So, Robert, you're saying that in the following quote, Jim is omitting the difference in atmospheric force on the valve seat?

another_jim wrote:What's the difference between altitude and a similar pstat (or relief valve) adjustment?

In a vacuum, a machine set at 1.2 bar gauge (2.2 bar absolute) would still show 1.2 bar, but be running at 1.2 bar in actuality; and as it rose in altitude it would run through this range of 2.2 to 1.2 bar absolute. Why is using the pstat to make the same adjustment different?

Would I be correct in assuming that a machine where temperature is controlled via a sensor like a thermocouple hooked to an electronic switch would seek the same temperature, but the OPV (over pressure valve) would need to be set higher?

The boiler temperature of a system containing only water and water vapour depends on absolute pressure.

Thus a temperature adjusting device would always produce the same absolute pressure.

However ordinary pressurestats and pressure relief valves depend on difference.

rpavlis wrote:However, the boiling point in an open container is lowered at higher altitude. It is about 92-93 at 2500 metres.

The interesting thing is the shot pressure. As the lever cavity fills, there is going to be a little flash boiling, but once the pressure has equalized, it will stop Once pressure is applied manually, there is no boiling at all. Flash boiling can only occur when the water exits the puck. This is probably not an issue at 2500m

So, couldn't the overheating problem of the group could be solved at sea level with a pstat adjustment or weakening the spring of the old relief valves?

another_jim wrote:So, couldn't the overheating problem of the group could be solved at sea level with a pstat adjustment or weakening the spring of the old relief valves?

In Millennium machines the PSTAT adjustment prevents an overheating "problem"* and allows temperature surfing through the range of coffees by using half pumps to differentially introduce hot water to the top of the group. In second generation pre-Millennium machines adjusting the relief valve would theoretically work, but then you might be venting steam very fast so it could run dry quickly if left on. There are other ways to reliably control the temperature of those that are outside the scope of this thread. Robert can tell us more about his first-generation machine as mine is still awaiting restoration.

* Temperature stability is less critical than temperature control. I want the group very hot for light roasts and quite cool for dark ones.

On the p-stat equipped machine changing the setpoint to a lower setting will lower the boiler pressure and temperature. As this is a saturated system the pressure and temperature are bound together. Once the p-stat is set changing the external atmospheric pressure will result in an internal pressure/temperature change. The pressure gauge indication is also impacted by these external pressure changes.

It seems as if an agreement has concluded?

The boiler pressure will always be constant.
Although the Pstat would need to be adjusted to correct bar absolute.

rpavlis wrote:on Yesterday, 3:40 pm
If one be much above 2500 metres I suspect we would be unable to maintain high enough temperature for proper espresso extraction.

another_jim wrote: on Yesterday, 5:30 The interesting thing is the shot pressure. As the lever cavity fills, there is going to be a little flash boiling, but once the pressure has equalized, it will stop Once pressure is applied manually, there is no boiling at all. Flash boiling can only occur when the water exits the puck. This is probably not an issue at 2500m

Now I do hear some boiling at the beginning, IF I pull the shot tooo slow. Sometimes it does anyways. ( Just trying to figure all this out )
At the end of the pull, it always boils, so I try to end the shot just before this occurs and drips in.

[creative nickname] wrote: on Yesterday, 5:42 am

Our family has a vacation home in Colorado at around 2500 meters above sea level. I make espresso up there using a 1976 Pavoni, and it is, as you say, the perfect tool in that environment. I find that slightly darker than usual roasts work best, however.

I'm slightly always leaned that way, I was wondering why everyone else here is leaning more to lighter roast.
I suppose the difference in elevation is influencing this factor?




Lighter roast prefer a hotter group, although darker roast use a cooler group temperature.

Too high an altitude use a darker roast, or go for the buttered tea like a real Sherpa. Gary.. need to use that line in a signature .

Boiler pressure/temperature will not be constant as the p-stat is responding to the differential pressure across the p-stat diaphragm which is boiler pressure on 1 side and atmospheric pressure on the other. The atmospheric pressure decreases with increasing altitude. The actuation point is determined by the absolute value of this differential pressure and the opposing pressure of the set point adjusting spring. The spring pressure is not affected by atmospheric pressure, it is strictly mechanical in this case. The sum of the atmospheric pressure force and set point spring force on 1 side of the diaphragm is opposing the boiler pressure force on the other side. Reducing the atmospheric pressure force will result in the boiler pressure force moving the diaphragm to the mechanical set point at a lower actual boiler pressure/temperature at a higher altitude than it will at sea level.

+1

Think of air as water. The deeper, or closer to our sea level, you go the higher the column of air above you and the more weight it exerts on everything around you. Just like when you dive into water - at 1' depth you don't notice the water pressure much, but at 10' you can definitely tell there's a column of water above you exerting pressure around your whole body and the ears start feeling pressure no matter which way your head is turned.

-edited for some clarity and hopefully correctness-
When you move up in altitude you also loose a percentage of the air column above you, resulting in lower pressures working on everything. Water boils at a lower temperature as a result. PSTATs will still run relevant to atmospheric pressure, but that atmospheric pressure is no longer 14.5/14.7PSI of sea level.
A PID would still yield the same temperatures, and the same absolute boiler pressure.

Air doesn't seem to us as a heavy object, but that atmospheric column is quite high and it does exert quite a bit of force. Over 14 pounds of air happens to be hovering over me at this very instant!

another_jim wrote:So, couldn't the overheating problem of the group could be solved at sea level with a pstat adjustment or weakening the spring of the old relief valves?

Running my Faema Faemina at 0.3 bars on the low setting at a few hundred feet above sea level does seem to solve any overheating problem.

I'm loving these explanations. I bet some of the posters in this thread could explain to me why a full balloon weighs more than an empty one. I know from experiments that it does because the air inside is not weighless, but since the column of air is pressing down on the balloon either way it doesn't seem to make sense.