How to detect leaks during leak testing

Technical Guide for the Vacuum Leak Tester · AT2E France

The principle: two pressures, one equilibrium

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

Packaging type	pressure ratio
Atmospheric	Inside = Outside. No change in the atmosphere.
Positive pressure	Internal > external. Carbonated beverages with added CO₂.
Negative pressure (partial vacuum)	Inside < Outside. Hot packaging: As the product cools, the condensed vapors reduce the internal volume and create a vacuum.

If the seal is compromised, the pressures equalize according to the natural behavior of gases. If a container is not properly sealed, air—and therefore microorganisms—can enter the product, posing a direct risk of contamination. How many flat sodas would you drink? A tight seal is essential.

The two fundamental essays

1. Negative pressure test (vacuum test)

This is the standard method for leak testing liquid products. The container is placed upside down or on its side in the chamber, ensuring that the liquid inside completely covers the sealing area – the closure or lid. The external pressure is then reduced below the internal pressure of the container.

This pressure difference – higher inside than outside – creates a balancing force. If the seal is faulty, liquid will leak out. 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 within the chamber. The gradual reduction of external pressure allows air bubbles to precisely indicate the pressure threshold at which the container begins to leak.

A crucial difference: A leak involves visible liquid escaping; a microleak only allows gas to pass through – the opening is too small for water molecules, but permeable to smaller gas molecules. A container without a visible leak is not necessarily 100% airtight.

2. Positive pressure test

In this test, air is blown into the chamber to create an external pressure that exceeds the internal pressure of the container, thus reversing the pressure gradient. The results are evaluated based on the deformation of the container: a weak seal will fail under the influence of the external force.

This test is particularly suitable for simulating distribution and transport conditions. A container sealed at altitude – where air pressure is lower – is exposed to higher external pressure upon reaching its destination at sea level. For every 1000 meters of altitude loss, the air pressure increases by approximately 100 millibars.

Practical example: A customer packaged yogurt in Acapulco (at sea level) and shipped 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 containers would break was determined. This allowed the process to be adjusted.

One chamber, two attempts

A fully specified vacuum leak tester can monitor both negative and positive pressure with a single device. Many manufacturers only offer vacuum testing, but those who operate at different altitudes or work with various container types require both. This chamber covers that entire range.

Calibration of the pressure sensor

The most important variable to calibrate is the pressure sensor, which monitors the applied vacuum or overpressure. It is recommended to install a hermetically sealed reference manometer in the chamber and verify that the sensor registers correct readings at several pressure points. The manometer must be hermetically sealed; otherwise, a vacuum will penetrate and prevent the accurate detection of external pressure changes. The recommended minimum calibration frequency is six months.