Gregory Bite Valve Cover Optic Blue One Size. However these Camping Gear Accessories by Gregory still need air and your bite valve needs it to perform its best. That's why Gregory designed these Backpack Accessories with an integrated tether to keep the cover in place. This is especially useful when you're inhaling deeply or in a situation where you need to protect your hose's bite valve from getting clogged with debris. To use this device you need to have a Gregory DryLock Magnetic Bite Valve Cover but that's the extent of its functionality. To fit this cover on any hose with outer diameters up to 0.5in or 13mm simply thread the tether onto the hose and slide the bite valve cover over the hose after the bite valve is closed. Then slide the tether onto the center of the hose where the bite valve is inserted and slide the cover over the bite valve and twist to close. When you're done using the device thread the tether to the nearest ground to seal the cover back in place. Then you can pop the valve into your backpack's hose and the cover stays in place until you take it off. For full-size hoses check out the Gregory Bite Valve Protective Cover on Amazon. Specifications for Gregory Bite Valve Cover: Manufacturer: Gregory Size: One Size Weight: 0.14 oz Pack Application: Day Hiking Color: Optic Blue Hydration Compatible: Yes Fabric/Material: HDPE/EVA Height: 1.1 in Features of Gregory Bite Valve Cover Bite valve cover with integrated tether Compatible with Gregory's DryLock Magnetic Bite Valve Thread tether over hose before bite valve to secure cover Fits up to a 0.5" / 13mm outer diameter hose Package Contents: Gregory Bite Valve Cover Gregory 136978-5583: Bite Valve Cover Optic Blue One Size