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Particle Counting
The equipment types that are most relevant to particle counting are hydraulics, compressors, refrigeration compressors, turbines, automatic transmissions, natural gas engines, robotics, injection moulding machines and "filtered" bearing or gear systems. Particle counting is also idea for diesel fuel, solvents, water-based hydraulic fluids and lubricants.
New oils from these units can also benefit as it tells you how clean the oil is when first introduced to the system. Many people aren't aware of the fact that a new oil may actually have a higher particle count than used oil from the machine. This is because oil can pick up dirt during the manufacturing process, which gives you an indication of the quality control applied during the process. Unwanted contaminatnts can also be picked up during transport and distribution as new oils are transferred from container to container.
If a gear system is filtered then particle count data may be prove useful. However, if the gearbox is not filtered, particle counting won't provide as much useful information as would other tests such as PQ/FW (Particle Quantifier / Ferrous Wear) or DR (Direct Read) Ferrography. Diesel engine oil is black (due to the soot particles) and requires techniques other that laser particle counting. FW/PQ, DR Ferrography and Wear Debris Analysis are probably wiser choices. (WDA is a vital tool in failure and warranty issues).
Oil Cleanliness
Making it crystal clear...
Cleanliness is a term used to describe the relative quantity of contaminant particles present within a given system. A useful way to gauge the cleanliness of a fluid is by referring to a number of internationally-agreed standards. Every machine has its own optimum cleanliness level. This level will be based on a number of factors, the main ones being keeping the machine at optimum efficiency and the costs of maintaining cleanliness.
Cleanliness doesn't just refer to the particles that you can see - there are also some that are
invisible to the naked eye! Have a look at the comparison chart below.
The most common standard used to rate cleanliness is "ISO 4406". The table to the left shows the numbers the make up an ISO Code that are used to represent the number of contaminant particles present in 1ml of oil. The whole ISO code is a three-number code that summarises the cleanliness of your oil. This code relates specifically to the number of particles present that fall within certain size ranges. These ranges are: >4 µm, >6 µm and >14 µm in size.
1 µm = 0.000001 m (1 x 10-6 m) = 0.001 mm. The symbol '>' is used to mean 'greater than'.
The unit 'µm', pronounced 'micrometer' is commonly referred to as simply 'micron'.
What does the ISO code mean?
Let's use the example ISO code: 23/21/16
23 / 21 / 16
The first number in the code refers to all particles found that were greater than 4 microns in size but were less than 6 microns.
For example, suppose we analyse an oil and it gives us readings of...
72,064 particles > 4µm;
541 particles > 14µm
...we refer to the table to find the ranges in which these numbers fall and then we read the corresponding code. This results in an ISO Code of 23/21/16.
The Effects of Poor Oil Cleanliness
Poor oil cleanliness can:
• Cause damage to interacting components via abrasion;
• Dramatically reduce component life via erosion;
• Cause blockages to critical oil flow paths;
• Cause damage to servo or proportional valves;
• Increase quality problems;
• Increase customer dissatisfaction;
• Reduce market perception;
• Damage business opportunities;
• Raise costs.
What Failures Are Likely Due To Poor Oil Cleanliness?
Sudden or catastrophic - this is caused when even a small number of particles invade a critical space and create a torque forces large enough to cause a seizure or fracture which is irreversible, e.g. piston pump.
Intermittent - similar to that above but usually caused by smaller size structures. Intermittent will ultimately lead to sudden or catastrophic failures. Typical examples are temporarily blocked or unseated spool/poppet valves.
Degradation - This is typically characterised by flow erosion, abrasion, polishing and general wear.
- Oxidation;
- Sulphation;
- Nitration;
- Soot.
A typical infrared spectrum is shown to the left. From this, we can determine
the overall condition of the oil by assessing the following four quantities:
Oxidation
is the reaction of Oxygen from within the air with the oil. This reaction
usually occurs as a result of high running temperatures or the presence of water, acidic compounds or other contaminants.
An increase in oxidation also brings about an increase in the viscosity which can lead to further problems (see the viscosity
article).
Sulphation is usually a result of abnormal blow-by in diesel
crankcase engines. It can also occur due to the use of high-sulphur fuels. A high sulphation value indicates the
presence of sulphur oxides and sulphuric acids. Engine over-cooling and also constant short journeys can result
in an increase in sulphation. Sulphation correlates inversely with the TBN (see TAN & TBN section).
Nitration is similar to oxidation but concerns the reaction of Nitrogen from within the air with the oil.
The nitration process is linked to high running temperatures and can form such organic compounds as nitrate esters.
High nitration can lead to the formation of grease-like sludge deposits and varnishing.
Soot is produced as a result of the incomplete combustion of fuel. It is made up of black, impure particles
of carbon. High soot can occur due to an incorrect fuel-air ratio in the combustion chamber of an engine and also a
blocked air filter. Faulty injectors can also lead to an increase in soot. High soot levels can cause
sticking rings, piston damage and bore polishing.
ICP-AES stands for Inductively Coupled Plasma - Atomic Emission Spectroscopy. This technique uses an inductively coupled plasma flame (shown left) to produce excited atoms and ions that release electromagnetic radiation of different wavelengths. Each element of the periodic table has its own unique wavelength. The detector within the ICP detects both its wavelength and its intensity and uses these to calculate the relative amounts of each element present within a sample of oil.
The ICP reports the amount of each element present within the oil in ppm (parts per million).
An ICP works by using a complex system of prisms, mirrors and detectors. The following is a brief outline of how an ICP works out element concentrations (even down to an accuracy of a single ppm) in a sample of your oil in about 20 seconds!
- - The plasma emissions from the torch (in the form of white light) are focussed through a series of mirrors.
- - The positioning mirrors self-adjust in order to maximise the light sensitivity.
- - A collimating mirror directs the white light beam onto an Echelle grating.
- - The Echelle grating diffracts the monochromatic light into its constituent wavelengths.
- - From here, it travels through a prism which cross-disperses the light beam into its constituent wavelength orders.
- - The resultant beam from the prism forms a 2D image called an Echellogram which shows a cascade of wavelengths varying from 167 - 785 nm (nanometer). (1 nm = 1 x 10-9 m......one millionth of a millimeter).
- - Then, a camera mirror focuses this image onto an extremely sensitive detector chip which is cooled to -35 °C to improve accuracy.
- - Finally, the computer calculates the amounts of each element present within the sample based on the wavelength patterns on the detector chip.
