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Tips for Controlling Brew Temperature on a Home Lever Espresso Machine

Postby drgary on Wed Dec 19, 2012 12:02 pm

In another thread I suggested some ways to control temperature on a home lever. I'm posting that primer here as an introduction so it's easier to find.

A Note on Toggling Heat Switches on Vintage Home Lever Machines

This is my understanding of how to achieve better temperature control of a home lever. I've written this to assist those comparing dissimilar machines, perhaps without experience using them. The idea of temperature control with a home lever machine* may seem difficult at first glance but I don't find it all that hard.

There's a preparatory phase of bringing the machine up to temperature. With a pressure gauge, thermometer or temperature strip, this is easy to tell. Say you have a Pavoni or Elektra with a pressure gauge. You can watch it go beyond shot-pulling pressure/temperature (one indicates the other**) and settle back down in range, somewhere between .8 and 1.2 bar for pressure. The second phase is when it reaches temperature. Water has been fully heated in the boiler but now the group is hot to the touch. A thermometer, thermocouple or temperature strip on the group indicates cruising temperature, which differs slightly across machines from the exterior measurement to what occurs within the brew chamber. But you can quickly learn the temperature range to shoot for by taste. To get to this temperature faster you can flush water through the group and steam wand to eliminate lower-density air as Robert Pavlis instructs in his wonderful thread on Dalton's Law. Otherwise over time air tends to leak and give way to steam and hot water that more efficiently transferring heat to the brew chamber. Now the group is up to temperature as a heat sink and you toggle heat switches to control temperature within a range.

Temperature Control and Steam, with/without pressurestat

Home lever machines equipped for steaming may carry sufficient steam at the top of the boiler that no additional heating is required. This is the case with machines like the Olympia Express Cremina, Elektra Microcasa a Leva, Millennium La Pavoni, which all have PSTATs. Machines without a pressurestat often have a dual heating element or a hotter mode using a single element. That hotter mode produces robust steam but temperature is too high for pulling shots. One learns to bleed off steam to lower pressure and return to brew temperature. With the High heating mode off, heat will radiate and temperature will drift back to shot-pulling range. Again, with some means of measuring temperature one can consistently pull shots within a desired temperature range. I'm not the first to assert this and read a nice thread about it by Fullsack here, using a Pavoni to pull shot after shot.

To heat the group by a few degrees on a manual lever, you can lift the lever halfway for a few moments. This allows hot water into the group to increase temperature. If you're using a thermometer on the group you can watch the temperature go up a few degrees to dial in your shot. Andypanda taught me this method on a Cremina. I may use a few of these pumps to get the shot temperature just right. On my Millennium Pavoni I sometimes dip the portafilter in cool water to adjust group temperature.

* Any home lever machine can overheat or miss the mark for temperature. That's the difference between home levers and commercial levers that are designed to be left on indefinitely and have a massive group act as a heat sink and a large boiler that help stabilize temperature. I'm suggesting that temperature control with any quality home lever machine isn't that hard to learn if you pay attention to the issues above and put in some practice.

** There are ways of correlating steam pressure with temperature described elsewhere (ask Dr. Google). I haven't gone into detail here.
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Postby jimskelton on Wed Dec 19, 2012 1:55 pm

OK, I'm really curious about lever machines like the Pavoni or Elektra with a gauge. I have a microcimbali vintage (with the 2 switches) and it seems fairly straightforward. The 300 watt element in theory keeps the water temp around 200 and when you switch on the 1000 watt one, it raises the temp and produces steam. The group is surrounded by boiler water, so is pretty much prewarmed.

I tried out a Pavoni with a gauge and was wondering how it could pull shots with consistent temperatures, since in theory, if the boiler is pressurized to .8 bar, the water temp will be around the 240-250 range. I can see that pulling some water through the group is necessary to warm it up since the group is suspended away from the boiler. But once you start pulling 250 degree water through it, won't you burn your shot? Or is it engineered to dissipate heat fast enough to lower the temp to the ideal 200 degrees by the time it reaches the portafilter? If this is the case, if you pull shots too frequently, does the temp rise too much?

If anyone has temp data from one of these machines I would be interested in seeing it. I have a dual probe thermometer on order, so next time I visit my brother-in-law (who owns a Pavoni) I'll be connecting it up to his boiler and group to see the relationship between boiler temp, group temp, and the frequency of pulls through the group.
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Postby rpavlis on Wed Dec 19, 2012 4:09 pm

The next time I order some brass stock I plan to order a piece of the correct diameter to make a temperature probe to go INSIDE the piston of La Pavoni groups. I want to bore out a rod so that a thermocouple can measure the piston temperature. It is easy to do, I just do not have a piece of metal stock the right size on hand. To me that seems to be the really important temperature.

