Gauge Performance Impact, Scan Speeds and Critical Issues Missed
The challenges faced when selecting a digital thickness gauge. As its name suggests a digital thickness gauge measures the depth of a material. While there are a variety of digital thickness gauges on the market, it is important to understand performance can very significantly between gauges and if not understood, this will severely impact measurements and detection of issues.
Example cases of critical component failure following routine corrosion inspection is often mistakenly believed to be corrected by the costly investment in Ascan type gauges and retraining of the inspection teams. Yet, how many companies stop to consider the gauge performance could be the true cause of such problems.
In purchasing a digital thickness gauge most consideration is based on the documented measurement range and price. The lack of understanding and appreciation for the performance difference is hard to demonstrate on a specification sheet.
Practically, almost every gauge can measure a simple flat surface step block, or a flat bottom hole! However, in real life corrosion is not flat, in real life inspection speed is critical.
Performance of the Gauge
The screen will flicker on some gauges for example between 0.01/0.00. Others will remain perfectly stable. Which one is best?
Consider the action of the thickness gauge. Electronically pulsing a crystal, which in turn vibrates a sound through the measurement material, waiting for the return echo to convert back to an electrical pulse and then to a number on the screen. The material has grain structure, there is couplant below the transducer. All these factors will impact the smoothness of the reading and cause a natural flicker in an ultrasonic thickness gauge.
To achieve a stable non-flickering reading, techniques such as smoothing, filtering, averaging must take place inside the gauge. This in effect will slow the response time of the gauge. A nice smooth display will probably result in a much slower responding gauge. Slower gauges have less ability to show vital imperfections and causes of concern.
Example
Dakota gauges are running at speeds beyond many in the market, To try and explain the importance of performance we tested a thickness gauge at the low rate of 4Hz reading rate, at 16Hz reading rate and Scan mode of 200Hz reading rate.
A 1mm hole was drilled into the steel plate. The hole was deliberately not flat bottomed it was standard angled drill bit, angled to reduce obvious reflections and simulate a more realistic measurement scenario.
The same 5Mhz 0.25” dual transducer was passed over the region above the hole at each reading rate setting.
The 4Hz set gauge had difficulty to locate and measure the hole and required additional manipulation to find it.
The 16Hz set gauge improved the locating and measuring time. Less manipulation and searching required.
The 200Hz scan mode of the Dakota ZX6-DL was by far the best, the ultrafast scan speed enabled the gauge to pick up the reflection sooner and for a longer duration enabling clear and obvious detection.
Please note, there are many gauges in the NDT market working below 4Hz, this was the lowest rate Dakota could demonstrate.
Conclusion
Higher speed gauges send multiple additional reflections to the surface quickly enabling better coverage and detection, ultimately offering an improved probability of detection.
Dakota gauges used in this application:
For more details on how the Dakota ZX Corrosion Thickness Gauge can give an advantage to your inspection process.