Leak Testing: A Detailed guide you should never miss


NDT Testing methods are the best way to find out discontinuity in materials. Where Leak Testing is one of them inside the NDT Methods.

If you are here to know about the leak testing, Then you landed on the exact helipad with your helicopter. Just Kidding !!

Here you will get the complete information about the leakage test, from scratch to end. Only you need to come with me and I will take you to the rail of Knowledge.

So, I invite you to come and join me and let me start the flow.

Before going deep, I will start with basics, it will help you further in the long run. So, be with me and scroll slowly and gently.

What is leakage?

The escape of liquid or gas from a pressurised or evacuated enclosure or system via imperfection such as holes, cracks, is Known as leakage.

In General, When liquid or gas is coming out due to cracks or holes, can be said leakage is formed.

The leak needs pressure differences to generates flow from higher to lower pressure.


Knowing about leakage, a question may arise in your mind that how does leakage form? So, Here is the answer-

How does Leakage form?

When gas or liquid travels with pressure, it generates pressure on the material and it reduces the holding strength over time. This may one of the most reasons for leakage formation. 

Material Aging and corrosion is the most another reason of leakage formation. 

In both scenarios, Cracks, Holes and other forms of imperfection can be seen on the material.

More on Leak.

After knowing the leakage and its formation, I am heading to the mainstream of the article, ie, What is leak testing and more on road. So, Scroll gently and slowly.

What is leak Testing?

As the name suggests, A form of Non-Destructive Testing, Where the material is tested to check leakage to maintain the proper functioning in the long run. Known as leak testing or leak test.

It is used to detect if the product includes any defects that could allow any leakage

Leak testing is measured upon the leak limit of material or specified by the client in general, As I will discuss leak limit later on in this article.

Stay in touch.

Principle of Leak Testing-

Basically, Leak works on the principle of Thrusting

Where Any object or fluid is pushed quickly with force, Known as Thrust and Thrust per unit area is known as pressure.

Where you know, Fluid is thrown out from the defect with force, If any area of material is allowing leakage due to any kind of discontinuity.

Units of leak Testing-

The most commonly used units of leakage rates are-

  • Standard Cubic centimeter per second(std.cm3/s)
  • Standard atmosphere cubic centimeter per second(atm.cm3/s)
  • For vacuum leak test- torr litre per second(torr l/s)
  • SI Unit is Pascal cubic meter per second(Pa m3/s)

Leak rates are generally measured in terms of the quantity of gas leaking in one second.

Leak Limit-

Here is a list of substances with an acceptable leakage limit-

Substances Acceptable Leak Limit(std.cm3/s)
Water 1×10-3
Gasoline from Storage Tank 1×10-4
Gas from Pipelines 1×10-5
Leakage in Tanker 1×10-6

These are some substances with an acceptable leak limit, considered during the leak inspection.

After discussing the basics of leak testing, I am heading to the procedure and methods of the leak test. 

I hope, you are still there and flowing with words of knowledge. If so, stay tuned and scroll gently. 

Here is the procedure and methods of leakage testing-

Procedure of Leak testing-

There are Three major steps is taken to perform leak testing-

  1. Locating the leak
  2. Measuring the leakage rate
  3. Monitoring the leakage for maintenance of the system
  1. Locating the Leak– The first and foremost step is to identify the location of the leak, Where the leakage is happening in the specimen due to any kind of the discontinuity. Once, exact leaking location is identified, further steps can be taken.
  2. Measuring Leakage rate- Then, the rate of leakage is measured based on the fluency of the fluid coming from the specimen, How fast or slow, the fluid is coming out from identified leakage location in the specimen.
  3. Monitoring the leakage for maintenance of the system– This is the most crucial step, where the leakage is monitored for a certain period of time to check, whether it is in acceptable limit or permissible limit or it is causing some issues ahead. Based on the monitored data, a further decision is taken, whether to call for maintenance or leakage is within the permissible limit. 

I hope you doing good. Now, Knowing about various steps procedures, Now it’s time to head to methods of the same. 

Methods of Leak Testing-

There are several leak testing techniques elaborated on below. Have a look-

  1. Ultrasonuc leak testing
  2. Bubble leak testing
  3. Dye penetrant leak testing
  4. Ammonia colourimetric leak testing
  5. Pressure Decay leak testing
  6. Differential pressure decay leak testing
  7. Helium leak testing
  8. Vacuum Leak testing

1. Ultrasonic Leak Testing-

Ultrasonic leak test is generally used to detect leakage in Gas in high pressurized pipelines. Gas produces an ultrasonic sound When escaping from the pore. That ultrasonic sound helps to find out leakage in the specimen with approximate sensitivity of 10-3 atm cc/s.

when gas comes to the leakage area, it generates turbulence in the flowing path producing a broad spectrum of sound called ‘white noise’ in the leakage area. Where larger the leakage intensity produces greater noise and similarly ultrasonic sound.

And these Ultrasonic sounds are measured through an ultrasonic leak detector in ultrasonic leak testing. The ultrasonic leak detector is used to detect in various industries like internal combustion engines, air brakes systems, bearings, hydraulic components and as well as in piping systems.

