How to detect leaks during seal integrity testing

Technical guide for the vacuum leak tester · AT2E France

The principle: two pressures, one equilibrium

Each container has an internal pressure and an external pressure. Depending on the packaging process, this relationship takes one of the following three forms:

Packaging type	Pressure relationship
Atmospheric	Internal = external. No change in atmosphere.
Positive pressure	Internal > external. Carbonated drinks with injected CO₂.
Negative pressure (partial vacuum)	Internal < external. Hot packing: when the product cools, the condensed vapors reduce the internal volume and create a vacuum.

When the integrity of the seal is compromised, pressures naturally seek to equalize according to the basic behavior of gases. If a container is not properly sealed, air—and with it, microorganisms—can penetrate the product, creating a direct risk of contamination. How many flat sodas would you drink? Seal integrity is not optional.

The two fundamental essays

1. Negative pressure (vacuum) test

This is the standard method for verifying the seal of liquid products. The container is placed upside down or on its side in the chamber, always ensuring that the internal liquid covers the sealing area—the cap or lid. The external pressure is then reduced below the internal pressure of the container.

By creating this pressure differential—higher inside than outside—a compensating force is induced. If the seal is faulty, the liquid will leak. The test is performed dry: the leak becomes visible without opening or cutting the container.

For products containing gas (carbonated beverages, nitrogen-filled containers), the test is combined with immersion in water inside the chamber. As the external pressure is gradually reduced, the air bubbles precisely indicate the pressure threshold at which the container begins to leak.

A critical distinction: a leak allows visible liquid to escape; a micro-leak only allows gas to pass through—the opening is too small for water molecules but permeable to smaller gas molecules. A container with no visible leakage is not necessarily 100% airtight.

2. Positive pressure test

In this test, air is injected into the chamber to generate an external pressure greater than the internal pressure of the container, thus reversing the differential. The results are evaluated by observing the deformation of the container: a weak seal will fail under the effect of the external force.

This test is particularly relevant for simulating distribution and transport conditions. A container sealed at altitude—where atmospheric pressure is lower—will be subjected to higher external pressure when it reaches destinations at sea level. For every 1,000 meters decrease in altitude, atmospheric pressure increases by approximately 100 millibars.

Real-life case: A client packaged yogurt in Acapulco (sea level) and distributed it to Toluca (2,660 m). The lids arrived detached. Using a vacuum chamber, the altitude conditions at the destination were simulated, and the exact pressure threshold at which the container would break was identified, allowing the process to be adjusted.

One chamber, two attempts

A full-specification vacuum leak tester can monitor both negative and positive pressure from a single unit. Many manufacturers only perform vacuum testing, but those distributing at different altitudes or working with various types of containers need both. The chamber covers this entire range.

Pressure sensor calibration

The critical variable to calibrate is the pressure sensor that monitors the applied vacuum or overpressure level. The recommended procedure is to place a hermetically sealed reference pressure gauge inside the chamber and verify that the sensor accurately records values at several pressure points. The pressure gauge must be hermetically sealed; otherwise, a vacuum will enter it and prevent accurate detection of external pressure variations. The minimum recommended calibration frequency is every six months.