foopresso wrote:...lack of an OPV acts just as a different pressure calibration but which one can compensate with good tamping ?

For an espresso machine equipped with a vibratory pump, an over-pressure valve sets the maximum brew pressure, but it doesn't come into play if you're pulling volumes near a double espresso; this is because the vibratory pump's effective pressure is inversely proportional to the flow rate. If you're pulling short ristrettos, the OPV opens to keep the maximum pressure below what it otherwise would be at that flow rate (around 11 bar for a ULKA pump, for example).

That's the short answer; see I still don't get it: Why adjust the OPV? for the long answer.

foopresso wrote:how much time must we add on an Oscar from a cold start to extraction compared to if the machine had the valve ?

A vacuum breaker doesn't affect the warm up time per se. Basically it automates the release of "false pressure" when the machine warms up. If you're there to purge a bit of steam 5 minutes after the machine is powered on, there's no difference in final warm up time. That's the short answer; see Can someone please explain false pressure? for the long answer.

foopresso wrote:what machine design is inherently more temperature wise stable ? HX or single boiler ?

Obviously an espresso machine with a dedicated brew boiler holds an advantage in terms of temperature management over one that requires cooling flushes. See Espresso Machines 101 for the long answer. See Why are HX espresso machines so popular? and any of the "HX vs. double boiler" debates that come up every couple months for the much, much longer answer (Can I brew at exact temperatures with HX espresso machines? is one such thread).

Yes to your first question and no to the second.

Tamping harder/softer doesn't dramatically impact the flow. Try it yourself tamping moderately and then tamping like a gorilla. In my experience, the latter may initially pour more slowly, but will catch up with the pour rate of the more moderately tamped puck. My theory is the tamp pressure can affect the initial wetting of the puck's surface and thus the early pour speed. As I've said many times before, the tamp is way down on my list of important contributors to exceptional espresso. Distribution and the correct dose are significantly more important.

To make things more complicated; a well adjusted OPV may make a dramatic difference on some machines, and have only a negligible effect on others. I experienced a large increase in ristretto and singles quality when tuning the OPV on my E61 machine down to 9 bar; but saw no improvement when I retrofitted an OPV to the Elektra Semiautomatica.

If you are at all handy, retrofitting an OPV on a machine that doesn't have one is under $100 in plumbing (including a PF mountable gauge) and about an hour's work. You do not even need access to the water tank for the waste hose, since that can be fed back into the pump's suction side (although this will increase the noise the pump makes substantially)

Hi, there.

As I'm restoring an Oscar, I could add something here. The neplax valve at Oscar's group doesn't open even around 14 bar. This valve is new, the pump is old.

I've added an adjustable OPV but can't say anything about improvement yet. But I beg to differ from Dan - even at normal doubles the pressure rise to 12 bar (I've installed a pressure gauge and considering the Ulka pump specs, that makes sense). But as far as I remember Andy's post, and I can't find the thread, the flow is not directly proportional to pressure - at some point the flow goes down, maybe because the higher pressure pushes more fines down the puck and the they restrict the flow.

A vacuum breaker is easy to fit, and it's just a convenience. About the power, mine is 115V, and the heat element is around 10 Ohms. But here the power line is 127V and I get more than 1500W and 12A, so the wires go hot! But 100 to 200W won't make such a huge difference.

And finally, about temperature stability, I've played with the machine and a Scace device and it's very hard to find a good routine, besides the fact that the temp profile is rising ramp, for instance from 92°C to 97°C! I've added the flow restrictors they use at Musica and now the machine is much better... Easy to use, just a little flush to get 94°C shots, or 1 to 4 seconds more if you need colder shots. As far as I've measured, no more than 1°C change.

Márcio.

Bezzera BZ07? It's a machine I'd like to test someday, as it could be an addition to the poor market of prosumer in Brazil, at a reasonable price.

There are a lot of threads about it, including temperature behavior. Note that it hasn't a thermosyphon HX, the group is heated by a cartridge element and the HX water goes directly to the group. But the opinions about it are very positive and the price around Europe should be less than 1000 EUR!

Márcio.