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From the workbench 2
Pressure Switches aka P-Stats
What?

A pressure switch, or pressure stat, is used within most espresso coffee machine to control the heating elements of the
boiler.  Remember that in Heat Exchanger (HK) machines  this switch controls your 1 boiler but in a Marzocco it
controls the rear (steam) boiler only.   In HX machines this small switch is one of the critical factors in regulating your
brewing water temperature.

How?

The pressure inside the boiler continues to rise as the water is heated by the heating elements.  All pressure switches
are wired in the
NC or Normally Closed position.  

In
HX machines the pressure inside the boiler usually idles around the 1.1-1.2bar area.

In
Marzocco machines the pressure is slightly higher up around the 1.5bar mark.  The reason it can be set higher is
because
HX machines at that same level will have a higher than optimum brewing temperature.

Pressure switches usually comprise of 2 units - the diaphragm and the electrical switch.  The diaphragm has two
functions - it must be able to flex in response to steam pressure and it must maintain a leak-free seal. Connected to
the diaphragm is usually a rod or pin that will activate the microswitch.  To regulate the pressure at which the
diaphragm and rod can move (i.e.  switch open & close) is a spring.  The spring acts against the diaphram meaning
that the correct amount of steam pressure is required to move rod.  When steam pressure decreases the spring
returns the rod to the original (closed) position.

This procedure of heating and cooling, opening and closing of the circuit and power on and off we refer to as the
p-stat cycle.

The length of the cycle between OPEN and CLOSED we refer to as the
deadband. For example your p-stat may
OPEN at 1.2 (elements off) and then close (elements on) at 1.0.  We would say it has a deadband of 0.2 bar.
Common p-stats inside commercial and consumer machines

There are currently 3 major pressure switch brands currently being used at the moment for espresso coffee machines,
Sirai, CEME and MA-TER.
The SIRAI switch

The Sirai switch is the big-daddy of them all.  99% of commercial machines made all over the world use this switch. The
switch is used because it is rated up to 30A 220V and therefore can handle powerful elements without the need for a
contactor switch.   It is however a huge beast - weighing it at almost 1kg.  

The switch has one adjustment screw - that of the open/close value.  The deadband of this switch does increase over
time and becomes rather large - as much as 0.8 bar in my experience.  The teflon membrane does harden over time
(causing the deadband to increase) and requires replacement.  Sirai are very sensitive to their exact placement in the
machine.  Too high placement will cause the membrane to harden quicker.  







The switch has a cover that slides off revealing the 2 row of 3 contacts.  Wiring is usually fed through from the bottom.
Some switches have bushings whilst others do not. The Sirai switch is usually 3 times higher than that of the CEME.  
The pipe fitting is  male thread 1/4 inch.  

In many applications I have had machines use the same Sirai switch for over 3 years without trouble.   My main
concern is the membrane hardens very quickly on Marzocco machines.  
CEME switches

The CEME switches are certainly a breath of fresh air after using the SIrai switches for several years.  These are more
commonly found on small consumer and domestic machines, but most notably these have been used on all La
Marzocco machines leaving the factory since early 2004.  

The main advantage over the SIrai is that it is a cheap component and very small.  It also has a more durable metal
membrane.  The disadvantage of the switch is that it cannot handle any large currents.  The rating on the side says
10Amp but I would not use it for anything over 1300w (6 amps).  In such case we need to use a magnetic contactor
switch or a Solid State Relay.  







I have been installing these switches onto LM machines now for several months and am pleased with the results.  The
deadband of the p-stat is very tight and an idling machine is practically clunking away all the time as the contactor
switches on and off.  

The switch has 2 adjustment screws as can be seen above.  The middle one adjust the open/close value and the
smaller screw adjusts the deadband.

These are sealed units, meaning that repair of the switch or membrane is impossible.  

YOU MUST COVER/INSULATE THE (NO) NORMALLY OPEN  CONTACT!
MATER switches

These switches have been used for many years in Isomac machines and a few other professional domestic machines.  
They are very similiar to the CEME in size and rating.  The difference is that the micro-switch looks like it can be
replaced and is easily removed from the unit by undoing the 2 screws.  
Problems and maintenance

Because the switches are basically 2 parts - electrical switch and diaphragm you will have two basic problems.

Diaphragm + Rod/PIN

The diaphragm in Sirai switches will harden over time.  As noted before this has been an issue with LM machines.
What usually happens is that the once flexible teflon disc will become brittle and hard.  9 times out of 10 it will cause
the p-stat to stick in the closed (elements on) position.     

Problem - hardened diaphragm  Remedy - buy a rebuild kit and replace the diaphragm.

Problem - safety valve blowing  Cause - hardened diaphragm or limescale buildup on metal membrane   Remedy -
replace diaphragm or descale the metal diaphragm + push rod.

Problem - heating elements switching on properly  Remedy - clean the contacts with contact cleaner.


Emergency treatment - If your p-stat has started to stick in either the open or closed position then try giving the
diaphragm some exercise!  Turn the adjustment screw several times in both directions to get the diaphram flexing
again and the rod moving.  It is a bit of a cowboy job BUT good enough form when the new switch arrives.
The bare facts
 
Contacts
Rating
Range
Deadband
Diaphram
Rebuilds?
Weight
SIRAI
3 Pole NC
3 Pole Common
30A
220V
0.5-1.4
Bar
0.2

(yeah right)
Teflon
YES
Kits avail.
780g
CEME
1 Common, 1 NO, 1 NC
10A
220V
0.2-6.0
Bar
0.1-0.8 Bar
Metal
No
40g
MATER
1 Common , 1 NO, 1 NC
10A
220V
0.1-2 Bar
unknown
Metal
No
40g
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.


Typical Sirai placement.
Same installation.
The Sirais are huge.
Top row on contacts visible.  NC position.
Adjustment spring.
Just 6cm high.
3 Contacts NO, NC and Common
Same pipe fittings as the Sirai.
A MATER inside an Astoria CKE
Adjustment screw visible on the top.
The details printed on the side.
The microswitch on the side.