Atomic SV1 Contour Snorkel Product Details: The SV1 Contour is the perfect for any use; snorkeling, diving and free diving. The letters SV stand for Scupper Valve. How it works? When the diver clears the Atomic SV1 Contour Snorkel a small watertight tube located inside the snorkel body is designed to keep the breathing airway path free of collected water droplets. The one way valve or Scupper Valve simply allows water to drain into a lower reservoir each time the diver exhales. The result is SV a crisp clearing, hydrodynamic, snorkel without being big, bulky, and uncomfortable to use making the Atomic SV1 Contour Snorkel a great choice!