Vacuum method is the most representative of the differential method of a test method. The test principle is to use the sample to separate the permeate chamber into two separate spaces. First, both sides of the sample are evacuated and then filled to one side (A high pressure side) Absolute pressure) of the test gas, while the other side (B low side) is to maintain a vacuum state, the sample on both sides of the formation of 0.1MPa test gas pressure. The gas permeation of the test gas enters the low pressure side through the film and causes a change in the pressure on the low pressure side. The amount of gas permeation (GTR) of the test gas can be calculated using the formula by measuring the amount of pressure on the low pressure side with a high precision vacuum gauge. The relevant standards are ISO 2556, ISO 15105-1, ASTM D 1434 (M method), GB 1038, JIS K 7126 (A method) and the like. The gas permeation (GTR) provided by ISO 15105-1 is calculated as follows: Where: ...