Metalworking / 4 min
Concentration Monitoring in Metalworking Fluids
Water-based metalworking fluids (coolants) used in machining operations are responsible for cooling the workpiece, lubricating the cutting tool and clearing away chips. However, the performance of these fluids depends heavily on correct bath management — and concentration monitoring in particular.
Published by: Rotek Technical Content Team
Content is published based on manufacturer technical documentation and equipment requirements as the basis for final selection. This is general guidance and does not replace equipment-specific manufacturer approval.
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- - Why Concentration Matters
- - Using and Reading a Refractometer
- - pH and Bacteria Control (Bath Life)
- - Water Hardness and System Contamination (Tramp Oil)
Why Concentration Matters
Concentration (the ratio of oil to water) must stay within the manufacturer's recommended range. Too low a concentration (below 2-3%) causes rust on the workpiece, shortens tool life and encourages bacterial growth. Too high a concentration (above 10%) reduces cooling performance, contaminates the machine, raises cost and can cause skin problems (dermatitis) for the operator.
Using and Reading a Refractometer
The most practical measurement tool at the machine is an optical refractometer. It measures the fluid's refractive index via a Brix reading. The reading is multiplied by that fluid's specific 'refractometer factor' (typically 1.0 to 2.5) to calculate the actual concentration percentage (%).
pH and Bacteria Control (Bath Life)
A healthy coolant emulsion typically has a pH between 8.5 and 9.5. A pH dropping below 8.0 indicates the emulsion is shifting toward acidity and that bacteria (usually foul-smelling anaerobic bacteria) are starting to multiply in the system.
Water Hardness and System Contamination (Tramp Oil)
Way oils or hydraulic oils leaking into the system (tramp oil) form an anaerobic layer on top of the coolant, accelerating bacterial growth (the 'Monday morning smell'). In addition, the hardness of the make-up water — due to calcium and magnesium minerals — can break down the emulsifiers in the coolant, causing saponification.
Frequently Asked Questions
Is it correct to add only water or only oil directly when topping up?
No. Neat oil or water alone should never be poured directly into the machine's sump. The make-up mixture (pre-mix) should be prepared separately, in a container outside the machine or with an automatic mixer (always adding oil to water), and then added to the sump.
Why does coolant smell bad?
The bad smell results from a layer of waste oil (tramp oil) accumulating on the bath's surface cutting off aeration, allowing anaerobic bacteria to grow and release sulfur gas. The fix is to skim off the surface waste oil and correct the pH.
How do I prevent coolant from causing rust?
To prevent rust, you should keep the concentration regularly at the manufacturer's recommended optimum level (typically 5-7%) and ensure there is no bacterial degradation in the machine.
What is a refractometer factor?
Different fluids have different optical refraction properties. This factor, which varies by the oil's density, is listed in the TDS. If your refractometer reads 3 and the oil's factor is 1.5, your actual concentration is 4.5%.