About Automatic Leak Testing
1. Introduction to Leak Testing
Leak testing is an essential procedure employed in various industries to ensure product integrity, safety, and performance. By identifying and quantifying leaks, manufacturers can maintain consistent product quality, protect users from potential hazards, and ensure reliable long-term performance. The purpose of leak testing is to:
- Detect manufacturing defects.
- Ensure that products meet specific sealing criteria.
- Reduce product returns and warranty claims.
- Adhere to regulatory requirements.
The primary question in leak testing is determining the acceptable leak rate. This is defined by the maximum allowable leak rate that ensures the component's functionality.
Components are classified based on their leak rate. If the leak rate is less than the defined limit, it's considered tight; otherwise, it's leaking. The units of measurement are most commonly expressed in volume of air per time interval, such as standard cubic centimeters per minute (ccm/min).
For instance, visual inspection underwater (also known as dunk tank leak testing) is an accurate method for leak detection, given its executed with precision and the component is visible throughout the test. If the leak rates detected through this method need to be identified by automated leak testing equipment, sensitive gas detection methods, like helium-based ones, might be required.
In a traditional dunk tank leak test, the air leaks out of the test part and bubbles are formed which are visible to human eye. Air bubble will typically rise once they reach a diameter of 1 mm. Thus, if this occurs every minute, then a leak rate of 0.001 cm3/min can be calculated. Assuming that the component we are testing is a rubber car tire that has an interior volume of approx. 40 liters, such a leak would cause the tire pressure to fall by about 0.1 bar or 1.5 PSI within 5 years!
If instead we examine a scenario where air bubble were to be produced every 3 seconds, the corresponding leak rate would be computed at 0.01 cm3/min. The above-mentioned pressure decay in the tire will then occur within one year. Surely, if a tire were to leak at that rate, it would lead to more regular gas station visits to fill up the tire pressure and might not be acceptable from a quality perception basis.
Given the fact that dunk tank testing requires human ocular inspection, which is costly and can lead to human error, and the long duration of test times required for smaller leak rates, the modern manufacturing process cannot rely on dunk tank leak testing.
Given the above, it’s clear that depending on the function of the part that is being tested, what is a small or large leak rate is relative. The requirement for a component to be leak-tight is always in relation to its intended operational environment. The testing method, pressurization level, and acceptable leak rate must be defined based on these operational conditions.
Dry Air leak testing has a significant advantage of being mess free and more accurate for some applications, such as testing of hydraulic systems. Imagine manufacturing a hydraulic pump or cylinder and filling it with oil to then pressurize, only to find that the system is leaking!
Common Leak Testing Methods in Mass Production
- Underwater Visual Inspection (Dunk Tank)
- Automatic Air Leak Test using Pressure Decay or Mass Flow
- Helium Sniffing at Atmospheric Pressure
- Integral Leak Test at Atmospheric Pressure
- Integral Leak Test under Vacuum
We will examine the Automatic Air Leak testing, as it is the most common form of leak testing in modern manufacturing seetings. When using automatic Air Leak Testing methods such as Pressure Decay and Mass Flow, the four critical parameters in leak testing remain the same.
2. Measuring Phases for Dry Air Leak Testing
Both Pressure Decay and Mass Flow Dry Air Leak Testers rely on test equipment that control the pressurization of the part and either measure the pressure and/or flow, in order to give a pass fail. In general, the phases are common between both types of Dry Air Leak testing.
The Four Phases are:
1. Filling Phase: The component is pressurized.
2. Balancing Phase: The system achieves stability. Turbulence from the filling process settles, and temperature changes in the test air balance out.
3. Measuring Phase: The escaping air causes a pressure drop in the test volume, which is measured and displayed.
4. Venting Phase: The system is depressurized.
3. Pressure Decay Leak Testing
Pressure Decay leak testing involves measuring the drop in pressure in a test piece over a defined period. If a leak is present, the internal pressure will decrease at a rate proportional to the size of the leak. In order to determine leak rate using Pressure Decay, you need to know the test pressure, acceptable leak rate, test volume, and test duration.
The acceptable leak rate varies for different applications. It's influenced by factors like the material of the component, its design, and the operational stresses it will undergo. Leak rate values are always associated with a specific test pressure. The pressure applied during testing usually mirrors the subsequent operational conditions.
The Pressure Decay Leak Test, a meticulous and efficient method for leak detection, is an integral part of quality control in manufacturing. It operates on a straightforward principle: a component or system is filled with air or another gas, and any decline in pressure, as indicated by a sensitive pressure transducer, signifies a leak.
During the testing process, the object under examination is sealed and pressurized, then isolated from the pressure source. The pressure transducer, a critical instrument in this test, continuously measures the internal pressure. The core of this method lies in the precise detection of pressure changes over a specified period. If the pressure remains stable, the component is deemed leak-free. However, any reduction in pressure, referred to as pressure decay, points to potential leaks.
