Steripen Adventurer Opti UV Water Purifier: when camping, backpacking, hiking, if you won't have access to the power grid for several days a time, this is the water purification system for you. The Adventurer Opti UV water purifier is essential gear for thousands of adventurers and features a unique technology " an optical eye senses water to ensure safe use and effective treatments. The revolutionary optical water sensor doubles as an integrated LED flashlight for added convenience on the trail or around the campsite: hold down the activation button for three seconds to illuminate the darkness. Rugged design and construction trusted by thousands of endurance athletes. Adventurer Opti is powered by two CR123 batteries making it a lightweight water purification system and keeping it operating effectively in even cold environments. The long-lasting UV lamp purifies up to 8,000 liters! Works in containers with a minimum 1.75 inch diameter opening. (Like Nalgene wide-mouth bottles one liter bottles.)