Max Dissolved Oxigen Saturation over a range of user-specified values for water temperature, barometric pressure, and salinity or specific conductance.

Max_Dissolved_Oxigen.c

// --------- Max Dissolved Oxigen Saturation ------------//
//
// Equation for the Henry coefficient as a function of temperature and salinity is used to calculate values 
// for unit standard atmospheric concentrations (USAC) in freshwater and seawater in equilibrium with air at a total pressure 
// of 1 atmosphere. It is estimated that the possible error in the new USAC values is no greater than $\pm 0.1%$ and probably less. 
// Tables and equations are presented for obtaining accurate USAC values in the ranges $0^\circ < t < 40^\cir C and 0 < S < 40$. 
// Simple procedures are given for calculating standard atmospheric concentrations at pressures different from 1 atm. 
// 
// Reference https://water.usgs.gov/software/DOTABLES/
// Dissolved oxygen (DO) solubility over a range of user-specified values for 
// water temperature, barometric pressure, and salinity or specific conductance.
// Low memory usage function for Arduino, esp 8266 who can estimate the DO,
// useful for water testing, hydroponics and aquarium automation etc

// Functions
float getMaxDO(float wt, float pres = NULL, int ppm = NULL) {
  float sat,tk, bp, sal, sc;
  
  // compute Kelvin temperature
  tk = wt + 273.15;

  // compute oxygen solubility in freshwater at 1 atm and input temperature
  sat = exp(-139.34411 +(1.575701E5 +(-6.642308E7 +(1.2438E10 -8.621949E11/tk)/tk)/tk)/tk);

  // if have pressure data
  if (pres) {
    
    // convert bp to atmospheres
    bp /= 1013.25;
      
    // compute pressure correction
    float u, theta;
    u  = exp(11.8571 +(-3840.70 -216961/tk)/tk);
    theta = 0.000975 -1.426E-5 * wt +6.436E-8 * wt * wt;
    sat *= (bp - u)*(1 - theta * bp)/((1 - u)*(1 - theta));
  
  }

  // if ec
  if (ppm) {
    
    // will use the original dotable variable name
    sc = ppm;
    
    // convert ppm to salinity
    sal = 5.572E-4 * sc + 2.02E-9 * sc * sc;

    // compute salinity correction
    sat *= exp(-1 * sal * (0.017674 +(-10.754 +2140.7/tk)/tk));

  }

  return sat;  
}

void setup() {
}

void loop() {
  // your vars
  float my_wt = 25.00;
  float my_pres = 1010.00; // input pressure expressed in millibars
  int my_ppm = 1000; // input your TDS PPM 
  
  float MaxDO; // our return var
  
  MaxDO = getMaxDO(my_wt, my_pres, my_ppm); 
  
  Serial.print("Max Dissolved oxygen (DO) solubility of ");
  Serial.print(MaxDO);
  Serial.print("mg/L at ");
  Serial.print(my_wt);
  Serial.print("°");
  
  if (my_pres) {
    Serial.print(" with ");
    Serial.print(my_pres); 
    Serial.print("millibars");
  }
  
  if (my_ppm) {
    Serial.print(" and at ");
    Serial.print(my_ppm); 
    Serial.print(" TDS PPM");
  }
  
  Serial.println();
}