general rule of thumb is to check brew pressure with a portafilter mounted pressure gauge (Scace II or other) rather than the machine one. Pressure at the brew head is what you're actually looking for.

EricC wrote:I have just re-checked the flow rate of my GS/3 and double checked the reading on the pressure gauge and without the portafilter it is only reading at 7 bar, nowhere near 9 bar.

I don't understand how you can get a reading of 9 bar without some sort of obstruction to influence the pressue, be it coffee or a backflush blank.

When others are checking their free flow rate what pressure are you reading on the built in gauge on the GS/3?

My GS/3 is set for 9 BAR free-flow pressure on the built-in brew boiler gauge.

I believe that for a given setting of the pump bypass valve, the tubing diameter between the boiler and group head dispersion block, including the .6mm gicleur (flow restrictor) in the group feed tube, establishes the free flow pressure. In answer to your question, you can get a free-flow pressure of 9 BAR simply by adjusting the pump bypass valve so that the built-in gauge reads 9 BAR during free-flow.

9 BAR on the built-in brew boiler pressure gauge has been the value in effect when I've done all of my flow rate measurements, on both GS/3s. At 9 BAR free-flow, the pressure rises to 9.5-10 BAR against a blind filter. I believe once that level is reached, the 3-way valve caps the pressure. I checked the blind filter pressure at the group head with a homemade PF pressure gauge, which uses a liquid-filled gauge recommended by Greg Scace, and it reads the same as the built-in brew boiler pressure gauge. That's what should happen when comparing blind filter pressure readings at the brew boiler and group head -- i.e., they should be exactly the same.

When pulling a shot, the built-in pressure gauge climbs to about 9.5 BAR. With my homemade PF pressure gauge adjusted to a flow rate of approximately 70 ml in 30 seconds, which I believe is the standard shot simulation flow rate used with a Scace II, the pressure at the group head reads about 8.5 BAR. I believe it's typical for the pressure difference between the boiler and group head to be about .5-1 BAR, so this looks normal to me. 8-9 BAR seems like a good pressure for pulling shots, so that's why I've been setting the built-in gauge to 9 BAR. I suspect this is why the value 9 is highlighted on the gauge.

7 BAR free-flow pressure on the built-in gauge seems low to me, but that doesn't necessarily tell you what the pressure at the group head will be when pulling a shot. If your built-in gauge pressure rises to 9 BAR when pulling a shot, then I would think the pressure at the group head is running 8-8.5 BAR. The GS/3 instruction manual says to use the bypass valve to adjust the pressure to 8-10 BAR when pulling a shot, so that's the target. As long as you get pressure in that range at the brew boiler when pulling a shot, you should be in the right range at the group head.

However, the fact that our free-flow pressure differs by 2 BAR could explain the difference in the free flow rate that we're measuring. I'd be interested to know what flow rate you get if you adjust the bypass valve for a free-flow rate of 9 BAR. You might also want to see how that setting affects your shots.

BTW, I'm not quite sure what you mean by talking about the pressure with the portafilter removed. There should be no detectable difference in the free-flow pressure reading on the brew boiler pressure gauge with the (empty) portafilter in place or not in place, nor should there be any difference with the diffusion screen and screw removed. The screen, screw, basket and PF don't provide any significant restriction to the flow. Of course, when everything is in place with coffee in the basket, the pressure will rise.