Advice want: slow extraction with Breville Double Boiler BES920 + Smart Grinder Pro
I've got a Breville BES920 Double Boiler and a BCG820 Smart Grinder Pro at home. I've had the Smart Grinder Pro for around 6 months, which I was originally using to make Aeropress. Around 2-3 months back we got a BES920 at my work, and after a few weeks of learning I was able to make consistently good coffee, so we decided to get one for home when Harvey Norman were doing a clearance deal.
At work, with the exact same models of grinder+machine, I'm able to make a consistently very good flat white. When pressing the single shot button on the BDB at work, as soon as pre-infusion finishes, the pressure gauge usually goes up to just above 9, then hovers there, and coffee starts to flow a few seconds later. The 30s timed extraction usually leads to the right volume, and adjusting the grind has the expected effect on flow and volume.
At home, however, if I use the same settings on the grinder and machine, after pre-infusion, the pressure gauge goes straight up to the top of the "optimal" range (around 10), then drops back slightly to around 9.5, and it often takes 5-10 seconds before anything comes out. When it does, it's often a slow drip, not a consistent flow, and once or twice nothing has come out at all. To counter this, I've increased the grind coarseness a few notches (so it's around 13 vs 9 on the same model grinder work), but I don't get a "good" looking extraction in terms of volume and rate unless I go up to 15-16 on the grind, at which point the coffee seems to have less flavour and body.
I've been buying the same beans at home as we use at work, and have gone through a few bags both at home and at work whilst trying to work out what's wrong with my home setup, so I don't think variations in the roast/bag of the beans is a factor.
Does anyone have any advice on whether this is likely due to the grinder or the machine? Is it possible that the Smart Grinder Pros aren't objectively calibrated consistently, so 15 on my grinder could be the equivalent of 9 on another Smart Grinder Pro?
I've tried cleaning the grinder (with the brush that came with it) and backflushing the BDB a bunch of times. One thing that's may be completelyunrelated that I've noticed is that on the BDB at work, when I lock the group handle, it always quite easily goes a little way past "perpendicular", whereas at home there's a lot more friction, and it usually locks slightly before being perpendicular to the machine.
Many thanks in advance!