What Is A Leak Test?

A leak test is used to determine if an object, product, or system functions within a specified leak limit. Leaks occur when gas or liquid flow through an object via an imperfection or manufacturing defect such as holes, cracks, weak seals, etc. Leaks always flow from higher pressure to lower pressure; leak testers use pressure to generate and monitor that flow.

Common Test Types Include:

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Other Leak Test Types Include:


  • Bubble/Submersion – This test simply putts pressure on the part, then dunks the part under water and looks for bubbles. This is a very common test
  • Hard Vacuum (Helium) – A part is charged with helium and then placed in a chamber. The Chamber has a vacuum pulled on it. A sensor in the chamber then detects any helium leaving the part and entering the chamber.  Helium is used because of the molecule size; helium gets out of smaller holes than air or nitrogen can.
  • Helium Accumulation – Helium is light, so it rises. This method puts the part under helium pressure and then captures the escaping helium in an umbrella shaped cone as it rises.
  • Helium Sniff – Similar to Helium Accumulation but the operator uses a wand to sniff the part and find where the helium is leaking out of the part.
  • Helium Spray – Vacuum is pulled on the part through a tester equipped with a helium sniffer. Helium is then sprayed around the part. Any air leaking into the part is picked up by the helium sniffer inside the tester.
  • Tracer Gas – This is another name for helium testing where other gases, such as hydrogen, are used.  Smaller gas molecules are used to find smaller leaks.
  • Ultrasonic – This method relies on higher-pressure air creating a sound that is picked up with a high sensitivity microphone.  Imagine the air creates a hiss that is detected by its frequency. 

A leak test can be performed on a wide variety of applications.

Leak Rate

A Leak rate is expressed as a volume per unit time. The rate is found by measuring the change in pressure multiplied by the volume and dividing that by the change in time multiplied by surrounding atmospheric pressure.

Leak Rate Equation

atm = Atmospheric pressure (psia)
V = Test volume (cm3)
Δp = The decay in pressure during test time
Δt = The amount of decay time (min.)
sccm = Standard Cubic Centimeters per Minute

For example:
Leak rate = .02psi/0.05min * 50cm3/14.7psia
Leak rate = 0.4 * 3.401
Leak rate = 1.36 sccm

Leak Rate Calculator

Atmospheric Pressure:

Leak Rate:

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Our leak test specialist are happy to answer any questions you may have about test types and which tester would best suit your products needs. 

Which Test Type Is Best For My Application?

The test type is dependent upon your product and its application. For example, only a chamber test can be run on an enclosed object whereas objects with a single opening may use multiple other tests. The most common tests include Pressure Decay (PD), Vacuum Decay (VD), Occlusion (Occ), Mass Flow (F), Burst (B), Crack (C), and Chamber (PR).

Pressure Decay

Pressure decay is one of the most widely used methods of leak testing in manufacturing and is ideal for small, sealed components with a single access port. In this test, a product is attached to a leak tester and filled with air. Once pressurized, the air source is valved off and the pressure is allowed to settle. During the test any decrease in air pressure over time signifies a leak.
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Pressure Decay Test Diagram:

Mass Flow

The Mass Flow test measures flow rate through an object. Once the object is attached to the test port it is pressurized with regulated air. The flow is  then measured with a Mass Flow Sensor. Objects that have a flow rate that falls between the max flow value and the min flow value pass while those that do not, fail. Mass flow is normally used with parts that have two ports open to atmosphere.
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Mass Flow Test Diagram:

Vacuum Decay

Vacuum decay tests are the inverse of the same principle, simply creating a negative pressure instead of a positive pressure.

Vacuum Decay Test Diagram:


Measures the passage of air through an object and signifies if the object’s passage is blocked.

Occlusion Test Diagram:

Downstream Occlusion

An occlusion test which includes a downstream release for more precise occlusion test results.

Downstream Occlusion Test Diagram:


A destructive pressure test that measures the maximum pressure at catastrophic failure.

Burst Test Diagram:

Pressure Cracking

Similar to a burst but more sensitive, often used to detect events such as a valve opening.

Crack Test Diagram:


Used to find leaks in sealed packaging or devices that do not include an opening for filling.

Chamber Test Diagram:

Specialty Tests

In some cases, the standard leak tests are not enough. At Zaxis, we offer several high precision specialty tests to ensure product viability. Using one or more specialty tests, in concert with standard leak tests, can provide the highest level of quality assurance for the development and manufacturing of any product.

Specialty Tests Include:

  • Backpressure Flow – While holding the flow rate constant, measure the back pressure created by the part

  • Pressure Increase – Pressurize one side of a multichambered part and inspect for an increase on the adjacent side

  • Valve Reseat – Measure the amount of pressure at a check valve closure (used in conjunction with the Crack test that will find the opening pressure)

  • Creep (Ramping Fill) – Ramping fill pressure decay test that can be set for multiple stages. Similar to a burst test, with allowances for expansion of the part material

  • Pressure Exercise – Exercise a check valve with positive pressure prior to a crack test

  • Vacuum Exercise – Exercise a check valve with vacuum pressure prior to a crack test

Speak to a Specialist

Our leak test specialist are happy to answer any questions you may have about test types and which tester would best suit your products needs.