Most thermoses rely on double-wall or even triple-wall construction, and the space between each wall is vacuum-sealed. Air is effective at transferring heat. If there’s less air between two objects (that aren’t touching), then the gap between them does a better job of resisting heat transfer. Simply stated: The less air, the less heat flow. Insulated windows work the same way.
You have two or three panes of glass joined in one sealed assembly, and each pane of glass is separated not by air but an inert gas that’s a poor conductor of heat. Scottish physicist and chemist James Dewar noticed this effect and, in 1892, invented the vacuum flask, a container with an inside and outside wall separated by a vacuum. Dewar’s flask was the forerunner of today’s thermos. The gap between the container’s inside and outside walls doesn’t have to be wide, it just needs to be absent of air to be effective.
Thermos Care And Use
It’s simple to maintain a thermos. Try not to drop it or bang it around, which may break its vacuum seal. Keep it clean, and make sure to remove and give the lid’s gasket a good scrub every once in a while. Most thermoses aren’t dishwasher-safe. Sometimes their lids are, but the bottle may not be rated to withstand the thermal cycle of a dishwasher. Two of the bottles we tested are, but when in doubt, hand wash. For the bottles with a narrow mouth, you might find a bottle brush helpful.
While we won’t do a deep dive into thermodynamics here, suffice it to say that a wide range of factors can influence the temperature of the liquid in a thermos. To keep the contents hot or cold, stash the thermos in a place that’s hot or cold. Say you want your coffee to stay hot on a winter day, keep the thermos in a pack surrounded by something that will insulate it. Or leave it in your car, especially if there’s sun coming through the windows. If the contents should stay cold, put the thermos in the shade or a cooler. Be mindful of where you set the thermos down. For example, placing it on ice (say you’re ice fishing) will increase the rate of heat flow from the thermos to its surroundings. Your coffee or cocoa will cool off more quickly. Yes, a thermos with an insulated lid and a vacuum seal at its base resists heat loss or gain more than other designs. But regardless of the construction, with a little common sense you’ll reduce the rate at which the thermos reaches temperature equilibrium with its surroundings.
OtterBox Elevation 36-Oz. Growler
The Elevation is an incredibly impressive thermos with strong thermal retention and a good lineup of accessories. We tested the 36-ounce model with the screw-top lid. Otterbox has an entire system of lids that work with this bottle, though, including an easy-flip (but not as water-tight) sipping lid and an even more robust thermal lid like you might see on traditional insulated bottles. We like that the lids can mix and match with other Otterbox vessels. The screw-top has a great handle and is easy to open and close. It’s not the easiest to sip out of on the move, but we’ll overlook it thanks to the impressive insulation. Outside of the more traditionally designed Stanley thermos and the Avana Beckridge, the Otterbox had the best insulation for both hot and cold, and it was still easy to use.
Stanley 25-Oz. Adventure
The Stanley Adventure is the most “traditional” thermos we tested. It has a classic thermal design with a very small opening, a cap that screws into rather than over the bottle’s opening, and a cup cover. This means it’s far from easy to drink from on the fly, like in the car on your way to work. In order to do so, we had to find somewhere to out the cup cover, then unscrew the supremely tight lid and hold it as we drank through the bottle’s small opening.
Avana 32-Oz. Beckridge
We’ll admit we were first drawn to the Beckridge for the sleek brushed metal exterior. But it’s more than a pretty bottle. It ranked among the top performers in our thermal testing, and Avana adds thoughtful touches. The Beckridge matched the insulating power of the Elevation in the cold test and bested it and the Adventure in the hot test. But its innovative easy-to-drink-from spout, which has a straw and a regular brim for big gulps, lends itself more to colder liquids.