I’m not sure if there’s a more respected name in the world of water filtration than Mountain Safety Research. If there is, it’s probably their sister brand Platypus, who introduced the first mainstream commercial gravity filter which we reviewed in 2013. Their newest water purifier is the Guardian, another gravity filter that offers an unparalleled high-volume system for expeditions, emergencies, and large groups.
MSR Guardian Gravity Water Purifier Features:
- Meets NSF protocol P248 military testing standard for removing viruses (99.99%), bacteria (99.9999%), protozoa (99.9%), sediment and microplastics from “worst-case” water conditions.
- Two-stage purifier includes activated carbon to reduce chemicals and unpleasant tastes and odors.
- 10 -liter reservoir is easy to fill at sinks and streams alike; covers daily water needs for drinking, cooking and hygiene.
- No backflushing required—simply opening the purge hose flushes the system to clean it and maintain flow rate.
- Up to 1 liter in 2 minutes with optimal reservoir hang height of 6 feet, 2.5x faster than the next leading gravity purifier.
- Rugged, military-grade construction. Compact system fits easily in carry-on luggage and smaller packs.
- NSF P248 Testing Standard: The most rigorous scientific certification for proving a treatment device safely removes claimed contaminants from challenging wilderness water sources.
- Guardian devices are the market’s only purifiers to pass the military standard P248 testing for virus removal.
- Weight: 1lb 3oz
- MSRP: $249.95
Big protection in a big package
The Guardian gravity filter is essentially a remake of their top-shelf Gravity pump, which was well known in the industry as being one of the most rigorous filter options available. The gravity iteration takes the work out of filtering, and offers a massive 10L reservoir to reduce the number of trips you take to the stream, lake, or fetid swamp.
Let’s talk a little about filtering efficiency, first. The Guardian uses the hollow fiber technology that was developed a few years ago and has been improved in various iterations since then. Basically, it uses tiny holes to strain out bacteria, viruses, protozoa and other contaminants. This is called size-exclusion filtration, and it’s the same idea as the ceramic filters that I started off backpacking with.
There are a few advantages with the hollow fiber technology. For one, it’s not a big ceramic hunk, so it won’t crack as easily in the field. It also can treat murky water that would confound a UV filter or overwhelm chemical treatment. It’s also light for its weight, which essentially means that you can pack more surface area into the same package: with size exclusion technology, more surface area means more filtration per unit of time. That’s why these filters are able to filter quickly and with just the force of gravity to drive them.
So that’s the background – let’s talk about this filter. The first thing to know is, it’s big. The actual filter body is about a foot long and the whole package weighs over a pound. But what you get out of that is a Cadillac: there’s a large reservoir, a built-in purge tube, around 6 feet of tubing, and a one-size-fits-all water bottle adaptor with included cover.
In many ways, this is built for groups and expeditions. While the dirty bag has a 10L capacity, you can fill that up to whatever size you need, and the snap-in connectors are compatible with other MSR and Platypus bladders if you wanted a smaller package. The great thing about the large size is that I could source water once for my small party and have a ‘tap’ easily accessible throughout the day. This is so great for small or medium parties where hydration is critical (as it often was during this long, hot summer) and you want to make it psychologically and physically easy to hydrate.
The dirty bag itself is very well constructed, with a tough, abrasion-resistant base that still looks like new despite the number of times I slammed it down on rocks and roots. It seals with a roll-top closure and has a separate strap for hanging, which is a nice touch. I foolishly let the bag fall off of a 5-foot drop while loaded, and while the seal burst the bag itself wasn’t harmed and still works like a charm (although I don’t recommend doing that).
The effectiveness of the filter is hard to argue with. It is true military-grade technology that is rated to remove bacteria (easy), protozoa (easy) and viruses (really really hard) from your water. This is a two-stage purifier with activated charcoal, meaning that many unpleasant tastes and odors will be removed as well.
Because this is a purifier and not a ‘mere filter’, flow speeds are a touch lower but still very good. With clear water and the appropriate hanging setup, you can achieve 1 or even 2L per minute. I would say that, in my tests of clear water which rarely involved the optimal hanging height, I often saw slower filtration rates of 0.5L/min. Your mileage will vary depending on the quality of your source and whether you’re able to find the perfect spot to hang it.
There are some interesting quirks to the technology. For one, the fibers shouldn’t dry out: that means that you need to leave the tubing connected and instead use a chemical to disinfect the filter before long-term storage to prevent biofilm growth. It’s a simple, important bleach cleaning found here under step 5. For another, there’s an integrated purge tube which should be used with each fill-up to purge air from the line. The procedure is easy and found at the same link above.
Note that you need to purge the filter before its first use and, when you do that, you’ll flush out about 0.5L of grey water containing charcoal dust and glycerine. It’s harmless but yucky. One of the major selling points of this as a military or professional-grade filter is that you can perform a field check to test the filter’s integrity: the procedure is also described in the manual but it essentially lets you see if the filter integrity is compromised by a test that passes air from the bladder through the filter. It’s easy, quick, and a great way to provide peace-of-mind if, say, the filter freezes overnight or suffers a large drop. Or a bear attack. Or a teething puppy attack.
- Probably the gold standard backcountry or emergency-preparedness commercially available water purifier
- Enormous filtering capacity with a thoughtful and well-integrated system
- Simple operation that requires a small amount of education to use correctly
- Scales well from small groups to large ones
- Activated charcoal second stage results in more pleasant drinking
- Can handle murky, cloudy and silty water
- It is relatively large and heavy
- I rarely achieved the same flow rates as MSR promises, and as a true purifier it runs slower than a filter
The Bottom Line: MSR Guardian Gravity Filter
It’s easy to recommend the Guardian gravity purifier for the right user. If you want heavy-duty purification for lots of people, and you’re willing to pay a premium for the military-grade filtration ratings and construction, this is a great product. If you take care of it, it will take care of you. Groups and individuals looking for lighter options in most backcountry settings could make do with another, lighter MSR or Platypus gravity filter like the AutoFlow or the updated Platypus GravityWorks. But if you need the highest level of purification, this product is for you.
Buy Now: Available from REI.com