Coffeenoobie Senior Member Joined: 11 Dec 2011 Posts: 2,320 Location: PNW Expertise: I like coffee
Espresso: N S Oscar Grinder: Vario W
Posted Thu May 3, 2012, 7:09pm Subject: Over pressure question on Oscar
So, I am watching my Oscar closely after the repair and I never see water come out through the OPV on the left side of the group head even when I back flush. I see some droplets of water in the tube so it is not dry over there but I never see water flow. Shouldn't I see water coming out of that tube with the blind filter in and the pump running?
Coffeenoobie
Buying advice: GRINDER GRINDER GRINDER. Don't cheap out on the grinder. My coffee treasure map... Click Here (maps.google.com)
epengr Senior Member Joined: 1 Apr 2012 Posts: 46 Location: Saskatoon Expertise: Just starting
Espresso: NS Oscar Grinder: Baratza Vario
Posted Thu May 3, 2012, 7:24pm Subject: Re: Over pressure question on Oscar
I'm NO expert, I don't even have the Oscar I just bought yet, but as I understand from too many hours scouring the forums is that the Oscar doesn't have an OPV (which is typically between the pump and the HX), the release valve before the group on the Oscar is actually more of a safety release and is set quite high. It won't open in normal operation - even back flushing.
The Oscar keeps it simple and relies on the fact that the pump will put out ~9 bar if the flow rate is about right for pulling a double in ~25s. You shouldn't need to worry about pressure unless you're play with ristrettos, etc.
germantownrob Senior Member Joined: 2 Dec 2007 Posts: 2,018 Location: Philadelphia Expertise: I love coffee
Espresso: Duetto 3, A Dead Oscar Grinder: Vario-W, Preciso w/Esatto,... Drip: Brazen Roaster: Diedrich IR-1, HT B
Posted Sat May 5, 2012, 5:22am Subject: Re: Over pressure question on Oscar
Hi Helen,
You should see some flow when doing a back flush, it usually is towards the end of a 10sec back flush and it is not like the tube fills rather there is a trickle flow. It is possible you have some scale or debris stuck in there or some mechanical problem with yours. It does seem when I do a chemical back flush I get more flow, you are handy so maybe pop the thing off and have a look and a clean or take a small amount of descale powder and do some back flushing with that.
I have mine set around 12 bar, when I adjusted lower I did not like the results, to much water lose which resulted in lowering the temp stability during a shot.
BTW I used to use the conical group gaskets that made a better seal for me but I can no longer find them and have to use the flat gaskets which tend to allow a slight pressure release especially during a back flush, I will have to pop the hood and have a look at the opv tube to see the flow and will report back.
I have never seen a full tube or anything like from the three way valve releasing pressure after a tight shot or a back flush. Now when I lowered the opv there was an increased flow but the tube was never full, it pulses as well or at least it does on mine.
I would love to see some data on tests done with a ulka pump and what pressure it puts out over a 40sec period. I have only read what people have posted claiming they are not putting out a constant pressure but I have nothing to back this up. If this is true I look at it this way, if they put out a max of 16 bar but not constantly and the opvset at 12-13 bar is only seeing peak pressure a short time in 30sec and the opv is not really releasing that much volume of water. However when set to 9-10 bar and pulling a 3oz triple there could be close to 1 oz of run off going to the drip tray on the Oscar with a tight grind. You have seen my post before of my desire to upgrade from Oscar to a rotary pump, this is a big reason, however I am still very satisfied with the results I get and the return for upgrade in taste will be small, that is why I drag me feet for an upgrade. To be honest you are a big reason I have not upgraded yet, you keep me appreciating the Oscar, so thank you.
Coffeenoobie Senior Member Joined: 11 Dec 2011 Posts: 2,320 Location: PNW Expertise: I like coffee
Espresso: N S Oscar Grinder: Vario W
Posted Sat May 5, 2012, 10:26pm Subject: Re: Over pressure question on Oscar
Well, now I feel pressure! Maybe 10 bars :)
But Oscar has some good points. The fact that he is so ugly you can paint him or cut holes in the shell for gauges and you don't feel guilty for doing that. I like the idea of tricking him out for fun. I really like that he is lighter weight and that my bread bags will not melt if my husband puts them too close to him. And he is easy to clean. I think if I had the shiny one I would get all OCD about him and worry about scratches if I moved it. With Oscar I am bringing him to a vacation home this weekend to impress the in laws. I can't see me self doing that if he was one of the all shiny and heavy machines.
