The coffee versus the OPV

Beginner and pro baristas share tips and tricks for making espresso.
walter-

#1: Post by walter- »

I have a Breville Dual Boiler (or more correct a European version of it). The previous owner ran the machine with the OPV set to 8Bar; I ran it for several months like this but changed the OPV some time ago. Now it is set a bit too high and pressure goes up to 10 bar (whatever that really means in the brew head). I have consumed several coffees on this setting. Mostly 17.5grams of coffee, 1-2 ratio, 25-30 seconds brewing. I am happy we the result as such.
Recently I noticed for some coffees, the pressure only goes to 9 bars and I still end up with a 1-2 ratio, 25-30 seconds. And with good tasting results. In this case I assume the OPV plays no role and it is only the coffee that defines the highest pressure.
Until now I though making espresso is always against the max pressure defined by pump and OPV -at least at the start of the brewing. But now I see that for some coffees it is actually the coffee that defines the max pressure. The OPV no longer plays a role. And for other coffees I still hit the 10 bars if I want to stick the recipe.

Do other people have this experience and what are your thoughts here? Was I just wrong in expecting that the pump/OPV are the one that set the max pressure. Or are there extreme cases where f.e. only 7 bar is reached and a 1-2 ratio is still kepped (here maybe a need for extra dosing is needed)?

Walter

iyayy

#2: Post by iyayy »

opv is, and over pressure valve. it just bleeds if you exceeds the pressure buildup. it doesnt control ur pump output.

bdb reads pressure at the group. so if your puck can only resist 7 bar pressure before it starts to flow, your gauge will only show 7 bar.

if your grind is too fine and able to resist, say 12bar of pressure, ur opv will kick in so that it doesnt go above your 10bar settings. but u'll likely see the shot choked and the gauge will stay at 10bar.

if you grind fine enough to have just 10bar pressure, you may see it initially hits 10bar, but as the shot starts to flow it the gauge may decrease since puck cant maintain the pressure and water can flow through.

for rotary pump it seems you can set both opv and pump.. er pressure? im not sure if this is pressure or flow, since flow affects pressure too. im also on bdb anyways, but they have extra settings, and can set the pump to be lower than opv (opv never triggers, hence zero function).
you will still need the puck resistance regardless.

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Jake_G
Team HB

#3: Post by Jake_G »

iyayy wrote:for rotary pump it seems you can set both opv and pump.. er pressure? im not sure if this is pressure or flow, since flow affects pressure too.
Nice explanation overall.

For rotary pumps, the bypass valve on the pump sets the brew pressure and the expansion valve (which is an OPV with a specific purpose) protects the components in the brew circuit from exceeding their rated operating pressure during warmup and recovery periods, where cold water that has entered the boiler (or heat exchanger) must expand as it gets up to temperature. The bypass valve on the rotary pump can't help with this, because there is invariably a check valve somewhere between the hot brew boiler and the pump outlet - hence the expansion valve.

Setting the expansion valve to a lower setting than the pump bypass is "fine" but wastes a lot of water. The expansion valve dumps hot water to the drain tray, whereas the pump bypass sends cold water from the pump outlet right back to the pump inlet, and wastes zero water. If you had the pump bypass set to 9 bar and the expansion valve set to 8 bar, you'd glood your drain tray in a hurry. Expansion valves are typically set to 12 bar, as this is safe for the brew components and higher than most folks would knowingly set their brew pressure. This ensure safety with minimal waste.

Cheers!

- Jake
LMWDP #704
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buckersss
Supporter ♡

#4: Post by buckersss »

Thanks Jake, I found a few threads on here with similar info on rotary pump issue but this is very helpful. If expansion valve is normally set to around 12bar, is the pump by pass normally set higher?

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Jake_G
Team HB

#5: Post by Jake_G »

Pump bypass is typically 9 bar or less.

This establishes your brew pressure, so long as the puck can sustain it.

Most espresso machines measure pump pressure ahead of the gicleur in the group, and are actually pretty good at labeling it as such - "pump" pressure vs "brew" pressure.

The BDB is one of relatively few machines on the market that measures brew pressure after the gicleur, so you see more questions about brew pressure changing with different coffees on Breville's machines than on most others. The pressure against the back of the gicleur, which is usually pretty steady, is what the "pump" gauge shows regardless of what is happening at the puck.

Cheers!

- Jake
LMWDP #704

walter- (original poster)

#6: Post by walter- (original poster) »

Tnx all so far for the feedback.
iyayy wrote: ..
if you grind fine enough to have just 10bar pressure, you may see it initially hits 10bar, but as the shot starts to flow it the gauge may decrease since puck cant maintain the pressure and water can flow through.

....
It is maybe this that I want to understand;
"Stronger" puck that can for example hold a lot of bars but when the opv kicks it will brew at for example 10 bars and then the pressure will drop because of the puck erodes.
"Less strong" puck, where the brewing will start at 9 bar and the pressure will also drop.
For the 'less strong' puck will the pressure drop indeed be faster? And would that be relevant.

I haven't really checked how long a puck remains as 10bar or 9 bar of ho faster it drops; mostly I am happy when pressure stays up to +/- 7 bars.

But I still want to understand if other people manage to run their recipe even with the puck is not holding the max pressure of pump/opv (for example in the case opv 10 bars, puck holding 9 bar).