REVIEW: Guardian Water Purifier from MSR

The MSR Guardian water purifier is claimed to be market-leading technology in the field of water filtering and purifying. The US military funded the multi-million-dollar and multi-year research and design that went into the Guardian, wanting a water purifier that could be used by US soldiers anywhere in the world. Some might have qualms over the political decisions that have led to US forces being deployed in various places around the world, but just like the climbers of the post WWII generation getting access to ex-War Department nylon ropes and steel carabiners, I think we can appreciate the civilian spin-off from this military technology.

Testing the MSR Guardian in the Peak , 211 kb
Testing the MSR Guardian in the Peak
© Toby Archer

The Guardian is called a purifier as opposed to “just” a filter because filters have a pore size that can’t catch viruses. It seems filters have a pore size of 0.2 microns (1 micron is 0.001 mm). This will be small enough to filter out bacteria and protozoa but not viruses. The MSR Guardian has a filter made of “advance hollow fiber” with pores sizes 0.02 microns, which is small enough to catch viruses as well. Reading up on this (here and here), it seems that viruses aren’t commonly transmitted in water unless the water is polluted with human fecal matter. Therefore it may seem that this isn’t a huge consideration unless you are travelling in a developing country with poor sanitation. But speaking for myself, the sickest I have ever been in the mountains was, unhelpfully, whilst halfway up Observatory Ridge on Ben Nevis in early summer conditions. The cause was almost certainly drinking water polluted by other campers around the CIC hut and, rapidly, my body was trying to evacuate whatever bacteria or virus (probably norovirus) I had picked up, from both ends, almost simultaneously. Try doing that in the late snows of Zero Gully, having been lowered off a runner on the steep bit of the ridge. It is not an experience I care to repeat and it could have been avoided by using a filter or putting a chlorine tab in my water bottle.

The Guardian screwed onto a 1-litre bottle, 139 kb
The Guardian screwed onto a 1-litre bottle
© Toby Archer
In use on a sheepy Peak District stream, 124 kb
In use on a sheepy Peak District stream
© Toby Archer

I was asked to try out the Guardian and write this review reasonably swiftly, so my opinion comes with the following caveats: I am not a microbiologist or similar with access to a lab or microscopes to test that the purity of the water coming out of the filter is what MSR claims it is. I, like most users, have to trust the company’s claims and US military standard that the Guardian has been tested against successfully. All I can say is that I’ve now used the Guardian on a number of hikes and backpacks where I drank only water I had filtered with it, and have been absolutely fine after doing so. Maybe the water from those sources would have been fine untreated, I don’t know, although in at least two cases it was from water sources I would not have trusted without the filter.

In use

So mainly I can report back on the what the Guardian is like to use (easy) and a few thoughts on its size and weight (not insignificant) and cost (also not insignificant!). MSR says the Guardian will purify 2.5 litres of water a minute.

Here's a short film of the filter in use:

In the video I wasn’t trying particularly hard, and I filled a litre bottle in less than a minute - so MSR's 2.5-liter-per-minute claim sounds about right. This sort of rate is quite a lot faster than many filters. 

But what the Guardian does that is perhaps unique is to take about 10% of the water it purifies and use that to backwash the filter. This is expelled down the second tube which runs parallel to the ‘in-tube’. What this means is that there is no cleaning necessary to keep the unit going, even when filtering water with lots of mud or other sediments in it. I’ve used a ceramic filter in the past which required regular disassembling and endless scrubbing with a toothbrush to keep it going; this became rather tiresome and always meant some risk of cross contamination when cleaning. The self-cleaning feature of the Guardian is very impressive.

Also, the tube that you drop into the water has a pre-filter on it which takes out bigger pieces of sediment and other stuff floating in the water. The shape of this pre-filter means you can put it into very shallow amounts of water and pump from there successfully into the purifier.

Size and weight

Packed up, it's about the size of a 1litre bottle
© Toby Archer

The Guardian is neither light nor small. All packed up it is about the size of a litre water bottle and weighs 490g. There are lots of water filters that are both much smaller and considerably lighter, and of course a few strips of chlorine tabs weigh next to nothing, but those filters will not remove viruses as the Guardian will. You can think of the filter as weighing only the same as 500ml of water and with the filter you can get water from anywhere and hence carry less. This may be true in many places but, of course, you might want or need to camp somewhere not close to water, in which case for some hours of your hike you could be carrying a couple of litres of water AND the half kilo of purifier.


