#include <math.h>

int DustSensePin = 13;
unsigned long duration;
unsigned long starttime;
unsigned long sampletime_ms = 30000;//sample 30s ;
unsigned long lowpulseoccupancy = 0;
float ratio = 0;
float concentration = 0;

void setup() 
{
    Serial.begin(9600);
    Serial.println("Starting Air Sensor, please wait for readings...");
    pinMode(DustSensePin,INPUT);
    starttime = millis();//get the current time;
}

void loop() 
{
    duration = pulseIn(DustSensePin, LOW);
    lowpulseoccupancy = lowpulseoccupancy+duration;

    if ((millis()-starttime) > sampletime_ms)//if the sample time == 30s
    {
        ratio = lowpulseoccupancy/(sampletime_ms*10.0);  // Integer percentage 0=>100
        concentration = 1.1*powf(ratio,3)-3.8*powf(ratio,2)+520*ratio+0.62; // using spec sheet curve
        Serial.print(lowpulseoccupancy);
        Serial.print(",");
        Serial.print(ratio);
        Serial.print(",");
        Serial.println(concentration);
        lowpulseoccupancy = 0;
        starttime = millis();
    }
}