Many of you use and love thermos flasks because of the convenience they offer, especially if you want to eat home-made meals while at work, drink hot coffee during the entire shift, or if you need a refreshing drink during a hot day.

We notice that thermos flasks help a lot with keeping warm drinks/soups hot and cold drinks cold for more than eight hours, but not everyone knows how does a thermos flask work. Believe it or not, the process and the science behind it is very interesting and useful to know.

Inside a thermos flask

A thermos flask is made of a smaller container that is put inside of another, bigger one. These two bottles are connected at their neck. What happens with the space between the two flasks? That’s where the vacuum is.

The vacuum could be considered the crucial part of the thermos flasks and the real answer to the question how does a thermos flask work.

Under the smaller flask, there’s usually an insulated material that serves as a support, so the neck of the flasks doesn’t get damaged. That support is insulated because otherwise, it would affect the vacuum and therefore, the temperature of the liquid inside the smaller flask.

The smaller bottle is coated with either glass or stainless steel if the flask is more durable. After the vacuum, there’s another coat of glass or stainless steel with a reflective, silver surface.

Then it’s possible to find an additional insulation layer. Not every thermos flask will have this layer, but the ones that do have it will retain the temperature of the liquid for an even longer period of time.

The bigger bottle is usually made of plastic but it can also be made of stainless steel if you need a really durable vacuum flask. On top of the flask, there’s a screw-on stopper that stops the hot drinks from getting cold via convection.

Whenever you open the stopper, the heat will escape a bit but your beverage will still remain hot for a really long time.

As you can see, even though a thermos bottle looks very simple, and all you have to do is choose the beverage, pour it inside the flask and close it with the stopper, thermos flasks are actually made of many layers that are there for many different reasons.

Why are all these layers important?

To answer this question, we must talk about what causes the hot beverage (or any hot liquid) to become colder during a relatively short time.

As you may know, heat is a type of energy. This energy can travel in three ways – conduction, convection, and radiation. That’s why your beverages in other containers or conventional cups lose the heat and get cold in a matter of minutes.

All of the layers, together with the vacuum around the smaller container in the flask, make this movement much slower, and that’s why the liquid inside can stay warm for more than 10 hours.

The screw-on stopper stops the convection, or in common words, the hot air. If you’ve wondered why you can see the smoke while the hot beverages get cold, the answer is the convection.

The heat is moving up, and you can easily see it at least in the beginning while the difference between the temperature of the drink and room temperature is big.

Conduction is possible when things touch, and the heat is transferred to the other object. For example, when you leave a cup with hot coffee on the table and the table becomes warmer as well.

That’s why the vacuum is essential – it slows down the conduction. Since there’s no such thing as a complete vacuum, this should actually be called a partial-vacuum, and that’s why the heat, although slowly, eventually disappears.

What stops the radiation? The reflective layer after the vacuum reflects the radiation that’s trying to leave the flask and makes it go back to the beverage, keeping it warmer for hours.

What about cold drinks?

You’ve probably noticed that even though you hold the thermos flask in your hand, and even if it’s in the sun for a long time, the beverage inside stays as cold as it was when you poured it inside the flask.

Not only do these layers and the vacuum stop the heat from leaving the flask, but they also stop the heat from entering the flask as well.

The most important thing is, again, the vacuum that stops the heat transfer via conduction (touch) from the outside world to the beverage inside. This is especially important when you’re holding the flask in your hand, or if your flask is packed inside your bag and surrounded by other things.

In that case, the flask touches another surface, and if there weren’t for vacuum, the beverage would become less chill since the heat would be transferred.

When it comes to radiation from the sun that could make the beverage warmer, the same reflective surface will stop the radiation from affecting the temperature of your drink.

Convection isn’t such a “threat” when it comes to cold drinks, but the hot air can still affect the temperature a bit, and therefore, the screw-on stopper is very important.


As you can see, it’s not too complicated to understand how does a thermos flask work. Even though not everyone will completely care for the science behind it, it can be understood enough to appreciate the convenience of thermos flasks even more.

About the Author James S

{"email":"Email address invalid","url":"Website address invalid","required":"Required field missing"}