static float absoluteHumidity(float temperature, float relativeHumidity)

{

    static const float A = 6.112f; // hPa

    static const float Tn = 243.12f; // °C

    static const float m = 17.62f;

    return 216.7f * (relativeHumidity * 0.01f * A * std::exp(m * temperature / (Tn + temperature)) / (273.15f + temperature));

}

 

void perspirationEngine(unsigned long timeStampMs,

                        float temperatureAmbientSensor, float relativeHumidityAmbientSensor,

                        float temperatureSkinSensor, float relativeHumiditySkinSensor,

                        float *perspiration)

{

    /* compute the time delta from the timestamp, the time delta will slightly vary but this is desireable for the filter computation */

    static unsigned long timeStampMsLastCall = 0;

    static float delta_t = int(timeStampMs - timeStampMsLastCall) / 1000.0f;

    timeStampMsLastCall = timeStampMs;

 

    /* relative humidity offset of ambient sensor */

    static const float RH_OFFSET = 0.0;

    /* temperature offset of ambient sensor */

    static const float T_OFFSET = 0.0;

    /* distance between sensors in m */

    static const float D_SHT = 0.008f;

    /* diffusion coefficient @ 25°C in m^2/s */

    static const float DIFF_COEF_25_DEGREES = 2.6e-5f;

    /* emperical constant, slope of temperature vs calibration factor */

    static const float T_CORRECTION_FACTOR = 0.04f;

    /* diffusion coefficient temperature dependent */

    float DIFF_COEF = DIFF_COEF_25_DEGREES * T_CORRECTION_FACTOR * temperatureSkinSensor;

    float CONVERSION_FACTOR = 1.0f / D_SHT * DIFF_COEF * 3600.0f;

    /* calibration factor of device */

    static const float CALIBRATION_FACTOR = 1.5f;

 

    /* define time constant for low pass filter and apply filtering on both temperature values */

    static const float TAU = 30;

    temperatureAmbientSensor = firstOrder_lowPassFilter(temperatureAmbientSensor, TAU, delta_t);

    temperatureSkinSensor = firstOrder_lowPassFilter(temperatureSkinSensor, TAU, delta_t);

 

    /* use adjusted temperature and relative humidity to calculate absolute humidity */

    temperatureAmbientSensor += T_OFFSET;

    relativeHumidityAmbientSensor += RH_OFFSET;

    float absoluteHumidityAmbientSensor = absoluteHumidity(temperatureAmbientSensor, relativeHumidityAmbientSensor);

    float absoluteHumiditySkinSensor = absoluteHumidity(temperatureSkinSensor, relativeHumiditySkinSensor);

 

    if (absoluteHumiditySkinSensor > absoluteHumidityAmbientSensor) {

        *perspiration = CALIBRATION_FACTOR * CONVERSION_FACTOR * (absoluteHumiditySkinSensor - absoluteHumidityAmbientSensor);

    } else {

        *perspiration = 0.0f;

    }

}

static CircularBuffer<float, 2> firstOrder_lowPassBuffer;

 

float firstOrder_lowPassFilter(float x, float tau, float delta_t)

{

    static float alpha = delta_t / (tau + delta_t);

    float y = alpha * x + (1 - alpha) * firstOrder_lowPassBuffer.get();

    firstOrder_lowPassBuffer.add(y);

    return y;

}