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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.

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

Step 1. Convert voltage reading to sensor resistance: Rsensor = Rload / ( Vcc - Vout) * Vout

Step 2. Normalise against resistance under air: Rsensor / Ro(Typical Resistance in Air)

Step 3. Compare normalised result against sensor response diagram:

Sensor Response Characteristics

Normalised Resistance (Rs/Ro) vs VOC's (ppm)
CO (ppm) = 10 (0.09721 - log(Rs/Ro)) / 0.6483


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.


  • 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.


wasp_voc.txt · Last modified: 2021/01/10 23:16 (external edit)