CRW Senior Member Joined: 24 May 2012 Posts: 35 Location: PA Expertise: I love coffee
Posted Thu Dec 27, 2012, 3:09pm Subject: Gaggia Classic OPV
should I be regularly seeing water releasing from the OPV tube back into the water reservoir on my Gaggia Classic Coffee? I haven't ever seen this happen. If I understand how the OPV works, I'd think I'd at least see it when backflushing, since the full pressure has nowhere to go.
How are you determining flow? The tube may be full of water without bubbles. One way to approximate pressure to 9 bar at the OPV is to plug the PF, such as blind, and then measure and adjust outflow volume. Remove the tank and put the water uptake tube in a glass of water and the OPV outflow tube in an empty glass and see the flow out for a 20 second interval.
The flow rate at 9 bar is substantial, about 200 - 300 ml per minute if I read the graph correctly.
If the group is occluded, this the flow out the OPV. Similarly, if the OPV is set for 9 bar and about 2 oz flows from the group in 25 sec in a typical brew, the the remainder of the 9 bar flow is out the OPV.
If the OPV is set at high pressure then the flow will be low out the OPV. You can see from the graph that the flow for 15 bar is very low.
Sounds that way. Is the OPV still factory set? If the pump puts out 15 bar and the OPV is at 16 bar, just an example, then no output even with a blind basket. Similarly for any set of numbers when the OPV is set over the pump. How old/used is the machine. Could the pump be tired? How difficult is it to choke the machine?
You could make a pressure device and find out.
You could open the OPV 1 turn and probably get it to function and then set it back to original. From what I read and found on my Classic, it took about 270 degrees open to drop to 9 bar.
Or, if it brews fine, you could disregard all of this :)
I think the machine is about 10 yrs old. I got it used. I have no reason to believe that the previous owners would have adjusted it. They didn't seem too coffee savvy. I myself am fairly new to quality espresso machines, and I'm trying to learn all I can about this one. I guess to really find out the pressure I would need to rig up a gauge.
When I used a Gaggia Classic, the mount of water that was released from the tube varied quite a bit depending on how find a grind I had and how firm a tamp I used. I had a bag of rather course pre-ground that left a most puck and almost no discharge; a full filter would give little discharge as well.
Finally getting back to this. I opened up the OPV, it seemed to be cranked pretty tight. It was especially hard to open the outer part with a socket wrench. So I opened it a few turns, yeah I probably should have paid attention to where it started, but oh well. After opening it a few turns, I was able to get a steady stream of water to run out of the discharge/return line when using a blind filter. So I know at least that the valve works. I tightened it back a little, so it trickles a little after it's been running for 10 sec or so.
My objective now has to be to determine approximately how much pressure I have before making any final adjustments. Because at this point I am questioning if my pump is OK or if it is weak. Does a vibe pump get "weak"---still running, but at a lower pressure? Or does it just fail? If I rig up a pressure gauge, and it shows more than 9-10 bar, I can work with the OPV. If it makes less than that, I just need to replace the pump before I go any further.
My concern for the pump is because it just doesn't seem to be able to push out espresso at 20-30 sec. per shot. I know the grind isn't fine enough to choke it on its own. I'm using a Hario Slim Mill hand grinder. At its finest setting it can make powder. I have it set a few clicks above that, because it just won't run any way else. I know the coffee isn't too dark and oily. A while back I was using a blade grinder and just grinding it to death, and it could make a shot, maybe not the best, but it would go. I've had issues the past few months with this machine, air in the lines, pump locking up, etc., discussed on another thread. I thought after I descaled it it was working better. I know my steam valve is leaky, but that wouldn't be affecting the brew pressure, right?
well I found out that if the OPV is too loose, then it can't build up steam pressure for the milk. As it started building up steam, it suddenly all dribbled out the return tube back into the reservoir. Tightened it back down a bit and it's making steam again. Tomorrow I'm headed to the store to build my makeshift pressure gauge and see what this machine can really do...
You can quantitate the flow and learn from it. Remove the tank and put the intake tube into a full glass of water and the overflow tube into an empty glass. Using a blind basket will divert all flow into the overflow. At 9 bar the flow is about 260 ml/minute. Run about 15 seconds and look for about 65 ml. You can tell in the first few seconds whether you are way to fast or slow and then adjust. Of course empty the overflow to measure each time and keep the pickup glass well supplied. You can do a 30 second run as you get close. I would not do this for prolonged times and let the pump cool/rest adequately between. I did that and got close and fine tuned after I got the gauge. You have to guess a little and use the longer run to get close. As you may have noticed, the pump takes a few seconds to fully pressurize the system and begin overflow so you have to interpret the flow a bit. You could start the time at first flow and time 15 seconds of actual flow, not just pump on time. It is just an approximation.
Steam can build up 3+ bar of pressure at about 280 - 300F. If steam caused OPV push back, you are way too loose. Click Here (www.jgbhose.com)
There are several thread on the topic of fresh coffee freshly ground, and the need for a proper grinder that will grind consistently and be able to choke the machine and have fine adjustment. I guess than can wait until you are sure that your machine/pump works. Of course the pump and machine could be ok then think grinder and coffee.
I looked at that pump repair page before. My pump isn't an Ulka, it's an Eaton, and it can't be disassembled like in the blog article. The outer casing is one piece. Where did you get the flow figures? When I run the pump straight into a measuring cup I get about 170 ml in 15 sec.
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