Leak Testing by Vacuum — Key Points

Vacuum Leak Testing

• Method: leak testing by vacuum. Provides a quick indication that a leak exists but does not locate the exact leak.
• If you cannot pull a deep vacuum or it won’t hold a deep vacuum, there’s a leak somewhere; other methods are needed to locate it.
• A key limitation: inward pressure under vacuum can hide leaks at joints (e.g., a poorly brazed fitting with a pinhole) whereas positive pressure would reveal them.
• Vacuum provides a small differential against atmospheric pressure: atmospheric pressure is $14.7 \text{ psi}$, so the pressure differential is limited when near vacuum.

Why a Micron Gauge is Needed

• Standard vacuum gauges read from 0 to 30 inches of mercury (inHg) with relatively large increments (e.g., two inches per step). This makes reading very close to a perfect vacuum impractical.
• A micron gauge measures much smaller units: a micron is one one-thousandth of a millimeter of mercury, i.e., $1 \mu\text{m} = 0.001 \text{ mm}$, and there are $25{,}400 \mu\text{m}$ per inch of mercury (1 inHg).
• To reliably assess near-perfect vacuum, we need a micron gauge to read in the range well below the coarse vacuum gauge’s resolution.

Using a Micron Gauge: Important Readings and Concepts

• Typical target: pull a vacuum below $500 \mu\text{m}$, then isolate (valve off) the vacuum pump from the system and watch the micron gauge.
• When the pump is running, the reading will be very low; after isolation, monitor how it changes.
• Vacuum pump oil can absorb moisture; if the oil is dirty or previously exposed to moisture, it slows evacuation. Use fresh dehydrated oil for each system.
• If, after isolation, the gauge rises to the atmospheric level $760{,}000 \mu\text{m}$, you have a leak.
• If the gauge rises to around $1{,}550{,}000 \mu\text{m}$ and then stops at some level between there and full vacuum, moisture is present and water vapor is boiling off.
• Many digital micron gauges read up to only about $25{,}000 \mu\text{m}$; you may need a higher-range gauge. Analog gauges can read to atmospheric pressure; example used in this material is the Avion $AV-760$ (digital) gauge.

Reading Scales and Chart (Conceptual)

• A chart (time vs. microns) helps visualize vacuum behavior over time. Key reference points (from an adaptation of an Apion chart):
  • Start at approximately 0 microns (for simplification; not achievable in practice).
  • $500 \mu\text{m}$: recommended evacuation level to maintain.
  • $20{,}000 \mu\text{m}$: at this level, water boils off; if the gauge rises to 20{,}000 and then stops, there is no leak—moisture boiling off.
  • $25{,}000 \mu\text{m}$: many gauges stop reading here.
  • $50{,}000 \mu\text{m}$: vapor pressure corresponds to about 100 degrees (environment-dependent).
  • If the gauge rises and continues to rise to atmospheric ($760{,}000 \mu\text{m}$) without leveling off, there is a leak.

Interpreting: Is It a Leak or Moisture?

• If the micron level rises and then plateaus around $20{,}000 \mu\text{m}$, moisture in the system is boiling off—no leak detected.
• If the level rises and does not plateau until it reaches atmospheric pressure ($760{,}000 \mu\text{m}$), a leak is present.
• In ambiguous cases, a light rod gauge can help distinguish moisture from a real leak.
• If a leak is confirmed, you’ll need to use other leak-detection methods to locate and repair it.

Practical Notes

• The Avion $AV-760$ digital micron gauge is used in this example; there are both analog gauges and other digital models with higher ranges.
• Always ensure pump oil is fresh/dehydrated to avoid artificially long evacuation times.
• Moisture presence and boiling off can mimic leakage in early readings; use the plateau behavior on the micron gauge to distinguish.

Quick Reference Values

• Atmospheric pressure: $14.7 \text{ psi}$
• Deep vacuum goal: < 500 \mu\text{m} before isolating the pump
• Leak indication if isolation reading: $760{,}000 \mu\text{m}$
• Moisture boiling off indication: plateau near $1{,}550{,}000 \mu\text{m}$ (and higher, up to atmospheric)
• Gauge reading limits: many digital gauges read up to $25{,}000 \mu\text{m}$; use higher-range or analog for higher readings

Summary

• Vacuum leak testing provides a quick yes/no on leak presence but cannot localize the leak.
• Use a micron gauge to measure near-vacuum levels; interpret readings to distinguish leaks from moisture.
• Proper oil condition and using higher-range gauges improve accuracy and reliability in diagnosis.