Why use temperature control?
Coffee tastes differently when brewed at different temperatures. If the brewing water is too cold, the coffee will taste sour. On the other hand, if the temperature is too high, the coffee will taste overly bitter. Obviously, it is quite important to brew at the correct temperature.

So how do we attain the correct temperature?

Thermostats
Almost every espresso machine is equipped with a thermostat that controls the temperature. In the best (but certainly not the most common) case, the thermostat is adjustable. The most common thermostat type is the mechanical one, from the most simple with bimetal technology into more advanced ones with expanding gas and membranes (like the ones in electric stoves).

There are a couple of major problems with all mechanical thermostats though:

• Their "on/off" nature. The heating element has either 100% power applied to it or no power at all (0%), which makes it hard to keep the temperature close to the desired value.
• Sluggishness. When the temperature reaches the desired value, it usually takes a number of degrees until the thermostat switches off, thus the temperature may reach many degrees above the desired value in the tops. The same goes when the temperture falls. It may take several degrees below the desired temperature before the power is turned on again. Even in renown espresso machines, the temperature can swing +/- 10 °C around the desired value.
• Hysteresis. Thermostats normally have a built-in difference in the "turn on" and the "turn off" temperature. This is to make sure that it won't turn on and off all the time. But the side effect is that it is theoretically (and practically) impossible to stabilize at the desired temperature.

Please look here for a temperature curve of typical (or even better than average) mechanical thermostat.
 
Temperature surfing
Many (home-) baristas have learned to live with the shortcomings of mechanical thermostats by using a technique called "temperature surfing". Basically, the method involves the following steps:

1. The water pump is turned on until the boiler has turned on (by the thermostat), often indicated by a lamp on the espresso machine.
2. The water pump is stopped and the barista waits a pre-determined amount of time (based on experience) to let the water tank's temperature fall to the desired value.
3. The espresso is brewed.

The idea of the above procedure is to make sure that the starting temperature of the brewing process is always the same. The downsides of this method are obvious:

• It is cumbersome and takes a lot of time.
• You have to do the process all over again for the next cup.
• There is no guarantee that the temerature is accurate anyway.

There is a variant on temperature surfing, suggested in the home-barista.com forum, where a digital thermometer is attached to an E61 group head to show when the temperature is right. Here is a direct link to that forum post.

Electronic Temperature Control
There are several techniques for electronic temperature control, and one of the more sophisticated ones is called PID control. A PID controller continously measures the actual temperature and compensates the difference from the desired temperature by applying just the right power on the heater, depending on how big the difference (i.e. error) is and how fast the system responds to the applied power. By using this technique it is possible to keep the temperature within +/- 0.1 °C in a well-tuned system!

You could compare PID control to driving your car. Let's say you want to attain and keep a certain speed. First, you might apply full gas (100%) but then as you are approaching the correct speed, you will probably reduce the gas a bit rather the turning it completely off (0%). You probably also feel a bit in advance how much you need to reduce the gas. When you approach an upward slope, you will probably also know when to apply more gas (a bit in advance), not to lose speed.

This means that you, as the driver, know the "process response" and continously calculate and apply the correct amount of gas to keep the desired speed.

A PID controller used for controlling the temperature sort of works in the same way. When you tune a PID controller, you tell it what the process response is. Then it knows exactly how much power to apply to attain and keep the desired temperature.

Read more about PID control here

Note: An affordable PID controller, specialized for espresso machines, will be developed on this site in the future.