Post By MrJack
Pressure profiling hack - Any downsides?
So today, after wasting almost 200g of a Guatemala SO, I worked out a little trick to reduce the brew pressure and finally ended up with a solid shot in the cup before I ran out of the beans.
The trick was to open the hot water slightly during the extraction which regulated the pressure to basically whatever I wanted it to be. The more I turned the hot water knob, the more it reduced the boiler pressure which in turn resulted in lesser pressure in the brew head.
Given that how simple it is, I don't think I am the first one to have discovered this trick but I can't find anything on this forum on this subject. Even after trying many keywords on google, I can't find anything.
The appliance I use is a Breville BES920 dual boiler. I call this an appliance as it always will be in my eyes as I learned the art of coffee on a LM Linea while working in a cafe back in my uni days. Anyway, if I can do this with my appliance, I'm sure people with HX or other DB espresso machine can do this too.
And if you can, then what has been your experience?
Are there any downsides of using this method?
The only downside I can think of is that the pump will be working above the OPV threshold which may mean that it will be working at full throttle during the brew cycle while you are tweaking the pressure. This may result in reduced life of the pump but I can't see why that would be a problem in a home environment especially if you allow enough rest time between each brew cycle.
I'm surprised your 920 lets you get away with that. Mine and every other one I've used stops the shot if you open the hot water valve enough for it to register it's open. You can get a tiny bit of water flowing before this point usually, but not much.
Otherwise I'd be very curious to try it out. Sounds like an interesting idea.
Yep. This is a thing. Used to play with this a bit when I had a 900.
Sorry I should have mentioned that it was done using the "Manual" button. Pressing and holding the manual button starts the pre-infusion, as soon as you release, the pump goes to full pressure. You can then use the hot water dial to tweak the pressure.
You are absolutely right that if you try this method with the single or double programmed buttons, it stops the pump as soon as you open the water.
Do the HX and other prosumer DB machines stop if you open water?
How does opening the hot water tap (which is fed by the steam boiler) affect the pressure at the group (which is fed by the brew boiler)?
If I open the hot water tap while pulling a shot on my BZ99 (HX machine) the boiler autofill usually kicks in, which diverts flow away from the group - but there is no degree of control...
The 900/920 is obviously configured to refill steam boiler as it is drained. So some of the flow will go to refill boiler instead of the shot.
I would say this is not possible on most HX /DB machines as the boiler is not refilled during shot. Some rotary pumps do but the pump has enough flow to do both.
I have read people doing this in the Sunbeam 6910 also.
Works great for flow rate on the Via Venezia but screws up the already pretty poor temperature control :-)
Thanks for the responses everyone.
MrJack, what you said makes perfect sense so I did a little more digging and found a simple schematic/P&ID representation of my machine's internals (attached).
This backs up your thinking that opening the water valve will kick in the steam boiler pump but since brew boiler's path has no leakage, the brew pressure shouldn't drop.
I am now more confused and curious as to why it happens. The flow rate reduces and the pressure gauge shows reduced boiler pressure proportional to the water valve opening rate. It goes up to full pressure as soon as i close the valve and the response is pretty much instantaneous i.e nearly zero lag between water dial adjustment and the pressure/flow change.
If this can't be done on other machines then another theory I have is that it may not have anything to do with the mechanical portion of the machine but the electrical components instead. If the hot water is drained, the heating element in the steam boiler (Thermistor controlled) demands more power and the control board limits the current to the 15bar pump as it may have been set to never exceed a certain threshold from the main supply.
I wish I could ask Breville to comment on this but I am sure they wouldn't want to share the internal working recipe of their machine to consumers.
Search on home barista. A guy there went quite far into researching this on his 920