Air Pressure Leak Testing for Welded Bag Seams
Air pressure leak testing seals a bag, introduces a small controlled pressure and measures how quickly that pressure falls. Because air is roughly fifty times less viscous than water, it escapes through channels a short water dip would never reveal, and it does so in seconds rather than minutes. The method returns a number instead of a wet spot, which makes it the only common leak test that supports statistical process control. It also leaves the unit dry, so a passed bag goes straight to packing. Use it for high-volume screening, and keep a water dip in reserve for finding where a failure actually is.
Table of Contents
Principle and Sensitivity
The principle is a mass balance. Seal the bag around a single port, introduce air until the internal pressure reaches a chosen set point, isolate the volume from the supply, and measure the pressure over a fixed observation window. If the pressure falls faster than the allowed limit, the volume is losing air and the seam or the closure has a path to the outside. If it holds, the assembly is intact to the sensitivity of the setup.
Sensitivity comes from the physics of the test medium. Air is roughly fifty times less viscous than water, so it moves through a fine channel far more readily, and modern pressure transducers resolve changes far smaller than a human eye can see on a piece of wetted tissue. In practice a well-built bench rig detects channels that would need many minutes of submersion to show up in a dip test, which is why the two methods are not equivalent in severity.
That difference has to be managed rather than celebrated. If your specification was written around a water dip and you switch to air testing without changing the acceptance language, you will start rejecting units that would have passed the older method. Set the air test limit by correlation: run both methods on the same population, find the decay value that corresponds to your dip criterion, and write that number into the work instruction.
The method sits alongside the other detection options rather than replacing them. Visual inspection at the station remains the cheapest screen, and a water dip remains the best way to locate a leak once air testing has flagged it. Our overview of seam leak detection methods places all four techniques in one decision framework, and the welding and testing capability page shows where each sits in the production flow.
Test Pressure Selection
Test pressure is a compromise between sensitivity and risk. Higher pressure drives more flow through a given leak, which makes the signal larger and easier to resolve, but it also loads the weld in peel and can open a marginal joint or permanently deform the bag. Lower pressure is gentler and safer for the product but produces a smaller signal that needs a better transducer and a longer observation window to read reliably.
For soft goods the working range is low. Most welded bag constructions are tested at a pressure that is a small fraction of the hydrostatic head the fabric achieves under ISO 811 or AATCC 127, because the purpose is to find an open path rather than to prove the material withstands pressure. Validate the value for each construction: pressurize good units and known leaking units, confirm the separation between the two populations, and confirm that good units return to shape afterwards.
Stability at the set point matters as much as the value chosen. The rig should reach the target pressure, allow a short stabilization period for the bag to settle and for temperature to equalize, and only then begin the measurement. Starting the observation window immediately after filling is one of the most common causes of noisy data, because the first seconds are dominated by the bag expanding rather than by leakage.
- Choose the lowest pressure that still separates known good units from known leaking units.
- Confirm good units return to shape and that no weld is loaded into peel at the set point.
- Fill, stabilize, then measure: never start the observation window during filling.
- Re-validate the pressure whenever material thickness, weld width or bag volume changes materially.
- Record the set point on every report, because decay limits are meaningless without it.
Fixture Design for Irregular Shapes
A soft bag is not a rigid test volume. When you pressurize it, it changes shape, and a change in volume produces a change in pressure that has nothing to do with leakage. This is the central problem of fixture design. The answer is to constrain the bag inside a cavity or between plates that hold it close to its natural geometry, so the pressurized volume stays constant and the pressure reading reflects leakage rather than expansion.
The port is the second design problem. Every bag needs a sealed entry point for air, and that entry must not itself leak. Purpose-made quick-connect fittings that clamp over a roll-top opening, a valve or a zipper port give repeatable sealing; improvised fittings taped in place by hand are the largest source of false failures on otherwise sound rigs. Design the seal so it can be made in one motion, and verify the seal itself with a blank or a known-good reference unit at the start of each session.