Also, I have not seen anyone mention it, but if the temperatures be above the boiling point of water, one can observe the espresso boil as it emerges from the bottom of the portafilter. When one sees it boil, one knows it was too hot! You can dip the bottom of the group into a ramikin of cold water before making the next shot should that occur. (And draw the cold water in and discharge it.)

I have programs to convert vapour pressure to temperature and the reverse. They are written in c, I wrote them years ago. They will compile with any c compiler, I think, at least any standard Unix one such as OsX, Linux, BSD, etc. They are short and simple. If anyone want them, let me know, remember you need a c compiler to use them.
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Postby homeburrero on Wed Dec 19, 2012 4:11 pm

jimskelton wrote:in theory, if the boiler is pressurized to .8 bar, the water temp will be around the 240-250 range.

Yes, that is roughly correct. At 0.8 bar gauge at sea level (1.8 bar absolute), water boils at about 241F, and at 5000ft altitude, (where 0.8 bar gauge is only about 1.6 bar absolute) at about 236F.

jimskelton wrote: Or is it engineered to dissipate heat fast enough to lower the temp to the ideal 200 degrees by the time it reaches the portafilter?

That's the idea. In the La Pavoni, the engineering to try to achieve that has gone through a couple major changes over the past 40 years.
jimskelton wrote:If this is the case, if you pull shots too frequently, does the temp rise too much?

Yes. Is a well known problem, especially in the machines that don't have a sleeve inside the cylinder. Pavoni recognized this and went back to an earlier design with the "Millennium" group (introduced in ~2000). This group lets the hot water feed into the space inside the brass group but outside a plastic cylinder that is the brew chamber. When the lever is raised on these the water that has already been cooled somewhat by the group enters the brew chamber. The space inside the group and outside the brew chamber, with the lever fully raised, accommodates about 40 ml of water (I measured it - I'm such a geek.)

jimskelton wrote:If anyone has temp data from one of these machines I would be interested in seeing it.

Me too, just because I find engineering problems interesting. I would really love to have a long conversation with a couple of old timer design engineers from Pavoni at a nice coffee bar in Milano. I'll need to learn Italian. But I'm not sure that analysis and reverse engineering are of that much practical use in pulling a good coffee from these things.

jimskelton wrote:I can see that pulling some water through the group is necessary to warm it up since the group is suspended away from the boiler.

The Pavoni group is bolted to the boiler, and on some vintages ('74-'00) also has a vent between the top of the brew cylinder and the boiler. They will come up to temp (even above desired temp) if left on long enough. Most people leave them turned off until they are wanting a coffee because they can be ready to pull in only 5-10 minutes. Group-warming flushes are fast and effective, each little flush can warm a too-cool group by 5 degrees. There are tons of home remedies for cooling the group when necessary (cool portafilter, Orphan Espresso's third shot gadget, cool wet towel, putting a ramekin of cool water at the group and lifting the lever, etc.)

P.S.
If you have a pStat Pavoni and it doesn't run 0.8 bar, don't fret - Pavoni's specs (http://www.lapavoni.it/PDF/CAT_LEVER.pdf) indicate that the pStat's of the current machines are set in the 0.7-0.8 bar range. I think lower is better provided you're steaming well enough. With a gauge and an on/off switch you can have the best of both worlds - brew at a low pressure then turn it back on for powerful steaming.
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Postby peacecup on Wed Dec 19, 2012 4:29 pm

FWIW the "club" style home lever machines, Sama/Ponte Vecchio Lusso, Olympia Club, Bezzera Family, effectively eliminate the group overheating issue. The groups on these machines are not bolted directly to the boiler, and stay much cooler.

The Sama group is heated via a thermosyphon just like some of the big boys. These machines are amazingly compact little engineering feats, with excellent control of brew temperature.

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Postby kursiv on Wed Dec 19, 2012 6:59 pm

peacecup wrote:FWIW the "club" style home lever machines, Sama/Ponte Vecchio Lusso, Olympia Club, Bezzera Family, effectively eliminate the group overheating issue. The groups on these machines are not bolted directly to the boiler, and stay much cooler.

Olympia Club falls somewhat outside this category as I see it as its group is bolted to the boiler much in the same way as the Cremina.
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Postby rpavlis on Thu Dec 20, 2012 10:00 am

On the 1974-2000 group La Pavoni lever machines heat is transferred from the boiler to the group by three different pathways: (1) direct heat conduction through the metal (brass) from the boiler. (2) Once the system is bled of air, including the space above the group cylinder, "live" steam constantly condenses above the piston, keeping it at the boiler temperature, usually about 115. Heat conduction occurs downward through the piston and the cylinder walls toward the brewing area. (3) When brewing water enters the cylinder its initial temperature is about 115, heat is transferred from it to the group, because the group is not this hot.

