Checking Cutting Fluid Concentration

Maintaining the correct concentration of a cutting-fluid emulsion is essential for its performance and service life. The emulsion’s composition depends on a precise balance between water and oil-based concentrate, and even small deviations from this ratio can reduce its lubricating and cooling properties.

The concentration of the cutting-fluid emulsion must remain within the percentage range recommended by the manufacturer to ensure optimal performance.

If the concentration is too high, the emulsion may become sticky, increase the risk of allergic reactions upon skin contact, and raise fluid consumption. An overly concentrated emulsion can also transfer heat less effectively and be more prone to foaming.

If the concentration is too low, it can promote the growth of bacteria and fungi, cause corrosion on workpieces and internal machine surfaces, and provide inadequate lubrication. In addition, the service life of tools and the system may be significantly reduced.

Concentration measurement should be included in every machine shop’s regular maintenance program. Measurements should be taken at least twice a week, and preferably more often if the system is continuously topped up with water or new fluid.

Measuring Concentration with a Refractometer

The concentration of cutting fluid is measured with a refractometer, a precision instrument based on optical refraction. A refractometer measures how much the fluid bends light—the more oil or dissolved substances the fluid contains, the higher its refractive index and the higher the reading on the instrument.

Before measuring, the refractometer must be calibrated with the same water used to dilute the cutting fluid. To calibrate it, place clean water on the measuring prism and adjust the zero point on the scale using the fine-adjustment screw on the side of the instrument until the scale reads 0%. This ensures that the measurement reflects the actual concentration.

Refractometer Factor and Calculation

Because the optical properties of the oils and emulsifiers in different cutting fluids vary significantly, the refractometer reading alone does not indicate the actual concentration. Each cutting fluid has its own refractometer factor, which is listed on the product data sheet or package label.

This factor indicates the number by which the measured value must be multiplied to obtain the actual concentration percentage. The refractometer factor typically ranges from 1.0 to 2.0, although it may exceed 3 for some products.

Example:

If the refractometer reading is 5% and the cutting fluid’s refractometer factor is 1.8, the actual concentration is calculated as follows: 5 × 1.8 = 9%

This gives the actual concentration of the emulsion and indicates whether the system is at the correct concentration or whether water or concentrate needs to be added.

Interpreting Measurement Results and Practical Tips

After each measurement, the result should always be compared with the concentration range recommended by the manufacturer. If the emulsion is too dilute, add concentrate in small amounts and mix thoroughly. If the concentration is too high, dilute the system by adding a weaker emulsion premixed with water—never plain water—to maintain the stability of the mixture.

Regularly monitoring and documenting the concentration helps identify deviations at an early stage. When the emulsion concentration is kept under control, machining quality, tool life, and machine reliability remain consistent and optimal.