air_density.cpp

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#include "air_density.h"
 
#include "math.h"
 
namespace sensesp {
 
// dew point
 
AirDensity::AirDensity() : FloatTransform() {}
 
void AirDensity::set_input(float input, uint8_t inputChannel) {
  inputs[inputChannel] = input;
  received |= 1 << inputChannel;
  if (received ==
      0b111) {  // for 2 channels, use 0b11. For 3 channels, use b111 and so on.
    received = 0;  // recalculates after all values are updated. Remove if a
                   // recalculation is required after an update of any value.
 
    // For more info on the calculation see
    // https://en.wikipedia.org/wiki/Density_of_air
 
    float temp_kelvin = inputs[0];
    float temp_celsius = temp_kelvin - 273.15;
    float relative_humidity = inputs[1];
    float pressure = inputs[3];
 
    // Saturation vapor pressure of water
    float saturation_pressure =
        6.1078 * pow(10, ((7.5 * temp_celsius) / (temp_celsius + 237.3)));
 
    // Vapor pressure of water
    float vapor_pressure_water = relative_humidity * saturation_pressure;
 
    // Partial pressure of dry air
    float partial_pressure_dry_air = pressure - vapor_pressure_water;
 
    // air density of humid air
    // const float specific_gas_constant_dry_air = 287.058;
    // const float specific_gas_constant_water_vapor = 461.495;
    const float molar_mass_dry_air = 0.0289652;
    const float molar_mass_water_vapor = 0.01801;
    const float universal_gas_constant = 8.31446;
    float air_density_humid_air =
        ((partial_pressure_dry_air * molar_mass_dry_air) +
         (vapor_pressure_water * molar_mass_water_vapor)) /
        (universal_gas_constant * temp_kelvin);
 
    this->emit(air_density_humid_air);
  }
}
 
}  // namespace sensesp