If you use a refractometer to measure the concentration of your finishing compounds and other fluids, do you know what you’re really measuring? Hint: The refractometer reading is NOT the percent concentration—not unless you are checking for sugar or table salt in water. Let me explain.
Have you ever noticed that a spoon, straw, or other straight device in a glass of ice tea appears to be bent? Next time you have a glass of iced tea, note the apparent angle of the bend in the straw before adding any sugar. Then, add a little sugar and note the change in the angle. Add more sugar, and it will appear to bend even more. The more sugar in the tea, the more the light is refracted, or bent, as it travels through the liquid.
A refractometer measures this deflection. Austrian scientist Adolf F. Brix (1798-1870) made a scale which correlates those measurements to the percent of sugar in the liquid. The values given on this scale are properly named degrees BRIX (°Bx). There are other scales for specific uses, but the BRIX scale has become the one of choice for measuring industrial fluids. It is important to know that, in the case of the iced tea, there is a straight-line relationship between the percentage of sugar and the degree of light refraction. If you dissolve 25 g of sugar in 75 g of distilled water, the solution will read 25°Bx; 5 g of sugar in 95 g of distilled water will read 5°Bx, and so on.
If you are measuring sugar in distilled water, the BRIX number and the percentage of sugar will always be the same. This is also true for some other solids, such as table salt. Other ingredients in the water will influence the reading. For example, a little soluble oil will increase the reading.
The BRIX scale can also be used to determine the concentration of most industrial fluids. Unless you are measuring sugar, however, the BRIX number is NOT the percentage concentration, or percentage of solids. The relationship between BRIX number and solids content will be a straight line, but not necessarily in a 1:1 ratio as with sugar. Because this fact is so widely misunderstood, the refractometer is perhaps the most misunderstood scientific instrument used in manufacturing.
That said, it is very easy to determine concentration of metalworking and cleaning fluids if you properly use a refractometer. Start by learning the BRIX number for a specific concentration. For example, prepare a 50:50 mix of the product you will be measuring. Do this in the make-up water you normally use rather than in distilled water to ensure that the base BRIX value of the water is included. Let’s suppose this mix reads 10°Bx. That means, because of the straight-line relationship of these readings, that the full strength product will have a reading of 20°Bx. Now, prepare a simple straight-line graph with percent on the vertical scale and °Bx from 0–20 on the horizontal scale. Draw a straight line between 0,0 and 100,20. Label the vertical scale “Percent Concentration” and label the horizontal scale “Refractometer Reading.”
This graph can be used to learn the percentage concentration of your working fluid simply by taking the BRIX reading from the refractometer and finding that value on the graph. Suppose, for example, your fluid has a refractometer reading of 2.0°Bx. Using the graph in the example, you will see that the concentration is 10%.
There are a few other things you must understand to get useful information from your refractometer. First, you must prepare a graph as described above for each fluid you will be testing with the refractometer. Typical water-based products will have a full--strength value of 15–25°Bx. Soluble oil products will have somewhat higher values, because oil has a very high refractive index. The point is, do not construe BRIX value as concentration, and do not use the same graph for different fluids.
The second thing to understand is that contaminants in the fluid will result in false high readings. This makes it desirable to get the sample you are testing from below the surface of a tank, and after any filtration. The best place for obtaining a sample in most operations is directly from the line going to the process. This is the compound feeder line on mass finishing machines, the washer nozzle on washing machines, and the fluid nozzle on cutting and grinding machines. In situations such as dip tanks, use a syringe that can get a sample from below the surface to avoid any floating oils, and sample above the bottom to avoid any stirred-up sediment.
The third consideration is to clean the stage on the refractometer with fresh water and a clean, soft cloth or lens tissue. Many factories have eyeglass cleaning stations or eyewash stations conveniently located on the production floor. This is often the easiest place for cleaning and servicing the refractometer. It is best to use a blotting action, rather than a wiping action, on the stage. Do not use a dirty shop cloth that can deposit oil or other soil on the stage.
As I said earlier, the refractometer may be the most misused, misunderstood instrument on the shop floor. That is because so many people think that the °Bx value is the same as percent concentration of their fluid. If you ask someone, “what percent concentration are you using?”, they may respond, “I always use 4%.” When asked how they measure that, they may show you their refractometer and tell you they keep the reading right on 4%. Often, it turns out that the full strength product may have, say 22°Bx, and the 4°Bx solution is actually 4/22 × 100, or 18.2%! Not only can this lead to problems such as residue from too much compound, but think of the cost when using perhaps four or five times as much as intended for the application.
A practical solution to this dilemma is to set the standard for solutions based on the refractometer reading. This avoids all discussion about the BRIX scale and how to make and interpret a graph. I recommend using R= the appropriate number, with R being the reading on the refractometer.
What do these things look like, and what should you buy? It happens that with this month’s column, PF is using new photographs of the writers. And, it happens that the photo is of me holding a hand-held refractometer. So, you can see what we both look like.
Prices range from about $100 for a hand-held to almost $10,000 for a laboratory grade bench model. Start by looking at units that are calibrated on the BRIX scale. They are available with different scale ranges such as 0–10, 0–30, and some read to more than 90. My suggestion is to have a 0–10 scale instrument for water based fluids, and a 0–30 for soluble oil fluids. You should be able to get what you need for under $300.
Q. I am in continuous disagreement with my vibratory finishing operator regarding the flow rate of compound solution going into our 10 ft3 bowl vibrator. Can you give me some guidelines and supporting arguments? H.B.
A. The quick answer for your machine is to keep the flow between 10 and 15 gal/hr. Studies have shown (using ceramic media) that cutting rate is very slow at flows from 0–0.25 gal/ft3/hr. After that, cutting rate increases rapidly until the flow rate reaches 1 gal/ft3/hr. It then levels off, perhaps climbing slightly, to .5 gph/ft3 flow rate, after which cutting increases very slowly to flow rate of about 6.0 gal/ft3/hr.
This study depended on good drainage. When the drains do not keep up, cut rate drops off rapidly, becoming once again nearly as poor as the 0.5 gal/ft3/hr performance.
This data makes a good case for keeping compound flow rate between 1 and 1.5 gal/ft3/hr. There is, however, more to the story. Many operators will report that the parts are not clean at these low flow rates. They are right, unless the compound is put into the machine in the right places. In your size machine, there is probably only one drain. In that case, best results will be obtained when putting the compound in with a single nozzle located 6–12 inches upstream from the drain. When machines have more than one drain, place a single nozzle the same distance upstream from each drain. Divide the total flow rate among the nozzles, favoring the last nozzle before the unloading point with a higher percentage of the total.
Compound recirculation and filtration, method of unloading, and other factors also must be considered. The suggested flow rate for your machine (10–15 gal/hr) is most cost-effective.