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 Mon May 7, 2012, 10:27am Subject: Re: Over pressure question on Oscar
Thanks Andy, that explaination allowed me to wrap my head around this. Let me ask you this, do you think that the ulka pump with a pressure gauge and a flow restrictor allowing 2oz in 30 sec without the use of an opv the pump would put out continuous pressure with multiple tests? It's not that I am doubting you rather I just find it hard to imagine such a small pump ( compared to a rotary pump) can be that precise, hopefully I am wrong.
Posted Tue May 8, 2012, 7:02am Subject: Re: Over pressure question on Oscar
What I have tried with some success ... is to put a resistor in series with the Ulka pump to reduce the output of the pump. This is a little trial and error and it isn't precise/consistent since the resistors heat up and vary their resistance (a light dimmer might work better).
I was using the resistor with a switch to bypass it so I could do a 4bar preinfuse. But when I was trying different value resistors I did notice that I could get a range where I was getting 9 bars without any water bleeding (or only a drop or two) from the OPV. (every pump is different and large wattage resistors aren't easy to find - radio shack isn't the best place - 20 ohm/25watt resistors would be a good place to start)
But with a vibe pump - this would still only work for one specific shot volume. If you tune it for 9 bars while pulling a 2oz shot then you wouldn't have enough pressure when pulling a 3oz shot and you'd still have too much pressure when pulling a 1oz shot. That's why the OPV works so well with the vibe pump ... and why vibe pumps advertise 16 bars ... they put out more than required and let the OPV regulate it so you get the right pressure at different flow rates.
Rotary pumps don't act the same way because they are more like a propeller on a boat (well they are an impeller) they are pushing steadily against the water in a different way ... but the vibe pump is like a bicycle pump compressing the water in pulses inside a sealed compression tube (with springs and valves) ... so resistances (flow restrictors/coffee puck) in the water path become very complex and interactive with the springs/valves/pistons in the pump.
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 Tue May 8, 2012, 7:36am Subject: Re: Over pressure question on Oscar
What I didn't mention above is the sound my pump makes while pulling a shot and it has done this since new, the pump fluctuates it's sound through out the time of the shot. When up against a blind the pump ramps up then goes nearly silent which I assume is creating full pressure according to Andy's great visual description above. My assumption of the up and down sound of the pump during the coarse of a shot is that the pump is putting out different pressures in this time. I do not see myself messing with flow restrictors or watt restrictors since I get great shots regardless and I vary from doubles to triples from 1-3.5 oz. In my mind with a HX machine there is no way around affecting temp stability by using the opv to lower the pressure and even question how stable an opv is at giving constant pressure stability at least with the Oscar.
My next machine will have a rotary pump so I can set it to the pressure I want and not have to worry about it anymore.
Posted Tue May 8, 2012, 8:10am Subject: Re: Over pressure question on Oscar
If the OPV is positioned in the cold water line (right at the output of the pump or between the pump and the HX) it won't affect temp stability.
If your pump is sounding erratic ... there are several possible problems you might have with your particular pump. There are springs/valves in there and all it takes is a spec of grit to mess things up. And since the pump vibrates so intensely sometimes a blockage gets stuck and then freed up and then stuck again.
Posted Tue May 8, 2012, 8:21am Subject: Re: Over pressure question on Oscar
germantownrob Said:
Let me ask you this, do you think that the ulka pump with a pressure gauge and a flow restrictor allowing 2oz in 30 sec without the use of an opv the pump would put out continuous pressure with multiple tests?
An Ulka vibe pump (without any OPV) that is working correctly will put out a constant pressure and flow against a constant resistance. With an actual coffee puck, the resistance of the coffee changes during the shot (flows faster and faster towards the end of the shot). But if you replaced the coffee puck with a non-changing resistance (say a Scace thermofilter with the calibrated flow - or a adjustable needle valve on your PF) ... you would see a consistent pressure over time with the pump running. A vibe pump shouldn't be run continuously as it will overheat - but you could run it for 60 seconds and the pressure shouldn't vary as long as the resistance against the flow doesn't change. (with a properly operating vibe pump and consistent voltage to the pump)
The vibe pumps change dramatically in sound - especially as they wear - depending on the resistance to flow. They get very quiet when pushing against a blind basket because the oscillating piston almost completely stops moving. When there is less resistance to flow (the puck fractures or channels - or you are at the end of the shot and the puck isn't resisting very much) the piston in the pump is oscillating wildly and the more wear it has, the more is slaps around and makes a racket.