And because he is used I feel good about tinkering on him. I think if he was new I would have been scared to crack the case. (and I have learned so much more about espresso machines by working on him)
Coffeenoobie
Buying advice: GRINDER GRINDER GRINDER. Don't cheap out on the grinder. My coffee treasure map... Click Here (maps.google.com)
germantownrob Senior Member Joined: 2 Dec 2007 Posts: 2,018 Location: Philadelphia Expertise: I love coffee
Espresso: Duetto 3, A Dead Oscar Grinder: Vario-W, Preciso w/Esatto,... Drip: Brazen Roaster: Diedrich IR-1, HT B
Posted Sun May 6, 2012, 8:03am Subject: Re: Over pressure question on Oscar
I know what you mean! I will not get rid of Oscar after I upgrade, he is just to darn easy to take places or to put outside for summer parties.
Helen you really should consider roasting, I think you would get very good at it, there is something very special about producing roasts for your coffee needs. Best hobby I have ever had!
Posted Sun May 6, 2012, 8:19am Subject: Re: Over pressure question on Oscar
germantownrob Said:
I would love to see some data on tests done with a ulka pump and what pressure it puts out over a 40sec period. I have only read what people have posted claiming they are not putting out a constant pressure but I have nothing to back this up.
I think you may be misunderstanding what you've read about the Ulka pumps. It isn't that they are inconsistent and change over the time of the shot. They are consistent (as long as they aren't worn out) in how they pump - they just have a normal/natural relationship between pressure and rate of flow.
Imagine the pump pressure as a water bucket 10 feet above you with a hose dangling down from the bottom. Put your thumb over the hose so no water can flow out the end of the hose. It now has maximum pressure (because there is no water flowing - your thumb has blocked all the flow just like a blind filter is blocking the flow in an espresso machine doing a backflush) and minimum flow (nothing is flowing). If you take your thumb completely off of the end of the hose you now have maxium flow (all the water is rushing out as fast as it can) and minimum pressure (there is no pressure because there is no resistance - nothing blocking the flow).
So those are the two extremes --- and then everything in between (low pressure and high flow - medium pressure with medium flow - high pressure low flow) as you vary the resistance (how hard you press your thumb over the end of the hose - or how fine you grind your coffee).
Don't know if that example helps or not .. but the vibe pump acts the same way in that the pressure will be inverse to how much liquid is allowed to flow. The more liquid flowing, the lower the pressure will be. \
They work best when they have an actual OPV in line between the pump and boiler - because the pump is capable of putting out higher pressure at higher flow rate than desired for espresso (it'll put out 16 bars when nothing is flowing - and it'll put out about 12 bars when flowing the equivalent of a double shot of espresso). So having an OPV between pump and boiler allows you to bleed off just a trickle of water during the shot and end up with 9 bars to the coffee puck - because the pump can put out more than you need, the OPV is able to regulate it so you get close to the same pressure regardless of whether you're flowing a 1oz ristretto or a 3oz lungo. Without the OPV, with a vibe pump you would be seeing very different pressures for those two shots.
Coffeenoobie Senior Member Joined: 11 Dec 2011 Posts: 2,320 Location: PNW Expertise: I like coffee
Espresso: N S Oscar Grinder: Vario W
Posted Sun May 6, 2012, 9:53am Subject: Re: Over pressure question on Oscar
Thanks AndyPanda I totally understand that now thank you very much.
Rob, great suggestion I would love to roast, I do think I would be good at it - I like to express myself with food prep but alas my nose is borked. Without a nose I think I could not blend coffees or tell when they are perfectly roasted. :( If I had a single wish I would wish my nose worked properly. And I read espresso roasting was the hardest of all. So I will just buy other's fine coffees. Besides, Oscar is taking all my spare cash right now so a roaster would not be something I can afford at this time.
Coffeenoobie
Buying advice: GRINDER GRINDER GRINDER. Don't cheap out on the grinder. My coffee treasure map... Click Here (maps.google.com)
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