"After pulling a sample from a piece of equipment, will the oil sample change within the sample bottle? If so, what is the longest amount of time we should allow the sample to sit?"
The moment an oil sample is taken from a machine, the sample and the oil in the reservoir begin to experience different environments. As you might imagine, at that point the properties of the two bodies begin to deviate from one another.
The greater the period between sampling and testing, the greater this deviation will be. Yes, the properties of the sample can change over time. However, the in-service oil is more likely to experience significant change.
Once taken, samples should be tested as soon as reasonably possible. To determine a time limit for shipping samples, consider the scope of the oil analysis program as it applies to the specific component. In particular, the sampling frequency should be considered.
If the sampling frequency is monthly, the samples should be tested within days. If the frequency is six months, immediate testing may not be so critical. Generally, one week is a reasonable maximum to allow samples to sit.
It is not uncommon for samples to sit for weeks or even months before they are sent for testing. After several weeks, the sample is no longer representative of the oil in the machine, and the value derived from testing is significantly diminished.
One of the common delays for shipping samples is the associated paperwork. This process can be overcome by working with the lab to set up preprinted sample labels and maintaining an accurate database of the machines and their oil analysis history. Once the system is set up properly, the labels may require no more than the unit ID number and the sample date.