Could someone please explain how an Olympia Cremina group head does not overheat like a pavoni? Also as I really love levers, which other semi available lever machines out there have a group head that does not over heat a la Pavoni. All info much appreciated, Thanks Patrick.

Hey Patrick, the Olympia, while possibly more temp stable than the La Pavoni, will overheat. After a warming flush to heat up the group, and one shot, most of us let the group cool off - which means turning off the machine or soaking hte group in ice water - before pulling another shot. The Olympia Cremina, like the La Pavoni Professional, raises boiler water to steaming temperature (250F) and the group head leaches 50 degrees or so out of the water. Very soon, the group head cannot act as efficient a heat sink and the shot will taste bitter. Even the La Pavoni Europiccola will eventually overheat. There are a couple of temp stable lever machines (that are heat exchanging). For instance the Gaggia Achille.

zubinpatrick wrote:...overheat like a pavoni? Also as I really love levers, which other semi available lever machines out there have a group head that does not over heat a la Pavoni. All info much appreciated, Thanks Patrick.

The group on a Cremina is bigger than a Europiccola and so helps stabilize the temperature. In this case, more brass = better stability. A properly adjusted Cremina will overheat eventually, but no where near as fast as a Pavoni.

Other levers use other means to stabilize brew temps. My experience with the other levers is limited to what I've read about them on HB so I'll leave others who are more knowledgeable to chime in with more info.

Jon

zubinpatrick wrote:... as I really love levers, which other ... lever machines .... have a group head that does not over heat...

The Ponte Vecchio Lusso (available with one or two groups), a spring-lever, has a group that is not bolted to the boiler (i.e. no metal-to-metal contact) and so no heat is conducted to it; its only source of heat is convection, from pulling the shots and from a thermosyphon which keeps the group warm and stable.

Another alternative is the manual-lever Gaggia Achille, a heat-exchanger whose group temperature can be managed by the flushing of water.

Regards
Timo

Oddly enough, I find that most of the heat that warms the grouphead, unless standing on for 30 minutes, is not from conducted heat from the boiler. It comes from the brew water with each shot. Of course, this is a problem with these single boiler machines (Pavoni, Cremina).

Jarno wrote:Oddly enough, I find that most of the heat that warms the grouphead, unless standing on for 30 minutes, is not from conducted heat from the boiler. It comes from the brew water with each shot. Of course, this is a problem with these single boiler machines (Pavoni, Cremina).

I don't doubt what you say, but I think it is the combination of conducted and convected heat that is the real clincher. With a machine whose group receives both conducted heat and convected heat, you cannot simply wait for the group to cool down after pulling a couple of shots, because more heat is continually being conducted out from the boiler; the group gets heat-saturated and cannot sink enough of it away to put the shot in sweet territory (unless the barista becomes proactive and cools the group with a wet towel). On the other hand, with the Lusso, you can simply wait for the group to cool down, because its only source of heat is convection; if the group gets hot from pulling three or four shots in a fairly short time, the thermosyphon flow slows down automatically, because the rate of flow is governed by the laws of convection.

Regards
Timo

My experience with the La Pavoni Pro (pre millenium) was group heating from conducted heat was MUCH greater than it is on my Ponte Vecchio Export (also directly bolted to the boiler). By the time the boiler on the LP was up to temp the group would be hot to the touch. When the PV gets up to temp the group is still cool. I'm not sure if this is due to differences in the connection, or in the relative heat sink capabilities of the group. I do think the PV group is a better heat sink, more brass, and a greater surface area to volume ratio. Also, setting the pstat lower can increase the time it takes to overheat the group.

PC

I'd like to see what clever methods people have for cooling the group those machines (like pavonis) that heat up. I've always thought an extra portafilter with cool water running through it between shots would work well.
mbach

I'll volunteer mine. I am actually anal retentive about cleaning my pavoni in between shots, so it heats up a lot faster than most, because I actually put a little piece of styrofoam in the filter basket to make it into a blind filter, and then I wriggle it around with the lever up to kick out the loose grinds... a method adapted from backflushing wriggles on E61 machines.

