Vibe pumps and grouphead pressure.

[Danny]

Posted on CG.com as well.....

I've had my machine (Isomac Tea) for a good few months now and I've been really happy with the results thus far.
The learning curve was steep, being a coffee newbie, it took me several frustrating weeks to understand how the variables worked.

I still have a long way to go..

I notice that the shots I pull consistantly finish at about 18-20 seconds with a smaller amount of espresso as a result. I've tried a lot to get better results but it was not until I had a real barista (one that takes the job seriously too) in my house that I discovered I am getting pretty much the best I can from the beans I am working with. I can do better when I roast myself, but for a few reasons that is not an option for me right now. Instead I am using coffee that I know to be reasonably fresh.

To my question....

I see the guage reading about 10.5 during the pull. Since this is pretty much the only variable I have yet to change, I wonder if someone out there knows if it is possible to adjust this on the Isomac HX vibe machines. I think I'd have to replace the overflow valve with one at a different pressure setting or is there some way I can adjust it? Maybe there is some other way I can alter the pressure in the group. I must confess that I dont tend to watch this guage very often even when I tamp less hard or grind finer.

Maybe the pressure is not so critical and I need to find a different bean supplier? At any rate I am interested in seeing what happens when I can bring the pressure down.

-D

[phil]

Carolyn?

[HughF]

Yes, you can do this. I don't think it's a massively difficult mod either. Not sure about warranty issues if your Tea is new though. Try searching alt.coffee for OPV or pressure mod. I think people settled at 9 bar. I think Jim Schulman was one of those who did mod his Isomac.

Cheers,

Hugh

[Danny]

Yeah he has mentioned this on CG... Thanks for the info.
I pulled the cover off last night before I posted and I expected a visible adjustment, like the pstat.

I see now that the OPV is adjusted via the inside of the overflow line. I will give it a go...

[carolynb]

You've obviously found the pressure by-pass valve. The inner part (that the silicon tube fits on to) simply screws in clockwise to increase the pressure and anticlockwise to decrease the pressure setting. You might need to turn it with a pair of pliers. But I wouldn't recommend this adjustment! The pressure by-pass on the Isomac machines is set to come into play at around 10 to 11 bar. The point is that if the pressure builds up above this the excess pumped water escapes via the by-pass valve. If you reduce the setting on the by-pass valve so that it cuts in at 9 bar you will suceed in limiting the pressure of your shot to 9 bar but it is curing a symptom of the problem, not the cause.

If your pressure is going above 9 bar then you either have your grind set too fine, or too much coffee in the filter basket (or both!). If you set the pressure by-pass valve to cut in at 9 bar it will be difficult to see when your grind is too fine because the pressure gauge will never go above 9 bar. What do you mean "the shots I pull consistantly finish at about 18-20 seconds"? Is this the point at which you have obtained 1 fl.oz. of coffee and crema (2 fl.oz. from a double basket)? Or is it the point at which the crema is becoming lighter in colour? The important thing is to set the grind so that you get 1 fl.oz. (2 fl.oz. from a double basket) of coffee and crema in 20-25 seconds. The pressure should then be around 9 bar (give or take a little bit). When filling the filter basket make sure it is clean and dry to start with and then fill it to level with its rim with loose ground coffee straight from your grinder. Don't tap or shake it or press in the coffee while filling it. This should give you the 14 to 17 grammes you need (double basket). Then tamp very firmly (you need a proper tamper - not the plastic thing that is supplied with the machine!).

Hope this helps you!

[cleverdic]

Carolyn,
I know this question has been asked many times on other sites but I'll ask the question here and see if we get a consensus.
Do you measure the time interval from the point that the coffee starts flowing or when you switch the pump on

Richard

[phil]

That's a very interesting question.

I've seen a lot of people say "from the first drops of coffee appearing". Well my challenge to that is that I can get my TD to nearly choke, so that it takes 20 - 30 seconds for the first drops to appear. Agreed I've made a mistake if that happens, but nonetheless if I were to wait another (up to) 30 seconds for what would undoubtedly be a very short ristretto then water would have been in contact with coffee (or at least some of it) for 50 - 60 seconds, which is by conventional wisdom far too long and should give a fairly bitter result as the warter soluble bittering elements come out in force.

