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:
- contamination (increase or decrease);
- oxidation (increase);
- thermal cracking (decrease).
Oil appearance (transparency, deposits) can provide information on a possible alteration, in particular contamination by another fluid or solid.
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.
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:
- chemical reaction (hydrolysis) on additives in the formula;
- oxidation catalyst;
- creation of an emulsion;
- corrosion of mechanical parts, etc.
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.)
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.
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.
This measurement, also called Conradson, represents carbonaceous residue of oil after combustion.
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:
- to take samples under standard conditions (Standard NF E 48-650);
- to use only a specific vial (Standard NFE48-654 and NF E 48-653) provided by the laboratory with the"PARTIC" product.
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.).
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 transition temperature between phases of calefaction and boiling (theta 1). Its increase may come from oxidation. In this case, there is an increase in the total acid number. Its decrease may be due to consumption of the tempering accelerator additive.
- The transition temperature between phases of boiling and convection (theta 2). Its decrease may be due to a presence of water.
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:
- Decrease in the bearing capacity of the oil film;
- increase the speed of oxidation by increasing oil-air contact on the surface;
- increase of compressibility of oil where a rise in temperature and worsening of oxidation occurs, disruption of the functioning of a hydraulic control;
- cavitation risk.
Anomalies of oil de-aeration can come from:
- contamination (silicone or other pollutants);
- ageing oil;
- a combination with another oil.
Foaming is characterised by the volume of foam and by its persistence. Foaming can cause:
- oil losses by overflow of a crankcase or a tarpaulin;
- it promotes oxidation by air-oil contact surface increase;
- a very strong foaming may result in a deactivation of the oil pump.
Excessive foaming may be a result of:
- Return to the tarpaulin above the oil level.
- From a volume of the oil load. Little consideration of flow and oil pressure.
- From the need to add some anti-foaming agents.
- From contamination.
- From an air inlet in the oil circuit.