PID vs. Pstat

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Many of us have heard that in order to make good espresso you need an espresso machine fitted with a PID controller. The real question is, why? Is it true that a PID is the only way to achieve the temperature stability for a great espresso shot? To understand the advantage of a PID system, let’s take a look at both systems to understand how they work.

A pressurestat is a mechanical device that uses the temperature to push on a rubber diaphragm and open a switch. In practice, this means that the expanding air from the heated boiler ‘pushes’ open a switch, turning off the heating element and allowing the system to begin to cool. The pressurestat can be adjusted to open at different pressures depending on the machine in question but is typically set to open somewhere around 1 to 1.2 bar. Now the machine begins to cool via the natural process of radiation, losing heat to the surrounding environment. This continues until the pressure on the rubber diaphragm is no longer enough to hold open the switch. The switch closes, completing the circuit and turning on the heating element. Now the espresso machine heats up again until the temperature can again ‘push’ open the switch.

This whole process puts the espresso machine in a constant cycle of heating and cooling - this isn’t a secret. Espresso machine manufacturers are well aware of this heating cycle and have many innovative ways to try and increase the temperature stability of their espresso machines. These can range from adding more metal to the group head to increase the thermal inertia of the brew unit, to adding additional water flow to allow water to be mixed in a controlled ratio to help maintain an ideal temperature for extracting espresso. These technologies have varying levels of success in temperature stability but are, inarguably, necessitated by the pressurestat. Good or bad, there you have it.

The PID system solves for the cyclical heating and cooling a pressurestat causes by using mathematical algorithms to predict and counteract the thermal inertia the espresso machine experiences. If you run the hot water tap, the PID system will know that it will be taking in cool water from the fill valve and will begin to micro-fire the heating element, cycling it on and off hundreds of times a second, in order to heat the machine before it has had a chance to cool. The PID system uses a logic panel to identify variation from a set point. This means that when an espresso machine is heating up and approaching the target temperature, the heating element will ‘reduce power’ (increase the “off” time and decrease the “on” time) to less than full power so that it does not over shoot the target temperature. In a perfect world with a perfect espresso machine, this would mean that the temperature remains perfectly constant and you get the right temperature every time. Unfortunately, due to restrictions of the real world, we still see some slight variation from events like boiler filling or loss of steam pressure given that the system cannot always flash-heat water as quickly as it would like (due to heating element wattage restriction). These events can cause dips in temperature, but they are far less pronounced than they are in a pressurestat system.

If a heat to time graph for a pressurestat produces a sine-wave, then the introduction of a PID system would reduce the amplitude of that wave. Said another way, the temperature would always remain closer to the set point by a significant factor. Tighter temperature control should mean better espresso more of the time.

The question remains, is a PID controller a requirement for good espresso? In short, no. A PID system is not a requirement, though it is a boon. The tighter control provided by a PID system will make it easier to guarantee that you will get a perfect shot each and every time - it is just science. However, there are many things that baristas have been doing for eons to produce fantastic espresso using mechanical espresso machines.

One of these methods for creating consistent espresso is to run hot water out of your espresso machine until you see the heating element come on. Then, count up from there and pull your shot, say, ten seconds after your element came on. Since your element always turns on at the same temperature, and your machine should heat at the same speed every time, this will allow you to learn your machine and come very close to hitting your temperature mark with enough practice.

For many people, a pressurestat remains a good method to use to control the internal temperature of your espresso machine. For some, who have put in the time and can taste the difference, a PID system will improve your experience by either simplifying your process to get a perfect extraction or by increasing the precision with which you operate your espresso machine.

All in all, your grind setting, coffee freshness, ability to get a good tamp and cleanliness of your espresso machine will have far more effect on the taste of your espresso than will the precision with which you are able to control temperature. If you are new to the espresso world, take your time and enjoy learning about all of the areas mentioned above before you take on mastering the precise temperature you use to extract that sweet brown nectar we call espresso.