Irregular shapes add two more requirements. Webbing, internal dividers and foam inserts occupy volume and can trap air in compartments that are not connected to the port, so a leak into an isolated pocket will not show in the reading. Support the bag so that straps and hardware do not press into the shell and temporarily close a leak path. Both problems are easier to solve if the fixture is designed around one construction at a time rather than as a universal rig.
| Fixture element | Function | Common failure |
|---|---|---|
| Constraining cavity or plates | Holds volume constant so pressure change means leakage | Bag allowed to balloon, producing pressure drop with no leak present |
| Sealed quick-connect port | Repeatable air entry with no leak at the fitting | Hand-taped improvised fittings that leak and cause false rejects |
| Support for straps and hardware | Prevents external parts pressing the shell closed | Hardware masking a real leak during the test |
| Reference volume or blank | Verifies rig integrity at session start | No reference check, so rig drift goes unnoticed |
| Pressure transducer and logger | Resolves small decay over the observation window | Transducer range too wide to resolve the limit |
Decay Rate to Leak Rate Conversion
A decay reading is only comparable between units if you account for the size of the volume being tested. The larger the bag, the slower the pressure falls for the same size of hole, because there is more air to lose. Converting the observed pressure drop into a volumetric leak rate removes that dependency and lets you compare a twenty liter duffel with a five liter pouch on the same scale.
The conversion follows directly from the ideal gas law. Multiply the internal volume by the pressure drop, divide by the observation time, and divide again by absolute atmospheric pressure. Written in consistent units, the leak rate equals volume times pressure drop divided by time times atmospheric pressure. Report the result in milliliters per minute and the number becomes portable across products, fixtures and factories.
As an illustration, a twenty liter bag that loses 0.05 kilopascals over a thirty second window is losing roughly twenty milliliters of air per minute at atmospheric pressure. Run the same calculation on the next size up and the leak rate is directly comparable even though the raw decay looked slower. This is the number to trend on a control chart, because it is independent of which fixture and which product size generated it.
Set the acceptance limit from data rather than from intuition. Build a population of known good units, measure their leak rates, and set the limit above the upper tail of that distribution with margin for normal process variation. Then confirm that units with deliberate defects of the smallest size you care about fall clearly outside the limit. A limit that was never validated against either population is a guess that will generate disputes.
Temperature Compensation
Gas pressure tracks absolute temperature, and the effect is large enough to swamp a genuine leak signal. A sealed volume that warms or cools by a single degree changes its pressure by roughly a third of a percent, which on a low-pressure test is comparable to the decay from a real defect. This is why a bag coming straight off a welding machine, still warm from the process, will show a falling pressure as it cools even when it is perfectly sealed.
The cheapest fix is procedural. Let units reach thermal equilibrium with the test area before testing, and do not test a warm bag. Standardize the dwell time in the test area, keep the area away from direct sun, doorways and welding machines, and record the ambient temperature next to the reading so a suspicious cluster of failures can be checked against a temperature excursion.
Where cycle time does not allow thermal stabilization, compensate in the instrument. Measure temperature in the test volume and correct the reading, or use a differential arrangement with a sealed reference volume of known integrity exposed to the same conditions, so thermal effects cancel. Both approaches cost more than a procedural control, so apply them only where the throughput justifies it, and validate whichever you choose against known good and known leaking units.
Advantages Over Water Dip
The first advantage is throughput. An air test runs in seconds, does not require the bag to be packed with absorbent material, and does not require the unit to be unpacked, dried and re-checked afterwards. On a program of several thousand units the labor saved is substantial, and the space saved by not needing dip tanks and drying racks is often larger than the footprint of the test rig itself.
The second advantage is data. A dip test returns a wet spot or a clean sheet; an air test returns a number. Numbers can be charted, trended and used to detect process drift before units start failing, which is the difference between a test that sorts product and a test that controls a process. Over a production run of weeks, that early warning is worth more than the pass or fail decisions themselves.
The third advantage is that the unit stays dry and clean. A bag that passes can go straight into packing with no drying step, no water spotting and no risk of mold in transit. For products with foam inserts, electronics compartments or padded dividers, avoiding immersion is not just convenient, it removes a whole class of secondary damage. Pair the air test with the pre-shipment inspection plan and the whole release decision happens on dry units.
