Measuring oil pressure on a compressor is essential for ensuring proper lubrication and preventing damage. The most common method involves determining the net oil pressure, which is the difference between the oil pump discharge pressure and the crankcase or suction pressure. This differential pressure is what effectively pushes oil through the lubrication system.
The Two-Gauge Method for Net Oil Pressure
Based on standard practice for many compressor systems:
Net oil pressure can be read by connecting two pressure gauges and comparing the readings. This comparison provides the crucial differential pressure.
- Gauge 1: Should be connected to the oil pump discharge. This measures the pressure at which the oil is leaving the pump.
- Gauge 2: Should be connected to the crankcase or the suction service valve. This measures the system pressure that the oil pump must overcome.
Calculating Net Oil Pressure
The net oil pressure is calculated by subtracting the crankcase/suction pressure from the oil pump discharge pressure:
Net Oil Pressure = Oil Pump Discharge Pressure - Crankcase/Suction Pressure
Why Measure Net Pressure?
While the absolute pressure at the oil pump is important, the net pressure is critical because:
- It indicates the effective force available to circulate oil throughout the compressor's moving parts.
- It accounts for the system suction pressure, which acts against the oil pump's effort.
- Low net oil pressure suggests potential problems like a worn oil pump, clogged oil filter, or low oil level, jeopardizing lubrication.
Oil Pressure Safety Controls
Compressors are typically equipped with safety switches that monitor this net oil pressure.
- If the differential (net oil pressure) is lower than a predetermined safe threshold (e.g., 0.63 Bar as mentioned in some systems), the oil pressure switch should trip, shutting down the compressor to prevent damage due to lack of lubrication.
Practical Steps for Measurement
When measuring oil pressure manually using gauges:
- Ensure the compressor is running under normal operating conditions.
- Locate the appropriate test ports for the oil pump discharge and the crankcase/suction side.
- Connect calibrated pressure gauges to these ports.
- Read the pressures on both gauges simultaneously.
- Calculate the net oil pressure by subtracting the suction-side reading from the pump-side reading.
- Compare the calculated net pressure to the compressor's specifications or the required differential pressure for safety switch activation.
Always consult the specific compressor manufacturer's service manual for exact test port locations, required pressures, and procedures.
