We'll just say it up front, MSR has designed what may be the perfect water purifier for small groups, capable of purifying ten liters of "worst-case" water at a rate of about one liter every two minutes, and with a filter rated for 3,000 liters of lifetime service. We know you have questions, like, what the hell is "worst-case" water, and why is it in scare quotes? Fear not, we have answers, so read on!
The Guardian Gravity was designed for the US military, and they needed something capable of dealing with the nastiest water they might find in the field, which – given where they've been operating the last two decades – can be pretty nasty. The military has specifications for everything. Your skull, for example, is officially a Brain Housing Group, and if you want to know exactly what is wrong with it, join the US Army and their friendly Drill Sergeants will tell you. The specification for individual and small unit water purifiers is NSF P248, a more rigorous extension of the P231 standard.
MSR sums up the third-party testing process: "The full NSF Protocol P231 test lasts for 10 days. Five of those days challenge a filter with tap water; the other five with what’s called 'worst-case' water—containing elevated levels of organic matter, turbidity, pH and salt, and at nearly freezing temperatures. In addition, the test challenges the filter at the end of the filter’s life to ensure it’s still removing pathogens up until the last moment its cartridge is full. Every MSR filter is tested to the full 10-day protocol."
So, we have an honest-to-uncle-sam water purifier, they've tested the bejesus out of it, and we are fully confident it will take out viruses (99.99%), bacteria (99.9999%), protozoa (99.9%), sediment and microplastics. How's it work in practice?
First of all, the packaging is just right. A zippered bag with mesh sides for drying perfectly fits the purifier, which unrolls ready to use, no assembly required. Well OK, a little disassembly is required, as you follow the instructions clearly printed on a large tag inside the bag: detach the water hose, fill the clearly labeled DIRTY WATER bag, roll it shut 4x, hang it six feet above your clean water container, re-attach hose, purge it, rinse bottle adapter, attach bottle adapter to clean water container and you're good to go. You get pretty fast at it in practice, and there are also detailed instructions for storing, cleaning and performing an integrity test.
There is a pre-filter in the bag to catch larger sediment, and the two-stage filter cartridge has a carbon filter that works very well to make the clean water odorless and tasteless. Reminder: do not detach hoses from filter cartridge, and do not disassemble cartridge or attempt to dry it out, as it needs to maintain a damp condition for the integrity of the filter. Also, when hanging try to stretch it out to the maximum hose length with a vertical drop. The Guardian will work at lower heights and at an angle, but slower. We found a liter every two minutes to be possible with the perfect height and angle, but a liter every 2.5 to 3 minutes is probably more achievable under normal field conditions. That's still less than 30 minutes for 10 liters of pure water.
After all this, the bottom line is that the MSR Guardian Gravity scores pretty much perfect marks on capability, usability and portability, which at 1lb 3oz is pretty damn good for a 10L purifier that requires no pumping. Even the affordability we consider pretty good, given its efficacy and long service life. If you're going out in small groups, you're not going to find a better purifier than the MSR Guardian Gravity.