# More Pavoni experiments. What is the correct group pressure and temp?



## rich987

I now know that my La Pavoni has good seals and I also know what 7 bar feels like on the lever, after testing with a pressure gauge fitted to a portafilter.

Is 7 bar a good target pressure to aim for? I doubt the machine is solid enough to cope with much more weight on the lever, you can see it flexing and the bolts on the group to boiler are all wrong for heavy downwards loads...

I read that 9 bar is the optimal pressure, but i think that would be pretty tough on the machine!

Regarding temperature, 92 C is optimal, correct?

I put a thermocouple inside the basket to measure the temp and another on the side of the group head to see what that gets to.

I can't upload any of the plots for some reason Error #2038, but basically the moment the group gets above 80 C, the coffee puck goes over 92 C.

After 3 pulls, the group is getting toward 90 C and the coffee is hitting 100 C.

From a cool start, the group is at about 50 C. A preheat pull brings it to 65 C and then a proper pull with the portafilter on takes it up to 80 C but the coffee doesn't get past 80 C.

I'd say is almost impossible to get the coffee temp to stay at 92 C unless let the boiler at water drop to around 95 C, but this means no pressure.

I may add another thermocouple to monitor water temp in the boiler.


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## rich987

Here are the graphs of coffee puck temp vs group temp:

cold group









warmer group









hot group









very hot group


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## jimbojohn55

Now that's interesting - what year is your pav ? pre or post millennium ?

as you say it the temp at the coffee puck that matters - it would back up me getting some great shots at 80 deg


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## mathof

I used a La Pavoni for many years and a Londinium for about the past three. These are my observations about group and brew water temperatures.

1) The group gives different readings at different spots, so you want to choose a place to mount the sensor of your thermocouple and keep it there.

2) With levers, brew water temperature is not constant. I aim for a differential of around 96C to 91C with lightly roasted coffees, and 93C to 88C with dark roasted coffees. But of course these temperatures vary with degree of roast level and, of course, taste.

3) The temperature of the brew water is also influenced by the boiler pressure, higher pressures yield higher water temperatures.

4) Having fixed your boiler pressure, and found a way to read group temperature, you then have to carry out a number of tests to establish the relationship between group temp and brew water temp. If you at some point change the location of the thermocouple sensor or the boiler pressure, you have to run some more tests to establish the new relationships. I worry only about maximum temperatures when doing these measurements.

Matt


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## mathof

I should add that when I made measurements on my 2nd generation, pre-millennium Europiccola, I found that the brew water temperature rose during the shot rather than fell, as is the case with the Londinium. The large commercial group would seem to act as a more efficient heat sink than the relatively tiny group on the La Pavoni.

Matt


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## rich987

jimbojohn55 said:


> Now that's interesting - what year is your pav ? pre or post millennium ?
> 
> as you say it the temp at the coffee puck that matters - it would back up me getting some great shots at 80 deg


Yes indeed. I've read about people letting the group go to 95C which must be way too hot, but maybe it depends on the coffee...

Mine is a 2002 with a pressure stat, single switch, plastic liner and piston.


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## rich987

mathof said:


> I used a La Pavoni for many years and a Londinium for about the past three. These are my observations about group and brew water temperatures.
> 
> 1) The group gives different readings at different spots, so you want to choose a place to mount the sensor of your thermocouple and keep it there.
> 
> 2) With levers, brew water temperature is not constant. I aim for a differential of around 96C to 91C with lightly roasted coffees, and 93C to 88C with dark roasted coffees. But of course these temperatures vary with degree of roast level and, of course, taste.
> 
> 3) The temperature of the brew water is also influenced by the boiler pressure, higher pressures yield higher water temperatures.
> 
> 4) Having fixed your boiler pressure, and found a way to read group temperature, you then have to carry out a number of tests to establish the relationship between group temp and brew water temp. If you at some point change the location of the thermocouple sensor or the boiler pressure, you have to run some more tests to establish the new relationships. I worry only about maximum temperatures when doing these measurements.
> 
> Matt


Hi Matt,

In this case, I am measuring the temp of the coffee puck, in the middle (centre and mid depth). The group temp is measured on the right side of the bell, just where it flares out. I would imagine the temp is reasonably even, given it's brass and chrome plated. The junction with the boiler is probably a little hotter.

You can clearly see more hot water coming in when I do a few short pumps to get any air out.

