Row of thermos

When buying a vacuum flask there are several factors that are useful to take into account to help you make a decision. While you cannot actually “borrow” a flask to take home for a test, there are things you can look for and questions that you can ask to ensure you make a good buy. Once you get your new flask home, you can test it to ensure that the claims made about the product are true. If not, then hopefully you bought your flask at a store with a solid returns policy.


1. Workmanship and Finish

This is the first thing to check when selecting a flask. Eyeball the flasks available. Workmanship should demonstrate good quality. Flasks should be free from any manufacturing defects such as rocking bottom, sharp edges, uneven neck and brim and have a uniform shape and size. The stopper and cup should loosen and tighten easily, and fit well. Also consider aspects of the flask like carry handles and bags, and the quality of the plastics and metals used in construction.


2. Brimful Capacity (Volume)

The capacity of the flask when filled to the brim should correspond with its nominal capacity. If the capacity claimed is 500ml, its actual capacity can be more or less by about 10% and still be acceptable i.e. 450-550ml. You may not be able to test this in store but do so when you get your purchase home. If there is a salesman about you can perhaps test his commitment to the sale. Either ask him how the capacity measures up, or do what a very pushy friend of mine did, and have him get some water for a test!


3. Leak Proof

This is another test that you can do at home, or if you find an accommodating salesperson. Half fill the flask with water and lock the stopper. Shake the flask vertically for about a minute. Any leaks fails the test.


pouring flask 24. Heat Retention and Thermal Insulation

When you get your new flask home you should immediately put it through this test. Not only to make sure you have bought a good product, or because you need to know what to expect from the flask, but also because the anticipation is fun. Fill your flask with water that has been brought to the boil, and then left to stand for 5 minutes. Check the water after 1, 5 and 24 hours. If you have a thermometer of some kind, that can read temperatures around 100 Celsius/220 Fahrenheit, then use this to make measures at the start and after each time period. If you have no thermometer then you will have to make your best judgement.

Thermal insulation is also a part of this test. Since the walls of a flask are joined a the neck, some heat will always escape. If you feel the outer surface of the flask during the heat retention test, especially with stainless steel flasks, it might be warm to the touch. This is an indication of how much heat is escaping at the neck. If it feels hot, then it is unlikely that the flask will pass the retention test as too much heat is passing through the neck into the outer flask. This is an indication of cheap or defective materials and construction.


5. Thermal Shock

This test is only necessary for flasks and jars with glass liners and helps to determine if the liner is in any way defective. You may want to avoid this test as there is a small chance of breakage (which is kind of what you want to test). Pour water at about 27 Celsius/ 81 Fahrenheit into the flask. If you don’t have a thermometer, that is about what we would call lukewarm if you felt it. Seal and let it stand for 5 minutes. Now fill with near boiling water, seal and let it stand for 5 minutes. Repeat again with water at 27C/81F. Check carefully for damage. The glass flask passes the test if it remains undamaged.


6. Sensory Tests

You can also conduct a sensory test to assess the taste, flavor and overall acceptability of a hot liquid like tea or coffee poured into the flask. Pour hot tea into the flask and then drink some after 30 and 60 minutes while paying attention to any changes in the taste. It can happen with stainless steel flasks that the flask imparts a slight metallic taste, but this is usually before the flask is well used. There may be slight taste changes but they should not be unpleasant. If they are, you should probably return the flask and bring it to the stores attention as it could indicate a manufacturing problem.