=====  Waspmote Nitrogen Dioxide Sensor  =====

====  Quick Data  ====

  * Model: e2v MiCS-2710
  * Measurement range: 0.05 ~ 5ppm
  * Air resistance: 0.8 ~ 8kΩ (typically 2.2kΩ)
  * Sensitivity: 6 ~ 100 (typically 55, ratio between the resistance at 0.25ppm and in air)
  * Load resistor: 2kΩ, Gain: 1, Supply voltage: 1.8V DC
  * Some approximate Nitrogen Dioxide ppm levels for reference (US EPA Guidelines)
    * Urban Areas: 0.010-0.100 ppm
    * Air quality standard: 0.053ppm
    * Public Warning (Visible Urban Haze): 1.2ppm
    * Significant Harm Level: 2.0ppm

====  Conversion Method  ====

**Converting to Concentration 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

  - Convert voltage reading to sensor resistance: **R<sub>sensor</sub> = R<sub>load</sub> / ( V<sub>cc</sub> - V<sub>out</sub>) * V<sub>out</sub>**
  - Normalise against resistance under air: **R<sub>sensor</sub> / R<sub>o</sub>(Typical Resistance in Air)**
  - Compare normalised result against sensor response diagram:

====  Sensor Response Characteristics  ====

^ Normalised Resistance (Rs/Ro) vs NO2 (ppm) ^
| {{:technical:sensors:internal:no2_response.png|}} |
| NO<sub>2</sub> (ppm) = 10<sup>(log(Rs/Ro) - 2.702) / 1.865</sup> |

====  Example  ====

Measurement of 1.277v is observed (from our databases)

  * Rs = 2000 / (1.8-1.277) * 1.8
  * Rs = 6683
  * Rs / Ro = 6683 / 2200
  * Normalised resistance = 3.12. Compare 3.12 to the sensor response graph
  * This reading lies below the logarithmic line, but using the extrapolation data in the links below this is approximately 0.05ppm, which seems correct (higher than average) given this was taken from a sensor near the basement carpark.

====  Notes  ====

  * A standard Rs/Ro reading will fall outside the logarithmic line of the sensor response, since the lower limit is at a ppm of 0.2 which is above air quality standards. See the links below for an extrapolation for smaller readings.
  * The affect of wiper resistance in the circuit is not accounted for, so RL could be slightly higher, affecting the overall calculation.

====  Downloads  ====

  * {{:technical:sensors:internal:mics-2710.pdf|Manufacturer Datasheet}}
  * {{:technical:sensors:internal:gases_sensor_board_2.0.pdf|Libellium Gases Guide v4.8 (See Page 28 for MiCS-2710)}}
  * {{:technical:sensors:internal:mics.pdf|Logarithmic extrapolation of sensor response for smaller readings}}
  
====  Links  ====

  * [[https://www.libelium.com/api/waspmote/de/dc9/classWaspSensorGas__v20.html#a6adecc3646770842238aa97792bffc66|Libellium API containing resistance calculation function with approximate wiper resistance.]]