Vacuum Testing Basics: What, Why, and How

Why and How to Vacuum Test

Watertight enclosures vary in length, size, and material, and each configuration has specific rated pressure resistance limits. The pressure resistance depth refers to the maximum operational depth as defined by product design and verified through testing. However, any modification to the enclosure—particularly the addition or adjustment of components like cable penetrators—can impact the seal integrity.

Each time the watertight enclosure is opened, resealed, or modified, there is a risk of seal degradation or failure. While hydrostatic pressure testing is the definitive method to verify depth resistance, it is not always practical for routine checks. Frequent hydrotesting can also reduce the service life of the equipment by increasing the number of pressure cycles. Therefore, a non-destructive vacuum test is often used to quickly verify sealing performance without introducing stress to the unit.

Key inspection points before testing:

• Verify all O-rings are properly installed and inspect for wear, aging, deformation, or cracking.
• Check O-ring contact surfaces for scratches, debris, or contamination.
• Ensure the enclosure body and end cap are free from cracks or warping.
• Inspect cable penetrators for pulling or strain that may have compromised the potting compound (gel).
• Verify locking caps on Enclosure Vents, Vacuum Plugs, and Connectors are fully seated and tightened.
• Ensure all screws and threaded fittings are secure.

Visual inspection alone cannot guarantee a watertight seal. A quick, harmless vacuum test is necessary to verify the seal integrity. Vacuum testing is widely used because it is fast, simple, non-destructive, low-cost, and provides accurate indications of leaks or improper sealing. It is also commonly used for ongoing monitoring when the equipment is equipped with internal temperature or humidity sensors. Changes in these sensor values can trigger alarms, power disconnection, or return-to-surface operations if water ingress is suspected.

Note that vacuum testing only confirms whether a seal is properly seated and leak-free at the moment of testing. It does not certify the system’s ability to withstand external hydrostatic pressure.

In addition to seal verification, vacuuming provides other operational benefits:

• The resulting atmospheric pressure differential helps keep the end cap and flange securely in place, often eliminating the need for additional mechanical locking.
• Reduced internal air volume minimizes humidity and the potential for condensation, which can otherwise interfere with sensitive electronics.

Procedure: How to Perform a Vacuum Test Using a Vacuum Plug

1. Install a self-sealing vacuum bulkhead (Vacuum Plug) onto the end cap.
2. Ensure the manual vacuum pump is functioning properly by checking for any air leaks in the pump body.
3. Connect one end of the air hose to the manual vacuum pump, and the other end to the copper plug attached to the Vacuum Plug assembly.
4. Insert the copper plug into the bulkhead and begin pumping. A properly sealed plug will remain inserted without falling out due to vacuum suction.
5. Continue pumping until the vacuum gauge reads approximately 15 inHg (inches of mercury) (a standard test level for seal verification, not equivalent to rated depth pressure).
6. Hold the pressure steady and monitor the gauge for several minutes. A pressure drop of no more than 0.5 inHg indicates a good seal.
7. Once confirmed, remove the vacuum plug and screw on the sealing lock cap to close the port.

Recommended Products for Vacuum Testing:

Subsea M10 Self-Sealing Vent & Vacuum Plug
The M10 plug supports vacuum leak testing and long-term negative pressure retention inside subsea enclosures. Its built-in self-sealing valve allows it to maintain vacuum even after the pump is disconnected. Ideal for preventing condensation and preserving internal vacuum during storage or deployment. Includes bulkhead, copper plug, and lock cap. Requires a manual vacuum pump (not included).

Subsea M8 Vent & Vacuum Plug
The M8 plug provides a basic solution for manual venting and short-term vacuum leak testing. It works with the M8 Enclosure Vent (sold separately) and must be manually sealed after testing, as it does not self-seal. Best suited for quick checks, enclosure servicing, or disassembly. Requires a 5 mm hose vacuum pump (not included).

For applications requiring continuous vacuum retention, the Subsea M10 Self-Sealing Vent & Vacuum Plug is the recommended choice.

Image: Vacuum Testing