Thread Content
Leakage rate testing, also known as airtightness testing, is a test used to determine the airtightness of a component or its ability to allow fluid flow through it. Its applications are very wide, mainly covering industries such as consumer electronics, auto parts, military affairs, aerospace, and medical equipment. Currently, airtightness testing primarily relies on three principles: pressure-based, flow-based, and vacuum helium testing ； Among them, the pressure type is further divided into direct pressure type and differential pressure type. Pressure-based detection methods can currently identify leakage rates in the range of 10^-3 pa*m3/s, while helium detection can detect rates ranging from 10^-3 to 10^-12 pa*m3/s. Traditional industries in the mid-to-low end segment make use of pressure-based detection equipment more often; such equipment is relatively inexpensive, and there are many manufacturers offering it. Some precision industries opt for vacuum helium testing, but it is much more expensive, and the testing process is also relatively complicated. Compared to the above two types of detection devices, the development of flow-based detection devices has remained sluggish. Most flow-type airtightness testing devices available on the market today can detect a minimum flow rate of 0.1 ml/min, and this value is quite unstable. Therefore, there is a significant gap in the market for reliable volumetric air-tightness testing equipment, which relies mainly on imports. Why is the development of flow-type leak detection equipment in China currently so lagging behind? First of all, we need to understand that the measurement and control of micro-flow rates is inherently a global challenge. The core sensor used in traditional domestic flow-based air-tightness testing equipment is the flow sensor itself, whose accuracy, resolution, and repeatability are relatively poor; as a result, the testing performance of such equipment is inferior to that of differential pressure-based testing devices. For this reason, few people use flow-based testing equipment, which also leads to many manufacturers being unwilling to invest time and resources in developing such devices. However, as product manufacturing techniques improve, it has become necessary to measure the leakage flow rate with high accuracy for many key components. As a result, some suppliers have begun to explore mid-to-high-end flow-based leak detection equipment. To date, no company in China is capable of producing standardized micro-flow leak detection equipment. Here, microflow typically refers to values below 0.1 SCCM. However, such flow-type leak detection equipment abroad has already been well developed, and its price is relatively high; as a result, domestic manufacturers can only sigh in frustration. Here, it’s pretty clear that the core of the problem lies with the sensors. We have surveyed most of the manufacturers in China that produce equipment for leak detection; they also manufacture flow-based detection devices. However, in order to reduce costs, these manufacturers often use flow sensors with poor performance. This approach of sacrificing performance for cost reasons results in the flow-based leak detection equipment having relatively inferior performance. How should the core sensor for flow-based airtightness testing equipment be selected? I. Examine repeatability, that is, stability. The accuracy of traffic detection is a very important aspect. We all know that the volume of gases expands when heated and contracts when cooled; therefore, in different testing environments, the leakage rate measured for the same component under test will change as the ambient temperature and pressure vary. Especially in micro-flow detection, the already minimal flow rate can cause defective items to become qualified ones as a result of thermal expansion and contraction, leading to such situations where defective items turn into qualified ones. And here, the laminar flow differential pressure mass flow meter must be mentioned. Laminar pressure difference mass flow meters have significant advantages in micro-flow measurement, which requires analysis based on their design principles. The principles of fluid motion are well known: even when a gas flow of very low volume passes through a pipe, there is a pressure difference between the front and back ends of the pipe. As long as the sensor has sufficient sensitivity and accuracy, it can detect this pressure difference. Based on the principle of the laminar flow differential pressure mass flow meter (Hagen-Poiseuille’s law), the volumetric flow rate can be calculated once a pressure difference is available. The laminar flow differential pressure mass flow meter is equipped with temperature and pressure sensors that can automatically adjust the volumetric flow rate under actual operating conditions to a standard condition volumetric flow rate (by standard conditions we generally mean 1 atmosphere pressure and 20°C). Therefore, the standard condition volumetric flow rate measured by the same laminar pressure difference mass flow meter at different testing environments for the same leakage point is a constant value, which greatly improves the repeatability and accuracy of the measurements. II. Consider the length of the testing time. Some imported brand thermal mass flow meters are also capable of detecting very low flow rates, such as 0.5 SCCM, but the detection time is relatively long, which greatly reduces the efficiency of detection. Products with low pressure loss for laminar flow differential pressure mass flow meters can effectively solve this problem, and experiments have shown that they can greatly improve detection efficiency. III. In terms of measuring very low flow rates: Thermal mass flow meters currently have difficulty measuring extremely small flow rates, and this is largely related to their design principle (the principle of heat conduction). We all know that thermal mass flow meters rely on heating the capillary to create a pressure difference in order to determine the volume flow rate; the smaller the flow rate, the finer the capillary needed. Capillaries have now been made as fine as possible, which makes it difficult to achieve further advancements in the field of measuring and controlling very small flow rates. However, the laminar flow differential mass flow meter has significant advantages: it is capable of detecting flow rates as low as 0.001 SCCM, with errors kept within the range of 1% RD (reading error) to 0.2% FS (full-scale error). Therefore, under positive pressure conditions, a laminar pressure difference mass flow meter can achieve leakage rate detection at the 10^-5 Pa·m3/s level. IV. In terms of low-pressure and negative-pressure detection: Due to its design, thermal flowmeters experience relatively high pressure losses, which makes them less suitable for use in environments with low pressure or negative pressure. In some specific industries, the required pressure is 3 KPa or even below 1 KPa; if the pressure loss inherent in the flow meter is greater than or close to this value, it is essentially difficult to obtain accurate measurements. In such operating conditions, the pressure loss of the flow meter must be less than 1 Kpa, or even much less than 1 Kpa; in these cases, only a laminar flow differential mass flow meter can be used. Based on our current understanding of the market, as the performance of domestic spare parts and sensors improves, more and more companies are willing to invest time and resources in developing high-end equipment in order to challenge the monopoly held by foreign high-end testing equipment in the domestic market. In the field of leakage rate detection, the promotion of laminar flow differential pressure flow sensors is an inevitable trend!