Pump pressure on new Giotto Rocket v3

[profmontoya]

Hi all,

Got new Giotto Rocket v3 (Hx machine). Only 2 weeks old. Vibrating pump model. Two pressure gauges, boiler pressure and pump pressure. This morning something went astray. Filled portafilter as usual, fitted to group head and switched on pump. Coffee took ages to come through, and then still only one drop at a time, pretty much for the whole 30ml. Pump gauge remained in sweet spot (approx 9 bar). NB I realise its pump pressure and not group head pressure.

My question. If I over filled, grind too fine, or tamped too hard, why doesn't the pump push out over 9 bar. My friends machine (Sunbeam or Breville DB) pumps up to 12-13 bar if the above factors are quite right. I know 9 bar is the sweet spot. Should I leave things as they are or should I try adjust the pump (or valve) to deliver more pressure. Ultimately I know I have to correct my technique.... Just wondering

Thanks
Carlos

[Bosco_Lever]

Coarsen your grind or reduce your dose.
Doubt it is your pump that is the issue.

[Jonathon]

Hi Carlos. This is perfectly normal, assuming you ground too finely or overfilled. The Giotto has an OPV (or two) between pump and HX, which limits pressure to 9 bar, or whatever it's set at.

So although the pump is quite capable of putting out 12 bar or more, anything more than 9 bar is released by the OPV.

Cheers
Jonathon

[profmontoya]

Thanks guys. Just as I suspected.

However my query is quite specific. Should I or shouldnt I fiddle / tweak/ adjust the OPV to allow (hypothetically) more than 9 bar pressure during extraction.

Appreciate that I would then need to pack, grind and tamp just right to hit the 9 bar sweet spot... but that is another issue.

[Bosco_Lever]

It is your machine, you can fiddle with it as much as you like.

Each time you fiddle, you change the goal posts and have to make adjustments in your dose and grind, to compensate.
Learn how to dose and grind to achieve a perfect pour that tastes good. Dose and grind are the simple fundamentals of espresso (but greatly misunderstood by many). Once you understand your machine and its capabilities, you can fiddle or fine tune it. I would suggest a bit more than two weeks of use to master it.

If I bought a brand new Giotto and had to adjust it to get a perfect pour (after 2 weeks), it would be on its way back to the place of purchase for a refund.

[Robbks]

After upgrading from a machine with no OPV to one with OPV fitted, was quite a learning curve again for me.
The first few shots were not much good at all. ground too coarse, but packed too hard and over-dosed.
so while i was getting 60ml doubles in 30 seconds, the machine was returning a lot of water to the reservoir and the coffee was average.
less dose, finer grind and lighter tamp, I get that perfect pour with just the slightest dribble returning to the tank.
I did find that I could change the dosing a little and not get that return at all, but still maintain the 9bar pressure, the drinks were not as good,

[ibiza888]

It is your machine, you can fiddle with it as much as you like........
If I bought a brand new Giotto and had to adjust it to get a perfect pour (after 2 weeks), it would be on its way back to the place of purchase for a refund.

+1
Hi Carlos, Congrats on your new machine! Did you buy from our CS sponsor? If yes, it had been tuned at the right sweet spot that you shouldn't adjust any OPV setting before you adjust your grind/dose/fill/tamp skills. otherwise, no consistency is achieved. Good Luck!

[ericshum]

My friends machine (Sunbeam or Breville DB) pumps up to 12-13 bar if the above factors are quite right. I know 9 bar is the sweet spot. Should I leave things as they are or should I try adjust the pump (or valve) to deliver more pressure. Ultimately I know I have to correct my technique.... Just wondering
Carlos

Hi Carlo,

It can get a little tricky trying to compare machines. I'll start at a very broad level - the equation of pressure, p = F/A.
You can see from that equation that area, A plays a part in this.

Depending on which model you are comparing against, the size of the basket might be different. If your friend's machine is one with a 53mm basket then you can easily expect a higher pressure be used to push through the puck (assuming other factors are consistent, i.e. dose amount, grind size and tamp pressure). The force, F is will depend on the resistance within the puck itself - Dose amount and grind size basically affects the flow rate in a significant manner. From what I have observed, tamp pressure only changes the 'preinfusion' by a few seconds.

As a side note, I think the Breville DB has a factory setting of 10bars at the OPV. 9 bar might be optimum for most commercial machines / environments, but it really depends on how far you want to take your 'brewing' of coffee. You can experiment with different brew pressure, grind size and dose amount for a particular bean some the same roast batch and that will give you an indication of what changes when you vary each variable.

Hope this helps.

Cheers,
Eric

[ericshum]

Should I or shouldnt I fiddle / tweak/ adjust the OPV to allow (hypothetically) more than 9 bar pressure during extraction.

I don't think there's a hard and fast rule - but you're missing one variable here - what is the flow rate that you are bench-marking your shot against? 30s?
I think 9bar brew pressure / 58mm double basket / 27s - 30s extraction time / 20g dose amount is a good place to start. Try to identify what you like or don't like about your cup and tweak accordingly brew parameters accordingly.

[MrJack]

Hi Carlo,

It can get a little tricky trying to compare machines. I'll start at a very broad level - the equation of pressure, p = F/A.
You can see from that equation that area, A plays a part in this.

Depending on which model you are comparing against, the size of the basket might be different. If your friend's machine is one with a 53mm basket then you can easily expect a higher pressure be used to push through the puck (assuming other factors are consistent, i.e. dose amount, grind size and tamp pressure). The force, F is will depend on the resistance within the puck itself - Dose amount and grind size basically affects the flow rate in a significant manner. From what I have observed, tamp pressure only changes the 'preinfusion' by a few seconds.

As a side note, I think the Breville DB has a factory setting of 10bars at the OPV. 9 bar might be optimum for most commercial machines / environments, but it really depends on how far you want to take your 'brewing' of coffee. You can experiment with different brew pressure, grind size and dose amount for a particular bean some the same roast batch and that will give you an indication of what changes when you vary each variable.

Hope this helps.

Cheers,
Eric

Eric, that "equation for pressure" (i.e. F = P * A) is a hydrostatics equation and isn't really relevant in hydrodynamics (flowing fluids). Area is still relevant however, as it influences velocity (to which many flow phenomena, including frictional pressure drop, are correlated).

Consider that the pressure drop across an OPV is only approximately constant, and this is only until the valve is fully open; any additional flow will then result in an increase in pressure drop (and you will see a greater pressure at the puck).

It seems (from information I've seen online) that cheaper OPVs often have less flow capacity. Perhaps the your friends machine simply has a lower capacity OPV (or doesn't have one at all)?

[sprezzatura]

I just serviced a Giotto yesterday. The inline particulate filter had junk floating in it. The filter is transparent so if there's anything in there you'll see it: I realise it's new but all sorts of swarf (plastic anyway) and debris can foul it. If you're poking around in there (safely: machine off; unplugged; desteamed; cool) have a look.