About Stainless Thermos
Hear the word “thermos” and an image comes to mind of construction workers on lunch break. But a thermos has more to offer than keeping coffee lukewarm on a job site. These containers also keep things cold. The best ones do so for longer and have other features, such as wide-mouth openings that make them easier to fill, clean, or add ice. They’re lightweight, durable, and more leak-proof than those that came before. We put a wide crop through the testing wringer to determine which tick those boxes.
How Does a Thermos Work?
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 don’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 the 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.
How We Tested
We put these thermoses to the test to see which will keep your hot liquid hot the longest, and which will protect that nice chilled drink from heating up.
The hot test: We filled each thermos to the very top (as high as we could without spilling) with 190-degree Fahrenheit water. We then closed the lids and let them sit for 24 hours. Before opening, we shook them to distribute any hot water that may have risen to the top, then tested each with the same thermometer that measured the starting temperature. We then emptied the bottles and let them sit for three hours so that all parts of the bottles would return to room temperature, without retaining any heat from our earlier tests, before continuing with Part 2.
The cold test: We filled each bottle to the very brim with chilled water, all thermometer-confirmed at a cool 46 degrees. Again, we sealed up the bottles and let them sit for 24 hours. The next day we shook them, opened each, and checked the temperature.
Using these measurements, we calculated the heat loss for our hot test and heat gain for our cold test. For both, a smaller number is preferred. We also used these bottles as you will, seeing how easy they are to drink from, close, open, and toss into a bag. We wanted to see how well they’d fit into our life to know how well they’ll fit into yours.
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