After reading Eric's post in this Considering upgrade from HX to double boiler espresso machine, i started playing with my Isomac Tea pressurestat.

My goal is to theoretically pull a shot without water dance. To do so one would thing that lowering the boiler pressure to the target brew temp would do just that. So i've lowered my boiler pressure so it fluctuates between 0.6 and 0.75 bar. Theoretically at this is between 187 and 198 deg. F, or so.

Oddly though, i'm still getting water that flash boils when i pull from the group. That doesn't make much sense to me! The thermocouple in the group shows it's idling at about 201 or so (it's only been on for about 25 minutes)

Any ideas?

thanks Eric. I agree on the accuracy of the pressure gauge, but i figured it would be accurate enough that if it showed 0.75 bar it would be well below boiling!

jasonmolinari wrote:My goal is to theoretically pull a shot without water dance.

Boiler pressure settings and the resultant brew temperature for one HX espresso machine don't translate well to another, even if they both have E61 groups. That's because manufacturers tune the thermodynamics of their equipment by altering the HX volume, HX injector length, HX/cold water mixing, thermosyphon restrictors, and delivery/return leg lengths/diameters to produce the desired temperature mixing. Eric's data for the Quickmill will be especially difficult to apply to your Isomac since the Quickmills vent water through the over-pressure valve from the hot side of the HX whereas yours vents cold water immediately after the pump.

jasonmolinari wrote:Oddly though, i'm still getting water that flash boils when i pull from the group. That doesn't make much sense to me! The thermocouple in the group shows it's idling at about 201 or so.

See this Saturated Steam Table. Your boiler pressure reading of 0.7 is 1.7 bar absolute; the water's temperature is thus 239°F. The grouphead measurement is lower because the thermosyphon is not 100% thermally efficient and the whole system is radiating heat. Still, the cooler grouphead may not be enough to initially cool the super-heated HX water below the boiling point of water at room pressure, hence a moment of flash boiling.

In other words, the idea of lowering the boiler pressure isn't to fine-tune the HX per se, but to have the grouphead idle near the desired brew temperature. A short flush of flash boiling may be just enough to equalize the group, since it's going to be hotter at the flat back of the grouphead than the tip of the saucer.

ah hah. Got it. Thanks Dan

The PID on my Anita's boiler makes the adjustment much more accurate and with much more narrow range around my set temperature. Still, its probably been a year since I bothered to lower the temperature to where I could pull shots without flushing. See Ian's fine thread HX Heaven or 1½ Boiler for details of the protocol. It takes my machine if I recall about 7-10 minutes before the temperature is stable enough for the next drink. That's not acceptable even in a leisure morning and so I don't use low boiler temperatures anymore.
However, I still utilize my PID to fine tune the boiler temperature based on either ambient temperature in the room (less frequently) or more often to make it easier to pull shots from 198 all the way up to 203* or so. Eric's thermometer grouphead probe is an ESSENTIAL tool for HX users and I find better to control temperature the the lower boiler method. With the PID to fine tune I can use reasonable flushes to get a wider range of pull temperatures with good profiles. I think this is a underestimated feature of putting a PID on an HX. The other of course is reliability.

Finally- Dan mentioned the QM feature of running water back to the reservoir via the HOT side. I find this the single poor design feature I found for the machine. It cools the water in the HX too fast on shots longer then 25-28 seconds and ristrettos need care to prevent the temperature from cooling too much at the end of the shot because of hot HX water returning to the reservoir via the overpressure valve. I'm sure there are good reasons why they made the design as they did and perhaps Eric or Dan can point them out for my education.

+1

I'm currently running my boiler ~0.5bar for walk-up 198.5°F Caffee Fresco Ambrosia 8g singles. I use On / Off mode on the PID, rather than controlling the boiler to double-0-nought stability. On the Vibiemme, the HX thermosyphon performs better with an oscillating, mimicking pStat, rather than flat line boiler temperature.

See Scale, HX Temperature Stability and a bit o' PID