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Robo-Pavoni; learning to make espresso machine from scratch

Postby matadero210 on Mon May 28, 2007 10:40 pm

Hi All,

This thread is a continuation of my work with the PID Pavoni: positive-pressure-pid-pavoni-world-domination-begins-t3767.html

It is also a break from my second thread, http://www.home-barista.com/grinders/home-made-grinder-t3880.html, while I finish the drawings.

In the PID Pavoni thread, I added a PID controller and brew-pressure gauge to my Europiccola. The main defect in that work was the need for an expansion tank in the water circuit. When the lever is raised, about 45ml is displaced from the head back into the boiler. This is, in turn, displaced back into the expansion tank, where hot and cold water may mix. In the robo-Pavoni design, the boiler has enough dead airspace to eliminate the expansion tank. Thus, no mixing and temperature instability.

The other design feature is stability. The Pavoni is not really up to the rigors of 9 bar. It flexes and tries to move. A second hand is nearly always used to hold the machine in place. This design seeks to allow 1-handed operation.

The design itself: take a Pavoni head, build an adaptor to connect to normal plumbing. Add a machined plate with water inlet (this is a plumb-in system), thermometer, and heater. Mount all in a brazed steel base.

Image
This picture shows (left to right) the SSR, the Pavoni head with brew pressure gauge (this is a different head, the PID Pavoni is in service), the PID controller (from auberins.com, no affiliation), the chassis and boiler. Note that the colors are all messed up: the boiler is copper, the base plate brass, and the legs steel.

Image
This picture shows the plate with mounting holes (4), recesses for brazing the boiler, and three threaded holes for water inlet (1/8npt), thermometer (1/8npt, PT100 RTD from auberins.com), and 1kw heater (1/2npt, mcmaster.com). The PID controller is 1/32--a real tiny one. Note how badly I suck at brazing. One of the purposes of the project is to sharpen my fabrication skills for the grinder project above and for building my dream machine from scratch.

Image
This picture shows the adapter for mounting the Pavoni head on the 1.5" k-type copper pipe boiler. Note that the final through-hole will be drilled only after the adapter is soldered or brazed into place. The Pavoni head will screw into the adapter just like on a normal machine, but the adapter will be soldered onto the boiler just above the 2x1.5" flare, about 1/3 of the way up the boiler. The steel chassis is only partly brazed together--I ran out of oxygen over the long weekend and had to stop.

Image
This picture shows the bottom of the plate and a nice view of the brew-pressure gauge.
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Postby matadero210 on Mon May 28, 2007 10:48 pm

For those who are interested, I'm using qcad, a nice open-source auto-cad-like program. The adaptor flange and base plate drawings:

Image


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www.espressoparts.com: espresso machines, grinders, brewing equipment & parts
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Postby hbuchtel on Tue May 29, 2007 6:01 am

Cool, I'm looking forward to seeing the end result!

A couple questions-

How are you going to control the amount of airspace in the kettle?

Why did you decide to go with a tall and thin kettle?

Where along the body of the kettle will you mount the group?

Will you try to control the direction of the cold water flow as it enters the kettle?

Have you thought about setting up a pavoni-head-gauge-adding service? ;) People could send you their pavoni groups, you could send it back with a gauge attached... :!:

Henry
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Postby matadero210 on Tue May 29, 2007 10:48 am

Hi Henry,

On to the questions:

1. I'll continue to use my regulated cold water supply, but without expansion tank. Its usually set for 7-10psi, or 0.7 bar. In that case, under half the boiler will be filled with water. As long as the water level is above the group attachment point, both the Pavoni piston and backfill ports will be immersed in hot water. The head itself will also have a good thermal connection. The airspace can be controlled by varying the cold water pressure regulator. I'd rather not add a schraeder valve and bike pump....

2. I'd rather have the kettle thicker, but off-the-shelf copper pipe doesn't get much bigger. Also, water level might get rather close to the ground (leaving less space for the cup). I might consider using 4" cast iron instead, but I wanted to keep everything metalurgically compatible. If I went with cast iron, I'd need SS fittings and a dielectric between the head and boiler. 4" brass or copper pipe is $$$$.