This technique's appeal stems from its rapid execution, typically concluding within seconds, making it a favorite in industries where speed is crucial. The automotive sector, for instance, relies heavily on this method to ensure the integrity of components that must remain airtight. Similarly, in the medical field, devices that require sterilization or contain fluids are routinely tested for leaks to guarantee patient safety. The consumer goods industry also benefits from pressure decay testing to ensure the durability and performance of products.
Furthermore, the pressure decay test is not only swift but also non-destructive, meaning that it doesn't compromise the integrity of the item being tested. This characteristic is vital for maintaining the functionality and quality of the final product.
2.2 Formulae and Calculations
Test Volume: VT = (PATM × VL × tm) / (Δp × 60)
Leak Rate: VL = (Δp × 60 × VT) / (PATM x tm)
Measuring Time: tm = (Δp × VT × 60) / (ATM x VL)
Where:
VT = Test volume in cm³
VL = Leak rate in std cm³/min
PATM = Atmospheric pressure in Pa
tm = Measuring time in seconds
(p x V) / T = constant
p x V = constant
p1 x VT = p2 x VT + pAt x VL
∆p = p1 - p2
VL = (∆p x VT) / pAt
3. Differential Pressure Decay
Differential Pressure Decay Leak Testing is a sophisticated and precise approach, widely employed in the manufacturing sector to validate the integrity and safety of products. This method involves introducing pressurized air or gas to the item being tested.
At its core, pressure decay testing traditionally uses a single sensor to gauge pressure changes within the product, with a notable drop signaling a potential leak. However, this absolute pressure decay technique has limitations. Notably, it requires a sensor capable of monitoring both the pressure buildup during initialization and minor fluctuations during the testing phase—often a challenging combination to find in one sensor. Moreover, sensitivity levels must be exceptionally high; for example, at a test pressure of 8 bar, a sensor must discern changes as minute as 0.001 mbar to detect minuscule leaks.
Another complication arises from the adiabatic process. As the product reaches the test pressure, a rise in temperature can occur, leading to a natural pressure drop as the heat dissipates. This can be mistakenly interpreted as a leak by the testing instrument.
To overcome these challenges, the differential pressure decay method utilizes two sensors. The first is an absolute sensor, spanning the range from atmospheric pressure to the test pressure. The second is a differential pressure transducer (DPT), finely tuned to detect the smallest leaks during the test phase.
An innovative feature of this method is the concurrent pressurization of a reference volume or a known good item alongside the test piece. The DPT measures the pressure variation between the two, allowing for a much more sensitive leak detection due to the reduced range of pressure differences it needs to monitor.
This dual-sensor system also compensates for the adiabatic effect, as any temperature-induced pressure changes affect both the test item and the reference equally, thereby not affecting the differential pressure reading.
When selecting a reference volume for the Furness Controls range of production line leak testers, there are three options:
1. The leak detector’s internal volume can be adequate for many test items, simply by sealing the reference port.
2. For larger items, an external reference volume that closely matches the test item's volume may be employed for greater accuracy.
3. For the most precise measurements, especially in cases where temperature stabilization takes longer, a known good item that has been verified leak-free by another method can serve as the reference.
Each of these options ensures that the differential pressure decay method remains one of the most sensitive and accurate for detecting leaks, safeguarding product quality and consumer safety.
4. Standard Mass Flow Leak Testing
The Mass Flow Leak Testing technique begins with the initial stage of pressurizing a reference volume, often denoted as a VEX, with air designated for testing. This pressurized air is then utilized to fill the component undergoing examination, ensuring it reaches the requisite test pressure. In contrast to the pressure decay approach, the mass flow method stands out for its ability to instantly provide leakage measurements.
The mass flow sensor, integral to this process, detects and quantifies any leakage in real-time. This instantaneous feedback is particularly advantageous for components produced in large quantities or when the production process demands rapid cycle times. The real-time data collection streamlines the testing process, allowing for swift identification and rectification of leaks, thereby enhancing the efficiency of the production line.
This methodology is especially beneficial for testing scenarios where time efficiency and rapid throughput are essential, making it a preferred choice for manufacturing environments with high output demands.
5. Continuous Mass Flow Leak Testing
For leak rates greater than 500 cc/min, continuous Mass Flow Leak Testing is a highly precise method for assessing the integrity of products that manage the flow of gases or liquids, such as engine components, household appliances, gas meters, heating, ventilation, and air conditioning (HVAC) systems, and certain medical devices. When product variability exceeds a 5-10% margin, this approach excels by accurately determining if the flow rate adheres to the specified standards for a particular application.
Key Advantages of Mass Flow Leak Testing:
- Provides immediate feedback on the integrity of the product.
- Non-destructive in nature, preserving the test item's condition.
- Capable of identifying even minor leaks with high precision.
- Versatile enough to handle different product sizes and variable flow rates.
- Eliminates the need for calibration against a standard or master part.
- Operates consistently under stable air pressure conditions.
