Choosing the right environmental dust monitor can be hard. There are different types of dust sampling methods – high volume, low volume, optical, beta attenuation – and there are different levels of certification too. We always get asked – how does this equipment compare to that equipment? What does equivalent mean and how does that compare to reference? We wrote this blog to explain, hopefully it helps.
For the purposes of comparison, we put popular dust monitoring instruments into four different performance classes: certified reference, certified equivalent, certified indicative, and indicative. The terminology is based on terminology that is commonly used in the United States and Europe.
The main difference between instruments from the different classes is the cost (purchase, and operational cost) and accuracy. Generally speaking the higher the accuracy the higher the cost.
The diagram below gives a quick visual of the different types of environmental dust monitors on the market including our own Dust Sentry and Dust Profiler instruments.
Certified Reference Instruments
The two instruments shown in the Certified Reference class are the high volume sampler and the Partisol. There are many other instruments which have reference certification, but these are the most common.
To be classed as Reference, instruments must employ the gravimetric method of particle measurement. A gravimetric method is a quantitative measure based on the mass of the solid.
Although this is the most accurate method of particle measurement it has three big disadvantages compared to methods from other classes.
- The method is not real time. The data is acquired only after weighing a filter which can take several days or weeks. So the gravimetric method cannot be used when immediate action is required following a dust event, for example suppressing dust from demolition using sprinklers.
- The data is usually provided as a daily average. It is not possible to understand the differences between the dust levels in the morning compared to the dust levels later in the day. That matters because environmental dust will peak at certain times of the day e.g. during high wind, or when vehicle movements are high.
- The method requires manual weighing of filters on a daily or weekly basis. The sample must be physically removed by the operator then sent off to a lab for analysis. This means the operational cost is very high compared to the automated methods.
In summary, reference instruments are very accurate but they are expensive to operate and can’t provide an hour by hour view of environmental dust levels.
Certified Equivalent Instruments
Equivalent methods are so called because they have been shown to have performance that is equivalent to Certified Reference instruments. That does not mean they perform exactly the same, but they do meet some objective pass/fail criteria set by the US EPA, MCERTS, TUV or similar regulatory body.
The three instruments shown in the Certified Equivalence class are the TEOM from Thermo Fisher Scientific, the BAM from Met One, and the EDM 180 from Grimm. There are many other instruments which have indicative certification, but these are the most common.
Several different measurement methods have been certified as equivalent. Each method has unique challenges and sources of error. All instruments in this class are more expensive than instruments in the Certified Indicative class. They can also be more expensive than instruments in the Certified Reference class.
Equivalence methods have two main advantages compared to reference methods:
- They can offer data at hourly intervals. These instruments can be configured to provide data as hourly averages, rather than daily averages. That means data from one part of the day can be compared to data from another part of the day.
- The methods do not require manual intervention to weigh a filter. These instruments can run automatically for longer periods of time compared to the Reference methods so the operational cost is lower.
The equivalence methods have some disadvantages:
- Many of these instruments require additional sheltering from the weather so they are not ready for installation out of the box. This adds to the total installation cost of these instruments. The additional bulk can limit the number of possible installation sites.
- These instruments cannot provide minute by minute resolution. Although these methods provide higher temporal resolution than the reference methods, they still not provide minute resolution. So they are not suitable for applications where an immediate response to environmental dust is required. Researchers may also value minute by minute data.
- These instruments cannot easily be integrated with additional sensors such as wind speed and direction, this integration often requires extra work to be done during installation such as configuration of data loggers which adds cost to the installation for these instruments.
In summary, Certified Equivalent instruments are accurate and provide hour by hour data, but the total installation cost can be high, and they cannot provide minute by minute data.
Certified Indicative instruments
Indicative means that the data has been shown to be indicative relative to Certified Equivalent instruments. Indicative is a term used more often in Europe where there is a formal definition of the criteria for Indicative instruments. The formal criteria led to the creation of the world’s only standard for indicative dust monitoring instruments, MCERTS.
Several manufacturers (including Aeroqual) have been able to demonstrate performance to meet MCERTS standard for indicative dust monitoring. The standard requires a full 40 day field trial of two indicative dust monitors against two reference or equivalent dust monitoring instruments. In addition the manufacturer must pass a factory inspection to ensure that quality systems are in place that will ensure instruments are produced of a consistent high quality.
There are many environmental dust monitors on the market which do not have any performance certification. They include the TSI Dust Trak and the Dust Guardian from Casella.
Certified Indicative instruments have several advantages compared to Certified Equivalence instruments:
- The optical sensor they use is relatively low cost, meaning the instruments can be very affordable. Being affordable means more people can afford to use them; it also means that some people can buy three Indicative instruments instead of just one Equivalent instrument. This increases data quality by increasing spatial density (i.e. you can measure at more than one location so you have a better idea of what’s really going on).
- Data is available minute by minute. The optical measurement method is real-time so the data-logging interval is 1 minute and sometimes better. High temporal resolution is invaluable to researchers and to those who need to respond immediately to high dust events.
- The optical sensor used is often robust and therefore the instrument can go for long periods before the sensor must be returned to the factory for servicing (up to 2 years in the case of the Dust Sentry).
- The instruments can be fully integrated stations complete with internal data-logging, communications, and sometimes a power supply. Buying an all in one package is convenient and allows you to know the total cost of installation upfront.
One disadvantage of Indicative instruments is that they may need calibration against a Reference instrument. It isn’t always required but if it is, it adds cost to the initial commissioning of the instrument.
In summary Certified Indicative instruments have significant cost advantages. They are affordable and easy to use. Certified instruments have been shown to meet formal accuracy and quality standards. Finally, servicing requirements can be lower and that keeps the total cost of ownership down.
How to choose?
It depends on your application and budget. For an affordable and reasonably accurate environmental dust monitor then a Certified Indicative instrument is a good choice. Where high accuracy is required and cost is not a barrier then you should consider a Certified Reference or Certified Equivalent instrument. If daily data is enough then Certified Reference might be right for you. If you need minute by minute data then you must have an Indicative monitor.
In many cases a mix of instruments can offer the best of both worlds. This blog post talks about the merits of mixing Reference with Indicative instruments in construction and mining applications. The Reference instrument gives highly accurate daily readings; the Indicative instrument gives minute by minute data that can be checked against the Reference data to ensure traceability.
If you still need help choosing, talk to a professional. Like us – we’re always happy to help!
Aeroqual’s range of dust monitoring equipment designed for continuous real-time measurement of ambient dust and particulate matter outdoors can be viewed here
John is a PhD who joined Aeroqual eight years ago and has been there since the birth of the AQM 60 compact air monitoring station. He blends deep technical knowledge with real world application of sensor based air monitoring. And he enjoys sharing this knowledge with partners and customers wherever they are in the world.