Pump Espresso MachineThe pump espresso machine uses an electric pump in place of the piston in the manual espresso machine.
When you read espresso machines specifications you will often come across things like "15 bar pump" or "18 bar pump". These numbers are irrelevant. The pressure is always reduced to about 8 to 10 bars for the actual brewing. Whether the pump can produce 15 or 18 bars is of no importance.
There are two different mechanisms that the espresso machine's pump can use: vibration pumps and rotary pumps.
Vibration PumpsMost home espresso machines use a vibration pump. Vibration pumps are not suitable for intensive high volume use. They cannot be used repeatedly for a long period because it could overheat and fail. Most home espresso machines have a protective mechanism that shuts off the pump when it is starting to get overheated - that avoids any possible damage to the pump.
The pressure that the vibration pump produces is inversely proportional to the flow of water. What this means in practice is that when you pull a really long shot then the pressure will be considerably lower than when you pull a single shot. It does not make a noticeable difference when you pull a double shot. But if you make for example café crema which is about 8oz then the brewing pressure will be only around 5 bars.
Rotary PumpsRotary pumps are used in most commercial machines and also in some higher end home machines. With the rotary pump the machine is plumbed into your water supply so is saves you the pain of having to refill the water reservoir. The rotary pumps are larger than the vibration pumps and require an electric motor to drive them. Some espresso machines have the rotary pump attached externally. One thing to bear in mind about rotary pumps is that they can fail if they run dry even for a very short time - that's why they need to be plumbed in.
The rotary pumps are usually quieter than the vibration pumps. They are also considerably more expensive.
Does rotary pump produce a better espresso?Some people claim that the rotary pump produces a better espresso than the vibration pump. There are different views of this. Some think that rotary pump makes a smoother crema, others claim that the vibration pump makes more crema. Some people think that the vibration pump gives the espresso more bite, others that they make the taste less transparent.
Yet other people - including myself - think that the pump makes no difference to the taste of a single or double shot of espresso. It makes no difference to the amount or quality of the crema either.
Further ReadingBoiler Types for Espresso Machines
Semi-Automatic Pump Espresso Machine