The Procedure of Mass Flow Leak Testing:
1. The product is filled with air to the required test pressure and held until the internal pressure stabilizes. As equilibrium is reached, the mass flow required to maintain pressure decreases.
2. When the testing phase begins, the pressure might fluctuate momentarily until the system balances out at the desired pressure level.
3. At the test's conclusion, the flow sensor quantifies the leak rate, determining the volume of air needed to sustain pressure throughout the test period.
Ensuring Successful Mass Flow Leak Testing:
To guarantee precise measurements, the right testing equipment must be chosen for each specific application. A successful mass flow test is contingent upon:
- The supply of clean, dry air to avoid contaminants that could skew results.
- A consistent source of pressure to ensure accurate and repeatable readings.
- A stable testing environment where the part under test does not deform under pressure.
- A flowmeter that is correctly scaled for the expected range of flow rates to be measured.
While mass flow testing is often the optimal choice under these conditions, consulting with an application engineer is advisable to discuss the particularities of the application, its requirements, and the desired outcomes. This ensures that the selected leak testing method aligns perfectly with the operational goals and the nature of the products being tested.
6. Leak Testing Equipment: Comprehensive Solutions
6.1 Measuring Procedures
In automated air leak testing, the component is subjected to a pressure difference to detect air escape. The escaping air quantity can't be measured directly, only its effect can be observed.
Two primary measuring procedures are:
Pressure Measurement: The component is pressurized and then isolated from the pressure source. The subsequent phase involves checking for any changes in the test pressure due to leakage.
Mass Flow Measurement: After pressurizing the component, it remains connected to the pressure source. The measurement phase involves checking if air is entering the component due to leakage.
6.2 Effects of Temperature and Pressure Over Time
As temperature changes, the pressure in the test volume also changes. For a leak test panel to display a value indicative of a tight part, an isothermal state must be achieved. However, reaching this "perfect balance" requires a significantly longer duration than the typical cycle times in industrial volume production.
For example, if a part is taken from outside in the winter and tested without allowing the ambient and component temperature to equalize, then it will usually lead to a large positive pressure.
6.3 Compensation Filling (Shock Filling)
Compensation filling involves applying a filling pressure slightly higher than the test pressure during an additional pre-filling phase. As the transition from the pre-filling phase to the filling phase occurs, the filling pressure is reduced to the test pressure. This causes the air in the test volume to cool down, compensating for the "imperfect balance". By adjusting the filling pressure, it's possible to achieve a reading indicative of a tight part even after a short balancing phase. This effect is influenced by the material of the test part and its thermal conductivity.
Special Measuring Procedures: Volume-Dependent Leak Test
Closed hollow bodies without any openings for air entry are tested in a bell jar. Once placed inside the jar, the cavity between the test part and the jar's interior is pressurized. This pressure is generated by an expansion volume from which pressurized air expands into the bell jar during the filling phase. The ratio of the expansion volume to the cavity's volume, along with the pressure in the expansion volume before filling, is chosen such that the test pressure will be achieved in the cavity unless the test part has a significant leak. During the measuring phase, fine leaks are detected using the pressure decay method, while significant leaks are identified through pressure monitoring.
6.4 Other Influencing Factors
System Leak or Creep
The design of the leak test fixtures, tightness of pneumatic connections and quality of selected components (such as regulators and valves) all play a significant role in determining the success of a leak test.
Creeping can also refer to the movement of air within the component, which can influence the test results as the pressure works it ways through the part. If the part has lots of crevices, nooks and cranny's, having a long enough settling time is essential to account for this factor when designing the test procedure, in order to ensure accurate results.
Unstable Volumes
The test volume can undergo changes during the test due to the influence of the test pressure. This is a result of the materials or construction utilized.
A change in the test volume, caused by the test part "breathing," can create a false impression of a leak during the measuring phase. The measuring sensor cannot differentiate between a volume increase and actual leakage.
Shrinkage, which might be caused by the pressure forces exerted by the sealing element, can compensate for any leakage. To prevent the sealing elements from "breathing," the rubber seals and sealing forces should be adjusted so that the sealing elements can no longer shift when sealed.
Hidden Testing Spaces
Hidden testing spaces can lead to misinterpretations. For instance, a hidden space might be connected by a non-return valve that opens during the filling phase but closes once the filling pressure is reached. This means it's disconnected during the leak test, resulting in potential leakages in this space not being detected.
Vaporization and Condensation Processes
Moisture on the surface can seal up leaks, preventing their detection during the leak test. These leaks might then reappear during subsequent operational conditions. Therefore, dry test parts are essential for the leak test.
Additionally, moisture in the testing space can cause vaporization and condensation processes during the filling and venting of the test part, which can distort the measurement result.
Leaking Pneumatic Measuring Equipment
The leak testing equipment itself can sometimes be a source of leaks. It's essential to differentiate between a leak originating from a porous area of the test part and leaks in the sealing.
If multiple rejects occur in quick succession during production, it might indicate issues with the testing equipment, such as a worn rubber seal or a leak in the test system.