Recommendations for pressure setting on OPV requested

[ristrettobrain]

This is a follow on to my previous post. So now, assuming that I figure out how to adjust the OPV on my ECM Giotto Classic, at what pressure do those with more wisdom than I suggest I set the OPV on my machine?

Any experience with various settings will be appreciated.

Thanks in advance,

Mike

[Abe Carmeli]

Set it to 10 bars, if you are using a portafilter mounted pressure gauge with no flow. This translate to 8.75 bars with 60 ml flow (a double) and 9 bars with 50 ml flow (a double Ristretto). So, in your case, set it to 10.

[ristrettobrain]

Abe,

Thanks for the advice, I am glad I asked the question . I was getting the idea that 8.5 to 9 bar is a good range but didn't realize the conversion factor to account for flow. But I must ask, why is this? If the puck is offering, say, 8.75 bars of resistance why would it matter if the OPV was set to relieve pressure at some point above that? Say, 9 bar, 10, or 12.5?

On a tangent, I have appreciated your and Jim's M3 grinder reports and photos. The M3 would be represent a massive investment on my budget but as something that would be used often and, hopefully, last decades it may make sense. Therefore ongoing reports will be highly useful, e.g., the belt slippage issue. Issues related to long term durability are important. Lastly, I must add that the shot photos you posted on that thread are spectacular. I must be going off of the espresso deep end because I'm thinking man that's gorgeous, it would make a great poster!

Mike

[HB]

You know you're getting lazy when you resort to quoting yourself:

Keep in mind that the OPV determines the maximum pressure during the extraction; the puck determines the minimum. So the OPV not opening isn't an issue for double espressos since the pump flow rate works out to around 9 bar anyway (as explained earlier in the thread). If you're pulling ristrettos (or "accidental ristrettos" as Jim sometimes calls them), it's another story. Then the OPV determines the maximum pressure by venting excess water back to the tank, thereby reducing the overall pressure.

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. Many are set to around 12 bar from the factory to protect the machine's plumbing, not regulate brew pressure.


(image courtesy of Ulka S.p.A.)

[HB]

On a tangent, I have appreciated your and Jim's M3 grinder reports and photos...

Follow on discussion merged with existing "Bench" thread of the Versalab M3 grinder.

[JonR10]

Follow on discussion merged with existing "Bench" thread of the Versalab M3 grinder.

Shoot - I had intended not to actually join IN the discussion of the M3, but rather to comment ON the discussion. Oh well, it was just an opinion anyway and everyone knows what THOSE are worth...

[HB]

Sorry Jon... give a moderator some new buttons to twirl and they will feel compelled to use them.

[barry]

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. Many are set to around 12 bar from the factory to protect the machine's plumbing, not regulate brew pressure.


(image courtesy of Ulka S.p.A.)

which line are we looking at here?

[HB]

The Ulka website doesn't specifically say what the three lines represent. My guess is the solid line is the median and the dottted lines are the acceptable error limits.

[barry]

okay, then are you guys pulling huge doubles? it looks to me like a "normal" sized double would push in at 10 to 11 bar.

[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.

[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"

[HB]

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?

[barry]

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"

[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.

[barry]

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"

[HB]

here's what we need to know...

Sorry, I'm not sure what the question is anymore.

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."

[barry]

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

[barry]

Sorry, I'm not sure what the question is anymore.

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]

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.

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