Ultimate PID: Gas

By Michael Teahan

Gas is a rare thing for espresso machines, but it wasn’t always. It should be pretty clear that I appreciate old stuff. Not merely because I have a lot in common with an original E61 Faema —technology was more organic, a simple straight line from challenge to solution. Sometimes we get so far away from our origins we lose sight of how things were designed to work. The solution now is to throw a processor at something and connect it to the internet.

How we heat espresso machines exemplifies this perfectly.

Why Gas?

Modern espresso machines developed after the World War II were getting pretty popular, pretty large, and electricity wasn’t cheap. Especially when you needed a lot of it. Most bars and cafes from before the war already had gas lines in place from the old vertical machines and changing to a machine fitted with gas was pretty simple. While machines were always fitted with heating elements, the gas kits were pretty standard. The lever machine in our shop has a gas kit installed. We have never used it, —it just came with it. You can have both running at the same time with no issues and it makes for a pretty bulletproof production machine.

Shortcomings

Before discussing the genius of gas heating systems, I want to get the caveats out of the way.

Gas is slow. Unlike forced air heaters with exchangers designed to extract every calorie of energy from gas, espresso machines boilers are unfortunately very aerodynamic. Heat likes to flow up and around the boiler and combustion gases have to go somewhere. While electric heating elements are 100% efficient, gas burners aren’t. It is super awesome for keeping espresso cups nice and hot, though.

It’s even worse on outdoor espresso carts. A windy day can mess with the flame so badly that the machine never wants to get hot.

Then there is the whole fire thing. Open flames don’t have an exhaust problem and the combustion is pretty complete, but the notion of having a fire on your front counter can be a little off-putting. To be fair, though, more espresso machines have exploded than burned down a restaurant.

And fracking. Fracking sucks.

Why Gas is Awesome

All electric elements are digital (binary); they are either on or off. Pressure switches are the least stable, allowing a boiler to drop a few degrees before coming on with full power. It’s like having the cruise control on your car push the pedal to the floor and then coast. Repeatedly.

Even with proportional integral derivative (PID) controllers, elements are still binary. The controllers create a semi-sinusoidal response curve depending upon temperature sensor output. Latency in the element and the sensor requires that the controller develop an algorithm to calculate the shape of the response curve to achieve a stable temperature. It has to learn how to do it. The problem is that the algorithm used to maintain temperature isn’t designed to respond to steaming three pitchers in a row while the boiler refills or pulling 14 shots of espresso when the busload of Italian tourists pulls in front of your shop. It wants to hit a target accurately, but not necessarily quickly.

Gas is analogue. The flame is precisely proportional to the pressure in the boiler. As the steam pressure drops the gas valve opens in a linear response in real time. The low flame setting uses the inherent inefficiency of the burner to maintain stable pressure just like the PID but responds changes more efficiently. Because no algorithm is involved, it responds to changes in demand and usage profiles in ways a PID cannot. 

This tight response to pressure changes results in equally precise temperature control. A sensor samples the temperature at a specific location and depends upon the thermal conductivity of the whatever its in to provide feedback to a controller. The relationship between pressure and temperature, however, is perfectly aligned. Sampling pressure conveys the average temperature of the entire system, not a particular place. Precisely regulating system temperature allows the system design to deliver coffee consistently as engineered.

But What About…

 Using pressure sensors to drive a modified PID controller? Solid- state pressure sensors have made their way onto a few machines but I honestly don’t know if they incorporate the coding to maintain pressures and respond to fluctuations or merely tighten the hysteresis of a conventional pressure stat. 

Besides, a complete gas kit costs about as much as a solid- state pressure sensor and doesn’t need a CPU. 

At the End of the Day…

Fire is awesome. Having a window on the back of your machine with a steady blue and orange flame for all to see creates a lot of attention and (most likely) isn’t going to catch anything else on fire. More importantly, in tandem with a conventional pressure stat, you can enjoy fast recovery and better thant PID stability without having to re-engineer your machine. 

Most importantly, looking at how gas is used to heat the machine can change the way we use electrical components to heat modern machines. 

Old is cool because it works.