All new particle monitors and particle profilers are calibrated at the Met One factory against a standard aerosol (NIST traceable monodisperse spheres). The factory calibration is traceable and results in good data quality when the monitor is operated in accordance with Aeroqual’s servicing and maintenance procedures.
Periodically, you need to remove the particle monitor or particle profiler from your monitor and return it for Met One factory service and calibration.
- Particle monitor - every 24 months (2 years)
- Particle profiler - every 12 months (1 year)
Field calibration isn’t a substitute for factory service and calibration. Factory servicing is essential maintenance that ensures the health of the monitor’s optical engine. With factory calibration it’s possible for the optical engine to last indefinitely. For the optical engine to be properly calibrated, it may be necessary to replace some high performance components such as the laser, detector or solenoid inside the engine. If required these parts can be replaced multiple times, ensuring the long life of the optical engine.
The optical engine will be returned to you with a new Met One calibration certificate. You’ll need to re-install the optical engine and enter the unique calibration parameters from the calibration certificate using Aeroqual software.
If you monitor has been offline for a significant period of time, we also advise calibration before beginning any new projects.
The turnaround time is 30-45 working days.
You only need to return the optical engine - don’t return the pump module, electronics module or sampling system.
If you have a spare optical engine, you don’t need a separate Aeroqual Cloud license if I want to use it during factory calibration of the original engine. The license is applied to the monitor’s serial number, not individual modules.
For more information on factory calibration, contact your nearest Aeroqual distributor.
Field calibration of your particulate monitor is typically not required. However, it may be appropriate if you have very high data quality objectives, such as:
- Supplementary monitoring to a reference network (so inter-comparison to reference is important)
- Research studies that need to demonstrate traceability to another monitor
- Consultancy projects that need to demonstrate traceability to another monitor.
In summary, field calibration of Aeroqual monitors is recommended when comparison or traceability to reference monitors is an important factor in the monitoring project.
A comparison with a reference instrument can also be undertaken when your Aeroqual monitor is first installed. This ensures readings from the Aeroqual monitor are adjusted for the local particle type, color and morphology.
Gravimetric instruments are considered the most accurate, but BAM or TEOM instruments deliver data faster and don’t involve lab work. See the Important considerations section at the end of this page for more information about the benefits of each instrument type.
The adjustment or correction is commonly known as a ‘K-factor’.
Data is collected from both the reference instrument and for the Aeroqual monitor. Then the data is plotted and a calibration curve (K-factor) is calculated.
The K-factor is then applied to the monitor, using the Gain setting for each PM channel:
In the following example, the Aeroqual monitor is installed outside a shelter. Inside the shelter is a gravimetric instrument. Data has been collected for 2 weeks.
The graph to the left shows the time series of the two monitors. The graph to the right shows the slope calculation. In this example, the Aeroqual monitor is reading higher than the reference instrument. The slope of the curve reference vs Aeroqual monitor is 0.6568. So a gain of 0.6568 needs to be applied.
This depends on your data quality requirements. A suggested time is two weeks. Two weeks should allow a representative number of days to provide a calibration graph. If less accuracy is acceptable, the number of days can be reduced. The shortest recommended calibration period is one day (24 hours).
Range of K-factors
Because of the significant variation in optical properties of particles, K-factors can also vary significantly. An Aeroqual monitor may read higher than the reference instrument or it may read lower. K-factors my vary by as much as 0.3 to 5.0.
Once a K-factor is established for a given site, you shouldn’t need to repeat the calibration as long as you’re confident the site’s particulate composition hasn’t changed.
Using poor-quality reference data for calibration will result in poor quality PM data from your Aeroqual monitor. Aeroqual recommends that calibration only be carried out using high-quality reference PM data. If you don’t have access to a reference monitor or aren’t confident you can access a well-run reference monitor, you should always rely upon the factory calibration.
- Location of monitor: The Aeroqual monitor and the reference monitor need to be located at the same site preferably with their inlets within 10m of each other and must be sampling at the same height. At reference sites using a monitoring shed, on the rooftop of the shed is usually the best location.
- Monitor set up: Both the Aeroqual monitor and the reference monitor need to be set up in accordance with the manufacturer’s recommendations, with correctly set flow rates and fresh filters. If you are using a gravimetric method reference monitor, you must follow appropriate filter conditioning and weighing methods.
- Duration of co-location: As a rule, longer is better, as this will give you more data points and therefore more confidence in your field calibration. It’s important that the co-location is carried out during the same environmental conditions expected during deployment. 1-2 weeks is preferred and no shorter than 48 hrs.
- Reference monitor selection: Selection of the right reference monitor is critical for a successful K-factor correction. Ideally, reference monitors should be operated in accordance with local reference method specification. Monitor methods are listed below, in descending order of preference:
- GRIMM / BAM / TEOM: Are best as they are continuous methods, meaning more data can be collected in a shorter period, giving a more accurate K-factor. They report data in near real time, often 30 minute or 1 hour intervals, which means, in one day you might get 24 data values you can compare. BAM and TEOM data can be noisy at hourly averages, so consider using 24 hr averages if particle levels are low. This is Aeroqual’s suggested method, as lots of data can be collected quickly, with no need for expensive lab work.
- Gravimetric methods (Partisol, low-volume samplers using 47 mm filters): A gravimetric instrument is the most accurate, but it means you need to run the co-location for much longer as they operate on 24-hr averages (ie. 1 data point per 24 hrs). Gravimetric instruments draw a known volume of sample air onto a filter that’s then sent off to a lab to be weighted (i.e. (X) ug dust / (X) m3 of air). Filters must be appropriately prepared and transported. Lab and field blank filters should also be used. Balance accuracy for pre and post weighing.
- Other non-reference methods (optical, open-path): Existing instrumentation can be used to establish variance between Aeroqual and a known monitor. This method can be used to assess new versus old instrumentation but is not a calibration to a reference monitor.
- High-volume samplers: Not recommended due to documented variance between these and other methods and potential for filter contamination.