When measuring the pressure on a Rotary Pumped machine, the pressure does not agree with the machines own pressure gauage
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The article specifically refers to measurements taken with a pressure measuring portafilter (on rotary pumped machines), sometimes the machines own guage can flicker making an accurate reading difficult, it's innacurate or the machine may not have a pressure gauge fitted. It is quite a good example of the physics of water and how it can affect results to a surprising degree. Often you will see a forum post such as:
When I attached a blind PF with pressure gauge (for a comparison check of the Alex panel mounted brew gauge), I got the exact same reading as on the Alex panel gauge. But then the PF gauge started to slowly rise up to about 12 bar after a little over one minute while the Alex gauge stayed solid at the original value. I repeated the test and the results varied so I somewhat dismissed it, but maybe it needs to be considered again
or
I did some live pressure testing and the gauge showed just slightly less than the Alex panel gauge during a shot. It was consistent during the shot also. Only against a blind PF (no flow) did the group pressure rise while the panel gauge remained constant.
The post above referred specifically to the Izzo Alex Duetto, but will actually happen on any coffee machine with a Rotary pump!
This is a very common observation and I don't believe I have ever seen the reason stated on the Internet. It's true it does happen. If you put a pressure measuring portafilter on an HX machine or a Dual Boiler machine with a rotary pump….you will see an initial rise in pressure up to say 9 bar (if that's what the pump is set to) and after a brief pause it begins a slow and gradual rise…..often it can even get to 12 bar in some cases (in the Duettos case around 1 bar higher than the pump pressure is set to). It's especially obvious in rotary pumped machines with a brew pressure gauge. It is actually the cause of many a person with a pressure measuring portafilter having some of the following concerns:
- They think their rotary pumps bypass is failing
- They think the inlet water pressure is too high
- They think their machines gauge is reading incorrectly
- They adjust their machine to an incorrect pressure….normally too low
- They say nothing and hope it doesn't matter
First thing to say is don't worry almost certainly nothing is wrong and the behaviour is normal for a Rotary Pumped machine with Heat Exchanger, single or double boilers..
Of course if this rise doesn't happen, then actually you could have a problem see izzo-alex-anti-backflow-also-called-non-return-valve. Of course, please ensure you read and understand the rest of the article, before assuming you have a problem with the anti back-flow valve.
Why Does it happen
I was pretty sure I knew what was happening (as I have noticed this effect for years) and had my theories and in a discussion with Martin (Mole), we walked through exactly what would be happening and tested the theory on my own Duetto. This was useful as going over the whole thing made this article easier to write.
The brew circuit on a rotary pumped machine should have an anti back-flow valve. This is not often the case with a Vibe pump machine as the vibe pump has one built in. This anti back-flow valve is after the rotary pump. The brew pressure measurement to be accurate for a Rotary pumps output, should actually be taken just after the pump and before the anti back-flow valve. When you lock in a pressure measuring portafilter, you are creating a closed system (between the 1 way valve and the portafilter). This closed system contains within it the brew boiler and of course it's heating element. This closed system is subject to all the laws of physics, especially expansion of water with heating. See image below (click to enlarge).

If we walk through what happens on the Duetto (with the steam boiler off).
- The Pressure Measuring portafilter (PMF) is locked into the group (creating a closed system)
- The pump starts and pressurises the portafilter as water fills the air space
- 9 Bar is achieved (or whatever the pump is set to)
- The Water in the brew boiler cools a little partly because some cold is added and partly because it is always cooling and heating slightly
- As the water cools it contracts…but the pump simply adds more water to maintain 9 bar (the right way through the 1 way valve)
- The heating element comes on, the water expands (but it can't go anywhere, because the one way valve creates a closed system)
- The displayed pressure in the portafilter rises above 9 bar
The larger the initial temperature drop, or more aggressive the PID settings (if they cause an overshoot) and the larger the rise.
The walk-though on an HX machine is similar, but the effect is MUCH larger
- The Pressure Measuring portafilter (PMP) is locked into the group (creating a closed system)
- The pump starts and pressurises the portafilter as water fills the air space
- 9 Bar is achieved (or whatever the pump is set to)
- The Water in the heat exchanger cools a lot because it's only around 100CC in volume, some cold water is added and partly because it is always cooling and heating slightly via the thermosyphon
- As the water cools it contracts…but the pump simply adds more water to maintain 9 bar (the right way through the 1 way valve)
- The water in the HX then heats up quite rapidly (because it's in a really hot steam boiler) and expands a lot, but it can't go anywhere, because the one way valve creates a closed system.
- The displayed pressure in the portafilter rises above 9 bar…in some cases up to 12 bar where the OPV is often set to vent
Of course all the above results apply if the machine is on and up to temperature (with the Duetto, the continual heating of even a machine that's just switched on will actually show the rising pressure)
If you doubt that water expansion can produce this effect, there is a simple check you can do. Test the Duetto with a PMP and watch the slight rise above the machines brew pressure gauge on the panel, or if the panel gauge does not agree exactly with the PMP, then the gradual rise above the brief pause where it displays peak pressure. Then simply switch off the PID (press and hold the right hand button)….and repeat the test. You will find you no longer get this gradually creeping rise in pressure above the maximum. (of course the steam boiler should be off and cold for this test)
So what does all this mean and why is it important!
- Firstly if you want to set the brew pressure in a machine, if it is an HX machine it should be cold….and then if the pressure measurement is done quickly enough, you should not get much of a heating effect in the HX
- Twin boiler machines should have the heating element in the brew boiler switched off (if possible) e.g. Vibiemme, Duetto etc.. With the Duetto the steam boiler should also be switched off (and cold).
- If you can't remove the effects of heating (e.g. La Spaz Dalla Corte etc..), then take the max pressure before the gradual creep up as your brew pressure
- Manufacturers fitted pressure gauges should never measure the brew pressure within the sealed part of the brew circuit (after the one way valve), where it can be subject to these effects (in rotary pumped machines)
- If you don't see this happening, your one way valve might be letting by! see izzo-alex-anti-backflow-also-called-non-return-valve
- This doesn't happen in most Vibe pumped machines, they are subject to a different effect (although the Izzo Vivi may have an anti back-flow valve, so might show this effect).
- If the Gauge on you Duetto, or any other machine that directly mesures it's pressure after the Rotary pump, but before the anti backflow valve, is accurate, then you can use it to adjust your machines brew pressure accurately and ignore any of these effects (as long as your machine has a rotary pump and 1 way valve)..
So remember, don't lower your brew pressure on these machines so that the maximum the gauge reads (after a prolonged period and a creeping rise in pressure) is 9 bar, if you do then your likely to have a brew pressure around 8 bar in the Duetto and as low as 7 bar in an HX machine….in fact the more efficient the HX the larger the effect!1
The Vibration pump machines are subject to a different effect completely when using a PMP and the result vs the machines own pressure gauge are quite different. The fluid physics are a little more complex, but we do have a theory that simply needs testing to verify, before the article is written.
Thanks to Martin (Mole) in helping clarify these ideas and testing the thories with me.