The manufacturing industry has little to no tolerance for error. That means suppliers often need to know the exact composition of the metallic alloys they’re dealing with.
Portable optical emission spectrometers (OES) have gained popularity in PMI testing. This is likely due to their portability and ability to provide the full chemistry of a metal including true carbon measurement.
QLX1 Portable LIBS Laser OES from QuantoLux
In this post, we’ll cover questions around portable OES including:
What is an optical emission spectrometer (OES)? An optical emission spectrometer, or OES, is a spark- or laser-based system designed to accurately determine the elemental composition of metals by burning the metal surface and measuring the light wave emissions coming from the plasma created.
And specifically for portable OES:
- History of portable optical emission spectrometers
- Pros and cons of using portable optical emission spectrometers for PMI
- Costs of using portable optical emission spectrometers for PMI
- Mistakes to avoid when using a portable optical emission spectrometer
- Portable optical emission spectrometer brands
Let’s dive in!
Psst: This post is based on a podcast with co-host Chris Carolan. To hear this episode (and more like it), subscribe to The Manufacturing Show on Apple Podcasts, Spotify, or wherever you listen to podcasts.
What is optical emission spectroscopy (OES)?
First thing’s first: If you’re unfamiliar with positive material identification (PMI), take a gander at this post. Then come right back! Reviewing PMI testing will help you better understand optical emission spectroscopy.
Now, what exactly is optical emission spectroscopy (OES)?
OES starts with an electrical or laser-based source to charge atoms within a metallic sample. When the atoms within a particular metal get excited, they emit a trademark light.
This light, or optical emission, requires a small piece of the sample to be heated up to thousands of degrees Celsius. What’s good at heating stuff up? Lightning and lasers!
Spark OES sample stand configuration
Essentially, OES uses electricity or lasers to vaporize the sample’s surface, charging up its atoms and creating a plasma. Then, the metallic sample shows its true colors. The light a sample emits is consistent with its chemical composition.
So, you’ve seen the colors that the metallic sample produces when heated. Now, it’s time to measure the intensity of each of those colors.
Fun fact: Iron emits over 8,000 different wavelengths!
The emitted light passes into the spectrometer. The OES spectrometer is then able to grade each separate light into element-specific wavelengths. A corresponding detector measures the intensity of each wavelength (aka, each light’s intensity).
Subsequently, the intensity of each wavelength is proportional to the concentration of their respective elements.
In other words, the nickel wavelength measures at a high intensity? There’s a lot of nickel in the sample. The wavelength corresponding with copper is weak? There’s not much copper in there.
Thirdly in OES testing, a computer system processes the intensity data. Through a predefined calibration, the system is able to generate elemental concentration levels.
Ta-da! You now know the overall composition of the sample. That means you can accurately compare it to a metal alloy grade library or database to make sure it’s the right stuff. (Ex. Stainless Steel 316)
History of portable optical emission spectrometers
As the knowledge around positive material identification began to expand, so did OES methods. Here’s a quick history of where the portable OES got its start.
Using a flame to test metal
In the early 20th century, manufacturers used a flame as the light source for OES testing. The flame would heat up the sample’s atoms, producing a light consistent with its components.
Ever heated up a penny before? You probably saw a greenish-blue light emitting from it due to the copper. Using a flame for OES testing is the same concept.
From benchtop to mobile
Of course, there had to be a more accurate way of measuring the composition of metals.
Enter, arc spark OES (or, spark OES). This metal testing method came about in the 1950s and 60s because of its increased excitation capabilities.
Gotta keep those atoms excited, right?
In its early days, spark OES was primarily used in metal production labs where they needed to know the full chemistry and composition. The only spark OES spectrometer available was rather stationary. But, it got the job done.
As PMI testing became prevalent outside of the lab, portable OES was developed to support the need for testing materials on the shop floor or out in the field.
A laser can replace the spark as the source that's vaporizing the metal.
Recently, the advancement of laser technology has allowed lasers to take the place of sparks in the vaporizing process.
Pros and cons of using portable optical emission spectrometers for PMI
Considering a portable optical emission spectrometer for your business? Take note of the following pros and cons we've collected through our experience serving the metals supply.
Pros of using portable OES for PMI
With a portable optical emission spectrometer, you can…
- Measure a broader, wide range of elements with greater sensitivity, accuracy, and precision than handheld equipment.
- Determine the material’s full chemistry to allow for better inspection and grade identification of mission-critical components in the field.
- Accurately measure the intensities of tough, light elements such as carbon, boron, sulfur, phosphorus, and nitrogen.
The analytical performance makes it the go-to technology to acquire full metal chemistry outside of the lab.
