When deciding how to make your espresso, the weight of grounds used is the primary variable. Everything else relies on it or flows from it. Most people call it the ‘dose’ and depending on your style it can be anywhere from 5-25 grams.
Deciding on a dose is important, but keeping it the same all day is crucial.
If you’re making coffee on a traditional espresso machine, systems must be developed to keep the dose in check. Scales are the most common dose control method in the specialty industry. Slightly less accurate scraping tools and finger methods are less prevalent. With scales, the barista can monitor the amount of grinds in the portafilter as often as desired; using that information to adjust the timer on their grinder or the depth of their scraping tool.
In a super-automatic machine the scale method is quite tricky. Placing a scale sensor (load cell) inside a box next to moisture, vibration, and electrical noise is an expensive and difficult task. Luckily, there are some other options.
Instead of weighing the dose, we can also measure the volume of grinds after they’ve been tamped. You could also call this measuring the puck height. Once tamped, ground coffee’s density becomes quite consistent and can be used as a proxy for weight. Of course a barista can also tell if their tamper is sitting higher or lower than usual. This is handy but certainly not precise. When a machine is measuring tamper height it’s sensitive to changes of 0.1mm. This translates to being able to detect changes as small as ~0.1g.
Once measured, the dose can be controlled by adjusting the grinding time. More grinding time equals more coffee and vice versa. By looking at the last few shots, a computer can easily see trends in the numbers. Perhaps the average dose is too high and grind time needs to be reduced by a few hundredths of a second. Or if the last dose is too high following a steady stream of correct doses it can wait for more data before making an adjustment - just like a barista would.
This control loop is quite simple and predictable. It’s also infinitely patient and disciplined - always striving for the perfect dose.