There is an interesting trick one can pull in this machine. The extraction pressure can be varied from maximum down to 2 bar or so by opening or closing the hot water valve. On my machine, the pressure is set to 10 bar into a blind filter. This gives a normal extraction pressure of 9 bar. With a few minutes of practice, I can set the pressure at anywhere between 2 and 9 bar to an accuracy of +/- 0.25 bar or so. Response is almost instant, so you can easily control the pressure at any phase of the extraction. It is a little dicey below 2 bar as the pump can shut off prematurely. The valve control is pretty sensitive but the knob is situated in such a way that you can easily get very fine control.

Bezzera Strega - Second Look

The Jim Schulman article above shows how to perform difficult espresso extractions by varying extraction pressure using a Bezzera Strega lever machine. I suspect one could duplicate his technique on the BES900.

It is likely that the same technique could be used on any machine which is plumbed similarly to the BES900.

When I do this, I normally catch the expended water from the tap in a tall glass to keep it out of the way and avoid filling the drip tray. I also pull a blank shot right after to expend any coffee which may have been forced back into the boiler. I don't know if this trick will cause any problems with the machine.

I don't really have the skill or experience to tell what effect this has on the taste, but there seems to be some interest in being able to vary the extraction pressure and this seems to be an easy way to do it.

If this truly works - and is not simply an artifact of the gauge's sensor location or operation - this capability will spark some serious interest and this will become a long, long thread.

In fact, the idea could spur a new class of pressure-bleeding mods and regulators for quite a few machines. It is starting to appear that time and other constraints may cause pressure profiling to be, ironically, more practical in the home environment than cafes, so mods for the lower-cost home-barista rigs could prove the most useful. Thanks for posting this information.

Does the temperature remain stable this way? It seems you will be bleeding off hot water and effect the extraction temps.

According to Mark Prince's writeup on the Breville, the pressure gauge measures very closely what the extraction pressure is. From his first look on CoffeeGeek.

This is also where the gauge up front comes into play and, like I said early on in this First Look, is so much more relevant on this machine than it is on most home espresso machines with a pressure gauge. First, the response time from this gauge is quick -- very quick. It's also very accurate. I ran a Scace II measuring device on the machine and the pressure measured in the portafilter was very close to the gauge's reading. This is a first for me, especially in the lack of lag time. Quite impressive stuff, and the gauge is this accurate because of where the pressure is measured (as close to the dispersion screen as is possible with engineering).

So it is likely that the pressure is correct.

The temperature display does not blink, which I would assume means that the temp is stable. Looking at the boiler photos from the above review, there is lots of water at temp to draw from. Can't tell unless I measure it but my gut feel is it is stable - certainly for 40 seconds or so.

Metatron wrote:The extraction pressure can be varied from maximum down to 2 bar or so by opening or closing the hot water valve... I don't really have the skill or experience to tell what effect this has on the taste, but there seems to be some interest in being able to vary the extraction pressure and this seems to be an easy way to do it.

I haven't tried it, but others have reported similar experiments like Rancilio Silvia Preinfusion Technique. Bleeding the pressure with a variable flow OPV would be better for temperature stability (i.e., bleeding water before it enters the boiler to reduce the effective brew pressure, not after).

i just tried this. the pressure gauge does respond which is obviously worth exploring. quick summary of what i found:

my opv is set near 10
- pre infusion for 6s
- i open up the hot water tab for the first 20s ramping from 0 to near 6bar
- i closed for the last 15s as keeping open longer caused my temp to start blinking - i'm not sure how much the temp dropped but my SV was 199F

HB wrote:bleeding water before it enters the boiler to reduce the effective brew pressure, not after

That's how I modified my commercial HX, without involving an OPV, originally posted here.