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Add rzero in constructor, correct atmospheric co2 level for 2019 #9

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1225a05
Ability to define RZERO value.
BukinPK Sep 21, 2019
7a88fbd
add rzero in constructor, correct atmospheric co2 level for 2019
BukinPK Sep 21, 2019
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22 changes: 12 additions & 10 deletions MQ135.cpp
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Original file line number Diff line number Diff line change
Expand Up @@ -11,6 +11,8 @@ the datasheet but the information there seems to be wrong.
@section HISTORY

v1.0 - First release
v1.1 - Add rzero param in constructor,
correcting atmospheric CO2 level for 2019 year
*/
/**************************************************************************/

Expand All @@ -20,20 +22,20 @@ v1.0 - First release
/*!
@brief Default constructor

@param[in] pin The analog input pin for the readout of the sensor
@param[in] pin The analog input pin for the readout of the sensor
@param[in] rzero Calibration resistance at atmospheric CO2 level
*/
/**************************************************************************/

MQ135::MQ135(uint8_t pin) {
MQ135::MQ135(uint8_t pin, float rzero) {
_rzero = rzero;
_pin = pin;
}


/**************************************************************************/
/*!
@brief Get the correction factor to correct for temperature and humidity

@param[in] t The ambient air temperature
@param[in] t The ambient air temperature in Celsius
@param[in] h The relative humidity

@return The calculated correction factor
Expand All @@ -60,7 +62,7 @@ float MQ135::getResistance() {
@brief Get the resistance of the sensor, ie. the measurement value corrected
for temp/hum

@param[in] t The ambient air temperature
@param[in] t The ambient air temperature in Celsius
@param[in] h The relative humidity

@return The corrected sensor resistance kOhm
Expand All @@ -78,22 +80,22 @@ float MQ135::getCorrectedResistance(float t, float h) {
*/
/**************************************************************************/
float MQ135::getPPM() {
return PARA * pow((getResistance()/RZERO), -PARB);
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] t The ambient air temperature in Celsius
@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);
return PARA * pow((getCorrectedResistance(t, h)/_rzero), -PARB);
}

/**************************************************************************/
Expand All @@ -112,7 +114,7 @@ float MQ135::getRZero() {
@brief Get the corrected resistance RZero of the sensor for calibration
purposes

@param[in] t The ambient air temperature
@param[in] t The ambient air temperature in Celsius
@param[in] h The relative humidity

@return The corrected sensor resistance RZero in kOhm
Expand Down
11 changes: 7 additions & 4 deletions MQ135.h
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Original file line number Diff line number Diff line change
Expand Up @@ -11,6 +11,8 @@ the datasheet but the information there seems to be wrong.
@section HISTORY

v1.0 - First release
v1.1 - Add rzero param in constructor,
correcting atmospheric CO2 level for 2019 year
*/
/**************************************************************************/
#ifndef MQ135_H
Expand All @@ -23,8 +25,7 @@ v1.0 - First release

/// The load resistance on the board
#define RLOAD 10.0
/// Calibration resistance at atmospheric CO2 level
#define RZERO 76.63

/// Parameters for calculating ppm of CO2 from sensor resistance
#define PARA 116.6020682
#define PARB 2.769034857
Expand All @@ -36,14 +37,16 @@ v1.0 - First release
#define CORD 0.0018

/// Atmospheric CO2 level for calibration purposes
#define ATMOCO2 397.13
/// On august 2019 from https://www.co2.earth
#define ATMOCO2 409.95

class MQ135 {
private:
uint8_t _pin;
float _rzero;

public:
MQ135(uint8_t pin);
MQ135(uint8_t pin, float rzero=76.63);
float getCorrectionFactor(float t, float h);
float getResistance();
float getCorrectedResistance(float t, float h);
Expand Down