The detection efficiency of the permeability test

  • Detail

Abstract: by analyzing the factors affecting the detection efficiency of permeability test, this paper shows that the number of test samples is the key point restricting the detection efficiency, and proves that there should be no obvious difference in the test time between vacuum method and sensor method. Combined with Labthink vac-v2, this paper introduces the improvement made by the differential pressure permeability test equipment to improve the detection efficiency and the remarkable results achieved

key words: differential pressure method, permeability, detection efficiency

differential pressure method is the basic method in permeability testing, and vacuum method is the most widely used method in differential pressure method. In the past, the technology of key components required by this technology was relatively backward and the accuracy was limited, which affected the detection accuracy and efficiency of the overall equipment. Therefore, there was a view about the "low test accuracy and poor detection efficiency" of differential pressure method equipment in the past. With the progress of testing technology, the testing accuracy of differential pressure method has been greatly improved, and has reached 0.01 ml/m2 · 24h · 0.1MPa, or even lower level, which is comparable to the isobaric method, and the detection efficiency has also been significantly improved. However, with the popularization of barrier detection and the aggravation of detection tasks, higher requirements are put forward for the detection efficiency of current equipment. This paper will analyze the improvement direction of the detection efficiency of differential pressure method combined with the test characteristics of differential pressure method, and introduce the actual achievements now

1 . The influencing factor of detection efficiency

the low detection efficiency was a very common evaluation of the differential pressure method test equipment in the past, and it also revealed two main shortcomings of the differential pressure method equipment at that time: first, the test time was long; Second, the number of samples is limited. These two points directly restrict the improvement of testing efficiency, so if you want to improve testing efficiency, shorten testing time, or increase the number of samples

1.1 test time

the representative method of differential pressure method - the test time of vacuum method includes the vacuum pumping time and the establishment time of osmotic balance. The test shows that too short vacuum pumping time will affect the test results. On the one hand, during the test, the "gas out" of the vacuum pipeline is counted as the test gas penetrating through the sample (the pressure sensor does not distinguish the type of gas), which leads to large test data and unstable test data. On the other hand, although the material has been pretreated, there will still be some impurities on the surface of the material and gas will permeate into it. The length of vacuum pumping time will affect the elimination of impurities on the surface of the material and the gas that permeates into it. The more thorough the vacuum pumping is, the better the elimination effect is, and the more stable the test data is. Long term tests have proved that the volume of the test chamber is related to the time required for vacuumizing. If the volume is reduced, the time required for vacuumizing will also be shortened. However, for vacuum equipment, when the vacuum pumping time is too short, it is difficult to maintain the pressure of the system. Moreover, the vacuum pumping time is also required in the standard. The national standard GB 1038 requires that the vacuum system should be continuously vacuumized for more than 3 hours after reaching the vacuum degree required by the standard

here is the test time of isobaric method. The test time of sensor method, a representative method of isobaric method, is also divided into two parts: system purge equilibrium time and penetration test time. The requirements for purging time in the test standard are very clear, and it is stated in many places that this "purging time" will never be very short. Moreover, the purging time in standard ASTM D is divided into dehumidification and purging zero point. The purging time is equal to the vacuum pumping time in vacuum method. Labthink blue light studied the reasonable purging time of the system when developing the oxygen sensor detection equipment, and believed 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, so as to obtain a higher test accuracy

it can be seen that the preparation time (vacuumizing time of vacuum method and purging time of sensor method) before the beginning of infiltration is equivalent in the two methods. As for the establishment time of osmotic equilibrium, it mainly depends on the permeability of the sample. However, because environmental factors will affect the process of establishing osmotic balance, the length of osmotic balance time is obviously affected by environmental factors. The most obvious is the influence of temperature change on the infiltration process. The more intense the temperature fluctuation is, the longer it takes to reach the osmotic equilibrium. Nie Zuoren said to the public that correspondingly, the worse the repeatability and accuracy of the test data are. It should be noted that environmental changes will affect all permeability test methods, including differential pressure method and isobaric method, and the degree of this impact varies with different test methods. Therefore, if the permeability testing equipment has a stable function of temperature and humidity control, it is more conducive to the stability of test data

to sum up, the test time of the two test methods is equivalent. Some literatures unilaterally believe that the isobaric method has a short test time, which has been proved to be inconsistent with the actual situation through practice

1.2 number of samples

as mentioned above, it is very difficult to improve the detection efficiency only by shortening the test time. You are welcome to pay attention to us. Therefore, if you can detect as many samples as possible in the same period of time, the detection efficiency can be greatly improved. However, the permeability test of materials is a kind of microscopic test, and the tightness of the test system is the basis for evaluating the accuracy and data reliability of the equipment and the ability to calibrate, correct parameters and normal experiments (extensometers are selected if large deformation is required). When the number of test samples is not one, the probability of possible leakage points in the whole test system is much greater than that when there is only one sample, and the occurrence of system leakage will seriously affect the test data. For example, in the vacuum method, because the direct detection parameter is the pressure in the test chamber, and the pressure sensor cannot identify the gas, if the system leaks, the infiltrated gas will be mixed into the test gas penetrating through the sample and cannot be distinguished by the sensor, resulting in the increase of the detected pressure value and the test failure. Therefore, although the multi cavity detection has a significant impact on the detection efficiency, it is very difficult to realize the multi cavity vacuum detection

2 . The improvement of Labthink vac-v2

from the previous 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, it is difficult to be less than 5 hours. Therefore, to shorten the test time as much as possible, we can only work hard on shortening the establishment time of osmotic equilibrium. As mentioned above, this time is objectively determined by the sample, but the stability of the test ambient temperature will affect the length of this time, so we can try to shorten this time by improving the stability of the ambient temperature. Labthink vac-v2 adopts an external temperature control system, which effectively realizes the high-precision and rapid rise and fall of the temperature in the test chamber through the water circulation principle, so that the temperature control efficiency can be effectively improved. At the same time, reducing the temperature control area is only for the scope of the test chamber, which greatly strengthens the temperature uniformity in the test chamber. The temperature control accuracy has reached ± 0.1 ℃, avoiding the gap between the measured temperature and the temperature in the chamber

the vacuum method equipment Labthink vac-v2 overcomes the technical difficulties of the equipment structure. It can detect three different samples at the same time and issue independent test data. Its detection efficiency is more than three times that of the single cavity detection equipment, and is comparable to the multi cavity sensor method equipment. In addition, the sealing performance of vac-v2 test chamber is excellent, and the vacuum pumping time is expected to be shortened after the ability of the vacuum pumping system is improved, so the detection efficiency still has room to grow

3 . Summary

vacuum method, as the basic permeability test method, has always been the most widely used method, but the low detection efficiency in the past has always been a bottleneck in the actual use of this detection method. Nowadays, with the development of high-precision vacuum technology and the success of multi cavity testing technology, the detection efficiency of vacuum method equipment is equal to that of multi cavity sensor method equipment, which can fully meet the actual detection needs of heavy detection tasks and lay a more solid foundation for the comprehensive application of vacuum method

reprinted from: Languang Laboratory

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