What are the differences between NDT and Condition Monitoring?

There are lots of ways that you can ensure your site is working well. But there can be quite a bit of confusion about the different types of inspection services, and one, in particular, is the difference between condition monitoring and non-destructive testing. Understanding the difference is vital to ensure that your site is functioning safely.

What is condition monitoring?

Condition monitoring (CM) is the practice of monitoring an output of a machine in order to assess its performance. For it to count as “monitoring” and not just “condition assessment”, several readings must be taken over a period of time and the data analysed to determine if there are any changes. The outputs that are normally measured include noise, vibration, pressure, and temperature amongst others. They can indicate whether a machine is working as it should. Keeping an eye on these measurements can then help engineers determine if a machine needs maintenance or repair.

What machinery is monitored?

Condition monitoring only works on machines that move, as they have outputs that can be measured easily. Equipment such as pumps, internal combustion engines, and electric motors are examples of assets that can be monitored in this way.

Normally, a site splits its equipment into three categories – critical, essential, and general purpose/balance of plant machinery.

Critical machinery is items without which the site cannot function. For example, a gas turbine within a power plant. Normally, these items are monitored continually.

Essential machinery includes items that are a significant part of the process, but if they fail, the process can still continue.

Equipment that is deemed general-purpose or “balance of plant” can be monitored periodically with a handheld device. The data is fed into a computer where an analyst or computer program analyses the information. The measurements have to be made reasonably frequently for this to create enough data to analyse.

In the case of periodic testing of general-purpose equipment, the line between condition monitoring and non-destructive testing can potentially blur in people’s minds. If the asset is only being periodically measured, people can think that non-destructive testing has taken place. But the true measure of whether a test is condition monitoring or non-destructive testing is based on what is being measured.  If the periodic measurement is of the machine’s output – for example, its vibrations – this still constitutes condition monitoring.

How is it monitored?

In cases where an asset is a critical component, condition monitoring equipment tends to be in situ, monitoring the asset at all times. There are different ways of monitoring equipment, based on the item in question. These techniques include:

  1. Vibration Analysis
  2. Oil Analysis
  3. Thermography/Infrared Thermography
  4. Motor Circuit Analysis
  5. Ultrasonic Monitoring/ Acoustic Analysis
  6. Radiation Analysis
  7. Laser Interferometry
  8. Electrical Monitoring
  9. Electromagnetic Measurement
  10. Performance Monitoring

In the case of Vibration Analysis, as each moving part of the machine moves and therefore vibrates at its own frequency, the technician can often determine the exact location and type of fault based on the frequencies emanating from the machine. This allows gradual wear and tear to be identified and a schedule of maintenance to be put in place weeks in advance, meaning there is controlled and limited disruption to site maintenance.

Other techniques use a “baseline” measurement of optimal output to determine whether the machine is working correctly. Any deviation from these measurements alerts the technician that there may be a problem.

How is it different to non-destructive testing?

1.       Condition monitoring can only work on moving parts

Condition monitoring can only work on equipment that moves, as there has to be a continual output that can be monitored to determine whether the equipment is working correctly. If a piece is static (like a pipe), non-destructive testing has to be used to determine the condition of the asset.

2.       Condition monitoring measures continually

Non-destructive testing assesses the condition of a piece of equipment at one fixed point in time. The test can detect weaknesses that may become faults in the future. Condition monitoring collects data points along a timeline, stitching the information together to form a picture of the machine’s performance. This gives a very accurate measurement of the machine’s performance, allowing the technician to determine when a flaw might happen and to take steps before this takes place.

3.       Condition monitoring often requires NDT to find the fault

Although Vibration Analysis can often determine the exact location of the flaw, many of the CM tests cannot track down and identify where the fault is located. This is where non-destructive testing must be used so that the flaw can be found and fixed.


Condition monitoring and non-destructive testing are both essential parts of site maintenance and safety. Both have to work together in order to generate a complete picture of the condition of the site’s equipment.

Responsive testing that delivers assurance that your equipment is safe.

Contact us for a quality testing service with a fast report turnaround. We can respond to any challenge.


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