By Tanner Vandenberg, CEO, FlexPak Leak Detectors Inc.
ASTM D4991 is the Standard Test Method for Leakage Testing of Empty Rigid Containers by Vacuum Method. It checks whether an empty bottle, jar, can or jerrican, with its closure fitted, leaks under the kind of pressure difference that can happen during air transport. It is written for rigid containers that carry some hazardous liquids under the UN TDG and the ICAO Technical Instructions, and it tests up to a limit of about 95 kPa (13.8 psi).
Often shortened to ASTM 4991, it is the ASTM vacuum leak test for rigid containers. It sits under ASTM Committee D10 on Packaging (subcommittee D10.21), and the current edition is D4991-25.
Overview of ASTM D4991
Understanding the Standard
The standard answers one question: at what pressure difference does this container and closure start to leak? You test the container empty, so the result reflects the container and its closure, not the product. You also test it as it will ship, because the method can pressurize a container without modifying the closure or the body.
The Vacuum Method Explained
The empty, closed container is immersed in an ethylene glycol and water solution inside a transparent test chamber. A partial vacuum is drawn and increased gradually. As the pressure outside the container drops, the air inside pushes outward. A leak shows up in one of two ways:
- Bubbles escaping from the container while the vacuum is applied.
- Test fluid inside the container after the chamber returns to normal pressure, because the fluid was drawn in through the leak.
The result is the point at which leakage starts, up to the method’s limit of about 95 kPa.
What Changed in ASTM D4991-25
The 2025 revision tightened how deep the container sits in the test fluid. Section 6.4 moved from “no more than one inch under the surface” to “no less than one inch below the surface,” as Westpak summarized. If your written procedure still says “no more than one inch,” update it.
Significance of ASTM D4991 Leak Testing
Importance for Regulatory Compliance
ASTM D4991 is a test method, not a regulation. The rules that matter for air shipments of hazardous liquids are the UN Recommendations on the Transport of Dangerous Goods and the ICAO Technical Instructions. D4991 is one recognized way to test rigid containers against the pressure difference those rules are concerned with. Check the exact requirement for your material and packing group in the regulation that applies to your shipment.
Why Air Transport Is the Risk
At altitude, the air outside the container is at lower pressure than the air sealed inside it. That difference pushes on the closure and the container walls. A cap that is tight on the ground can weep in flight, and a leaking hazardous liquid in an aircraft hold is a serious safety problem. D4991 reproduces that pressure difference in a chamber, where a leak costs a container instead of a shipment.
Enhancing Consumer Safety
Most of the people protected by this test never see the container: cargo handlers, flight crew and anyone downstream of a spill. Testing the container and closure together, the way they ship, catches the combination that fails, such as a closure torqued too lightly for that bottle.
Cost-Effective Practices
Finding a leaking closure design in the lab is cheaper than finding it after a rejected or damaged shipment. It also gives you a documented result to show a shipper or customer who asks how the packaging was qualified.
Scope and Application of ASTM D4991
Industries and Products Affected
D4991 applies to anyone shipping liquids by air in rigid containers, and especially hazardous liquids. Typical specimens include plastic and metal bottles, jars, cans and jerricans with screw, lug or snap closures. The standard notes it may not suit some packages, such as those with paper cap seals, because the test fluid can quickly damage them.
Comparison With Other Standards
D4991 is easy to confuse with other vacuum-based packaging tests. Here is how they differ:
| Standard | What it tests | Container state |
|---|---|---|
| ASTM D4991 | Leakage of rigid containers under vacuum, up to about 95 kPa | Empty, closed, immersed in test fluid |
| ASTM D5094 | Gross leakage of liquids from containers with threaded or lug-style closures | Filled with liquid |
| ASTM D3078 | Leaks in flexible packages by bubble emission | Flexible, with headspace, immersed |
| ASTM D6653 | Effects of high altitude on packaging systems (conditioning, not detection) | Packaged as shipped |
For how these fit alongside the other packaging standards, see our guide to ASTM packaging standards.
Key Procedures in ASTM D4991 Testing
Preparation of Test Samples
Test containers empty and clean, with the closure applied the way it will ship. In practice that means using the closure method and torque the closure maker, container maker or shipper specifies, since an under-torqued cap is one of the most common causes of a leak. Several specimens can be tested at once if the chamber leaves room to watch each one clearly.
Execution of Vacuum Test
With the containers at least one inch below the surface of the test fluid, increase the vacuum gradually and watch for a steady stream of bubbles from any container. Record the vacuum level at which leakage starts, or that none occurred up to the level you are testing to. After returning the chamber to normal pressure, check each container for test fluid inside.
Challenges and Considerations in Leak Testing
- Reaching the full differential. The pressure difference a vacuum chamber can create is limited by the atmospheric pressure where you test. At a high-elevation facility the air is thinner, so the maximum achievable differential is lower than at sea level. Check that your equipment and site can reach the level your requirement calls for before you rely on the result.
- Trapped air is not a leak. Air clinging to threads or the outside of the container can release as isolated bubbles. A leak shows as a steady, repeating stream from one spot.
- Packages the fluid can damage. Paper cap seals and some liners may not survive the test fluid, as the standard itself notes.
How FlexPak Fits
FlexPak leak detectors are transparent vacuum chambers built for immersion testing, the same style of setup D4991 describes, and they let you see exactly where a container is leaking. The maximum vacuum you can reach depends on the configuration and your site, so if you are testing to a specific differential such as the 95 kPa limit in D4991, tell us your requirement and elevation and we will confirm whether our equipment reaches it before you buy.
ASTM D4991: Quick Answers
What is ASTM D4991?
It is the ASTM test method for leakage testing of empty rigid containers by vacuum, used mainly to check containers for hazardous liquids shipped by air.
Is ASTM D4991 required for air shipping?
Not by itself. The requirements come from the UN TDG and the ICAO Technical Instructions. D4991 is a test method you can use to show a rigid container meets the pressure-difference concern those rules address.
What is the difference between ASTM D4991 and ASTM D5094?
D4991 tests an empty rigid container under vacuum. D5094 tests a container filled with liquid for leakage past a threaded or lug-style closure.
Not sure which test your container needs? Tell us the container, closure and how it ships, and we will recommend a setup and send a quote within 24 hours.