Determination methods for monitoring the cooling lubricant


concentration of emulsions/solutions


The adherence to a specific ingredient concentration has a considerable impact on the operating behaviour of cooling lubricants and is therefore, together with the determi- nation of the pH value, a key indicator.


The concentration of a water-mixed cooling lubricant may be possibly subject to significant fluctuations during use. The reason for this is the oil discharge through abrasion, swarf and work piece, oil entry from external oils as well as the vaporisation of water.


Therefore, in order that the concen- tration of coolant remains within the functional limits during use, regular monitoring of the coolant concentration is necessary.


1. Determination using hand refractometers


The determination of concentration using hand refractometers is available for practical use on-site. By using refrac- tometers, the determination of the ingredient concentration can be defined simply and quickly.


A few drops of the sample to be examined are placed between the prismatic glass. When held against the light, the indicated value to be read (refractive index in °Brix) is identified on a scale using a bright-dark boundary.


This reading value is then multiplied by the specific factor for the product (can be inferred from the technical datasheet), whereupon the relevant concentration is converted.


Indicated value [°Brix] X FP = cooling lubricant concentration [%]


The measurement procedure is based on the principle of total reflection of beams of light that appear beneath a certain angle on the liquid layer to be examined and/or measured and are refracted depending on the concentration. At the time of measuring, a bright-dark boundary line that can is easily readable appears on the scale of the instrument.


Note: A zero line calibration must be carried out with water before the sample measurement. Any adjustments must be carried out using a set screw at the front part of the hand refractometer. The hand refractometer is well suited for determining the concentration of solutions, since there is a linearity over a wide concentration range between the refractive index and coolant concen- tration. In contrast, the emulsions form a dispersed system where the size of the oil drops varies significantly during use.


As long as the emulsion is still in use, a very clear bright-dark boundary can be read from the refractometer.


To that end, 100 ml of the emulsion to be examined is placed in the acid flasks and after adding acid (salt acid) the separated, non-water-soluble part determined. In this case, the coolant concentration can also be determined using a factor. Of course, this method cannot be used for solutions.


This method may be applied if determi- nation using a refractometer were to result in misinterpretations. A problem with this determination is that external oils are also recorded which make it more difficult to determine the exact concentration.


The concentration is calculated from the reading value (°Brix) and the relevant concentrate factor or with the aid of a diagram.


During use, the emulsions are noticeably dispersed in a more rough-textured fashion. Through the ocular of the hand refractometer, a more or less diffusive image can be seen possibly resulting in a misinterpretation of the ‘true’ concen- tration.


Either a specially specified support emulsifier must be added or the cooling emulsion must be thinned.


2. Determining concentration using acid cleavage


Unlike determination with the hand refractometer, this is a laboratory method. In the case of this method (DIN 51368), the oil quantity and/or the proportion separable with acid is measured.


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LUBE MAGAZINE No .103 JUNE 2011


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