Regular testing and documentation of coolant is mandatory.
This is especially critical for water-mixed coolants, as they are particularly susceptible to microbial contamination.
Contaminated coolants pose a wide range of health risks. Respiratory illnesses, skin conditions, and eye irritation are the most common health issues associated with contaminated coolant.
Production issues and defective parts are often caused by dirty or degraded coolant.

To maintain the functionality of coolant, it is essential to monitor and control all operationally relevant parameters. These must be checked regularly to ensure the proper function of cooling. If deviations occur, appropriate corrective actions can be taken in time.

The most important testing parameters at a glance (details to follow):

Oil concentration of the coolant
pH value
Nitrite content of the coolant
Contamination level of the coolant

Oil Concentration of the Coolant

Coolant is a stable emulsion of water and mineral oil, with the water content being significantly higher than the oil content. The oil concentration is crucial for the coolant’s functionality and the specific properties it is designed to provide. Therefore, maintaining the correct oil ratio is essential.

Tramp oil that enters the system—such as oil from components, slideway oil, hydraulic oil, etc.—alters the overall oil content in the coolant. This tramp oil can severely impair the coolant’s performance.

Moreover, tramp oil often floats on the surface and can block oxygen from reaching the coolant, creating ideal conditions for anaerobic bacteria to thrive.

The oil concentration is measured using a refractometer, which is an optical measurement tool.

Tramp oil is commonly removed using an oil skimmer, provided the oil floats on the surface. If turbulence in the coolant tank prevents the oil from floating, a bypass oil separator must be used. This device calms the coolant flow so that the tramp oil can rise to the surface and be removed using a skimming process—such as an oil skimmer, or an overflow edge.

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Checking the pH Value of the Coolant

The pH value of coolant provides key information about the overall condition of the fluid and what processes are occurring within it.

A high pH value often indicates an excessive oil concentration. It may also signal the presence of alkaline foreign substances, such as cleaning agents used on machines or parts.

A low pH value can indicate a low oil concentration, but may also point to more serious issues such as bacterial contamination—often accompanied by unpleasant odors—or the introduction of acidic substances, including some cleaning agents. Acidic mixing water can also be a cause of a low pH level.

The pH value is typically measured using test strips, which change color to indicate the pH of the medium.

To correct an abnormal pH value, the underlying cause must first be identified. If the deviation is due to an incorrect oil concentration, it can usually be corrected by either adding oil or removing tramp oil.

Bacterial contamination, however, may require the use of bactericides or, in severe cases, a complete replacement of the coolant.

Measuring the Nitrite Content of the Coolant

The nitrite content of coolant, like the pH value, can be measured using test strips or test rods. Monitoring nitrite levels is essential, as excessive nitrite concentrations can pose health risks, which must be avoided under occupational safety regulations.

The nitrite content in coolant is often dictated by the nitrite level of the water used to mix the emulsion. Therefore, nitrite levels can only be reduced if the content in the mixing water is also lowered. You can contact your local water utility provider to find out the nitrite concentration in your water supply.

Reducing Nitrite Levels in Water-Mixed Coolants

If the nitrite concentration exceeds 20 mg per liter, appropriate measures must be taken to lower the nitrite level. When this threshold is surpassed, the cause of nitrite formation should also be identified and eliminated.

The following actions are suitable for reducing nitrite levels:

Replacing the coolant, either completely or partially

Adding inhibitors to prevent nitrosamine formation

Contaminants in Coolants

The introduction of contaminants into coolants can significantly impact the machining process they are used for. Typical signs of excessive particle contamination include reduced tool life, defective surface finishes on machined parts, and accelerated bacterial growth.

Contamination levels can be measured using membrane analysis. In this method, the coolant is drawn through a filter membrane. The particles remaining on the membrane can then be examined under a microscope to determine their size and composition. The number of particles on the membrane provides an indication of the overall contamination level in the system.

To counteract this, a filtration system is essential. In machining centers, return-line band filters are commonly used to easily remove particles from the coolant. Additionally, suction filters are installed to protect pumps.

