cycles can be determined for the particular engine, brand of oil and their use, all of which affect oil degradation. This test is carried out by stretching a filter paper tightly over the top of a convenient tin or other receptacle and held in place by means of an elastic band and then dropping a blob of the oil onto the filter paper and leaving it to spread. Over a short period of time, the oil slowly spreads out over the surface of the paper and drips through leaving any contamination behind as a series of coloured, concentric rings. A fresh sample of oil should be dropped onto the paper at the same time as the sample being tested for comparison purposes. The procedure is simple:
- Withdraw the engine or gearbox dipstick and allow a generous drop to form on the end. Deposit the oil drop on the test sheet by lightly touching the end of the dipstick on the medium.
it is usual to look for water with a special test. On site, a simple hot plate or crackle test may be used but in a lab the Karl Fischer test is applied. The crackle test only tells the investigator that water is present but not by how much. The Karl Fischer test gives the investigator the quantity in a given sample and so provides more insight into how badly the lubricant is suffering from too much water. It is good practice to carry out a crackle test on site before taking a sample to the lab for a full, in depth Karl Fischer test.
Lubricating Oil On-site Tests
The diagnostic or maintenance engineer should pull out the dipstick and check that there is sufficient oil in the sump by reading the oil level against the marks thereon. It should be noted that lubricating oil in a compression ignition engine is always black and, in a petrol engine, a dull yellowy-brown colour. He can then make some simple on site
80 | The Report • March 2020 • Issue 91
tests to check the quality of the lubricating oil in the sump. There are three of these tests:
• the feel test. • the drop test. • the crackle test.
The Feel Test: For this test, he should rub the sample on the end of the dipstick between thumb and forefinger. The oil should feel smooth to the touch. If there is anything wrong with the lubricating oil this will be instantly discovered as it then will feel gritty.
The Drop Test: If he feels this grittiness between his thumb and fore finger, the diagnostic or maintenance engineer should then see if the oil requires further analysis by making a simple drop or dispersions test. The drop test provides quick and accurate visual confirmation that it is time to change a lubricant. Further, if the results of such a test are compared at regular intervals, oil change
- Leave the test sheet in a horizontal position for a few minutes and then compare the sample to a sample of clean unused oil similarly treated.
- Backlighting samples enhances the sample results. This is done by simply holding the sheet up to a light or the sky.
The crackle Test: This test has been used as a reliable indicator of emulsified water, a go/no-go test. However, with practice and keen eyes and ears, the procedure can be advanced considerably and made more quantitative. Rather than simply listening for the crackle (which is due to scintillation), by adding visual observation of vapour bubbles, a rough indication of the amount of moisture present can be obtained. The revised method is referred to as the visual crackle. Success in using the procedure depends on practice with varying moisture concentrations in different common fluids. A laboratory syringe and a paint shaker can help create the experimental suspensions. While
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