SteriPEN Classic 3 (with pre-filter)
You get the new improved Classic (it's watertight now!) and the pre-filter together in this package. The Classic 3 itself uses four AA batteries (not included) which will last for up to 50L (lithium batteries will last for up to 150L). The UV bulb, which disassembles the DNA of any nasties in the water, will last up to 8000 activations and after that SteriPEN will replace your unit with a new one (just be sure to register your product with the company when you purchase it).
Using UV light to purify water has been around for decades and is how many municipalities treat their water. The Classic 3 is easy to use: simply take off the cap, press the activation button (once or twice for .5 or 1L), insert into the pre-filter opening, turn the bottle upside down, and gently rock to circulate light through the bottle. You don't have to use the pre-filter if your water is free of debris, you can just stir the Classic 3 around in your glass or bottle until the LED indicator flashes to tell you it's safe to drink. That's the nice part - your water is instantly safe, no waiting around for chemicals to work.
The pre-filter fits on Nalgene and other 32oz wide-mouth water bottles.
Weight (with batteries): 178g (6.28 oz)
Dimensions: 186 x 43 x 43 mm
Minimum bottle diameter: 22mm
Effective 99.9% against: bacteria, viruses, and protozoan cysts (such as giardia, cryptosporidia)