intro.
Fancy coffee machines are extremely expensive and basic models do not offer much in the way of instrumentation. For my first foray into the coffee world, I wanted to see what was happening without spending hundreds (or thousands) on a professional coffee machine.
Good coffee starts with predicability. Pressure and temperature must be regulated accurately to extract the perfect flavors and texture for espresso.
This Delonghi pump espresso machine works well for a first machine but left me desiring more accurate temperature control and pressure feedback.
finale.
Here is the last picture first, showing the final features. I've added a pressure gauge and temperature controller. The pressure is mostly the same for all coffees using the built-in coffee holder but I'll be fixing that soon by moving to a bottomless portafilter.
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| pump espresso machine with added temperature controller and pressure gauge |
inizio.
These are the stock internals of the coffee maker. The purpose of this picture is to help me figure out how to reassemble everything and reverse engineer the wiring. Here we can see two separate temperature controlled switches which control power input to the boiler. One switch is for "brew" and the other is "steam". I'll be replacing the "brew" temperature switch with an external temperature controller.
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| stock boiler |
potenza.
Here I am working out the wiring details. Later this will help to insert the temperature controller. I wanted a temperature controller to perform my own (less professional) experiments with extraction temperature. The front panel selector sends power to the stock temperature switches which are NC or Normally Closed until the desired temperature is reached, then they open and disconnect power to the boiler.
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boiler wiring
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pressione.
Espresso pressure is one of the most important factors in making a great cup of espresso. The pressure is created by the pump in an espresso machine and it forces hot water through the coffee grounds at a high pressure. This pressure is what extracts the coffee’s oils and flavors, giving espresso its signature taste.
If the pressure is too low, the espresso will be weak and watery. If the pressure is too high, the espresso will be bitter and have a burnt taste. The perfect pressure for espresso is around 9 bars (130 psi). Fancier espresso machines have a pressure gauge to give the operator some feedback about this metric.
This machine has a spring limit setup inside the boiler which forces the pressure up to 90 psi before exiting into the coffee. The pressure is predictable but not adjustable.
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| exploring pressure gauge location |
calore.
The temperature controller requires an external relay, in this case an SSR or Solid State Relay to switch on and off the boiler. This replaces the "brew" temperature switch from the boiler. I left the "steam" switch in place to allow an easy switch to "steam mode" from the front panel switch. You can see the heatsink for this SSR the first picture of this post. It turns out the heatsink is not necessary for heat dissipation. It does help hold the SSR in place and looks awesome anyway...
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| added temperature control relay |
filtro.
A portafilter is a key part of any espresso machine. It’s the part that pulls the espresso shots out of the machine. It’s also where you add the beans to the machine.
The Delonghi "2 cup" filter has one tiny hole at the bottom which makes it a "pressurized" filter. The entire contents are pressurized up to ~160psi during extraction. As long as the coffee is not overly packed, this pressure level is reliable. If the tiny hole clogs, the boiler pressure races above 200 psi while no coffee is coming out.
termine.
The next steps are to add the "bottomless" portafilter and perfect the tamping technique. With this non-pressurized portafilter, the tamped coffee grounds will more directly affect the extraction pressure. A finely ground coffee with an accurate tamping should produce an extraction around 130psi.
