Measuring group static brew pressure versus flowing

[HooHaw]

Should I allow a flow (eg, 2 oz-25 s) when measuring group pressure? Or is measuring static pressure the correct method?

I've been lurking for about a year now and as I understand it group pressure is taken with no flow (or leaks).

[HB]

Bob suggested this setup:

It seems to me that if your going to measure brew pressure and make adjustments, then you need to know the exact pressure that you have while pulling a shot. Most portafilter gauges measure only the static pressure at the brew head when the by-pass valve has opened. That is not the pressure you are getting when you are brewing espresso. The attached photo is a simple mod I made to the gauge I bought from Chris Coffee. I can adjust the needle valve to allow precisly 2 oz of water to flow in 25 seconds while reading the pressure. On my machine, the resulting pressure is 8 bar, as seen in the inset.



here it is in use:

The static pressure reading for vibratory pumps runs slightly higher than the actual brewing pressure, on the order of 0.5 bar. The difference for a rotary pump machine's static versus flow rate reading is even smaller.

[DavidMLewis]

The static pressure reading for vibratory pumps runs slightly higher than the actual brewing pressure, on the order of 0.5 bar. The difference for a rotary pump machine's static versus flow rate reading is even smaller.

Depends on whether or not the machine has a gicleur. With one, the difference can be much larger.

Best,
David

[robinje]

The pressure gauge on my Anita measures roughly 0.2 bar lower when pulling a shot versus using a blind filter. If there's a flaw in my technique where I dose too little, don't tamp sufficiently, or grind too course, the difference is on the order of 0.3-0.5 bar. I assume this is because the puck is not offering as much resistance (back pressure) as it does when my technique is "better". I have my brew pressure set at 9.0 bar using a blind filter. From the factory, it was set around 10.5 bar, but changing it was a very simple procedure.

Hope this helps...

Jim

[HooHaw]

I guess if one takes pain to measure temperature with flow then one should also do so with pressure, right?

Thanks for the link Dan.

More tweaking to the PF gauge then! It won't cost much more...

[gscace]

Bob suggested this setup:


The static pressure reading for vibratory pumps runs slightly higher than the actual brewing pressure, on the order of 0.5 bar. The difference for a rotary pump machine's static versus flow rate reading is even smaller.

dunno how I missed this thread. Actually the difference in pressure can be very large. On one vibe pump equipped machine that I measured, the difference between flowing at the rate I set for thermofilters was about 65 psi compared to static. That's 135 psi static and 70 psi under flow conditions. The manufacturer was very interested to learn this and his quick attention vastly improved the coffee.

Rotary pump machines with little flow restriction may see little pressure drop. Small diameter gicleurs, long flow paths, small passageways, and proximity and configuration of group solenoids all affect pressure drop under flow conditions. Sweeping the difference under the rug is a very bad idea, because it can vary widely and complicate machine setup needlessly. I propose that folks measure under flowing conditions. It will become much easier to do so in the very near future.

-Greg

[old442]

I propose that folks measure under flowing conditions. It will become much easier to do so in the very near future.

-Greg

Would this be in reference to the device I saw on your counter next to the fridge?
If so I want one!

Kurt

[gscace]

Would this be in reference to the device I saw on your counter next to the fridge?
If so I want one!

Kurt

Yup.

-Greg

[cannonfodder]

Now you did it, the cat is out of the bag. What device?

[gscace]

Now you did it, the cat is out of the bag. What device?

I'm gonna make a version of the thermofilter that's a complete technician's diagnostic tool for setting brewing parameters. So it'll do the normal temperature thing, and also measure pressure under the flow conditions specified in the WBC machine testing protocol. I may make other flow rates available. I'm may also offer an electronic pressure transducer option, with logging capability. There's an option for NIST traceability for the pressure measurement from the gauge manufacturer, so if folks have need for that (such as devices used to set up competition machinery), then it'll be available. It's a pretty sweet device, actually. It'll make it so that people can do a better job of machine setup and quality control, because the measurements get done under conditions more approximating real use, and it uses a much better pressure gauge than is currently being supplied on static pressure measuring systems.

-Greg