If you own an MSR Guardian™ Purifier, you may be wondering why you’re not supposed to dry out its hollow fiber filter cartridge before storage. This can be confusing if you’re familiar with the MSR AutoFlow™ and HyperFlow™ microfilters, which also use hollow fiber technology and require that you do actively take steps to remove their cartridges and dry them out before long-term storage. With the Guardian no such action is required. More, the Guardian’s hollow fibers will stay properly moist inside the pump housing, with no steps needed from you. So why do the Guardian’s hollow fibers need to stay wet inside the housing? Unlike the AutoFlow™ and HyperFlow™, which are microfilters, the Guardian is a purifier. This means it removes viruses in addition to bacteria and protozoa. To remove viruses (which are far smaller than bacteria and protozoa), the Guardian had to be built with revolutionary hollow fibers. These fibers are made of special materials which happen to be hydrophobic (literally: “afraid of water”). During the manufacturing process, we ensure that the filter is wetted properly, and it remains sufficiently wet as you use it—unless, that is, you purposefully dry it out. If the fibers dry out, they will “refuse” to accept water again. The pump will become hard to operate or may not pass water at all. The safety of the filter is not compromised—the fibers won’t crack and the pores in their walls won’t expand or constrict; they can still block pathogens. But the pump itself will simply stop working. It's important to leave the filter cartridge inside the pump's housing (unless you are replacing the cartridge at the end of its life). If the Guardian's cartridge is left inside the housing, hollow fibers will stay properly moist, with no action needed on your part. Performing a simple sterilization of the Guardian purifier before and after long-term storage will keep it delivering fresh-tasting water. Learn how to sterilize your purifier in the instruction manual located at the bottom of this page.