Vacuum Pump Procedure Quiz

Ten questions on evacuation, micron gauges and decay tests. Pick an answer to see whether you are right and why. Related reading: The Code: vacuum.

1. The main reasons to evacuate a system are to remove:

   1. Oil and refrigerant
   2. Air (non-condensables) and moisture
   3. Dust and debris
   4. Flux and solder

   Non-condensables raise head pressure, and moisture forms acids and can freeze at the metering device. A deep vacuum boils the moisture off and pulls the air out.

2. Vacuum depth is measured accurately with:

   1. The low-side compound gauge
   2. A micron gauge
   3. A thermometer
   4. A clamp meter

   A compound gauge cannot resolve such low pressures. A micron gauge reads in microns of mercury, and 1,000 microns is one millimeter of mercury.

3. Where should the micron gauge be connected for the most accurate reading?

   1. At the vacuum pump
   2. At the system, as far from the pump as practical
   3. On the refrigerant cylinder
   4. It does not matter

   At the pump, the gauge reads the pump's vacuum, not the system's. At the far end of the system it shows what the system has actually reached.

4. Removing the Schrader valve cores during evacuation:

   1. Slows the process down
   2. Greatly speeds evacuation by removing a major restriction
   3. Is not allowed
   4. Lets air into the system

   Valve cores and their depressors restrict flow. Pull them with a core removal tool that keeps the system sealed, and use short, large-diameter hoses.

5. A common target that many manufacturers specify for a deep vacuum is:

   1. 5,000 microns
   2. 2,000 microns
   3. 500 microns or lower
   4. 29.92 inches of mercury on a compound gauge

   Many installation instructions call for 500 microns or lower, followed by a decay test. Use the manufacturer's value for the equipment.

6. After the pump is isolated, the micron reading rises quickly and keeps climbing without leveling off. This usually indicates:

   1. A leak
   2. A normal result
   3. Too much oil
   4. A completely dry system

   A steady, continuous rise means air is getting in. Find the leak with a nitrogen pressure test.

7. After the pump is isolated, the reading rises and then levels off. This usually indicates:

   1. A leak
   2. Moisture or outgassing still in the system
   3. A perfect vacuum
   4. A failed micron gauge

   Moisture keeps boiling off until the pressure reaches its vapor pressure, then the rise stops. Keep evacuating until the reading holds.

8. Before pulling a vacuum on a new installation, the system should be:

   1. Pressure tested with dry nitrogen to confirm it is leak-free
   2. Charged with refrigerant
   3. Run for an hour
   4. Left open to the atmosphere

   A standing nitrogen pressure test finds leaks quickly and safely. Pulling a vacuum on a leaking system wastes time.

9. Why change vacuum pump oil often?

   1. Oil contaminated with moisture limits how deep the pump can pull
   2. Old oil changes the refrigerant type
   3. Only to make the pump quieter
   4. Manufacturers require it every ten years

   Moisture and contaminants in the oil reduce how deep the pump can pull. Fresh oil keeps it working properly.

10. After evacuation, the vacuum should be broken with:

    1. Outdoor air
    2. The system's refrigerant or dry nitrogen
    3. Compressed shop air
    4. Oxygen

    Air puts moisture and non-condensables straight back in. Never use oxygen in a refrigerant system, because it can react violently with oil.