One must two different OG readings

runningonempty · October 8, 2011, 1:45pm

I blended 3.5 pounds of Clover Honey and water to make one gallon. Using both a hydrometer and a refractometer I took SG readings. I was expecting to get a value from both that was reasonably close. They were not. The refractometer has both a brix and SG scale with ATC and I checked the calibration of the device using distilled water. It was spot on. The SG was 1.118 using the refractometer. The SG using the hydrometer was 1.134. I allowed the hydrometer time to settle and checked it multiple times to ensure that I read it correctly. The ambient room temperature was in the range for the refractometer.

Can anyone offer an explanation as to why there is such a large discrepancy between the two devices?

medsen_fey · October 8, 2011, 10:50pm

That is a rather large difference.

Did you check the calibration of the hydrometer by testing in distilled water?
By the way, what was the BRIX reading from the refractometer? Perhaps the conversion formula being used is not the most accurate.

runningonempty · October 9, 2011, 12:06pm

I did check the calibration of the refractometer using distilled water and it was “spot on”.

The Brix value from the refractometer was 30.2 which correlated to the 1.118 SG value on the refractometer’s SG scale.

I found an SG to Brix table online and based upon that table a Brix of 30.15 correlates to a 1.130 SG which would be verry close in value to the hydrometer value I got of 1.134.

The brix to SG scale on this refractometer seems accurate up to brix 5. After that the brix to SG values start to diverge such that a brix at the high end of 30 equals 1.116 on the refractometer’s SG scale and NOT 1.130 as indicated in the conversion table. I did notice that this refractometer states “Brix % Wort” and the bottom of the scale. Would this make a difference when making wine/mead?

This refractometer has ATC so it should make adjustments appropriately. However the hydrometer does not so how would a room temp of 81 effect the hydrometer’s results.

Suddenly confused.

medsen_fey · October 10, 2011, 8:29pm

Using your hydrometer at 81 will cause it to read .001 or .002 below the actual value depending on your calibration temp. Warmer solutions are slightly less dense.

Did you test the hydrometer in plain water to see if it is accurate? Hydrometers can also have a significant amount of error in some cases and you can compensate for it if you know it.

You might look in the manual of the device to see what kind of formula they are using for the Brix to SG conversion, but it is apparently not accurate at higher gravities. There is a old rule of thumb that gravity points = 4 X Brix, which actually works ok for gravity that you may find in beer brewing, so that if you have a batch with 10 Brix, the SG would be around 1.040. Such a simplified formula become more and more inaccurate as the gravity becomes higher. A calculator like Vinocalc

can give you better results. You can also program a formula into a spreadsheet to use.

Vinocalc uses the following:
brix = 143.254 * sg3 - 648.670 * sg2 + 1125.805 * sg - 620.389
sg = 0.00000005785037196 * brix3 + 0.00001261831344 * brix2 + 0.003873042366 * brix + 0.9999994636

One more thing to remember about refractometers is that they will give you inaccurate readings once fermentation starts as the refractive index of alcohol is higher than that of a sugar/water solution. With a finished batch, you may get a hydrometer reading of 1.000 while the refractometer will still be reading 10 Brix. Vinocalc and other calculators have some formulas to make a correction for this, though they could be refined a bit more. You’ll want to use those formulas or you’ll be thinking a batch is stuck when it is not. At the end of fermentation I always like to get a gravity reading rather than trusting the refractometer number.

I hope that helps.

Medsen

runningonempty · October 11, 2011, 8:16pm

I filled a test tube with filtered water at 81F and checked the hydrometer. Its value was 1.002. This would agree with your response. Therefore my hydrometer’s original OG of 1.134 would actually be 1.132. Correct?

In summation:

can I assume my refractometer’s Brix values are correct and I should ignore the SG conversion.
use the Vinocalc website to obtain an SG value based upon my refractometer’s Brix value.
always use a hydrometer to get the OG and FG values.

With respect to using a refractometer to determine FG I have used a spreadsheet available at this website: http://morewinemaking.com/learn_vids/vids_refract . It provides an interesting video with two links below the video for the spreadsheet for beer OR wine conversion. This spreadsheet does require an accurate OG Brix value.

Again thanks for taking the time,

Ron

medsen_fey · October 12, 2011, 4:45pm

Yes, your hydrometer looks like it is reading a couple of gravity points too high.

I think the Brix reading you are getting are probably accurate, but keep in mind that the accuracy is usually no more than 0.1 Brix so you will get slightly different readings each time if you repeat it, and you are taking sample of 1 drop and there may be some drop to drop variation depending on how well mixed things are.

I always like to confirm my final reading with a hydrometer. When I start batches, I’m comfortable using the refractometer alone.