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Key attributes: Leakproof, practical design enables one-handed drinking, wide mouth for easy filling and cleaning This flask has a familiar design with a classic ‘pop’ stopper that makes for easy pouring. With cheap, inferior flasks these are often prone to leaking but with this Outwell version the component seems well-made and reliable. It certainly didn’t leak on test. It’s a simple, inexpensive flask that makes use of a classic tried and tested design. The wide mouth makes the flask easy to fill and clean, while the cup’s plastic liner ensures you won’t burn your mouth. On the other hand, it is fairly bulky for a 750ml flask, with a small cup compared to some other designs. The external finish is also a bit slippery and prone to scratching. Full Specifications My gripe with this particular genuine Thermos brand flask (watch out for fakes) is that it is plastic on the outside and glass inside. The cups are plastic, too, rather than stainless steel. But like the Stanley it is BPA-free, and it kept liquids hotter than the other Thermos flasks I tried, which were composed primarily of stainless steel. It weighs 890g, which is lightweight considering its vast capacity. My hot water was 93.6°C at 8:25pm, and I already had my suspicions, because the exterior was markedly hot compared to the others – a sure sign of heat escaping. At 8:25am it was 34.8°C – a significant temperature drop of 58.8°C after 12 hours. Unlike the other flasks, my cold water and cubes didn’t get colder – it got hotter. The marketing blurb says it keeps contents hot for 28 hours, or cold for 90 hours, which based on these results I can’t really see happening.
If you want to carry your flask long distances, it’s a lightweight option – weighing 400g (14 ounces) with canteen cap, or 380g (13.4 ounces) with sports cap, while empty. When single-use plastics started getting a bad rap, S’well became the drinks bottle of choice for a host of celebrities and ‘influencers’. But they’re not all style over substance – the design consists of a triple-walled construction incorporating a copper layer for improved insulation. This flask is lightweight and practical – the ergonomic shape makes it easy to hold, the really wide mouth makes for easy drinking, filling and cleaning, and the grooved lid makes it easy to open, even if screwed on tightly. It’s sure to draw admiring glances if you leave it on your desk at the office. It’s just a shame that the capacity is a bit small, and it’s relatively expensive for a little flask. Full Specifications How do Thermos flasks work? In short, they use a double walled construction, with the air removed from between these walls, forming a vacuum. Heat struggles to pass through the vacuum, meaning liquid stays at a roughly constant temperature.KEEP YOUR DRINKS HOTTER FOR LONGER: Made from 304 stainless steel and BPA Free food grade plastic this double walled vacuum insulated flask is designed to keep your tea, coffee or soup hot for up to 12 hours or keep chilled drinks including wine, beer or carbonated drinks cool for up to 6 hours. Flasks are more 'smell prone' than 'stain prone' – none of the flasks we tested had any staining after cleaning, but some retained a faint coffee smell. The Stanley Classic is the Daddy of all insulated flasks, and with good reason. My hot water was 84.6°C at 7:51pm, and at 7:51 am it was 70.8 °C – a temperature drop of a mere 13.8°C after 12 hours (far less than its competitors, as detailed below).
We hand-washed each flask with hot soapy water and let them air dry overnight, then ranked them on smell. You probably wouldn't describe the scientific rigour as laboratory standard – but the tests did replicate the kind of environment you'd actually use these flasks in, either filling up your tea or coffee at the start of the day, or icing water. And the results were certainly interesting. While the cold water tended to record similar temperatures across products (with the exception of Klean Kanteen, which I'll get to), there was a significant degree of difference when it came to heat retention. My conclusion is that if you really care about keeping your caffeine hot, you should choose your insulated flask wisely.Other useful design features to look for include a wide mouth, which makes for easier cleaning and filling (especially if you want to add ice cubes to your flask). In addition, many larger flasks incorporate ‘twist and pour’ spouts, so you can pour from the flask without having to completely remove the stopper. An integrated cup is also useful. Smaller flasks often feature lid designs that allow on-the-go swigging or one-handed operation. The original and, judging from the results of our temperature test, still the best. This stainless steel flask is very durable, with a silicone base to protect against dents. The two-part stopper closes securely yet pours easily and has an integrated cup. In terms of outright heat-holding performance for up to 8 hours, nothing can touch the Ultimate Series. It was the best-performing flask we tested overall. It is relatively lightweight too. And if you’re concerned that the 500ml capacity will be too small, it’s worth noting that Thermos also makes a larger 900ml version. Full Specifications A big part of the appeal of the Stanley is that it makes you feel like an intrepid camper, thanks to the distinctive green branding. With a lifetime warranty, the iconic flask will survive even the wildest of adventures: since it was founded in 1913, Stanley bottles have survived -70 degree winds, 4,000 foot drops, speeding bullets and category 5 hurricanes (please do not try this yourself). Unlike many flasks, it is also dishwasher safe – a big bonus in my book. The lid doubles up as a decent-sized cup and provides extra insulation; a handle on the side makes it easy to lug around and grip while battling the elements, but there’s no chance of leakage. While primarily made up of stainless steel, some parts are plastic – but it is BPA-free (an industrial chemical used in water bottles and similar that many people prefer to avoid).
My hot water was 91.3°C at 8:36pm; at 8:36am it was 75.7°C – a temperature drop of 15.6°C after 12 hours (not that much more than the Stanley, and still piping hot enough for a decent brew). My cold water was 4.4°C at 11:13am, and as with the Stanley, 12 hours later it was even colder – at 1.7 °C. My hot water was 92.3°C at 8:05pm, and by 8:05am it was 57°C – a temperature drop of 35.3°C after 12 hours. It’s still “warm” – just not as warm; and certainly not pipingly hot. My cold water with ice cubes is 5°C at 11:02 and 12 hours later its 3.5°C.We dropped each flask five times from 90cm to mimic a dropped flask. Those with large dents that inhibited function or ability were rated low.
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