How do you measure the “concentration” of a metalworking fluid? The “concentration” of a metalworking fluid is the measurement of specific parameters or components in the product. For a given product there can be several different measurements of “concentration” that provide different numbers. Many times it is important to test concentration by more that one concentration test method. Each concentration method measures a specific chemical, group of chemicals or fluid characteristic. Typical concentration measurements are:

Total Alkalinity (TA)
Total alkalinity is a pH based titration that uses 0.1N HCl to titrate to a pH of 4. The result is reported in percent concentration by alkalinity. This concentration method measures all alkaline type materials (which are chemicals that have a pH above 7 and up to 14) in the mix. Some of the typical alkaline materials in metalworking fluids are amines, biocides, caustic soda (NaOH), and corrosion inhibitors. Alkaline components tend to stabilize biological control and enhance ferrous corrosion control.  Excessive alkalinity will decrease mildness of the product.  Additives such as caustic soda (NaOH), amines, corrosion inhibitors, and certain microbicides may increase the alkalinity.

Total Alkalinity is used as the primary concentration measurement for some synthetic metalworking fluids and cleaners, but may also used as a secondary method for semisynthetic and soluble oil type fluids. Typically alkaline materials tend to “build up” in metalworking fluids, so over time the alkalinity concentration will increase. Contaminants such as floor cleaners, parts washer chemicals, or rust inhibitors may also contribute to higher alkalinity. A large increase in alkalinity would be an indication of adding new concentrate or a possible contaminant. A large decrease in alkalinity may be an indication of a system being diluted with water.

Some titration methods measure ”long-chain” fatty acid component of metalworking fluids. Fatty acids are a key component of semisynthetic and soluble oil type fluids. Titrations may be used to determine the fatty acid content, which represents lubrication, emulsification, and corrosion inhibiting properties of the fluid. High water hardness and certain metals will react with fatty acids to create “soap scum”. This results in depletion of the fatty acid, requiring additional concentrate makeup. 

Other titrations measure “short-chain” length fatty acids. This specialized titration is used on some synthetic fluids. This method helps monitor corrosion inhibiting and lubrication properties of the fluid. Again, hard water or metal soaps can deplete fatty acids in the product.  Typically, these titrations are done in laboratory environments and not on the plant floor.

The refractometer is a hand held optical instrument that is used to measure the refractive index of a liquid. The refractive index is the degree that light is bent when passing through a liquid. The various components of metalworking fluid mixes (oil, organic and inorganic chemicals, total dissolved solids, etc.) provide a unique number on the refractometer scale, (Brix scale) typically from 0 to 15 %. The operator places a few drops of metalworking fluid mix on the refractometer prism and then views the scale through the eyepiece. The boundary line from light to dark is read as the refractometer reading. The refractometer reading must be multiplied by the refractometer factor to provide the % mix concentration. For example, many soluble oil mixes have a refractometer factor of one, so if the refractometer scale reading is 5, then the actual mix concentration would be 5%. Most synthetic and semisynthetic fluids have refractometer factors higher than one. For a synthetic, if the refractometer factor is 2 and the scale reading is 3, then the mix concentration would be 6%. MC=F x SR, where MC is the mix concentration %, F is the refractometer factor, and SR is the refractometer scale reading.

It is important to calibrate the refractometer with water (typically the same water used for fluid makeup) prior to testing the metalworking fluid, to insure accuracy. Some refractometers are also temperature compensated for greater accuracy.

It is important to know that high levels of tramp oil and dirt will make reading the refractometer very difficult. In addition, certain metalworking fluid mixes do not provide clear readings, even as a new mix. The advantage of the refractometer is that it is a simple and quick measurement that requires no additional chemicals. The disadvantage is that contaminants may interfere with the reading and provide a reading greater than the true concentration (especially semisynthetics and soluble oils).  The refractometer works well for a “fresh charge”, but its accuracy deteriorates with time.

How often should concentration be measured?
For most individual machines, the concentration should be measured each day or a minimum of once per week. Concentrate and water are added as needed to adjust the fluid volume and concentration in the tank. It is recommended that anytime concentrate or water are added, the concentration should be measured about two hours later to insure that the fluid is within recommended parameters.

For central systems, the concentration is typically measured each day, ideally by using a titration method recommended for a particular metalworking fluid. For example, the Total Alkalinity method would be recommended for most synthetics and a fatty-acid method would be recommended for most semisynthetic and soluble oil products.  Most large suppliers will offer a service for routine samples from central systems to be sent to their lab to check all routine operating parameters, including concentration. In many cases there is more than one concentration test method reported on the central system sheet.  Biological, tramp oil, and dirt load can also be monitored at that time.

Why do the fluid concentration tests give different values?
For a metalworking fluid mix, several concentrations may be reported, such as Total Alkalinity, fatty acid and Refractometer. These concentrations may provide similar numbers when the fluid is a fresh mix. However, over time certain components may increase and others decrease. These components in the fluid may vary over time due to depletion or microbial activity. With proper makeup and control of a metalworking fluid, the components will stay “in balance” so that there will not be a preferential depletion of one component compared to others. Concentration measurement is subject to interferences (oils, cleaners, metals, fines, water hardness, etc.) and measurement error that leads to different readings.

What happens if the concentration is not within specifications?
For each application there is an ideal concentration range to provide the optimum fluid performance. Since each particular fluid system undergoes continuous change (carryoff and evaporation losses), the concentration measurement is a “moving target”. The goal is to maintain the fluid within a range of values. As a fluid ages, it may be necessary to change the control limits that are listed on the central system sheet.

Are there specific components in a metalworking fluid that require testing?
Your fluid supplier may recommend that certain fluid components need to be checked in addition to concentration. For example, specific tests for bactericide content, Extreme Pressure additives (such as chlorine or sulfur), and ions (such as chlorides and sulfates) are examples of special tests for central systems. A good fluid supplier will be able to track these items over time to predict performance properties or changes in the fluid.

Any additional questions or concerns?
You may want to confirm with your supplier that you are using the best concentration method for your particular metalworking fluid. It is also important to insure that the correct sample size and control factors are used for the titration method used.

Is there a single concentration check best for my metalworking fluid?
For most individual machines, a Refractometer test is all that’s needed for concentration control. For central systems, typically a chemical titration that measures alkalinity and fatty acids will be recommended as the primary test method used to control thefluid system. For central systems, several concentration tests are typically used to monitor the fluid.  A good fluid supplier will be able to monitor nearly every component in the fluid to some extent, but these tests are very expensive to do on a regular basis.

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