Chert wrote:I don't follow the point that insulation of the boiler would lead to a higher temperature of the group or greater heat transfer into the group. The central relation seems to be temperature and pressure governed by the setting of the pressure stat.

If it isn't too late to comment on this - that's true, but I think you would agree that if you could raise the temperature of the boiler itself (as opposed to its contents), assuming the boiler is attached to the group and that it's massive enough to make a difference, that excess temperature will transfer to the group.

So (it seems to me) it comes down to whether insulation really does make the boiler much hotter. My intuitive notion is that it depends on how effectively the contents maintain temperature. If temperature can be maintained at the inside surface of the boiler, then I would expect the boiler temperature will be maintained as well, since the boiler metal is much more heat conductive than the air around it. But if the temperature next to the inside surface drops, as heat is conducted out due to lack of insulation, of course the boiler temperature will drop as well.

Of course that's just idle speculation. I regard our uninsulated boiler as a auxiliary heat source for the kitchen. We have fewer hot days on this side of the mountains - may not have any this year.

I've found that my measurements seem poorly reliable perhaps because I have not been able to make a consistent contact with the group. To be reproducible, the location and contact would have to be the same. yesterday I measured temperatures at 1.0, .85 and .7 bar measured on the pressure gauge 1-2 hours after dialing down the Sirai pressurestat. The temperature at the group remained basically 70C with some fluctuations to 72C. That doesn't make much sense.

I probably just need to do more homework about temperature measuring and temperature and pressure relationships. I should be able to calculate the temperature of the water within the boiler at the current pressure. Is the pressure gauge relative pressure? Then using a table one would check at what temperature water boils at atmosphere plus the current gauge setting. E.g. for gauge set at .7 bar add 1 bar so look up boiling temp at 1.7 bar 114 C = 238 F. whereas at 2.1 bar (1.1 on the gauge) the 121 C = 250 F.

Here is a picture of the parts of an extra CMA group I have. I've pondered how to measure during brewing without going around the portafilter or making a point for leaks. I think it would be interesting to study the profile during shots with alterations in the other parameters. Any suggestions?

donn wrote:If it isn't too late to comment on this - that's true, but I think you would agree that if you could raise the temperature of the boiler itself (as opposed to its contents), assuming the boiler is attached to the group and that it's massive enough to make a difference, that excess temperature will transfer to the group.

So (it seems to me) it comes down to whether insulation really does make the boiler much hotter. My intuitive notion is that it depends on how effectively the contents maintain temperature. If temperature can be maintained at the inside surface of the boiler, then I would expect the boiler temperature will be maintained as well, since the boiler metal is much more heat conductive than the air around it. But if the temperature next to the inside surface drops, as heat is conducted out due to lack of insulation, of course the boiler temperature will drop as well.

Of course that's just idle speculation. I regard our uninsulated boiler as a auxiliary heat source for the kitchen. We have fewer hot days on this side of the mountains - may not have any this year.

Donn,

You very well may be right. It does make sense that if the heat transfer occurs through the metal which is hotter because insulated against ambient temperature, then the group may be warmer. Thanks for sharing in my idle speculations.

Hello Flint, I too have had a lot of speculations about grouphead temperature in my newly acquired Conti Empress, that seems to have a tendency to idle at too high temps. As your CMA also seems to be, according to your pics, the Empress is a "dipper-fed" lever. The group is directly attached to the boiler, no termosyphon. What matters most for the grouphead temperature IMO is: 1) boiler pressure, and yes, the table corresponds well 0,7 bar should be around 114C. 2) Heat transfer from boiler to group, determined by contact area, and any material (gaskets) that alters the thermal conductivity 3) dissipation from the grouphead, surface area or active cooling/warming.

You can lower group temp by lowering 1)
Or by changing 2), meaning changing gaskets to a material with lower thermal conductivity or of greater thickness

3) is difficult to change if you don't have a termosyphon loop..

I think that for some of the vintage lever machines, these were built in a time where Asbestos gaskets were frequently used, and it has a very low thermal conductivity, effectivly blocking excess heat transfer, so we need some way to compensate for that, either using thicker gaskets or of materials with equivalent low thermal conduction.
I have done some searches, and so far PTFE/Teflon is the material that I have found with the properties we want for gaskets with the lowest thermal conduction, but would be glad to hear other suggestions!

Best regards and good luck with further experiments

Ulrik

I've been getting wildly different results lately with my 1 group so I picked up a cheap thermometer with thermocouple. I have the OE temp strip and after warm up the left square is starting to change color. I've had the p-stat set to 1.05 to 1.25. Today before work I threaded the end of the TC up a spout on the portafilter, pulled the lever down, and got a reading of 210 degrees F. I haven't had a chance to try the TC in boiling water yet but I don't think it's that far off. I didn't take notes but after one particular flush/rest cycle I got temps around 200 degrees F but they weren't repeatable. Yet.

Anyway, after all that I turned the p-stat down to about .9 to 1.1 but I have a feeling I'll turn it down a little more tomorrow and do a lot of testing.

. . . so I picked up a cheap thermometer with thermocouple. . . .

Check your thermocouple AND meter in a "covered pot of steam" and calibrate with respect to YOUR barometric pressure and altitude. This would be a good read - Olympia Cremina Temperature Study, Part 1

Yep, that's the discussion that got me motivated. And then I recalled this one that deals specifically with the Astorias.

Ok, so the thermometer/thermocouple are within 1 degree F of boiling temp here in Los Angeles and I think I might have time to do some testing this weekend.

But, after my first two shots today which were bitter and hot, I turned the pstat down to .8 to 1.0 and let it sit for an hour. When I came back out to the garage I pulled two shots in a row on it and both were stellar. Environmental temp today was around 65 degrees.

So I'm looking forward to seeing what the numbers are tomorrow.

My machine is in the basement, running with the covers off at the moment, boiler at 1.2 bar.
At least, I'm guessing 1.2 bar. When I was futzing with the gauge when adding a vacuum breaker, it took a small tumble and doesn't read zero anymore

I'm puttering with measuring the Astoria group again because I finally fashioned a portafilter/basket/thermocouple arrangement to measure the water temperature above the puck.

Using already extracted grounds to hold the thermocouple above the puck with the wire passing through a hole I drilled in the basket a took a few measurements. There was enough variability in the range of temperatures during an extraction that there is no point in sharing most of the measurements. One needs a measurement device that allows a consistent and very slow flow during the measurement. With a newly packed PF/TC basket I measure a peak temperature of 92 C which declines during the extraction. So I did learn an approximation of maximum extraction temperature at that boiler setting.

The coffee I am currently enjoying has a recommended brew temperature of 200-201 F (app 93 C). So I upped the brew pressure to 1.05 bar. I'm glad I did because the intensity of floral/spice flavors in the resulting extraction was superlative. But I tried to take some readings with the higher boiler pressure and instead spewed grounds through the hole in the filter basket - twice. Not sure if the higher boiler pressure or insufficient quantity of coffee around the TC wire is to blame.

These tests confirmed one other impression I have had about the commercial lever group. If one keeps the group valve open more than 3-5 seconds, the water temperature above a tightly packed portafilter peaks and declines.

These tests confirmed one other impression I have had about the commercial lever group. If one keeps the group valve open more than 3-5 seconds, the water temperature above a tightly packed portafilter peaks and declines.

I'm not sure if that would apply to levers with thermosyphon systems. My Brasilia grouphead stays very hot from thermosyphoning, so maybe it wouldn't experience the thermal profile your Cafethema, (Astoria et al). I have no temperature measuring equipment so I'll not be able to see for myself.

Flint, bring your probe next time you visit.