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July 10, 2020 /
App – Pressure & Flow

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 puck 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 here.

Understanding the interactions between pressure, flow, and resistance can be difficult. Baristas are used to adjusting pump pressure 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 pump pressure alone isn’t enough to control pressure in the puck
  • 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 puck doesn’t only depend on the pump pressure. If you set the puck resistance to zero, then the water just shoots right through and no pressure can build up. As you increase the puck 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.

The resistance from the puck during an extraction is variable, so the pressure will vary with it. This tells you right away that changing the pump pressure isn’t enough to get a specific pressure or flow rate through the puck.

 

The Flow Restrictor

If you set the puck 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 puck 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 puck resistance is lowest — in other words, during pre-infusion and towards the end of the shot.

 

Pressure and Flow profiling

Pressure profiling machines have two ways of working. They either change the pump pressure at specific times during the shot, like the Synesso MVP Hydra or the Sanremo Opera, or they try to maintain specific pressure at the puck, 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 pump pressure to maintain a specified pressure at the puck. 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 puck erodes. Try maintaining a fixed pressure at the puck in the app, and you’ll see the pump pressure 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 pump pressure 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 Pressure and Flow in our latest online course called Advanced Espresso, only available with a Barista Hustle Unlimited subscription.

 

A few notes about the app

If you change the puck pressure setting in the Pressure and Flow app, then the pump pressure 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 pump pressure will drop accordingly.

The simulation of puck 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 puck itself is never completely consistent. In a real puck, the resistance would change according to the amount of extraction, the viscosity of the liquid in the puck, and the pump pressure, among other factors. The app keeps this simple so that you can focus on the fundamentals.

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3 Comments

  1. lukasunzh

    Great app, love it so much and it’s very practical.
    I just have a little question here, firstly, we always say that to serve the purpose of achieving certain extraction goals, we can manipulate the puck resistance(grinding size) by observing the influence air density has on the bean density. And in the last paragraph here, it shows that the viscosity of the liquid in the puck has an influence on the puck resistance as well. I know people always say that there is no direct relation between viscosity and density. However, I feel it may has a relation somehow in this case. Is that true or I’m just being stupid, LOL

    Thank you guys, whoever responds to this, and stay safe fellows 🙂

    • BHLearn

      Hi Luka, thanks for the kind words! The liquid in espresso is more dense as well as more viscous than water, if you exclude crema. But it’s the viscosity that matters in terms of how much resistance to flow there is in the system. To be clear, the viscosity of the espresso isn’t changing anything in the puck, it’s just harder to push a more viscous liquid through the puck.

      • lukasunzh

        Thank you so much for the highlight. It’s quite clear to me now. 😀

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