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In almost every case the answer is that the two instruments are not measuring the same air. Particle concentration varies sharply over short distances and over seconds, so two devices a metre apart, or one near a wall and one in open space, are sampling genuinely different conditions and both can be right. Once that is ruled out by putting them inlet to inlet, the remaining gap comes from how each was calibrated, how each responds to humidity, and how each converts counts into a mass figure. A broken sensor is well down the list of likely explanations.
Two instruments sampling different air is the usual answer
A room is not a well mixed box. Air arrives from a vent, falls down a cold window, rises off a radiator and a human body, and leaves under a door, and the particle field rides on all of that. Settling adds a vertical gradient that gets steeper as particle size increases: a ten micron fragment falls at a few millimetres per second, so near floor concentrations of coarse material run well above head height concentrations, while sub micron material is effectively suspended indefinitely and mixes with the airflow.
Surfaces make it worse. Put an instrument within a few centimetres of a wall or a tabletop and it samples the boundary layer, where velocity is low and deposition has already stripped out part of the coarse fraction. Put another in open air a metre away and it samples the bulk. Both report correctly and the pair disagrees by an amount that depends on nothing but geometry.
Then there is time. Open a door, walk across a carpet, switch on an extractor, and concentrations move within seconds. Two instruments with different averaging windows, or simply with their sample cycles out of step, will show that transient at different heights and at different moments. A side by side comparison started by pressing two buttons several seconds apart is not a comparison at all during any period when something is happening in the room.
Calibration aerosols differ, so the conversion differs
An optical counter is taught the relationship between pulse height and particle size using a reference aerosol of known diameter. The classic choice is monodisperse polystyrene latex spheres, which are uniform, round and have a well characterised refractive index. Other makers use a different reference, and the choice propagates into every reading the instrument will ever produce, because the particle sizing ladder is anchored to it.
Real room aerosol resembles the reference material very little. It is a mixture of sizes, shapes and optical properties, with soot absorbing light strongly, mineral dust scattering differently again, and water droplets behaving closest to the idealised sphere. An instrument anchored to a pale spherical reference will undersize dark absorbing particles, placing them in lower channels, while one anchored to a different reference will place the same particles elsewhere. Neither instrument is faulty and the two will not match on a smoky afternoon even though they matched on a clean morning.
The reference used is almost never published for instruments in this class, and it is not stated in the listing bullets for any of the counters here. What is published is the channel structure, which at least tells you where the thresholds sit. The six channel air particle meter states six size channels and an active pump at 2.83 litres per minute. The cleanroom and HVAC counter states the same channel set and the same flow, plus storage for more than two million groups of measurement data through a Type C interface. Matching thresholds remove one source of disagreement without touching the calibration question underneath.
Humidity swells particles before the laser sees them
Many of the components of indoor aerosol are hygroscopic, meaning they take up water from the surrounding air. Salts, many organic compounds and the products of combustion all do this, and the uptake accelerates steeply once relative humidity climbs past about seventy percent. A particle that has absorbed water is physically larger and optically closer to a pure droplet, and the instrument has no way to know that most of the new volume is water.
The consequence runs in one direction and affects both displayed numbers. Swollen particles move up into higher channels, so coarse channel counts rise. The mass calculation then multiplies those inflated diameters by an assumed density, so the microgram figure rises faster still, because mass follows the cube of diameter. Two instruments in the same humid room will both overstate the dry particle load, and they will overstate it by different amounts if their channel boundaries differ, because the swelling pushes particles across thresholds that are not in the same places.
This is why a comparison run deserves a humidity note alongside it. Both of the six channel instruments mentioned above state integrated temperature and humidity sensing, which means the context can be captured without a second device. A morning comparison in a dry room and an evening comparison in a steamy one are not the same experiment, and the difference between them is not an instrument fault.
A side by side check that separates instrument error from room variation
The point of a structured comparison is to switch off the room so that only the instruments are left varying. The sequence below does that, and the table shows what each result implies.
- Place both inlets within a few centimetres of each other, at the same height, away from walls, vents and people, and leave the room undisturbed for several minutes before starting.
- Start both at the same moment and use the longest sample period each instrument offers, so counting statistics are not confused with disagreement.
- Run at least three consecutive samples and keep every figure, not the average, because the spread between repeats on one instrument sets the scale against which the gap between instruments must be judged.
- Swap the physical positions of the two instruments and repeat the whole set.
