Vertical Axis is °P Actual
Horizontal Axis is °P Measured
It looks like both the refractometer and the hydrometer work equally well for measuring original gravity of a wort. The refractometer used for this comparison has a demarcation resolution of 0.2°P while the hydrometer was marked every 0.5°P. The resolution limit of the refractometer is better, but the actual accuracy of the tools were both about the same at 0.5°P
Hydrometers work by measuring the relative density of a liquid. Brewers use this density assuming that it a solution of sucrose and water to determine the sugar content. Refractometers measure the refraction index, and like with a hydrometer, the assumption is made that the solution is water and sucrose. Because sucrose has a very similar density, and refraction index when compared to other sugars this is a reasonable assumption.
Compound

IOR

Density

Water

1.333

0.9982

Sucrose

1.561*

1.587

Ethanol

1.361

0.789

Maltose

1.54
 
Glucose

1.54
 
isopropanol

1.3776

0.786

Methanol

1.3288

0.7918

*Value of a theoretical 100% solution by volume used for calculating the Brix value.
There are some common sources of error between the two devices. The sample must be well homogenized before a sample is taken. Stratification is especially common when working with liquid malt extracts.
There are some sources of error unique to the refractometer. Because a very small sample of wort is used any contaminate left on the prism will add error to the measurement. Cleaning the prism after use is important so that sugar residue does not skew then subsequent readings.
Some of the tools to preform this test can be found at the bottom of this page. The graphs below were produced using serial dilutions. The starting volume was 100ml of a 23 Plato wort. Each dilution was achieved by removing 10ml of wort and replacing it with 10ml of water.
Vertical Axis is the Percent Error
Horizontal Axis is °P
Vertical Axis is the Error Magnitude
Horizontal Axis is °P
The graph below was created with ten discreet dilutions of the same 23° Plato wort. Because of the smaller sample size that was prepared only the refractometer was used for measurements.
Vertical Axis is the Error Magnitude
Horizontal Axis is °P
If you're willing to perform some more tests with the refractometer, it would be nice to know how accurate it is to measure final gravities.
ReplyDeleteI know that a formula (involving the OG) has to be applied to the measures to compensate for the alcohol content. Testing the reliability of this formula (or formulae? are there more than one?) could be interesting.
By the way, I really appreciate all the tests you perform: they are really needed to vanquish all the mythology surrounding homebrewing
That's actually the direction I am headed. The next few posts start to focus on the chemistry and math behind these calculations.
ReplyDelete