diff --git a/src/MQUnifiedsensor.cpp b/src/MQUnifiedsensor.cpp
index 8bac1a5..f845095 100644
--- a/src/MQUnifiedsensor.cpp
+++ b/src/MQUnifiedsensor.cpp
@@ -139,7 +139,7 @@ int MQUnifiedsensor::readPPM(int m, int b) {
   double sensor_volt = this->getVoltage();
   double RS_gas; //Define variable for sensor resistance
 
-  RS_gas = ((5.0 * 10.0) / sensor_volt) - 10.0; //Get value of RS in a gas
+  RS_gas = ((VOLT_RESOLUTION*RLValue)/sensor_volt)-RLValue; //Get value of RS in a gas
 
   _ratio = RS_gas / this->_R0;   // Get ratio RS_gas/RS_air
 
@@ -147,7 +147,7 @@ int MQUnifiedsensor::readPPM(int m, int b) {
   double ppm = pow(10, ppm_log); //Convert ppm value to log scale
   return floor(ppm);
 }
-double MQUnifiedsensor::calibrate() {
+int MQUnifiedsensor::calibrate() {
   //More explained in: https://jayconsystems.com/blog/understanding-a-gas-sensor
   /*
   V = I x R 
diff --git a/src/MQUnifiedsensor.h b/src/MQUnifiedsensor.h
index 408b76f..2b6f853 100644
--- a/src/MQUnifiedsensor.h
+++ b/src/MQUnifiedsensor.h
@@ -12,7 +12,7 @@
 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;
+double RS_air, _m, _b, _ratio;
 
 /************************Hardware Related Macros************************************/
 
@@ -45,6 +45,20 @@ double RS_air,_R0, _m, _b, _ratio;
 #define RatioMQ303CleanAir 1  //RS / R0 = 1 ppm    
 #define RatioMQ309CleanAir 11 //RS / R0 = 11 ppm    
 
+//From the calibration program we can obtain the R0 value for each sensor
+#define R0_MQ2 1 //R0 after 48 hours pre-heating the sensor
+#define R0_MQ3 1   //R0 after 48 hours pre-heating the sensor
+#define R0_MQ4 1  //R0 after 48 hours pre-heating the sensor
+#define R0_MQ5 1  //R0 after 48 hours pre-heating the sensor
+#define R0_MQ6 1   //R0 after 48 hours pre-heating the sensor
+#define R0_MQ7 1 //R0 after 48 hours pre-heating the sensor
+#define R0_MQ8 1   //R0 after 48 hours pre-heating the sensor
+#define R0_MQ9 1  //R0 after 48 hours pre-heating the sensor
+#define R0_MQ131  1 //R0 after 48 hours pre-heating the sensor
+#define R0_MQ135  1//R0 after 48 hours pre-heating the sensor
+#define R0_MQ303  1  //R0 after 48 hours pre-heating the sensor
+#define R0_MQ309  1 //R0 after 48 hours pre-heating the sensor
+
 /***********************Software Related Macros************************************/
 
 #define ADC_RESOLUTION 10 // for 10bit analog to digital converter.
@@ -80,7 +94,7 @@ class MQUnifiedsensor
     int readSensor(String nameLectureRequeired = "", bool print = false);
     int readPPM(int m, int b);
     
-    double calibrate();
+    int calibrate();
     double getVoltage();
     double stringToDouble(String & str);
 
@@ -89,7 +103,7 @@ class MQUnifiedsensor
     
 
   private:
-    int _pin, _type; 
+    int _pin, _type, _R0; 
     String _MQ[19], _nameLectureRequeired;   
 };