International Standards for Metal Thickness Gauges

As we learned in previous articles, metals coating weight and thickness measurement gauges provide precise, real-time measurements that help meet specifications of various applications while maximizing raw material use. Thickness gauges are primarily used to look for dramatic deviations in thickness – which could affect the mechanical properties of the finished product.

International organizations like the ASTM, Japanese Standards Association and others provide guidance on not only alloy chemistry tolerances, but sheet dimensional tolerances and mechanical properties as well. For Instrument Suppliers, the International Electrotechnical Commission (IEC) has produced standards to provide guidance and definition for specific terms and tests used. The standards act as a consistent scale to compare one instrument to another without confusing nomenclature obscuring the true performance of each.

The Nuclear Instrumentation Technical Committee (IEC Technical Committee 45) produced IEC 61336 “Thickness measurement systems utilizing ionizing radiation – Definitions and test methods.” The first committee release was in 1983, and an update was drafted in 1996. The document is available for purchase from the IEC at http://webstore.iec.ch/webstore/webstore.nsf/ArtNum_PK/21703?OpenDocument — as such we cannot reproduce it here. The Standard first defines common testing parameters in order to clarify what specific words mean. For example the “mean response time” may be called the “first time constant” by some, and something else by others.

Image above: Graphical representation of gauge response to an instantaneous thickness change

The fixed definitions avoid any ambiguity in interpretation. They additionally provide guidance in setting up and carrying out the tests defined in the second part of the standard.

The standard dedicates no less than nine terms to clarify time based parameters. Some are related to defining the time associated with the time taken to respond to a change, while others are related to the digital processing of signals. This is particularly critical with the advent of high speed data processors. Incoming data can be manipulated and processed by advanced filters to mask or hide true statistical variations. Radiation measurements are statistical by their very nature. All noise figures should be quoted with a reference to the number of Sigmas, or confidence levels (CL). Most gauge manufactures present 2 sigma (95% CL) noise figures, but not all.

Straightforward data processing provides for predictable results and a better representation of the process dynamics. If a change occurs in the process, advanced filtering may portray a portion of the change, but not the full change. Process engineers and their AGC algorithms may over react, or under correct thanks to the manipulated data.

In order to ethically improve the speed of the sensor response to change, without increasing the statistical noise on the measurement, the physical characteristics of the sensor need to be optimized for the application. The raw signal must be shielded to remove as much electrical noise as possible. In an ideal instrument, the radiation detector output is digitized right away. The analog detector signal is converted to a digital number within a few millimeters of its origin. This practically eliminates the possibility of electrical noise impinging on the signal. In comparison tests, following the IEC 61336 guide, the noise on the new designed detector improved by a factor of 30%.

Additionally, users of this type of system can benefit further by taking advantage of the detector’s ability to operate at a 1ms mean response time. At this speed, and with the reduced noise, process engineers have the tools to analyze data at high speeds, revealing mill chatter and other higher frequency anomalies.

In a typical rolling mill that produces sheet steel at 250 um, the noise at a 10 ms mean response time might be on the order of +/-0.10% (2 sigma). With the improved signal processing of this system the noise will drop to +/-0.07% (2-sigma). For a mill that produces 2,000,000 tons a year, that translates to a savings of almost $360,000 in raw material alone (using HRC price at $600USD per ton).

An additional benefit to digitizing the signal so early in its journey to the AGC system is speed. Once digitized, the data can be processed without the time consuming ADC / DAC conversions. The reduction of a few microseconds of process delay time can assure the AGC has time to fully correct any strip thickness deviations. This can result in higher quality product.

NOTE: To read additional details about High Speed X-ray Thickness Gauges, including illustrations, read the white paper, Optimization of a Cold Rolling Mill with a High Speed X-ray Thickness Gauge.