The OverPressure Valve (OPV) or Expansion Valve
The Over pressure Valve (OPV) also known as an Expansion Valve, is an often neglected, but very important part of your machine. Vibration pumps produce around 15-18 bar unregulated, depending on type. This is far too high for best espresso extraction. Unfortunately, far too many machines, have totally unregulated pumps (these tend to be at the cheaper end of the market). However some of the more expensive and popular models also have no real ability to regulate pressure or a fixed OPV, which cannot be adjusted.
This is important, as when reading this article you should understand that although some OPV valves are better than others, it does not mean these manufacturers are producing machines that are “faulty” or “substandard”, when they contain an OPV that is not perhaps as good as others. If the OPV does the job and regulates pressure to whatever the manufacturer presets it to at the factory….then its meets the specifications they publish. If you want a machine that can be regulated down to 9-10 bar, you need to understand the limitations of some of the OPV valves fitted to various machines.
I have written an short article (click on the image of the .pdf file) with photos for those interested in learning more about this critical area of your machine.
I received some intelligent and well thought out feedback about the article on the OPV and felt I should include it below. It makes some valid points and is well worth considering in conjunction with whatever equipment is installed in your own machine.
I just reread your article on the OPV. A marvellous piece that I have directed several of my friends to. Your comments regarding the quality and lack of adjustability as regards the Fluidotech OPV raised my concern since my new Dalla Corte mini came with both a Fluidotech pump and OPV. Despite expectations, however, I seemed to have no problems adjusting it down. Actually, in order to get 9 bar (10 using the pf gauge) I had to turn it in quite a bit.
After some pondering I think I found the explanation for both the problems of the Fluidotech when mounted on the Brewtus machines and why it seems to work on the Dalla Corte. Look at the attached brew pressure profiles of the Fluidotech and Ulka pumps respectively. Whereas the Ulka max out at about 15-16 bar and then has a fairly straight approx 45 degree curve down, the Fluidotech exerts a maximum of 12 bar pressure and has a much steeper curve down.
Thus, an OPV used in conjunction with an ULKA pump, as used in the Brewtus, would need to relieve much more water than one used in conjunction with a Fluidotech pump. The problem is thus not necessarily that the Fluidotech OPV can't cut it but rather that they were never intended for use with the ULKA of CEME pumps. From my initial series of measurements with the pf gauge, the max pressure seems to be the same every time, with a steady ramp up and no flutter. Whether it will stand the test of time, however, remains to be seen….1