Repeatability in LIBS Testing: Why is it critical?

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When it comes to precision measurements, accuracy and repeatability are the dynamic duo of measurement confidence. While they are equally important, in this blog we will focus on the importance of repeatability in the field of alloy verification, using the new handheld Laser Induced Breakdown Spectroscopy (LIBS) technology.

Confidence is what it’s all about

Repeatability in LIBS measurements refers to how close a series of measurements from a single alloy is, when taken successively, using the same analyser, and run by the same operator. Another term for repeatability is test-retest reliability; when you retest or re-measure an alloy sample, you should get essentially the same result, with a value close to the 3 sigma precision, or inversely, within approximately the 3 sigma error value. Since LIBS requires a different spot for each test, the homogeneity of the alloy may add some error in the repeatability. This is due to potential non-homogeneous alloy factors such as inclusions.  MnS and TiC are examples of typical inclusions. In such cases the amount of Mn or Ti would read as outliers from normal “expected” deviation (3 sigma error band). When the same measurement is not achieved, and the problem is not heterogeneity or inclusions, then we say that there is analyser variability or lack of reproducibility. Variability can be caused by the quality of the analyser being used, the lack of proper calibration of the analyser, poor training/incorrect use by the operator, or environmental factors like room temperature, ambient pressure, or simply the inherent instability of the analyser design.

So, why is repeatability crucial to the LIBS technology?

The LIBS test results are almost always critical data points that determine either the safety or economic value of the material being tested. Whether you are verifying an installed part or fabricating with that part, if the measurements taken are not reliable then the eventual service life of the structure or safe operation of the process may not be reliable either. Variance from engineered specifications will result in irregularities in the end product or in worst case, a malfunction that endangers life, the environment and the bottom line of an operation. Repeatability in your measurements helps to ensure quality in your process and your outcome.

The importance of calibration

A repeatable measurement is a consistent measurement, and more often than not, inspectors rely heavily on consistency for ultimate precision. One way to help check, maintain, and guarantee repeatability of your measurements is to conduct wavelength calibration and response calibration followed by repeatability tests. A repeatability test is specifically designed to check the repeatability of one person, one device, and one sample, in one environment. Before you begin your daily routine testing, calibrate your analyser using the set-up samples provided by the manufacturer. Then define a certain set of operating modes and conditions that will remain constant throughout the testing. These include: the operator log-in, the mode of operation of the analyser (averaging if desired), data fields to populate, units of readout, and element/alloy display options.

Other conditions to note are environmental conditions, the experience of the operator, the sample preparation, the locations and method of conducting the measurement and the amount of data you want to collect. In general, the more data you collect, the better. After collecting data, you can analyse a range of descriptive statistics in order to assess the repeatability of your measurement process. Regularly conducting repeatability tests is a good idea for any measurement facility.

Repeatable measurements for material verification are easy to achieve using new fast, accurate, portable LIBS analysers for elemental analysis – including carbon detection.

Source: Thermo Fisher Scientific – Analyzing Metals

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