Gas Sensors MQ Series Principle

[natong]

The virgin sensor require to run for 24 hours (the 1st time initialization).
The heater require 5V 800mW (160mA) which is a bit hot while working.

Updated: The MQ-2 sensor resistance (R_S) will vary between 3 - 30 kohm while measuring gas. I don't known what it mean. Normally when measuring the clean air, the MQ-2 sensor resistance (R_S) just only 10 kohm and reduce when found the gases. Look at the graph (post #4), if R_O is fixed, the R_S should be lowest to 0.3 kohm. The prefered range should be 0.3 to 10 kohm.

The MQ-2 sensor resistance will working as the Voltage Dividers.

[natong]

R_S while measuring gas can find from V_OUT and the fixed R_L.
The best R_L value for the entire sensitivity range can find from the equation below:

[natong]

Updated: If we choose R_{S (min)} = 0.5 kohm and R_{S (max)} = 10 kohm, the best R_L will be 2.2 kohm.

These are the R_L calculation from the R_S max & min range:

R_{S (min)} = 0.5 kohm
R_{S (max)} = 10 kohm

The best R_L value should be 2.2 kohm which give the analog V_OUT 649 scales/spreads. It is the highest resolution.

[natong]

(Reserved)

[natong]

To calibration, we need to find the R_O value while measuring the clean air.
Since we known the ratio of the clean air from the graph is 9.83, then we can calculate R_O from the equation below:

Now we can find the concentrations (ppm) of any gas from the graph from R_S/R_O ratio.

[natong]

The FC-22 Gas Sensor Module use R_L = 1 kohm.
These module will give 589 spreads.
The module also power down the heater by connected the resistor 5.1 ohm.

PS: Some modules use R_L = 5 kohm which give 589 spreads too.

[natong]

const int MQ_PIN = A5;
const float RL = 1.0;    // kohm (FC-22 module = 1.0 kohm)
const float CLEAN_AIR_RATIO = 9.83;    // Taken from datasheet graph
float RS;    // kohm   
float RO;    // kohm
int ADC;    // 0 to 1023
  
ADC = analogRead(MQ_PIN);
RS = ( (float) RL * (1023 - ADC) / ADC );

RO = RS / CLEAN_AIR_RATIO;    // Do only once while measuring clean air.

[Sijuro]

Hello natong,

The Rs/Ro ratio of this kind of sensors are measured in a static value of RL, so i thought that Rs/Ro ratio change if you change RL. For example, i'm using MQ-7, and in datasheet show that the Rs/Ro ratio is for a RL value of 10k, and in my case, i'm using FC-22 too and i'm a little confused.

Kind regards.