Evacuate System With Vacuum Pump

Vacuum pumps suitable for removing air and moisture from A/C systems are commercially available. A specification for system pump-down used here is 28 to 29 1/2 inches vacuum. This reading can be attained at or near sea level only. For each 1000 feet of altitude this operation is being performed, the reading will be 1 inch vacuum lower. As an example, at 5000 feet elevation, only 23---24 1/2 inch of vacuum can be obtained. The system must be completely discharged before it can be evacuated. Damage to vacuum pump may result if pressurized refrigerant is allowed to enter.
1. With hand gauges connected into system, remove cap from vacuum hose connector. Install hand gauge manifold center hose to vacuum pump connector. Open low side gauge manifold hand valve only.
2. Ensure low side gauge is calibrated correctly. It should be reading zero. If not, adjust calibration.
3. Evacuate system with the vacuum pump until the low pressure gauge reads at least 28 inches of vacuum. Continue evacuating system for an additional 15 minutes for routine system servicing or 20 to 30 minutes, if any parts have been replaced.
4. When system evacuation is complete, close low side gauge manifold hand valve, then turn vacuum pump "OFF."
5. Check ability of system to hold vacuum. Watch low side gauge to see that gauge does not rise at a faster rate than 1 inch vacuum every 4 to 5 minutes. if low side gauge rises at too rapid a rate, install partial charge and leak test. Evacuate system again.
6. If system holds vacuum, charge system with refrigerant.