How to Detect Leaks: Vacuum and Pressure Leak Testing Explained
Updated: 2 days ago
Every sealed container has an internal pressure and an external pressure. A leak tester changes the pressure around the container, inside a test chamber, to reveal a faulty seal. This guide explains the principle, the two fundamental tests and the calibration of the pressure sensor.
The principle: two pressures, one equilibrium
Depending on the packaging process, the pressure ratio between the inside and the outside of a container takes one of three forms:
Atmospheric: inside = outside. No change in the atmosphere.
Positive pressure: inside > outside, for example carbonated beverages with added CO₂.
Negative pressure (partial vacuum): inside < outside, for example hot-filled 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. Air, and with it microorganisms, can then enter the product, which is a direct risk of contamination. For a carbonated drink, the loss of pressure also means a flat product.
The two fundamental tests
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, so that the liquid inside completely covers the sealing area: the closure or the lid. The pressure outside the container is then reduced below its internal pressure.
This pressure difference, higher inside than outside, creates a balancing force. If the seal is faulty, liquid leaks out. The test is performed dry: the leak becomes visible without opening or cutting the container.
For products containing gas (carbonated beverages, nitrogen-flushed containers), the test is combined with immersion in water inside the chamber. As the external pressure is reduced gradually, air bubbles show precisely the pressure threshold at which the container begins to leak.
A crucial difference: a leak lets visible liquid escape, while a micro-leak only lets gas through. The opening is too small for water molecules, but large enough for smaller gas molecules. A container without a visible leak is not necessarily 100% airtight.
The vacuum test also simulates transport to a higher altitude, where the air pressure is lower. Near sea level, the air pressure changes by approximately 100 millibars for every 1,000 meters of altitude. Practical example: a customer packaged yogurt in Acapulco (at sea level) and shipped it to Toluca (2,660 m), and the lids arrived detached. The vacuum chamber reproduced the air pressure at the destination and determined the exact pressure threshold at which the containers failed, so the process could be adjusted.
Positive pressure test
In this test, air is blown into the chamber to create an external pressure higher than the internal pressure of the container, which reverses the pressure gradient. The result is evaluated from the deformation of the container: a weak seal fails under the external force.
This test simulates the opposite transport situation: a container sealed at altitude, where the air pressure is lower, is exposed to a higher external pressure when it reaches its destination at sea level.
One chamber for both tests
A leak tester equipped for vacuum and pressure checks both negative and positive pressure with a single device. Many manufacturers only offer vacuum testing, but companies that ship between different altitudes or work with various container types need both. On the AT2E LT-ST and LT-PLC leak testers, a pressure model or a pressure and vacuum model (pressure up to 1000 mbar) is available as an option.
Calibration of the pressure sensor
The most important variable to calibrate is the pressure sensor, which monitors the applied vacuum or overpressure. Install a hermetically sealed reference manometer in the chamber and check that the sensor reads correctly at several pressure points. The manometer must be hermetically sealed: otherwise, the vacuum enters it and it can no longer detect the changes in external pressure. The recommended minimum calibration frequency is six months.
AT2E leak testers
LT-ST Leak Tester (standard model): vacuum adjustable up to -999 mbar, 2 stages of vacuum and holding time on a touch screen.
LT-PLC Leak Tester (PLC model): up to 16 user-defined test cycles with up to 4 steps each.
AMVT-1 Automatic Vacuum Leak Tester: automatic judgement of vials, bottles, caps and cans from the vacuum difference.
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AT2E is an ISO 9001 certified company (AFNOR Certification).
Request a quote: info@at2e.com
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