When should you stop trusting the brew pressure gauge on your espresso machine?

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Samcanadian

#1: Post by Samcanadian »

Recently I've noticed that my pressure gauge on my Bezzera BZ10 shows 9bar for shots that seem somewhat quick. For instance, I'll pull a shot that only takes 20-21 seconds to get to 2oz, but the pressure gauge show a solid 9 bars. If I pull a shot that's closer to 10-11 bars, it seems to have a more suitable flow rate and take around 25-30 seconds.

I guess the obvious answer is to ignore the pressure gauge and just watch the flow/blonding, but at the same time I would like to definitively figure out if there's something wrong with my gauge or if it's just me.

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Randy G.

#2: Post by Randy G. »

"When should you stop trusting the brew pressure gauge on your espresso machine?"
When you find out it has been going out behind your back with your TDS meter!

But seriously folks, The gauge is a good learning tool and good for diagnoses of possible problems, but experience wins out here. It's like when you own a car for a long time. The slightest change in a sound or feel of the wheel is enough to know something is different. Even a two pound change in tire pressure can be sensed. The sound of the pump, the issue of espresso from a bottomless portafilter, the time it takes to blonde are all excellent indicators to anyone who is one with their machine.

A quick flow? Stale coffee, overdosing or underdosing, and other factors to be checked, but the pressure gauge can be like a red light in the dash display to "CHECK ENGINE." Or maybe check the driver for BAC? :wink:
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kolu

#3: Post by kolu »

BZ10 pressure gauge is one of the better located gauges in the machines world. It's directly connected to the output of 3-way valve. So you should trust it. The problem actually is that the OPV in your Bezzera is set (well, it comes that way from the part manufacturer, not from Bezzera) to 12 Bar. I would first suggest you open the machine and loosen the OPV so you are reading 10 Bar with blind filter.
So again - your gauge is probably right. If you have good flow around 11 Bar then the flow at 9 Bar should be, and is, faster.

Samcanadian (original poster)

#4: Post by Samcanadian (original poster) replying to kolu »

This is very helpful. I'll take a video this morning and post, but I bet this is my problem.

Samcanadian (original poster)

#5: Post by Samcanadian (original poster) »

Here are a couple examples, actually.

The first one is one I took a while ago (That I forgot about)


The other is one I just took this morning.

Samcanadian (original poster)

#6: Post by Samcanadian (original poster) »

After watching these videos, it's pretty obvious I need to tighten my grind. I suppose the pressure gauge at 9 has been what's throwing me off lately, and if what @kolu has been saying is true, I'm pulling all these shots at closer to 8bar than 9.

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kolu

#7: Post by kolu »

Well, the problem is that vibration pump pressure is very dependent on the flow, as opposed to rotary vane pump with balanced bypass, so the usual "rules of espresso" are bit different and the expansion valve, that primary serves the safety function, could actually be used as a pump pressure setting valve. Hence the recommendation to decrease the pressure down to 10 bar with blind filter. I really recommend it, it takes only 4 allen key screws (3 mm) on the bottom and than with 16 mm spanner you can easily get the OPV on the right side of the machine.
If you don't mind opening it, of course.
Also, BZ10 have quite big free flow (water debit) with its 1,2 mm gicleur so lowering the pump pressure makes it less prone to channeling.

Samcanadian (original poster)

#8: Post by Samcanadian (original poster) replying to kolu »

So just to clarify, I have a couple options

1. Assume that the "9 Bar" my pressure gauge shows is actually more like 7-8 bar, and shoot for a reading of closer to 11 for something that's actually closer to 9 bar.

2. Turn down the pressure to 10 bar, so that not only does it give me a more realistic reading on the pressure gauge but also minimizes the pump pressure so it doesn't blow apart my puck when I actually pull the shot?


I feel like #2 is the option I really should go with, for not only a more accurate reading but to really optimize the machine's ability. Option #1 is just me needing to understand that what's showing as 9 bar is really more like 7-8?

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kolu

#9: Post by kolu »

Your pump is able to get you almost 15 Bar at 0 flow, but the pressure drops significantly as the flow increases. There is OPV limiting the pressure to 12 Bar so what you get is pressure increase with decreasing flow up to max. 12 Bar and it's not gonna get higher even in case of no flow, as the pump output is "taken away" by the OPV.
Your pressure gauge is directly at the "top of the puck" so what you "see" there could be very well called "real brew pressure" (like on KvdW Spirit of last generation or KvdW Slim Jim, Slayer V3, ... as an example). If you see there 9 bar, you are extracting at 9 bar.

In more common commercial/traditional machines, the pump is rotary vane pump, gives 10 Bar pressure and between the pump and the "top of the puck" is a gicleur (a jet, sort of a resistance from fluid dynamics point of view), typically 0,6-0,8 mm. This jet then actually bears the 0,5-1,0 Bar drop of pressure between the pump output (10 Bar) and the brew pressure (9 Bar or whatever the Italians call for as an "espresso").
In your machine, there is no such a part (1,2 mm gicleur is like... nothing, just a huge hole, almost like the size of 3 way valve port anyway) so there is no pressure drop from the pump output and the whole brew pressure is hugely dependent on flow through the puck, which is... em... not okay, as the Ulka EX5 delivers way too much flow at 9 Bar. And that is the point where your machine "diverts" from "espresso" standards. You can drop the OPV setpoint and get easier to the "right 25 sec/60 ml" extraction at the right 9 bar pressure as the excess flow provided by the pump at that pressure will be just diverted back by the OPV.

If you search the forum for OPV, Gaggia, Silvia and pressure, you can find many examples of successful tuning of similar home machines. I would, again, recommend you to tune down the OPV to 10 bar with blind filter as a good start point.

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kolu

#10: Post by kolu »

wow, and I need to give you the short one also for #1 - no, your 9 bar is really 9 bar. you pressure gauge doesn't care about your OPV setting or your pump at all. it's just measuring the pressure at certain point in the system.