Checking cooling lubricant – When it is necessary and how it works

Cooling lubricant test plan according to DGUV Standard 109-003

There are specifications for testing water-mixed cooling lubricant that must be respected – and which help you to take all the necessary steps to ensure the quality of the cooling lubricant. DGUV Standard 109-003 (formerly BGR/GUV-R 143) and Technical Standard for Hazardous Substances (TRGS) 611 define specifications both for the initial test when mixing water-mixed cooling lubricants and for the subsequent quality tests. These specify certain test aspects and maintenance measures for the test plan, the results of which must be recorded in a measurement protocol.

The measurement protocol should at least include the aspects “perceptible changes in the coolant”, “pH value of the cooling lubricant”, “nitrite content”, “use concentration” and the “nitrate content of the mixing water”. How exactly you should proceed when checking these aspects is explained below.

How can I check the cooling lubricant?

pH value of the cooling lubricant

The pH value of a water-mixed cooling lubricant should be slightly alkaline and ideally lies between 8.5 and 9.3. Deviations from the target value are harmful to health in both directions: If the pH value is below the target value, this promotes the formation of carcinogenic nitrosamine. If it is above, skin irritation or damage can occur on contact. As a rule, the pH value decreases in the course of use; if it is too high, the concentration of cooling lubricant in the mixture is usually too high.

You can easily check the pH with pH test strips or indicator sticks. It is important to use test strips or sticks specially adapted to the alkaline range, as these allow more accurate measurements. It is also important to make sure that the test strips are dipped in the filtered coolant, i.e. in a clean emulsion, and not in floating oils or other films.

Alternatively, you can also check the pH value of the cooling lubricant with a pH measuring device. This must be calibrated before each measurement. And here, too, make sure to insert the electrodes in the clean cooling lubricant.

If the pH value of the cooling lubricant is too low, it can often be easily adjusted upwards by adding atmospheric oxygen. You can also regulate the pH value upwards by adding a pH-increasing additive or cooling lubricant concentrate. This is particularly advisable in the case of large deviations of more than 0.5 points. A downward correction can be achieved by reducing the concentration of water-miscible cooling lubricant, e.g. by partial replacement with freshly prepared water-miscible cooling lubricant.

Nitrite content of the cooling lubricant

The nitrite content of water-mixed cooling lubricants must not exceed 20 mg/l. Otherwise, the formation of nitrosamines is increased, which are carcinogenic; in addition, respiratory diseases may occur.

You can easily check the nitrite level using nitrite test sticks. If the value is high, the source of the nitrite must be identified as soon as possible and further input prevented. One source could be bacteria that break down nitrate and convert it to nitrite. You can regulate the nitrite value by adding freshly prepared water-mixed cooling lubricant or by adding an inhibitor.

Use concentration of the cooling lubricant

The concentration of the water-miscible cooling lubricant in the mixture should normally be 4-8 %. You can check this with the help of a hand refractometer. For this measurement, the water-mixed cooling lubricant must not be contaminated. Otherwise it must be filtered before the test.

The refractometer’s mode of operation makes use of the refraction of light in liquids. Depending on the concentration of the cooling lubricant in the test liquid, the light emitted by the refractometer is refracted at a different angle and captured on the light incidence plate. A line is created. Its intersection with the measuring scale of the instrument is your value – given in Brix (refractive index).

If the value is difficult to read despite pre-filtration, you can also dilute the test liquid with the same amount of water. Then you only have to multiply the value by 2. By the way, the refractometer can be calibrated simply with a drop of tap water (then the value
should be 0).

You can then calculate the concentration as follows: Measured value in Brix * individual refractometer factor of the cooling lubricant (see product information).

As an alternative measuring method, you can also carry out an acid titration. If both methods give very different results, the exact concentration can be determined in the laboratory.

If the value deviates from the standard, you can correct it by adding higher or lower concentration of the cooling lubricant.

Microbial contamination of the cooling lubricant

For testing microbial contamination (by germs, fungi, bacteria and yeasts) you do not need a measuring device in the first step. It can easily be checked during the daily inspection. Foaming, phase separation, floating biomass and biomass films as well as a noticeable, usually strong smell indicate a contamination.

In addition, if an increased nitrite concentration or a too low pH value is detected during the weekly check, you should also check the cooling lubricant for a germ contamination, which can be responsible for such deviations.

If you find the above-mentioned deviations in your lubricant, it is worth measuring the colonisation of the lubricant by microorganisms. Immersion sample carriers, also called dip sampler, are used for this. The organisms settle on them and thus make the contamination visible. It should be noted that a certain number of germs will always be present in water-mixed cooling lubricants. It is only necessary to keep the growth of the microorganisms within a certain range that allows compliance with the important limit values.