|
This article was first published in 2001.
|
Last week we looked at interpreting the results gained by analysing oil with ICP Spectroscopy, while this week we move onto Particle Quantification Index (PQI), Viscosity measurement, and testing for the presence of water.
Particle Quantification Index
In this test, each sample is passed over a sensor which measures the bulk magnetic content of the oil. As iron is the major wear element in virtually all components, the PQI is really a measure of how much iron is present in the sample, the amounts of other magnetic elements being negligible. The PQI makes no mention of size - the bigger the number, the more iron. What the PQI is communicating could be said to be grams of iron per litre of oil.
Iron and PQI relation-ship
| Situation |
ICP Iron
(ppm) |
PQI |
Inference |
Wear Profile |
| 1 |
Low |
Low |
Few wear particles |
Normal wear profile |
| 2 |
High |
Low to medium |
Lots of small particles, few or no large ones |
Accelerated wear
Dirt entry (abnormal) |
| 3 |
Low |
High |
Few small particles, many large ones |
Fatigue |
| 4 |
High |
High |
Lots of particles of all different sizes |
Serious wear likely, catastrophic failure possible |
The PQI, unlike the ICP (see last week's story), does not have particle-size limitations. As such it does not give us any indication of the sizes of the particle. Casting our minds back to the example mentioned in the previous article of a ball bearing ball in a sample: a solid ball bearing and the same one ground to powder should give the same PQI.
Used in conjunction with the ICP iron reading, the PQI is invaluable in making an estimate of the distribution of wear particle sizes. The table above illustrates this relationship. 'High', 'medium' and 'low' are relative concepts and should be interpreted in the context of other samples in the component's history. Situation 2 has various possible origins. It can be typical of a component experiencing accelerated, but not abnormal, wear, ie the component is working harder than normal. The classic illustration of this may be found in comparing the wear readings of differentials of identical trucks in different operations, for example short and long-haul operations. The truck hauling cane through muddy fields with its differentials locked can be expected to wear more than its counterpart cruising on a highway.
Differences in what can be considered 'normal wear' for each situation can be very major.
Viscosity
There are two types of viscosity:
- kinematic
- dynamic (or absolute)
Oil analysis concerns itself almost exclusively with the former. Kinematic viscosity is measured in centistokes (cSt) and is a measure of a fluid's resistance to flow or, more simply, its thickness. It must always be quoted at a stated temperature as a fluid's viscosity will change with temperature. At 40°C a 200 cSt oil is thicker than a 100 cSt one. As oil temperature increases, viscosity decreases.
The company for which the author works carries out a viscosity measurement at 40°C on every sample. A viscosity measurement at 100°C can also be carried out. The process is simple: a glass tube (the ends of which are kept open to the air) is immersed vertically in a bath at the required temperature; oil is introduced at the top and, as it flows down, it is brought up to the correct temperature. Its flow is then timed between two marks. This time measurement is converted to a viscosity.
There is another property of an oil related to its viscosity. This is the viscosity index (VI). It is known that as the temperature of an oil increases, its viscosity decreases. The VI of an oil tells us by how much it is going to thin out. A monograde oil has a lower VI than a multigrade, which tells us the monograde will tend to thin out more than the multigrade with increasing temperature. For example, a typical SAE 30 monograde and a typical SAE 15W/40 multi-grade can both have a viscosity at 40°C of 100 cSt. But at 100°C they have respective viscosities of 10 and 15. The way to determine the VI of an oil is to measure its viscosity at both 40 and 100°C.
Interpretation of changes in viscosity
| Component |
Viscosity change |
Cause |
Engine |
Increase |
Overheating. (May or may not be accompanied by oxidation)
Sludging (poor combustion or overextended use) |
| Decrease |
Fuel dilution (marine engines fired with heavy fuel oil)
Severe water contamination |
Other Components |
Increase |
Overheating
Grease contamination
Severe water contamination
General breakdown of the oil
Mixture of oils |
| Decrease |
Contamination by a volatile substance
Breakdown of VI improver additive (particularly noticeable in transmissions filled with a multi-grade)
General breakdown of the oil |
The table above illustrates some of the causes of changes of viscosity. It is important to note that concurrent conditions can mask the effects of changes of viscosity. Fuel dilution accompanied by overheating could leave the viscosity reading looking normal. Once again the importance of accurate submission information is underlined here. Perfectly good oil may be recommended for changing due to large differences between the viscosity of the oil described on the submission form and the viscosity of oil actually in the component. Furthermore, an engine described as having an SAE 30 or a SAE 15W/40, but actually running an SAE 40 or SAE 20W/50, may go unchecked for fuel dilution, as the decreased viscosity resulting from fuel dilution may compare favourably with the normal viscosity of the described oil. During diagnosis test labs will usually allow the viscosity of an oil in use to vary by approximately 30% either side of its starting viscosity before commenting.
Water
Water is one of the more common contaminants. It can be introduced into a system via internal coolant leaks, high-pressure hose cleaning procedures, or by condensation from the air as the system cools.
Water has several negative effects on the performance of oil:
- It causes rust, which in turn contaminates the oil.
- The load-bearing capacity of water is not as high as oil, so wear is promoted as water replaces the oil film.
- In engines it tends to flash to steam as the temperature rises rapidly in the bearings, effectively steam-cleaning them.
It is important that water contamination be kept to the absolute minimum. Seals and breathers should be regularly inspected and maintained. Pressurised cooling systems need to be pressure-tested from time to time to confirm their integrity. Low concentrations of water due to condensation may be evident in samples that were not taken at the operating temperature of the component. As the lab assumes every sample is taken correctly, taking a cold sample could lead to unnecessary oil changes.
Engine samples are screened for water using FTIR analysis and every other sample is screened for water using the crackle test. This test involves putting a drop of oil onto a steel surface which is maintained between the boiling points of water and oil. If the oil drop contains water, it spits and crackles, hence its name. The crackle test can detect water contamination of less than 0.1%. If a sample fails the crackle test, the actual water content is measured.
The water test involves mixing calcium hydride with the oil. This reaction generates hydrogen gas, the volume of which is measured and converted to a percentage water content of the oil. Once again, tentative limits for water contamination are used (see the table below) although these will vary in situations of abnormal or unusual usage. Water should not be relied upon as an indication of an internal coolant leak, particularly in engines. It tends to evaporate off at normal operating temperatures.
Water limits
| Component |
Limit (%) |
| Engine |
0.0 |
| Drivetrain |
1.0 |
| Transmission |
0.5 |
* Ashley Mayer is technical consultant for the Wearcheck Division of Set Point Technology. WearCheck Africa is the leading oil analysis company in Africa serving the earthmoving, mining, freight, passenger transport, rail, aircraft and marine industries, as well as the industrial sector. http://www.wearcheck.co.za
|