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MQ135.cpp
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MQ135.cpp
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/**************************************************************************/
/*!
@file MQ135.cpp
@author G.Krocker (Mad Frog Labs)
@license GNU GPLv3
First version of an Arduino Library for the MQ135 gas sensor
TODO: Review the correction factor calculation. This currently relies on
the datasheet but the information there seems to be wrong.
@section HISTORY
v1.0 - First release
*/
/**************************************************************************/
#include "MQ135.h"
/**************************************************************************/
/*!
@brief Default constructor
@param[in] pin The analog input pin for the readout of the sensor
@param[in] rzero Calibration resistance at atmospheric CO2 level
@param[in] rload The load resistance on the board in kOhm
*/
/**************************************************************************/
MQ135::MQ135(uint8_t pin, float rzero, float rload) {
_pin = pin;
_rzero = rzero;
_rload = rload;
}
/**************************************************************************/
/*!
@brief Get the correction factor to correct for temperature and humidity
@param[in] t The ambient air temperature
@param[in] h The relative humidity
@return The calculated correction factor
*/
/**************************************************************************/
float MQ135::getCorrectionFactor(float t, float h) {
// Linearization of the temperature dependency curve under and above 20 degree C
// below 20degC: fact = a * t * t - b * t - (h - 33) * d
// above 20degC: fact = a * t + b * h + c
// this assumes a linear dependency on humidity
if(t < 20){
return CORA * t * t - CORB * t + CORC - (h-33.)*CORD;
} else {
return CORE * t + CORF * h + CORG;
}
}
/**************************************************************************/
/*!
@brief Get the resistance of the sensor, ie. the measurement value
Known issue: If the ADC resolution is not 10-bits, this will give
back garbage values!
@return The sensor resistance in kOhm
*/
/**************************************************************************/
float MQ135::getResistance() {
int val = analogRead(_pin);
return ((1023./(float)val) - 1.)*_rload;
}
/**************************************************************************/
/*!
@brief Get the resistance of the sensor, ie. the measurement value corrected
for temp/hum
@param[in] t The ambient air temperature
@param[in] h The relative humidity
@return The corrected sensor resistance kOhm
*/
/**************************************************************************/
float MQ135::getCorrectedResistance(float t, float h) {
return getResistance()/getCorrectionFactor(t, h);
}
/**************************************************************************/
/*!
@brief Get the ppm of CO2 sensed (assuming only CO2 in the air)
@return The ppm of CO2 in the air
*/
/**************************************************************************/
float MQ135::getPPM() {
return PARA * pow((getResistance()/_rzero), -PARB);
}
/**************************************************************************/
/*!
@brief Get the ppm of CO2 sensed (assuming only CO2 in the air), corrected
for temp/hum
@param[in] t The ambient air temperature
@param[in] h The relative humidity
@return The ppm of CO2 in the air
*/
/**************************************************************************/
float MQ135::getCorrectedPPM(float t, float h) {
return PARA * pow((getCorrectedResistance(t, h)/_rzero), -PARB);
}
/**************************************************************************/
/*!
@brief Get the resistance RZero of the sensor for calibration purposes
@return The sensor resistance RZero in kOhm
*/
/**************************************************************************/
float MQ135::getRZero() {
return getResistance() * pow((ATMOCO2/PARA), (1./PARB));
}
/**************************************************************************/
/*!
@brief Get the corrected resistance RZero of the sensor for calibration
purposes
@param[in] t The ambient air temperature
@param[in] h The relative humidity
@return The corrected sensor resistance RZero in kOhm
*/
/**************************************************************************/
float MQ135::getCorrectedRZero(float t, float h) {
return getCorrectedResistance(t, h) * pow((ATMOCO2/PARA), (1./PARB));
}