In our previous blog post, we gave a quick run down on our recently purchased digital refractometer from AliExpress. In this post, we're going to outline a quick side by side comparison we did to compare the results when measuring a simple solution of dextrose and water using a standard floating hydrometer, and our new digital refractometer.
After mixing up the sugar solution in a glass pyrex jar (to simulate our wort), we poured a sample into the floating hydrometer tube and floated the hydrometer and took a reading. We then took a small sample from the hydrometer tube and used this on the digital refractometer. After each test, we returned the solution from the floating hydrometer into the pyrex jar, then added more dextrose (or in some cases added water to re-dilute) to get different and randomised levels of sugar in the solution. I didn't measure the amount of dextrose used, just simply "free poured" then stirred it in thoroughly.
Interestingly, we found we were getting more accurate results by adding 5-6 drops onto the digital refractometer which is contrary to what we found in our previous/initial testing of the device.
Our first comparison after adding only a small amount of dextrose, sees a reading of around 1.008 on the floating hydrometer and 1.005 on the digital refractometer. As you can see from the display of the digital refractometer, the temperature of the water was quite cold at a little over 15c. The floating hydrometer is calibrated at around 20c so using morebeer's hydrometer temperature correction calculator, the actual reading would be 1.007, meaning a difference of 2 gravity points between them.
Our next comparison shows a value of 1.028 on the floating hydrometer and 1.026 on the digital refractometer. Our sample temperature is now a more reliable 19c, so once again we have a difference of 2 gravity points.
For the next side by side test, we unfortunately didn't break the meniscus on the floating hydrometer sufficiently before taking our photo, so we'll go off the reading of 1.052. Our digital refractometer read 1.049 giving us a difference of 3 gravity points.
Next we revisited our previous reading where we failed to break the miniscus on the floating hydrometer properly (by diluting our 1.080 sample). This time around we had a reading of around 1.050 on the floating hydrometer and a surprisingly low 1.044 on the digital refractometer. A difference of 6 gravity points which is significant.
Next up we had 1.036 on the floating hydrometer and 1.034 on the digital refractometer. Once again, giving us a difference of 2 gravity points.
| Floating Hydrometer | Digital Refractometer | Difference |
|---|---|---|
| 1.007 | 1.005 | -0.002 |
| 1.028 | 1.026 | -0.002 |
| 1.052 | 1.049 | -0.003 |
| 1.080 | 1.080 | 0 |
| 1.050 | 1.044 | -0.006 |
| 1.036 | 1.034 | -0.002 |
| 1.054 | 1.053 | -0.001 |
The table outlined above shows the digital refractometer consistently had a lower gravity reading than the floating hydrometer. For the majority of tests, the reading was within 2 gravity points of the floating hydrometer. The advertisement for the digital refractometer states it's accurate to within +- 0.001 gravity points, and given floating hydrometers aren't necessarily 100% accurate either, this all seems reasonable.
We'll write off the test with a difference of 6 gravity points as an outlier - perhaps the digital refractometer screen wasn't cleaned properly, or the sample was otherwise contaminated or diluted which may have affected the reading.
Our initial concern was that perhaps in the range of 1.050, the digital refractometer wasn't accurate - but was more accurate at higher gravity readings such as 1.080, however our last test at 1.054/1.053 indicates this isn't the case.
What can we take away from this? We think it demonstrates that the readings are reasonably accurate, especially for homebrewers where having measurements 1 or 2 points off really aren't that big a deal. We're not calculating alcohol percentage for tax and labelling/packaging purposes after all so the implications of a slightly inaccurate reading are almost negligible. The convenience of much lower sample sizes is a major benefit in using the digital refractometer, but I think we'll end up taking measurements with both for the next few batches to see how they compare.
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