The Bard once said: "To flow control, or not to flow control, that is the question." Okay, Shakespeare never said that, but if espresso were around in the Elizabethan era, perhaps Hamlet would have acted with precision and haste if he'd had a delicious espresso. Alas, we will never know, and mostly everyone ended up dead; using flow control won't kill you...we promise. There is a sneaky, counterintuitive little fact about brewing with flow control that may have thrown off every "profiled" shot you've ever pulled. Yet it doesn't matter how thoughtful the profile was or how precisely you used the paddle; if you ignore this piece of physics, you might as well be pulling shots blindfolded.
For those who only have the attention span to read short blocks of text or watch the Sopranos entirely via TikTok clips, here's the short version: the pressure your pump makes and the pressure your coffee actually experiences are two totally different numbers, and most espresso enthusiasts start off their flow control journey by profiling with the wrong numbers in mind.
Where "Nine Bar" Comes From
To find this ghost in the machine, let's start with how a classic espresso machine measures pressure. Every machine has a pump, either vibratory, rotary, gear, etc. The pump draws cold water from the reservoir or a water line, sends it through a short maze of tubes and pipes, and pushes it into a boiler to heat to brew temperature. From there, the water heads to the group head, meets the coffee puck, gets forced through it, and lands in your cup as espresso.
Almost every modern espresso machine, at least the non-consumer-based machines, is calibrated at the factory to deliver 9 bar by default. How do we know the pump is delivering 9 bar? The pump pressure gauge. One handy way to do a pressure test is by popping in your blank backflush disk (the basket with no holes) and act like you're pulling a shot. Check the pump pressure gauge and see how much pressure your machine is putting out.
Pull a shot and the pressure gauge needle, depending on which pump you have, will climb quickly to 9 bar, or have a slower ramp-up, sit there for the length of the shot, and fall back to zero when your shot is complete. The traditional 9-bar espresso. All's right with the world.
Or is it?
A Classic Pressure-Profiled Shot
With that in mind, we put a LUCCA E61 flow control unit on an M58 Sunto and pulled a shot with a textbook pressure profile: a 4-bar pre-infusion for the first 10 seconds, a slow ramp up to 9 bar, a hold, and a ramp back down at the end. Tapering off the pressure at the end of your shot accounts for the extraction breaking down, when fewer volatile compounds and solubles remain to extract.
Yet, the result was channeling carnage. If you had just installed what you thought was a silver bullet for consistent extractions and got that, you'd be pretty disappointed. It was a pretty tough morning dialing in. Sometimes I need my coffee before I have my coffee.
Two Gauges, Two Different Numbers
About 9 seconds into that disaster of a shot I pulled, the brew pressure gauge on the group is sitting at 4 bar for the pre-infusion, exactly as planned. Just below, the pump pressure gauge still reads 9 bar. No fear! There is a perfectly reasonable and entirely non-paranormal explanation. The two gauges are the same kind of simple pressure sensor, but they measure pressure in two different places.
- Brew pressure gauge: measures pressure inside the group, just before the espresso puck.
- Pump pressure gauge: trace its line back through the machine, and you'll find it measures pressure immediately after the pump.
At that moment in the shot, there is a 5-bar pressure gradient between the pump and the portafilter. The pump gauge doesn't tell you what's going on inside of the group head chamber.
The Nine-Bar Shot Is (Mostly) a Lie
So, if there's a gradient with flow control, how big is it without flow control? To find out, I swapped the flow control mushroom for the standard one and pulled another shot. Brew pressure moves on its own. It starts low, rises as pressure builds, and then slowly drops as the puck loses resistance through the extraction process. The actual brew pressure is almost never at 9 bar. With some recipes, you won't hit 9 bar at all.
I guess this is why they say you should never empirically test and verify your heroes, in a manner of speaking.
The ghost that was wrecking those flow-profiled shots was just a pressure gradient that nobody had really noticed before. Profiling a shot to hit 9 bar at the group didn't recreate the classic shot profile; it overloaded the puck with more pressure than a standard shot normally sees, making everything more finicky and every symptom of channeling worse. Crisp white tee ruined.
The Solution: Aim Lower
The fix is dead simple: The classic "nine-bar" shots we thought we were pulling were probably closer to 8 bar at the group. So aim for 7 to 8 bar at the peak of your shot instead of 9.
How you get there depends on the system:
- Pressure control, like on the LUCCA Tempo: this is super easy. Set the pressure control valve so the gauge reads 8, and it won't exceed it. We have a handy video on the perfect profile on the LUCCA Tempo.
- Flow control, like on the M58's E61 group head: expect a little more feathering of the paddle mid-shot to hold your target.
One more thing the standard shot I pulled illustrated: the flow control gauge was never sitting at 9 bar for long, if at all. Take that as nature's permission to get a little more flowy with your flow control.
The Recipe I've Been Using
Here's what I've been leaning on lately for a 1:2 brew ratio, reading pressure at the group:
- Hold 4 bar for the first 15 seconds.
- Ramp up to 8 bar by 20 seconds.
- Hold 8 bar until 30 seconds.
- Ramp back down to 4 bar from 30 to 35 seconds, then stop the shot.
That extra-long, low-pressure infusion gets slow, even flow through the entire puck before the higher pressure is fully applied to the puck. This has resulted in incredibly delicious, consistent shots. Try it yourself and get back to us about your results. We'd be happy to hear from you.
Happy shot pulling!