I had one pretty good profile/shot using a medium roast where I was able to slowly preinfuse around 4 bar, turn flow up to 8 bar, then taper off. Then I ran out of beans and switched to a light roast. Now I'm having trouble building pressure beyond 4-5 bar. Am I correct in assuming this means I need to grind finer to increase resistance?

What pressure drop would one see normally, in the absence of flow control, due to puck degradation?

That is what I'd try.

Most pump machines are a pressure source so you see a flow increase, not a pressure drop due to natural puck erosion. It is very dependent on the bean and grinder, in my experience. There can also be a secondary effect with the puck "clogging" later in the shot, dropping the flow.

It sounds like you've got your flow control set so that the valve is providing more restriction than the puck later in the shot. This is neither bad or good. There are people pulling shots from light-roast coffees in the 4-6 bar range. If you can reliably repeat your shots, explore, take good notes and figure out what works well for you.

Jeff wrote:It sounds like you've got your flow control set so that the valve is providing more restriction than the puck later in the shot.

I think it's the opposite, I've been unable to raise pressure beyond 4-5 bar even with the valve totally open. That's why I think I need to grind finer!

I'm getting 9 bar again, but have the grind so fine that it's coming out quite clumpy and irregularly from the Oro Xl. Maybe I need to back off a bit and try dosing higher. In any event, i've done some reading about lower pressure extractions and maybe 6 bar isn't so bad.

There's one thing you should try. Pull the mushroom out and make sure the o ring at the bottom is not damaged.

This will cause all manner of erratic behavior with the flow control device.

Don't see why it coming out as clumpy should deter you from grinding that fine.

If you have a wdt it shouldn't be a problem after good puck prep