Using ammonia sensor – MQ137 to detect when a toilet needs cleaning?


  1. Standard detecting conditions required are 20°C ± 2°C and 65%± 5% relative humidity(RH) with 95%RH being the maximum.
    The relative humidity in toilets can very often go above 95% thus, affecting the readings.
  2. Detecting scope of an MQ137 sensor is 10 ppm to 300 ppm
    Our required functional data lies very close-to or below the lower limit of the sensor. (To give an idea of the scale of magnitudes: Ammonia at about 30 ppm can cause eye/nose irritation)
  3. Household and industrial cleaning solutions may contain ammonia.
    This can trigger false positives.
  4. An added power consumption of ~800 mW on the board due to the presence of heating coils that are required for the sensor to operate.
    We might need to alter the onboard power supply if it was designed for a specific current rating.
  5. Not selective to one particular gas, it can be affected by other gases like CO2, NOx, alcohol, smoke etc.
    Since the MQ135 measures gas concentration based on changes in conductivity, it can be affected by other gases like CO2 ( CO2 is denser than air and hence does not rise up easily – since sensor is placed at head level it is more prone to being negatively affected by CO2) or alcohol which is found in antiseptic solutions.
  6. The sensitivity of the sensors will be reduced when spattered or dipped in water.
    There is always a possibility of the sensor to come in contact with water (as it is exposed).
  7. Prolonged exposure to adverse conditions (like high humidity) will permanently affect the sensor’s performance.
    Hence they might need periodic calibration


The reliability of using the MQ135 sensor for a valid data point is questionable. There are too many factors at play in harsh environments like toilets.
Need to test it in the real world before I conclude anything.


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