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An air quality monitor is a collection of separate sensors sharing one case. Particulate is measured optically by shining a laser into a stream of air and counting the flashes as particles cross it. Carbon dioxide is measured by how much infrared light the air absorbs at a specific wavelength. Formaldehyde and other single gases use electrochemical cells, while the volatile organic compound figure usually comes from a heated metal oxide element that cannot identify any gas at all. The index on the front is arithmetic performed on one of those channels.
Every channel on the display is a separate sensor
Listings for multi channel units are unusually honest about this once you read them as a parts list. The LifeBasis eleven in one air quality detector states an infrared carbon dioxide sensor, a laser particle sensor, a semiconductor sensor, a photoelectric sensor and a temperature and humidity sensor, and reports an index, carbon dioxide, three particulate size classes, a particle count, formaldehyde, volatile organic compounds, temperature and humidity from that set. Eleven numbers, five sensing technologies, and a case.
That matters because the quality of a monitor is not a single property. A unit can have a good infrared carbon dioxide channel and a nearly meaningless volatile organic compound channel, and no overall figure of merit captures that. Judging one means judging each channel against what its physics can deliver.
| Channel | Sensing principle | What is measured directly | What is calculated |
|---|---|---|---|
| Particulate mass | Laser light scattering | Pulse rate and pulse height | Size distribution, then mass per cubic metre |
| Particle count | Laser light scattering | Pulse rate above a threshold | Counts per volume from the sample flow |
| Carbon dioxide | Infrared absorption | Light reaching a detector through a path | Parts per million from a calibration curve |
| Formaldehyde | Electrochemical cell | A small current across an electrolyte | Concentration from a sensitivity figure |
| Volatile organic compounds | Heated metal oxide | Resistance of a heated film | An equivalent concentration of a reference gas |
| Temperature | Thermistor or integrated sensor | Resistance or a digital value | Degrees, with a case offset applied |
| Humidity | Capacitive polymer film | Capacitance between electrodes | Percent relative humidity |
| Air quality index | No sensor | Nothing | A piecewise conversion of a pollutant figure |
How a laser counts particles it cannot resolve
An optical particle sensor contains a laser diode, a dark chamber, a photodetector placed off the beam axis, and a fan or heater creating a steady flow of air through the beam. A particle crossing the beam scatters light in all directions, and a fraction of that light reaches the detector as a brief pulse. The number of pulses per second indicates how many particles are passing. The height of each pulse indicates roughly how large the particle was, because bigger particles scatter more light.
Both of those inferences have limits. Scattering intensity depends on particle size, refractive index and shape, so a soot flake and a water droplet of the same diameter do not produce the same pulse. Below about 0.3 micrometres the scattered signal falls away faster than the detector can follow, which is why a channel labelled for the smallest particles is often derived from the size distribution rather than counted directly. And the step from a counted distribution to a mass concentration in micrograms per cubic metre requires assuming a density for the particles, which is a manufacturer choice rather than a measurement.
This explains the single most common complaint about these instruments, which is two units disagreeing in the same room. They are not disagreeing about what they counted. They are disagreeing about what to multiply it by. The Vetouch monitor notes and the HSCLPM monitor notes record what each listing publishes about its own optical channel, which is usually less than a buyer would want.
Infrared absorption and why carbon dioxide gets the good sensor
Carbon dioxide absorbs infrared light strongly at a wavelength near 4.3 micrometres, and very little else in indoor air does. That gives a measurement that is specific to the gas by physics rather than by inference. A non dispersive infrared sensor puts an infrared source at one end of a small chamber, a narrow band optical filter and a detector at the other, and measures how much light survives the trip. More carbon dioxide in the path means less light arriving, following an exponential relationship between concentration and absorption.
The weakness is not specificity, it is baseline. The source ages, the chamber walls dull, the optics collect dust, and the amount of light arriving in clean air slowly changes. Instruments compensate either with a second reference channel at a wavelength nothing absorbs, or with automatic baseline correction that records the lowest reading over a period of days and assumes that value was outdoor air. The second method is cheaper and fails in rooms that never reach outdoor concentration. The Temtop M2000C monitor bundle states carbon dioxide, particulate mass in two size classes, and export of readings as comma separated values over a universal serial bus, and that export is what lets you see whether a baseline has crept over weeks rather than guessing.
