Manufacturing Engineering ran an article on materials testing using some of the more portable instruments available (see “Testing the Metal,” ME, December 2018). The article pointed out that Positive Material Identification, or PMI, is important for manufacturers checking incoming material. It also vital for installed equipment, like pipes in plumbing, and heating. At my company, Watts Water Technologies, validating material properties to ensure they are composed of the correct elemental composition is an important part of our work.
The article discussed some of the popular portable instruments used for PMI. While important, users should also be aware that using more than one device in a strategy of “escalation of testing” is crucial under certain circumstances. One of these is ensuring pipes can safely carry drinking water, a core element of our work at Watts Water Technologies. Throughout manufacturing, there must be similar circumstances where the metal in use must be precisely as specified for health and safety reasons.
Three Common Technologies
The three technologies we commonly use at Watts are X-ray fluorescence (XRF), optical emission spectroscopy (OES), and wet chemistry. Each has a different level of required sample preparation time, measurement time, statistical confidence, and cost. The advantage of XRF is no sample preparation, two- to 30-second measurement time, highly portable handheld models, and low cost. Also, it is the only non-destructive test method. XRF is significantly faster than the other testing methods, provides reasonable accuracy and is used primarily for verification or screening purposes. Watts uses the Bruker Titan 600 Handheld brand of XRF analyzers. It is typical to get high confidence readings in as little as 15 seconds. In my experience, this is true checking for the compositions of a complex, important alloy like C87800 that contains copper, zinc, tin, and lead. This is known in the plumbing industry as a low-lead material.
Optical Emission Spectrometry
When a higher level of accuracy is required, we turn to Optical Emissions Spectrometry (OES). However, OES is not as portable or convenient as a handheld XRF, requires significant sample preparation time and longer total measurement time, is higher cost and often requires an operator to have a higher level of expertise. There are some semi-portable cart type OES units, but typically it is a stationary laboratory instrument. Finally, the top of the line for accuracy is wet chemistry, which involves sample digestion, takes hours to receive results, and is a destructive test.
A simple analogy in comparing these methods is to think of each as a judge in a court room. Comparing accuracy to severity and impact of a court case, an XRF unit would be like trying a case in a local traffic court, an OES would be like trying a case in state court, and wet chemistry would be like trying a case at the Supreme Court of the United States. Think of the expense of the lawyers and the time to prepare as the case is escalated. So it is with test escalation.
Like anything, the method that is simple, cheap, and yet gets the job done is by far the preferred method. That would point to heavy use of handheld XRF analyzers. I have seen incoming parts primarily inspected all over the world using XRF to determine if the amounts of lead in an alloy are acceptable based on NSF Standard 61 (which covers compliance with the Safe Drinking Water Act of 2014). After a very quick random sort of parts, normally to some type of AQL incoming inspection method, quick validation can be reliably achieved. So, why would you want to escalate?
It is important to understand that lead is not a homogeneous substance. The elements mixed into an alloy are not perfectly homogeneous, which can sometimes create a detection challenge. The challenge is when one company says the parts are within specifications and another says the parts do not meet the specifications.
Back to Fundamentals
What to do? First, go back to the fundamentals of using an XRF, the least costly inspection method. Four operating points are important in getting a confident reading.
The first point is getting a large enough sample size, by checking enough of the sample to ensure that all of it is sound. The second point is reviewing the XRF analyzer’s calibrations and settings. Each brand has different calibrations, and some have large performance differences depending on many variables. Some even have significant hardware differences. Creating readings of the entire periodic table of elements is just not practical, so different brands cover a wider range of elements or emphasize some elements over others.
The third point is to check the measurement time of each reading. Increasing the measurement time of the reading—thereby gathering more data—can and will provide better confidence in your results. For example, one approach is increasing the reading from five to 30 seconds.
Finally, the fourth point is to make sure that the device you are using is clean and in good working condition. Most brands today have a thin plastic lens, or window, which, if contaminated or damaged, can provide false readings during short measurement times, such as five seconds.
If after doing all of this you still have a large discrepancy, change the technology and move up to OES. This is not a trivial decision. Besides the added cost and time, OES leaves spark burns on the element. However, each element is detected at a different timing rate, which is known to provide better results.
Normally, after a proper OES test with multiple readings from the same part you can conclude that the results are a certification of the measurements. The challenge we see is the part size in relation to the opening in the OES. If the parts are very small, this test does not work and you will need to escalate yet again to wet chemistry. OES can also be performed incorrectly. Equipment not being cleaned properly, equipment not functioning up to the original equipment specification, and sometimes changing of parts for maintenance in equipment can create some issues.
Moving on to Wet Chemistry
The final escalation level is wet chemistry. The one thing to keep in mind with wet chemistry is that the person using this test method must be well trained and needs to understand the importance of the test results.
A quick note on laser induced breakdown spectroscopy (LIBS), another PMI technology now being offered in a portable form. While this technology is relatively new, it is fast and can be accurate; it does a good job with some materials.
Our industry has moved toward these simple key analysis methods. Those of us using these tools and methods must understand the importance in detecting the elements of the materials used in the drinking water industry.