Are you sure you do not mean a half a bar? As in about 7.5 psi? I've never seen a machine with gradations more frequent than tenths of a bar. How is this being measured? If what is being claimed is a dramatic taste difference by .75 psi, then I have something yet to experience. For now, I'm not convinced.

Just because machines don't have gauges to display that degree of detail doesn't mean it isn't relevant.

My experiments show the same results as John's work does.

I was confusing the brew pressure with the boiler pressure, which this thread is really about. And I agree that .05 bar in boiler pressure correlates with nearly a degree and a half F., and that is most certainly detectable.

It is our feeling that these are not minor flavor changes. We have learned, just this summer, about just how sensitive this process is. We were getting lots of variations in the cup that we thought was the natural result of the variability in the beans from one shot to the next or one day to the next. What it was was the small variations in brew parameters that we didn't imagine would affect the flavor, and that we therefore paid no attention to from shot to shot or day to day. Now we have serious repeatability.

We are not imagining things - nor exaggerating - when we say that the flavor change of .05 bar on the M3, 100 rpm grind speed on the M3 grinder, or 1-2 degree change on the Sivetz roast temperature on one of three beans in a blend - is a very serious flavor change. Any single one of these can make or break the flavor in a cup. And any one of these is repeatable.

I consider myself an average espresso lover and thus assume such minute subtleties would either be beyond my abilities to detect or acceptable variance. To put it in basic terms by way of example... I like red grapes more than white grapes, but I'll happily eat either for lunch. But if I'm really in the mood for grapes and we only have bananas in the house, well, I'm disappointed. I am struggling to grasp if the differences between such minute pressure changes would mean nicely ripe bananas, hard green bananas, or simply red grapes / white grapes. I don't mind a little serendipity in my espresso as long as it's within an acceptable range. In contrast, based on the writings of some professionals, I imagine them flying into a rage if the temperature is off by 0.4F from the previous extraction. That's not me.

More seriously and directly to your point: Is the tight control you describe within .05 bar of a fixed pressure, or do you intentionally vary pressure through the extraction to produce a "pressure profile"? Jim Schulman postulated on the positive effects in Received wisdom about brew pressure (excerpted below) and I'm wondering if his findings are consistent with your own.

Dan I think that the fruit tastes analogies might work, but I think to refine the enquiry.

But first let's go back to the ability of you to detect the flavor variations. As I have stated, no one has yet to fail to identify them. You might not, but I would find it hard to believe that someone with your interest in flavor would fail. A .1 bar change was even clearly noticed by a young woman who doesn't drink or like coffee. We didn't think to try her on .05.

The difference is not like red or white grapes, which if both of fine quality give equal pleasure. It is more akin to the difference between wonderful grapes and adequate/slightly inadequate grapes of any kind. Then within further revisions of pressure one will find a whole panoply of different flavors - some appealing and some not appealing and they all are difficult to describe, but clearly different.

Bananas. Are you familiar with the possibility of finding a very fine ripe banana that tastes and smells perfectly as you imagine a ripe banana should and then another time finding a ripe looking banana that lacks all that quality. It looks the part, the texture is a little tiny bit off, but it just doesn't deliver the banana it promises. Almost tasteless and a bit mealy. This might take .1 bar.

Limes. Sometimes a lime is really brimming with limey sweet/acidic juice. More often today one finds limes which look like limes but deliver less juice and the flavor is certainly citric, maybe something of lime, but hardly satisfying.

Finally. I don't expect that any others have experienced precisely what we have - in fact I would be shocked! No one should expect to be able to hotrod a home or commercial machine in their spare time and have something that is on a par with what we did. Think F1 car versus very nice saturday night modified stock racer. While no one but home espresso fanatics were paying much attention to espresso quality, Versalab threw over 15,000 hours and $140,000+ into creating the most nutso espresso machine imagined. We are not stupid. If we could have accomplished 60% of the M3 by tricking out an available machine we would have. But we felt we saw too many problems with existing equipment so chose to go our own way. Then our development continually revealed new levels of clarity at each design revision. So we kept at it and at it. To some very fine levels of detail.

We presently use a fixed pressure brew.

when you changed the brew pressure, did you change the grind as well?

rfc wrote:I was confusing the brew pressure with the boiler pressure, which this thread is really about. And I agree that .05 bar in boiler pressure correlates with nearly a degree and a half F., and that is most certainly detectable.

No, you had it right the first time. John is talking about flavor changes resulting from a change in extraction pressure of 0.05 bar.

barry wrote:when you changed the brew pressure, did you change the grind as well?

No. We did not need to. Flow through the bisket is pretty self-compensating providing the grind and pack are good.

John wrote:Flow through the bisket is pretty self-compensating

That's good, John. Illy calls it the "coffee cake," most people around here call it the "puck." But I kind of like the "bisket."

HB wrote:Presumably the same logic applies to pressure: Does it not vary from the top of the puck to the basket's exit? Excuse my lack of knowledge of fluid dynamics, but how could a small difference of 0.05 bar produce a taste difference when that same extraction pressure is "experienced" mere millimeters below the uppermost surface? I suppose therein lies the danger of applying my limited engineering knowledge to taste; if indeed any Joe or Jane off the street would appreciate the taste difference with ease, I can live with not understanding why it matters and simply accept that it does.

I've seen some research that shows pressure is less important as pressure goes above 9 bars, but more important if it is too low. However, as I learn more about coffee and as I'm able to refine my process control and technique I learn more about these things and I'm not sure that the methodology provided in the study I read is without fault. It's certainly fertile ground. With respect to pressure distribution in the cake, the pressure profile is less complicated than the temperature profile. Pressure drop across the cake should be relatively constant in the flow cases here, since the velocities are fairly low through the cake and since there is supposed to be some headspace above the cake, which should result in constant pressure across the upper surface of the cake. In this case, pressure drop should be linear across the resistance (the cake) and pretty simple, although I'd add the deeper baskets drop pressure differently than shallow ones, since the resistance to flow occurs over a longer distance.

WRT temperature uncertainty, John's comments are correct, and people generally sweep the uncertainties under the rug. As far as thermofilters are concerned, right now I'm makeing them with only type T thermocouples, since type T is the best one to use for boiling wire temperature and I'm using probes constructed of "special limits of error" wire. That cuts down uncertainty as much as I think is economical, since most folks can't afford calibration services and I don't think they're really necessary anyway for what we are doing.

What gets confusing when one discusses measurements is the difference between accuracy and precision. One of the usual things that affects accuracy in thermocouple measurement with cheap readout devices is the level of care in temperature compensation of the readout device. If you really want to kick ass with thermocouple measurements, you make the measurements with a high accuracy voltmeter, and sink a referemce thermocouple junction in a well-insulated bath of distilled water ice. In this case, the voltage measurements are referenced to the ice point, and you can do very good thermometry. It ain't practical in real life and it's expensive (an Agilent 3458A is around $10k these days), so we make do. The way cheap readout mfrs make do is to use a thermistor to measure the temperature of a small block of copper inside the readout device, assume that the "reference junction" is at the temperature of the thermistor, and compensate accordingly. the thermistors used for this may not be very accurate, in the case of cheap readouts, and the degree of correlation between the actual reference jcn temp and the temperature measured by the thermistor may be pretty crappy. Howeve, one can get very repeatable answers if one recognizes this and takes some practical steps to minimize the problems. Good things to do are to keep the readout device away from heat or cold sources, don't handle it when making measurements, and always use the same readout device. In this way, you'll get good reproducibility from pretty cheap gear.

-Greg