E61 Group Pressure Correlation to Brew Pressure - Page 3

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Tonefish

Postby Tonefish » Jun 14, 2018, 6:52 pm

Holy Moly, you've got some spreadsheets going ... want to share them? I've done a fair bit of VBA spreadsheet boosting:D
Jake_G wrote:As long as the OPV is closed, the pressure and flow relationship during a flush are driven by your pump curve, which is a little on the low side (flow should be more like 220mL/30 seconds @ 3.5 Bar or pressure should be more like 6 Bar at 180mL/30s) but this could be due to the PI chamber "stealing" some of the water during the test. @ 3.5 Bar, the PI valve should be cracked, so that's a definite possibility... However, the shape of the chart above is based on the Cv of your gicleur. If you get a smaller gicleur, the pressure drop will increase, and the pump will follow its curve to deliver less water. The shape of the curve above will get steeper (higher pressure) and shorter (less water debit means we stop at maybe 4.5 mL/s instead of 6).

For some reason I thought we were calculating the size of the gicleur. What is the equation behind this curve? Also, my pressure here at 6000' is under 0.82 bar so the delta P would be higher (not sure if you included that or how much difference it could make).

This assumes a constant pump pressure of 9 bar.

My pump pressures during a shot are 9.3-9.4 bar and during a blind flush, 9.6-9.7 bar.

So what this is saying is that IFF your puck is so porous or eroded that it flows 5mL/s (Super Lungo!), the pump will only be able to muster around 6.8 bar to the inlet of the gicleur. The gicleur will then have a pressure drop associated with this flow of just under 2.5 bar, so the resulting pressure at the puck is only 4.4 bar. You can see how the OPV flattens out the pump curve for flow rates of less than 4.5mL/S and leads to much better (or at least predictable) performance in the typical range of espresso brewing.

Never wanted a Super Lungo (always ristretto), but I suppose that if I did I'd want the pressure to reduce anyway when I'm drawing blonde, right? :P

Thanks for the crunching, and the results, but I'm skeptical by nature until I see what is behind the curtain :shock: so to speak. 8)

Cheers and Thanks!!
LMWDP #581 .......... May your roasts, grinds, and pulls be the best!

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Jake_G

Postby Jake_G » Jun 14, 2018, 7:14 pm

Tonefish wrote:Holy Moly, you've got some spreadsheets going ... want to share them? I've done a fair bit of VBA spreadsheet boosting:D

Happy to share.

Tonefish wrote:For some reason I thought we were calculating the size of the gicleur. What is the equation behind this curve? Also, my pressure here at 6000' is under 0.82 bar so the delta P would be higher (not sure if you included that or how much difference it could make).

I would have used the diameter to approximate Cv. Water debit is a better way to calculate it directly. The equation is nothing more than solving the Coefficient of Flow formula for pressure drop and plotting against flow. Since your gauge pressure references your local atmospheric pressure, the pressure drop calculations are "corrected" for your altitude.

Tonefish wrote:My pump pressures during a shot are 9.3-9.4 bar and during a blind flush, 9.6-9.7 bar.

This will bump the puck pressure up by 0.3 to 0.4 bar and move the knee point to the left on the pump-corrected chart. The shape of the curve will remain intact for the most part.

Tonefish wrote:Thanks for the crunching, and the results, but I'm skeptical by nature until I see what is behind the curtain so to speak.

Understood. And you're welcome!

Cheers!

- Jake

Tonefish

Postby Tonefish » Jun 14, 2018, 8:59 pm

Jake_G wrote:The equation is nothing more than solving the Coefficient of Flow formula for pressure drop and plotting against flow. Since your gauge pressure references your local atmospheric pressure, the pressure drop calculations are "corrected" for your altitude.

Got it! And I'm caught up now. I understand your plots and know how the curves are calculated and built. I'm wondering about the linear assumption on pump pressure and puck pressure reduction as the Ulka capabilities reduce. Is this because the pump performance reduces approximately linearly as well? I notice the debit flow rate allows around 5.5 bar on that pump curve, so there are definitely some appreciable plumbing losses between it and the shower, assuming the pump is performing to spec.

So would you say this is likely a 0.6mm gicleur? That flow/pressure looks about right from plots I see over at BH.

Thanks again for the kickstart in getting me worked through this!!

Happy Pulls!!
LMWDP #581 .......... May your roasts, grinds, and pulls be the best!

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Jake_G

Postby Jake_G » Jun 15, 2018, 12:32 am

Tonefish wrote:I'm wondering about the linear assumption on pump pressure and puck pressure reduction as the Ulka capabilities reduce. Is this because the pump performance reduces approximately linearly as well?

Yes. The pressure reduction is still equal to pump pressure minus pressure drop, but the pump curve is dominant, even though the pressure drop is ever-increasing as flow increases. I approximated the slope based on the Ulka curve and your measured water debit. Vibe pumps are very linear creatures since the spring itself does the pumping. The coil just recharges the chamber. As such, the spring can only push until the back pressure matches the force of the spring. Truly a pressure-compensated variable displacement pump of the simplest sort.