- Record relative humidity at the start and the end, and note anything that happened in the room during the run.
| What the results look like | Most likely cause | What to do about it |
|---|---|---|
| Gap stays with the position after the swap | Room variation, not instrument | Move both away from the surface or vent causing it and repeat |
| Gap follows the instrument after the swap | Genuine instrument difference | Record a fixed scaling factor and use it only for trend work |
| Repeats on one unit scatter as widely as the gap | Sample too short for the concentration | Lengthen the run until repeats tighten, then compare again |
| Counts match, mass figures diverge | Different conversion assumptions | Compare counts only, and treat the mass figures as separate estimates |
| Gap widens as humidity climbs | Hygroscopic growth, different thresholds | Compare only at similar humidity and note it with the reading |
One further cause belongs on the list for dirty air. When concentration is very high, two particles can cross the beam close enough together to be read as one larger particle, which is called coincidence. It depresses counts and inflates sizes, and because it depends on the sampled volume per unit time, two instruments with different flow rates reach the onset of coincidence at different concentrations. The Eujgoov dust particle counter states a built in micro vacuum pump and a fast response, and its upper limit before coincidence becomes significant is not stated in the listing bullets, which is normal for the class.
What agreement between two monitors would actually look like
Stop expecting matching digits. Two optical counters of this type agree well when their counts sit within roughly a fifth of each other in stable air, when they move in the same direction and by a similar proportion when the room changes, and when the ratio between them stays roughly constant across several days. That last property is the valuable one, because a stable ratio can be carried as a correction and a wandering ratio cannot.
Mass figures deserve a looser standard still, since each instrument stacks its own assumptions on top of counts that already differ. Treating the two microgram readings as independent estimates of the same quantity, rather than as a measurement and an error, is both more honest and more useful. The same pattern shows up with gas instruments for different reasons, and why two carbon dioxide monitors show different readings covers that case where drift and reference calibration dominate instead.
Where this leaves a practical decision is simple enough. Use one instrument consistently for any comparison that matters, keep the second as a sanity check rather than as a referee, and record the conditions well enough that a future reading can be set against today without guesswork. Background on the instrument side of the question is collected in why particle sensors disagree, the fixed and portable options are surveyed in the air particle monitor overview, and the mass focused end of the category is covered in the PM2.5 monitor overview.
Frequently Asked Questions
How close is close enough for two monitors to agree?
For optical instruments of this class, counts within about twenty percent of each other in the same air, after a proper side by side run, is a normal and acceptable result. Mass figures can sit further apart because each instrument applies its own conversion assumptions on top. Perfect agreement is not the goal and would be slightly suspicious. What matters far more is whether the two move together when the air changes, because a pair that tracks each other can be used for comparison even if their absolute figures differ.
Does humidity really change a particle reading?
Substantially, and it is one of the most underrated effects in the whole field. Many common aerosol components absorb water as relative humidity climbs, so a particle physically swells. A swollen particle scatters more light and gets sized into a higher channel, which raises both the count in the coarse channels and the calculated mass. The same dry material measured at low humidity and at high humidity can produce readings that differ by a large factor, with no change whatsoever in how much solid matter is suspended.
Should I trust the newer instrument?
Age is weak evidence either way. A new instrument has not yet accumulated deposits on its optics or lost pump displacement, which favours it. An older instrument that has been used regularly in known conditions has a history you can reason about, which favours it instead. The question worth asking is not which is newer but which one can state its sampling conditions, since an instrument that publishes a flow rate and a channel convention can be checked and one that publishes neither cannot.
Can I calibrate one monitor against another?
You can scale one to the other, which is useful, but you should not call it calibration. A true calibration refers an instrument to a traceable standard, typically a reference aerosol of known size and concentration. Scaling one consumer instrument onto another simply transfers the errors of the reference unit onto the second one and then hides them behind a correction factor. It is a reasonable way to make a pair of monitors comparable for trend work, and it is not a basis for any absolute claim.
Why do the counts agree but the mass figures differ?
Because counts are observed and mass is computed. If two instruments detect a similar number of particles in similar channels, their optics and airflow are broadly consistent. The mass figures then diverge because each applies its own assumed density, its own representative diameter for each channel, and its own treatment of the particles that straddle a definition boundary. Agreement on counts with disagreement on micrograms is actually the most informative outcome, because it isolates the discrepancy to the conversion.