pH Value and Bacterial Concentration of Cutting Fluid

The chemical balance of a cutting fluid is one of its most important properties. In particular, monitoring the pH level and bacterial concentration is essential for keeping the emulsion stable, protecting machines and tools, and maintaining a pleasant, odor-free work environment.

The importance of pH

Monitoring the system’s pH level is a key part of cutting-fluid maintenance. pH indicates how acidic or alkaline the fluid is, directly affecting its corrosion protection, lubricating properties, and microbial growth.

If the pH level drops, it often indicates that the concentration is too low or that microbial growth has begun in the fluid. Metabolic byproducts produced by bacteria and fungi acidify the fluid, causing its protective properties to deteriorate rapidly.

The pH level of most cutting fluids should be at least 9.0. In a new, correctly mixed emulsion, the pH is typically between 9.0 and 9.6, although the exact value varies by product.

The pH should be measured at least once a week, and the results should be recorded in a log along with the concentration measurements. If the pH falls below the recommended level, the emulsion should be analyzed and, if necessary, corrected by adding concentrate, raising the pH with a pH booster, or replacing the fluid.

How to measure pH

The pH can be measured with a pH meter or pH test strip.

• A pH test strip is a quick and easy option. Dip the strip into a clean sample and compare its color with the color chart provided on the package.
• A pH meter provides an accurate numerical reading and is particularly suitable when measurements are taken regularly or when the system contains several different emulsions.

When taking a measurement, it is important to collect the sample from clean emulsion rather than from the surface of the tank, where tramp oil or other contaminants may distort the result.

Measuring bacterial concentration

Bacteria are one of the main causes of cutting-fluid deterioration. They lower the pH, weaken corrosion protection, produce unpleasant odors, and can form slippery biofilms that clog pipes and filters.

Bacteria multiply rapidly if the system contains “dead spots,” stagnant fluid, or organic contamination to which they can attach. For this reason, bacterial concentration should be measured regularly—at least once a week.

The most common method is a contact-plate test, in which a special test strip is dipped into a fluid sample and incubated at room temperature. After two days, the number of spots that have developed on the strip indicates the level of bacterial activity.

If the bacterial concentration is high, the fluid’s properties deteriorate rapidly. In such cases, biocide treatment, pH adjustment, or, if necessary, thorough system cleaning and fluid replacement may be required.

Summary

Managing pH and bacterial concentration is essential to cutting-fluid maintenance.

• pH ≥ 9.0 ensures corrosion protection and emulsion stability.
• Regular bacterial monitoring prevents fluid acidification and unpleasant odors.

When these measurements are taken regularly and deviations are addressed promptly, the cutting fluid lasts longer and machining quality remains consistent. At the same time, unnecessary downtime and maintenance costs are reduced.