Misunderstandings between the hottest differential

2022-08-08
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Misunderstanding between differential pressure method and isobaric method in test efficiency

Abstract: by analyzing the test time of permeability test, this paper shows that the purification time of the system should be included in the range of test time, and only the extraction of differential pressure method meets the standard: the empty time needs to be included in the test time, and it is very wrong not to include the purging time of isobaric method in the test time. At the same time, this paper also introduces the factors that affect the osmotic balance

key words: permeability, detection efficiency, system purification

for the test efficiency of differential pressure method and isobaric method, some people think that the test efficiency of isobaric method is higher than that of differential pressure method. This understanding is wrong. Its formation is related to the wrong understanding of the test time and test efficiency of permeability test. This paper will analyze the testing efficiency of permeability testing methods according to the characteristics of material permeability testing, and analyze the testing efficiency of differential pressure method and isobaric method from the current testing technology

1 . How to calculate the efficiency of permeability test

to evaluate the test efficiency of a test method, it is necessary to clarify the test time of the test method. Different from other physical tests, the air permeability test requires not only the state adjustment of the test sample, but also a period of adjustment time for the air permeability test equipment, which includes not only the preheating time of the equipment, but also the purification time of the system. No matter what method is used, the pollution of the gas in the test chamber of the permeability testing equipment and the gas pipeline caused by the clamping of the sample cannot be avoided, and the gas impurities (non test gas) will interfere with the penetration process of the test gas to the sample, thereby affecting the test results. Therefore, before the gas penetration process begins, the impurity gas in the test chamber and the gas pipeline must be purified

what is the relationship between the purification time of the system and the test time? In fact, this is the root of the misunderstanding of the efficiency of permeability test, that is, there is no clear definition of the test time of permeability test, which makes it very random whether the purification time is included in the test time. For example, the vacuum pumping time of differential pressure method is included, while the purging time of isobaric method is not included. The test time of permeability test should include the purification time of the system and the establishment time of osmotic balance. It is not allowed to evaluate the test efficiency only by adding and comparing the test time of isobaric method with the test time of differential pressure method and vacuum pumping time

1.1 purification time

the purification of the system adopts different methods for different test methods, such as vacuumizing the test chamber and related gas circuit in the differential pressure method, and purging the test chamber and gas pipeline with large flow carrier gas in the isobaric method. Therefore, the purification time is also different for different test methods. For the differential pressure method, the purification time of the system is the vacuum pumping time before the beginning of the infiltration process, while for the isobaric method, it is the purging time of using carrier gas (generally nitrogen) to purge the entire test chamber and gas path before the test gas is introduced

the long-term test shows that the volume of the pressure difference method test chamber is related to the time required for vacuumizing. If the volume is reduced, the time required for vacuumizing will also be shortened. The vacuum pumping time is also required in the standard. The national standard gb/t 1038 requires that the vacuum system should be continuously vacuumized for more than 3 hours after reaching the vacuum degree required by the standard. The requirements for purging time in the isobaric test standard are very clear. The purging time is related to factors such as the test sample and the thickness of the sample. Moreover, the standard ASTM D divides the purging time into two parts: dehumidification and purging zero point. It has been explained in many places that this "purging time" will never be very short, and the time generally required is comparable to the vacuum pumping time in the differential pressure method. Labthink blue light studied the reasonable time of system purging when developing isobaric detection equipment. The test proved that after introducing the purged carrier gas into the sensor for continuous purging for 24 hours, it can be considered that the oxygen content in the system has reached a very low state. In this way, the purging can get ideal results and accurate test results. In this way, for most polymer materials, the system purification time required for the test is longer than the time for penetration to reach equilibrium

in order to improve the efficiency of the test, it is proposed to shorten the purification time. However, what impact will the gas pollution caused by shortening the purification time have on the test? LAN Guang laboratory once set up a test subject for the impact of the length of purification time on the test. For the isobaric method, we use to shorten the purging time, and for the differential pressure method, we use the shrinkage method to follow the development trend of the household appliance industry to shorten the vacuum time, and then compare the test results with the results obtained according to the standard test steps. It can be seen from the comparison of test data that as for the differential pressure method, with the shortening of vacuum pumping time, the establishment time of osmotic equilibrium has no obvious change, and the test data shows an increasing trend, which has a greater impact on the test of high barrier samples and a smaller impact on the test of medium and low barrier samples; For isobaric method, the reduction of purging time leads to the extension of the establishment time of osmotic equilibrium, and the repeatability of test data is also significantly reduced. The reasons for these conditions are analyzed as follows: for the differential pressure method, due to the influence of vacuum pumping time, the impurity gas in the test chamber and gas pipeline does not achieve the ideal purification effect, and the impurity gas interferes with the penetration process of the test gas, resulting in large test data and poor data stability; For the isobaric method, shortening the purging time will cause the confirmation error of the test zero point, and the purification effect of the impurity gas is not ideal. The impurity gas remaining in the system will affect the infiltration process, resulting in a significant decline in the accuracy of the test. In addition, due to the shortening of purging time, some impurity gases still remain in the system and can only be carried away by the subsequent test gas, so the establishment time of osmotic balance will be significantly increased. It can be seen that shortening the purging time will not shorten the whole test time

1.2 establishment time of osmotic equilibrium

establishment time of osmotic equilibrium, that is, the test time used to detect the permeability of the sample, that is, the time required from the time when the penetration has not occurred to the time when the gas is stable, and the length of this time mainly depends on the permeability of the sample. Using differential pressure method and isobaric method to detect the same kind of materials, the required osmotic equilibrium establishment time is very close, and there is no obvious difference. Generally, the following rules are followed: it takes about 2 hours for low barrier materials, 2 hours to 6 hours for medium barrier materials, and more than 6 hours for high barrier materials

in the actual test, it is necessary to determine whether the osmotic balance is reached according to certain judgment conditions, so as to issue the test data. However, the length of the establishment time of the osmotic balance is obviously affected by environmental factors, especially the influence of temperature changes on the osmotic process. The more intense the temperature fluctuation is, the longer the time it takes to reach the osmotic balance is, and accordingly, the worse the repeatability and accuracy of the test data are. This environmental change will have an impact on all permeability test methods, and the degree of impact will not vary with different test methods. Therefore, the equipment with temperature and humidity control function is superior to the equipment without temperature and humidity control in terms of test data repeatability and test efficiency

2 . Summary

whether in terms of test methods or specific equipment, the detection efficiency of differential pressure method is equal to that of isobaric method, which can meet the actual detection needs of detection tasks. In some literatures, the view that isobaric method has short testing time and high testing efficiency is very one-sided, incorrect and inconsistent with the actual situation

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