For your reading pleasure, here are a couple more tests to measure the effect of varying pump pressure on the rate of espresso extraction. Last time the results suggested that increasing pump pressure above ~8 bar actually decreased the extraction rate, and I wanted to verify this result.

In the first experiment, I kept dose and grind constant and simply measured how much espresso I got in a 25 second extraction at various pressures. Once the pump came up to full pressure, the pressure was maintained as flat as possible. In particular, I was looking at the lower pressure realm, because some data in Illy's book suggested the the flow rate might go up around 5 bar. I saw no evidence of this:



In the second experiment, I kept the dose and grind constant and set the flowmeter to terminate the shot after 48 ml of water had been pumped into the boiler. This resulted in shots that were fairly uniform in volume (19.0-21.4 grams). To get the average flow rate, I divided the volume in grams by the seconds of flow (total seconds minus 8.5 seconds dwell time). This data suggests that the flow rate peaks somewhere around 8 bar:



Notes:
1. I've been saying for years that it's the pressure/volume relationship in the first few seconds of the extraction that is most critical in determining flow rate. I still believe this, so I kept the parameters in the initial 4 seconds of each extraction constant.
2. I tasted all the shots, especially in the second test, where similar volumes permitted realistic comparisons. It would be very easy to draw hare-brained conclusions from these, but I believe I'd have to run a lot more shots with a blind taste panel to get any definitive results.
3. I also have a hare-brained theory for why espresso seems to come out best around 8-9 bar, but I'll save it for when I can actually prove it (if ever).
4. I bought a new 1.5% pressure gauge to calibrate my setup. I believe the pressures in the graphs aren't perfectly accurate, but they're pretty close to what the cake is actually seeing.

Wow! What a terrific set of measurements.

Well my old economist self is pleased. Seems the flow, ceterus paribus, is maximized at the 8 to 9 bar range; just where one would expect the curve to have its maximum given that this is the brewing range used for the last 50 years.

I also believe the dwell time is probably where all the interesting action is. The fines migrating downward are in a race against the particles swelling from water absorption and locking them in (Think Indiana Jones fines dodging flood waters and ever enlarging boulders). Presumably, the flow rate around 10 seconds after the flow starts (this is roughly where I decide to dump a gusher or dripper) will reflect how close the fines got to forming a barrier in that initial race.

But does this mean the fines are getting locked in higher up in the puck at 8.5 bar than at 5 or 12 bar?

another_jim wrote:The fines migrating downward are in a race against the particles swelling from water absorption and locking them in (Think Indiana Jones fines dodging flood waters and ever enlarging boulders).

Couldn't have said it better myself.

another_jim wrote:Presumably, the flow rate around 10 seconds after the flow starts (this is roughly where I decide to dump a gusher or dripper) will reflect how close the fines got to forming a barrier in that initial race.

But does this mean the fines are getting locked in higher up in the puck at 8.5 bar than at 5 or 12 bar?

The fines seem to be an "x factor" whose migration is highly dependent upon the preinfusion.

As far as the 5 bar, 8.5 bar, 12 bar extraction rates go, they may be explainable through the interaction of two conventional, yet competing factors:
(1) higher pressures normally produce higher flow rates through a fixed resistance
(2) but the resistance isn't fixed, because higher pressures compact the cake, reducing its porosity, resulting in lower flow rates.

I keep wanting to make an intelligent comment on this thread, whose initial post I read a number of hours ago. But it escapes me.

So I am reduced to saying, "thank you for your efforts, Andy," and don't interpret a lack of further commentary, on my part or on the part of others as disinterest. Rather, we lack the intellectual horsepower to say anything that gives the appearance of being an intelligent response.

ken

Ken Fox wrote: don't interpret a lack of further commentary, on my part or on the part of others as disinterest.

Ken, thanks for your generous comments! But the problem with these Pump/Pressure posts of mine is that so far, they reside in the realm of theory only. I'm hoping that eventually I'll stumble upon something that'll actually be of use in making better espresso (see photo of actual espresso below). Until then, this is probably just of interest to geeks and nerds like me.

Thanks Andy for the work!

I won't pretend to understand the theory completely but I think I get the gist of it. It's interesting to consider the actions of the fines with respect to blooming especially with pre infusion and the overall effect on puck density/resistance also relative to pump pressure.

From what I've read pulling a lever shot doesn't generate as high of a pressure as 8+bars. Less gets extracted and that would account for a smoother, maybe sweeter shot? That seems to be what your results are showing.

It's a difficult thing to measure, I think, when extraction/flow rates seem so sensitive to grind composition, dose, distribution and packing. What comes to mind is ensuring ideal conditions with some sort of imaging device coupled with some sort of high frequency shaker. The imaging device could scan the grind distribution to activate the shaker until a uniform distribution/density is achieved. An automatic tamper set for pressure could finish with a theoretically perfect tamp.

mrgnomer wrote:From what I've read pulling a lever shot doesn't generate as high of a pressure as 8+bars. Less gets extracted and that would account for a smoother, maybe sweeter shot? That seems to be what your results are showing.

I don't own a lever machine, so I have no experience there. But if extracting less (or extracting at lower pressure) was the secret of smoother, sweeter shots, pump machines would have no problem doing that....

AndyS wrote:Until then, this is probably just of interest to geeks and nerds like me.

There may be more geeks here than you can chuck a puck at.

You, Ken, Jim et al. are truly appreciated.

Many Thanks.

Andy -

Thanks for the time you took in running these tests.

I have read the particular sections of Illy's book you mention and, to be honest, I still trying to figure out what he is trying to illustrate.

The illustration is reproduced here for discussion purposes:


Another idea would be to video a scale readout during the brew process and graph the data much as you did with the Silvia flat-liners you did previously.

EricS wrote:Another idea would be to video a scale readout during the brew process and graph the data much as you did with the Silvia flat-liners you did previously.

Ken has this really small 0.1 gram scale from Hongkong. You could put something like this under the cup, along with a timer, while you video the pour, to get a proper output volume estimate. You already have the flow meter for input volume; so perhaps there is some gold in looking at how the input and output curves relate. Perhaps there are differences in the amount of water the puck absorbs at different pressures.

One suggestion: at this point you aren't looking for the best shot, just the way the system behaves. A wider range of pressure (e.g 4 bar for a low, and 14 bar for a high) may give a better idea of the curve.