possible error in the calculation formula for _RS_Calc = ((_VOLT_RESOLUTION*_RL)/_sensor_volt)-_RL;

[berryPiTech]

I think there is an error in the library source code (https://github.com/miguel5612/MQSensorsLib/blob/master/src/MQUnifiedsensor.cpp):

The variable _VOLT_RESOLUTION in the equation below should be 5V, not _VOLT_RESOLUTION, which is 3.3volts in case of esp32.
_RS_Calc = ((_VOLT_RESOLUTION*_RL)/_sensor_volt)-_RL; //Get value of RS in a gas

The above equation (line 148) is supposed to be the implementation of this equation in this link (https://www.jaycon.com/understanding-a-gas-sensor/).

RS = [(VC x RL) / VRL] – RL

The variables are in figure 6 of the link above.

Specifically Vcc=5V is the vcc to drive the heater of the sensor. It can't be 3.3volts, which is too low to drive the heater.

The correct formula should be:
_RS_Calc = ((_VCC*_RL)/_sensor_volt)-_RL;

where VCC = 5volts.