PMI or Positive Material Identification is a key part of many industries and has several applications like avoiding dangers in the chemical petroleum sector, maintenance, in the construction businesses, and also in the power plants. There can be production loss if the right alloy is not used in the applications. The threat to public safety, well-being, and health are, of course, always more important.
Several organizations like the Occupational Safety and Health Administration of the US, American Petroleum Institute, Hazardous Materials Safety Administration, Bureau of Safety and Environmental Enforcement, National Transportation Safety Board, the Dept. Department of Transportation Pipeline, and others too have recommended that PMI programs be implemented in respective areas.
XRF and PMI
Handheld XRF is an extremely useful methodology for PMI. Modern electronics, fundamental science, digital computing all come together here to provide a system that is accurate, precise, and also simple. The technician carrying out NDT or non-destructive testing can get good elemental analysis and also match the alloy grade. There is hardly any need for specimen preparation as well. All this can be achieved merely in 30 seconds. A lot of commercial alloys can be identified in just 2-3 seconds. Extensive testing is carried out as this is so easy and quick.
PMI is critical and so the NDT technicians sometimes wonder whether the XRF instruments are accurate and precise. Can they really be confident about the measurements they make? In this article, we will study this in detail. We will also look at the precision and accuracy of XRF.
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Accuracy in the Measurements
The process of XRF is extremely accurate. It can measure almost all common alloy metals like nickel, copper, iron, and others to below 0.01%. You can measure and detect most elements in periodic tables using the XRF, but the commercial alloys have just a few of them. It will be difficult for lighter elements and it will take longer as well for finding elements that are heavier. But the effect is minimal.
There can be inter-element interference given the metal’s density in most cases and the X-ray emission’s energy proximity of the alloy elements. In spectroscopy, this is known as “matrix effects”. The limits of detection can therefore vary based on the sample type and elements present. The limits of detection are conservatively reported given the complex composition of several commercial alloys. For a further discussion of limits of detection XRF.
The XRF’s accuracy for grade matching in PMI is rarely dependent upon one element. In most alloys, there are several elements for grade matching. For instance, identifying correctly stainless steel depends on the alloy requirements for iron, chromium, nickel, titanium, and specific allowances for manganese, copper, and molybdenum. This grade identification doesn’t just depend on the accuracy of a single element (in the case of chromium, for example) but on measuring different elements (iron, nickel, titanium, for example, along with the chromium). The identification will be more accurate.
Precision in the Measurements
You can have high accuracy (close to the bull’s-eye) but the precision can still be low. Precision can also be high, but the accuracy low. The measurements are considered to be precise when they show the same results on repeat tests, independent of whether they show the true value. In other words, “Is the measurement reproducible?” A properly designed and calibrated XRF equipment can provide a measurement, which should be precise and accurate, both.
Reporting Accuracy, Precision in XRF
XRF handheld devices are usually very accurate common element alloys.
This figure shows a typical readout on an XRF instrument. Here, the measurement of the composition is reported in the second column of the chemistry table on the analyzer screen. Each reported value in column 2 is an average of multiple measurements. Although a test can be completed in 30 seconds (or less), the instrument actually measures the sample hundreds to thousands of times in this timeframe (hundreds to thousands of X-rays). Using the aforementioned analogy, this is equivalent to throwing hundreds to thousands of darts at your target. You can see the avg. of these measurements in the second column.
Since the instrument is measuring the sample multiple times in a single test, it will show the closeness of the measurements to one another (that is, the precision). Ideally, all the measurements should be the same (meaning, the darts will be on top directly). In reality, however, this is often not the case due to the complexities and the configuration of the electronics. The measurements can be close to one another (high precision) but are rarely identical. Always there will be a deviation or spread between data points that follow a normal distribution or “bell” curve.
The X-axis represents the value of the measurement. The Y-axis represents the number of times that value is measured (meaning, the frequency of measurement). The green bar reflects the average of all these measurements, coinciding with the top of the bell curve. The width of the curve reflects the measurement’s spread or deviation. The smaller the spread, the greater will be the precision. The measurement’s precision or deviation is reported as the ± value. The precision indicates how the measurements were tightly clustered to one another.
XRF provides a fast and effective way of identifying alloys and their element compositions. This is important in the context of PMI reporting that those who are NDT technicians realize the relationship and differences between precision and accuracy. It is always best to use standard operating processes and report the correct measurements and also the precision as well as the testing conditions. This is the way XRF will be a great PMI too and will make industrial operations safe.
Author: Sarah Hagi