=====  Waspmote VOC's Sensor  =====

====  Quick Data  ====

  * Model: e2v MiCS-5521
  * Gases: CO, Hydrocarbons, Volatile Organic Compounds (full list in downloads section)
  * Measurement range: 30 ~ 400ppm
  * Air resistance: 100 ~ 1000kΩ (typical 100 kΩ in air at 20°C / 50% RH )
  * Sensitivity: 1.8 ~ 6 (typically 3, ratio between the resistance at 60ppm and at 200ppm of CO)
  * Load resistor: 100kΩ, Gain: 1, Supply voltage: 2.5V DC
  * Some approximate Carbon Monoxide ppm levels for reference
    * Natural Concentration: 0.2ppm
    * Australian Air Quality Standards: 9.0ppm
    * Life Threatening: 400ppm
    * Undiluted car exhaust: 7000ppm

====  Conversion Method  ====

**Converting to Concentration of CO 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> = R<sub>load</sub> / ( V<sub>cc</sub> - V<sub>out</sub>) * V<sub>out</sub>**

Step 2. Normalise against resistance under air: **R<sub>sensor</sub> / R<sub>o</sub>(Typical Resistance in Air)**

Step 3. Compare normalised result against sensor response diagram:

====  Sensor Response Characteristics  ====

^ Normalised Resistance (Rs/Ro) vs VOC's (ppm) ^
| {{:technical:sensors:internal:voc_response.png|}} |
| CO (ppm) = 10 <sup>(0.09721 - log(Rs/Ro)) / 0.6483</sup> |

====  Example  ====

Measurement of 0.9258v is observed (from our databases)

  * Rs = 100000 / (2.5-0.9258) * 0.9258
  * Rs = 58811
  * Rs / Ro = 58811 / 100000
  * Normalised resistance = 0.59. This result lies well under 10ppm minimum on the graph, so the CO concentrations are safe.

====  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 30ppm which is above air quality standards.
  * 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.
  * Typical resistance in air changes between sensors, and required calibration for mroe exact measurements.

====  Downloads  ====

  * {{:technical:sensors:internal:e2v-mics-5521-datasheet.pdf|Manufacturer Datasheet}}
  * {{:technical:sensors:internal:voc-sensors.pdf|}}
  * {{:technical:sensors:internal:gases_sensor_board_2.0.pdf|Libellium Gases Guide v4.8 (See Page 41 for MiCS-5521)}}
  * {{:technical:sensors:internal:an4-using-mics-sensors-for-alcohol-detection1.pdf|Application of MiCS-5521 as a breathalyser}}