I've attached a pressure gauge to the bottom of my PF and discovered that my brew pressure is just slightly exceeding the 200psi limit of the gauge. So this means my brew pressure is close to 14bar. Way too high as I understand it. I've read some of the links on adjusting the opv on the oscar but mine being a newer oscar they seem to have done away with a hex nut on the opv and gone with a completely round brass fitting, making adjustment difficult if not impossible. How then do I lower my brew pressure? I don't want to adjust the pstat do I? I did a basic thermometer test of the brew temp and it's only just above 140f. (60c) Mind you I was just holding my small frothing pitcher under the brew head and measuring the water temp in the pitcher, not ideal I know but it still seems low. I'm a bit confused what to do now. My brew pressure seems way too high and my brew temp seems way too low. I'm able to pull good shots mind you so I'm just not sure what to think.

Thanks,

nixter wrote:How then do I lower my brew pressure?

That is indeed a high maximum pressure, though you should measure with a constant flow rather than against a blind basket (see Building a Portafilter Pressure Gauge for more details). It's difficult to tell from your photo, but it looks like an OPV similar to the Andreja Premium where you must turn the barbed portion of the fitting; see Chris' Coffee Service FAQ #13: Andreja Premium & Anita: Adjust brew pressure.

nixter wrote: I don't want to adjust the pstat do I? I did a basic thermometer test of the brew temp and it's only just above 140f. (60c) Mind you I was just holding my small frothing pitcher under the brew head and measuring the water temp in the pitcher, not ideal I know but it still seems low.

The brew pressure and boiler pressure (temperature) are not related. The Styrofoam cup test (excerpted below) is accurate within a few degrees. Of course it's important to remember that Oscar is an HX espresso machine, so the brew temperature is dependent on the flush amount (explained in detail in HX Love).

barry wrote:foam cup method:

take a small foam cup which will fit up against the group gasket (6oz is about the right size), and push a dial stem thermometer through the side about an inch up from the bottom. angle the thermometer stem downwards across the cup and wedge it into the opposite bottom corner. to measure brew water temp, remove the portafilter from the machine and hold the cup firmly against the group gasket on the underside of the brewhead in such a way that you can see the thermometer dial. run about 2oz of water into the cup and watch the thermometer dial for the maximum temperature reached. it's easy to get burned doing this, so be careful not to let hot water run onto your hand.

PS: Although your Oscar is the newer model, the thread Nuova Simonelli Oscar - how to lower the brew pressure? has helpful background information.

Thanks for the reply Dan.

Regarding my pressure test...Even though my test was blind I think it's still relevant. If I adjust for a max blind pressure of 10 bar then won't that eliminate the possibility of brewing under too much pressure? For example if I adjust to 10 bar and pull a shot and my machine clogs up, I'll know that my grind is too tight or I've over dosed. The way things currently are set I could be pulling shots in the correct amount of time but under far too much pressure, exaggerating the chances of channeling or other problems. This is just my logic based on my limited knowledge so far, correct me if I'm wrong.

n

nixter wrote:If I adjust for a max blind pressure of 10 bar then won't that eliminate the possibility of brewing under too much pressure?

I still don't get it: Why adjust the OPV? explains in detail, but the short answer is yes. The slightly longer answer is that at double espresso flow rates and the appropriate pump, the OPV is unnecessary. The Elektra Semiautomatica and Salvatore are two high-end espresso machines without OPVs.

Well after reading that it looks like my logic is sound. I think I'll try lowering the setting. I just have to figure out how to grip the valve as these newer Oscars don't have a hex nut valve, they are round. Maybe it's easier when cold. When I tried yesterday the fittings were still hot. I wonder how much turning is involved.

n

The larger over-pressure valves on the Quickmills require slightly more turning compared to the smaller ones on the Isomacs:


Photo of an Isomac Zaffiro's OPV

From the photos you've posted, I can only speculate, but it looks like a smaller spring type to me. If you remove the retainer clip on the exit tube, you may be able to turn the barbed fitting with a pair of pliers. No need to use a gorilla grip.

Just be sure to use plastic jaw pliers or something to protect the barb

Well I made an adjustment and didn't have to grip the barb. The first few cranks with the pliers were stiff as a loctite like product had been used on the threads. I originally made 4 full revolutions which brought me down to 120 psi so a couple more tweaks and gauges later I arrived at a fairly accurate 145psi reading judging by the middle of the blurry needle. I made one test shot in a triple basket just for kicks and things were noticeably improved. I had just guessed at coarsening my grind for this first test and must have guessed right. I had a nice smooth flow with a massive cone and ridiculous amounts of crema. The flow was a touch fast but I tweaked that this morning with good results. The taste was more subtle and easy than i'm used to from my machine. I'd call it more "drinkable" for sure.

I'm very happy with my new 10bar opv setting. I understand the logic behind those who say the puck dictates the brew pressure but I think things become a little more tricky when dealing with overdosed doubles and triples. The chances for error in creating "x" bar of pressure with that much coffee, while throwing in grind plus tamp as variables are huge in my opinion. I'd guess even exponential when dealing with increased resistance due to coffee mass. Anyways, reducing my pressure has allowed me to coarsen my grind while keeping my flow rate slow enough for proper extraction. The coarser grind will mean less resistance, channeling, jetting, and otherwise wasted shots I'm hoping.

I'll update in a few more days after more shots.

cheers for all the help, especially Dan.

n

Hey Nixter, I recently got a NS Oscar as well.

That "OPV" at the brew head, at least in my european model did not seem adjustable in any way. Apparently NS doesn't use this in a traditional sense, but rather in a safety sense and as far as I know they are set to 16 bar.

Obviously from your post, there may be some differences between your model and mine. When I tried to loosen mine, I got no reduction in pressure, and eventually I got some leaking.

There are a couple of issues with using this valve as a traditional OPV.

1. It empties into the drip tray. So you are losing water from the system, and constantly having to empty the tray.
2. Its on the hot water side of the boiler - so you are impacting the thermal stability of the machine and potentially the brew temperature.

I fitted a Izzo Vivi OPV on the cold water side of the boiler, following some nice pictures that Greg Irwin (a poster on coffeegeek) had sent me. ChrisCoffee had fitted a vacuum breaker and OPV to his machine, and I followed this schematic. This OPV empties into the reservoir, and does not affect thermal stability.

I must say, it was a bit of a PITA to fit. I'm no plumber, but with the proper fittings, more than a little bit of PTFE and perseverance I got there in the end.

Hi David, would you mind forwarding me the links to the schematics? Does you Oscar have the sirai pstat or the older one? I heard the Euro Oscars didn't get the upgrade. What's your take on the vacuum breaker? I've heard conflicting theories. Some say you MUST vent steam right at the initial warm or it will cause incorrect readings by the pstat while other say that as long as you vent the steam at some point, let the heater come on and then off, you're fine. Thanks for the replies in both my threads. I think you're bang on about my opv being too loose now.

n