# OPV Modification: Why?



## Blue_Cafe (Jun 22, 2020)

Why do some reduce espresso machine pressure limits by modifying the machine pressure relief valve (OPV or Over Pressure Valve)?

Commonly known as the OPV mod,

What are you actually trying to achieve?

I ask this because from a process flow perspective, (using free flow baskets) the only thing generating pressure is the puck. Without it, the pressure in the system is zero.

Your puck generates (well, causes) the pressure. Nothing else.

So, by reducing the set pressure on the safety valve, you are effectively putting a ceiling on the pressure that the puck can generate.

This is then, a blind process. You have no idea during operation if your puck is too weak (high flow rate) or soo strong (low flow rate) except by looking at the output from the portafilter.

Pump flow rate is dependent on the pressure (head) its pumping into. The pump curves are fixed, so the only thing changeable is the head its pumping into (the puck), or the OPV lifting.

*In theory, get the puck right and the OPV shouldn't engage. The OPV only works when the puck is wrong, no?*

So, If this is true, would it not be better to put a pressure gauge on the pump instead and monitor that during extraction? wouldn't this be a lot more useful?

Is the OPV mod a safety gate for poor technique, rather than a essential in the set up of the machine?

For those using the OPV mod, do you see the OPV opening (ie do you actually see water) during the shots?

I am just learning, so its an interesting topic for me. Any papers or proofs on the OPV mods would be welcomed!


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## NJD1977 (Dec 16, 2014)

Keeping the pressure through the puck at a capped 9bar takes one of the variables out of the mix. It enables you to dial in your grind more easily.

E.g. you're aiming for a 25s shot at 1:2 at 9bar. All you have to adjust now is the grind size. But what if you were getting 1:1 at 13bar. Now you have to get the pressure down first, then the length of shot adjusted.


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## NewboyUK (Jul 14, 2018)

Also depends on the machine.

Your not touching the safety valve at all and would potentially void the current certificate on the machine.

On some more basic machines the pump pressure isnt adjustable so you are altering what the pressure at the puck would be before the valve opens.

On machines that has a adjustable pump - you would use this for expansion pressure i believe.

Take a tank fed domestic Fracino. It has a 15bar (i think) that you cant adjust - so you adjust this OPV it so it opens and recycles the excess pressure back into the fill tank so you get 9 bar at the puck.

Someone poke me if im wrong


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## Nikko (Aug 20, 2014)

Blue_Cafe said:


> *In theory, get the puck right and the OPV shouldn't engage. The OPV only works when the puck is wrong, no?*


 No.

As you say the pump curve is fixed. You would not need an OPV only if at your chosen brew pressure, say 9 barg, the pump flow corresponded with your desired flow rate, say 1.5 g/sec. Most pumps have greater flow than what is needed.


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## DavecUK (Aug 6, 2013)

Blue_Cafe said:


> 1. So, by reducing the set pressure on the safety valve, you are effectively putting a ceiling on the pressure that the puck can generate.
> 
> 2. This is then, a blind process. You have no idea during operation if your puck is too weak (high flow rate) or soo strong (low flow rate) except by looking at the output from the portafilter.
> 
> ...


 1. No you are limiting the pressure the pump can generate this in turn affects what the puck can flow.

2. Well you have a point in that the 9 bar was chosen as an arbitary average from other types of machine, it wasn't actually calculated.

3. There are other factors but yes the pressure head it's pumping into does greatly affect the flow rate capability of a vibration pump

4. Because a pump can flow much more than required, without an expansion valve to manage the pressure at 9 bar or whatever it's set to, you will probably get far too high a flow rate, tighten up the grind and up goes the pressure. Think about a hosepipe a pressure washer and water usage.

5. That's not really how pressure in a system works

6. No

7. They will if it's set up correctly yes

By limiting the maximum pressure you also change the flow dynamics of the coffee puck, in way which you might not expect and often very negatively where high pressures are used as it can compress the puck down and restrict flow/encourage channelling.


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## NewboyUK (Jul 14, 2018)

DavecUK said:


> Well you have a point in that the 9 bar was chosen as an arbitary average from other types of machine, it wasn't actually calculated.


 Im speaking to local coffee roasters regarding this - I can pull better shots at 4bar than they can at 9 🤔


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## MediumRoastSteam (Jul 7, 2015)

NewboyUK said:


> Im speaking to local coffee roasters regarding this - I can pull better shots at 4bar than they can at 9


Lots of people rave about 6 bar. I tried, not my thing.

Sent from my iPhone using Tapatalk


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