Chert wrote:I just recall that the probe on top of a puck of coffee made readings that I could not fathom. I seem to remember thinking that the thermometer could not adjust quickly enough as the water moved. If I were willing to ruin a few more thermocouples, I could try again and grind finely enough so that the water would stay above the puck for at least ten seconds.

A thermocouple's speed largely depends on the thermal mass of the junction point; a tiny beat thermocouple will react much faster than one encased in a large probe.

Chert wrote: Both lever groups I have worked with have superheated water behind them (Pavoni and CMA) so that if the water can pass rapidly through the puck, I expect temperatures over 100c to hit the coffee. An open boiler design, as some folks have and describe here, would seem to allow gravity extraction followed by pressurized extraction from the lever without much change in temperature.

Now this is interesting, and it is interesting to think of what happens to the superheated water in a lever group. Is there any reason to think that superheated water is actually making large scale contact with the coffee?

The lever is brought down, the 125c water starts to flow, hits the group and starts cooling (while heating the group). The shower screen keeps the coffee from mixing in any large quantity in the chamber. The water from the boiler fills the chamber until it is full, pressurizing it with boiler pressure. There's nowhere for the air that previously occupied the chamber to go, so it is there too, compressed.

The water will cool as it goes through the group screen under the boiler pressure, starting the preinfusion. It further drops in temperature as it hits the coffee. Then the lever is raised and the rest of the water is forced under spring pressure through the puck.

It might be interesting to bury a thermocouple in the coffee (tamped into the coffee) to see what temperatures it sees during preinfusion. Of course the thermocouple's very presence will alter the water flow at this stage.

Darn Heisenberg!

I guess a Scace tool might be the best we can model, but it measures the water as it would sit on top of the puck, not in it. I can't imagine a large difference though there might be convection effects in such a configuration which would throw off the reading.

One thing that the lever group has over a pump machine in this regard is that it allows a hot preinfusion followed by a cooler extraction, because the water for the extraction is pulled from the boiler and cools during preinfusion. Trying to do a hot preinfusion with an unflushed pump HX machine would do a hot preinfusion followed by a hot extraction. The temperature of a lever group isn't constant, that is clear - it would be interesting to log the temperature during a shot as well, and graphing the changes during the whole process.

Note that on its way to the brew chamber, the boiler water passes through a narrow tube - in the boiler chamber, but connected to the group, so I expect it would be a few degrees cooler at that end. If you bring the lever down without a portafilter in place, and let the water pour out, you'll get super heated water, but just if you draw only the volume of a shot, it should be cooler. Like with the hot water tap - first couple cc's are not nearly as hot as the water that follows them - but the hot water tap heats up more after use because it isn't as massive as the group heads.

Interesting discussion - here's my 2 cent:



The superheated boiler water ist not immediately entering the group, as there is already some water present (see left image) - the dipper tube and the group channel are filled with water left over from the previous shot and therefor slightly cooler than the boiler water, because not actively heated. The steady temperature of this water is proportional to the group temperature, which itself has a given heat loss via its surface and the surrounding air temperature and a given heat win depending on shot frequency and temperature and, as it is directly bolted to the boiler, it depends higly on the surface temperature of the boiler. You can see the influence of this connection when comparing with other machines using the same group bolted to the structure instead of the boiler as Nuova Simonelli or Izzo, which tend to need much higher boiler settings (up to 1.5bar)
That said, you will notice that an insulated boiler will result in a higher boiler surface temperature for a given water temperature and therefor in a higher temperature win of the group, leading to overheating problems especially in times of frequent shots. Uninsulated boilers will be more forgiving in this aspect as that is the way they were planned and calculated to be used.
When you open the group lowering the lever (right image) this water is pushed forward by the boiler pressure, touching the coffee first, while superheated water is going through dipper tube and group channels, thus exchanging heat with the group and cooling down on its way.
Neither on my 1grp Astoria nor on the 2grp Wega overheating is a problem, using 0.9-1.1 bar on the Astoria and 1.0-1.2 on the Wega. This deadband of the pressostat means a temperature band of approx. 2° boiler water temperature, a band with which I can easily live without thinking about PIDing the boiler.

So perhaps removing the insulation will help to get a better thermal stability on the groups ...

Best regards
Emil

Just a nit, but not only is the group bolted to the boiler, it is part of the boiler. So surface temps notwithstanding, there is direct contact of boiler steam with the back side of the group, which will set up a convection pattern in the boiler at idle, keeping the group heated.

And any water in the dipper tube is going to be heated by this steam/boiler water as well.

Otherwise, all good points. Im glad you pointed out that the water in the sleeve at idle isn't being actively heated, so maybe it's a cooler preinfusion. I'll have to think about this.

As full disclosure, I'm only speaking notionally here; my lever group is now fully disassembled, and I've never pulled a shot with it. Gotta get those seals/parts ordered so I can try this puppy out

I think everyone may be trying to get the same answers from different types of machines. Some machines the grouphead is directly attatched to the boiler, some the grouphead is mounted seperatly to the frame of the machine and plumbed, and even those can vary between brands.

Basicly a person is trying to hit a specific temp that will result in a good extraction.

The group is at "x" temp, the water is at "x" temp, and when it all comes together at the point where the extraction occurs it needs to be at "x" temp.

Obviously if the group is mounted remote to a steel frame there will be more heat loss by the time the water gets to the group, compared to machine where the group is directly attatched to the boiler.

As Clint mentioned even as the temp changes thru the day can effect the end result and thats where good barista skills come into play, taste, observe, adjust. Sounds like he has a great setup for sure...


I think the "ideal" would be a gravity fed, totally imersed group that could be held to whatever exact temp the user needed

ANeat wrote: Basicly a person is trying to hit a specific temp that will result in a good extraction.

The group is at "x" temp, the water is at "x" temp, and when it all comes together at the point where the extraction occurs it needs to be at "x" temp.

Obviously if the group is mounted remote to a steel frame there will be more heat loss by the time the water gets to the group, compared to machine where the group is directly attatched to the boiler.

ANeat wrote: I think the "ideal" would be a gravity fed, totally imersed group that could be held to whatever exact temp the user needed

But is temperature stability what you want from a lever group, or is it the inherent temperature characteristic that helps lend that lever character to the shot? That is, if you achieved total temperature stability with a lever group, would the shots pulled from this machine taste more like pump machine espresso?

Right now in my learning curve I would like to have a little more temp stability, as in being able to pull multiple shots without worying about the temp falling out of the optimum range (up or down).

I think many of the commercial lever machines with a remote boiler achieve that,

Im not sure if its the swing in temp or the pressure curve that contributes to the "lever flavor" Probably everything, but as to what % that the inrush of hot water into a somewhat cooler group adds to the flavor I dont know.

Just one of the nuances that makes its all fun really

Interesting thread....FWIW my dipper tube lever has the groups bolted to the boiler, yet even after flushing the group exterior never gets close to the boiler ext., temp. In fact the group sheds so much heat it is a bit of a challenge to get the water hitting the puck hot enough (measured by taste only as I have no scace etc....)
I will eventually get a dig thermo so I can at least get the temp of the shot on exit.
My experience with cars etc...leads me to remind that metal will shed tons of heat even without fins etc.....