La Marzocco Linea PB volumetric woes

Hello!

I have been having some massive issues with the volumetrics on our Linea PB. Not matter what we do, we can’t get them to stick. They are having variances of several grams, regardless of what we do. There are also inconsistent with how inconsistent they are.
There is also the issue of regardless of of grind size, dose weight or yield, one of the groups will always set off the flow meter. I know this is there intentionally but it will always just be on the one group. Currently we are running everything on manual with scales, but I would love to get them back up and running

We have tried:
Cleaning the injectors
Having the volumetrics replaced
Having the whole group replaced
We have a new pump with a new larger expansion tank.
New RO with new membranes and filters
Luckily we looked at our general recipes across the board which involved lower dose, lower yield, quicker shots, a drop in temperature and pressure.
The whole machine has been taken for a service and we found the replacement machine had the same issue.

I have tried turning it on and off again.

It would be great to know if anyone else has had similar issues to this, and even better if they got they fixed! We are a very busy shop and it is affecting bar flow a lot.

Any thoughts would be super helpful!

I owned a 2 group PB but never actually used it (I flipped it instead), so I could be WAY off base in my suggestions to help. That being said, do you have the ‘pre-infusion pulse’ enabled or disabled? It’s not a traditional pre-infusion as it hits the puck with full pressure in very quick increments. The reason I ask is because the Linea PB groups all share the same brew tank, so if the pulse is ENABLED with only 1 group active, it should be fine and all is well, but as soon as you activate a 2nd or 3rd group it effectively wastes the pulse (since the tank is shared) … I’m no expert, but maybe that could effect the volumetrics?

What grinder are you using?

So we starter with roburs originally but have recently moved over the Mythos one.

The problem has been there with both of them. Though when we swapped to Mythos I don’t think it became worse as much as it became more obvious.

Thanks!!

And you are able to adjust in the second menu part?
Is this the same as pre-wet? As that is the only option we have.

Thank you!

The problem with your beverage weight variance doesn’t lie with your volumetrics - they are functioning as intended (especially now after being replaced!). Instead, your Mythos, and the 20°C swing in grinds temperature is the cause.

Provided there is a steady flow of coffee going through the Mythos, and it was either left on overnight or allowed 30+ minutes to “heat” up, the grinds will exit at around 30°C during your seasoning shots, move to 35°C after around 10 or so constant shots, then gradually climb to 40°C as you get busier. At your peak time, once the burr heat has fought past the fans constraining or cooling the grinds, you’ll start hitting 50 - 52°C, and up to 55°C if you’re getting hammered.

This change in grinds temperature changes the grind profile (if grind setting is kept constant, as you heat up you’ll have a decrease in fines, yet they will increase in size) and also changes the grind profile behavior in the basket. This all leads to a curve in resistance of the puck to pressurized water. At the start of the day you’ll have little resistance, you’ll grind fine to increase it, the burrs will heat up, eventually you’ll over shoot the mark, grind coarse, then at your peak time you’ll either stay coarse or grind finer. The set amount of water you have programmed for your volumetrics will either have limited access through the puck at high resistance, or essentially flow straight through the puck at low resistance. This manifests in you hitting your target yield (at the start of the morning, low grinds heat) or being over your target (during your peak, high grinds heat). This is assuming you have a constant supply of coffee through the grinder during the day. As their are natural fluctuations in the ebb and flow of customers, there’s also an ebb and flow in grinds temperature, leading to fluctuations in resistance of the puck.

What I do to combat this (we don’t have scale integration with our Opera yet, so rely on volumetrics - and we are very busy: 50-60kg over five days, majority between 7-11am) is to use volumetrics in tandem with intermittent scale use. I change the grind based on my yield - if it’s over, I increase resistance and grind fine, if it’s under I decrease resistance and grind coarse. I don’t worry about shot times, I let that take care of itself, allowing it to range from a ~28 second shot at the start of the morning (low grinds heat, high resistance) to a ~38 second shot during peak times (high grinds heat, low resistance). I’ll check maybe every third or fourth shot off the line, adjusting the grind setting according to the weight.

I was in the same boat as you, frustrated with what I saw as inconsistent volumetrics. In the end I found it was the grinds temperature that was causing the inconsistency, and this was simply a byproduct of their functionality - burrs get hot, the heat affects the grinds. It’s taken a change in mindset, but this has improved workflow and consistency massively. Hope this helps.

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Thanks for the suggestions!

If your using “pre-infusion” this could be an issue. If your water pressure varies, your shot volumes will as well. The way pre-infusion works, is that without the pump turning on the solenoids open at the group allowing static line pressure to enter the shower. As pressure fluctuates, your shot volumes will as well.

An unlikely issue, could be a ground issues. I’ve had accounts which the ground (at the building) is sporadic which is required for a lot of the electronics to function properly. Its highly unlikely but not improbable.

You say you replaced the group head. Do you mean the group cap? the group head is welded to the brew boiler and can’t be replaced.