What exactly does a brew pressure gauge measure?

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BaristaMcBob

#1: Post by BaristaMcBob »

I see people using special portafilters with pressure gauges, despite the machine having a brew pressure gauge. Does the machine's built-in pressure gauge not measure the pressure in the puck?

Jeff
Team HB

#2: Post by Jeff »

They typically measure the pressure somewhere between the pump and the group head and usually at the end of a long, thin tube.

There are many flow restrictions between where most machines measure pressure and in the basket. Even those that measure it at the group often don't measure it in the basket, but very near to the basket. With a typical machine, you've got at least a gicleur with a flow-dependent pressure drop across it (the E61 flow-control devices are effectively adjustable gicleurs). With a classic E61, you add in a preinfusion chamber.

That long, thin tube also acts as a dampener, which slows the response.
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baldheadracing
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#3: Post by baldheadracing »

BaristaMcBob wrote:I see people using special portafilters with pressure gauges, despite the machine having a brew pressure gauge. Does the machine's built-in pressure gauge not measure the pressure in the puck?
In addition to what Jeff said, machines with rotary pumps typically have a pump pressure gauge, not a brew pressure gauge. A pump pressure gauge for a rotary pump will read 9 bar (or whatever) when the pump is running, even if no portafilter is inserted.

BaristaMcBob (original poster)

#4: Post by BaristaMcBob (original poster) »

Interesting. Well, the reason I'm asking is because most home machines have an ULKA vibe pump. And most machines produce espresso around 10 bar. If I adjust my grind so that the extraction occurs around 10 bar, it's a gushing mess. I can set the grinder to get a perfect 25-second shot, but the pressure reads 13 to 14 bar. It's a Bezzera BZ10. On Youtube, Luca Bezzera said that such machines are supposed to brew at 13 bar with an ULKA.

So, that means that my BZ10 operates normally, but doesn't explain the following:
- Why is 13 bar beyond the 'red' zone on the pressure gauge if it's supposed to be the sweet spot?
- Why do other prosumer machines brew at lower pressures with the same vibe pump?

So, this leads me to think that my pressure gauge is measuring something other than the basket pressure.

baldheadracing
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#5: Post by baldheadracing »

There has been endless discussion of Bezzera and their high extraction pressures. It has to do with which is more important - pressure or flow - and Luca Bezzera was talking about the goal of maintaining flow; the higher pressure is a consequence of the ULKA's pressure/flow curve, not the goal. Bezzera rotary pump machines are set at 9bar.

A few years ago, Rancilio gave up fighting the Internet, and lowered the pressure on the Silvia (and Silvia Pro). Bezzera, and Rocket, and Gaggia, and Elektra, and ... are more stubborn. Some American importers deliberately spec or adjust the machines that they sell to 9 bar for the American market, leaving the higher pressure for other markets. The customer is always right, especially if they read it on the Internet.

I think three things are certain: 1) espresso is a lot easier to pull if the pressure is adjusted lower; 2) espresso may well taste better at lower pressures for many people because of it is easier to pull; and 3) coffees that are a lot lighter than traditional Italian espresso roasts taste better at lower pressures - lower than 9 bar, actually. (... and there are people who have a personal preference for espresso pulled at lower pressures, and also machines that are deliberately setup to pull lower than 9 bars, but those are different issues.)

The pressure gauge shows red as they are common parts used in many machines.

From what I remember, on a BZ10, what you see is what you get, subject to Jeff's caveats. Try a lower pressure if you're curious.

Finally, the Elektra/BZ group does pretty well at higher pressures: Elektra Microcasa Semi Automatica - Appendices
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BaristaMcBob (original poster)

#6: Post by BaristaMcBob (original poster) »

Really interesting. When mfgs lower the pressure, it's done by adjusting the OPV, right? And so also diverts part of the flow, I assume.

randytsuch

#7: Post by randytsuch »

BaristaMcBob wrote:Really interesting. When mfgs lower the pressure, it's done by adjusting the OPV, right? And so also diverts part of the flow, I assume.
My machine (opv adjusted to 9 bar by me) has a tube out of the opv that goes back into the water tank. So yes, water is diverted, and in my case, ends up back in the tank.

I would guess for a plumbed in machine, it would divert to the drain.

Side note, in case any finds this interested, I recently added an electronic pressure gauge (cheap ebay chinese one) to my HX machine. Added a Y connected a few inches after the pump, then about 1 foot of tubing from Y to the sensor.

It measures a very steady 9 bar if the OPV is controlling the final pressure, which I think makes sense.
Just added ability to lower pressure from pump, and pressure is much less steady like this.
I'll create a dedicated thread on my machine in the near future.

Randy

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BaristaMcBob (original poster)

#8: Post by BaristaMcBob (original poster) »

I read the Elektra link. He says that he get's more control over the taste by varying grind and dose rather than tinkering with boiler settings.

I'm beginning to think that I should leave my Bezzera alone. It pulls perfect shots and if it didn't have a pressure gauge I wouldn't think to question the operation.

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cafeIKE
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#9: Post by cafeIKE »

There is 1.2mm restrictor [JET M6 X 6 HOLE D1.2 OT] in the BZ10 water circuit. BZ10 Parts Diagram which is about 4x the area in a [Vibiemme] e61.

At std flow 30ml in 30s pre gicleur, my pressure about 10 bar, post 9¼. One issue with measuring pressure @ constant flow rates is puck don't flow at a constant rate.

P.S. Perfect shots are judged by taste, not volume and flow. In 25 years, I've had about 10, split evenly between shops the world over and chez moi.

Pressino

#10: Post by Pressino »

The quick answer to the OP's question is that a brew pressure gauge measures the pressure in the brew chamber, which is the space that contains the water just above the coffee puck. The pump is applying pressure to the water that enters the brew chamber, supposedly at the pressure measured by the brew pump pressure gauge. In E61 machines most brew chamber gauges are installed in the channel that supplies water to the screen. It's important to realize that the pressure in the brew chamber is going to vary based on the changing condition of water flow through the puck during extraction. If you were to watch the brew and pump pressure gauges when you have a blind portafilter in place you will see both gauges reading about the same pressure. If you watch the pump pressure gauge during an extraction it should remain relatively constant (usually at whatever pressure you set the OPV). 8)