Sounds pretty great, huh? Well, there are also a few drawbacks to think about when using a traditional, portable spark OES analyzer.
Cons of using a traditional, spark-based portable OES for PMI
Drawbacks of using a traditional, spark-based portable OES include…
- It’s very sample preparation intensive — not just point-and-shoot testing like the handhelds.
- You have to know what the base material is first in order to prepare the analyzer properly.
- They require a sufficient amount of knowledge and training to use.
- They’re high maintenance. (Understatement?)
- Although it’s no longer stationary, a traditional, spark-based portable OES is still pretty big and bulky.
- A traditional, spark-based portable OES requires argon for every test.
Pro tip: Laser-based OES provides all the chemistry and sensitivity you need with almost none of the cons of traditional, spark-based OES.
Next, we’ll discuss the costs of using a portable OES for positive material identification.
Costs of using portable optical emission spectrometers for PMI
It’s safe to say purchasing your business’s own portable optical emission spectrometer isn’t going to be cheap. Even so, the consequences of failing to conduct positive material identification are even costlier.
Silver Eagle Refinery Explosion
From the CSB: "On the evening of January 12, 2009, 2 refinery operators and 2 contractors suffered serious burns resulting from a flash fire at the Silver Eagle Refinery in Woods Cross, Utah. The accident occurred when a large flammable vapor cloud was released from an atmospheric storage tank, known as tank 105, which contained an estimated 440,000 gallons of light naphtha. The vapor cloud found an ignition source and the ensuing flash fire spread up to 230 feet west of the tank farm. On November 4, 2009, a second accident occurred at the Silver Eagle Refinery in Woods Cross, Utah, when a powerful blast wave - - caused by the failure of a 10 inch pipe - damaged nearby homes."
A pipe failure that could have been avoided with proper PMI testing.
It’s a necessary evil, some would say, to gain access to a portable OES and conduct proper PMI testing. The reliable analysis and quality control is always worth it.
Purchasing a portable OES will put you out anywhere from $35-$60K, sometimes more. You may be able to budget in the initial cost; however, traditional, spark-based portable optical emission spectrometers require more maintenance than most other machines.
On-going maintenance costs of traditional, spark OES will put a big dent in any operating budget. With constant Argon gas usage, electrode cleaning and sharpening, and other consumables that need to be maintained, maintenance costs can easily reach thousands of dollars per year.
Beyond the actual dollars, the time and effort it takes to constantly maintain and operate these analyzers should also not be overlooked or underestimated.
Depending on the repair, plan on spending $7-$20K for the maintenance of spark OES analyzers. A few more factors you should plan on include
- Transportation to and from the maintenance provider — these things aren’t easy to move.
- The cost of the manufacturer service rep visit. Hourly service rates of $150-$300 per hour.
- The fact that owning a portable OES doesn’t lead to many opportunities when you’re ready to sell it.
To keep costs down and increase convenience, consider renting your spark-based portable optical emission spectrometer (or, switch to a laser-based system).
A one-month rental of a portable spark OES system usually runs between $7,000 and $10,000.
Mistakes to avoid when using a portable optical emission spectrometer
To attain the best PMI results with your OES metal analyzer, avoid these mistakes during testing:
- Failing to do proper sample preparation. To achieve the cleanest burn possible, the metallic sample should be flat, free of corrosion, and in direct contact with the OES electrode or laser.
- Poor maintenance of the electrode. Along with many other parts on a traditional, spark-based portable OES, the electrode absolutely needs to be cleaned and sharpened routinely in order to provide proper elemental analysis.
- Impure argon gas. Ensuring the argon gas is ultra pure grade will result in more efficient burns. Low-grade argon can allow moisture into the system and require costly, full system flushes and failures if accidentally used.
- Not allowing the analyzer to adjust to its environment. Any time a traditional, spark OES is moved to a different environment or is paired with a new system, it needs time to adjust.
If you don't have strong enough light signals, you're not going to be able to read the elements.
Avoid making these mistakes when using a portable OES and there should be smooth sailing.
Portable optical emission spectrometer brands
Brands that supply portable optical emission spectrometers include
- Hitachi (formerly Oxford) for spark-based OES systems (PMI MASTER Smart)
- SPECTRO for spark-based OES systems (SPECTROTEST)
- QuantoLux for laser-based OES systems (QLX1)
In addition to these listed brands, there are more European OES providers entering the market.
Master your metal testing
Choosing the right tool for PMI testing is crucial for numerous reasons. OES is a cornerstone technique for determining the complete chemistry of a metallic alloy.
In an industry that requires little to no margin of error, it’s worth it to fully understand all the PMI tools at your disposal and remember to...
Never stop testing your metals.