It was suggested to me on my own forum that I post about this article on other forums....so here it is, hope it's of interest/useful to some

I often see a similar statement on forums about some keen individual ready and willing to try and adjust his/her vibe pump machine down to 9 bar pressure. Some of them even try this without a pressure gauge, thinking that they can measure the total volume of OPV run-off and calculate the pressure accurately that way (i'm not going to go into details, but you cant reliably do this). Even more baffling to me is that people are willing to do this with often limited or no knowledge of how this simple device actually works....yes they have a vague idea, but may have never seen inside one.

Then later you often see them posting about other problems, seemingly totally unrelated to the OPV valve, but actually caused by their attempts to adjust brew pressure! Things like:

• The group goes cold after a while
  
  I pull the group lever down (switch the pump on whatever) and for 5 or 6 seconds no water flows out of the group and when it does it's too cool
  
  I have a small amount of water under the machine after it's been on a while

It always seems a good idea at the time and only requires the twist of a nut, or the simple turn of a screw. Unless you know whats really going on though, it can be an area for problems So I have written a short article on OPV valves, you can read much more and see photos on this link to article

Nice article Dave! I wish I would have found it before I went all out and installed a relief valve in my unregulated vibe machine. Check it out.
Hopefully the manufacturers will pick up on the idea that OPVs are important.

Well, all credit to you for actually installing the relief valve (nice work)

oofnik wrote:Nice article Dave! I wish I would have found it before I went all out and installed a relief valve in my unregulated vibe machine. Check it out.
Hopefully the manufacturers will pick up on the idea that OPVs are important.

would you mind sharing the parts / source you used to splice in the OPV ?

My main machine is an expobar, but I have an isomac venus. I really want to put a new OPV in it (the current one is located after the boiler and shared with the steam pressure) regular shots taste fine, but ristrettos come out slightly cold and sour - the extra pressure from the finer grind blows hot water out the boiler.

whenever i bring this up, people try to 'educate me', so here's the preventative snippit: a regular grind / shot is geared for 9bars of resistance which gives a great shot in 25s. the finer ristretto grind creates 11-13bars of pressure, and you need to rely on an OPV to regulate the pressure -- or else its bitter.

2xlp wrote:would you mind sharing the parts / source you used to splice in the OPV ?

Sure, everything was ordered from McMaster-Carr's online catalog. The valve is part # 8088K14. It only comes in 1/2" pipe size so I just got 1/8" adapters along with the rest of the fittings to get it all arranged properly. It's a tight fit due to the size of the valve but I'm sure you could make it work somehow

thanks.

i ended up ordering an expansion valve from chriscoffee - it wasn't much more, and i needed some other stuff from him.

i can't wait until all my parts arrive.