Here's a question that I received in a PM here that seems worth posting along with the response:
Dan Streetman wrote:Greg,
I am wondering about when using a scace 2 what your experiences have been in the descrepency from the pressure gauge on machines to the scace reading at the scace.
I am asking because recently a customer had a tech adjust their pressure using the scace 2, and he adjusted it to read 9 bars on the device. While I don't believe this is necessarily wrong, I am unsure if this is desirable, or if we should be getting a different reading because of flow restriction etc. They are using a La Marzocco FB-70 Hybrid, with the .08mm gicluers.
Anyway the main reason I am skeptical is that, the coffee has not tasted as good since the adjustment, and a couple of the barista's have commented about the machine making a funny squeaking noise. I also notice that since the adjustment the pump pressure will hit 12 on the gauge, which should activate the excess pressure release into the drip tray, and I am not sure what effect that is having on the extraction.
Cuvee Coffee Roasting Company
Scace 2 reads pressure immediately downstream of the dispersion screen (or upstream of the coffee cake), as you prolly already know. The pressure immediately upstream of the coffee cake is the most useful pressure value when we are talking about machine setup for specific coffees. All espresso machines experience pressure drop between the pump and the group. Energy gets expended pushing the hot water through tubing, solenoids, and small drilled passageways and the energy used to push the water through the plumbing system shows up as pressure drop. Different designs from different manufacturers use different plumbing schemes, and different designs locate the pressure gauge at different points along the flow path. So if machines A and B are different designs, one can't just adjust the pressure by looking at the machine's internal gauge and assume that the brewing pressure at the top of the coffee cake in machine A will be the same as the pressure at the top of the cake in machine B. The brewing pressure is also affected by group gicleurs and the amount of lime scale present in the system. Small gicleurs and the presence of scale increase pressure drop between the espresso machine's pressure gauge and the group. Any of a range of gicleur sizes may installed in otherwise identical machines (for example, a Linea AV with no gicleurs compared to one with 0.6mm gicleurs), and scale formation may vary considerably from one machine to another, or even from one group to another in multi-group machines. So similar pressures at the internal gauge of presumably identical machines will not necessarily produce identical pressures at the top of the cake.
The whole fluid flow thing has another wrinkle that produces pressure errors - static vs. dynamic pressure. If one measures pressure at the group using a gauge connected to a blind filter, a very common industry practice, the measured pressure will be higher than the actual pressure when brewing coffee. The difference can be quite large. I once was asked to do a technical review of a machine that used a vibe pump with an integrated pressure relief valve. The difference in pressure at the group between static case (measured with a blind filter), and pressure when brewing coffee was 4 bars - HUGE.
1) You should see a pressure discrepancy between Scace 2 and internal pressure gauges because water is flowing through the espresso machine and pressure drop occurs between the upstream internal gauge and the group. The Scace 2 should read less than internal gauges. FWIW, the pressure gauge used in Scace 2 has an accuracy spec of +- 2 psi at normal brewing pressures.
2) Adjusting pressure to 9 bars at the group using Scace 2 may not produce the best result. This is analagous to blindly setting your temperature controller to 203.1 degrees. You must correlate the pressure value to what tastes best. Then use the Scace 2 as a quality control tool so that your machine ( or machines if you are maintaining machines at different locations) produces the same pressure and temperature at the cake as it ages.
3) The pressure value from the Scace 2 is transferable between machines of different design / manufacturer because the indicated pressure is independent of upstream plumbing effects.
Lemme know if you have other questions.