This beautiful and interesting brew method has been around since before 1827. Often when someone first sees a vacuum pot brewer, they think of laboratory chemistry. The common vertically aligned, two-compartment contraption that begins with water on the bottom and coffee on the top certainly presents an image of scientific mystique. Apply some heat and the water moves to the top chamber, through a tube, and mixes with the coffee. Remove the heat and the now-brewed coffee returns to the lower chamber while the coffee grounds remain on top, thanks to a filter nestled in place at the top of the tube.

All of this sounds very complicated. One might even think the name, siphon pot, alludes to how it works. Unfortunately, no siphoning is occurring using this brew method, making the name rather fallacious. Let’s explore just how this brew method works and discover why they should always be called vacuum pots and not siphon pots.

To begin, some physics and chemistry

When enough energy is added to a liquid, the liquid converts into a gas. When enough energy is lost from a gas, it converts into a liquid. When water has reached a temperature of 212°F (100°C), it has enough energy to convert to a gas. For the purposes of this conversation, energy is going to be in the form of heat.

A given amount of liquid takes up less volume than the same amount of it in gaseous form.

The gas phase of an object is less dense than the liquid phase. When the two phases are in the same container, the gas will rise to the top.

A gas that is trapped in a tight space, i.e., one that is under pressure, tries to eliminate that pressure. It will do this by stretching its container (think of plastic wrap on a dish that has been heated on a stove or microwave—it puffs up), moving to a place where it has more space (think of air rushing out of a balloon), or, if there’s enough pressure generated, it will break the container (think of a coffee can or brick that wasn’t degassed before sealing the container). 


Lifting the water

When heat is added to the water-filled lower compartment (usually via flame, contact with a hot surface, or via a halogen lamp), the energy is transferred to the water molecules.

When enough energy is transferred, some of the water molecules convert to a gas (steam).

The steam rises to the top of the lower compartment and begins filling up the empty space.

Once the upper space is filled, the steam begins exerting pressure on the container wall and the pool of liquid beneath it. When the pressure exceeds that of atmospheric pressure (think of air as filling the space between the ground and outer space; atmospheric pressure is the weight of all that air pushing down on the earth), the steam pushes the liquid water away to make more room for itself. The water only has one place to go—up. It is pushed into the upper compartment through the tube. Note that only the steam is at a temperature of 212°F (100°C); both pools of water are much cooler in the beginning and both will require some time to reach an optimal brew temperature.

Did you know?

Coffee causes about 29 percent of people to have a bowel movement within minutes of drinking it.

As a point of interest, this is similar to the way that electric drip machines move water from their reservoirs to a point above the coffee bed. Electric drip machines heat the water at the bottom of the reservoir, converting some of it to steam. The steam then carries the water to the top of the machine where it escapes through the showerhead.


Brewing the coffee

Eventually, most of the water is moved to the upper compartment. It is held there by the steam in the lower compartment. Some water remains in the lower compartment and is a source of new steam. This new steam carries heat to the upper compartment where it will condense and transfer its heat to the pool of water.

Some brewers wait for the upper pool of water to reach proper brewing temperature before adding the coffee while others begin heating the water with the coffee already in the upper compartment. Each of these methods requires its own brewing protocol because of the differences in water temperature, contact time, and agitation. In either method, it is important to remember that the steam will constantly be heating the upper pool of water.

Consequently, it is advisable to lower the heat input to limit the amount of heat transferred to the brewing mixture since the water can become too hot and over-extract the coffee.

The influx of new steam to the upper compartment not only transfers heat but it agitates the brew, speeding up the brewing process. Thus, coffee brewed using this method takes less time than most other brewing methods.


Filtering out the grounds

When the brewer (person, not equipment) decides the brewing is complete, the heat is removed from the lower compartment. As the steam in the lower compartment cools, it condenses back into water. Since the liquid form takes up less volume than the gas, a void is left where the gas was. This void is a partial vacuum that is now at a negative pressure in the lower compartment. The coffee in the upper compartment moves into the lower compartment to equalize the pressure. The filter nestled in the upper compartment permits the water to flow down, but keeps the grounds on the top.

What do we call it?

This method of brewing/brew pot takes its true name from the creation of the partial vacuum: the vacuum pot. I don’t know when or where it began, but this brew method gained the additional, erroneous name, siphon pot. It is erroneous because there is no siphoning occurring in this method, no matter what physical shape the pot takes on (there are other shapes where the two compartments are not vertically aligned). 

A siphon (noun) is usually a tube or pipe in an upside-down “U” shape. However, the “u” is lopsided where one end is much longer than the other. To siphon (verb) is to use the tube to move liquid from a higher location to a lower location, with the liquid moving up the bend and then down to the lower compartment, without the need for a constant input of energy. The short end of the “n” is placed in the higher compartment and the long end in the lower compartment. The process begins with the tube being full of liquid (this is where energy is required), then placed in the starting, higher location. The liquid will flow freely from the lower end of the tube, and, so long as the output end of the tube is below the starting location, the flow of liquid will occur on its own.

Implications for the cup profile

This brew method is a fun presentation of some basic scientific principles. It also tends to be well-regarded as a method of brewing coffee. While the vacuum itself probably doesn’t impart any influence on the taste of the beverage, the method does offer two unique aspects that likely do influence the taste.

First, while the coffee is in the upper compartment brewing, the heat from the rising steam allows the temperature to be held constantly at the proper brewing temperature. 

Other brew methods begin with properly heated water but the water quickly cools as it comes into contact with air and the coffee bed. How this influences the taste has yet to be documented.

Second, there is always a small amount of water that remains in the lower compartment. When the coffee returns to the lower compartment, it mixes with this water and becomes diluted, a process unique to this brewing method. This, too, needs exploration but it seems reasonable to guess that it is analogous to adding a few drops of water to a whisky. 

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