specific_conductivity.h

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double viscosity_ratio(double temperature_celsius)
{
    const double A = 1.1278;
    const double B = 0.001895;
    const double C = 88.93;
 
    double delta = 25.0 - temperature_celsius;
    double log_ratio = (A * delta - B * delta * delta) / (temperature_celsius + C);
 
    return std::pow(10.0, log_ratio);
}
 
double tuned_b(double measured_conductivity, double temperature_celsius)
{
    // Normalize inputs
    // range: ~0.05 to 1.5
    double ec = measured_conductivity / 100000.0;
    // range: 0 to 1
    double t = temperature_celsius / 50.0;
 
    // Polynomial model: b = intercept + coef1*ec
    //                        + coef2*t + coef3*ec*t
    //                        + coef4*ec^2 + coef5*t^2
    double b = 0.67132 + 2.93970 * ec - 0.90973 * t - 11.85994 * ec * ec + 13.66141 * ec * t -
               2.06555 * t * t + 12.93730 * ec * ec * ec - 26.03585 * ec * ec * t +
               14.13775 * ec * t * t - 1.48252 * t * t * t;
    // Clamp - Limit to physical bounds
    return std::fmax(0.68, std::fmin(b, 0.88));
}
 
double calculate_specific_conductivity(const double measured_conductivity,
                                       const double temperature_celsius)
{
    if (measured_conductivity < 8000.0)
    {
        // Linear model for low EC (<8000 µS/cm)
        return measured_conductivity / (1.0 + 0.0191 * (temperature_celsius - 25.0));
    }
    else
    {
        double b = tuned_b(measured_conductivity, temperature_celsius);
        double mu_ratio = viscosity_ratio(temperature_celsius);
        return measured_conductivity * std::pow(mu_ratio, b);
    }
}