Fixed MQ-131 Issue

examples/MQ-131/MQ-131.ino

@@ -17,6 +17,8 @@
   https://github.com/miguel5612/MQSensorsLib_Docs/blob/master/static/img/MQ_Arduino.PNG
   Please take care, arduino A0 pin represent the analog input configured on #define pin
 
+  Note: high concentration MQ-131 sensor.
+
  This example code is in the public domain.
 
 */
@@ -83,11 +85,12 @@ void setup() {
   if(calcR0 == 0){Serial.println("Warning: Conection issue founded, R0 is zero (Analog pin with short circuit to ground) please check your wiring and supply"); while(1);}
   /*****************************  MQ CAlibration ********************************************/ 
   MQ131.serialDebug(true);
+  Serial.println("Ignore Ratio = RS/R0, for this example we will use readSensorR0Rs, the ratio calculated will be R0/Rs. Thanks :)");
 }
 
 void loop() {
   MQ131.update(); // Update data, the arduino will be read the voltage on the analog pin
-  MQ131.readSensor(); // Sensor will read PPM concentration using the model and a and b values setted before or in the setup
+  MQ131.readSensorR0Rs(); // Sensor will read PPM concentration using the model and a and b values setted before or in the setup
   MQ131.serialDebug(); // Will print the table on the serial port
   delay(500); //Sampling frequency
 }

src/MQUnifiedsensor.cpp

@@ -143,6 +143,24 @@ float MQUnifiedsensor::readSensor()
   //if(_PPM > 10000) _PPM = 99999999; //No negative values accepted or upper datasheet recomendation.
   return _PPM;
 }
+float MQUnifiedsensor::readSensorR0Rs()
+{
+  //More explained in: https://jayconsystems.com/blog/understanding-a-gas-sensor
+  _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 = this->_R0/_RS_Calc;   // Get ratio RS_air/RS_gas <- INVERTED for MQ-131 issue 28 https://github.com/miguel5612/MQSensorsLib/issues/28
+  if(_ratio <= 0)  _ratio = 0; //No negative values accepted or upper datasheet recomendation.
+  if(_regressionMethod == 1) _PPM= _a*pow(_ratio, _b); // <- Source excel analisis https://github.com/miguel5612/MQSensorsLib_Docs/tree/master/Internal_design_documents
+  else 
+  {
+    // https://jayconsystems.com/blog/understanding-a-gas-sensor <- Source of linear ecuation
+    double ppm_log = (log10(_ratio)-_b)/_a; //Get ppm value in linear scale according to the the ratio value  
+    _PPM = pow(10, ppm_log); //Convert ppm value to log scale  
+  }
+  if(_PPM < 0)  _PPM = 0; //No negative values accepted or upper datasheet recomendation.
+  //if(_PPM > 10000) _PPM = 99999999; //No negative values accepted or upper datasheet recomendation.
+  return _PPM;
+}
 float MQUnifiedsensor::calibrate(float ratioInCleanAir) {
   //More explained in: https://jayconsystems.com/blog/understanding-a-gas-sensor
   /*

src/MQUnifiedsensor.h

@@ -30,6 +30,7 @@ class MQUnifiedsensor
     //user functions
     float calibrate(float ratioInCleanAir);
     float readSensor();
+    float readSensorR0Rs();
     float validateEcuation(float ratioInput = 0);
     
     //get function for info