Conti Prestina Espresso Machine Restoration 101 (Completed and Indexed) - Page 67

Equipment doesn't work? Troubleshooting? If you're handy, members can help.
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drgary
Team HB

#661: Post by drgary »

No, the hose connects to the top of the vacuum breaker. As pressure is building and the air pocket starts to move through the machine it carries water with it. This soon becomes steam instead of air. With enough steam pressure the valve closes and stops sputtering. I've attached the hose to the top of the vacuum breaker to carry that discharge into the drip tray. Remember the vacuum breaker isn't a stock part of the machine but is something I've added so I can connect the machine to a timer and have it warmed up by the time I go to the kitchen in the morning. If I turned it on in stock condition it wouldn't fully warm up because the PSTAT would shut off power to the heating element because it doesn't know the difference between air pressure and steam pressure. I would then need to discharge air from the steam tap and group head, and then the machine might take another 20 minutes to come up to stable brewing temperature.

Here's how I understand this interconnected group of parts. The PSTAT is part of a closed system connecting the steam pipe coming out of the top of the front plate with the manometer, the PSTAT (aka pressure switch), overpressure valve (OPV) and vacuum breaker. That system is closed when it's in equilibrium at brew temperature. The OPV sits on top of the manometer knuckle as a safety device and normally stays closed. The vacuum breaker also connects to the manometer knuckle via a T fitting and closes after pressure builds to a certain point. Check out the pictures above and you'll be able to visualize that system.
Gary
LMWDP#308

What I WOULD do for a good cup of coffee!

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drgary
Team HB

#662: Post by drgary »

My day job has been taking time lately, but I did have a chance to do some more testing and prep of final assembly. First I bent a small piece of pipe and inserted it in the vacuum breaker discharge tube to ensure all of the discharge goes into the tray. I brought the machine up to temperature with no leaks and this new snout works just fine. The latex tube secures it without further fastening.

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I also measured and cut the melamine insulation. It doesn't smell at all and doesn't discharge any particulates and it's easily cut with scissors. It has very little tensile strength, so it tears easily. I'll have to be careful when strapping it into place.

I brought the machine up to temperature and measured boiler surface temperatures at 110C top and bottom. This means I'm free to mount the heat safety switch anywhere that's convenient. That switch, which works with the Microcimbali Liberty and is sold by OE, is rated for 150C. That will be easy to install along with the insulation, probably Tuesday.

I've received a small and expensive tube of Loctite 2046 and will replace the current Loctite 567 thread sealer with this food-safe thread locker. The 567 gives off an artificial citrus smell I don't like when the steam wand is activated. The new thread locker should keep the steam and water taps from turning.

The pilot lights are now held in place with silicone sealant.
Gary
LMWDP#308

What I WOULD do for a good cup of coffee!

zubinpatrick

#663: Post by zubinpatrick »

drgary wrote:Close Call This Morning

The thermal fuse failed (stayed intact). So I've ordered a heat safety switch from Orphan Espresso, one where I can strap the bottom onto the front plate of the boiler to have double protection.
Know that a thermal switch or fuse must be placed so it will easily overheat if necessary. Use a GFCI whenever possible.

Added: To put this in perspective, a fried element would not be a catastrophe, just expensive and time wasting. At this point I know the machine so well I would still have a fine machine near the end of restoration.
FWIW my need with thermo overload devices is that yjey rarely trip soon enough to protect anything....or if they do trip soon enough they trip too soon and create no heat situations. Getting them in the correct zone to trip before fire or element failure seems almost impossible. On my Pav for instance the thermo switch did not fail on an boiled dry boiler....or soon enough to stop a fire.

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drgary
Team HB

#664: Post by drgary »

The thermofuse is a crude device. I think it didn't melt because contacts shorted by water tripped the GFCI first. The heat safety switch I'm about to mount on the boiler is rated for 150C. The boiler reaches 110C + when it's normally up to temperature. That seems worth installing, don't you think? It's this device:

http://www.orphanespresso.com/Resettabl ... _3328.html

BTW I did have a thermofuse melt and save the heating element in my Isomac Amica when the boiler gasket began leaking.
Gary
LMWDP#308

What I WOULD do for a good cup of coffee!

jonny

#665: Post by jonny »

Protecting an element in a dry boiler is nearly impossible. The element heats up super fast and blows before it has any time to heat the boiler in the slightest. Maybe it will work if the fuse/breaker is very near the heating element, but even then, I doubt heat transfer is fast enough. My belief is that these fuses/breakers are not meant to protect anything in this situation, but more to keep a boiler full of water from exploding. With water in the boiler, the water controls the rate that the element heats up and the water conducts heat to the entire boiler to set off the switch wherever it may be on the boiler. If no water, yeah, you are probably toast.