3. About 1/3rd up from the bottom is where the group will mount.

4. No. What I learned from my previous machine is obvious in retrospect. Once the machine fills and heats, operating the lever just short of opening the piston fill port, what I called 'dry pumping', allows circulation of hot water behind the piston, heating the group. When 'dry pumping', the air gap expands and contracts and the pressure varies, as 45ml is pumped in and out. The air gap is designed to be at least 4x45ml. Then, when ready to pull a shot, the lever is raised all the way up. That displaces 45ml to the boiler, which then returns to fill the piston. Only when the lever is depressed (and the piston disconnected from the boiler) does cold water enter the boiler. So--is a check valve needed at the cold water port? Maybe, but the regulator on the cold water port seals up when back pressure is exerted, so only a tiny amount of convective mixing will happen. I'll use tubing rated to 90C. I am slightly concerned that my thermometer is very short and near the inlet. I have longer, t-type thermocouples, but I wanted to experiment with PT100, which Suyi and auberins.com claims is the most accurate/sensitive for this application.

5. I have. This one I nearly spoiled, but caught just in the nick of time. So it would have to be on a best-effort, if-I-screw-up-you-are-hosed basis. The second one took about 1/2 the time as the first (30min compared to over an hour)--I'll bet I can get it down to 15 min with practice.

raj
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Postby espressme on Tue May 29, 2007 7:53 pm

Thanks for going where no man has gone before! :)
sincerely
richard
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Postby matadero210 on Sat Jun 09, 2007 10:10 pm

Hi All,

The new system is up and running. Firsts shots are excellent, although there is at least one variable yet to dial in.

Image

Image

A couple of construction details:

1. Originally I planned to solder the flange, but many thought it would be too weak, so I brazed it instead.
2. You can't really see them, but fiberglass washers are used (and SS bolts) to thermally isolate the hot brass and copper from the cool steel frame.
3. The PT100 thermister does give +-0.2C after the inital auto-tune. I hope for slightly better after I calibrate the 100C to boiling and tweak the PID parameters by hand.
4. The brew-pressure gauge is the same design as my previous PID Pavoni system.
5. There is no thermosiphon. The brew water is in direct contact with the (back of the) head, and even without dry-pumping the head sits seemingly at brew temperature.
6. There is enough room (~130ml) of expansion space in the top of the boiler to allow use without an explicit expansion tank. Note in the photo the regulator is now by itself. The system is connected with a sanitary quick-disconnect between boiler and regulator. I'll put an OPV in there too when it comes from mcmaster.
7. The anodized aluminum controller box houses an SSR, the PID controller, and the power switch. Its pretty tight in there, but I should add a fuse. I always use GFI outlet for espresso machines.

In operation, I noticed the same thing as in the previous system: if the regulator is set too low, the pre-infusion is fine but the shot is short. I think it takes a modest pressure to force the air down and out before the puck seals. This is a Pavoni design flaw. In my next machine the water inlet will be at the bottom of the piston, not the top.

Obviously I need to tend to some cosmetic details, but I must say that I'm very pleased with the system. The major design goal, stability, is fully achieved. You can pull a 10bar shot WITH ONE HAND!

raj
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Postby hbuchtel on Sun Jun 10, 2007 5:47 am

:shock: :D

That is fantastic!

How about a video?

I think you are mistaken about the air getting trapped in the piston chamber. If you think about it, the water will go straight to the bottom of the chamber, right? Once it does so there is no way for the air to get out ... unless it is good at diving :wink: I can only guess that the considerable amount of air which is trapped in there just gets compressed... which might be part of the reason you are seeing a difference in volume with the different preinfusion pressures.

Regards,

Henry
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Postby matadero210 on Sun Jun 10, 2007 7:43 am

Hi Henry,

I think you are exactly right--my point is that Pavonis always work that way, both in my system and out-of-the-box. I've never understood why or how it works in either case, just that I observe that more pressure yields 45ml shots, less pressure yields 30ml shots (1 pull).

Will work on the video,

r
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Postby hbuchtel on Sun Jun 10, 2007 9:58 am

matadero210 wrote:Hi Henry,

I think you are exactly right--my point is that Pavonis always work that way, both in my system and out-of-the-box. I've never understood why or how it works in either case, just that I observe that more pressure yields 45ml shots, less pressure yields 30ml shots (1 pull).

Will work on the video,

r


Sorry, I misread your original post, re-reading it I see your comment "before the puck seals." I'm curious though, why do you think having the inlet at the bottom of the piston chamber would help?

Maybe you could drill another hole for air release like you did with your pressure gauge? :lol: One with a little valve that would seal at a pre-infusion pressure... :roll:

Henry
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Postby matadero210 on Sun Jun 10, 2007 4:18 pm

Hi Henry,
In my ideal machine, the piston inlet and outlet would both be at the bottom of the cylinder, both with check valves. That way, raising the piston would suck water into the piston (no air), and lowering the piston would pump water out. The grouphead check valve could be a flapper, if the piston is in the grouphead, or a normal check valve if the piston is isolated from the group. This would only work for an unpressurized system (otherwise, the positive pressure would always flow out the group).

r
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