For example, “you wouldn’t want to use these devices to disinfect apple cider,” he said. “The light wouldn’t penetrate.”

The lamp that creates the ultraviolet light, which has a wavelength of 254 nanometers, is good for 10,000 treatments  about 2,500 gallons of water  said Miles Maiden, inventor of the SteriPen and the chief executive of Hydro-Photon. The batteries will last for about 100 treatments before they need to be replaced.

Ultraviolet rays, of course, can be damaging to eyes and skin, so the product comes with safety features. The SteriPen has electrodes or water sensors in its neck that prevent its turning on until the device is completely immersed in water, Mr. Maiden said. After that, the ultraviolet light is securely contained: the surface of the water acts as a mirror, blocking the ultraviolet light from escaping, and UV beams cannot pass through standard container materials.

SHANNON DAVIS, associate editor of Backpacker magazine in Boulder, Colo., liked the liquid-crystal display and the microprocessor of the new SteriPen Journey. “The display shows you that you’ve left the device in there the right amount of time,” Mr. Davis said, “and that adds confidence.” He also liked the snug fit between the pen and standard plastic bottles that hold, for instance, one or two liters of water. “The fit is so tight that you can insert the pen in the bottle,” he said, and then swirl the bottle vigorously without spilling a drop.

Meridian Design, of San Jose, Calif., also makes a portable ultraviolet water purifier, the mÜV ($49 at the company Web site, uvaquastar.com) that is rechargeable, said Dan Matthews, president of the company. The unit is in limited production, Mr. Matthews said. “We’re testing them individually before they are released to full marketing,” he said. “We’re not ready to make thousands of them until we’re comfortable we’ve worked out the kinks.”

Jamie DeBenedetto, a hiker and trail leader who lives in Phoenix, tried the mÜV and prefers it to the iodine tablets she usually carries in her backpack for purification of water. It is easy to recharge, she said, “and the water tastes a lot better than it does with iodine.”