Capacity: 592ml/20oz / Temperature test: 0 hrs 90°c, 90 mins 81.7°c, 4 hrs 72.4°c, 6 hrs 65.1°c, 8 hrs 59.9°c, 24 hrs 36.2°c / Double wall vacuum stainless steel construction / Screw in stopper with leakproof flip top lid / Powder coat finish / BPA-free and phthalate-free / Dishwasher safe (hand wash recommended). BUY NOW: EARTHWELL.COM 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. We’ve reviewed a selection of the best thermos flasks, ranging in size from 470ml to a whopping 64oz (1.8 litres). Bigger flasks usually retain heat better than smaller flasks, but are typically bulkier and heavier. We’ve compared their performance with our own temperature test. What Is The Best Thermos Flask?

Like almost any decent bit of outdoor kit, a good vacuum flask should also be robust and durable. All the flasks we tested are made of high-quality 18/8 food-grade stainless steel that resists dents, dings and scratches. The trade-off with steel flasks is that they can be heavy, although modern designs are shaving weight all the time, and in some cases, they now weigh little more than a standard plastic drinks bottle. 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.

S’well Roamer 64oz Flask

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. Capacity: 600ml (20.3oz) / Temperature test: 0 hrs 90°c, 90 mins 79.6°c, 4 hrs 66.5°c, 6 hrs 58.3°c, 8 hrs 51.6°c, 24 hrs 22.2°c / Stainless steel construction / Wide mouth opening / To help you choose which one to invest in, I tried and tested a selection of flasks that promise to keep your beverages colder and hotter for longer, measuring how much heat was retained or how cool they kept respectively over a period of 12 hours. 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.

Founded in 1997, we’re the UK's largest dedicated outdoor clothing and equipment retailer and have been offering our customers unbeatable VALUE for money and outstanding quality for 25 years.The wide mouth means it’s also suitable for soups and smoothies to take to work, and it will certainly keep them either sufficiently cold or hot until lunchtime. It's available in a range of colours, which is fun, from canary yellow to baby blue via three shades of pink. 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

YOUR INEXPENDABLE COMPANION AT ALL TIMES - The practical KAAVE vacuum flask for on the go, at home, in the office or outdoors. Highly durable thanks to the sturdy handle and bottom. A reliable companion that guides you through the day. Perfect for every tea and coffee enthusiast -Ideal for sharing thanks to 2 cups integrated into the lid - because sharing is caring.Capacity: 500ml / Temperature test: 0 hrs 90°c, 90 mins 89.3°c, 4 hrs 82.8°c, 6 hrs 79.2°c, 8 hrs 75.3°c, 24 hrs 50.9°c / Stainless steel interior and exterior / Cool-touch, no-slip finish / Protective silicone rings for ultimate durability / Two part stopper for smooth pouring. BUY NOW: THERMOS.CO.UK We hand-washed each flask with hot soapy water and let them air dry overnight, then ranked them on smell. 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. 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).