I am about to produce a frankenstein mod to my machine by adding a rotary pump that ought not be there. My setup precludes me from using a device to measure the pressure at the brew head, mostly because i don't have the pipe fittings lying around and a non-standard PF size. The rotary pump I do have lying around I was able to fit a pressure gauge to and it reads 120psi of pressure at the brew head w/o coffee. Any fellow engineers with more fluid dynamics experience may be able to help me here. I suspect that the flow rate of proper extraction will approximate a static pressure scenario and thus the indicated pressure at the pump will be very close to the extraction pressure at the puck (after a few feet of 1/4inch OD tubing). Is this a reasonable assumption? At least it should be able to improve upon an unregulated vibe pump, I hope.

kmills wrote:I suspect that the flow rate of proper extraction will approximate a static pressure scenario and thus the indicated pressure at the pump will be very close to the extraction pressure at the puck (after a few feet of 1/4inch OD tubing). Is this a reasonable assumption? At least it should be able to improve upon an unregulated vibe pump, I hope.

I'm not a mechanical /hydrodynamic engineer, but I can probably answer your question.

Your assumption appears reasonable to me. Because of the way that rotary pumps work, you should see a similar pressure for an actual shot. Rotary pumps are high flow, positive displacement vane pumps that are typically capable of much higher flow rates than an actual shot requires. These pumps have flow capacities in the neighbourhood of 1000 ml/min.

With the flow is restricted (e.g. machine gicleur, coffee puck), the pressure will increase until the bypass valve opens and recirculates the surplus flow within the pump. A properly prepared coffee puck will provide sufficient restriction to cause the pressure to rise to 120 psi at which point the pump bypass valve will open (to prevent the pressure from rising further). I am making the assumption that you connecting your rotary pump at a "normal" location in the water path, i.e. not bypassing the machine's gicleur.

The static pressure you are measuring represents the bypass valve setting which is about 120 psi for your pump. And for any reasonable flow rate, you should see roughly the same pressure during an actual shot. A 1/4" OD line is on the small side but should be fine as the water debit for an espresso machine is only something like 75 ml/first 10 seconds.

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Losses due to the pipe will be very minimal. Assuming 2oz over 30 seconds is ~0.03GPM which is nothing for 1/4" pipe.

Did your OPV open during your pressure test? You're only a little over 8 bar...

kmills wrote:I am about to produce a frankenstein mod to my machine by adding a rotary pump that ought not be there. My setup precludes me from using a device to measure the pressure at the brew head, mostly because i don't have the pipe fittings lying around and a non-standard PF size. The rotary pump I do have lying around I was able to fit a pressure gauge to and it reads 120psi of pressure at the brew head w/o coffee. Any fellow engineers with more fluid dynamics experience may be able to help me here. I suspect that the flow rate of proper extraction will approximate a static pressure scenario and thus the indicated pressure at the pump will be very close to the extraction pressure at the puck (after a few feet of 1/4inch OD tubing). Is this a reasonable assumption? At least it should be able to improve upon an unregulated vibe pump, I hope.

Hi:

I am an engineer. You'll get pressure drop on the way to the coffee. The bulk of the drop will be in small passageways and wherever the fluid flow changes direction substantially. It's hard to know the magnitude of the pressure drop without measuring it, but I'd guess it'll be on the order of 15 psi, if your pump installation is fairly normal.

-Greg

My machine has no OPV and thats the whole reason im puting in the rotary pump which has the adjustment screw. The 120 psi is the back pressure produced when flowing without coffee (very fast flow!). I was concerned at first that i would have to use this as some offset and add 9 bar to that to get proper extraction. After thinking about it for a day i realized that the dynamic pressure is some function of the fluid velocity and should be essentailly zero at the proper coffee extraction flow rate. I just wanted to confirm this with HB readers. So as it stand I will set it so that the pressure with practically choked machine is at 130 psi as indicated at the pump and assume this pressure will be the same at the brew head during normal extraction.

If you have a very high water debit (flow with no puck in the group), you may want to consider installing a gicleur in the range of 0.6-0.9 mm. High water debit can make espresso preparation a good bit more unforgiving (your machine will likely be much more predisposed to puck channeling).

Maybe I'll stick a needle valve in line if I see issues, Thanks!

My machine has no OPV . . .

Exactly what make and model do you have? You will very likely be disappointed with the espresso production because there may very well be other "gadgets" internal to your machine designed to work in concert with a vibration pump but NOT NECESSARILY a rotary pump.

. . . flowing without coffee (very fast flow!).

A reasonable free-flow of water with an empty portafilter is 450-500 ml/minute. What is your "very fast flow" ?

Nice thread!

In a Frankensteined home machine, a gicleur may not be possible; but people have used needle valves instead. Once you have everything set; adjust the needle valve so the dwell time (time from turning on the pump to seeing the first drop of espresso), is about 5 to 7 seconds; and it will be doing the same job the gicleur does in a commercial group.

There are two issues on pressure drop. One is measuring the pressure during the shot upstream from the group, as you have with a panel pressure meter; the other is measuring static pressure against a blind filter. Greg is being conservative; unless I'm misinformed, a 1 bar drop from pump to group during a shot is the maximum on a tight long water path. But you can get 1 bar and a bit more easily on rotary going from blind pressure to flow, and 3 bar on a vibe with a loose OPV.

Thanks for the help! Is the 5-7 seconds with or without coffee? My machine, embarrassingly enough, is a Saeco Classico with a home brew PID set up. I get to see a lot if really high tech scientific equipment get trashed because it doesnt make sense to fix them but they still have some pretty good parts in them. My ghetto machine is the beneficiary of some high end dumpster diving. Even my tamper is a high carbon steel ultra strong anvil from a 20k ton press! With a vario and PID i get pretty good shots from my home roast but i suspect the unregulated pressure is one of the last significant steps to better espresso, but fixing that wont help me justify buying a duetto.....