I am trilled to report that my Vibiemme Domobar Super Double Boiler has finally arrived.
Unfortunately I am still waiting from my Cimbali Max Hybrid, but it should arrive next week. In the meantime I've just been playing around and trying to get a better understanding of this beautiful machine.

There is one thing that worries me. There is a tiny leak at the very top of the group - water is seeping out very slowly and dripping down. It appears when I switch the machine on, however it does seem to stop after a few hours.... could be because the group gets hot that the water evaporates instead?

I also seem to have an irregular brew pressure, but this is where I need some help and explanation.

What should the brew pressure gauge read when the machine is in idle?

Currently it is at around 1 bar, however on others days it's also been at 4, 13 and a few other values. When I actually pull a hot (using pre-ground Illy - I know, I know, but I had to try something while I wait for my grinder) the pressure drops dramatically. Ok, let me give you some numbers that I got this morning with a blind.

Switch on - 1 bar
Blind - 4.5 bar
Idle - 4 bar (slowly drops to 2)
Blind (long one) - 5 bar them shoots up to 13 bar
Idle - 13 bar, dropping slowly over 5 minutes to 6 bar.

Unfortunately since I'm in Germany I'm relegated to the useless manual that VBM supplies so the only resource I have this this forum. There had been a few stories regarding this and it seems it could be the OPV spring.

I've tries to adjust the OPV adjustment screw, but because of the wild fluctuations in pressure (that still remain) I don't know if it did anything.

Any advice would be much appreciated!
Cheers.

congratulations on the new setup. I pre-ordered myself for a VBM DB, but I gotta wait till May for mine . Hopefully you get your grinder soon and get some video's up for us .

CaffeRoma wrote:What should the brew pressure gauge read when the machine is in idle?

The HX loop is a closed circuit when the machine is idle, so the pressure will naturally rise as the water heats and expands. The over-pressure valve serves as an expansion valve in this case, limiting the maximum system pressure to avoid damaging the plumbing.

Related topic... Mythbusters demonstration of an exploding water heater without an expansion valve:


Subsequently the gauge reading is only interesting during an extraction.

Blind (long one) - 5 bar them shoots up to 13 bar

The blind reading should be consistent. If it's not, then you may have one of the "bad" OPV springs Randy reported (link).

Thanks Dan,
Ok, so when idling, the pressure it not so important. Also, it seems I do have consistent blind pulls...basically for the first 4-5 seconds the needle hovers at around 4/5 bar then shoots up to 13 or 13.5. I assume this is the important part to watch.

Will my blind pulls differ in pressure from extraction ones? When I was using the pre-ground Illy, I was getting only gushers - way too fast, I would get 1oz single shot in just 6 seconds and the pressure stat was reading 6 bar.

Will it change once I get proper ground espresso with my Cimbali?

Can I set my OPV by doing blind pulls or should I do it with proper extraction pulls?

Thanks again!

A quick perusal of the Resources page or FAQs and Favorites will lead you to threads explaining how and why to adjust the over-pressure valve. In a nutshell, it sets the maximum pressure during an extraction. For most vibe pumps, the dead head reading against a blind basket is slightly higher than that during a real extraction. Assuming your model has the "good" OPV spring, I recommend starting at 10 to 9.5 bar for the Vibiemme. The 13 bar reading is too high, especially if you're fond of ristrettos.

As for the Illy preground... that's also covered in the FAQs and Favorites. Short answer: preground coffee = stale coffee. Illy's nifty nitrogen-flushed canisters theoretically slow staling while the coffee is warehoused and shipped, but it seems to accelerate staling once the can is opened. The whole bean Illy I sampled was painfully stale within a couple days of opening. Someday I would like to try genuinely fresh Illy coffee, I bet it is good...

CaffeRoma wrote:There is one thing that worries me. There is a tiny leak at the very top of the group -

Tighten the two socket head screws just a little. If that doesn't do it, tighten the top nut. If that doesn't do it, replace the seals up there. This should be a warranty repair.

I also seem to have an irregular brew pressure, but this is where I need some help and explanation.
What should the brew pressure gauge read when the machine is in idle?

When the brew switch/lever is off, the brew pressure gauge in not useful for anything. It is only functional when brewing espresso or backflushing and not at any other time.

