# Testing Gas Permeability Using Differential Pressure Method and Equal Pressure Method

Film permeability test can be classified into three parts namely, permeability test for organic gas, permeability test for water vapour, and permeability test for common inorganic gas. The auto gas permeability test represents the film permeability test of nitrogen, oxygen and carbon dioxide, which are common inorganic gases. This article examines and compares the techniques of film auto gas permeability testing.

## Auto Gas Permeability Test Method

Auto gas permeability testers normally use either equal pressure method or differential pressure method. The test principles and test conditions of these methods differ considerably.

## Vacuum Testing Method

The vacuum process method represents the differential pressure method. In this approach, the sample package divides the permeation cavity into two separate parts (Figure 1). The two sides are evacuated, with one side filled with testing gas of 0.1Mpa while the other side maintains the vacuum state. Side A denotes the high-pressure side.

A pressure difference of 0.1Mpa is realized from the testing gas present between the two sides. The testing gas then diffuses via the film, enters into the low-pressure side, and creates a pressure change. Using the below formula, a high precision vacuum gauge can be used to determine gas transmission rate (GTR). The result unit is cm3/m2 • 24h • 0.1Mpa.

Figure 1. Illustrative diagram of vacuum testing method

T represents the thermodynamic temperature or test temperature; Vc is the volume of low-pressure side; A is the effective transmission area; R is the gas constant; and dp/dt represents the pressure difference on low- pressure side per unit time once the transmission becomes stable.

The vacuum process has many testing standards such as ISO 15105-1, ISO 2556, ASTM D 1434 (M method), JIS K 7126 (A method) and GB 1038.

In the vacuum process method, the principle of negative pressure is applied to form 0.1Mpa of pressure difference between the two sides; however, the positive pressure difference principle can also be employed. Nevertheless, volume method is commonly used as this method eliminates the need to evacuate the cavity and also maintains the vacuum state.

Hence, this method reduces the complexity involved in the permeation test and also reduces the challenges associated with instrument development. These factors consequently reduce instrument manufacturing costs when compared to the vacuum process method.

## Equal Pressure Method

At present, the only method utilized in equal pressure method of permeation testing is chemical sensor testing. This method is predominantly employed for oxygen permeation testing. In this testing principle, the package is utilized to split the permeation cavity into two separate airflow systems, with one side filled with testing gas (A) and the other filled with dry nitrogen gas (B) (Figure 2). Both the sides have equal pressure, but a difference is observed in oxygen partial pressure. Under the principle of oxygen concentration difference, oxygen permeates via the film and enters into the sensor through nitrogen carrier gas.

Figure 2. Illustrative diagram of sensor testing

The package’s oxygen gas transmission rate (O2 GTR ) can be easily determined from the oxygen quantity quantified by the sensor in nitrogen carrier gas. The uncalibrated oxygen transmission rate can be tested using this method.

## Differential Pressure Method vs. Equal Pressure Method

Test principles and test conditions are different for both equal pressure method and differential pressure method. The result units also differ from each other. In theory, there is no difference between the uncalibrated data achieved from the two approaches; however, the comparison is inevitable after the apparatus of equal pressure method is calibrated with a standard film and the rectified factor is used in the formal testing.

Differential pressure method is generally applied in research and testing institutions. This technique has excellent versatility for different testing gas. Likewise, the volume method exhibits a good flexibility for testing gas.

The sensor testing method is constantly growing. The instrument’s corrected factor is not often effective and has to be calibrated on a periodic basis. The sensor also needs to be replaced when it deteriorates over time. In standard conditions, Labthink’s TOY-C1 features an oxygen sensor that lasts for 12 to 30 months.

## Conclusion

In material and quality testing of flexible packages, gas permeability testing plays an important role. With advances in science and technology, instruments employing different testing methods are constantly improved and updated. With broad testing and inspecting applications, the testing accuracy is also being continuously enhanced. For instance, Labthink VAC-V1 comes with a temperature controlling function between 5°C and 50°C, and TOY-C1 oxygen permeability tester allows complete functional oxygen permeability testing for flexible packages.

This information has been sourced, reviewed and adapted from materials provided by Labthink International, Inc.

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