Application NotesTechnical Documentation & Guides
Pressure Decay Leak Testing Fundamentals
What pressure decay leak testing is, how test theory and sensitivity work, and when to pick it over other methods.
Overview
Pressure decay is the most common production leak test. Pressurize the part, close it off from the supply, and watch for a pressure drop over the test window. Drop more than the reject threshold and the part leaks. The method is simple. Getting repeatable results out of it is not, and that comes down to knowing what actually moves the measurement.
Test Theory
The pressure decay rate is related to the leak rate by the Ideal Gas Law. For a fixed test volume at constant temperature:
- Leak rate (scc/min) = (ΔP × V) / (Patm × t) where ΔP is pressure change, V is volume, t is time
- Smaller test volumes provide higher sensitivity (less air to lose)
- Higher test pressures increase sensitivity proportionally
- Longer test times allow smaller leaks to be detected
- Typical sensitivity: 10⁻² to 10⁻⁴ scc/min depending on conditions
For air at standard conditions: Q (scc/min) = (ΔP × V) / (14.696 × t) where ΔP is in PSI, V is in cc, and t is in minutes. A 0.001 PSI drop in 100cc over 10 seconds = 0.041 scc/min leak rate.Test Cycle Phases
A pressure decay test runs in four phases: Fill (pressurize the part), Stabilize (let pressure and temperature settle), Test (measure the change), and Exhaust (vent it). Stabilize is the one that trips people up. Cut it short and thermal effects either hide a real leak or invent one that isn't there, so the result is wrong either way.
Factors Affecting Accuracy
Temperature is the biggest source of error. A 0.1°F swing in a 100cc volume at 30 PSI shows up as a pressure change the size of a real leak. Other culprits: adiabatic heating during fill (compressed air heats up), moisture evaporating off the surface, and flexible parts flexing under pressure.
Typical Applications
- Automotive component testing
- Medical device seal verification
- Consumer product waterproofing
- HVAC component testing
Related Products
LM-Series Instruments • Sentinel Series
Leak Test Method Selection Guide
How to pick a leak test method (pressure decay, mass flow, differential pressure, vacuum decay) for the sensitivity you need and the part you are testing.
Overview
Each leak test method trades off differently on sensitivity, cycle time, and the parts it suits. Picking one means weighing how small a leak you have to catch against cycle time, cost, and the part itself. LeakMaster instruments run several methods, so one box covers most of what a production line throws at it.
Method Comparison
The four primary production leak test methods, ranked by typical sensitivity:
- Mass Flow: 10⁻³ to 10⁻⁵ scc/min — measures actual flow to maintain pressure
- Differential Pressure: 10⁻³ to 10⁻⁵ scc/min — compares test part against reference
- Pressure Decay: 10⁻² to 10⁻⁴ scc/min — measures pressure drop in isolated volume
- Vacuum Decay: 10⁻² to 10⁻⁴ scc/min — same as pressure decay but under vacuum
When to Use Each Method
Pressure decay handles general-purpose testing where sensitivity needs are moderate and the fixturing is simple. Reach for mass flow when you want a direct leak rate in scc/min, or when test volumes run large. Differential pressure earns its keep when ambient temperature swings would throw off a plain decay test. Vacuum decay is for parts you cannot pressurize from the inside.
Part Characteristics to Consider
The part drives a lot of this. Rigid parts (metal castings, plastic housings) work with any method. Flexible parts (bags, tubing) can balloon under pressure, so drop the test pressure or go vacuum. Porous materials usually need a sealant or special fixturing. Large internal volumes need longer test times, or a mass flow method to keep sensitivity up.
Typical Applications
- Production line test planning
- Quality engineering specifications
- Test method validation
- Leak test process improvement
Related Products
LM-Series • Sentinel Series • Custom Test Systems
Integrating Leak Testers into Automated Production Lines
How to drop LeakMaster testers into an automated line: PLC comms, fixture design, and cutting cycle time.
Overview
Putting a leak tester on an automated line means getting it to talk to the PLC, hand off to whatever moves the parts, and seal into a fixture. Do it right and you get reliable results without adding much to the line cycle. Below: the communication interfaces, fixture design, and where the cycle time actually goes.
PLC Communication
LeakMaster instruments support multiple PLC communication options:
- Discrete I/O: Start, Pass, Fail, Ready signals for simple integration
- Serial (RS-232/485): Full parameter access and data retrieval
- Ethernet/IP: Direct tag access for Allen-Bradley PLCs
- Modbus TCP: Universal Ethernet protocol for any PLC brand
- PROFINET: For Siemens and other PROFINET controllers
Test Fixture Design
The fixture seals the part and plumbs it to the instrument. Three things matter: seal material (has to be compatible with the test medium and the part surface), seal force (enough to stop leaks, not so much it deforms the part), and internal volume (smaller means higher sensitivity and faster tests). Use quick-connect fittings and keep the tubing run between instrument and fixture short.
Cycle Time Optimization
Fill and stabilize eat most of the cycle. Bigger tubing and fittings shorten the fill. For stabilize, control the fill rate so the air heats up less, put regulators close to the fixture, and hold ambient temperature steady. Where you can, overlap the test with part handling to claw back throughput.
Typical Applications
- Automotive production lines
- Medical device manufacturing
- Consumer electronics assembly
- HVAC production testing
Related Products
LM-Series • Sentinel Series • Custom Test Fixtures