Once the system is bled of air #2 is the most important. One of these machines will become too hot if one simply let it stand too long after turning it on, bringing it up to temperature, and bleeding it. Making shots in quick succession can result in raising the temperature much too high as well.

Because this all involves heat transfer acting over time one can get decent temperature control by careful timing.

With a pressure switch La Pavoni one can turn on the machine and wait for the heat to turn off. If one touch the group at this time it will be only slightly warm, not at all hot. One can open the steam knob a few seconds, and then without closing the valve, raise the group handle momentarily all the way to the top so that a small amount of water escapes. This will drive all the air in the top of the group into the boiler, where it will be swept out, since the steam valve is open. Now steam pours into the top of the group and it will become almost instantly too hot to touch.

When the pressure switch turns off again the temperature in the top of the group will be from 115 to 120, depending on pressurestat setting. Now heat begins to conduct downward to the brewing area. I leave the porta filter in place. In between two and three minutes the brewing area should be near the right temperature. If one wait too short of an interval it will be too cold, and one will get a spongy pull because the incoming water will not vaporise enough to drive out the air. If one wait too long the espresso will tend to be bitter, and the espresso will emerge from the portafilter boiling. One can instantly recognise that it is boiling! It means the next time one does this one should make the shot earlier!

One can cool off the group by immersing it in a ramekin or wet towel or other technique before making subsequent shots. One needs to use the same technique all the time, and be very very meticulous about timing everything. (One also needs to have the beans ground properly too.)

With the two switch la Pavoni machines the only thing that needs to be bled is the group. Once the machine starts to hiss loudly with both elements on, one can pull the group lever too the top as with the pressurestat machines, and it will also almost instantly get hot. It will, in fact, heat faster, because the initial steam is hotter, because the air "self bleeds" through the pressure valve. Once it is bled one turns off the 800 watt element, and again one needs to wait, but the wait interval is shortened because the boiler was already fully up to temperature.

I have to confess that I use temperature measuring techniques to set up what works, and then once I have the time parameters, I proceed "by the clock."
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Postby drgary on Thu Dec 20, 2012 10:57 am

On a La Pavoni Millennium machine with the pressurestat set at 0.85 bar, after initial heat up as described by Robert or if simply left on long enough so the air bleeds out, one can turn it on and leave it on. Multiple shots can be pulled in succession without overheating by simply dipping the portafilter in cool water and locking it into the group. I showed that in this thread long ago.* Currently with that machine I've refined the technique by adding a Teflon group-to-boiler gasket so there is a narrower temperature swing at the group -- not necessary but convenient -- and I measure the temperature with a thermometer whose probe tip is taped to the group head. With this setup I can make different types of coffee, needing different temperatures in close succession. It's a matter of being familiar with what group temperature those coffees require.


* Sorry, I haven't had the time to do a measured temperature study. I'll just say it works and if someone else has time to do a study that would be great.
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Postby rpavlis on Thu Dec 20, 2012 1:00 pm

Making espresso with the La Pavoni machines "by the clock" works admirably when one is making espresso just for oneself or one other person that is planning on drinking the espresso at the same time. It works very badly when one is making espresso for a group of people that are coming to one at irregular intervals. It is important to keep track of the timings, because ambient temperature matters a bit and it tends to change with seasons. One can keep track and change things the next time! My present time interval is between two and three minutes between the group air purge and the pull. Less than that gives a "spongy pull". More time is fine up to about ten minutes, I find, and then it is too hot.

I made between 40 and 50 cups of espresso at a party about ten days ago. Because of the setting, I just kept turning them out, one after another, this worked fine. I knew just when to cool with a water filled ramekin, my favourite cooling technique. It would NOT have worked so well if I had had to make them at irregular intervals. I had TWO Europiccolas, it is amazing how efficiently one can make espresso that way!
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Postby coelcanth on Thu Dec 20, 2012 1:26 pm

drgary wrote:One learns to bleed off steam to lower pressure and/or do cooling flushes to return to brew temperature. With the High heating mode off, heat will radiate and temperature will drift back to shot-pulling range. Again, with some means of measuring temperature one can consistently pull shots within a desired temperature range.



i'm not sure it's possible to have any sort of cooling flush on my 1960s Europiccola..
any time water is pulled through the group seems to only raise the temperature.
the only appreciable way to cool is externally using the portafilter or a wet towel
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