Technical Glossary

You will find a detailed explanation of terms commonly used in our sector.


This is the kinematic viscosity achieved with a standard temperature (40°C for industrial oils.) Viscosity is a fundamental quality that can vary due to: 

Visual appearance

Oil appearance (transparency, deposits) can provide information on a possible alteration, in particular contamination by another fluid or solid.

Insoluble content

This measure indicates the quantity of solid impurities (in weight %) retained by filtration on a millipore filter of 5 microns. These impurities can come from external solid contamination or wear metals and contribute to increasing the rate of wear of the material. It must be noted that these 'insoluble' particles; are suspended in the oil and that this measure may be totally different from the contents in elements determined by plasma emission spectrometry.

Content in elements

The method used to obtain the content of elements is the plasma emission spectrometry. This analysis allows us to quickly determine, in a single measurement, concentrations of the various chemical elements present in oil. The result is expressed as 'ppm'; or Parts Per Million, or in mg/kg.
1mg/kg = 1 ppm = 0.0001% or 10000 mg/kg = 10000 ppm = 1%.
Plasma spectrometry doses only chemical elements present in the form of particles smaller than 5 microns. These chemical elements can come either from additives present in oil, or from contamination or wear elements.

Water content

Different methods for the determination of water content exist and differ by units of expression of the result: either WT % or in ppm. The maximum allowable moisture in oil depends on its nature (Hydraulic, coolant, etc.), the criticality of the lubricated circuit and according to operating conditions. The presence of water can have different consequences on both the properties of oil, as on the lubricated equipment: 


In all cases, water contamination is an anomaly which must be remedied as soon as possible (decanting, filtration, centrifugation, partial or total purging, draining, etc.)

Acid index

This measurement provides the number of mg of potash (KOH) necessary to neutralise the acidic compounds present in the analysed oil. Its regular monitoring may give an indication of the oxidation of an oil charge: Total acid number increases with oxidation. However, you should know that some families of additives have a naturally high total acid number even without any degradation. For example, the Antisure of type ; 'Zinc Dithiophosphate' (hydraulic oils) or the Extreme-pressure-type Phosphorus-sulphur (Reducing oils). New oil with these families of additives will therefore have an elevated total acid number at the beginning of its service.

Flash point

This measurement indicates the temperature (in °C) to bring the oil sample so that its vapours will ignite momentarily (Issuance of a ‘Flash') in contact with an ignition source. This value characterises the volatility of oil and provides an indication of the maximum operating temperature of the oil in an open jug. A decrease of the flashpoint can highlight degradation by cracking, or contamination by a solvent.

Carbon Conradson

This measurement, also called Conradson, represents carbonaceous residue of oil after combustion.

Particle count

This analysis is intended to monitor the cleanliness of oils used in hydraulic circuits of high criticality. The result is expressed in the form of class of contamination representing the distribution of particles according to their size. In order for this measure to have a meaning, it is imperative:


It should be noted that this measurement of a non-negligible cost is irrelevant in the case of oils with a visual contamination (disorder, water, etc.).

Dielectric strength

The dielectric strength or insulation strength is the property that the insulating oil has to prevent the formation of an arc under the influence of an intense electric field. This important feature depends essentially on the cleanliness of oil. It is lowered by the presence of water and suspended solids. It allows the appropriateness of a drying and filtration treatment to be decided.


Drasticity of a hardening oil represents its ability to cool a metal mass previously heated to a high temperature. Drasticity is representative of the oil dipping power. The monitoring of the drasticity of oil in service allows ensuring that its action on the mechanical characteristics of the parts has not changed. We define drasticity from two characteristic temperatures: 



The time of de-areation characterises the ability of oil to free air previously dispersed. When air is introduced into an oil by mechanical agitation or by blowing, etc., it can form foam on the surface. Aeration of an oil can have disadvantages: 


Anomalies of oil de-aeration can come from:


Foaming is characterised by the volume of foam and by its persistence. Foaming can cause: 


Excessive foaming may be a result of: