Dye Penetrant Testing(DPT): Defination, Principle, Procedure,types

A significantly old and widely used NDT Method, Liquid Penetrant test(LPT) or Dye Penetrant Testing(DPT) or Penetrant Testing(PT) followed by various industrial sectors to detect discontinuity in their Test object to get quick results before applying it.
In this article, you are going to get, all about Liquid penetrant testing from Scratch to end. The only destination to get knowledge about the same.
So, Sit back and relax, Scroll with to get all about LPT in front of you at your fingertip. So, without wasting more time, Let’s Get started…

What is Liquid Penetrant Testing(LPT)?-

A Non-Destructive Method to inspect surface-discontinuity in test specimen using Penetrant and Developer, Known as Liquid Penetrant Testing.
LPT is also known as Dye penetrant testing(DPT), and Penetrant Inspection(PI), as I discussed above.
As you go through the definition, You saw the terms- penetrant and developer. Here is the meaning of those terms, Given below-
Penetrant- Penetrant is liquid form Spray material, usually comes in red colour, used to spray on test object to sip in the flaws on the object surface. 
Developer- A liquid form white coloured spray material, used to Suck-out sipped penetrant from the flaws on the object’s surface. It is usually sprayed from the height of 8 to 12 inches.
As you know the basic terms of Dye penetrant testing, Now it’s time to discuss the principle of LPT. 

Principle of Liquid Penetrant Testing-

LPT works on Capillary Action. This is the basic principle of Dye penetrant testing.
Sipping of Penetrant in flaws or you can say, the narrow opening area is based on the Capillary action. Due to this action liquid gets slipped inside the flaws. 
Now, you may wonder what is capillary action, then relax and scroll to the next line. 
Capillary Action- A process of liquid travelling to a narrow opening area without applying any external force due to intermolecular attraction is known as Capillary Action.
Fig.- Capillary Action in Water
<img data-src="cappilary-action.png" alt="Capillary Action">

To know more about Capillary Action. Click Here

Where Dye penetrant Testing can be used-

DPT can be used for various Non-porous materials, like-
  • Plastic
  • Glass
  • Metals
  • Ceramics etc.  

These are some non-porous materials, Where DPT is used to inspect flaws. Mostly, Metals are considered to inspect through DPT.

 Materials Used to Perform Dye Penetrant Testing-
The various liquid penetrant testing materials or test kits are needed to perform DPT-
  • Penetrant
  • Developer
  • Cleaner
  • Cotton Cloth
Now, It’s time to discuss the procedure of liquid penetrant testing. So, Are you ready to uplift your knowledge to the next level in simple and understandable language, without any fancy or heavy words?
So, Here is the procedure of Dye penetrant Testing-

Liquid Penetrant Testing Procedure-

Here is the guide to liquid penetrant testing steps or Procedures-
Surface Cleaning- 
Before performing the DPT test, the Surface must be cleaned and there must not be any dust, rust, oiling or grease or any other foreign substances on the surface of the test object. This is one of the important steps in DPT to get an accurate result. So, It needs to be done carefully and precisely.
<img data-src="Suface Cleaning.png" alt="Suface Cleaning">
Applying penetrant- 
After cleaning the surface thoroughly, We apply penetrant, A red coloured liquid spray, Applied on the surface evenly. Due to Capillary action, penetrant seeps down inside the flaws. 
Now, Surface is left till Dwell Time.  
Dwell TimeWhen test specimen is left for a certain period of time, to get penetrant seeped inside to flaws thoroughly, and that time duration is Known as Dwell Time.  
Dwell TIme can be, From 05 minutes to 60 minutes or more than that, depending on the type of penetrant, the material used and size of flaws.
p <image src"Penetrant Applied.png" alt="Penetrant Applied">
Removal of Excess Penetrant- 
In this step, Excess penetration is removed thoroughly. 
Cotton cloth can be used to clean the excess penetrant until we stop getting red colour on the cloth.
Keep in mind, do not clean in zig-zag direction instead of clean it in uni-directional or straight in-line to the applied penetrant to avoid any chances of removal of sipped penetrant inside the flaws.
The cleaner can be used to clean the surface more precisely. But, it should not be applied directly to the surface. instead of directly applying on the surface, spray cleaner on the cotton and then use it to clean until stop getting penetrant colour on the cotton cloth.
Application of Developer- 
After successful removal of excess penetrant, the Developer is applied from 8 to 12 inches of height.
                        <img data-src="Application of developer.png" alt="Application of developer">
The developer makes a thin film on the surface of the test specimen. The developer works as a bloating paper, It sucks sipped penetrant out to the visible surface. 
<img data-src="Flaw Detection.png" alt="Flaw Detection">


After applied Developer, It is left for a certain period of time(ie, 10 to 60 minutes or more than that), Known as Developing time
Before Applying, the Developer must be shaken thoroughly.
After that, If any flaw will be present in the test specimen, can be view an indication in the red lines on the top of the surface after developing time.
<img data-src="Cavity Detection.png" alt="Cavity Detection">
In the last step of Liquid penetrant testing, the Surface is cleaned thoroughly 
and send to repair, if needed.
Fig. Dye penetrant test diagram
➤Here is the Video Guide to Know More, Give a watch before going for further reading-


➢These were the procedure and I think, you got an idea of How DPT is performed. Now, it is the best time to discuss penetrant types to get you to know more about penetrant. So, Here we go… 

Types of Penetrant used in Dye penetrant Testing-

Based on the characteristics and performance, Penetrant divides into Two categories-
  1.  Fluorescent Penetrant 
  2.  Non-fluorescent penetrant
1. Fluorescent Penetrant- 
This type of penetrant is used to detect flaws under ultraviolet light and needs a specific place to inspect.
➤This Video Guide may help you to understand more about Fluorescent Penetrant. Give a watch-


2. Non-fluorescent penetrant- 
This type of penetrant is used to detect flaws in broad light or under daylight conditions. The minimum requirement of light in this type of penetrant is 1000 LUX.
Based on penetrant removal application, Penetrant Classification-
  1. Water- Washable
  2. Solvent Remover 

1. Water- Washable-  This type of penetrant can be washed through the water.
2. Solvent Remover- This type of penetrant needs solvent to remove the dye on the surface.
✹ Based on the sensitivity, Penetrant Classified into five categories-
  1. Ultra-low 
  2. low
  3. Medium
  4. High
  5. Ultra-High Sensitivity
After getting knowledge of various penetrant types. we are moving to the advantages, Disadvantages, applications and limitations of Dye penetrant testing. You have a lot more to know till now. So, Scroll with me line by line.
First of all, Here is the Advantages of Liquid penetrant testing-


Here are some major advantages of Liquid penetrant Testing, Given below-

Quick- Startup- 

The main advantage of DPT is to QuickStart. No fancy or heavy setup is required.

Minimum size Discontinuity Detection-

Dye penetrant testing is able to detect the smallest of flaws presented in the test specimens, which humans can not see through.

Higher Sensitivity-

DPT has a higher sensitivity level with small flaws or discontinuity. It can detect the smallest range of flaws in the test objects.

Inspection of the wide range of materials-

DPT can inspect a wide range of materials, which comes in a non-porous materials range.

Low cast-

No heavy machinery or equipment is required to perform the test. Penetrant, Developer and cleaner are enough to start the inspection.



Here are some major Disadvantages of Dye penetrant Testing, Given below-

Surface-Discontinuity limitation-

The major disadvantage of dye penetrant testing is the limitation of inspecting surface flaws only. This can detect surface-level discontinuity, Subsurface is not detectable.  

Time-consuming procedure-

Due to Dwell time and Developing time, it consumes so much of time. Inspector needs to wait until dwell time and developing time.

Chances of Discoloring of Material-

There is a probability of some materials may be permanently discoloured due to penetrant and developer colour.

Non-porous materials Detection-

Only non-porous materials can be inspected through this method,. This is one of the major disadvantages of DPT.



Here are some Applications of Liquid penetrant Testing, Given below-
  • Pipelines service & maintenance
  • Aircraft component service
  • Gears and mechanical components


Here are some Limitations of Liquid penetrant Testing, Given below-
  • Multi-operational
  • Porous materials limitation
  • Multiple Cleaning

Acceptance Criteria of Dye Penetrant Testing-

ASME Boiler pressure vessel code(BPVC) section-8 Division-I gives a detailed guide for acceptance criteria for liquid penetrant testing.

Here are some major points overview of BPVC Sec-VIII DIV-I – 

Relevant Indication– An indication greater than 1.5mm can be considered as a relevant indication and indications can be ignored which is less than 1.5mm. 

Rounded Indication– Any indication length is less than three times of width and goes up to 5mm, can be accepted, And rejected when the value increases more than 5mm.

Several Rounded indications– >5mm but 4 rounded indications together with edge to edge difference of 1.5mm between 4 rounded indications can be rejected.

Elongated indication– This type of indication can be rejected regardless of size. 


Click Here to know more about Acceptance Criteria

Standard and codes of Dye penetrant Testing-

 Here are some Dye penetrant testing standards and codes procedure follows during the test-
ASTM E-165- Standard practice for liquid penetrant examination for general industry
ASME BPVC Section-V Article-6- Liquid penetrant examination


In 1890, Dye penetrant testing is started with kerosene oil, used as a penetrant and chalk dust mixed with alcohol, used as a developer.
In 1890, The name of this method was Oil and Whiting method. This method is used to detect large cracks in railroads.
Further, Much research is done to advance the process of LPT and capillary action is discovered and this theory became the basic principle of liquid penetrant testing.



One of the earliest and most straightforward non-destructive inspection techniques is the dye penetrating test (DPT), Liquid Penetrant Testing(LPT) or Dye penetrant testing(DPT) is a widely used and low-cost inspection method among Other NDT methods. This method can detect flaws, which beyond the human eye’s capacity to see.
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Before you go, Here are some liquid penetrant testing Q&A sessions, Have a look at these too.
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