What's causing the pressure change?

Primarily the condensation of steam due to the thermosyphon flow within the heat exchanger and secondarily, the heat loss from the boiler surfaces and connecting tubing.

Eric's chart and explanation are misleading in that they reference p-stat deadband variance over time rather than pressure gauge reading at any particular time.

It is a graph of boiler pressure gage readings (actually a much more accurate gage) attached to the steam wand. The graph was constructed from a video. Pstat deadband and the variation in boiler pressure as a function of time are identical.

So it's a graph of the variation due to mechanical p-state deadband and at some later time, the graph would have peaked again?

Absolutely.

The graph will repeat ad infinitum as long as the machine is simply sitting there at idle. Obviously, different machines/configurations would have different graphs but all would follow the same GENERAL pattern in that heating element on time is 4-7 seconds and off time is about 90-120 seconds.

What counts is a change from initial use to operation months or years following.

There is also the issue that pressure correlates to the maximum temperature any of the water is at, not necessarily to the temperature at the location you care about. At any given time, there is no single "boiler temperature".

homeburrero wrote: Yes. If you flush really long and lock in and pull really fast you can get a too-cold brew even if your boiler is set way high. Having said that, it is easier to get predictable temperatures (and saves water) if your pStat is set lower.

I don't understand how this happens, so hopefully you can clarify based on my HX knowledge. As I understand it, water passes through the tube and collects heat as it does so. That heat should be a constant for a given boiler pressure. For machines with lower pressures or slower heat exchangers, we wait before brewing so that the water in the path heats up more than it would just being pumped through (as we would for a flush and go). So if that is true, how could a high boiler pressure cause cold brew temperature? Are there some HX systems that are resilient to actual heat exchange?

Gig103 wrote:So if that is true, how could a high boiler pressure cause cold brew temperature?

The high boiler setting does not cause a cold brew temp - it tends to cause the opposite. I was trying to make the point that you could get a cool brew despite a high boiler pressure setting. When flushing, cool water is flowing quickly from the reservoir or water line through the HX, cooling the group in the process. Then if you lock in and pull immediately, the water in the HX hasn't had time to heat up, and the group temp is still cool, so you could get a 190F shot despite the 260F temp of the water in the boiler. As others have pointed out, different HX machines will behave differently in how they actually respond to flushes and delays.

homeburrero wrote:a high boiler pressure setting. When flushing, cool water is flowing quickly from the reservoir or water line through the HX, cooling the group in the process. Then if you lock in and pull immediately, the water in the HX hasn't had time to heat up, and the group temp is still cool, so you could get a 190F shot despite the 260F temp of the water in the boiler. As others have pointed out, different HX machines will behave differently in how they actually respond to flushes and delays.

i agree about this, in my small experience with my machine, im really careful on doing cooling flush, if i think im doing cooling flush a bit longer then i have to wait a bit longer than i usually did. (fyi set my machine to 1.4 bar)
usually my routine are doing cooling flush just to remove the steam, and wait bout 10s or 15s if a draw to much water when doing cooling flush, then brew.
maybe from what i think based on my little experience, high steam pressure benefit from shorter rebound time? or is it not?

agree to on different hx machine will behave differently, in my case i think the thermal mass on the group head is inferior to that of the E61, so the portafilter away time become quite important.