=====  Waspmote Liquid Petroleum Gases Sensor  =====

====  Quick Data  ====

  * Model: Figaro TGS2610
  * Gases: Primarily Isobutane. Also sensitive to Methane, Ethanol, Hydrogen
  * Measurement range: 500 ~ 10000ppm
  * Resistance at 1800ppm (isobutane): 0.68 ~ 6.8kΩ
  * Sensitivity: 0.56 ± 0.06 (ratio between the resistance at 3000 and at 1000ppm)
  * Load resistor: 10kΩ, Gain: 1, Supply voltage: 5V DC
  * Some approximate gas ppm levels for reference
    * Iso-butane/Propane
      * Lower explosive limit (propane): 21000ppm
      * Higher explosive limit (propane): 104000ppm
      * Dizziness to asphyxiation: >100000 ppm (non-toxic - purely due to oxygen displacement)
    * Methane
      * Lower explosive limit: 50000ppm
      * Higher explosive limit: 150000ppm
      * Dizziness to asphyxiation: >100000 ppm (non-toxic - purely due to oxygen displacement)
    * Ethanol
      * Lower explosive limit: 33000ppm
      * Higher explosive limit: 190000ppm
      * Adverse Effects: 1000 ppm
    * Hydrogen
      * Lower explosive limit: 40000ppm
      * Higher explosive limit: 750000ppm
      * Adverse Effects: 10 ppm
      * Life Threatening: >150 ppm

====  Conversion Method  ====

**Converting to Concentration of Liquid Petroleum 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>s</sub> = ( ( V<sub>cc</sub> - V<sub>out</sub> ) / V<sub>out</sub> ) * R<sub>l</sub>**

Step 2. Determine sensor resistance at 1800ppm (isobutane) R<sub>o</sub>. If there is insufficient data at this stage to do this empirically, begin approximation by using the lower extreme of the expected sensitivity provided in the datasheet ~ 0.68kΩ

Step 3. Normalise against resistance at 1800ppm (isobutane): **R<sub>s</sub> / R<sub>o</sub>(Typical Resistance at 1800ppm (isobutane)**

Step 4. Compare normalised result against sensor response diagram:

====  Sensor Response Characteristics  ====
^ Normalised Resistance (Rs/Ro) vs LPG's (ppm) ^
| {{:technical:sensors:internal:lpg_response.png|}} |
| Hydrogen (H) (ppm) = 10 <sup>(1.825 - log(Rs/Ro)) / 0.4820</sup> |
| Ethanol (C2H6O) (ppm) = 10 <sup>(2.041 - log(Rs/Ro)) / 0.5030</sup> |
| Methane (CH4) (ppm) = 10 <sup>(1.640 - log(Rs/Ro)) / 0.4807</sup> |
| Iso-Butane (C3H8) (ppm) = 10 <sup>(1.206 - log(Rs/Ro)) / 0.3855</sup> |

====  Example  ====

Measurement of 3.3v is observed (from our databases).

  * Rs = ( (5 - 3.3) / 3.3) * 10000
  * Rs = 5152
  * Normalised = Rs/Ro = 5152/680 = 7.58
  * Normalised resistance = 7.58. This does not intersect with any gases and lies near the clean air value of 10, as would be expected.

====  Notes  ====

  * A standard Rs/Ro should read between 6 and 10, since this sensor only detects large and dangerous concentrations.
  * 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.

====  Downloads  ====

  * {{:technical:sensors:internal:2610pdf.pdf|Manufacturer Datasheet}}
  * {{:technical:sensors:internal:gases_sensor_board_2.0.pdf|Libellium Gases Guide v4.8 (See Page 33 for TGS-2610)}}