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Q. I was very interested in your article about refractometers, but I wonder if you are a little out-of-date? We have been using a digital refractometer for about two years, and we find it more reliable than our older optical unit. This is particularly true with solutions we maintain at 2.0 Brix or lower. Furthermore, operator interpretation is no longer a factor.


A. In response to your first question, yes, I am a little out-of-date. A famous old saying by an unknown author says, “We get too soon old, and too late smart.” Amen to that. That column on refractometers spurred more interest than most, and it deserves to be updated because fluid management is critical in mass finishing operations. This pertains to coolant and washer fluid management as well.

Soon after that column was published, I received a pocket-sized, digital refractometer from a reader who also thought I need some updating. I have three optical refractometers, made by three different manufacturers. Two are 0–30 scale, and one is 0–10 scale. All are more expensive brands. You see one of those with my picture. These three instruments are pretty old, the oldest, with 0–30 scale, has been with me for more than 25 years and has no temperature compensation. The other two are about 10 years old, and both have temperature compensation. I promptly set up a small test procedure to compare these instruments?with the?digital unit.

Using a mass finishing compound that is 20 Brix at full strength, I prepared three solutions: Neat, 25%, and 2% in distilled water. I then recorded Brix readings for each solution using each of my three refractometers and also the digital one.
This was a useful exercise, resulting in these suggestions: 1. Always use a temperature compensating refractometer; 2. When Brix values are lower than 3, use either a digital or 0–10 scale optical refractometer; 3. When Brix values are less than 1.0, use the digital instrument; and, 4. When using a digital model, fill the sample bowl to the brim and replace the batteries at least twice a year.

Here is an example of how critical this can be. Suppose the neat solution is 20 Brix and you maintain your use solution between 2 and 3%. This is from 0.4 to 0.6 Brix. A very small reading error, whether instrumentation or operator judgment, can lead to either rust problems or excessive compound usage. I know an operator who just cannot read the scale closer than ±0.5 Brix. Clearly, the digital device will be a good choice in low-Brix applications. Note, however, that the newer, more expensive optical units use optical glass prisms, and they are easier to read and have brighter displays than older units.
The digital refractometer occasionally changed readings after the first attempt with the same solution in the holding cup. I learned that taking three readings in a row resulted in the measurement settling down, usually on the third attempt. That third reading was accurate, and what I would use in an actual situation.

One final caveat in all this is that the market is being flooded with very cheap refractometers. These may be acceptable in some applications, but they have no place in manufacturing operations where quality control is essential. If you have any doubt, ask for a free trial or a money -back guarantee. You get what you pay for in these precision instruments. The pocket digital I tested is priced at about $300.