing around there were a lot of specific applications for them. For instance, I saw one made for wine, maple syrup, honey, nectar... Whether or not the differences are hugely substantial, I am not certain, but I would imagine it could have a result of the magnitude of which you are seeing.

As for fermenting beer, morebeer had a excel sheet with a built in formula that I use to monitor my fermentations with my refractometer readings. In the end, the mismatch can be a bit more extreme, but still within 6 or 7 points.

I would suggest borrowing another hydrometer and seeing how it compares to your own. You also may want to make a few standard calibrations, not just water, but a standard sucrose solution or two, and see how they fit.

 

[WoodlandBrew]

A similar refraction index table can be found for Maltose. The deviation seems to be less than 1% of the sugar density between the two.

 

[ajdelange]

AJ in your experience does that apply to digital refractometers also and are there any tips for using it correctly?

Yes, the relationship between RI and sugar content is not as robust as between SG (or density) and sugar content.

..and are there any tips for using it correctly?

Many of the tips are the same. The most important may be to be sure the sample is at the instrument's reference temperature. This may be a bit tricky as the ICUMSA formula (1) for pure sucrose uses a reference temperature of 28 °C whereas formula (2) for sucrose, glucose, fructose and invert sugars is based on 20 °C. The reason this is important is that beer has an RI vs temperature characteristic that is quite different from sucrose. For an inexpensive instrument that doesn't give the user any option to select sugar I'd assume formula (1) is in use and shoot for 28 °C unless, of course, the manual says to shoot for 20.

My precision hydrometer and ±0.1° Brix digital refractometer never get the same O.G. reading, even though I believe both to be accurate (refractometer tested with ±2 mg sucrose solutions, hydrometer with pure water).

The digital also can measure refractive index. Would that be a better measurement to use and is there a formula to convert it to wort S.G.?

The instrument measures RI and converts it to Brix (which is easily converted to SG). Were you to measure RI you could take control of the temperature question raised earlier as you could choose whether to implement Formula (1) or Formula (2). This gives you RI vs Bx. Now you must figure out how to find Bx from RI using that formula. Newton's method or root bisection are both easily implemented or, if you use Excel, the Solver will do the job for you. Finally, conversion of Brix to SG involves use of the inverse Lincoln Equation or root bisection inversion of the ASBC SG vs °P equation.

In case that's not clear, you would have to do the work the instrument currently does for you. If you feel you can do it better then measuring RI is indeed a viable option.