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Introduction
In the the mining industry, preventative maintenance is vital to avoid costly breakdowns and ensure the safe and efficient operation of critical equipment. A crucial aspect of maintenance revolves around the ability to measure a materials thickness accurately. Whether, monitoring wall loss thinning in a pipeline transporting a hazardous material, or surface corrosion of structural steel supporting vital machinery. The ability to measure and monitor a materials condition and predict a future maintenance schedule is pivotal.
This article will discuss selection of ultrasonic thickness gauges used in material measurement, outlining several considerations.
Principle
A thickness gauge uses a transducer to covert an electrical pulse into a sound wave, the sound wave travels through a material, reflecting off the back surface and returning to the transducer to be converted back into an electrical pulse. The time taken for the signal to travel back and forth is converted into a thickness value and displayed on the gauge screen.
When it comes to thickness measurement, there are 2 key considerations in selecting a gauge, choosing a “Digital” or “Ascan” display. Both options offer unique benefits and cater for various industry needs.
Digital Gauge
The Digital gauge has many advantages, notably their ease of use. This simplicity in operation has certain cost advantages for the gauge and for the requirement of trained and certification personnel.
Ranging from a simple “turn on and go”, to the more complex ability of multiple material calibrations, the Digital gauges offer a variety of additional features to meet specific measurement requirements. Interestingly, the feature often referred to as Data Logging (DL) is one of the most overlooked features by many, despite the considerable time and cost saving achieved from instant and traceable report generation.
Digital Concern
For general thickness the Digital gauge offers an adequate solution. However, a concern arises with all Digital gauges. The reported thickness is simply the reflection from the highest signal amplitude the gauge records. In the situation of localised corrosion such as, deep pitting and cracks for example, depending on their orientation, size and shape, there is a risk these may go undetected.
Corrosion has 2 distinct types, uniform and localised. Uniform corrosion with its smoother wall loss favours the Digital gauges. Localised corrosion comes in many forms. The danger for Digital gauges arises with measurement and detection in parts that might suffer certain types of localised corrosion and other internal issues.
Ascan Gauges
An Ascan, provides an additional layer of confidence in identifying a concern. Any variation in Ascan signal response highlights there is something that requires further investigation. The addition of an Ascan increases the possibility of defect detection. Gauges with Ascan’s usually have a much wider measurement ability than their Digital counterparts, The increased features enable customisation and optimisation.
The increase confidence the Ascan gauge provides does come at a small additional cost. The asset owner must decide whether the enhancements and added reliability offset the additional costs, in considering the specific measurement need.
Is it the Ascan or the Device?
In recent times, there is a trend towards Ascan type gauges in certain industries, because of the increase in confidence, traceability, and consequence of error. However, there is a good question to understand. Is the move to more Ascan gauges a result of the increased use of poor-quality Digital gauges?
A Digital gauge with slow response speed, can have a significant impact on measurement results. Often mistaken for a “nice stable display”, the significantly slower reaction speed can potentially miss indications when scanning across a part. Fast scanning speeds with increased response times, has a significantly increased potential to identify an indication.
There are a number of thickness gauges on the market today, offered at a range of prices, not all gauges are the same and certainly not in regard to performance.
DakotaNDT formally, Dakota Ultrasonic have been designing and manufacturing thickness gauges for over 30 years. Currently, they have over 20 types of quality thickness gauges, some general use and some very solution specific. DakotaNDT welcome any questions.
For more information on Elcometer and Dakota NDT, contact sales@bamr.co.za
