Oil analysis: what to sample, where and how often
Lab results are only as good as the sample. Choose the machines, the sampling point and the interval before you worry about the test list.
Why sample at all
Oil that has been in a machine reflects the condition of the machine. Wear particles and contaminants stay suspended in it, so a regular sample sets a baseline for normal wear, shows when abnormal wear or contamination starts and shows whether the oil is still fit to keep using. It can also justify extending an oil change safely.
Two goals for every sample
The sample must represent the oil in the system, and it must not pick up contamination during the sampling itself. A bad sample leads to a false alarm that stops a good machine, or worse, to a missed problem.
Pick machines by criticality
It is better to sample critical assets properly than to sample everything badly. Ask what happens if the machine fails, what the repair costs, whether it has failed before, whether there is a safety risk, and whether you are spending too much to keep it running.
| Situation | Typical interval |
|---|---|
| Extremely critical machine | Weekly |
| Critical machine | Monthly |
| Non-critical machine | Quarterly |
| History of failures or high contamination risk | Shorter than normal |
| New oil fill or bulk delivery | Sample after the change to confirm the right product |
Interval rule
The interval has to be shorter than the time between first detection and failure for the program to work as a predictive tool. If you only want condition based oil changes, set it against the oil service life.
Where to sample
Fit a sampling valve near the return line, gear set or bearing, with the tip about two inches (50 mm) clear of surfaces. Sample from a live, turbulent zone with the machine running at normal temperature. Never sample from a dead leg or from a drain valve at the bottom of the sump, where wear particles, dirt and water settle.
- Gearbox or bath lubricated machine: fixed valve with a short internal tube is best; drop tube is acceptable if the method is strictly the same each time.
- Hydraulic system: pressurised line upstream of the filter, ideally a low pressure point such as the case drain.
- Circulating system: after the pump and before the filter; the return line gives concentrated wear information.
- Keep the location and method identical from sample to sample, otherwise the trend is useless.