A hand held optical instrument called refractometer is used for this measurements. Additional advantage of this device is that they are applicable for measuring concentration in a wide variety of fluids. Since it is optical instrument, most important disadvantage is that the accuracy of measured results depends on fluid contamination, more contaminated fluids give less accurate results. Therefore, for a more precision measurements other techniques are recommended. The idea of this test is to destabilize cutting fluid emulsion and separate it in two layers one of which is water and other is present oily material.
Oily material is concentrated in the upper layer, while water is separated on the bottom.
Relation between volume of oily material and fluid volume represents the oil concentration. Detailed description of this testing method is covered with DIN 51368. This method is as accurate as refractometry, but it requires laboratory equipment sensitive to excess oil. If the contamination from machine lubrication system is presented into the sample this will have an influence on accuracy because oil volume will reflect both oil from the emulsion and tramp oil. In addition, this method can not be used for solutions.