You can also sometimes get a leak (an air leak - you won't see any drips) on the intake side of the vibe pump (so it doesn't keep the intake hose filled with water) and that can also make the pump very inconsistent in pressure and noise. It can only pump good pressure as long as the pumping chamber fills with intake water with each stroke. I've seen plenty of Ulka vibe pumps where the rubber nipple that the intake hose clamps onto has become loose on the pump itself (the hose is tight to the nipple but the nipple is leaking air where it attaches to the pump) and this prevents the pump from doing its job. Very common problem that a new pump won't fix if you use the same leaky nipple.
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 Tue May 8, 2012, 8:58am Subject: Re: Over pressure question on Oscar
AndyPanda Said:
If the OPV is positioned in the cold water line (right at the output of the pump or between the pump and the HX) it won't affect temp stability.
If your pump is sounding erratic ... there are several possible problems you might have with your particular pump. There are springs/valves in there and all it takes is a spec of grit to mess things up. And since the pump vibrates so intensely sometimes a blockage gets stuck and then freed up and then stuck again.
Ah Ha! That makes sense to put the opv before the HX inlet and then I could divert it back to the reservoir instead of the drip tray.
The erratic pump sound is very faint, it starts up after the ramp to full pressure and continues to the end of the shot, as flow increases during the shot I do not notice any audible changes in sound other then the slight fluctuation. Since my machine has done this since new I just thought this was how it was supposed to be. I will have to listen to some YouTube vids to see if I hear other vibe pumps making the same sound. Debris may be the cause since every once in awhile some charcoal particles to escape my Britta filter and could be lodged in the pump.
Is it worth taking the pump apart or better to just spend the $51?
Edit: just watched SCG's vid on the Oscar, the sound was decent and I do not hear what I hear on my machine.
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 Tue May 8, 2012, 9:12am Subject: Re: Over pressure question on Oscar
Coffeenoobie Said:
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.
Borked nose is only one of your senses and it is easily overcome with data from temp readings. Roasting City to City+ for espresso is the most challenging but with a roaster with control roasting to FUll City is not any harder then roasting for drip. It takes practice and a lot of tasting but eventually you get the results that suit your tastes and not what tastes right to some Roast Master that has to please many pallets.
Money, that is a different issue, good roasters with control cost a lot of it and they don't come up for sale used that often. Building your own can be done easily for under $200. Just saying, lol.
Most of the Ulka pumps are easy to take apart ... there isn't much in the biggest section of the pump, that is all electromagnet and the metal piston that vibrates back and forth and a big spring - nothing in that part needs to be cleaned usually. But the section that sticks out in front - that is where there are two very soft springs pushing two rubber plugs (either a ball shape or a mushroom shape usually) against two seats where they allow water to go towards the output but not come back in - one way valves. Those are where you probably have some grit intermittently plugging the hole or preventing the rubber plug from sealing the hole.
The Invensys pumps are made to not come apart - but all the Ulka pumps I've seen are pretty easy to take that valve section off to clean. The springs are very small and the check valves too .. so be careful to note how they were positioned and careful not to lose them.
I hijacked this photo from someone's blog ... hope they don't mind. Here is there URL in case you want to read their blog (I didn't read it - I just searched for a photo and this was the first good one I came across). Click Here (ulkapumprepair.blogspot.com) And notice that this particular Ulka pump has an OPV on the output (probably from a Gaggia - they come that way) which would be the easiest way for you to add an OPV to your Oscar.
I'm always happy when anyone is interested in this stuff. I find it facinating so I've spent a lot of time experimenting with these pumps and how the brew pressure changes in various types of machines. I'm glad my visual thought experiments help.
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