Home-Barista.com

An addition for the FAQ that I've posted elsewhere, with updates... How to adjust an espresso machine's pressurestat (steam boiler temperature). Most heat exchanger espresso machines have one of these pressurestats:

• Sirai pressurestat
• MA-TER pressurestat
• CEME pressurestat

(Click links above for photos).

All three have a large center screw for adjusting the pressure; the CEME has a smaller off-center deadband screw. To adjust the boiler pressure:

1. Verify the direction for increasing or decreasing the pressure by looking for ± arrows. Manufacturers can change among equivalent thermostat models from one run to the next and the adjustment direction may be different.
2. Make very small adjustments, equivalent to five minutes on a clock at most (MA-TER and CEME). The adjustment is much larger for Sirai pressurestats (i.e., one half turn = 0.1 bar). Verify the setting has stabilized by activating the pressurestat at least three to four times before making another adjustment.
3. The pressurestat "on" setting is the point where the heating element indicator light illuminates. These readings are slightly higher (lower) than the gauge's lowest (highest) point because the pressure continues to drop below the lower setpoint for a moment after the heating element comes on, and momentarily rises past the higher setpoint after it turns off.
   
   The CEME pressurestat has a smaller dead band screw (sometimes it is painted over). Do not adjust it. A common mistake is to turn it too far and break the pressurestat, which will not be covered under warranty. The deadband factory setting is 0.2 bar; if your machine's setting is markedly greater, call your vendor for assistance.

IMPORTANT: Unplug the machine when making this adjustment. Turning it off at the switch isn't enough -- unplug it.

Paul wrote:I recall reading some time ago that it is risky to attempt adjustment on an aged ceme or mater pstat due to the plastic weakening over time. Maybe a post from Jim S but my memory is a bit foggy. Would anyone care to comment?

I've read many reports of "sticky pressurestats," but don't recall specific problems adjusting older pressurestats. That said, pressurestats in general and the MA-TER and CEME type pressurestats in particular have delicate adjustments and are best adjusted once and forgotten. Ideally this is done at the factory or by the vendor prior to delivery to the customer.

TimEggers wrote:Dan nice write up but it leaves me wondering why specifically the machine has to be unplugged before making the adjustment. Can you enlighten us? What are the risks involved?

The main risk is electrical shock (see the lawyerly disclaimer below).

Turning the power switch off isn't the same as unplugging the machine. Power is still present at some junctions (e.g., the wires leading to the power switch and the switch itself, at a minimum), plus there's the chance the machine isn't properly wired or has been modified in an unsafe manner by a previous owner. Some espresso machines have an access hole to allow adjusting the pressurestat without removing the covers, but hopefully readers appreciate the wisdom of unplugging the machine before sticking a screwdriver into an electrical appliance.

DISCLAIMER wrote:THIS ADJUSTMENT HAS BEEN DESCRIBED FOR INFORMATIONAL PURPOSES ONLY, BECAUSE ESPRESSO HOBBYISTS HAVE EXPRESSED AN INTEREST IN IT. THIS ADJUSTMENT SHOULD ONLY BE CARRIED OUT BY QUALIFIED REPAIR TECHNICIANS. SUCH ADJUSTMENTS REQUIRE MANIPULATING AND/OR BEING IN CLOSE PROXIMITY TO CONTROLS AND CONDUITS FOR ELECTRICITY, VERY HOT WATER, STEAM AND HIGH WATER PRESSURE. THE RISKS OF FAILING TO FOLLOW PROPER SAFETY GUIDELINES AND PROCEDURES INCLUDE SERIOUS INJURY OR DEATH AND THE DESTRUCTION OF PROPERTY BY FIRE AND OTHER CAUSES. THE AUTHOR ACCEPTS NO RESPONSIBILITY FOR ANY INJURIES OR LOSSES RESULTING FROM ANY ATTEMPT TO PERFORM THESE ADJUSTMENTS.

In addition to the excellent safety tips that Dan points out (turning machine off AND unplugging it), here's a tiny bit of info:

The pressure adjustment screw on CEME pstats is oftentimes best adjusted with a jeweler's screwdriver after removing the appropriate sheetmetal. The reason for this is that, again, oftentimes, the access hole in the case does not line up 100% with the pstat adjustment hole. It is not that difficult to drop the screwdriver and have it land on or near some wiring that would otherwise be "hot" if the machine were not turned off AND unplugged.

Secondly, when you make an adjustment to the CEME pstat, you will invariably open the contacts and cause the heater to come on if the machine were switched on. The observed deadband (as compared to the true electrical deadband) on the pressure gage will get smaller as the machine heats up and the change in group temperature that a pstat change causes will typically take around 20-30 minutes.