The best thermos flasks can keep liquids hot or cold for a long time – over 12 hours in some cases. Look for a two-layer construction with an intermediate vacuum layer. This prevents heat loss by convection and conduction.

If you are searching for a great value thermos flask at the lower end of the budget range then we don’t think you can do much better than the SMARDY Vacuum Insulated Water Bottle. This is a compact size flask with a smallish capacity of 430ml – this is only enough for two cups of tea. Some thermos flasks use a cork or plug to seal the liquid in, but we really don’t feel this is an effective design. Our suggestion is to go with a screw cap and a clip-on outer cap which will double as a cup. Using a flask is actually also a very responsible thing to do in terms of recycling – apparently 99.75% of disposable coffee cups are not recycled! As we said before, vacuum insulation is important for keeping the heat energy inside your flask, and the Hydro Flask does this well. It is made of stainless steel that is extremely good for heat retention. The Thermos has dimensions of 31 x 8 x 8 cm and weighs only 515g – a great size for a day hike. A 1 litre capacity is about the optimum in our view – enough to comfortably serve two people with hot or cold drinks throughout the day without being too much of an impediment to carry.

The GSI MicroLite 1000 Twist Water Bottle is a vacuum insulated stainless steel flask that has a one litre capacity but impressively small size and density. It is extremely lightweight at 0.37kg which is less than half the weight of the Stanley 18-8 for the same capacity. You should carefully consider the required capacity of your flask before buying one. Have a think about how many people will require a drink from it, and who will be the person carrying it around. Is it for home use (some people take a flask to their home office in the garden), or carrying on a walk for rest and recuperation?

Hydro Flask are a fashionable US company who have made reusable flasks and water bottles immensely cool over on the West Coast of America. The Hydro Flask Wide Mouth is a quality vacuum flask employing some great tech, a lovely powder-coated exterior surface for great grip as well as excellent heat retention properties. The downside is that they are a touch on the expensive side – you don’t get this sort of style for free girl! Warranty: We always suggest buying a vacuum flask from a manufacturer who will offer a warranty of at least one year, and preferably more. A warranty acts as a safeguard for your flask against unexpected defects, and ensures that you won’t have to buy again if something goes wrong. If you’re planning on spending a wodge of cash on a flask, then go for the one with the warranty. Typical Heat Retention Times You may think choosing a vacuum flask is a simple matter of grabbing the shiniest example in the camping shop (if it’s open!) and hoping for the best. Well, unfortunately we’ve tried that approach several times and have frequently been disappointed. The Oneisall measures 34.5 x 11.5 x 11.5 cm and weighs 1.2kg. For a large capacity flask, those are not actually bad specifications. And it really does look the business! Beautiful lines as they say!Heat Retention is a major factor determining the usefulness of a thermos flask. It directly affects how long your drinks or food will stay warm or cold.

Capacity: 1 litre (33oz) / Temperature test: 0 hrs 90°c, 90 mins 85.5°c, 4 hrs 79.1°c, 6 hrs 74.3°c, 8 hrs 69.9°c, 24 hrs 44.5°c / Made of food grade 18/8 stainless steel that doesn’t impart or retain flavour / Lid doubles as a mug ensuring you always have a cup handy / Silicon sleeve / Drinking stopper opens and closes with a slight turn / Additional standard stopper for optimum heat retention included / Tapered design for easier handling / Large mouth opening is easy to fill and clean. BUY NOW: PRIMUSEQUIPMENT.COM 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. The temperature performance of the Smardy is quite praiseworthy. It gives around 12 hours for hot drinks and 24 hours for cold drinks. Certainly good enough for day trips in any conditions. The Milu Thermo Food Flask is a bowl-shape flask (16 x 10 cm) that is not only useful for keeping your drinks hot/cold, but also certain types of liquid-based food. You can keep soup, rice, noodles, fruits and even baby food in this great flask. All parents know that it’s often a problem carrying baby food at the right temperature. The 450ml flask Milu is a great solution to the problem. 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).The Smardy flask is manufactured with high-quality stainless steel that makes it extremely durable and we are confident it will perform well over the long-term. It utilizes a silver coat in the vacuum portion of the flask which enhances heat retention. After Stanley, I found the Thermos – a brand which has become synonymous with insulated flasks – to come a close second in terms of heat retention.

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. 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. 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. To get maximum performance from your flask, you should always pre-warm it before use. Fill it with boiling water and leave to stand for five minutes or so before emptying and refilling with your chosen brew – such as tea, coffee, soup or hot chocolate. Filling your flask right to the top will also help minimise heat loss. Temperature Test: Method Since the American manufacturer, Thermos, came to fame, the term has become slang for vacuum-insulated flasks. Although the terms are largely interchangeable, the original scientific phrasing is 'vacuum flask'.Another advantage of stainless steel it that does not leave a metallic taste in the mouth or residual odours. However, it is always worth cleaning your flask thoroughly with hot soapy water between uses. The heat retention capacity of the flask is exceptional. You will certainly be impressed with both the hot and cold performance. We were able to keep some tomato soup warm for 24 hours and on another occasion, some chilled orange squash remained cold after 36 hours. This is definitely enough for an overnight wild camping trip if you can handle the weight.