Introduction and physical principle
Vacuum Decay is the most widely used deterministic method for testing the Container Closure Integrity.
This method has been in use for several decades, and it is offered in advanced versions both for online and bench-top applications.
The Vacuum Decay finds applications in the most different packaging types: rigid or flexible, larger size (liters) or smaller (ml), metal or glass or plastic made, it is not color dependant, can measure samples filled with liquid or powders, even with minimal or no gas in headspace, the gas in headspace, if any, can be air or nitrogen or carbon dioxide.
Typically, online applications need to test at higher speed, and the sensitivity is inferior to the bench-top instruments.
Physical principle
The physical principle is relatively simple: assuming that you have a container with some gas in headspace, and assuming that the gas in headspace is at atmospheric pressure, you apply vacuum to the outside; if there is a leak, the gas inside will start to flow towards the outside space.
The container under test is enclosed in a vacuum-tight chamber that exactly matches as much as possible outer size and shape of the tested container; the dead volume outside the container is very limited.
For the test execution, vacuum is applied for a very short period of time, just as needed to reach the desired preset value of residual pressure, then the chamber is isolated by closing valves and for the following short time you observe the vacuum level in the chamber.
In case that you are testing on a defective container, the air that escapes throughout the ‘leak’ to the dead volume inside the chamber causes an increase of observed pressure, i.e. a vacuum diminution, hence the name Vacuum Decay.
From a conceptual point, the physical law that essentially rules this measurement is very basic: General equation of perfect gas (Clapeyron equation):
PV = nRT
Where:
P is observed pressure of the chamber
V is volume of the available space in the chamber (dead volume outside the sample)
n is the amount of gas in the volume V, expressed here as number of moles
R is a constant called Universal Gas Constant
T is absolute temperature in degrees Kelvin
The meaningful data are measured in the volume that is inside the measurement chamber but outside the tested container (the so called ‘dead volume’).
R, T and V remain unchanged during the measurement. The amount of gas/vapour in the dead volume increases from the beginning to the end of the test because the gas or vapour escapes through the leak of the container and goes to the dead volume. In practice, n increases.
As a consequence, P increases, to maintain the equation.
As Pressure increases, we speak Vacuum Decay.