Anyway, when I want a second or third shot with the machine on, I usually just hold two ice cubes against the grouphead till they melt. I do this prior to distribution and tamping, so it gets a chance to recover a little. This seems to work very well, as I almost never see any kind of burn-ring from overheating, and the coffee is also rarely sour from being too cold. It certainly works for me, and it doesn't feel like it's too much work.

Sometimes I think, "wow, I'd really like an Elektra!" 2 minutes later, I think, "If I were going to spend that kind of money on a machine, I go all the way and get a Quickmill Vetrano." IMO, my vintage but near-mint $200 La Pavoni can contend with just about any machine on looks, and beat the crap out of ANY machine in terms of looks per dollar spent. It's more trouble to make shots, but it's also about 1/10th the trouble to clean and descale, not to mention to take apart and service. I really like the fact with these machines that a simple mind such as myself could explain every part of this machine to you. No black boxes under the base!

I have owned a PV Export for a couple of months and like peacecup's the group head is not immediately hot to touch after it gets to correct working temp. Also like peacecup, I think is in part due the group head's connection and resulting poor thermal conduction paths to the boiler:

1. Thru the brass screws and nuts which have a high thermal conductivity but the total area of conduction is only about 1 cm square
2. Thru about 1.5 mm thick teflon gasket which has a large surface area but has a very very low thermal conductivity.
3. Via the walls of the group head brass syphon tube which has a high thermal conductivity but a very small cross section
4. From direct contact with the hot water in the boiler, but the area is small probably < 0.5 square cm because the teflon gasket's inner diameter is only slightly larger than the syphon tube. (note. only true when the boiler is more than 1/2 full and the boiler water is in direct contact with the group head )

Thermal conductivities W/cm-K
Brass = 109
Stainless steel = 17
Teflon = .002
Water = 0.58
Steam = 0.016 (at 125 degrees Celcius)

Now, a crazy idea! If I changed the group head studs and nuts to stainless steel, made the teflon gasket a tight fit on the syphon tube, and changed the syphon tube material to stainless steel would it be worth the effort?

Cheers
Geoffrey

I think the changes would probably imperceptible in the cup and nothing compared to what you could achieve by:

a) Switching off between shots and watching pressure gauge

b) Flushing portafilter with cold water

c) Wrapping head in cold wet cloth

.... assuming you need to crank out 3 or more shots in a row.

Cheers

Mike

The other alternative is an open-boiler (vintage) machine like my Mini Gaggia or a Caravel or La Peppina. The boilers on these machines aren't under pressure so the temperature in the water in the boiler is the temperature of the water that comes out and the group is mounted directly on the boiler. No overheating. You can also refill the machine while the boiler is on. The big trade-off (besides being a vintage machine and being a bit harder to find parts and get repairs which is really OK since these machines are usually simpler to begin with) is that there is no steaming. So you have to be fine with espresso only.

GB wrote:Now, a crazy idea! If I changed the group head studs and nuts to stainless steel, made the teflon gasket a tight fit on the syphon tube, and changed the syphon tube material to stainless steel would it be worth the effort?

Cheers
Geoffrey

Geoffrey, the stainless steel screws might further prolong the time it takes for (too much) heat to be conducted out to the group. A potable-water-safe high-melt-temp anti-seize coating applied to the SS screws would be a good idea.

Timo,

I think we are both suggesting the same thing for the same reasons but we are using different fasteners? The PVE group head studs are permanently threaded into the group head boiler flange. I would replace those studs with stainless steel and use stainless steel nuts to fasten the group head to the boiler flange but with a thicker teflon gasket.

Your point about using anti seize when fastening stainless to stainless is well taken - thanks.

Soon, I should have access to machine tools and hope to start doing this modification. But, the first job will be to mill the drip tray recess deeper for I am really tired of that thing sliding all over the place and always at the wrong time. The final project is to attach a pressure gauge to the boiler fill cap and tweak the boiler pressure.

Thanks
Geoffrey

michaelbenis wrote:I think the changes would probably imperceptible in the cup and nothing compared to what you could achieve by:

a) Switching off between shots and watching pressure gauge

b) Flushing portafilter with cold water

c) Wrapping head in cold wet cloth

.... assuming you need to crank out 3 or more shots in a row.

