Leak Check and Evacuation) Leak Testing by Vacuum

 

1. Introduction to Leak Testing by Vacuum

• Purpose: Provides a quick indication of a leak in the system.
• Method: If the system cannot pull or hold a deep vacuum, there is a leak.

2. Advantages and Disadvantages

• Advantages:

  • Quick indication of a leak.

• Disadvantages:

  • Doesn't Identify Leak Location: Only indicates that there is a leak somewhere in the system.
  • Potential to Be Fooled:
    • Inward Pressure: Atmospheric pressure outside and vacuum inside create inward pressure on joints. A pinhole leak may be hidden by the inward pressure, whereas positive pressure would reveal it.
    • Low Pressure: Atmospheric pressure is 14.7 PSI. In a vacuum, the differential pressure is only 14.7 PSI, which is much less than the pressure used in nitrogen testing (10 times higher).

3. Micron Gauge Use and Importance

• Purpose of Micron Gauge:

  • Measures very small increments as we get close to a perfect vacuum.
  • Provides accurate readings that standard gauges cannot.

• Conversion:

  • Inches of Mercury: 1 inch of mercury (Hg) = 25,400 microns.
  • Millimeters of Mercury: 1 millimeter of mercury (mmHg) = 1,000 microns.

• Standard Gauges:

  • Measure vacuum in increments that are too large to accurately assess near-perfect vacuums.
  • Typically, go from 0 to 30 inches Hg, but the smallest increment is 2 inches, which is 50,800 microns.

4. Using the Micron Gauge

• Target Vacuum Level:

  • Pull vacuum below 500 microns, then shut down.
  • Valve Off/Isolate: Isolate the vacuum pump from the system to see if the vacuum holds.

• Oil Quality:

  • Vacuum pump oil should be fresh and dehydrated.
  • Dirty or Hydrated Oil: Slows evacuation and extends process time.

• Reading the Micron Gauge:

  • Immediate Rise to 760,000 Microns: Indicates a large leak.
  • Stable Reading Between 1,500 and 50,000 Microns: Indicates possible moisture in the system.

5. Understanding Vacuum Levels

• Micron Gauge Accuracy:

  • Digital micron gauges typically read up to 25,000 microns.
  • Higher-level gauges are necessary for full diagnosis.

• Pressure and Boiling Points:

  • At 20,000 microns, water boils at 72°F.
  • At 50,000 microns, water boils at 100°F.
  • Rising to 760,000 microns indicates atmospheric pressure and a definite leak.

• Chart Representation:

  • Time vs. Microns:
    • 0 Microns: Ideal (impossible in the field).
    • 500 Microns: Recommended evacuation level.
    • 20,000 Microns: Boiling point of water at 72°F.
    • 25,000 Microns: Limit for many digital gauges.
    • 50,000 Microns: Boiling point of water at 100°F.
    • 760,000 Microns: Atmospheric pressure, indicating a large leak.

6. Identifying Leaks vs. Moisture

• Leak Identification:
  • A rising micron level that doesn't stabilize indicates a leak.
  • Stabilizing between 20,000 and 50,000 microns suggests moisture.