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.
If the compressor does not have an oil plug — like many hermetic compressors — taking an oil sample can be difficult. However, there is a solution if you think ahead and prepare for the need.
When the system is open to the atmosphere you can configure an oil trap. This will allow you to take an oil sample anytime you desire. The oil trap is simply an inverted tee with a Schrader valve placed in the suction line at least 6 pipe diameters downstream of any risers. An oil collector is then attached to the tee. The collector can be configured from a ½-inch sight glass with a shut-off valve on each end.
On one end add a ¼-inch female fitting with a Schrader depressor to the shut-off valve and on the other end a ¼-inch cap to its shut-off valve. Attach the collector to the tee while the system is open to the atmosphere. Open the shut-off valve connected to the tee and close the shut-off valve with the capped end. This will allow the collector to be exposed to system while the system is being evacuated and charge with refrigerant.
As the system runs and the refrigerant returns to the compressor, some of the oil will fall to the bottom of the collector. Monitor the process and when enough oil has been trapped, isolate the collector from the system using the shut-off valve and remove it from the tee. Place a ¼-inch cap on the Schrader valve when done to prevent damage to its threads and prevent any refrigerant loss, if the Schrader core were to leak.
If another oil sample is needed, simply place the collector back on the inverted tee with both shut valves initially closed. On the collector’s capped end, remove the cap and connect a vacuum pump to fitting, open the shut-off valve, and pull a vacuum on the collector. When done, close the valve and turn off the vacuum pump, then open the valve to the system and collect some more oil.
This method does require some extra time and materials on a job, but it will allow you to take an oil sample from a system without an oil plug.
Publication date: 12/3/2012