Metal oxide sensors and the number called TVOC
A metal oxide sensor is a thin film of tin oxide or a similar semiconductor held at a few hundred degrees by an integral heater. Oxygen adsorbed on the hot surface traps electrons and raises the resistance of the film. When a reducing gas arrives it reacts with that adsorbed oxygen, releases the electrons, and the resistance falls. The sensor is therefore a detector of reducing gases in general, and it responds to alcohol, cleaning products, cooking vapours, fragrances, solvents and the breath of anyone who has had a drink, without distinguishing between them.
Because the element cannot identify a species, the output is scaled against one reference gas and reported as an equivalent concentration, which is what the letters in a total volatile organic compound figure mean. It is an index of reducing gas activity. A rise means something changed, which is genuinely useful, and the absolute value should not be read as a concentration of any particular compound. What that number is and is not is set out in what a volatile organic compound reading means.
Electrochemical cells are the more specific alternative for a single named gas. A working electrode, a counter electrode and an electrolyte produce a current proportional to how fast the target gas diffuses in through a controlled membrane, so the response is specific and reasonably linear. The cost is a finite lifetime, since the chemistry is consumed, and sensitivity to temperature and pressure. The ZEMEOLLO ten in one air quality tester states precision electrochemical sensors for formaldehyde and volatile organic compound monitoring alongside a dedicated carbon dioxide technology, and quotes an error margin of less than five percent. Which channel that figure applies to, against what reference, and over what range is not stated in the listing bullets, which is the normal situation and the reason a single accuracy claim on a multi channel instrument should be read as a headline rather than a specification.
The index is arithmetic not a measurement
An air quality index is a published function that maps a pollutant concentration onto a bounded scale with named bands, defined by an environmental authority so that different pollutants can be compared on one axis. It is a transformation, not a reading. A monitor showing an index has measured a concentration and then applied the conversion, and a monitor that shows an index without showing the underlying concentration has hidden its own input.
Two practical consequences. The index on a consumer display may not be the index used by your national authority, since several definitions exist with different breakpoints, so a value is not comparable with an official local figure unless the definitions match. And the index is usually computed from the particulate channel alone, so a room with excellent particulate and a carbon dioxide concentration past 2,000 parts per million can show a comfortable index while the air is plainly stale.
What these listings publish about their own sensors
Reading a listing as a parts list is the most useful habit available to a buyer here. Named technologies are a good sign: infrared for carbon dioxide, laser for particulate, electrochemical for a named gas. Unnamed technologies on a channel with a specific sounding output are the warning. Sensor part numbers, response times, drift per year and the index definition in use are the four specifications that would settle most questions, and across these listings none of the four appears. Not stated in the listing bullets, consistently.
So judge a monitor by which channels it has a credible mechanism for, treat the volatile organic compound and formaldehyde figures as relative indicators rather than concentrations, and trust the carbon dioxide channel most. The Sculpriv monitor notes go through one unit in that order. Having understood what each channel is, the next question is what to do with the numbers, and the Sacnahe monitor notes show the same channel by channel reading applied to a different unit.
Frequently Asked Questions
Why do two monitors in the same room show different particulate numbers?
Because the mass concentration they display is calculated, not measured. Both count scattering pulses and infer particle sizes, then convert that distribution into micrograms per cubic metre using assumptions about particle density and shape that the manufacturer chose. Different assumptions give different numbers from identical air.
Which sensor in a multi channel monitor is the most trustworthy?
The carbon dioxide channel, if it uses infrared absorption. The measurement is specific to the gas, the physics is stable, and drift shows up as a baseline shift that can be corrected. Volatile organic compound channels on heated metal oxide elements are the least specific by a wide margin.
Does a monitor need a fan to work?
A particulate channel does, because particles have to be drawn through the optical chamber at a known rate. Gas channels can work by diffusion, which is quieter and uses less power but responds more slowly. A fan also stirs the air around the temperature and humidity sensors, which changes what they read.
What is the warm up time about?
Heated metal oxide elements have to reach their operating temperature before their resistance means anything, and electrochemical cells need their internal chemistry to settle. Readings taken in the first minutes after power on are unreliable on those channels, while the infrared and optical channels are usable almost immediately.
Can one sensor measure several gases?
A metal oxide element responds to many reducing gases at once and cannot tell them apart, which is why its output is reported as a combined index rather than as a concentration of anything specific. Separating gases needs either a separate specific sensor each or a laboratory technique such as chromatography.
Why does humidity change the particulate reading?
Many airborne particles absorb water and swell in damp air. A larger particle scatters more light, so the optical system sees it as a bigger particle and reports more mass. Readings taken at high humidity are commonly higher than the dry particulate content of the same air.