Checking the acidity and/or moisture content of refrigerant oil is one of the methods used to test if a refrigeration system is clean and uncontaminated. Many times this test is used after cleaning up a system from a compressor motor burnout, but it can be used anytime a system is suspected of being contaminated.
On compressors with an oil plug, taking this oil sample is no problem. Simply front seat the suction service valve and pump the compressor’s crankcase down to about 5 psig. Then electrically shut down the compressor and carefully loosen the compressor’s oil cap and allow the oil to seep around the cap and into a clean oil container. Do not take off the cap and allow the compressor crankcase to drop to atmospheric pressure.