What is the Direct Pressure Test Method?

The direct pressure test method (also known as the pressure decay method) is a commonly used approach for air tightness testing. The principle of air tightness testing is to use compressed air as a medium, apply a certain pressure to the inner cavity or outer surface of the product to be tested, and then use a highly sensitive sensor to detect pressure changes, thereby determining the sealing performance of the test product. The direct pressure method uses the pressure drop over time of the compressed air filled into the workpiece to determine whether the workpiece has a leak.

Working Principle & Testing Process

The working principle of a direct pressure air tightness tester can be summarized as follows: fill the workpiece with a certain pressure of gas, hold the pressure, and then determine whether there is a leak based on the “pressure change.”

The entire test process can be divided into the following four core steps:

1. Filling
   Clean compressed air is filled into the inner cavity of the workpiece through a pressure regulator until the set test pressure is reached. The test pressure is determined by the process specifications, generally ranging from a few Pascal to several megapascals.

2. Hold (Stabilization)
   The air supply is cut off, allowing the internal pressure of the workpiece to stabilize. During this stage, the instrument can first determine whether the workpiece has a major leak based on the pressure change.

3. Measurement
   A high-precision pressure sensor continuously monitors the internal pressure change of the workpiece, records the current pressure value, and reads the pressure again after a few seconds or tens of seconds. If there is a leak, gas escapes, the internal pressure decreases, and the sensor measures the pressure drop ΔP.

4. Judgment
   The two pressure values are compared. If the pressure drop exceeds the set threshold, the product is judged as unqualified; otherwise, it is qualified.

The direct pressure method also has two variants: positive pressure testing (pressurizing by filling gas into the product) and negative pressure testing (evacuating to create a vacuum). These modes can flexibly adapt to products with different structures.

Technical Advantages

1. Non‑destructive testing
   Using only clean compressed air as the medium eliminates the risk of liquid damage, making it particularly suitable for sealing tests of precision electronic products.

2. Quantitative judgment
   Not only provides a “pass/fail” qualitative judgment, but also accurately measures the leak rate value, providing a scientific basis for quality grading.

3. High cost‑performance ratio
   Simple equipment structure, relatively low cost, convenient operation, and quick to learn.

4. Fast testing speed
   Short test cycle, perfectly matching the fast‑paced requirements of modern production lines.

Technical Limitations

1. Temperature sensitivity
   According to the ideal gas law pV = nRT, temperature changes significantly affect pressure readings. The adiabatic compression during the filling process raises the internal temperature of the workpiece, and the pressure decreases during cooling. This “false leak” may lead to misjudgment. In production environments with large temperature variations, sufficient stabilization time must be reserved in the test procedure.

2. Influence of volume and deformation
   The larger the internal volume of the workpiece, the smaller the pressure change for the same leak rate, thus reducing detection sensitivity. Soft or thin‑walled materials expand and deform when pressurized, effectively “absorbing” part of the pressure, which can also cause misjudgment.

3. Unable to locate the leak point
   The direct pressure method can only determine whether a product has a leak, but cannot precisely locate where the leak is. To find the specific leak location, other methods such as bubble testing (water inspection) or tracer gas leak detectors are required.

Applicable Scenarios

The direct pressure method is suitable for products with a filling port or pneumatic interface, offering broad versatility.

Direct Pressure vs. Differential Pressure

In the field of air tightness testing, direct pressure and differential pressure are the two most mainstream technical approaches. The differential pressure method can be essentially understood as an upgraded version of the direct pressure method in terms of accuracy. It introduces a reference part (a non‑leaking standard) that is pressurized simultaneously with the test workpiece, and a high‑precision differential pressure sensor measures the pressure difference between the two, effectively canceling out interference from ambient temperature fluctuations, air supply pressure variations, and workpiece deformation.

The key differences are summarized below:

Selection advice: The direct pressure method is suitable for high‑volume, cost‑sensitive production lines with less stringent leak requirements. The differential pressure method is suitable for high‑reliability products with extremely high sealing requirements, such as new energy vehicle battery packs, automotive components, and aerospace parts.

Technology Development Trends

With the advancement of Industry 4.0, direct pressure testing technology is rapidly evolving toward intelligence and integration. New‑generation direct pressure air tightness testers already support million‑level test data storage and MES system connectivity, helping companies achieve full‑process quality traceability. At the same time, cutting‑edge technologies such as AI‑assisted visual leak point localization and multi‑physics coupling algorithm optimization are also accelerating deployment, further improving detection accuracy and intelligence levels.