www . refrigerationandaircon . co . za RACA Journal I July 2023 47
Technical
Simply put , PM2.5 refers to particles of 0.3 – 2.5 µ m in size and a claimed efficacy level of 99 % naturally means that the device must remove the particles in accordance with the claim .
In addition to the above , the efficacy is referenced in µ g / m ³ which clearly refers to a mass measurement test and not a particle count test which would determine the efficacy for a given particle size . 1
Size estimation : This is a critical factor in determining mass . Since the volume of a sphere is the cube of its diameter it is imperative that we arrive at a reasonably accurate estimation of the actual size of all the particles we see . The table below shows an example of PM calculation by size for a 6-channel sensor . For the sample period this sensor saw nearly 13 000 particulates in the 0.3 µ m channel and only 79 particulates in the 2.5 µ m channel , yet , the estimated mass of the 2.5 µ m particulates in the 2.5 µ m channel is more than 5 times as large . It has a huge impact on the quality of this estimation and so the more accurately a sensor can estimate the size of particulates the more accurately that sensor can estimate particle mass for particulates in a channel ( or multiple combined channels , as in PM2.5 ).
Channel Channel PM (µ m ) ( counts ) (µ g / m3 ) 0,3 12980 0,38 0,5 1413 0,28 1 181 0,45 2,5 79 1,93 5,0 12 2,34 10,0 9 14,04 installed on the inlet to attempt to remove the moisture from the incoming air stream in an attempt to improve sensor accuracy .
A PM ( Particulate Matter ) mass measuring particle counter is a device that is used to measure the mass concentration of airborne particulate matter in a given environment . Here ' s a brief overview of how a PM mass measuring particle counter works : 1 . Sampling : the particle counter draws in a sample of air from the environment being monitored . The air sample is usually passed through a filter to collect the particulate matter .
2 . Weighing : once the air sample has been collected on the filter , it is weighed using a microbalance . The filter is typically made of a material with a known mass , such as a Teflon or quartz filter .
3 . Data Analysis : the weight of the filter is measured before and after sampling , and the difference in weight is used to determine the mass concentration of the particulate matter in the air . The mass concentration is typically reported in units of micrograms per cubic metre .
Some PM mass measuring particle counters also use a laser-based method to estimate the particle size distribution in the sampled air , in addition to measuring the mass concentration . These instruments use a laser beam to illuminate the particles in the sample and measure the amount of light scattered at different angles to estimate the particle size distribution .
Claims of 99 % efficacy by mass do not mean the device is effective in removing the finer particulate matter at the same efficiency levels . For example , a single 2.5 µ m particle has the same mass as 600 0.3 µ m or 2000 0.2 µ m particles . An increased number of studies confirm that the fine particulates are not reported well using PM mass measurements . Efficacy by mass (µ g / m ³) should not be confused with efficacy by size ( micron ).
OTHER FACTORS When estimating mass , a number of factors impact this estimation . The density , refractivity , and even the geometry of particles passing through the sensor can impact the mass estimation . As you might imagine a dark , and very dense particulate will have a much lower mass estimation than a less dense and paler particulate even if they are of identical size . High relative humidity can also have an effect on particulates . They can act like seeds and absorb moisture growing in apparent size within a sensor . This is more pronounced outdoors in highhumidity environments . In such environments , heaters can be
The mass of the particles is estimated and based on general mass data because one cannot assume that all particles of an identical size will weigh the same .
Overall , PM mass measuring particle counters are used to test general environmental conditions and atmospheric air filters but are not suitable for testing air in cleanrooms nor for testing HEPA filters .
SO , HOW DID WE GET HERE ? Well , as we know from professional air quality testing , air quality measurement arose out of a need to control manufacturing environments in the cleanroom space . The companies that serve
www . refrigerationandaircon . co . za RACA Journal I July 2023 47