Limitations and False Passes
The method has a blind spot worth stating plainly: it only finds leaks that connect the pressurized cavity to the outside. A channel that runs from the interior into a sealed-off pocket, a delamination inside a laminate, or a leak into a compartment isolated by a divider will not show up, because the air it admits stays inside the measured volume. Map the internal geometry of the product and confirm every region of interest is actually connected to the port.
Blocked paths cause false passes. A leak can be temporarily sealed by water, by debris, by a fold in the material or by a strap pressed against the shell by the fixture. A cold weld that opens only when the seam is flexed is the classic case: it holds air on the bench and opens the first time the customer loads the bag. Where flex-induced failures are a known risk, add a flex cycle before the leak test rather than testing only in the as-molded state.
False rejects are the mirror image and are usually caused by the rig rather than the bag. A leaking port seal, a bag that is still expanding during the measurement window, a temperature gradient, or a fixture that deforms the shell will all produce decay on a sound unit. Control them with a reference check at the start of every session, a stabilization period before measurement, and a documented setup that does not change between operators.
| Failure mode | What you observe | Control |
|---|---|---|
| Isolated internal compartment | Leak into a pocket does not register | Map internal geometry and confirm connection to the port |
| Flex-opening cold weld | Passes on the bench, fails in use | Add a flex cycle before the leak test |
| Path blocked by debris or a fold | False pass on a defective unit | Inspect and orient units consistently before testing |
| Leaking port seal | False reject on a sound unit | One-motion sealed fitting plus a reference check each session |
| Bag still expanding during measurement | Decay with no leak present | Stabilization period before the observation window |
Inline vs Bench Implementation
Inline implementation puts the rig at or next to the welding station so every unit is checked as it is made. The benefit is immediate feedback: an operator who sees two consecutive rejects knows within a minute that something has drifted. The cost is a fixture robust enough for production use, an operator trained to use it correctly, and cycle time added to every unit. Choose inline where the product value justifies one hundred percent verification.
Bench implementation puts the rig in a controlled area and tests samples drawn to a sampling plan. The benefit is quality of measurement: stable temperature, a better transducer, a more careful operator, and no pressure to shorten the cycle. The cost is latency, because a process problem is discovered after a batch has been built. Choose bench for routine release against AQL 2.5 under ISO 2859-1, which is the default for most programs and the basis of our AQL 2.5 inspection plan.
Most mature programs run both, and the split is a commercial decision rather than a technical one. Development and qualification happen on the bench, where data quality matters most. Production release happens by sampling, with inline checking reserved for products that carry a submersion promise or a high-value payload. Our 50L dry bag OEM programme runs submersion-class products inline and general-purpose styles on sampled bench testing.
Whatever you choose, tie it to the commercial framework so the test has consequences. Sampling takes 6 to 10 working days, extending to 12 to 15 working days for complex constructions, and bulk production runs 35 to 50 days after approval. Terms are FOB Xiamen with T/T 30/70, and MOQ is 500 pcs per style and colourway. Build the leak test requirement into the custom development programme at quotation stage, not after the first lot is built.
Frequently Asked Questions
Q1: What is air pressure leak testing and how does it work?
A: The bag is sealed around a single port, air is introduced until the internal pressure reaches a set point, the volume is isolated, and the pressure is measured over a fixed window. If it falls faster than the allowed limit, air is escaping and the seam or closure has a path to the outside. The unit stays dry throughout.
Q2: Why is air testing more sensitive than a water dip?
A: Air is roughly fifty times less viscous than water, so it moves through a fine channel far more readily, and pressure transducers resolve changes far smaller than a person can see on wetted tissue. A leak needing many minutes to show in a dip is often detected by air testing in seconds, which is why the two methods differ in severity.
Q3: What test pressure should be used for a welded bag?
A: Use the lowest pressure that still separates known good units from known leaking units. Soft goods are tested at a small fraction of the fabric hydrostatic head, because the goal is to find an open path rather than to prove the material resists pressure. Validate for each construction and confirm good units return to shape afterwards.
Q4: How do you convert a pressure decay reading into a leak rate?
A: Multiply the internal volume by the pressure drop, divide by the observation time, and divide again by absolute atmospheric pressure. That removes the effect of bag size, so a twenty liter duffel and a five liter pouch can be compared on the same scale. Report the result in milliliters per minute.
Q5: Why does temperature affect the air leak test result?
