Why Two Instruments Might Give Inconsistent Results
So you’ve just bought a new particle counter, you’re testing it with your fleet, and… the counts don’t match. Does this mean there’s something wrong? What happened? Why is there a difference? And is there anything you can do? We’ll discuss all of this and more in this week’s handy blog.
Who’s Affected?
The first order of business is defining who will come across differing counts between two particle counters. Did you buy the same model of instrument from the same manufacturer? You’re in Group 1. Did you buy similar instruments from different manufacturers? You’re in Group 2. Did you buy different instruments from two different manufacturers? You’re in Group 3.
| Group 1 | I bought two of the same instrument from the same particle counter manufacturer. |
| Group 2 | I bought two similar* from two different manufacturers. |
| Group 3 | I bought two different instruments from two different manufacturers. |
*Similar here refers to the airborne particle counter’s sensitivity, flowrate, and the number of size channels.
These groups are necessary because the complexity of the answer changes. Groups 1 and 3 are fairly simple; Group 2 is not.
Airborne Particle Counter Group 1
This group offers the best chance for close agreement between airborne particle counters, specifically for cumulative and differential data agreement even when accounting for the manufacturing date. By buying two of the same instrument, you’ve eliminated data differences caused by calibration. Manufacturing consistency and quality control are the only limiting factors. You can minimize inconsistencies by being careful with how and where sampling is performed; however, even in this best-case scenario, it is possible for some variation in particle counter results.
Airborne Particle Counter Group 2
Absolute data comparisons are complicated, but suffice to say, similar instruments will perform differently even though they have the same key specifications. Why? Well for one, the design of an optical particle counter is a significant factor for many aspects of data collection and interpretation. Design impacts how the instrument will count and size particles. For example, the laser type and beam shaping in the sample region, optical detection and signal processing approach, and sample cell and flow delivery (including recirculation) are all choices made by the engineers of each instrument. The cumulative impact of these factors results in differences in the cumulative and differential data.
Calibration factors are also significant. Here are the main culprits:
- There are a variety of suppliers for the materials used to calibrate an optical particle counter.
- Monodisperse particles have different mean sizes and size distributions.
- It is common for specific part numbers from the same manufacturer to have specification differences batch to batch.
- The method to deliver the particles during the calibration can be a source of error.
- The type of particles used must be free from any actual contamination. Calibrators must also deliver them in the correct concentration when setting channel thresholds.
- The type of reference instrument used to calibrate the instrument under test will have an impact on its counting performance.
Particle Counter Specs on Paper vs Specs in Practice
More size channels mean more ways data can vary, and a small change in sizing will have a much greater effect on an instrument with higher resolution. A common approach to setting a particle counter’s channel size threshold during calibration, called “splits”, also plays a role. Adjusting and setting a size threshold to match a median is not an exact science and there are many variables associated with it. Taken together, there is a lot that can potentially contribute to data mismatch between similar particle counters from different manufacturers.
Airborne Particle Counter Group 3
Expecting data to agree between instruments bought from two different manufacturers is a recipe for disappointment. Instrument design, sensitivity, and the calibration process will all affect cumulative counts and the ability to determine the accuracy between samples taken in environments with variable-sized particles. As a result, we can’t even estimate the significance of the effects on counts. People falling into this group should consider this the next time they need to replace a particle counter in their fleet.
What’s Next?
Now that you know how instruments can differ, you have a better idea of why the counts shown by one particle counter will sometimes not match another. This may result in some changes in use, some re-thinking on what to prioritize in results, and how to interpret data. Most importantly, you can be better prepared about what to look for when you buy your next particle counter.
