So I read here:

Brew pressure and its effects on espresso

About the brew pressure gauge and the brew pressure being quite a bit different. I am wondering if this is still accurate information.

Interestingly enough my previous issue with my drop in brew pressure appears to be related to having set the OPV too high. When I get the gauge to read right at 9bar, everything works fine. But in the above referenced link, having the brew gauge reading at 9bar might actually be low.

When I set the OPV higher, then things change considerably. Now the machine does its thing up to around 9 bar, and then gets very, very quiet, and slowly increases the brew pressure back up to 9.5, or 10, or wherever I have it set. Also noted is the fact that extremely little or no water is coming back into the reservoir tank from the "over pressure"? line from the OPV (when the machine is operating normally, after it gets to 9bar, plenty of water comes back thru this line back into the reservoir).

The physics have not changed. Pressure drop depends on the machine, pump used, gicleur and other layout specifics.

-Greg

buzzmc wrote:So I read here:

Brew pressure and its effects on espresso

About the brew pressure gauge and the brew pressure being quite a bit different. I am wondering if this is still accurate information.

Interestingly enough my previous issue with my drop in brew pressure appears to be related to having set the OPV too high. When I get the gauge to read right at 9bar, everything works fine. But in the above referenced link, having the brew gauge reading at 9bar might actually be low.

When I set the OPV higher, then things change considerably. Now the machine does its thing up to around 9 bar, and then gets very, very quiet, and slowly increases the brew pressure back up to 9.5, or 10, or wherever I have it set. Also noted is the fact that extremely little or no water is coming back into the reservoir tank from the "over pressure"? line from the OPV (when the machine is operating normally, after it gets to 9bar, plenty of water comes back thru this line back into the reservoir).

On my two Vibiemme, there is a 1 bar delta between the two brew pressure gauges for about 8.5 bar on a puck / Scace.
DB gauge reads ~9.75 and HX reads ~10.75. When first measured in 2007 [ see Digital Pressure Adapter ], the HX gauge read about 1 bar high and has drifted up over time.

I usually see half a bar to one bar high on the gauge, depending on the machine. The gauges are not the most accurate or expensive. They are more of a good guide than an absolute. Choked or blind pressure will also be higher than what you get with a properly flowing puck. I would adjust my pressure one bar high with a blind basket. So if I wanted 9 bar, I would set it at 10 with the blind basket. Rotary pumps tend to behave a little different and run closer to the flow pressure when using a blind basket.

S'why I adapted a much more accurate manometer to my Scace Device. Turn out that the 8.5 Bar I was set at when I finally decided that I was still getting weird results was actually just over six. Set at almost eleven on the machine, I'm actually showing a tad over 8.8 on the Scace.

A quick question then. When I keep reading about using approx 9bar pressures, would that be measured at the puck (ala Scace type device) or at the group head (ala built in pressure gauge)?

That's an interesting question. I don't know for sure how the measurements were performed that produced the canonical 9-bar figure, and I don't think many folks do. Illy suggests a range of pressure that falls within his definition of espresso, so I suggest that you first realize that even though different pressure measurement methods may produce different answers, the range of "acceptable" pressure is pretty large, if you go by the definition. I've proposed that the most interesting values of P and T are the ones directly above the coffee cake, and i do my measuring there under a standardized set of conditions that mimic espresso brewing. That way you don't have to consider which machine produces what pressure drop between blind filter testing and actual brewing. Then I use my taste buds to figure out what i like. The measurements produce convenient numbers that allow you to replicate the brewing conditions you previously liked, in case something changes down the road in your equipment, such as pump failure, supply pressure change, pressure relief valve mal-adjustment, or whatever.

Generally the dialing-in process should be 3-steps:

1) Measurement of T and P should be performed to get you into the ballpark, say 200F and around 8.5 bars at the group with a Scace.

2) Critically taste the coffee and make changes to T and P according to what tastes best.

3) Measure and record T and P so that you may return to these values in the future if something changes in your setup.

I usually expect Optimum T to change depending on what I'm feeding my grinder, but I don't change pressure profile a whole lot, unless I have some very unusual coffee.

-Greg

Greg,

Something puzzles me about pressure measurements.

You've said that the flow meter in your Scace I and Scace II devices is calibrated (the hole is sized) to produce 70ml of fluid in 30 seconds at 9 BAR of pressure. But do you know you had 9 BAR at the puck when you did the calibration? If pressure affects flow rate, and vice-versa, don't you have a chicken-and-egg situation?

+1

Long before I got the Digital Pressure Gauge and, much later, a Scace, I'd backed into ~8.5bar on the puck by adjusting for taste, ignoring the pressure gauge indication.

Echoing Dave, typical prosumer gauges are cheap & cheerful. Initial and long term accuracy is highly suspect.
A quality vendor calibrates a machine with a Scace in the pre shipment checkout, but has no control over gauge drift.

On the Vibiemme Double Domo, after about 6 months, the pressure dropped rapidly over a few weeks. The OPV spring was the culprit. Since replacement, brew pressure and relative gauge indication have not shifted in over two years.