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"How long should you run a diesel generator before pulling an oil sample?"
As with any oil sampling procedures, it is recommended that the machine be currently running or recently shut down (less than 10-15 minutes), depending on how the sample is to be taken. Diesel generators are no different.
One of the most important times to take a sample on a diesel generator (or any machine) is during the first few hours of operation. The probability of failure is much higher at the onset of a machine's life due to the potential of infant mortality, which is a result of break-in wear and other possible but improbable faults during fabrication.
That said, soon after a diesel generator is initially run, a sample should be taken. At this time, and assuming fresh oil was just added, it is recommended that a sample be taken after the generator has been in operation for approximately one hour. This running time not only is early enough to provide the oil analyst an opportunity to catch any significant concerns before an issue worsens, but also long enough to anticipate potential break-in wear or other concerns present in the oil sample.
If the oil analysis is reported normal and without concern, then sampling intervals can increasingly be extended until a standardized sampling frequency is reached. This will be dependent on the operating conditions and manufacturer's recommendations. This frequency could be anywhere from a few weeks for continuously run, highly critical applications to up to a year for low critical applications with sporadic use. Environmental conditions will play a role in this decision as well.
For a used oil sample, the advice is nearly the same. Just be sure the sample has been taken when the generator is operating under normal conditions or immediately after it has been shut off while still at typical operating temperatures. In either case, the generator should have been running for at least an hour.
If the sample is taken from a sampling port, such as on the inlet of the filter housing, then sampling during running conditions is not a concern. If a sampling tube must be inserted near the machine's moving components, then you must sample soon after the generator has been shut off.
Try to have all the sampling equipment ready before shutting off the machine so you are able to pull the sample within 10 minutes of the machine being shut down. Remember, it is critical to obtain the sample in such a manner that it will be representative of the oil in the generator.