Fig. Ultrasonic Leak detector-

ultrasonic leak testing

The ultrasonic leak detector is used to detect in various industries like internal combustion engines, air brakes systems, bearings, hydraulic components and as well as in piping systems.

 2. Bubble Leak Testing

When the high pressurized specimen is placed under low-pressure fluid or water, it forms bubbles inside the water due to leakage inside the specimen, Known as Bubble Leak testing. Bubble leak testing is also known as the Water immersion test or Air bubble test.

10-3 to 10-4 atm cc/s is the estimated sensitivity of the bubble leak test.

There are basically three types of bubble leak testing. Those are-

  • Liquid Immersion
  • Liquid Film
  • Foam Application

Dye penetrant leak testing

When a Dye used in Dye penetrant testing, is sprayed to the suspected leakage area, it sucks dye inside due to capillary action, if leaks are present, it can be seen later on with a red mark on the surface. Showing in Fig.

It is a time-consuming method that involves a long waiting time to get the result.

Fig. Dye penetrant leak test-

dye penetrant leak testing

Ammonia colourimetric leak testing

In this testing method, Ammonia and developer are used as a chemical, Where Ammonia is used as a tracer gas.

During the procedure, Ammonia is induced in the specimen and the powdered form developer is sprinkled on the outer surface of the suspected leakage area.

Now, With a certain period of time, ammonia reacts to the developer resulting change in colour. This change in colour of the developer indicates a leak on there, where the change in colour is observed.

Pressure Decay leak testing

A specimen is pressurized by air or gas until set pressure. Then isolation valve is removed from the supply pressure. Then, pressure is monitored using a pressure gauge or sensor.

If air or gas leaks, pressure formed inside the specimen drops.

The leak rate can be calculated based on the change in pressure over a certain period of time. This test is also known as the Pressure change leak test.

Fig.- Pressure decay testing-

Pressure decay testing

Differential pressure Decay leak testing-

Differential pressure decay test is a more accurate method of Pressure decay test. where differential pressure transducer is used to detect the pressure differences between reference specimen and test specimen.

Due to the reference specimen, the pressure sensor range is reduced, resulting in more accurate results even for small leakage.


Helium leak testing

The most used method, Helium leak testing developed in 1940. In this test, two devices- mass spectrometer and sniffer probe are used.

This test can be performed with two modes

Sniffer probe Mode– The specimen is pressurized with helium gas and then the outer surface of the specimen is scanned through the Sniffer probe connected to the mass spectrometer to locate the leak position.helium leak testing sniffer mode

Accumulation Mode– In this mode of testing, the specimen chamber is pressurized with helium gas and then helium gas is evacuated. Then, the mass spectrometer is used to detect the presence of helium gas leaking into the chamber.

helium leak testing accumiulated mode

Vacuum leak testing

A specimen is placed inside the vacuum chamber. Now, total air inside the vacuum chamber is evacuated through the vacuum pump.
Now, Leak detecting gas is supplied in the specimen. If the leak is present in the specimen, gas will come out through the leakage pore, this gives the signal to the gas leak detector and the detector measures the leakage rate of gas and informs leakage.

This is Quick, More reliable and can find micro leaks also.

A vacuum leak test is used to inspect various components like- AC components, Airbag generators and igniters, Fuel systems, Heat exchangers, Heat pumps.

This video guide will help you to get more on vacuum leak test. Give a watch-

Are you there?

If so, I hope you are enjoying the reading. Now, These were the most discussed methods in Leak testing, Which I discussed in detail above. Now, I will move you to the advantages, disadvantages and applications of the leak inspection. So, stay with me and move your finger slowly.

Advantages of Leak testing-

Here are some advantages of leak detection test, Given Below-

  • Accuracy
  • Cost-Efficient
  • Relatively Fast process
  • Non-Destructive

Disadvantages of Leak testing-

Here are some disadvantages of leak inspection, Given Below-

  • Messy
  • Time-consuming
  • Continuous Monitoring needed
  • May Influence workflow


Here are some applications of leak detection, Given Below-

  • Pipe Lines
  • Boilers
  • Heat pumps
  • Heat Exchangers
  • Hydraulic systems etc.

Now, After knowing the pros, cons and application of leak tests, I am heading to leak test machines or systems. Stay tuned !!

Leak testing machines-

Here are some names of leak detection machines or systems, given below-

  • Hermetic(Airtight) enclosure and components
  • Hermetic Systems
  • Evacuated enclosure and components
  • Vacuum systems

Standards and codes-

Here are some Acceptance criteria of leak test, Given below-

ISO 20484:2017– Vocabulary of leak testing
ISO 27895:2009- Leak testing of valves
ISO 3530: 1979- Spectrometer-type and leak detector calibration

Know more on Standards


Leak testing is the commonly used method of NDT in various industries, to enhance productivity and reliability As I discussed above the same in detail.

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I hope to see you soon, Thank you for being with me.



Before you go, Here are some FAQs you should consider looking around-


Q. How do plumbers test for leaks?

Ans.- They use ultrasonic sound to detects leakage in plumbing.

Q. How long does a leak test take?

Ans. Leak tests can take 1 to 5 hours, depending on the methods of leak tests.

Q. Is there a tool to detect water leaks?

Ans. Now these days, there are many advanced water leak sensors available in the market, by which water leakage can be detected easily.


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