I am not aware whether there exists a specific standard for this type of operation. The usual procedure is to take a sample from a system in operation from the return oil, either through a dedicated valve or from the oil reservoir. What you are after is repeatability and consistency in the way subsequent samples are taken. That means that when a sample is taken from the reservoir it should be taken at the same depth and location every time and under the same conditions. Depending on the sample taking depth and the distance to filters, strainers or baffles results may vary considerably. Thus the most important thing is: the same procedure every time to guarantee meaningful results.
See: https://www.machinerylubrication.com/Read/650/used... for a general overview of sampling methods.

romke,

Thank you for your help!