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drgary
Team HB

#666: Post by drgary »

Hmmm. I think I'll attach that thing as a good luck charm, then. It can't hurt. Any other opinions out there? OTOH if I keep a thermofuse on the lead to the element and the wire heats up, that could melt the fuse and have some protective function. I just don't think it quite got there when the GFCI tripped. It did protect my Amica.
Gary
LMWDP#308

What I WOULD do for a good cup of coffee!

jonny

#667: Post by jonny »

Just to clarify, I do highly recommend the use of these thermal safety devices. They are not redundant in any way. They are one more safeguard that could save you a ton of money and even life. I think they just don't prevent what some people think they prevent. In a commercial environment, a dry boiler would be an anomaly, so to use a device for such a situation would be odd. I have never seen anything else used, but I wonder if there are any fast response devices that could save a heating element in the event of a dry boiler... I would suspect it would have to either be something that could actually measure the direct temperature of the heating element (internal or maybe surface), or it would have to function by other means, like electrical readings. I'm sure something of this nature would be either, if not both, expensive and invasive, and probably of the digital nature.

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drgary
Team HB

#668: Post by drgary »

Maybe some kind of PID controller with a temperature probe inserted into the boiler. It shouldn't be more complicated than that. Necessary? If the heating element blows in a machine of this value I will have another custom made and then create the PID system. :P
Gary
LMWDP#308

What I WOULD do for a good cup of coffee!

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drgary
Team HB

#669: Post by drgary »

Well, after that feedback and looking at where to attach the heat safety switch I decided to just reinstall the thermofuse. I covered it with heat shrink tubing, grasped that with cold pliers while shrinking the very ends with a match, and connected it with Rescue tape sealing where it mated with the heating element and PSTAT wires. I also surrounded the heating element contacts with Rescue Tape, which is rated for high temperature and is waterproof. It's commonly used for electrical contacts like this.

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Then I replaced the barb fitting for the inlet hose with a press-to-fit connection, mating 1/8" NPT with a 3/8" silicone hose from the FloJet pump.

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Next I painstakingly fit and cut the melamine insulation for the boiler. I left room at the bolts so they can be tightened from time to time. Next time I would consider a different insulation because although this stuff doesn't seem toxic, it's not stretchy and tears and powders easily. I fastened this down with very large cable ties obtained from McMaster-Carr.

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All of this fit very snugly in the case.

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Getting the case back on was a challenge as the clip nuts fell out a few times when the screws weren't exactly aligned. I had to be careful to not tear the insulation. Alignment in general was tight. But now it's fastened on, and I was able to check the size of the drip grate on an assembled machine.

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I also chose to smear some more Loctite 567 onto the steam tap threads as this almost held. I'll let it cure and the smell will fade.

All that remains is pressure testing to make sure that squeezing the case onto the PSTAT caused no leaks, and having the drip grate parts tack welded together. I'll also attach stainless steel edge molding to the back of the drip grate to help prevent water from leaking through in that spot, although even at this point, which the welded plate being slightly higher than the grate surface and drainage holes below that, it should be pretty leak resistant.
Gary
LMWDP#308

What I WOULD do for a good cup of coffee!

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drgary
Team HB

#670: Post by drgary »

The Home Stretch

Pressure testing went fine. This afternoon I took the drip grate parts to a welder. He removed the protective film and we eyeballed the parts and realized the film had covered the perforations sufficiently that when lined up the upper plate didn't fully reveal the intended perforations underneath, just by a fraction. He handed me a file and I worked the edge down for about 1/2 hour. Then he showed me a sample of how tack welding would leave scorch marks and maybe even bubbles or bulges on the surface and said these could not be entirely polished off. He was a nice guy and agreed with my choosing to use JB Weld, an epoxy, to fasten the pieces together. Then I went to a metal supply house and after an hour of searching came up with some perfect chromed brass L bracket that will create edges at the back and the sides of the upper plate. This evening I spent another hour filing off excess steel and creating a mild bevel on all exposed edges. Then I roughed up the side parts that will stick together.

Stainless steel takes an incredibly long time to wear way. It was a good time to do a meditation practice to be patient with the process, although at this point it all feels downhill. Tomorrow morning I'll apply dabs of JB Weld, then I'll very carefully align the upper plate and the main workpiece and tape, then clamp them together to cure for the day. I'll also cut the angle brackets to size for back and sides. The side part will hide where the two edges are joined. The angle brackets will hold the drip grate in place on the frame. And that should be it. Tomorrow I'll move my Prestina to the kitchen counter so I can start using it with the other half of drip grate I cut to free the workpiece even while I'm fabricating the finished grate.

At the welder I'd been holding the workpiece in one hand and steadying it against my body while filing. Resting the workpiece edge on the plywood table steadied it much better. In this process you need to pay steady attention. With a long file like this one seldom slips off the piece but I did manage to do so a couple of times and introduce minor scratches I later sanded out with #220 grit sandpaper. Sorry for the blurred photo.

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I also filed a narrow beveled edge and used coarse sandpaper to rough up the parts that will be glued together. The file is medium with one side single grooved and the other crosscut to wear away more material (stainless is still slow).

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The drip grate will be brushed steel to match the steel plates on the machine. The L brackets used for edging are chromed brass, so they'll match the chromed parts of the machine. It should all look nicely finished.
Gary
LMWDP#308

What I WOULD do for a good cup of coffee!