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=====  Waspmote Carbon Dioxide Sensor  =====

====  Quick Data  ====

  * Model: Figaro TGS4161
  * Measurement range: 350 ~ 10000ppm
  * Voltage at 350ppm: 220 ~ 490mV
  * Sensitivity: 44 ~ 72mV (variation between the voltage at 350ppm and at 3500ppm)
  * Gain: 7
  * Some approximate Carbon Dioxide ppm levels for reference
    * Atmosphere/Outdoor concentration: 380-450ppm (250ppm Pre-Industrial Revolution)
    * Indoor concentration: 400-1000ppm
    * Industrial workplace safety standard (Max 8 hours/day): 5000ppm
    * Serious health effects: >30,000 ppm
    * Human exhale: 35,000 to 50,000 ppm

====  Conversion Method  ====

**Converting to Concentration in Air (parts per million)**

The output of the waspmote to the database is a voltage reading with a gain of 7 applied. As CO2 PPM increases, the voltage output drops

Step 1. Determine the baseline voltage reading for 350ppm (EMF1). The datasheet suggests this range lies within 220-490mV but based on real-world observations this range can be much larger. One approximation method of baseline would be to average the upper quartile of readings and assume that is 350ppm - which is a reading of outstanding ventilation for an office building.

Step 2. Take your database reading (EMF2), it must be less than the baseline reading

Step 3. Divide both readings by the gain applied on the sensor, i.e. 7x

Step 4: Apply the equation **ΔEMF=(EMF1/GAIN)-(EMF2/GAIN)**. Multiply by 1000 for value in mV. Result must be positive and between 0-90mV. If not the baseline EMF1 is incorrect.

Step 6: Compare delta EMF to the sensor response diagram below

====  Sensor Response Characteristics  ====

^ Delta EMF (mV) to CO2 Concentration (PPM) ^
|{{:technical:sensors:internal:co2_response.png|}}|
| CO<sub>2</sub> (ppm) = 10 <sup>(ΔEMF+163.4)/64.25</sup> |

====  Example  ====

Measurement of 0.77097v is observed (from our databases in a heavily occupied classroom). An upper quartile average of 0.98616 is averaged over the previous few weeks.

  * ΔEMF = (0.98616 / 7)-(0.77097 / 7)
  * ΔEMF = 0.14088 - 0.110138 = 0.03074
  * ΔEMF = 30.74mV
  * Reading the sensor characteristics this approximates to ~1000ppm which is an upper limit expectation of a poorly ventilated classroom.

====  Notes  ====

  * Sensor is also sensitive to CO, Ethanol and H2, but sensitivity is too slight for meaningful readings of these gases without excellent calibration.
  * Sensitivty characteristics are slightly affected by humidity. Since the affliction would not modify result larger than the inherent margin of error we have not described this process in the conversion method. Refer to datasheet for Humidity Dependancy.

====  Downloads  ====

  * {{:technical:sensors:internal:tgs4161.pdf|Manufacturer Datasheet}}
  * {{:technical:sensors:internal:tgs4161dtl.pdf|Technical Information}}
  * {{:technical:sensors:internal:gases_sensor_board_2.0.pdf|Libelium Gases Guide v4.8 (See Page 25 for TGS4161)}}
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