Buying guide for Best large thermoses
Need to keep a pot of coffee hot for hours during a long workday? Heading up to a campsite on a chilly morning? Chances are you’ll use a large thermos or one of the thousands of variations on the original Thermos model. From the rugged 64-ounce models used at job sites or hunting camps to the sleek carafes perched on conference tables, the large thermos is an indispensable part of everyday life.
When considering a large thermos, you probably have a good idea of the style you’re looking for. The biggest concern is how well the thermos will insulate the contents and how long the liquid will stay hot or cold. Durability is a factor too. Whether camping, transporting refreshments to a soccer match, or setting up for a party, a large thermos has to stand up to frequent use and everyday bumps and drops.
Key considerations
Large thermoses are a must for family camping trips, parties, work sites, and anywhere that several people gather and need quick access to hot or cold drinks. Despite their size, most thermoses are portable (though full ones aren’t always easy to lug from one place to another).
These large containers hold at least 1 quart (or liter) of liquid, such as coffee, hot chocolate, soup, sports drinks, or iced tea. If you’re spending the day in a location where access to a microwave or refrigerator is limited, a thermos is an indispensable companion.
Considering how popular and essential thermoses are to everyday life, you might wonder exactly how they maintain temperatures so well.
How it works
All thermoses rely on a vacuum to slow the transfer of heat or cold from inside to out. Think of hot coffee in a paper to-go cup, for example. Set the cup on a table in a 70° room, and within a few minutes, the coffee will go from boiling hot to lukewarm. The heat is lost through the open top of the cup, of course, but even with a plastic cover, the heat loss is almost as rapid. That’s because paper and thin plastic are not great insulators.
Heat transfer happens all the way down to the atoms that make up the vessel holding a hot or cold liquid. Researchers found in the late 1800s that certain materials like ceramic (including glass) helped slow the transfer of heat from one place to another. They also found that air pockets in certain materials also slowed heat transfer because there were fewer atoms in these pockets.
A true vacuum is one that contains no atoms. This would be the ultimate insulator, but creating a true vacuum for a thermos would be really expensive. However, a partial vacuum can be created without much trouble.
How it’s made
Large thermoses are made in the same way as their smaller lunch box counterparts. An inner flask holds hot or cold liquids or foods. It’s surrounded by an outer container, or shell. The two containers are sealed together at the top of the thermos. Air is removed from the space between the two containers, creating a partial vacuum, and the outer container is sealed at the base.
This two-container design dates back to 1892 when inventor Sir James Dewar created the first vacuum flask. He placed one flask inside of another, joined them at the top, and removed as much air between the flasks as possible, creating a partial vacuum.
Dewar wasn’t interested in the commercial application of his invention (he was researching cryogenics), but two glassblowers from Germany saw its potential, and in 1904 developed the first commercial vacuum flask, calling the product a Thermos. Licensed versions of the Thermos were refined over the years, and the product quickly became a familiar name, as well as quite sought after by explorers, construction workers, and many more.
To further improve insulation, thermos liners today incorporate other elements and layers. For example, a reflective material coats the interior flask walls to reduce heat transfer via thermal radiation. An additional insulating layer may be included on the other side of the vacuum, against the outer wall of the thermos, to further slow heat transfer and help prevent condensation on the outside of the thermos.
Weak points
Mouth: Heat transfer happens more rapidly where the inner flask makes contact with the outer shell. In most cases, that’s the top of the thermos where the two components are joined. Manufacturers reduce this heat loss by adding insulating materials to the thermos cap.
Spacers: Large thermoses require much wider and longer vacuum flasks, which need to be supported by spacers between the exterior shell and interior flask. These spacers are an additional source of heat loss, which means large thermoses aren’t as effective at maintaining temperatures as smaller thermoses.
Did You Know?
Vacuum flasks were first developed to hold liquefied gases, such as liquid nitrogen, at the required extremely low temperatures.
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Features
Large thermoses that hold a quart or more of liquid like coffee, hot chocolate, or sports drinks have many features. Some are intrinsic to the design and function of the thermos, while others are fashionable touches. A few common features include the following:
Lid
Dispenser lid: Countertop coffee thermoses or carafes boast this feature, which incorporates a lever and straw to siphon liquid up to a spout.