However, these filters are not sufficient for thorough coolant filtration. It is recommended to supplement the setup with bypass filters to remove finer particles. Depending on the materials being processed, magnetic filters are an ideal solution—for example, when machining steel, cast iron, stainless steel, or carbide.
For non-magnetic materials such as aluminum, brass, or copper, bag or cartridge filters can be used to eliminate particle contamination from the system.

Disadvantages of contaminated coolants:

Reduced tool life

Defective surface finishes

Increased bacterial contamination

Regular Visual Inspection of the Coolant Is Crucial

A visual assessment often helps detect changes in the coolant that should be examined and addressed. Easily identifiable changes include:

Visual Inspection of the Coolant

Color changes

Foam formation

Residue buildup

Excessive oil film

If such signs appear, it is essential to investigate the cause and take corrective measures. Failure to do so may result in significant problems with the coolant and the entire system.

Moreover, bacterial contamination in coolants can lead to various health risks:

➡ Learn more at: https://friess-online.de/en/preventing-microbial-contamination-in-coolants/

Intervals for Coolant Inspections and Testing

Daily Checks

Daily checks before starting work include inspecting the appearance and smell of the coolant. Physical contaminants such as floating metal debris can often be detected visually. Microbial contamination caused by fungi or bacteria can be identified by an oily biofilm on the water surface. This contamination may also be noticed by a foul odor.

Weekly Measurements

The physical condition of the coolant is assessed weekly and documented in a test protocol (see test plan below).

Checking Coolant Concentration with a Refractometer

Calculating coolant concentration: Because testing coolant concentration using a refractometer is very simple, it can also be part of daily inspections to determine purity. A portable hand refractometer is the easiest tool for this measurement. The refractometer works by measuring the refractive index of light passing through the fluid, allowing the volume concentration of coolant to be determined easily.

Depending on the need, this concentration measurement can be performed daily but must be done at least weekly.

Intervals for Coolant Inspections and Testing

Daily Checks

Weekly Measurements

The physical condition of the coolant is assessed weekly and documented in a test protocol (see test plan below).

Detailed Coolant Testing Plan

The purity of the coolant should be recorded on a monitoring sheet. This provides an overview of the contamination levels of the coolants.

The following points should be recorded in the coolant testing list:

Date

Concentration

Nitrite content

Heavy metals

Free space for remarks to note any special observations

Additional points that should be noted on the monitoring sheet, which are not directly related to coolant testing but are important for clean documentation:

Person responsible for measurements

Machine/location of measurement (for larger systems)

Coolant product used

Refractometer factor

Tank size

Filling date

Documentation of Maintenance Measures

It is also useful to document maintenance measures, as the measured values provide insights into the efficiency of the technical devices used. Examples include oil skimmers or magnetic filters, which contribute to extending the service life of the coolant.

Testing Schedule for Water-Mixed Coolants

How Metalworking Fluids (Coolants) Are Tested in the UK

Standards and Guidelines:

Machining with metalworking fluids – COSHH (HSE, UK)

https://www.hse.gov.uk/coshh/essentials/direct-advice/metalworking-fluids.htm

Questions and Answers About Coolant Testing:

What is tested in the coolant?

For water-mixed coolants, weekly tests of pH value, nitrite content, and coolant concentration must be performed. Whether there are any abnormalities in appearance or smell can be checked daily with a smell test and a visual inspection.

What are the reasons why coolants must be changed?

The nitrogen compounds in coolants provide an ideal breeding ground for various bacteria. With prolonged use of coolants, an increase in bacterial colonization is to be expected. During coolant treatment stages, cleaning takes place which allows for an extended service life of the coolant. Bacteria-rich biofilms can be skimmed off using an oil skimmer. Nevertheless, coolants must be regularly replaced.

How is the current concentration measured?

The concentration of the coolant can be determined using a refractometer.

What to do if the coolant concentration does not meet the target value?

If the coolant concentration does not correspond to the specified target values, appropriate measures must be taken immediately. Besides coolant treatment and cleaning through various stages such as oil skimmers and magnetic filters, the following actions should be taken:

Too high coolant concentration:: Correct by adding coolant with very low concentration until the proper coolant concentration is reached.

Too low coolant concentration:: Correct by adding coolant with a high concentration to increase the concentration.

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