In the automotive industry, sealing is a critical quality parameter, and leak detection technology has evolved significantly over the years. However, several uncertain factors still influence the final test results. To address these challenges, temperature compensation systems are often employed in sealing inspections. Despite their complexity and limited effectiveness in some real-world applications, many leading automotive manufacturers have shifted towards natural or forced cooling to align with on-site environmental conditions, aiming to control the temperature gradient. This approach works best when the temperature fluctuations at the production site are minimal. Nevertheless, this does not mean that temperature compensation has been abandoned. In fact, some companies still use it to ensure accurate leak detection during unexpected environmental changes.
The leakage rate (Q) of a product's seal refers to the volume of gas entering the atmosphere per unit time under a given pressure. By analyzing the pressure changes caused by leaks, the leakage rate can be indirectly determined based on the flow rate and pressure variations. The formula for the leak rate is derived under conditions where the gas is incompressible, the temperature is constant, the measured volume remains unchanged, and the pressure change is negligible:
$$ Q = \frac{\Delta P \cdot V}{T_m \cdot P_{atm}} $$
Where:
- $ Q $: Leak rate at standard atmospheric pressure
- $ \Delta P $: Change in pressure drop
- $ V $: Volume being tested
- $ T_m $: Test duration
- $ P_{atm} $: Standard atmospheric pressure
To ensure the accuracy and stability of the formula, various environmental and operational conditions must be considered, including temperature, humidity, material properties of the workpiece, chamber stability, reliability of the sealing fixture, and internal airflow effects. While most of these factors can be controlled, temperature remains the most challenging and impactful variable.
Many manufacturers implement constant temperature and pressure control in their workshops, but ambient temperatures can vary significantly depending on the season and time of day. Additionally, workpieces may enter the testing environment with a significant temperature difference from the ambient air, especially after processes like cleaning or drying. If the temperature difference between the workpiece and the environment is large, it can lead to inaccurate leak measurements.
According to Charles’s Law, when the volume of a gas is constant, its pressure is directly proportional to its thermodynamic temperature. Therefore, if the temperature increases, the pressure rises, and so does the leak rate. Conversely, a decrease in temperature leads to lower pressure and a reduced leak rate. In mass production, where the workpiece volume and test time are typically fixed, the pressure drop becomes the most critical factor affecting the leak rate.
When a workpiece enters the sealed test equipment, any residual heat from previous processes can quickly raise the pressure inside the test chamber. As the test continues, the air inside the chamber heats up further, causing a significant increase in pressure and, consequently, an exaggerated leak rate. To ensure accurate results, the temperature of the workpiece should match the ambient temperature before the test begins.
Practical experience shows that a temperature difference of ±5°C is generally acceptable, as it minimally affects the test results. However, for components with stringent leakage requirements, even small temperature differences can have a major impact. For example, high-pressure oil passages in cylinder blocks or casing require precise measurements, where temperature deviations can significantly skew results.
An experiment demonstrated this clearly: a workpiece was tested with a 11°C temperature difference between itself and the ambient environment. The measured leak rate was 1.3 mL/min at the peak difference, dropping to 0.17 mL/min when the temperature stabilized. This 1 mL/min difference represented a 20% deviation from the allowable limit, exceeding the 10% accuracy requirement of the system.
This highlights the importance of temperature compensation. By using data acquisition systems and analyzing long-term test data, a linear temperature compensation value can be established. When applied, the test results become more accurate, even with temperature differences ranging from -10°C to +20°C.
As shown in the graph, the compensated data closely aligns with the true leakage value, while the uncompensated data shows greater variability. Temperature compensation plays a vital role in ensuring accurate leak detection, especially in environments where strict leakage control is required and temperature regulation is difficult. It enhances both the reliability of the testing process and the overall safety and quality of the final product.
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