When the water is heated over a fire, pressure builds inside the vessel, and the only way for it to escape is up the tube, through the coffee and out of the tube in the top. Since the end of the tube is under water, the pressure forces the hot water up through the tube.
There are some disadvantages to a machine like this. The pressure in the system depends on the temperature of the water, and the temperature required to build up enough pressure to force the water through the coffee might exceed the ideal brewing temperature
Nowadays, many espresso machines are equipped with pumps and are fairy simple to operate. A regular pump machine consists of the following parts:
Reservoir:
The reservoir holds the cold water used in the espresso machine. It is not pressure-tight or heated, and it is detachable.
Pump:
The pump draws water out of the reservoir and pumps it into the heating chamber at high pressure.
Heating Chamber:
The heating chamber is a sturdy, stainless-steel structure with a heating element built into a groove in the bottom. The resistive heating element, which is quite simply a coiled wire, is similar to the filament of a light bulb. The coil is embedded in plaster to make it more rugged. The heating chamber also contains a one-way valve that allows water into the chamber from the pump, but not from the pump to the water chamber.
Porta-filter:
The porta-filter is a small basket type, detachable part of the machine that holds the ground coffee. The ground coffee is packed inside the basket in a small removable screen. There are two spouts underneath the porta-filter from which the espresso comes out.
Steam Wand:
The steam wand, commonly used to heat and froth milk, is connected to the heating vessel. When the user puts the valve in the steam position, steam from the heating vessel is released out of the wand and into the milk.
Control Panel:
The control panel normally contains the on/off switch, two indicator lights and a control valve. Separate lights indicate whether the machine is on or off, and if the heating chamber is at optimum temperature. The valve is used to start the flow of water through the coffee in the porta-filter or to start the flow of steam from the steam wand. It also engages one of two micro-switches that control the pump and heating element.
Terms Used
Bar: It refers to the pressure rating used on pump driven espresso machines. 9 bar, the typical accepted pressure for brewing espresso is 8.8 atmospheres of pressure or 130 pounds per square inch. Almost every consumer espresso machine with a vibratory or rotary pump is capable of producing this pressure consistently.
Tamp: Tamping refers to the act of pressing and compacting a bed of loose, finely ground coffee, in preparation for brewing espresso. Different machines require different tamping methods. Steam powered espresso requires a leveling tamp, where as a piston lever, spring lever, and pump espresso requires a more compacting action. Some prefer a heavy tamping action (using 25 or more pounds of pressure), while others prefer a light tamping action (less than 15 pounds of pressure exerted).
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