Understanding ultrasonic thickness measurement

Ultrasonic thickness measurement has a role in almost all industrial applications. To better understand how the technology works, and how it can be applied, MEPCA sat down with technology specialist, Cygnus Instruments.

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How does ultrasonic thickness measurement technology work?

Taking ultrasonic thickness measurements is one of the most fundamental practices within a family of analysis techniques known as Non-Destructive Testing (NDT).

An ultrasonic thickness gauge (UTG) will allow an engineer to test the thickness of a material from one side only (as opposed to boring a hole in the material and using a calliper).

To operate the unit, a simple calibration is performed on the material to be measured and then ultrasound is passed in to the material via a transducer (sometimes known as a probe); the gauge measures the time taken for the ultrasound to pass through the material and return &#; and thus a thickness measurement can be calculated and is presented.

What are the typical applications for ultrasonic thickness testing in plant maintenance routines?

A thickness gauge can be used to measure any metal (or dense engineering material such as HDPE) and so the scope to involve ultrasonic thickness testing in plant maintenance is extensive. However, the most common application for the UTG is with pipework.

Pipework can be subject to myriad types of corrosion; galvanic corrosion, deposits, microbiologically influenced corrosion (MIC) and chemistry can all cause wall thinning in pipes. And, unlike some other assets within plant maintenance, pipework rarely stops working; as such erosion &#; due to flowing liquids (and air bubbles) &#; is a simple and ever-present cause of wall thickness loss. If ignored or forgotten, there is the threat of eventual pipe failure that can lead to potentially highly costly shutdown and, worse still, industrial accidents.

However, the simple act of using a UTG as part of regular, planned plant maintenance &#; or for more ad hoc inspections as and when needed &#; will help to ensure that wall thinning in pipework can not only be monitored, but also included as part of a predictive maintenance programme. Cygnus thickness gauges are equipped with the facility to record and report on findings. These reports can be exported (via cable, SD card or Bluetooth) and loaded into a spreadsheet, where they can be analysed and absorbed into a wider predictive maintenance programme.

So, can a UTG be used in situations where the items being tested are coated?

With most UTGs, it is not possible to measure through a coating simply and quickly. However, Cygnus Instruments does offer a particular ultrasonic technique called multiple echo. The benefits of this technique are that it is incredibly accurate, straightforward to use and that the method naturally ignores coatings (only the substrate that is subject to wear and tear is measured).

What about hazardous areas, are the testing instruments safe to use here?

 

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Again, UTGs cannot generally be used in hazardous environments. However, Cygnus Instruments manufactures the only UTG in the world that can be used in a Zone 0 hazardous environment. The Cygnus Intrinsically Safe thickness gauge has been a stalwart of the inspection industry for nearly two decades thanks to ATEX certification to Marking Group I & Group II.  

What are the key considerations when choosing an ultrasonic thickness gauge?

 

An ultrasonic thickness gauge must be simple to use; some UTGs can have very advanced features but more often than not this complexity is of little use to today&#;s engineer who wants to take quick, accurate measurements.

Alongside simplicity, accuracy is, of course, critical; and the multiple echo technique is extremely helpful with this issue. In other measuring modes, the observation and recording of the ultrasonic waveform (commonly known as the A Scan) and assisting functionalities such as MSI (Measurement Stability Indicator) can also help with gaining the most accurate results possible.

Finally, durability is also a key consideration. As with any product, ultrasonic thickness gauges are not all created equally, and a unit that has passed formal drop tests and is IP rated will naturally help extend the life of the UTG and ensure that, when it is needed, the gauge is functioning correctly.

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According to Hong Kong&#;s General Specification for Buildings, the specifications for finishes and coatings state that wall and ceiling finishes should have a maximum deviation of 3mm from a 1,800mm straight edge.

Additionally, the thickness of a wall render should be at a minimum of 10mm, while pipes that pass through roofs should be less than 50mm in diameter.

Stone slabs used for paving wall facings of fittings should not be less than 20mm thick, while slabs for paving surfaces shall not be less than 32mm thick.

For mosaic tiles, the thickness will depend on the type of mosaic tile. More specifically:

• Glass mosaic tiles shall be 4mm thick and regular in shape
• Glazed ceramic mosaic tiles shall be 5mm thick with square edges
• Unglazed vitreous mosaic tiles shall be 4 or 5mm thick with matching coved tiles

Recently in Hong Kong, Kwong Wah Hospital cable trunking coating was inspected by the Hospital Authority (HA). Although the thickness of the cable trunking coating does not pose additional risks to the hospital, insufficient thickness may limit the durability of trunking and shorten its lifespan. Thickness gauges are one of the many tools used to ensure that regulations and standards are being kept in buildings.

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