Improving the detection efficiency of differential pressure gas permeability testing equipment

The differential pressure method is the basic method in gas permeability testing, and the vacuum method is the most widely used method in the differential pressure method. Since the key components required by this technology were relatively backward and had limited accuracy, which affected the detection accuracy and efficiency of the overall equipment, the differential pressure method equipment in the past has always had the disadvantages of "low test accuracy and poor detection efficiency". In recent years, with the advancement of testing technology, the test accuracy of the differential pressure method has been greatly improved, and has now reached 0.01 ml/m2·24h·0.1MPa, or even lower, which is comparable to the equal pressure method, and the detection efficiency has also been significantly improved. However, in the face of the popularity of barrier testing and the increasing weight of testing tasks, the detection efficiency of the current differential pressure method equipment is still not so satisfactory. In response to this, Labthink has timely launched the Labthink VAC-V2 differential pressure gas permeability tester.

Factors affecting detection efficiency The reason why

"low detection efficiency" has become a very common evaluation of differential pressure test equipment in the past is that the differential pressure method equipment has two main disadvantages: long test time and limited number of specimens. This means that in order to improve the detection efficiency, the test time must be shortened or the number of specimens must be increased.

1. Test time

The test time of the vacuum method, a representative method of the differential pressure method, includes the vacuuming time and the time to establish the permeation equilibrium. Experiments have shown that a too short vacuuming time will affect the test results. On the one hand, during the test, the "outflow" of the vacuum pipeline is counted as the test gas that permeates through the sample (the pressure sensor does not distinguish the type of gas), resulting in large test data and unstable test data; on the other hand, although the material has been pre-treated, there are still some impurities on the surface of the material and gas infiltrates it. Generally, the length of the vacuuming time will affect the degree of removal of impurities on the surface of the material and the gas that has infiltrated it. In other words, the more thorough the vacuuming, the better the removal effect and the more stable the test data. Long-term tests have shown that the volume of the test chamber is related to the time required for vacuuming. If the volume is reduced, the required vacuuming time will also be shortened. However, for vacuum method equipment, if the vacuuming time is too short, the system is not easy to maintain pressure. Moreover, the vacuuming time is also required in the standard. For example, the national standard GB 1038 requires that the vacuum system should continue to be vacuumed for more than 3 hours after reaching the vacuum degree required by the standard.

The test time of the sensor method, a representative method of the isobaric method, is divided into two parts: system purge balance time and penetration test time. Various test standards have very clear requirements for purge time. In the ASTM D 3985-05 standard, the purge time is even divided into two parts: dehumidification and purge zero point, so that the purge time is comparable to the vacuum time in the vacuum method. When developing oxygen sensor method detection equipment, Labthink has studied the reasonable time for system purge. It is believed that only after the purge carrier gas is introduced into the sensor and purges continuously for 24 hours, it can be determined that the oxygen content inside the system has reached an extremely low state, so as to ensure a higher test accuracy.

It can be seen that the preparation time before the start of penetration of the two methods (vacuuming time of the vacuum method and purge time of the sensor method) is equivalent. As for the time to establish the penetration balance, it mainly depends on the air permeability of the sample. Since environmental factors will affect the process during the establishment of the penetration balance, the length of the penetration balance time is significantly affected by environmental factors. The most obvious is the effect of temperature change on the permeation process. The more drastic the temperature fluctuation, the longer it takes to reach the permeation equilibrium, the worse the repeatability of the test data and the lower the accuracy. It should be noted that environmental changes will affect both the differential pressure method and the equal pressure method, and the degree of influence will vary depending on the test method. Regardless of the method, as long as the temperature and humidity control stability of the air permeability test equipment is improved, it will be beneficial to the stability of the test data.

In summary, the test time of the two test methods is equivalent. Some literatures unilaterally believe that the test time of the equal pressure method is short, which is proved to be inconsistent with the actual situation through practice.

2. Number of specimens

As mentioned above, it is very difficult to improve the test efficiency by simply shortening the test time. If as many specimens as possible can be tested in the same period of time, the test efficiency can be greatly improved. However, the air permeability test of materials is a microscopic test, and the sealing of the test system is the basis for evaluating the accuracy of the equipment and the reliability of the data. When the number of test specimens is not 1, the probability of leakage points in the entire test system is much greater than when there is only 1 specimen, which seriously affects the test data. For example, in the vacuum method, since the directly detected parameter is the pressure in the test chamber, and the pressure sensor cannot identify the gas, if there is a leak in the system, the infiltrated gas will be mixed with the test gas that permeates through the sample and cannot be distinguished by the sensor, resulting in an increase in the pressure value detected and the test failure. Therefore, although multi-cavity detection has a significant effect on improving detection efficiency, it is very difficult to achieve multi-cavity vacuum method detection.

Improvement of Labthink VAC-V2

From the above analysis, it can be seen that with the current vacuum technology, in order to achieve the vacuum conditions in the vacuum method test, even if the vacuum pumping time can be further shortened by reducing the volume of the test chamber, this time is difficult to be less than 5h. Therefore, in order to shorten the test time, we can only work hard to shorten the time to establish the permeation equilibrium. Although this time is objectively determined by the sample, the stability of the test environment temperature will also affect the length of this period, so this period can be shortened as much as possible by improving the stability of the ambient temperature.

Labthink VAC-V2 uses an external temperature control system. Through the principle of water circulation, it can effectively achieve high-precision and rapid temperature rise and fall in the test chamber, which significantly improves the temperature control efficiency. At the same time, the temperature control area is reduced so that the temperature control is limited to the test chamber, which greatly enhances the temperature uniformity in the test chamber and avoids the difference between the measured temperature and the temperature in the chamber. The temperature control accuracy can reach ±0.1℃.

Labthink VAC-V2 vacuum method equipment overcomes the technical difficulties in equipment structure. It can test 3 different samples at the same time and issue independent test data. Its detection efficiency is more than 3 times that of single-chamber detection equipment and is comparable to multi-chamber sensor method equipment. In addition, the test chamber sealing performance of VAC-V2 is excellent, and its re-vacuum system capacity has the potential to be improved, which can further shorten the vacuum time and improve the detection efficiency. (end)