Recommendations for pressure setting on OPV requested - Page 2

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HB
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#11: Post by HB »

Are you including the water absorbed by the puck in your calculation and subtracting time for preinfusion? Let's say for sake of discussion that there's ~20ml of absorbed water and ~20 seconds of actual max pressurization time.

So at nine bar:
  • (250 ml / min) * (1 min / 60 sec) = 4.2 ml / sec
    20 sec * 4.2 ml / sec = 84 ml or...
    ~60 ml espresso + ~20 ml absorbed water
I haven't weighed a wet versus dry puck in awhile and of course there is a lot of guestimation, but it looks close enough for me.
Dan Kehn

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barry

#12: Post by barry »

oh, you evil evil man. ;) you made me pull out my calculus book.


the pressure graph is roughly a line of equation: y= -(15/650)x + 15,
or y = -.023x + 15. or x = (15 - y)/.023

the flow rate for a 60ml/30sec double, is actually about 75ml/30sec (i measured it for the thermofilter design), or 150ml/min. if we assume that full brew pressure (whatever it might be) doesn't occur until 5 seconds after the pump is activated (the 'ramp up time'), then we have a much higher flow rate for those first 5 seconds. brew pressure is determined by the lesser value of puck resistance or relief valve setting.

total volume should equal low (ramp up) pressure volume plus high (brew) pressure volume.

Vt = Vr + Vb



flow rate at 9 bar is, as you noted, 250ml/min.

flow rate for ramp up (0 to 9 bar) is an average of 450ml/min (650 max, 250 min).

if ramp up is 5 seconds, then Vr is:

(450ml/min) / (60 sec/min) * 5 sec = 37.5ml, or half the total target volume.


the 9 bar flow rate with a 25 second flow period will pass 104.1ml, giving a total shot volume of 141.6ml. a very long double indeed.

now let's hold to that 75ml number as the target for total volume. given the figure for Vr above, then Vb would be the same.

(37.5ml) / (25 sec) * (60 sec/min) = 90ml/min

plug that into the line formula (or look it up on the graph) and this gives a brew pressure of about 13 bar.

i'm sure that someone with better math skills can work all that out into a function where we only have to plug in the variables to work out what the pressure and flow will be for any given volume/time relationship.

now, if the relief valve is set for 9 bar, then part of that copious flow will vent to drain (or reservoir). interesting, though, that half the shot volume is poured into/through the puck before the pump comes up to full pressure. i think, perhaps, preinfusion is 3 or 4 seconds, instead of the assumed 5. i hope.



--barry "possibly wrong"

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HB
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#13: Post by HB »

barry wrote:now, if the relief valve is set for 9 bar, then part of that copious flow will vent to drain (or reservoir). interesting, though, that half the shot volume is poured into/through the puck before the pump comes up to full pressure. i think, perhaps, preinfusion is 3 or 4 seconds, instead of the assumed 5. i hope.
Professor Jarrett,

I appreciate you keeping me honest. Using the thermofilter to regulate the flow rate, I measured an overflow from the OPV for 20 seconds of ~50 milliliters at eight bar. Is that consistent with your calculations?
Dan Kehn

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barry

#14: Post by barry »

HB wrote:I appreciate you keeping me honest. Using the thermofilter to regulate the flow rate, I measured an overflow from the OPV for 20 seconds of ~50 milliliters at eight bar. Is that consistent with your calculations?
is that for a 60ml double in 25 seconds total time?

--barry "gotta have all the info"

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HB
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#15: Post by HB »

I didn't bother measuring the flow from the thermofilter under the assumption that it's constant (and calculable) at the stated pressure. The twenty seconds that I measured the overflow from the OPV was only once the group was fully pressurized so the question of ramp up / preinfusion wouldn't enter into your equations. Is there a flaw in that assumption?

PS: I'm glad you prodded me into measuring. Apparently Valentina's OPV spring has gotten tired over the past months and is allowing more water to escape than before. Looks like it's time to tweak it down a half turn. :?
Dan Kehn

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barry

#16: Post by barry »

HB wrote:I didn't bother measuring the flow from the thermofilter under the assumption that it's constant (and calculable) at the stated pressure.
<slaps hand to forehead> yeah, you did say that, didn't you? ;)

The twenty seconds that I measured the overflow from the OPV was only once the group was fully pressurized so the question of ramp up / preinfusion wouldn't enter into your equations. Is there a flaw in that assumption?
hhmmm...

here's what we need to know:

preinfusion time
maximum brew pressure
total shot time
total shot volume (or total flow volume if using the thermofilter)
opv overflow volume


i'm not sure using the thermofilter to determine flow characteristics with a vibe pump is appropriate for comparing with flow characteristics of a real shot. the thermofilter doesn't absorb water, so ramp up time should be quicker. from the chart, flow at 8 bar is 300ml/min, or 5ml/sec, so a 20 second pour should pass 100ml. if 50ml went out the opv, then you should have had 50ml in your shot glass.

--barry "or thereabouts"

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HB
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#17: Post by HB »

barry wrote:here's what we need to know...
Sorry, I'm not sure what the question is anymore. :oops:

On a related tangent, have you or Greg given thought to standardizing the measurement of pressure profiles? The thermofilter has definitely advanced the issue on the temperature front. Speculation about the merits of precise pressure control and declining profiles are bandied about by various luminaries (e.g., Jim Schulman's experiments with Variac vibration pump control, recently renewed interest in spring levers, discussions in the Brewtus Group, Chris Tacy's "clarity" musings, and of course the Versalab M3), but I'm unaware of any work towards documenting pressure profiling. My gut feel is that it will be the "next big thing."
Dan Kehn

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barry

#18: Post by barry »

barry wrote: total volume should equal low (ramp up) pressure volume plus high (brew) pressure volume.

Vt = Vr + Vb

let's modify this to:

total shot volume should equal ramp up pressure volume plus brew pressure volume minus overflow volume:

Vs = Vr + Vb - Vo

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barry

#19: Post by barry »

HB wrote:Sorry, I'm not sure what the question is anymore. :oops:
i was questioning this assertion:
The short answer is for true doubles, you're right, the OPV setting isn't important since the pump can't muster more than 9 bar at that flow rate.



On a related tangent, have you or Greg given thought to standardizing the measurement of pressure profiles?


not yet. every time i start thinking about measuring pressure, i run into expensive equipment and then wander off to another project. ;)

gscace

#20: Post by gscace »

barry wrote:

not yet. every time i start thinking about measuring pressure, i run into expensive equipment and then wander off to another project. ;)
Yeah, electronic transducers are expensive, but concepts are cheap. I'd do it exactly the same way as with the thermofilter, that is with the proper flowrate thru the portafilter.

-Greg