Particle counts and mass concentration: two numbers from one sensor

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Counts per volume and micrograms per volume answer different questions

Count concentration answers a question about how many discrete objects are suspended in a given quantity of air. It is written as particles per litre or particles per cubic foot, usually broken out by size channel, and it is the language of contamination control. A surgical suite, a paint booth, an optics bench and a semiconductor bay all care about objects, because a single object of the wrong size landing in the wrong place is the whole problem.

Mass concentration answers a question about how much material is suspended, written as micrograms per cubic metre. It is the language of public health guidance, because the dose that matters to a lung is a quantity of deposited material rather than a tally of events. Every published outdoor and indoor air quality target you are likely to meet is phrased this way, which is why a handheld instrument that only produced counts would feel useless to most people who buy one.

So manufacturers give you both. The Eujgoov dust particle counter states exactly this arrangement in its bullets: a particle counting mode and a concentration measurement mode, switched with one key. What the bullets do not say, and what almost no listing says, is that the second mode is derived from the first. The conversion factors are not stated in the listing bullets for any of these instruments, and they are the single largest reason two counters can agree on objects and disagree on weight.

How a scattering sensor turns pulses into a microgram figure

Inside the chamber a pump or a fan pushes a narrow stream of air across a laser beam. When a particle crosses the beam it scatters light, and a photodetector sitting off to one side sees a brief pulse. Two properties of that pulse carry information. The fact that a pulse happened at all is a count. The height of the pulse, which depends on how much light the particle threw sideways, is the only clue to size the instrument will ever get.

Pulse height is compared against a ladder of thresholds, and the particle is assigned to whichever size channel it clears. Because the instrument knows its own airflow, it knows how many litres passed the beam during the sample, and dividing counts by litres gives a count concentration directly. Nothing has been assumed yet beyond the relationship between scattered light and diameter, which is itself calibrated against a reference aerosol of known size.

The mass step is where the assumptions arrive. For each channel the firmware assigns a representative diameter, treats the particle as a sphere, works out the volume of that sphere, multiplies by an assumed density to get a mass per particle, multiplies by the number of particles in the channel, and sums across the channels that fall under the PM2.5 or PM10 definition. Four separate guesses sit in that chain: the representative diameter inside a wide channel, the sphere, the density and the size range cutoff.

The density and shape assumptions hidden inside PM2.5

Density is the assumption with the largest leverage, because it multiplies the whole answer. Most optical instruments assume something close to one gram per cubic centimetre, which is the density of water and a reasonable stand in for the droplet rich aerosols these sensors were tuned against. Real indoor aerosol is a mixture. Soot agglomerates from a candle or a gas hob are fluffy open structures with an effective density well below one. Crushed mineral dust from plaster, grout or concrete runs above two. A room full of plaster dust reported with a water density assumption will understate the real weight by more than a factor of two, while a room full of candle soot will overstate it.

Shape matters for a different reason. Optical sizing assumes a sphere because a sphere scatters predictably from every angle. A flake of skin, a fibre of cotton or a chain of soot particles presents a different cross section depending on how it happens to be oriented as it crosses the beam, so the same object can land in different channels on successive passes. Fibres are the worst case, since a long thin object can scatter like something far smaller than its actual length.

Refractive index completes the set. Light scattering depends on how strongly the particle interacts with the beam, and a dark absorbing particle throws less light sideways than a transparent droplet of the same diameter. An instrument calibrated on a pale test aerosol will size dark particles as smaller than they are, which pushes them into lower channels and reduces the calculated mass twice over.

Worked example: the same air described both ways

Take a single target of twelve micrograms per cubic metre and ask how many particles per litre it would take to reach that figure, if every particle in the air were a sphere of one diameter at a density of one gram per cubic centimetre. The volume of a sphere is one sixth of pi times the diameter cubed, and at that density a volume of one cubic micron weighs one picogram, so the arithmetic is short.

The spread across that final column is a factor of more than thirty thousand. That is the whole lesson. Mass is dominated by the largest particles present, number is dominated by the smallest, and the two readings on the screen are tracking opposite ends of the same distribution. It also explains a familiar frustration: vacuuming a carpet lifts coarse material and moves the microgram figure sharply while barely touching the count in the finest channel, and lighting a candle does precisely the reverse.

A gas instrument has no equivalent ambiguity, which is a useful contrast. The DANOPLUS carbon dioxide controller states a dual beam low drift sensor reading to plus or minus fifty parts per million, and parts per million is a ratio of molecules with no size distribution hiding inside it. There is one number because there is one thing to count. Particles do not offer that convenience, and an instrument that showed only one figure would be concealing a choice rather than simplifying one.

Which of the two numbers to record for the job in front of you

Write down the count when the question is about change. Before and after a filter replacement, with and without an extractor running, upstairs against downstairs, during a sanding job against an hour later: all of these are comparisons made with one instrument in one place, where the assumptions in the conversion cancel out because they are identical on both sides. Counts also carry more resolution for small changes, since the fine channels respond to events the mass figure barely registers.

Write down the mass figure when the question is about a published target, and write it down with the sampling conditions beside it. The six channel air particle meter states an active pump drawing 2.83 litres per minute and separate alarm thresholds for all six channels, which means a record from it can state the sampled volume rather than leaving it implied. A microgram figure without a sampling duration, a location and a note on what was happening in the room is not evidence of anything.

Two further habits keep the record defensible. Note the humidity, because swollen droplets scatter like larger particles and inflate both readings together. And repeat the measurement after a known interval, because optical sensors shift over time in ways that look like a real change in the room. The background on that is set out in what sensor drift actually is, and the practical consequences across instruments are gathered in why particle sensors disagree. If you are still working out which class of instrument suits the work, the particle counter overview and the Temtop particle counter writeup cover what the different form factors can and cannot report.

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