Customer:
I have 3 MQ137 sensors. We’re connecting them with 5V supply on the assigned pins and taking output from pin A0. We’ve measured that the load resistance on the PCB itself is 10K. We’re converting the A0 output into ppm by calculating Rs and dividing by R0 (Clean Air Rs).
The problem is inconsistent readings. Sometimes we see a linear increase when we supply 30 and 40 ppm to the sensor, but sometimes when it’s in a room it gives an inverse relationship with ppm. We need guidance on proper connection and calibration.
Originally submitted via email support on 21 October 2025. Migrated to the forum for community visibility.
Soldered Support:
The MQ series sensors have cross-sensitivity to other gases. Our library measures gas in ppb/ppm based on mathematical linear regression, which is a rough approximation inherited from the library we based our own on.
See the documentation at: MQ Gas Sensors – Measuring gas | Soldered Documentation
For better accuracy, use another device to measure the gas while taking raw analogRead values, then create a calibration table (for example: your measurement reads 10 ppm and analogRead gives 100, your measurement reads 15 ppm and analogRead gives 200, etc.). From this data you can derive a mathematical function that maps to true ppm values more accurately.
Customer:
Does the PCB include load resistance that I should measure the voltage on, or do I need to connect another load resistor?