Abstract: This paper introduces the relevant content of calibration in ASTM D 3985. Through actual experimental comparison, it proves that there are differences in the test data of the isobaric method and the differential pressure method. Moreover, in practical applications, the efficiency of oxygen sensors will change with the extension of the service time. Therefore, it is necessary to calibrate the isobaric method equipment. Keywords: Calibration, differential pressure method, isobaric method, data comparison. Because the isobaric method differs from the differential pressure method, the basic method for testing material permeability, in its testing principle, it affects the comparison of test data between the two methods. In addition, the oxygen sensor used in this method will wear out with the extension of the service time. Therefore, the isobaric method needs to be calibrated to determine the working status of the oxygen sensor and to appropriately correct the permeability test data system through calibration coefficients. However, there is also the claim that "isobaric method equipment does not need calibration and the original test data can be used directly", often citing statements in ASTM D 3985 as proof. In fact, this citation is incorrect. This paper will analyze the relevant content of ASTM D 3985 in detail and discuss the necessity of isobaric permeability test calibration. 1. ASTM D 3985 calibration describes reference membrane calibration as one of the most common calibration methods using the isobaric method. It uses a standard substance traceable to a standard value determined by the differential pressure method. Testing is performed on the equipment to be calibrated, and the test results are compared with the standard value of the standard substance to determine whether the equipment is functioning correctly and whether the sensor is experiencing wear. However, according to ASTM D 3985, this standard substance should be SRM 1470 material provided by the American National Standards and Technology Institute (NIST). The claim that "isobaric testing equipment does not require calibration and raw test data can be used directly" is often cited as evidence, specifically from ASTM D 3985: "Limited statistical data on correlations with Test Method D 1434 methods are available; however, the oxygen transmission rate of a standard reference material as determined manometrically by NIST is in good agreement with the values ​​obtained in the coulometric interlaboratory test using material from the same manufacturing lot." The oxygen sensor used in this test method is a coulometric device that yields a linear output as predicted by Faraday's Law. In principle, four electrons are produced by the sensor for each molecule of oxygen that passes into it. Considering that the sensor is known to have a basic efficiency of 95 to 98%, it may be considered an "intrinsic" standard that does not require... Calibration. (Translation: The oxygen sensor used in this testing method is a coulombic device that outputs a linear signal according to Faraday's law. In principle, each oxygen molecule entering the sensor generates four electrons. The sensor is known to have a basic efficiency of 95%–98%, which can serve as an "inherent" criterion without requiring calibration.) It is important to note that the coulombic oxygen sensor operates according to Faraday's law, which simply means that one oxygen molecule corresponds to four electrons. Changing the number of electrons corresponding to one oxygen molecule is impossible, so the standard's description that "the sensor does not need calibration" is reasonable. However, it should not be interpreted as "if the sensor does not need calibration, the equipment does not need calibration." These are two different concepts and should not be confused. Therefore, ASTM D 3985 introduces a reference membrane to calibrate the equipment. 2. Necessity of Isobaric Calibration 2.1 Lack of Persuasiveness in Single Data Point Comparison ASTM D 3985 compares the isobaric test data and the differential pressure test data for SRM 1470 material, concluding that the data obtained by these two methods have good consistency. The standard specifies that the isobaric method test data is 59.36 cm³ (STP)/m²·d·atm with a standard deviation of 1.21 cm³ (STP)/m²·d·atm, while the NIST standard data for the differential pressure method is 63.8 cm³ (STP)/m²·d·atm with a standard deviation of 0.4 cm³ (STP)/m²·d·atm. However, this comparison only considers test data for one material. Therefore, it can be considered a single-point comparison of the data systems of the isobaric and differential pressure methods, and cannot fully describe the relationship between the test data of the two methods. This is because it is possible for the data systems of several test methods to be very close within a certain measurement range, but the test results may show certain differences once they leave this range. Langguang Laboratory has conducted extensive work on the comparison of differential pressure and isobaric method test data, and some experimental results are listed in Table 1. Table 1. Comparison of Test Data between Differential Pressure Method and Isobaric Method[/align] Note: The unit for differential pressure method data is cm³/m²·24h·0.1MPa; the unit for isobaric method data is ml/m²·day. As can be seen from the data in Table 1, the test data from both methods show good consistency in the increasing trend of oxygen permeability of the tested materials. However, for specific materials, there are differences in the test data obtained by the two methods. The existing test data comparisons generally show the following characteristics: when testing high-barrier materials, the isobaric method test data is lower; when testing medium-barrier materials, the data from both methods are similar; when testing low-barrier materials, there is a significant difference in the data obtained by the two methods, with the isobaric method test data being lower. The closest range between the two methods is between 30 and 70. It can be seen that the single-point comparison using SRM 1470 in ASTM D 3985 falls within the range where the data from the two methods are closest. However, after comparing data over a wider range, it is clear that there are indeed significant differences in the original test data systems of the two methods. 2.2 The efficiency of oxygen sensors is not fixed. The efficiency of coulombic oxygen sensors is not constant during use. This is evident from the efficiency range of 95%–98% given in the ASTM D3985 standard. Oxygen sensors based on Faraday's law are consumable components; in other words, they have a limited lifespan. During the gradual wear and tear, the sensor actually consumes the KOH electrolyte and the rare metals of the positive and negative electrodes. As the chemical reaction continues, the amounts of electrolyte and metal electrodes change, and the sensor's performance and response time gradually decrease. When the sensor reaches its maximum lifespan, it needs to be replaced. 2.3 The isobaric method requires calibration. As the above analysis shows, because the raw test data from the isobaric method differs significantly from that of the differential pressure method over a wider range, and because the efficiency of oxygen sensors is variable, isobaric method equipment must be calibrated. The use of reference diaphragms further illustrates this point. 3. Summary The calibration method provided by the isobaric method standard mainly involves reference diaphragm calibration. However, because many countries do not issue reference membranes recognized by their national standard material institutions, and the data stability of reference membranes is time-limited, reference membrane calibration faces certain obstacles in practical applications. As mentioned earlier, isobaric calibration is essential, thus a widely applicable isobaric calibration method is urgently needed. Using a gas with a known oxygen concentration for calibration can effectively solve the problems currently encountered in reference membrane calibration; a similar application is already present in the German standard DIN 53380-3. The operational method for calibration using standard gases will be detailed in subsequent papers.