Fix Rs calculation for MQ303A

examples/MQ-303A/MQ303.ino

@@ -86,7 +86,7 @@ void setup() {
 
 void loop() {
   MQ303.update(); // Update data, the arduino will be read the voltage on the analog pin
-  MQ303.readSensor(); // Sensor will read PPM concentration using the model and a and b values setted before or in the setup
+  MQ303.readSensor(true); // Sensor will read PPM concentration using the model and a and b values setted before or in the setup
   MQ303.serialDebug(); // Will print the table on the serial port
   delay(500); //Sampling frequency
 }

src/MQUnifiedsensor.cpp

@@ -133,9 +133,12 @@ float MQUnifiedsensor::validateEcuation(float ratioInput)
   //Serial.println("Result: "); Serial.println(_PPM);
   return _PPM;  
 }
-float MQUnifiedsensor::readSensor()
+float MQUnifiedsensor::readSensor(bool isMQ303A)
 {
   //More explained in: https://jayconsystems.com/blog/understanding-a-gas-sensor
+  if(isMQ303A) {
+    _VOLT_RESOLUTION = _VOLT_RESOLUTION - 0.45; //Calculations for RS using mq303a sensor look wrong #42
+  }
   _RS_Calc = ((_VOLT_RESOLUTION*_RL)/_sensor_volt)-_RL; //Get value of RS in a gas
   if(_RS_Calc < 0)  _RS_Calc = 0; //No negative values accepted.
   _ratio = _RS_Calc / this->_R0;   // Get ratio RS_gas/RS_air

src/MQUnifiedsensor.h

@@ -31,7 +31,7 @@ class MQUnifiedsensor
     
     //user functions
     float calibrate(float ratioInCleanAir);
-    float readSensor();
+    float readSensor(bool isMQ303A = false);
     float readSensorR0Rs();
     float validateEcuation(float ratioInput = 0);