fixed calibration

src/MQUnifiedsensor.cpp

@@ -136,12 +136,25 @@ int MQUnifiedsensor::readPPM(int m, int b) {
   return floor(ppm);
 }
 double MQUnifiedsensor::calibrate() {
+  //More explained in: https://jayconsystems.com/blog/understanding-a-gas-sensor
+  /*
+  V = I x R 
+  VRL = [VC / (RS + RL)] x RL 
+  VRL = (VC x RL) / (RS + RL) 
+  Así que ahora resolvemos para RS: 
+  VRL x (RS + RL) = VC x RL
+  (VRL x RS) + (VRL x RL) = VC x RL 
+  (VRL x RS) = (VC x RL) - (VRL x RL)
+  RS = [(VC x RL) - (VRL x RL)] / VRL
+  RS = [(VC x RL) / VRL] - RL
+  */
   float sensor_volt; //Define variable for sensor voltage
   float RS_air; //Define variable for sensor resistance
   float R0; //Define variable for R0
   float sensorValue; //Define variable for analog readings
   sensor_volt = this->getVoltage(); //Convert average to voltage
-    R0 = sensor_volt / _ratioInCleanAir; //Calculate R0
+  RS_air = ((VOLT_RESOLUTION*RLValue)/sensor_volt)-RLValue; //Calculate RS in fresh air 
+  R0 = RS_air/_ratioInCleanAir; //Calculate R0 
   return R0;
 }
 double MQUnifiedsensor::getVoltage() {

src/MQUnifiedsensor.h

@@ -4,9 +4,16 @@
 #include <Arduino.h>
 #include <stdint.h>
 
+
 //Count of posible lectures
 #define lecturesAvailable 19
 
+/************************Global vars************************************/
+int VOLT_RESOLUTION  = 5.0; // if 3.3v use 3.3
+int RLValue = 10; //Value in KiloOhms
+int  _ratioInCleanAir, _PPM;
+double RS_air,_R0, _m, _b, _ratio;
+
 /************************Hardware Related Macros************************************/
 
 //Index in the nameLecture vector
@@ -82,10 +89,8 @@ class MQUnifiedsensor
     
 
   private:
-    int _pin, _type, _PPM, _lecturePosInArray, _ratioInCleanAir;
-    double RS_air,_R0 = 11.820, _m, _b, _ratio;
+    int _pin, _type; 
     String _MQ[19], _nameLectureRequeired;   
-    int VOLT_RESOLUTION  = 5.0; // if 3.3v use 3.3
 };
 
 #endif //MQUnifiedsensor_H