Pressure decay leak testing is a popular leak testing method used by manufacturers across the globe. In basic terms, pressure decay leak testing measures the drop in pressure on the interior of a part to identify the leak rate. This leak rate signifies either a passing or defective part based on set parameters.
The absolute pressure decay method is the most common and can be effective for many purposes, however, more manufacturers are discovering the benefits of a slightly different method; differential pressure decay leak testing. Depending on the application, using differential pressure can deliver improved resolution and faster cycle times than absolute pressure decay.
Learn about the differences between absolute pressure decay leak testing and differential pressure decay leak testing, including the best applications for both, below.
What is Absolute Pressure Decay Leak Testing?
In an absolute pressure decay leak test, air is introduced into the test component until it reaches the set target pressure. The air supply is then cut off. This isolates the pressure and the following loss in pressure (decay) is monitored over a set period of time. Absolute pressure transducers are employed in this test method to compare the pressure with absolute vacuum (or full vacuum) on the opposite side of the transducer. A leak is detected if there is a pressure drop beyond the specified test limit.
What is Differential Pressure (DP) Decay Leak Testing?
A differential pressure (DP) decay leak test uses similar parameters but uses a differential pressure transducer which tests for leaks by measuring a pressure change between a reference volume of air and the test part volume.
In a differential pressure decay leak test, a test component and a reference volume are pressurized to a designated target pressure. The air supply is then shut off using the fill valve, allowing the test pressure to stabilize across both volumes. The isolation valves for the reference air are then sealed, while the exhaust valve is opened to isolate the pressure on either side of the differential pressure transducer. Over the set test duration, the transducer compares and measures any pressure variation between the volume of the test component and the non-leaking reference volume as the part begins to leak. Again, a leak is detected if there is a pressure drop beyond the specified test limit.
Which is Better: Absolute Pressure Decay vs. Differential Pressure (DP) Decay?
Generally, differential pressure decay (DP) leak detection offers a higher level of sensitivity compared to standard absolute pressure decay tests, supporting increased Gage R&R, faster test times, and faster defect detection.
Below are a few examples that help illustrate how differential pressure decay offers higher accuracy:
Example 1: Comparing the “Clarity”
The picture on the left is divided into two sides. The left side of the image represents an absolute pressure decay measurement, with the right side representing a differential pressure (DP) decay measurement.
While a standard transducer can still provide an accurate measurement (left), it measures pressure loss at a lower resolution, so the final “image” (measurement) will not be as “clear” (precise and repeatable). In comparison, the differential pressure transducer offers a much higher resolution measurement capability, providing a more precise, “clear” image (a more precise and repeatable leak rate measurement).
Example 2: Comparing the Data
The above illustrations offer a comparison of leak rate measurements during an absolute pressure decay test and a differential pressure decay test, plotted in a graph format. It is evident that the measurements are much tighter and more repeatable in the differential pressure decay test results, which is due to the higher resolution measurement capabilities of the differential pressure transducer.
One of the ways differential pressure transducer offers this higher resolution measurement is that it helps eliminate the thermal effects that occur when pressurizing the part. Combined with a high-resolution 24-bit A-D circuit and a narrow range DP transducer, a differential pressure decay (DP) leak test system can achieve exceptionally precise measurements. This enables the test to reach stability faster, yielding a clearer signal with improved distinction and more precise data to swiftly differentiate between acceptable and faulty parts. Consequently, this results in a highly consistent and repeatable test process.
Which Parts are Best to Test Using Absolute Pressure Decay vs. Differential Pressure (DP) Decay?
Absolute pressure decay is often better suited for medium-sized parts using modest test pressures. If you’re testing around 10 sccm/10 psi and you don’t need to cut cycle time, there’s no major benefit to using differential pressure decay instead of absolute pressure decay.
However, if you’re looking to improve your cycle time and repeatability, particularly at higher test pressures (>30 psig) and/or lower leak rates (<1 sccm), differential pressure could present a wide array of benefits on your line. Below are some ideal applications suited to differential pressure decay leak testing:
- Testing large non-expanding parts and those with high leak rates (>5 sccm): Common parts and assemblies include EV batteries and drive systems, engines, transmissions, and coolant system final assembly testing.
- Testing parts with very small pressure loss, small leak rates (0.1-1 sccm): Common parts and assemblies include consumer electronics, refrigerants, and more.
- Testing at high pressures (>30 psig): This includes parts and assemblies like brake lines, hydraulic systems, fuel pumps, fuel lines, fuel fittings, oil passageways, and more.
- Testing pressure drop over time: This includes parts and assemblies in medical device manufacturing, like catheters, access sheaths, medical solution or fluid bags, and more.
Want to learn more about differential pressure decay leak testing? Watch the Pressure Decay Webinar.
How to Choose the Best Leak Test Method for Your Part
While we have provided a general overview of ideal use cases for differential pressure decay leak testing above, remember that the best solution will depend on the application and your specifications for the test. The leak test specialists at CTS can work with you to understand your required specifications and get you the best leak test solutions for your needs. We can support nearly all leak testing applications, with the most diverse portfolio of leak test instruments.
Have questions about your leak test? Contact the leak test experts at CTS!
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