Tuesday, 29 April 2008

The container originally developed by Sir James Dewar is officially known as a Dewar flask, but most of us call it by a trademarked product name "thermos". These containers slow down temperature change using the principles of convection, conduction, and radiation. Learn more about finding a good thermos.


The Basics

  • Thermoses all use the same insulating method to limit heat transfer and keep the contents hot or cold.

  • The interior of a thermos is typically made of glass and plastic or stainless steel. Both do the job, but:

    • The glass interior is cheaper and is better at reducing the conduction of heat, but can break easily if dropped. Look for one with a thicker inside wall.

    • Stainless is more durable, but more expensive.

  • A thermos with a large opening is better for soups and stews. A narrow mouth is better for beverages, and holds heat in better since there’s less room for it to escape.

  • The cap can make a big difference to how well the thermos works. Look for a deep, thick cap that reaches into the flask to prevent heat or cold from escaping. The deeper the threads on the screw cap, the better the seal.

Other Considerations

  • Pre-heat and pre-cool your thermos before you use it. It will work better. Fill it with boiling or ice water for 5 minutes first.

Be Aware

  • Never put carbonated beverages in thermoses due to pressure build-up

  • Never put a thermos in the dishwasher or microwave, which can both wreck the seal.

  • Don’t clean it with a scouring pad.

  • Don’t submerge a thermos in water.


We tested our thermoses in the extreme by filling with boiling 100°C water, burying them deep in the snow, and measuring the temperature after time. We also took another set to the slopes for some après ski, hot chocolate refreshment. We tested:

  • Aladdin (glass with snap-up spout): $9
  • Thermos Brand (glass with a stash top for condiments and a twist and pour spout): $11
  • Nissan by Thermos Brand (stainless steel): $30
  • Starbucks (stainless steel with push-button cap): $42

Buried in Snow Test

After 3 hours we dug up our thermoses to measure the water temperature:

  • Aladdin: just below 59°C
  • Thermos: 60°C
  • Nissan: 70°C
  • Starbucks: 70°C

Usage Test

  • All were easy to pour.

  • The ones with screw nozzles and push buttons were our favourites.

  • We also liked to drink from the larger cups, even though the smaller ones were more convenient to pack.

  • The stainless steel thermoses kept our hot chocolate hottest.


The Nissan by Thermos has a deep screw cap (for better heat retention), is durable stainless steel, is not the most expensive, and kept the liquid hottest.

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