Most of us run at 9 Bar.
Hope this helps.
I have a Rocket Giotto Premium Plus thats less than a couple of years old. It mostly works very well, but Id been noticing my first shot in the morning - warmed up for at least an hour, flushed and false steam pressure released - often pours slower than the afternoon. I havent quite put my finger on whether its an issue or not yet, but it compelled me to check the brew pressure with a home made portafilter gauge.
The gauge I built is a little better rated than the usual suspects Ive seen, but its pretty much the same setup as everyone uses. Except I didnt bother with a needle valve.
The gauge takes a little while to build to full pressure and finally hovers on 150 PSI (10.3 Bar), whether its hot or cold. Final pressure is steady and consistent.
I doubt my initial concern is related to the OPV setting, - and Im now thinking I dont have an issue at all - but is it worth dropping the pressure down a little anyway?
What does everyone else run their OPV at?
Most of us run at 9 Bar.
Hope this helps.
Welcome "Blackman".... [smiley=thumbsup.gif]
That pressure to a closed head sounds pretty close to me mate; I wouldnt touch it. If you had fitted a Needle Valve and controlled the flow rate to the same rate as for a Double Shot, Im pretty sure you you would be looking at very close to the nominal 9.0 Bar.... ;) 8-)
More likely, the change in pour rates is due to changing ambient conditions but leaving the grind setting static. Different relative humidity is the main culprit and you need to slightly alter the grind setting to suit... :)
Mal, that adds a rather ambiguous error vector into the calculation. Is it standard practice then, using a fixed portafilter gauge, to set the machines to say 10.5, with 1.5 bar allowed as a extraction pressure differential?
I set it to 9 bar on the nose, before your reply. Hard to tell what effect its had. I stupidly changed beans at the same time, so of course needed to change grind for that reason alone.
Crema is spotted nicely. Coffee is great, but it was pretty great before too. What should I be seeing/tasting if its set too low?
For anyone interested, its a pretty simple adjustment in itself but to get at the valve you have to take out the water tank, undo four screws - taking care not to drop them into the case, and unscrew the water tank holder (2 rusty screws). Getting it back together is a bit more fiddly, trying to align the screws with the water tank holder, but its not a huge deal if you swear enough and have a bandaid handy.
Edit: I see Greg Pullman reckons the factor is about .25 bar, which makes setting it at 9 Bar more like 8.75 Bar in the real world. Do you think 8.75 would be optimal, or would you still notch it higher?
Gday Bm.... :)
The OPV isnt a Digital device in that it isnt either On or Off. It operates over a narrow range of pressure from the time it cracks open until it is eventually fully open.
Also, when operating against a closed head, the pressure you observe is the pressure at that point in the system with no flow. Once you initiate flow, the pressure observed will decrease at the point that flow is occurring and as has been observed by several people who either own Scace Devices or their own version of one, the pressure at the Group Handle will move in concert with the rate of flow, up to around 1.0 - 1.5Bar in some machines.
This is due partly to the design of an individual hydraulic circuit and because the small Vibe Pump used in domestic espresso machines has a limited ability to supply water flow at higher delivery pressures. The pump characteristic as such is not absolutely linear in this respect either. As the pressure increases, the pump stroke shortens against the additional pressure resistance, therefore reducing water volume delivery. Here is a link to a PDF document from Ulka that contains the pump performance charts... http://www.ulka.it/admin/moduli/m003...g_ULKA_web.pdf
If the pressure differential on a Giotto is in fact only 0.25Bar, then I wouldnt worry about trying to move it up, I doubt that any detectable difference in the cup could be identified. Maybe the next time youre giving your Giotto a service with the covers off, you could tweak it back up a smidgen or even better, grab a needle valve to fit downstream of your pressure gauge device and set your machine, right on the button.... ;) 8-)