Converting to Concentration of Pollutant Gases in Air (parts per million)
The output of the waspmote to the database is a voltage reading, as can be seen from the circuit diagram below, this reading is affected by the input voltage, sensor resistance and load resistance.
To convert to ppm, the voltage reading is applied as follows
Step 1. Convert voltage reading to sensor resistance: Rsensor = ( ( Vcc * Rload ) / Vout ) - Rload
Step 2. Determine sensor resistance in air Ro. For an assumed “clean-air” value simply use the minima value that sensor has output over its lifetime at 20ï¿½C and 65% R.H.
Step 3. Normalise against resistance under air: Rsensor / Ro(Typical Resistance in Air)
Step 4. Adjust Rs/Ro for temperature/humidity sensitivity. Multiply by the result given in the dependency graph for the conditions at time of voltage reading.
Step 4. Compare adjusted normalised result against sensor response diagram:
|Normalised Resistance (Rs/Ro) to Pollutants (PPM)||Temperature/Humidity Dependancy|
|Hydrogen (H) (ppm) = 10 (-0.04887 - log(Rs/Ro)) / 0.08731||20°C @ 65%RH: y = 1.9335 * (x ^ -0.2511)|
|Ammonia (NH3) (ppm) = 10 (-0.07142 - log(Rs/Ro)) / 0.2612||20°C @ 95%RH: y = 1.2183 * (x ^ -0.4226)|
|Ethanol (C2H6O) (ppm) = 10 (-0.1040 - log(Rs/Ro)) / 0.4232||20°C @ 35%RH: y = 6.5753 * (x ^ -0.5398)|
|Hydrogen Sulphide (H2S) (ppm) = 10 (-0.5412 - log(Rs/Ro)) / 0.6423|
|Toluene (C7H8) (ppm) = 10 (-0.5158 - log(Rs/Ro)) / 0.5681|
Measurement of 1.8529v is observed (from our databases) with corresponding conditions of 17°C and 85% R.H. A minima for that sensor was observed a few months prior at 1.6 volts in ideal test conditions (20°C and 65% R.H)