=====  Waspmote Air Contaminants Sensor  =====

====  Quick Data  ====

  * Model: Figaro TGS 2602
  * Gases: Primarily: Toluene, Hydrogen Sulphide, Ethanol, Ammonia. Also sensitive to Hydrogen
  * Measurement range: 1 ~ 30ppm
  * Air resistance: 10 ~ 100kΩ
  * Sensitivity: 0.15 ~ 0.5 (ratio between the resistance in 10ppm of Ethanol and in air)
  * Load resistor: 10kΩ, Gain: 1, Supply voltage: 5V DC
  * Some approximate gas ppm levels for reference
    * Toluene
      * Typical City: 0.075 ppm
      * Adverse Effects: 500 ppm
      * Life Threatening: >2000 ppm
    * Hydrogen Sulfide
      * Odour threshold: 0.00047 ppm
      * Adverse Effects: 50 ppm
      * Life Threatening: >500 ppm
    * Ethanol
      * Typical Indoor: <0.5 ppm
      * Exposure while refuelling car: 50ppm
      * Adverse Effects: 1000 ppm
    * Ammonia
      * Typical Atmosphere: 0.006 ppm
      * Adverse Effects: 24 ppm
      * Life Threatening: 2500 ppm
    * Hydrogen
      * Typical Atmosphere: 1 ppm
      * Adverse Effects: 10 ppm
      * Life Threatening: >150 ppm

====  Conversion Method  ====

**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.

{{:technical:sensors:internal:sensor_resistance_circuit.png|}}

To convert to ppm, the voltage reading is applied as follows

Step 1. Convert voltage reading to sensor resistance: **R<sub>sensor</sub> = ( ( V<sub>cc</sub> * R<sub>load</sub> ) / V<sub>out</sub> ) - R<sub>load</sub>**

Step 2. Determine sensor resistance in air R<sub>o</sub>. For an assumed "clean-air" value simply use the minima value that sensor has output over its lifetime at 20�C and 65% R.H.

Step 3. Normalise against resistance under air: **R<sub>sensor</sub> / R<sub>o</sub>(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:

====  Sensor Response Characteristics  ====

^ Normalised Resistance (Rs/Ro) to Pollutants (PPM) ^ Temperature/Humidity Dependancy ^
| {{:technical:sensors:internal:ap2_response.png|}} | {{:technical:sensors:internal:ap2_dependancy.png|}} |
| Hydrogen (H) (ppm) = 10 <sup>(-0.04887 - log(Rs/Ro)) / 0.08731</sup> | 20°C @ 65%RH: y = 1.9335 * (x %%^%% -0.2511) |
| Ammonia (NH3) (ppm) = 10 <sup>(-0.07142 - log(Rs/Ro)) / 0.2612</sup> | 20°C @ 95%RH: y = 1.2183 * (x %%^%% -0.4226) |
| Ethanol (C2H6O) (ppm) = 10 <sup>(-0.1040 - log(Rs/Ro)) / 0.4232</sup> | 20°C @ 35%RH: y = 6.5753 * (x %%^%% -0.5398) |
| Hydrogen Sulphide (H2S) (ppm) = 10 <sup>(-0.5412 - log(Rs/Ro)) / 0.6423</sup> | |
| Toluene (C7H8) (ppm) = 10 <sup>(-0.5158 - log(Rs/Ro)) / 0.5681</sup> | |

====  Example  ====

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)

  * Rs = ((10000 * 5) / 1.8529) - 10000
  * Rs = 16985
  * Typical resistance in Air varies greatly sensor to sensor, we use our minima reading of 1.6v from that particular sensor as baseline "clean-air" resistance.
  * Ro = ((10000 * 5) / 1.6) - 10000
  * Ro = 21250
  * Normalised Resistance Rs/Ro = 16985/21250 = 0.799
  * As we can see on the temp/humidity dependency graph, 17°C and 85% R.H gives us a Rs/Ro modifier of 1.1x. New Rs/Ro = 1.1 * 0.799 = 0.8792
  * Modified Normalised resistance = 0.8792. On the response graph we can see that value intersects a Hydrogen component around 1ppm which matches a baseline reading with no notable traces of other pollutants (since their logarithmic lines do not pass through that Y-component) .

====  Notes  ====

  * A standard Rs/Ro reading should fall between ~0.8 and 1 to indicate clean air. A lower reading indicates concentrations of potentially dangerous gases that need further measurement.
  * If your Rs/Ro value intersects multiple gases on the response graph, it is impossible to determine which gas or gases the sensor is responding too without extra equipment.
  * Baseline typical air resistance should be taken in ideal conditions of 20°C and 65% R.H to avoid skew from the sensor dependancy on temperature and humidity.
  * Typical resistance in air varies widely between sensors making calibration of the sensor necessary for final application.

====  Downloads  ====

  * {{:technical:sensors:internal:2602pdf.pdf|Manufacturer Datasheet}}
  * {{:technical:sensors:internal:gases_sensor_board_2.0.pdf|Libellium Gases Guide v4.8 (See Page 36 for TGS-2602)}}