How To Determine Brew Pressure From Flow Rate??

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JavaJive
Posts: 25
Joined: Wed Aug 29, 2012 3:53 pm
Real Name: Greg
Equipment: Grimac Mini (aka: La Uno), Fiamma MCI 2 flat burr grinder, Westbend Poppery, Original Gaggia Baby
Location: South Florida

Postby JavaJive » Wed Oct 17, 2012 10:07 pm

Hi...

I've seen info on this in snippets in various threads over time, but
why not lets start from scratch?

No gauges at the moment, and a ballpark setting will do for now.
(Freshly rebuilt machine, with new/different/adjustable OPV.

Vibratory pump... Ulka EAX5.
E61/HX Machine.

What is the exact procedure, and how does one determine the correct volume
for the desired pressure result?

The Ubiquitous Ulka pump graph:
Image

Indicates a certain maximum volume from the pump total.

So:
With blind basket in portafilter, all the flow out of the OPV bypass = the total volume?

How does one determine the desirable volume?
How does one know how that translates to what goes through the brew group?
What about the portion that gets bled off through the OPV?

A million other questions, but best to let someone who knows the answer reply
than going on and on.

Thanks,
Greg

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Gui Marques
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Location: Belo Horizonte, Brazil

Postby Gui Marques » Wed Oct 17, 2012 11:39 pm

The pump chart shows how much energy the pump can deliver to the fluid (pressure energy) for a range of flows.

As you can see, the pump can operate over a range different flows, but for each flow rate, it can deliver a specific amount of energy (pressure).

To know at which flow rate the pump will operate, you need to know the other part of the equation, the one that tells how much energy the system will demand from the fluid when it flows from point A, at the pump exit, to point B (wherever you choose, it can be at the opv outlet, or at the end of the puck.

The system is everything in this A to B path: the piping, gicleurs, turns, the dispersor, the head screen, the ground coffee in the puck. All these things will dissipate energy from the water, reducing its pressure along the way (head loss).

The point where the energy provided by the pump equals the energy consumed by the system in the determined water path is the operating point of the pump. This point defines the pressure at the pump's exit, and also the flow rate in the system.

If the head loss does not change during the pump operation, this point will remain constant, and thus the flow rate.

However, here's where it becomes complicated. The paths pump-puck and pump-opv are different, they have different head losses and flow rate and pressure will be different. The puck is a weird animal. It's not homogeneous, and it changes as it gets soaked. To make things worse, the water may find "easier" paths across it, changing the total head loss offered by the puck and thus the flow rate.

So, you could put a blind basket**, time the volume of water coming out, calculate the flow rate and enter it in the pump's chart, to get the pressure. However, this would be the pressure at the pump's exit, and not the brew pressure. The real brew pressure is variable because of the puck.

Gui

**disclaimer: dont do this unless you know your equipment and what you are doing. If run for too long it can damage your machine.

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cannonfodder
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Postby cannonfodder » Thu Oct 18, 2012 12:21 pm

I don't think I have ever seen someone take so long to say you cannot, go buy a pressure gauge. To get the brew pressure at the brew group you need a pressure gauge fitted portafilter. They are easy to make and only cost a few dollars for the gauge. You just screw it onto your portafilter spout threaded fitting with a female adapter. There are posts on HB about how to make one.

Just do a search for pressure gauge portafilter. Here is one right out of the FAQ...

Zero cost adapter for portafilter pressure gauge
Dave Stephens

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JavaJive
Posts: 25
Joined: Wed Aug 29, 2012 3:53 pm
Real Name: Greg
Equipment: Grimac Mini (aka: La Uno), Fiamma MCI 2 flat burr grinder, Westbend Poppery, Original Gaggia Baby
Location: South Florida

Postby JavaJive » Fri Oct 19, 2012 12:12 am

Gui..
Thanks for helping me to clarify the concepts in my mind.

Dave...
Thank you for a short, clear, to the point answer.
I actually have about half the parts to build the more elaborate version
of the PF/Gauge.
(Just wanted a baseline setting till I've a chance to get to the store)

How'z about we split the difference?
I'd guess it would be reasonable, just for the purpose of getting in the ballpark
to set flow for around 230CC, yielding around 10 Bar....
10 bar to give something one can "live with" given the resistance of the remainder
of the brew system?

Greg

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Gui Marques
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Postby Gui Marques » Fri Oct 19, 2012 7:16 pm

That's because it was never my intention to say "you cannot, go buy a pressure gauge".

If that was my intention, I wouldn't even bother posting.

Gui


cannonfodder wrote:I don't think I have ever seen someone take so long to say you cannot, go buy a pressure gauge.
Zero cost adapter for portafilter pressure gauge

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erics
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Location: Silver Spring, MD

Postby erics » Fri Oct 19, 2012 8:08 pm

What is the exact procedure, and how does one determine the correct volume for the desired pressure result?

For sure, the pump performance curve for the EX5 has a band of acceptability but . . . a heavy majority of users who have measured same have ended up on the solid line.

I would simply measure the flow from your new OPV relief and come up with a ml/min number. The way I have done this in the past is to let the flow fill a Pyrex container to the 2 ounce mark and time how long it takes to hit the 6 ounce mark. The idea is to use a differential, not a start from 0 to 4. When this computes to 260 ml/min you are set correctly.

Gui provides an eloquent response - his only "error" is in saying that:
The real brew pressure is variable because of the puck.

The adjustment of the OPV sets the maximum pressure you would see . . . it's the flowrate that is variable because of the puck.
Skål,

Eric S.
http://users.rcn.com/erics/
E-mail: erics at erols dot com