Checking the concentration of cutting fluid
The correct concentration of a cutting fluid emulsion is a key factor for its performance and service life. The composition of the emulsion is based on a precise balance between water and oil-based concentrate, and even small deviations from this ratio can impair its lubricating and cooling properties.
The concentration of the cutting fluid emulsion must stay within the percentage balance recommended by the manufacturer in order to function optimally.
If the concentration is too high, the emulsion becomes sticky, increases the risk of allergic reactions when it comes into contact with skin, and leads to higher fluid consumption. An overly strong emulsion can also hamper heat transfer and increase the likelihood of foaming.
If the concentration is too low, it often results in the growth of bacteria and fungi, corrosion on workpieces and machine surfaces, as well as poor lubricating ability. In addition, the service life of both tools and the system are significantly reduced.
Measuring the concentration should be included in every machine shop’s regular maintenance schedule and should be carried out at least twice a week, or more often if the system is frequently topped up with water or new fluid.
Measuring Concentration with a Refractometer
The concentration of cutting fluid is measured with a refractometer, a precision device based on optical refraction. A refractometer measures how much the fluid bends light—the more oil or soluble components in the fluid, the higher the refractive index, and the higher the reading on the device.
Before measuring, the refractometer should be calibrated with the same water used to dilute the cutting fluid. Calibration is performed by placing clean water on the measurement plate and adjusting the scale to zero using the fine adjustment screw on the side of the device, until the scale reads 0%. This ensures the measurement result matches the actual concentration.
Refractometer Factor and Calculating the Result
Since the optical properties of different cutting fluids’ oils and emulsifiers can vary significantly, the refractometer reading alone does not indicate the true concentration. Each cutting fluid has its own refractometer factor, which is specified on the product data sheet or the package label.
This factor tells you what number to multiply the measured value by to obtain the actual concentration percentage. The refractometer factor usually ranges from 1.0–2.0, but for some products, it can exceed 3.
Example:
If the refractometer reading is 5% and the cutting fluid’s refractometer factor is 1.8, the real concentration is calculated as follows: 5 × 1.8 = 9%
This gives the true concentration of the emulsion, letting you know whether the system is at the right concentration or if more water or concentrate is needed.
Interpreting Measurement Results and Practical Tips
After measurement, always compare the result to the concentration range recommended by the manufacturer. If the emulsion has become diluted, add concentrate in small batches and mix thoroughly. If the concentration is too high, dilute the system by adding a weaker premixed emulsion, never just water, in order to maintain the stability of the blend.
Regular monitoring and documentation of concentration help detect deviations early. Keeping the emulsion strength under control ensures consistent machining quality, longer tool life, and reliable machine performance.
