Choosing an espresso machine is perhaps the most important buying decision you will make. It will be used to create most of your products (and most of your revenue). If you business is built around espresso drinks, it stands to reason that you should find the best espresso machine you can.
You may have heard that a well-trained barista should be able to produce excellent drinks on any espresso machine. While this is true to an extent, "well-trained" is a very necessary component of the statement.
Lets consider the job of an espresso machine: First it must force hot water through a bed of ground coffee at high pressure. Second, it must provide steam pressure for heating andtexturing milk for lattes, cappuccinos, and other milk based drinks. It all sounds simple enough, but coffee and milk are complicated substances. In order to achieve consistently excellent results, the pressure and temperature of the water used for extraction must be tightly controlled. Incorrect temperature is a common cause for bitter or weak espresso. A change as small as 1 degree can have anoticeable impact on the quality of espresso.
The availability of steam pressure, as well as the configuration of the valve, wand, and tip also have an impact on the end product.
There are essentially two "styles" of espresso machine. Heat Exchanger, and Dual Boiler.
Heat exchanger machines have a single boiler which is used for creating steam and heating water for brewing espresso. Steam is pulled from the top of the boiler. Water for brewing is heated with a heat exchanger, which is essentially a tube running through the boiler. Cold water is pumped into the tube, absorbing heat as it makes its way to the espresso on the other side. Ideally, it has hit the target brew temperature at that very moment.
Brew temperature is adjusted by changing the pressure of the boiler. Since temperature and pressure are directly related, raising the pressure of the boiler raises the temperature of the water coming out of the heat exchanger.
Technology exists to ensure temperature stability under constant use, most notably the e61 grouphead. The problem is that if the machine sits idle for any length of time, the temperatures of the water in the heat exchanger and boiler equalize. A steam boiler at a common setting of 1.33 bars will run somewhere around 250 degrees F (depending on atmospheric pressure). Pulling a shot of espresso under these conditions would mean subjecting coffee to temperatures well above boiling...not good for any blend. In order to work around this flaw, skilledbaristas develop flushing routines to bring the brew water back down to the target temperature. The amount of flushing required is dependent on idle time, and varies from machine to machine.
Some espresso machines employ two (or more) boilers. The steam boiler is controlled by a pressurestat, and is designed for creating steam for milk-based drinks, and as a hot water source for tea and americanos. The brew boiler is controlled by a thermostat or PID, and is used only for brewing espresso.
Dedicating boilers to steam production and espresso brewing allows finer control and better temperature stability. La Marzocco espresso machines use a saturated group, which keeps the group and portafilter hot at all times. Since brewing water is never super-heated, the barista does not need to worry about burning the espresso: the procedure for making a drink is the same regardless of idle time. Additionally, since steam boiler pressure no longer dictates brew temperature, the two can be adjusted independently: thebarista no longer has to sacrifice milk texture for espresso flavor.
The La Marzocco GB5 takes temperature one step further: it uses a PID controller to maintain brew temperature, and uses pre-heated water to fill the brew boiler.
The PID controller uses complex algorithms to keep the temperature exactly at the set-point by operating the heating element at the proper intensity. If you're interested you can read about the differences betweenPID and thermostat temperature control here.
Since the brew boiler does not contain any empty space, any water exiting the boiler must be replaced. In fact, the water coming into the boiler effectivly pushes the brewing water out through the puck of grounds. On almost all machines, this inlet water is cold, causing temperature fluctuations especially under heavy use. The GB5 fixes this problem by running the inlet line through the steam boiler first, which preheats the water. They even went so far as to mix a bit of cold water back in so that it's not too hot.
In an espresso machine we have an electric heating element in water and we are trying to control a given setpoint (maximum achieved temperature).
The THERMOSTAT is an electromechanical switch trying to control the on/off cycle of the heating element in water. The thermostat can turn off the heating element at the exact setpoint, but because the heating element is still hot (it can not cool instantly) and continues to heat the water causing a temperature raise above the ideal set point,( the problem). So what to do, position the setpoint lower so it turns off sooner then rises to the desired maximum, oh great now at least the water did not get to hot but it has to cool down to the lower setpoint before it will turn on. The best we can do now is to have a thermostat with a very narrow band width (the difference between on and off setpoints) and hope for the best in our cycle of heating and cooling.
PID can be described as a set of rules with which precisely regulates a closed-loop control system. In our heating element in water example the PID predicts when to control the on/off setpoint making corrections so the heating element does not under or over shoot the desired temperature.
How PID works without the math! Closed loop control system means a method in which a real time measurement of the process being controlled is constantly feed back to the controlling devise to ensure that the value which is desired is, in fact, being realized. The mission of the controlling device is to make the measured value, known as the process variable, equal to the desired value, usually known as the setpoint. The best way to accomplish this task is to use the control algorithm known as PID.
In its basic form, PID involves three mathematical control functions working together, the most important of these, Proportional Control the P, determines the magnitude of the difference between the setpoint and the process variable (known as the error), and then applies appropriate proportional changes to the control variable to eliminate error. Integral Control the I, examines the offset of the setpoint and the process variable over time and corrects it when necessary. Derivative Control the D, monitors the rate of change of the process variable and makes changes to the output variable to accommodate unusual changes.
Each of the three control functions is governed by a user defined parameter. These parameters can be adjusted to optimize the precision of control. The process of determining the values of these parameters is known as PID Tuning or (BIG MATH)!