The Espresso Machine Restoration site
A non-commercial site for those interested in espresso equipment repair and restoration.
From the workbench 2
What are they?

Flowmeters measure the volume of water going through the groups - the water used for brewing the espresso.  
Flowmeters are used on semi-automatic machines with the volumetric dosing buttons.  For Marzocco machines that
refers to AV machines and for CMA machines that is SAE or SAEP.
The impeller. Impeller inside the main body. Underside of the field sensor. You can see the parts how they are assembled.
How does it work?

Water enters the flowmeter when you have activated the keypad and the pump switches on.  As water enters it
causes the impeller to rotate.  There are 2 magnets in the top of the impeller and as these turn they pass through
the magnetic field causing a switch to close and then open.  This sends a pulse signal to the microprocessor.

So in actual fact when you "program the dosage" on your machine you are in fact programming how many times the
switch opens and closes sending a pulse to the microprocessor.

Some flowmeters have an LED on them that flashes along with the pulse as water enters the flowmeter.   
Problems and overhauling

Flowmeters suffer from the same problems as most other components and that is limescale and dirt.  

Problem - Lack of flow from the groups

Cause - Limescale and dirt will build-up over time and may have blocked the inlet of the flowmeter.  

Remedy - Turn water mains supply off and bleed off pressure.  Remove the flowmeter and dismantle.  Check the
inlet and outlet holes for blockage (scale, dirt, teflon tape) and clear if necessary.

Descaling and cleaning

Now if you must descale then you have to be careful since the field sensor is fragile - especially the earth wire that
will snap off if you are not careful.  What I would do is take it apart and be gentle with it especially when removing the
wires from the terminals.  Also take care not to break the o-ring since you should be able to use it again.

You can drop the main body into a descaling solution - but be warned the nickel plating may come off slightly.  I
would only leave it in a hot solution for about 20 mins.  You could also drop the impeller in there for a few mins to
remove some build up from around the magnets.

The field sensor is fragile.  I would try and remove the scale from the underside of the unit taking care not to get the
terminals wet or damaged.  So you must try and find a shallow dish so that the citric solution is only 1cm deep or so.

Sometimes the field sensor will need replacing if the PC is reporting a blockage.  A good flowmeter field senso
should have an ohm reading of 2.2k between the "+" and "0" terminals.  It should still be OK  if the reading is
between 1.8k and 2.4k.

By far the most common fault with the field sensor is corrosion on the earth wire and the terminals.  When these
corrode they snap off very easily and you must replace the unit.  
This website is created by Paul Pratt, Hong Kong 2004. If you would like to use any of the images or text I am sure I
will say yes, but please ask first!  

Email me here.

A complete flowmeter.
1 inlet 1 outlet.  Stamped into the brass with arrows.
Nickel plated brass.
A typical scale build up in a flowmeter.
A dirty impeller.
Underside of field sensor also dirty.
Build up of scale, dirt and sealant that must be cleared.
After descaling.
New left. Descaled right.
Underside of field sensor. New left and descaled right.