Safe Water Anywhere—A Guide to the MSR Guardian Gravity Purifier

guardian gravity purifier

Did you know that the original, award-winning pump-action Guardian™ Purifier has a sibling? Meet the Guardian™ Gravity Purifier. Years in the making, it uses the same advanced technologies as the original pump purifier in a high-volume, hands-free gravity design to deliver purified water on demand. Like its predecessor, the Guardian Gravity is a purifier, meaning it removes not just bacteria and protozoa but the tiniest pathogens and viruses. Viruses are of growing concern, even in the US, as more and more people visit the backcountry to the point that even the old standard of burying your waste is no longer enough to protect water supplies. And like the original Guardian, this gravity version uses 0.02-micron hollow fiber media to purify, but we added a carbon component to help pull out chemicals, tastes and odors as well.


The Guardian Gravity purifier was originally developed for the U.S. military, allowing service members to drink from challenging sources of fresh water virtually anywhere on earth. Requiring years of development work and sophisticated engineering, it is the only gravity purifier available that meets the stringent NSF P248 military testing standard. The purifier cartridge was originally designed as a drink-through system (i.e. attached to a drinking spout)—it is a true engineering feat to achieve a sufficient flow rate for drinking while using hollow fiber media small enough to block viruses.

It’s also the only gravity purifier to offer that level of certified protection with such a fast flow rate. It delivers up to a liter of clean water in two minutes—that’s 2.5 times faster than the closest competitor. A height difference of at least six feet between the dirty reservoir and your clean water receptacle is required for optimal flow rate, so hang the reservoir bag as high as possible when using the purifier.

water purifier on bikepacking trip
Photo by Scott Rinckenberger


Another exciting feature: this purifier requires no backflushing. Simply open the purge clamp to flush the system to clean it and maintain flow rate. MSR water lab tests show that regularly cleaning the filter greatly improves the lifespan.

filling dirty water bag for backcountry water purifier
Photo by Scott Rinckenberger

Like its high-tech predecessor, the Guardian Gravity Purifier is a state-of-the-art piece of gear that gives you advanced military-grade technology, a super-fast flow rate for an anti-virus gravity system and an unrivaled level of protection.

How to use the Guardian Gravity Purifier:

  1. Fill the reservoir with water and hang it as high as possible (a height difference of at least six feet between the bag and your clean water receptacle is required for optimal flow rate). Ensure that the shut-off clamp and purge clamp are both closed.
  2. Attach the inlet hose to the reservoir.
  3. To start the water flow, open the purge clamp and flush enough water to purge the air bubbles. This is highly concentrated dirty water, so ensure no drops get into your clean container.
  4. Close the purge clamp, remove the clean side cover and open the shut-off clamp.

The first time you use the purifier, you’ll need to flush out loose carbon dust and glycerin, which are harmless but don’t taste very good. This initial water will come out gray until the carbon dust has been flushed—this is normal.

  1. Once you’ve filtered about half a liter of water, attach a “Clean” container and allow it to fill with clean, purified water.
  2. When you’re done purifying, towel dry the parts and wrap them up like a burrito. Keep all the parts connected to prevent cross-contamination.

Before long-term storage (about two weeks or longer), disinfect the filter to prevent viral film growth which can decrease the flow rate (instructions included). It’s critical you do not disassemble the cartridge to try to dry it out—the cartridge must stay wet inside. If the internal fibers dry out they will no longer pass water. As long as you don’t remove the hoses from the cartridge, it will stay sufficiently wet inside during storage.

Do not drop or freeze the cartridge. The Guardian Gravity Purifier’s filter media is made of advanced, tiny hollow fibers. If you do happen to freeze or drop it after it’s been wetted inside, follow the included instructions for a safety check. This safety check is a hallmark of MSR design—because it’s impossible to see if a single hollow fiber has been damaged, this just-in-case feature offers peace of mind that your device still functions properly–something most competitors don’t provide.

To perform the Guardian Gravity Purifier integrity test:

  1. Disconnect the inlet hose from the reservoir and fill the reservoir with three to four liters of water.
  2. Fill the rest of the reservoir with as much air as possible.
  3. Roll the closure strap four times to seal it.
  4. Reconnect the inlet hose and lay the hose system on the ground in front of you so that it’s easy to view the entire inlet and outlet hose system.
  5. Flip the reservoir so that its hose port is down.
  6. Holding the closure strap, kneel on the reservoir and force water through the inlet hose. Make sure no air enters the hose.
  7. Open the purge clamp to purge all air bubbles from the cartridge.
  8. Close the purge clamp.
  9. Open the shutoff clamp while continuing to kneel on the reservoir.
  10. Fill the outlet hose with water.
  11. Flip the reservoir over so that its hose port is facing up.
  12. Kneel on the reservoir and force only air through the inlet hose all the way through to the cartridge.

Your goal is to force air against the fully-wetted fibers inside the cartridge. If a fiber is damaged, air will escape and reveal a steady stream of bubbles in the outlet hose. If you see a steady stream of bubbles in the outlet hose, we recommend repeating this test twice to ensure accuracy. Sometimes trapped air will escape, causing a few bubbles, but the cartridge is still good. What you’re looking for is a steady stream of bubbles in every test, which indicates that there is a damaged fiber and the cartridge is not safe. If the cartridge is compromised, immediately discontinue use and replace it.

water purifier for international travel
Photo by Scott Rinckenberger

Who is the Guardian Gravity Purifier for?

Since it delivers high volumes of purified water on demand and military-grade construction makes it extremely rugged, the Guardian Gravity Purifier is great for international travelers, backpackers, overlanders and those stocking emergency kits.

The 10-liter collection bag makes it great for groups who need enough water each day for drinking, cooking, and/or hygiene. Despite offering such a high capacity, the whole system was built for traveling. It packs down small to easily fit in a day pack and comes with a compact and ventilated zippered carrying case.

It’s easy to fill from both sinks and streams, offering incredible versatility for both international trips and local adventures. It has a universal bottle adapter that allows adventurers to use their favorite bottle, meaning they can stop buying bottled water on their adventures–a worthy goal for many seeking to decrease their environmental footprint. The cartridge lasts ~3,000 liters, which is equivalent to keeping 3,000 one-liter or 6,000 .5-liter disposable water bottles out of landfills and, ultimately, the ocean.

Behind the Guardian Technology

The military approached MSR with a number of complicated requirements. As such, it took us six rigorous years to develop the right technology. Every bit of the Guardian’s technology pushes the envelope. No other purifier physically removes viruses at such a fast flow rate, self-cleans and achieves military durability.

MSR’s water lab gives us the ability to test water filters and purifiers in a way that many other manufacturers can’t. It is imperative to have a biohazard level 2 lab because we can safely grow hazardous bacteria and microbes that users could encounter in the backcountry. Growing microbes on-site is a huge advantage for our product development and testing process— it allows us to make the most reliable, high-quality, innovative and durable products for the backcountry where you can’t afford to have gear failures.

Both the original pump and new gravity Guardian purifiers are unique. They are some of the only water purifiers available that meet the military-approved P248 standard for water treatment devices. P248 is a rigorous 10-day test that uses both clear water and water so contaminated it’s dubbed ‘worst-case water’. When you are backpacking in an area with crowded campsites and human waste potentially present near water sources or if you’re abroad in an area where sewage treatment isn’t well established, you need reliable, certified virus protection. The Guardian is the solution for clean water anywhere.

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Updated. Originally Published April 19, 2021.

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