Pour top: This feature allows you to pour liquid through a partially unscrewed lid, thereby keeping the remaining liquid hot.
Drinking cup: Classic thermoses have an outer lid that doubles as an insulated drinking cup. It is much easier than drinking directly from a large thermos.
Shape
Wide mouth: This spans the entire diameter of the thermos and is a good option for making hot soups and stews easier to serve.
Contours: Innovative thermos shapes cater to specific situations and are increasingly showing up in the marketplace.
Handle
The larger the thermos, the more likely it is to have a carrying handle, either on the side or the top, so it’s easier to move.
Durability
Thermoses designed for camping, hunting, or work sites have a steel outer shell that may have plastic or enamel molded onto the exterior. This coating resists scratches and small dents, and the steel protects the inner flask if the thermos is dropped.
Did You Know?
A thermos is sometimes referred to as a Dewar bottle after the scientist who invented the vacuum flask.
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Accessories
Coffee filter cone: Manual Drip Coffee Filter Cone
Brew coffee directly into your large thermos or carafe with this extra-large filter cone that fits most carafe openings.
Milk frother: Bodum Bistro Milk Frother
A must-have addition to the coffee aficionado’s thermos dispenser setup is a way to quickly heat and froth milk to add to that cup of joe.
Insulated coffee mug: Congela Insulated Coffee Mugs
Why let hot liquids get cold after pouring them out of your large thermos? Pour them straight into one of these insulated 18-ounce mugs instead, so your soup or coffee will stay hot even longer.
Pricing
Inexpensive: Serviceable large carafes and thermoses can be found for as little as $15 to $25, but you may sacrifice some heat retention, and they may not be very durable.
Mid-range: For a few dollars more, about $27 to $59, you’ll find a greater range of large thermos styles with an impact-resistant outer shell.
Expensive: Designer brands rule the $61 to $95 price range, but you’ll also find unique thermos designs like insulated casserole holders and campsite-tough French press carafes.
Did You Know?
Thermos was once a trademarked name, but “thermos” is so widely used that it is now a generic name in several countries including the United States.
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Tips
- Preheat or prechill your large thermos before filling it with your beverage. Fill it with hot water or ice and let it stand for at least 30 seconds. This helps reduce heat transfer while filling the thermos with your hot or cold beverage.
- Be careful not to drop your thermos. Dropping a carafe or thermos that contains a glass vacuum flask isn’t as risky as it was in years past, but try to avoid it. Use the thermos handle when shifting position or transporting a filled container to reduce the chances of dropping it.
- Check for a tight seal. To ensure a tight seal between the cap and the mouth of your large thermos, tighten the cap and then loosen for a moment before you tighten it again. This releases any trapped air that would keep the cap from being fully tightened.
- Clean your large thermos after each use. Do this as soon as possible after emptying it, and pay attention to cleaning the insulated cap and dispenser spout too.
FAQ
Q. Are thermoses limited to the original canister style?
A. No. In fact, in the past decade, thermos styles have branched out, and you can now show up at cookouts or campsites with a tapered-neck, growler-style thermos, a stylish French press thermos, or even a kettle-shaped, insulated casserole thermos.
Q. How long will coffee stay hot inside a 2-liter carafe with a dispenser spout?
A. That depends on the brand and the amount of insulation between the inner flask and outer shell of the carafe. Dispenser spouts should also have some degree of insulation because the most heat loss occurs at the mouth of the carafe. Check the manufacturer specifications, which should list the average time that hot liquids will remain hot within the carafe.
Q. Does preheating the thermos before adding a hot liquid keep it hot longer?
A. Yes, it really can help the thermos maintain temperature. And prechilling works for cold liquids as well! For one thing, when transferring hot liquid from one container, like a coffee pot, to the thermos, preheating helps reduce the amount of heat lost from the liquid during the transfer. To preheat your thermos, pour boiling water into it and let it sit for 30 seconds to a minute. Pour out the boiling water and replace it immediately with the hot liquid of your choice.
