We hear the terms all the time: “Vacuum Insulated, Vacuum Sealed, Airtight Construction”, but what do they really mean? Vacuum insulation can be applicable to a large variety of products, ranging from water bottles all the way to airplane cabins. When it comes right down to it, vacuum insulation is essentially a space within two walls of metal or other material that is void of air, allowing no sort of heat transfer to take place between the inside of the vessel and the wall between it and the outside.

A well designed Vacuum Bottle can maintain the temperature of its contents for an extended period of time, but only if it is properly constructed. In order to understand just how that works, we must first understand how heat and the transfer of heat works.

Double Wall Water Bottles use two steel walls with a vacuum between them in order to keep the contents of the bottle at the same temperature or close to the same temperature when they were first stored.

Conduction

Although for many the concept of conduction brings forward thoughts of electricity and wires, conduction is also a term associated with the way heat moves through matter. Conduction ONLY occurs when two things touch. An example of conduction would be holding a piece of ice. Before long your hand becomes cold, and the ice starts to melt. This is because your hand transferred heat into the ice cube, and the ice cubes temperature rose because of it, causing it to start melting.

Well how does this apply to a stainless steel water bottle? Well, every physical object is comprised of molecules, as well as the air around us. If we are in a cold room and have a hot cup of coffee sitting on a table, the heat will constantly be transferring from the cup of coffee into the cold air and even the table beneath it. An insulated stainless steel water bottle puts a barrier between the two different temperature molecules and “seals out” the heat transfer.

The vacuum is the actual barrier, since no air molecules or any molecules are present, no heat transfer can take place, allowing the contents of the bottle to stay at the same temperature or close to it for extended periods of time.

Convection

While conduction can only occur when molecules are touching one another, convection occurs because of the movement of liquids or gases. Now hang on, I know that may sound a bit complicated! However, convection is actually the most simple method of heat transfer, and one of the more common methods that you’d encounter during day to day activities. An everyday example of convection is boiling water in a pot on the stove. As the water begins to warm, it rises. Remember, warm molecules move quicker than colder molecules, and will always want to naturally rise up. While the warm water molecules are rising, the colder water molecules are being pushed to the bottom, where the warmer water used to be. This process continues until all the water within the pot begins to boil and is the same temperature.

As pictured, the heated water rises to the top while the colder water moves to take its place near the heat source.

When it comes to a Stainless Steel Water Bottle, Heat transfer by convection is prevented two ways:

1. Assuming the bottle is properly constructed and insulated, and not placed on top of a heat source, the temperature of the contents should be the same in every part of the bottle, ensuring convection does not occur. Contents that have a constant temperature across the entire surface area of the bottle will be the least prone to convection. If a liquid is stored in a bottle but the bottle is improperly constructed and not insulating fully, a temperature difference will occur in the liquid. Once this happens, convection takes place and it’ll only be a matter of time before the liquids temperature changes.
2. A Stainless Steel Bottle that has a high-quality, well designed, and effective seal for its cap or lid prevents any sort of difference in heat between the top and bottom of the bottles contents. If the lid on a stainless steel water bottle is not making a good seal, then there will be a temperature difference within the bottles contents, causing convection to take place.

Radiation

Compared to conduction and convection, radiation is the oddball out. Radiation heat transfer is heat given off by hot molecules. Sometimes, we can see radiation in the form of light, the most common example being sunlight. While most Vacuum Bottles are effective at preventing loss of heat from conduction and convection, only the more high quality ones can prevent loss of heat through radiation. This is because in order to reflect radiation waves back into the liquid, the internal wall of the water bottle must be professionally polished and buffed.

Below is an example of three grades of polished steel:

The inner wall of steel on the far right will do a much better job at reflecting radiation from hot liquids back into the liquid rather than into the bottle itself.

When it comes down to choosing a stainless steel bottle or thermos, take this into consideration:

• Is the bottle made from a high-grade of steel?
• Are the inner walls of the container polished or not polished?
• Does the bottle have a powder coating or paint on the outside?
• Is the bottle advertised as being double walled and vacuum sealed?
• What type of lids and caps is the bottle compatible with?
• Do these lids and caps make a tight and visible seal?

Not every bottle or thermos is designed or manufactured the same, and many manufactures cut corners when it comes to developing and testing bottles. Search the web and reference buyer reviews before making your decision.