With the blind filter in place, turn on the brew cycle. Wait. Pressure will hit about 4 bar and sit there for about three to four seconds or so, then ramp up to the brew pressure. That should be about 9.7 to 9.9 or so... in that range. The pressure reaching the coffee will be about 1BAR less. Adjust the OPV when the oressure is at its highest at that time.

The OPV valve should be easily adjusted and the adjustments very linear and smooth as you turn the screw with a blind filter in place and the brew cycle on. After adjusting, if you see varying pressures during brewing, PARTICULARLY HIGHER PRESSURE THAN THOSE TO WHICH YOU ADJUSTED THE opv, then the problem is probably the spring.

Randy G. wrote:The OPV valve should be easily adjusted and the adjustments very linear and smooth as you turn the screw with a blind filter in place and the brew cycle on.

I did try to make some adjustments and I noticed that the screw is incredibly hard to turn - it was painful getting in there. In reading some earlier posts regarding the Domobar it seems that with the 'new' screw the valve should turn easily.... is this true?

I did this while the machine was off, but I may try it while running as you say.

HB wrote:Short answer: preground coffee = stale coffee. Illy's nifty nitrogen-flushed canisters theoretically slow staling while the coffee is warehoused and shipped, but it seems to accelerate staling once the can is opened.

Yes, I agree completely. It was just a test of sorts - I have this great machine and no grinder yet but I still wanted to pull an extraction and see what I get.

I am lucky enough to live down the street from a renowned local roaster that does some wonderful espresso, so I'll have a steady supply of freshly roasted beans when I get the Cimbali.

I will keep you posted.
Thanks!

CaffeRoma wrote:I did try to make some adjustments and I noticed that the screw is incredibly hard to turn - it was painful getting in there. In reading some earlier posts regarding the Domobar it seems that with the 'new' screw the valve should turn easily.... is this true?

I have seen the grease they use thicken up. Try removing the screw completely, cleaning it off as well as the threads in the OPV body, and re-greasing (food-grade silicone grease or similar). The screw has an O ring for a seal and that is what makes it stiff to turn. The 'new' (proper) springs are actually stiffer and make it more difficult to turn the screw.

What I did to make the OPV more accessible was to bend the copper tubing just a bit, then I loosened the two fittings on the OPV to set it at a better angle to the access port. Even then, you still have to navigate around the micro switch for the reservoir level. This poor arrangement was one of the negatives I listed when I reviewed my machine (see full review on my website).

The screws are stiff, and personally I would leave it that way. If it turns easily, it could drift over time from the pump vibration. As a general rule, your maximum blind basket pressure will be 0.5 to 1.0 bar higher than a real shot, unless you are using preground Illy coffee. Setting the OPV to just over 9 bars should put you in the 8.5 bar range, 9.5 would get you around 9 bar.

Thanks Randy & Dave.... will keep everyone updated on my progress.

Dave,

I think that the spring force against the screw and the friction of the O-ring seal should be more than sufficient to keep the screw from moving positions. The lube should be there to keep the O-ring from being damaged during insertion and when adjusting the screw. Possibly, also to resist corrosion. I wiped the thick grease off the threads when I installed the new spring, while leaving a film of it there. The screw moved smoothly and even though the new spring had much more compression resistance, the screw moved more easily and the brew force has not changed in over 6 months. If the screw was able to move I would have expected the brew pressure to decrease over time as the screw backed off.... or not.

Sounds good. I have not had any wandering brew pressure issues either but I have not remove the OPV, just adjusted it. I had an Isomac that liked to wander on occasion and the OPV was relatively light to adjust. Just a concern but it sounds unfounded given your experience.

I just rechecked my machine with the blind basket, I am getting around 0.4 bar difference between the blind basket pressure and brew pressure with the onboard gauge. I would have to put the ScaceII on the machine to verify the pressure. The factory gauges on most home espresso machines tend to be of lower quality to keep the cost down and the VBM gauges are no exception. They are one of my few 'nice to improve' items on the Domobar Super. I am assuming that VBM retained the same OPV and gauges, along with most of the internal plumbing between the Domobar Super and the new double boiler. I don't know how the lack of a heat exchanger would affect the pressure settings but I would guess they would be close in offset. So if you are shooting for a 9 bar brew, you may want to set your OPV at 9.4-9.5 bar with a blank basket.