Fix MQ303A voltage offset bug

.github/pull_request_template.md

@@ -12,6 +12,8 @@ Please delete options that are not relevant.
 - [ ] New feature (non-breaking change which adds functionality)
 - [ ] Breaking change (fix or feature that would cause existing functionality to not work as expected)
 - [ ] This change requires a documentation update
+- [ ] I have updated the version in `library.properties` to reflect my changes (advance by 0.01 for minor changes, or a whole number for new features)
+- [ ] I have submitted the pull request
 
 # How Has This Been Tested?
 

CONTRIBUTING.md

@@ -20,3 +20,7 @@ Pull request will be check by three main authors who are linked in
 [REAMDE.md](https://github.com/miguel5612/MQSensorsLib/README.md) and they determine to merge to
 master branch. Later than assessment and debbuging procedures was completed, the contribution 
 will be released.
+
+## Versioning
+
+When making a pull request, please remember to update the `library.properties` file to reflect the changes you've made. If your change is minor, consider advancing the version by 0.01. If you're adding a new feature, you may want to advance the version by a whole number.

src/MQUnifiedsensor.cpp

@@ -142,10 +142,14 @@ float MQUnifiedsensor::validateEcuation(float ratioInput)
 float MQUnifiedsensor::readSensor(bool isMQ303A, float correctionFactor, bool injected)
 {
   //More explained in: https://jayconsystems.com/blog/understanding-a-gas-sensor
+  float voltageResolution = _VOLT_RESOLUTION;
   if(isMQ303A) {
-    _VOLT_RESOLUTION = _VOLT_RESOLUTION - 0.45; //Calculations for RS using mq303a sensor look wrong #42
+    // Issue #42: MQ303A requires a 0.45V offset for the RS calculation. The
+    // previous implementation permanently decreased _VOLT_RESOLUTION every time
+    // the sensor was read which produced incorrect results after multiple calls.
+    voltageResolution -= 0.45;
   }
-  _RS_Calc = ((_VOLT_RESOLUTION*_RL)/_sensor_volt)-_RL; //Get value of RS in a gas
+  _RS_Calc = ((voltageResolution*_RL)/_sensor_volt)-_RL; //Get value of RS in a gas
   if(_RS_Calc < 0)  _RS_Calc = 0; //No negative values accepted.
   if(!injected) _ratio = _RS_Calc / this->_R0;   // Get ratio RS_gas/RS_air
   _ratio += correctionFactor;