heat_index.cpp

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#include "heat_index.h"
 
namespace sensesp {
 
// heat index temperature
 
HeatIndexTemperature::HeatIndexTemperature() : FloatTransform() {}
 
void HeatIndexTemperature::set_input(float input, uint8_t inputChannel) {
  inputs[inputChannel] = input;
  received |= 1 << inputChannel;
  if (received ==
      0b11) {  // 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.
 
    // The following equation approximate the heat index
    // using both Steadman's and Rothfusz equations
    // See https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3801457/
    // Algorithm 1 (table 1) (algorithm in figure 3)
 
    float temp_fahrenheit = 1.8 * (inputs[0] - 273.15) +
                            32;  // dry-bulb temperature in degrees fahrenheit
    float relative_humidity = inputs[1] * 100;  // relative humidity in percent
 
    // step 1: if temperature is less than 40°F then heat index temperature is
    // dry bulb temperature.
    float heat_index_temperature;
 
    if (temp_fahrenheit <= 40) {
      heat_index_temperature = temp_fahrenheit;
    } else {
      // step 2: use algorithm to calculate heat index and us it if the result
      // is less than 79°F;
      heat_index_temperature =
          -10.3 + 1.1 * temp_fahrenheit + 0.047 * relative_humidity;
      if (heat_index_temperature > 79) {
        // step 3: calculate heat index temperature for other temperatures
        // Coefficients for temperature in Fahrenheit
        const double c1 = -42.379;
        const double c2 = 2.04901523;
        const double c3 = 10.14333127;
        const double c4 = -0.22475541;
        const double c5 = -0.00683783;
        const double c6 = -0.05481717;
        const double c7 = 0.00122874;
        const double c8 = 0.00085282;
        const double c9 = -0.00000199;
 
        // equation for heat index
        float heat_index_temperature =
            c1 + c2 * temp_fahrenheit + c3 * relative_humidity +
            c4 * temp_fahrenheit * relative_humidity +
            c5 * pow(temp_fahrenheit, 2) + c6 * pow(relative_humidity, 2) +
            c7 * pow(temp_fahrenheit, 2) * relative_humidity +
            c8 * temp_fahrenheit * pow(relative_humidity, 2) +
            c9 * pow(temp_fahrenheit, 2) * pow(relative_humidity, 2);
 
        // step 4: if humidity =< 13% and temp between 80°F and 112°F then
        // correct heat index temperature
        if (relative_humidity <= 13 && 80 <= temp_fahrenheit &&
            temp_fahrenheit <= 112) {
          heat_index_temperature =
              heat_index_temperature -
              ((13 - relative_humidity) / 4) *
                  sqrt(17 - abs(temp_fahrenheit - 95) / 17);
        }
 
        // step 5: if humidity => 85%  and temp between 80°C and 87°C then
        // correct heat index temperature
        else if (relative_humidity > 85 && 80 <= temp_fahrenheit &&
                 temp_fahrenheit <= 87) {
          heat_index_temperature =
              heat_index_temperature +
              0.02 * (relative_humidity - 85) * (87 - temp_fahrenheit);
        }
      }
    }
 
    this->emit((heat_index_temperature - 32) / 1.8 +
               273.15);  // Fahrenheit to Kelvin
  }
}
 
// heat index effect (warning levels)
 
HeatIndexEffect::HeatIndexEffect() : Transform<float, String>() {}
 
void HeatIndexEffect::set_input(float input, uint8_t inputChannel) {
  float heat_index_temperature = input - 273.15;  // celsius = kelvin - 273.15
  String heat_index_effect = "";
  if (heat_index_temperature > 54) {
    heat_index_effect = "Extreme danger";
  } else if (heat_index_temperature > 41) {
    heat_index_effect = "Danger";
  } else if (heat_index_temperature > 32) {
    heat_index_effect = "Extreme Caution";
  } else if (heat_index_temperature > 27) {
    heat_index_effect + "Caution";
  }
 
  this->emit(heat_index_effect);
}
 
}  // namespace sensesp