Quick Start – This app simulates the pressure and flow inside an espresso machine while a shot is running. To view the effects of these changes in real time during a shot, click on ‘Graph’ and then ‘Start’, and the app will simulate an espresso being pulled, showing you how the pressure and flow change as the resistance at the القُرص changes because of extraction. Change the pressure or flow restriction during the shot to see how pressure profiling and restriction profiling can affect flow during a shot.
Run multiple shots to see the graphs overlaid on top of one another and compare the effect of different changes. Click ‘Clear’ to clear the graph at any time.
You can also view a full-screen version of the Pressure and Flow app هنا.
Understanding the interactions between pressure, flow, and resistance can be difficult. Baristas are used to adjusting لضغط الضخ as their primary variable, but engineers often think of pressure as something that develops as a result of resistance, rather than the output from the pump.
The Pressure and Flow app explains these interactions in a visual way to help you develop an intuitive understanding of them, without having to memorise the equations underlying the physics. Once you have a grasp of it, you should be able to explain:
- Why لضغط الضخ alone isn’t enough to control pressure in the القُرص
- Why the flow restrictor mainly affects the beginning and end of a shot
- How pressure profiling works differently in different machines
- How restriction profiling machines like the Slayer affect flow during a shot
- Why flow profiling requires a machine to make adjustments many hundreds of times per second
In this video, we show you how to experiment with these settings in the app, and how to simulate the pressure and flow inside different types of espresso machine.
No Pressure Without Resistance
The first thing to understand is that the pressure at the القُرص doesn’t only depend on the لضغط الضخ. If you set the القُرص resistance to zero, then the water just shoots right through and no pressure can build up. As you increase the القُرص resistance, you’ll see that pressure starts to build up.
To conceptualise this, think of the difference between blowing through a straw and inflating a balloon: no matter how hard you blow, the straw doesn’t get pressurised. Whereas the balloon puts up resistance, so pressure builds up as you inflate it.
resistance from the القُرص during an extraction is variable, so the pressure will vary with it. This tells you right away that changing the لضغط الضخ isn’t enough to get a specific pressure or flow rate through the القُرص.
The Flow Restrictor
If you set the القُرص resistance setting in the app to zero, you can see that the flow restrictor has a big effect on flow. But if you put the القُرص resistance on the highest setting, you can see that the added resistance from the flow restrictor has barely any effect on flow. This means that the flow restrictor has the biggest effect at times when the القُرص resistance is lowest — in other words, during pre-infusion and towards the end of the shot.
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Pressure and Flow Profiling
Pressure profiling machines have two ways of working. They either change the لضغط الضخ at specific times during the shot, like the Synesso MVP Hydra or the Sanremo Opera, or they try to maintain specific pressure at the القُرص, like La Marzocco’s Strada.
The limitation of fixing pressure at the pump is that it doesn’t account for changes in resistance. In the Strada, however, the machine changes the لضغط الضخ to maintain a specified pressure at the القُرص. For example, if you set the machine to stay on 9 bars, the pump will keep working harder and harder to keep the pressure up, to counteract the loss of resistance as the القُرص erodes. Try maintaining a fixed pressure at the القُرص in the app, and you’ll see the لضغط الضخ has to continually increase to compensate for the loss of resistance.
Flow profiling machines approach the problem in a different way: they try to control the outgoing flow rate. To do this, they must constantly monitor the flow and adjust the other parameters to compensate. Try it for yourself: start a shot, and after pre-infusion ends try to keep the flow at a fixed rate — say, 2 ml/s — by adjusting the لضغط الضخ or flow restrictors during the shot. You’ll find that however quick your reflexes are, it’s almost impossible to get the flow rate perfectly flat.
For more detail on these exercises and the physics behind them, take a look at the chapters on الضغط and التدفق in our latest online course called المهارات الاحترافية لإعداد الإسبريسو - (قيد الترجمة), only available with a Barista Hustle Unlimited subscription.
A Few Notes About the App
If you change the القُرص pressure setting in the Pressure and Flow app, then the لضغط الضخ will adjust to supply the pressure you need. Flow is capped at 15 ml/s so if you lower the resistance too much, then the لضغط الضخ will drop accordingly.
The simulation of القُرص resistance is greatly simplified — the idea is just to give an idea of how unexpected changes in resistance change flow. In the app, changes to resistance during the shot are randomised, so that no two shots are exactly alike, to show you how difficult it is for espresso to be consistent when the القُرص itself is never completely consistent. In a real القُرص, the resistance would change according to the amount of extraction, the viscosity of the liquid in the القُرص, and the لضغط الضخ, among other factors. The app keeps this simple so that you can focus on the fundamentals.