Or do they - since there is so little water in such a restricted shot? Frankly when I've made such a shot and tasted it, it hasn't been perfect but it's often been drinkable. I made Allyson a shot for her cappa a couple of days ago that lasted nearly 2.5 minutes! That was when I over-filled too much. The coffee had barely dripped by 20 seconds. Then the postman rang the doorbell and I completely forgot what I was doing in my rush to see if the important letter I was waiting for had arrived. By the time I got back I had a cappa cup one third full of coffee, with a reasonable crema - didn't look badly over-extracted. I just bunged the milk and foam on it and gave it to her, being the loving husband that I am.

And Allyson said it was probably the best cappa I'd ever made her! Go figure! (of course the milk covers a multitude of sins).

Discuss.

[cleverdic]

Phil,
I love that. a 2.5 minute pour I would probably get away with that as well.

Richard

[carolynb]

You count the seconds from the time that the pump starts, so maybe you will not see any liquid for up to 9 seconds but stop the pump at 20-25 seconds. What you don't want is a "white tail" on the crema so that is always a good guide to watch for.

BTW Isomac machines cut out if the pump runs for more than a minute! Maybe their wisdom is that if the pump has been running for more than a minute you don't know what you are doing!!

However as with everything in life, learn how to do it by the recommended method, then play around like Phil suggests and hey presto you might actaully be able to improve on the recommended method - but if you hadn't had that base line to work from you wouldn't realise the scale of improvements you had made!

[Danny]

You've obviously found the pressure by-pass valve. The inner part (that the silicon tube fits on to) simply screws in clockwise to increase the pressure and anticlockwise to decrease the pressure setting. You might need to turn it with a pair of pliers. But I wouldn't recommend this adjustment! The pressure by-pass on the Isomac machines is set to come into play at around 10 to 11 bar. The point is that if the pressure builds up above this the excess pumped water escapes via the by-pass valve. If you reduce the setting on the by-pass valve so that it cuts in at 9 bar you will suceed in limiting the pressure of your shot to 9 bar but it is curing a symptom of the problem, not the cause.

OK, then why are the pumps on rotary machines adjustable? Or is this more a rate adjustment than a pressure adjustment (I guess the two are related)

It's really quite frustrating. I mean the shots that come out after 18 seconds taste good. I am not complaining about the quality at all. But when I change the variables to produce a 28 second shot, it's disaster. Over extracted, bitter, burnt....

If your pressure is going above 9 bar then you either have your grind set too fine, or too much coffee in the filter basket (or both!). If you set the pressure by-pass valve to cut in at 9 bar it will be difficult to see when your grind is too fine because the pressure gauge will never go above 9 bar. What do you mean "the shots I pull consistantly finish at about 18-20 seconds"? Is this the point at which you have obtained 1 fl.oz. of coffee and crema (2 fl.oz. from a double basket)? Or is it the point at which the crema is becoming lighter in colour? The important thing is to set the grind so that you get 1 fl.oz. (2 fl.oz. from a double basket) of coffee and crema in 20-25 seconds. The pressure should then be around 9 bar (give or take a little bit). When filling the filter basket make sure it is clean and dry to start with and then fill it to level with its rim with loose ground coffee straight from your grinder. Don't tap or shake it or press in the coffee while filling it. This should give you the 14 to 17 grammes you need (double basket). Then tamp very firmly (you need a proper tamper - not the plastic thing that is supplied with the machine!).

After tamping I get the level to more or less the ringline in the basket. I have tried a millimeter or two below this line as well.

I usually end the shot based on what the stream looks like. I can usually get pretty close to the mark with the lightening of the crema.

Try as I might I cannot get a dark stream to last past 20 seconds.

If I try and coarse up the grind, it turns watery.

Hope this helps you!

Sure, all input is valued

[phil]

Carolyn

Actually I wasn't advocating playing around (:!: ) I was just relating a range of experiences from the rich list provided by "cock-ups I have made"!