Cheers

Mike

+1 ...all work well with my Cremina

To test the theories discussed in my earlier post I performed two temperature tests on the Ponte Vecchio Export (PVE) as shown below:

In both tests the boiler was filled just to the top of the sight glass. The machine turned on, false pressure released and left until the heater idled on and off for about a minute. In each test the water from the grouphead (no filter basket) was collected into a styrofoam cup with a thermometer pierced into its side about 1/8" from the bottom of the cup. This thermometer was tested beforehand in boiling water at about 150 feet above sea level and read 210 degree Fahrenheit. All the numbers are as tested are in degrees Fahrenheit.

TEST 1 (4 oz of water per pull)
1. 2 minutes at idle from startup 191
2. 3 minutes later 188
At this point the water was below the opening in the boiler flange so I waited 30 minutes hoping to prove one of my theories
3. 30 minutes from sample 2 193
4. Last pull because boiler water is below the sight glass 190

Not many data points. Obviously the 4 oz water sample was too large so did a second test this time with less water and after the boiler was rinsed many times with cold water bringing the grouphead back to room temperature. The following test are 2 oz per pull.

TEST 2 (2 oz of water per pull)
1. 2 minutes at idle from startup 186
2. 3 minutes later 189
3. 3 minutes later 186
4. 3 minutes later 190
At this point the water was below the opening in the boiler flange but I did not wait 30 minutes. Just kept on sampling every 3 minutes until the boiler water did not show in the sight glass
5. 3 minutes later 191
6. 3 minutes later 192
7. 3 minutes later 191
8. 3 minutes later and water at bottom of sight glass 190

The experiments are not sophisticated. The numbers are as read and do not have the 2 degree offset due to the thermometer reading 210 degrees instead of 212 in boiling water. Also I would strongly suspect that the temperature the coffee grounds experience is higher due to the evaporative cooling of the water stream as it falls into the cup. Maybe this can be offset by pressure stat adjustment? And speaking of pressure stats, I do not know the boiler pressure but soon will and will post it accordingly.

The numbers are very interesting. There is no evidence to my theory that the water will be cooler when it gets below the opening in the grouphead flange. The flange modifications I suggested in my earlier posts are unnecessary. So much for theories

But I think this shows reasonable evidence that the grouphead of the PVE overheats very little during the of making coffee at regular intervals. Which I hope answers zubinpatrick's original question.

What is also interesting, this data correlates well with peacecup's and my experiences with the PVE. For we have both found from the cup the need to pull a couple of warming shots to get that loverly dark crema on the first shot. And the data also shows that we can most likely can get 5 more

The data in my previous post says more about boiler water temperature than it does about grouphead temperature. A more realistic test of grouphead heating requires thermometry of the water IN the filter basket.

Back to the drawing board.

Geoffrey

A simulation that wanted to approximate an actual duty cycle on the Ponte Vecchio would take into account its relatively small water draw. You shouldn't draw more water than the cylinder can hold + whatever small amount the coffee in the basket can absorb + whatever small amount you allow to dribble into the cup from a Fellini preinfusion. That's for a single. Adjust volume for a two-pull doppio. 40ml, IIRC.

Also, the PV basket is tall and narrow, relative to a broad, shallow 58mm basket. The temperature gradients in the puck will be different according to the basket shapes. So perhaps a tall narrow basket needs more than one probe?

The effects of temperature upon the extraction also vary according to pressure: n° degrees at 6 bar will be different than n° at 9 bar. So we need to bear these differences in mind too, if attempting to compare PV temps to, say, a pump machine's temps.

Timo,

Thanks for the thoughtful response. My reasoning for the 2 oz shot volume was two fold. Get more data points and pull the volume of my typical quick flush and a shot. But it was too crude an experiment to give much meaningful data.

Your points about two probes and pressure gradients are spot on. An ideal experiment probably requires at least two basket temperature probes and at least one pressure probe. And the basket would have to be packed with coffee for each test. Unfortunately such a set up is beyond my resources at present.

Thanks
Geoffrey