The Guardian is designed to screw onto wide mouth bottles. Nalgene bottles are the most well known, but MSR also makes them plus also making water bags with the same compatible openings. If you don’t have a Nalgene (or similar) you will need to buy one, and this adds another tenner onto the Guardian’s already significant cost of £295. This puts it into the same sort of price bracket as a decent tent or a top quality sleeping bag. If you are quite happy drinking out of mountain streams for nothing, then the best part of 300 quid seems like a huge amount. MSR’s argument is that global travellers can avoid buying bottled water in countries where the tap water is not safe and could actually save money as a result with the Guardian. They say that the Guardian will purify 10,000+ litres (yes, I’m not really sure what the + means either, I guess 10,000 at minimum) before the filter becomes clogged. So for most weekend hikers this would suggest many years of service from the purifier before you would need to invest in a new filter element. In that sense, either if travelling in less developed countries, hiking in areas with suspect water, in the long term the Guardian might actually be a rather good investment; but it is a big outlay. The unit is definitely built to last by the looks of it (think of those GIs and jarheads chucking them into their patrol packs); MSR claims it is drop proof from a couple of metres and will even resist damage from freezing – a problem for many filters.

But do you really need one?

I suppose the big question is whether you (or I) need a water filter at all? I have only used one in the past when travelling in India and in particular we invested in one for hiking and climbing in Himachal Pradesh – on previous trips to India and Nepal I had got by on bottled water and with chlorine tabs and iodine drops.

Do you always check what is upstream before drinking that water?, 141 kb
Do you always check what is upstream before drinking that water?
© Toby Archer

Living in Scotland, I drank from mountain streams quite happily with only that one bad experience on Ben Nevis. I’ve drunk from streams in Snowdonia and the Lakes as well without too much thought for many years. Living in Finland, I always happily took water from lakes without any trouble despite dire warnings from some locals who warned about industrial run-off. But having now spent almost two years living on the edge of the Peak District, I’ve been reassessing that approach. Even high on Kinder and Bleaklow there are always sheep around, and one stream I pumped water from last weekend on Bleaklow was right next to the reasonably busy Pennine Way, meaning human waste could perhaps be a risk. Here having some sort of water purification seems sensible if you don’t want to be limited to what bottles you can fill up and carry when passing by somewhere with a tap. But of course a fiver invested in chlorine tablets might be all you need in such cases.


The Guardian is a pretty impressive piece of design and technology; I think for someone or, perhaps even more so, for a small team of people going to a part of the world where you really can’t trust the water at all (see MSR’s film from Peru below, for instance), despite the relatively high initial investment, the Guardian would be superb. In the UK the case is less clear cut – particularly if, like me, you are often doing your hiking alone. One of the slower and less long lasting, but cheaper and lighter options would probably do ok here instead. Nevertheless, the Guardian has been a pleasure to use, and I’ve even drunk water pumped out of a black, slightly scummy half dried-up mini-pond at the bottom of a peat hag to no ill-effects. Having the Guardian has definitely opened up wild camping possibilities, due to not needing to be that close to a water source that at least looks like it is safe to drink from.

MSR Say:

With military-grade engineering and game-changing technology, the Guardian Purifier is simply the safest, easiest way to purify water virtually anywhere on earth.

MSR Guardian Purifier product shot, 86 kb

  • 2.5 litres per minute
  • Treats up to 10,000 litres
  • Hollow fiber technology: Unlike conventional hollow fiber technology, these advanced medical-grade fibers block viruses, the tiniest waterborne microbiological threats. Made in the U.S., these fibers are also more durable than others. They’re not damaged by freezing, and in our water lab, the purifier withstood drop testing (6ft onto concrete).
  • Self-cleaning: On every stroke, the purifier uses 10% of its water to flush the contaminants in its filter back into the source. This means, you’ll never hassle with backflushing or scrubbing cartridges to maintain its fast flow. 
  • Physically removes - Viruses like: Norovirus / Norwalk, Rotavirus, Hepatitis A; Bacteria like: E. coli, Shigella, Salmonella, Cholera; Protozoa like: Giardia, Cryptosporidium, Amoebae; Particulate like: Sediment, Silt, Dirt

Price: £295

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