Tonefish wrote: I notice the debit flow rate allows around 5.5 bar on that pump curve, so there are definitely some appreciable plumbing losses between it and the shower, assuming the pump is performing to spec.

Indeed. There is a very real possibility the PI chamber is mucking with the numbers. It would be an intersting test to run the pump for a minute and see if the measured water debit changes throughout the flush. Maybe one of your videos watching the scale and pressure gauge so you can chart it? :D

Tonefish wrote:So would you say this is likely a 0.6mm gicleur? That flow/pressure looks about right from plots I see over at BH.

My gut tells me it's 0.7mm, with some other explanation for the lower pump flow. I could very well be wrong, though!

Cheers!

- Jake

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Jake_G

Postby Jake_G » Jun 15, 2018, 10:22 am

Jim,

Is this photo an accurate of your hydraulic circuit?
Image
Image swiped from this coffeesnobs thread.

I'm confused by the lower solenoid on the nickel plated cross below the OPV which appears to feed the coiled capillary tube that must feed the brew pressure gauge. This is odd. The P-stat version of the PL-62 has the brew gauge tapped into the upper HX port, which is capped on the PID version.

Why switch the brew gauge?

Also notice the check valve at the bottom of the tee feeding the cross. There will be a modest pressure drop across the check valve, which would account for some of the reduced pressure of your flush relative to the pump curve, but this is typical of any HX machine design.

Also peculiar is that your brew gauge reads lower than your group gauge when the pump is not engaged. It seems the brew gauge solenoid is killing the pressure to the gauge unless the pump is active. What's odd is that it appears to be a 2-way valve, so how is the gauge returning to zero? I must be missing something here...

Cheers!

- Jake

Tonefish

Postby Tonefish » Jun 15, 2018, 1:46 pm

Jake_G wrote:I'm confused by the lower solenoid on the nickel plated cross below the OPV which appears to feed the coiled capillary tube that must feed the brew pressure gauge. This is odd. The P-stat version of the PL-62 has the brew gauge tapped into the upper HX port, which is capped on the PID version.

Why switch the brew gauge?

Yes, that's right. It caught DaveC by surprise too and he discusses it here during his review walkthrough and later in the "small details" section. It's a feature! :D

Also peculiar is that your brew gauge reads lower than your group gauge when the pump is not engaged. It seems the brew gauge solenoid is killing the pressure to the gauge unless the pump is active. What's odd is that it appears to be a 2-way valve, so how is the gauge returning to zero? I must be missing something here...

I vaguely recall a slight wiggle on that brew gauge when I start the trickle preinfusion but I'll check to verify that. I need to draw up a flow schematic to really understand this. Unfortunately I did not snap pics when I had the cover off last, but I will next time. :wink:

Thanks for your interest and Happy Pulls!!
LMWDP #581 .......... May your roasts, grinds, and pulls be the best!

Tonefish

Postby Tonefish » Jun 15, 2018, 1:48 pm

I can't get to the last page again, which is really annoying, so I'm having trouble tracking this thread unless I read it backwards while faking a reply. Sorry!
LMWDP #581 .......... May your roasts, grinds, and pulls be the best!

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Jake_G

Postby Jake_G » Jun 15, 2018, 3:16 pm

Tonefish wrote:I can't get to the last page again, which is really annoying, so I'm having trouble tracking this thread unless I read it backwards while faking a reply. Sorry!

Clear your cache and the issue will go away!

Tonefish wrote: Yes, that's right. It caught DaveC by surprise too and he discusses it here during his review walkthrough and later in the "small details" section. It's a feature!

Ah, In the walk-through, you can see that it is in fact a 3-way valve, with a dump tube back to the reservoir. When you turn the pump off, the gauge drops to zero. pretty unique!

Tonefish

Postby Tonefish » Jun 15, 2018, 3:53 pm

Jake_G wrote:Indeed. There is a very real possibility the PI chamber is mucking with the numbers. It would be an intersting test to run the pump for a minute and see if the measured water debit changes throughout the flush. Maybe one of your videos watching the scale and pressure gauge so you can chart it? :D

I went back to my video on that last debit measurement and it was a full 16s out of 30s until the flow settled down from spitting steam (again, my altitude boils at brew temps). So now I needed a pm pull so I ran the flush for one minute during warmup (145F group) and I also captured the PI drain flush at the end. I got 378 ml from the screen (in 60s) and 37 ml from the PI drain.
LMWDP #581 .......... May your roasts, grinds, and pulls be the best!

Tonefish

Postby Tonefish » Jun 16, 2018, 12:21 pm

Jake_G wrote:Clear your cache and the issue will go away!

Thanks Jake!! Worked like a charm. Cheers!
LMWDP #581 .......... May your roasts, grinds, and pulls be the best!