How to turn numbers on a screen into meaningful information.
A voltage is produced in the metal in the UVI-01 sensor when it is exposed to ultra violet radiation, by the photoelectric effect. As shown in the graph below, the greater the intensity of the radiation, the higher the voltage (it also depends on the resistance of the resistor used in the sensor's application circuit, shown by the multicoloured lines).
Nitrogen dioxide concentrations are a good measure of urban air pollution; NO2 plays a major role in atmospheric reactions that produce ground-level ozone, a major component of smog.[Clean Air Partnership]
It can be harmful because it can transform in the air to form gaseous nitric acid and toxic organic nitrates[CAP]- concentrations above 20ppm are classified as immediately dangerous to life or health and it can damage the lungs, cardiovascular system, respiratory tract and eyes[Airgas].
We tested the MICS-2710 sensor's response to NO2 concentration ourselves:
Carbon monoxide is another major urban air pollutant, and also part of a series of reactions producing petrochemical smog. It's mainly produced by incompletely combustion of fossil fuels, and concentrations can rise to lethal levels in closed environments. CO poisoning is the most common type of fatal air poisoning in many countries.
|35 ppm||Headache and dizziness within six to eight hours of constant exposure|
|100 ppm (0.01%)||Slight headache in two to three hours. Starts becoming dangerous to human health.|
|200 ppm||Slight headache within two to three hours; loss of judgment|
|400 ppm||Frontal headache within one to two hours|
|800 ppm||Dizziness, nausea, and convulsions within 45 min; insensible within 2 hours|
|1,600 ppm||Headache, tachycardia, dizziness, and nausea within 20 min; death in less than 2 hours|
|3,200 ppm||Headache, dizziness and nausea in five to ten minutes. Death within 30 minutes.|
|6,400 ppm||Headache and dizziness in one to two minutes. Convulsions, respiratory arrest, and death in less than 20 minutes.|
|12,800 ppm||Unconsciousness after 2-3 breaths. Death in less than three minutes.|
Our code takes the resistances of your gas sensors and turns them into gas concentrations according to our measurements, but if you'd like to have a go calibrating your AirPi's sensors yourself, here are the ranges we worked out.