   //Serial.println(Rs);
   //Serial.println(R0);
   //Serial.println(ratio);
-  Serial.println(concentration);
-  return concentration;
+  if(concentration>0)
+  {
+    Serial.println(concentration);
+    return concentration;
+  }
+  else
+  {
+    Serial.println("0");
+    return 0;
+  }
 }
 
 float readNO2()


// define MICS-4514 global variables
#define PRE_PIN          8
#define VNOX_PIN         A0
#define VRED_PIN         A1
#define PRE_HEAT_SECONDS 10
int vnox_value = 0;
int vred_value = 0;

  //Serial.println(Rs);
  //Serial.println(R0);
  //Serial.println(ratio);
  if(concentration>0)
  {
    Serial.println(concentration);
    return concentration;
  }
  else
  {
    Serial.println("0");
    return 0;
  }
}
