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Air Quality Solutions

Particulate Matter

SEN5X family of sensors from Sensirion is a platform of compact particle sensors. They have a >10 years lifetime. Because they use a fan to create a steady stream of air in the sensor chamber, they create 24dB noise at a distance of 0.2m. Common used parameters are PM1 (0.3-1 $\mu$ particles), PM2.5 (0.3-2.5 $\mu$), PM4(0.3-4 $\mu$), PM10 (0.3-10 $\mu$). They also measure Humidity and Temperature with an accuracy of $+/-4.5%$ and $+- 0.45\degree C$.

There are the following models available:

  • SEN50 Particulate Matter approx. $22.2
  • SEN54 Particulate Matter, rel Humidity, Temperature, VOC Index, approx. $28.1
  • SEN55 Particulate Matter, rel Humidity, Temperature, VOC Index, NOx Index, approx. $32.6

There are less expensive sensors available from Winsen

CO2

Sensirion makes accurate compact photo acoustic CO2 sensor. The sensor is negatively impacted by vibrations. Its not clear if the fan of a particulate matter sensor impacts this sensor.

There are the following models available:

  • SCD40 400-2000ppm, +/- 50ppm datasheet approx. $29.7
  • SCD41 400-5000ppm, high accuracy, low power datasheet approx. $37.1
  • SCD42 400-2000ppm, +/-75ppm datasheet no lonber available.

They also measure Humidity $+/-6/%$ and Temperature $+/-0.8 \degree C$ as the sensor signal is affected by those quantities.

There are less expensive sensors available from Winsen

Pressure

Bosch makes high quality pressure sensors with an altitude resolution in air of 25 cm.

  • BMP581, 30 -125 kPa, ultra low noise of 0.1 Pa RMS, Temperature range of -40 to 85C datasheet approx. $4.12

Humidity and Temperature

Bosch

Bosch makes fully integrated airquality sensor that is commonly used for indor airquality assessment. But their CO2 is usually an electric estimate and the integrated circuits don't measure particulates.

  • BME688 Pressure 0.6hPa, Humidity $+/-3%$ Temperature $+/-0.5 \degree C$

High Accuracy Temperature and Humidity Sensors

Many sensors need temperature compensation and some require humidity compensation. The Humidity and Temperature sensors that are integrated into other sensor are not of highest accuracy. higher quality sensors are available:

  • SHT40, Humidity $+/- 1.8%$, Temperature $+/-0.2 \degree C$, approx. $2.5
  • SHT41, Humidity $+/- 1.8%$, Temperature $+/-0.2 \degree C$, approx. $3.4
  • SHT43, Humidity $+/- 1.8%$, Temperature $+/-0.2 \degree C$, approx. $5.1
  • SHT45, Humidity $+/- 1%$, Temperature $+/-0.1 \degree C$, approx. $6.4

NOx & VOC

A variety of MEMS sensors can measure electrical equivalents of atmospheric gases through electro chemical reactions where the sensor element is heated and the presence of a gas reduces its resistance. Usually there is cross sensitivity within one element with other gases.

A typical Sensirion MEMS gas sensor consumes 3mA to measure VOC and NOX. It takes 10 seconds to detect a change in VOC and about 4 mins for NOx. Sampling is typically once per second.

  • SGP41 sensor, VOC Index 1-500, NOX Index 1-500, $8.96 datasheet

Carbon Monoxide, Amonia, Nitrogenoxide

Its difficult to find accurate and inexpensive Carbon Monoxide sensors. It is possible to calibrate less expensive sensors by measuring known levels of gases in a test chamber. For example, there should be no Carbon Monoxide in the atmosphere and a sensor signal outdoors indicates the sensor background level.

Carbon Monpoxide is a poisenos gas where as Nitric Oxide and Methane should be avoided indoors.

Amphenol produces an multi element MEMS gas sensor:

  • Amphenol SGX Sensortech, MiCS-6814 MEMS sensor $13 datasheet

This sensor has 3 elements each being heated and consuming approximately 30mA. The sensor's resistance is measured and relates to chemical reduction (CO), oxidation (NOx), and NH3 concentration.

Nominal resistance for NH3 is 10-1500kOhm, for NO2 0.8 to 20kOhm and CO 100 to 1500kOhm. Factory calibration is needed to measure absolute values since the nominal resistance varies significantly for each sensor.

Sensor Nominal $R_{0_{min}} $ Nominal $R_{0_{max}} $ $C_{min}$ $C_{max}$ $R_S/R_0 min$ $R_S/R_0 max$ low concentration
CO 100k 1500k 1ppm 1000ppm 4 0.01 high resistance
NH3 10k 1500lk 1ppm 300ppm 0.08 30 low resistance
NOx 0.8k 10k 0.05ppm 10ppm 0.8 0.07 high resistance

The Amphenol sensor does not have an integrated analog to digital converted and the resistance will need to be measured using a resistor network such as a voltage divider. Its necessary to adjust a voltage divider for each sensor so that the maximum sensitivity is achieved with a microcontroller's internal ADC. It usually is more important to detect low quantities than to measure accurately high levels of the analyte. The ESP32 AD converter has an internal attenuator and the minimum accurate measurement is 0.1V and the maximum 2.4V. It requires linearization and averaging to obtain an accurate value.

The example heater circuit in the datasheet of this sensor is for a 5V supply design and puts the CO and NH3 heater in series. For 3.3V operation, the heaters need to be connected independently, each with its own serial current limiting resistor. Current and nominal resistance of the heater can be obtained from the datasheet and the required serial resistor is as following:

Gas $I_{heat}$ [mA] $R_{heat}$ [Ohm] $V$ [V] $R_{serial_{5V}}$ [Ohm] $R_{serial_{3.3V}}$ [Ohm]
CO 32 74 2.4 82 29
NOx 26 66 1.7 126 61
NH3 30 72 2.2 95 38

Neopixel

To indicate status we use RGB LEDs controlled with 800kpbs serial signal. These LEDs come in different form factors usually 5x5mm but 1x1 and 2x2 are available with similar performance. Since each color can draw up to 5mA the current for one element is 15mA if white color is selected. With 12 LEDs and all emitting white, this would result in 200mA current consumption on the supply line. The serial signal is 3.3V compliant and the LED should work with 3.3V supply also, although 5V is recommended. datasheet