A: Gas pressure tracks absolute temperature, so a sealed volume that warms or cools by one degree changes pressure by roughly a third of a percent. That is comparable to the decay from a real defect. A bag that is still warm from welding will show falling pressure as it cools even when perfectly sealed, so standardize dwell time and record ambient temperature.
Q6: What causes false rejects in air leak testing?
A: The usual causes are a leaking port seal, measuring while the bag is still expanding, a temperature gradient across the unit, or a fixture that deforms the shell. Control them with a one-motion sealed fitting, a stabilization period before the observation window, and a reference check at the start of every session.
Q7: Can air leak testing produce a false pass?
A: Yes. It only detects leaks connecting the pressurized cavity to the outside, so a channel into a sealed-off pocket or an isolated compartment will not register. A cold weld that opens only under flex also passes on the bench. Map the internal geometry and add a flex cycle where flex-induced failures are a known risk.
Q8: Should air testing replace the water dip entirely?
A: No. Air testing is faster, dry and quantitative, which makes it the better screening tool, but it does not tell you where a leak is. Keep a water dip in reserve for diagnosis: use air testing to find that a unit fails, then dip or section it to locate the defect so the process can be corrected rather than just sorted.
Q9: Is inline or bench testing better for a production program?
A: Bench testing is better for measurement quality and is the right default for release sampling against AQL 2.5. Inline testing gives immediate feedback to the operator and suits products with a submersion promise or a high-value payload. Most mature programs use bench for qualification and development, then sample on the bench with inline checks reserved for critical styles.
Q10: How should the acceptance limit for decay be set?
A: Build a population of known good units, measure their leak rates, and set the limit above the upper tail with margin for normal variation. Then confirm units with deliberate defects of the smallest size you care about fall clearly outside it. A limit never validated against either population is a guess that will generate disputes.
Q11: Does air leak testing damage the welded seam?
A: Not when the pressure is selected correctly. Risk appears when the set point is high enough to load the weld in peel or to deform the bag permanently. Validate by pressurizing good units and confirming they return to shape, and re-validate whenever material thickness, weld width or bag volume changes materially.
Q12: How does air leak testing fit into an AQL 2.5 release plan?
A: Use it as the defect-defining test applied to every unit in the AQL sample. Draw the sample under ISO 2859-1 at general inspection level II, run the leak test on each sampled unit, and accept or reject the lot against the accept and reject numbers. Keep the leak results in the same lot file as cosmetic and dimensional findings.
People Also Ask
What is air decay leak testing?
Air decay leak testing pressurizes a sealed part with air, isolates it from the supply, and measures how quickly the pressure falls. A drop faster than the allowed limit means air is escaping through a seam, closure or pinhole. It is fast, dry and quantitative, which makes it well suited to high-volume screening of welded bags.
How sensitive is air pressure leak testing?
It detects channels far finer than a short water dip can reveal, because air flows much more easily through small openings and pressure change can be measured precisely. Practical sensitivity depends on the transducer range, the test volume and the observation window, so the limit must be validated against known good and known leaking units.
Why does a warm bag fail an air leak test?
Pressure in a sealed volume follows absolute temperature. A bag that is still warm from welding cools during the test, and the cooling lowers the internal pressure in exactly the way a leak would. Let units reach thermal equilibrium before testing, or compensate for temperature in the instrument when cycle time does not allow stabilization.
What pressure is used to leak test a dry bag?
A low pressure, well below the hydrostatic head the fabric is specified to, chosen to detect an open path without loading the weld in peel or deforming the bag. Validate the value by confirming it separates known good units from deliberate defects, and re-validate whenever the construction, material thickness or bag volume changes.
Can air leak testing find a leak in an internal compartment?
Only if that compartment connects to the pressurized cavity. A channel leading into a sealed-off pocket or a region isolated by a divider will not register, because the escaping air stays inside the measured volume. Map the internal geometry of the product and confirm every region of interest is connected to the port.
Should every welded bag be air leak tested?
Usually not. Statistical release against AQL 2.5 under ISO 2859-1 is the economical default. Test every unit when the product carries a submersion promise, when the payload is high-value, or when process data shows instability. Inline testing makes one hundred percent checks practical where the fixture and cycle time allow.