How to choose a flask: for hiking, work and kids

1. How a flask retains heat

A high-quality flask has two stainless steel walls with a vacuum between them. A vacuum doesn't conduct heat, so the drink cools mainly through the neck and lid.

A narrow neck retains heat the longest because the least heat escapes through it. The choice for drinks and a full-day hike.
A wide neck is easy to fill and wash, and you can eat from it. However, it retains heat for a shorter time.
The lid and stopper matter more than they seem. A stopper with a push-button or twist valve lets the drink out without having to open the flask completely. Heat only escapes briefly this way.

The specification “retains heat for 24 hours” is measured by the manufacturer under certain conditions, most commonly when fully filled and at room temperature. In freezing conditions and with frequent opening, every flask retains heat for a shorter time.

2. What size to choose

0.35–0.5 l: one person for a shorter trip, to work or to school. Fits in a backpack pocket.
0.75–1 l: a full-day hike, ski touring, cross-country skiing or two people for a shorter trip.
1.2 l and more: a family or a full day in freezing temperatures. Expect a higher weight; a full one-litre flask weighs over a kilogram.

Tip: a full flask retains heat longer than a half-empty one. It's better to get a smaller one that you will fill completely.

3. Food flask

For soup, porridge or pasta, a food flask with a wide neck is suitable. A spoon fits inside and it's easy to wash. Food in it usually stays warm for several hours, a shorter time than a drink in a narrow-necked flask.

4. For kids

Parents often ask on forums whether they can carry milk or ready-made baby food in a flask. In a warm environment, bacteria multiply quickly. It is safer to carry hot water in the flask and prepare the milk or porridge on the spot. For a child's flask, choose a robust lid that the child can open themselves, and a smaller volume so they can carry it.

5. Is a more expensive flask worth it?

The difference between a cheap and a more expensive flask is usually in three things:

the seal of the lid, so the flask doesn't leak in the backpack even when lying flat;
the quality of the stainless steel and welds, which determine how long the vacuum lasts;
a stopper and seal that can be taken apart, which can be cleaned so the flask doesn't smell.

When a flask stops retaining heat after years of use, the vacuum has been damaged, most often after a drop. It cannot be repaired.

6. To keep it warm for longer

Preheat it. Pour hot water into it for a few minutes, pour it out, and only then pour in your tea.
Fill it to the neck. The air at the top absorbs heat.
Open it only for a moment and close the lid immediately.
In freezing conditions, put it in your backpack among your clothes, not in a side pocket.

7. Cleaning and care

Rinse the flask after every use and leave it open to dry.
Take the stopper apart and wash the seal separately. Odour and residue linger exactly there.
You can remove tea and coffee residue with bicarbonate of soda or citric acid dissolved in hot water.
Only put the flask in the dishwasher if the manufacturer states it is safe to do so.
Do not pour carbonated drinks into it. Gas in a closed flask increases the pressure.