When adjusting the pump pressure for brew, how should one adjust this based off the machine's gauge if I do not have a Scace device? Should I be pulling a shot and adjust the pump to the desired pressure (ie. 9 bars) or should there be just free water running from the group without a portafilter? Or should I be running the group with a blind basket?

The manual says to do the adjustment while pulling a shot but after trying to find info online, I've read that La Marzocco suggests to do the adjustment with free running water from the group, was there a change in the gauge readout that changed the process? I've noticed that if I adjust while brewing with coffee grinds, the pressure drops by about a bar when it is just free flowing water.

Thanks!

Adjust the pump to your desired pressure with free flowing water . . . most definitely NOT with a blind basket with a rotary pump machine.

Like Eric said: adjust the pump bypass valve so that the coffee boiler gauge reads 9 BAR (or whatever pressure you desire) at free flow.

When the pump is set to 9 BAR at free flow, the gauge will rise 1-2 BAR when you pull a shot. This is due to resistance of the puck. But the 0.6mm gicleur drops the pressure at the puck down by an equivalent amount, so you get 9 BAR at the puck. I'm not sure that this is true at other pump settings, but I've confirmed it with a Scace at 9 BAR.

Got it, thanks both!

The owners manual say (on page 7) that the pressure should be adjusted to 9 bar with the machine operating and coffee in the portafilter

Yep, but seems like info online seems to be the same as others suggests, with free flowing water.

My rationale for adjusting the pump pressure while simply doing a flush through an empty portafilter or no portafilter (doesn't matter) is that you are now flowing substantially more water as compared to actual espresso making. Just to "throw out" some representative numbers for the GS/3, free flow is ~475 ml/minute whereas espresso brewing is ~ 120 ml/minute. The GS/3 pump is rated at 70 liters/hour (1167 ml/minute) which I believe is an assigned rating at 100 psi discharge pressure. I also believe the GS/3 uses a Fluid-o-Tech (FOT) pump - can someone provide some details like FOT model numbers?

As with all espresso machines fitted with rotary pumps, the overwhelming majority of the flow is simply redirected back to the pump's inlet.

For sure there is a difference between brew boiler pressure and the pressure that exists on the top of the coffee puck. But, this differential is so marginal as to be inconsequential during espresso production.

Makes sense. And I assume how the grinds are dosed could affect the readout if adjusting with grinds in the portafilter (ie. very coarse will result in a lower readout?).

No.

By turning the pump's adjustment, you are simply increasing or decreasing the amount of water that is being bypassed until the brew boiler pressure is at your desired value. The fineness of the grind and the amount of the dose will decrease/increase the flowrate through the coffee bed but the brew boiler pressure should remain the same.

This would be easy enough to verify.

erics wrote:My rationale for adjusting the pump pressure while simply doing a flush through an empty portafilter or no portafilter (doesn't matter) is that you are now flowing substantially more water as compared to actual espresso making. Just to "throw out" some representative numbers for the GS/3, free flow is ~475 ml/minute whereas espresso brewing is ~ 120 ml/minute. The GS/3 pump is rated at 70 liters/hour (1167 ml/minute) which I believe is an assigned rating at 100 psi discharge pressure. I also believe the GS/3 uses a Fluid-o-Tech (FOT) pump - can someone provide some details like FOT model numbers?

As with all espresso machines fitted with rotary pumps, the overwhelming majority of the flow is simply redirected back to the pump's inlet.

For sure there is a difference between brew boiler pressure and the pressure that exists on the top of the coffee puck. But, this differential is so marginal as to be inconsequential during espresso production.

Since the pump pressure setting is differential to the mains pressure (i.e. pressure at output = pressure at input + bypass spring force) - doesn't this reasoning depend on the pressure before the pump remaining constant whether during freeflow (no portafilter) and full pressure espresso pull? Doesn't have to be if one uses filtration or thin gauge tubing.