3 Reasons Your Nuclear Density Meter Is Losing Accuracy

23

OCTOBER, 2020

Nuclear Density Meter Losing Accuracy

nuclear density meter losing accuracy

Nuclear Density Meter losing accuracy? This article explores why and how to troubleshoot solutions.

You had to complete a hefty stack of paperwork in order to get your Nuclear Density Meter on-site. In addition to that, you and your team received certifications ensuring you knew how to handle, store, and dispose of it safely.

In spite of that, it all seemed worth it. The Nuclear Density Meter was providing you with valuable insights into the characteristics of the slurry running through your pipework.

However, when you checked the absorber plate recently, the meter was not showing the correct reading for your carrier fluid. It appears your Nuclear Density Meter is losing accuracy.

So what happened?

There are a few possibilities that could be impacting your nuclear density meter’s accuracy. This article will address possible culprits in order from most to least difficult. 

Diagnose Nuclear Density Meter Losing Accuracy

1. Isotopic Nuclear Disintegration

The first possibility is radioactive decay, also known as nuclear disintegration. Eventually, radioactive decay causes the strength of gamma rays emitted by a radioactive isotope to weaken over time. In essence, this happens as the nucleus of the atom decays. This results in the nuclear density meter losing accuracy over time as the strength of the gamma rays are weakened.

Half-Life

Surprisingly, the often used term “half-life” (symbolized by t1/2) isn’t referring to that period between waking and the first cup of coffee. Rather, half-life is the time it takes for the energy emitted by a specific radioactive atom to decay by 50%. Many nuclear density meters use a radioactive isotope called Caesium-137, which has a half-life of 30 years. Since radioactive decay is a spontaneous and random process (one of the most random processes in the universe, it turns out!), half-life is actually a guess based on average decay times. Therefore, calculating half-life is a less-than-exact science, especially as the nuclear density meter ages.

The technical specifications of your nuclear density meter likely indicate a “source decay compensation” built into the instrument. Let’s say we have an alternate universe where radioactive decay is deterministic instead of probabilistic. Let’s also say that, in this universe, the decay rate of a nucleus can be mapped linearly.  Consequently one could reliably calculate the exact decay along any point in the hypotenuse of a basic line graph. This would subsequently create a simple “source decay compensation” function, because some seventh-grade geometry will tell you that calculating a hypotenuse for this graph is as simple as c = √(a² + b²)

However, in our actual, probabilistic universe, radioactive decay doesn’t make things that easy. The function that calculates half-life is an exponential function which can be plotted along a curve, not a line. Since nuclei disintegrate on their own time, any variation from the average curve will create errors that exponentially accumulate over time, therefore resulting in a compensation function that strays wildly from the expected plot points along that curve.

Resolution

Thus we have identified one potential source for accuracy loss (and scientifically locked in the job security of nuclear density meter calibration technicians across the world.) Given that half-life is a side effect of the nuclear density meter’s principle of operation, there is no immediate resolution beyond replacing or upgrading the instrument.

nuclear density meter losing accuracy

Diagnose Nuclear Density Meter Losing Accuracy

2. Incorrect NUCLEAR DENSITY METER Orientation in Non-Homogeneous Slurry

The Nuclear Density Meter is using gamma rays to observe the density of a single slice of a slurry as it moves through the pipe. Therefore, if your slurry is not completely homogeneous, the orientation of the nuclear density meter becomes critical in taking a representative sample.

If you have a stratified slurry flow, with each layer having its own characteristics, then you have to be sure you are blasting those gamma rays through the correct portion of the pipe.

As show below, the varying configurations of slurry density which can impact the accuracy of the nuclear density meter depending on how meter itself is oriented within the process. Essentially, the nuclear density gauge works well within optimal conditions – however, if those conditions change, nuclear density meters lose accuracy.  

nuclear density meter losing accuracy
Dust Affecting Nuclear Density Meter Reading

Diagnose Nuclear Density Meter Losing Accuracy

3. Dust (That’s All… Just Dust)

In brief, any particulate matter that comes between the source and the detector of the nuclear density meter will interfere with the strength of the radiation received by the detector. Therefore, resulting in the nuclear density meter losing accuracy.

This hazard won’t concern you if your pipework happens to be in the clean room of a microprocessor plant.

However, if you’re reading this article, your application is likely to be immersed in dust, sludge, and unspeakable gunk of all varieties. As you know, industrial environments come with constant debris which are trying to find their way between your nuclear density meter source and the detector, skewing the results, and impacting your operational efficiency.

Consistent cleaning and maintenance is the only clear solution when dealing with dust affecting the accuracy of your nuclear density meter. Alternatively, you may consider upgrading to a newer technology which is not impacted by any of the factors detailed in this article.

Guide to replacing nuclear density meters

Although the reasons or timing may vary, most industrial measurement processes will
at some point need to consider replacing their nuclear density meters.

This guide details the options and process of replacing nuclear density meters.

Conclusion

To summarize, while the very principle of a nuclear density meter’s operation can impact the nuclear density meter losing accuracy, other factors can also increase the headache of excessively high or low density readings. Ultimately each of these – or a combination – result in the nuclear density meter losing accuracy over time.

Through process of elimination you can determine the root causes, which may lead you to the appropriate solution.

In some cases where a resolution cannot be found, the nuclear density meter must be replaced. For more information on that process, download our Guide To Replacing Nuclear Density Meters.

 

https://pubchem.ncbi.nlm.nih.gov/compound/Cesium-137#section=Related-Compounds

Half-Life and Activity

https://iowadot.gov/training/ttcp/training_manuals/NuclearGauge.pdf

https://redmeters.com/replaceanuke/

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