If the boiler is at nominal pressure, the water is at about 115C. It's going to cool some amount, depending on, a) how long it sits in the group, b) the temp of the group.

None of my plots show the temp of the coffee increasing as you pressurise (pull), but it is amazing how you can take the portafilter out, with the puck still in and dump it in cold water and the temp inside stays high for ages.

I am quite surprised how quickly the temp of the coffee comes up as the water hits it, given how well it retains heat after removal.

I am sampling at about 4 times a second, so that gives you some idea of time on the graph. The x axis is 1/4 seconds.


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## jimbojohn55

rich987 said:


> Yes indeed. I've read about people letting the group go to 95C which must be way too hot, but maybe it depends on the coffee...
> 
> Mine is a 2002 with a pressure stat, single switch, plastic liner and piston.


interesting - mines a 99 with a flake, I I I ...mean a Teflon piston and no liner, what I'm waiting on is trying the 73 pav with brass liner and piston, no pressurestat just pressure release valve and use of the smaller 200w element once its boiled -

I think your onto something - essentially the temp inside the puck during the pull


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## rich987

Yes it is interesting stuff. The mad scientist in me wants to understand the mechanics of whats actually happening in the machine rather than blindly following youtube vids...

The lever pressure is another eye opener... 20kg force to get close to 9 bar. I don't think many people actually achieve that because the machine is not really up to it, it's too unstable, the lever is not long enough and the base is too flexible.

I am in the process or making a pull pressure gauge to evaluate this more.


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## Rob1

Have you hacked the syphon tube so it sucks water up rather than relies on steam pressure to feed the group? It's something I'm interesting in doing when I finally rebuild the old pav I've got lying around in pieces.


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## jimbojohn55

rich987 said:


> Yes it is interesting stuff. The mad scientist in me wants to understand the mechanics of whats actually happening in the machine rather than blindly following youtube vids...
> 
> The lever pressure is another eye opener... 20kg force to get close to 9 bar. I don't think many people actually achieve that because the machine is not really up to it, it's too unstable, the lever is not long enough and the base is too flexible.
> 
> I am in the process or making a pull pressure gauge to evaluate this more.


I think its been calculated that 30Lb lever pull is optimal although many say its a myth and should be lower - is the pressure going to be different above the puck as below the portafil;ter, just a thought


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## rich987

jimbojohn55 said:


> I think its been calculated that 30Lb lever pull is optimal although many say its a myth and should be lower - is the pressure going to be different above the puck as below the portafil;ter, just a thought


I have the gauge in the portafilter exit hole, but no coffee in the basket when I tested. So the pressure I measured is the pressure developed by the piston. Obviously as the gauge is screwed in tight it means you get nowhere for the water to go and you get a solid hydraulic lock. You only get that once you do the Fellini move to expel any trapped air - just like bleeding brakes!

My measurements don't tally with the calculated charts. I may test again with a dead weight on the handle rather than trying to hold the lever down at a constant force, read the pressure and read the weight on the scale at the same time.

The calculated pressure charts show 13 bar at 20kg. I see around 9 bar.


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## rich987

Rob1 said:


> Have you hacked the syphon tube so it sucks water up rather than relies on steam pressure to feed the group? It's something I'm interesting in doing when I finally rebuild the old pav I've got lying around in pieces.


No not heard of that. I guess that means you can pull your coffee with the boiler at much lower temps and hence no pressure.

If the boiler water was at 92C and the group was fitted with a heater, so that it was also sitting at 92C, you'd have a perfect solution. (once you can get the water transferred by pumping rather than steam pressure).

I suppose this is exactly what more upmarket machines actually do.


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## rich987

Well after a few hours on the lathe, i now have a permanently installed pull pressure gauge.

6 bar is about the max you want to go to, before it all gets a bit dodgy...

Maybe with a lever twice as long, a base twice as thick and a machine that's bolted to the bench, 9 bar would be easy....









(ignore the nasty looking espresso! Just testing for leaks....)


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## jimbojohn55

Fascinating - so the max pressure is about 6bar - some go for lower pressure deliberately, I suppose the best guide is taste, but I'm certainly not trying to get the group to 90 anymore as 80 is getting me great results


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## rich987

jimbojohn55 said:


> Fascinating - so the max pressure is about 6bar - some go for lower pressure deliberately, I suppose the best guide is taste, but I'm certainly not trying to get the group to 90 anymore as 80 is getting me great results


Now I can experiment with different pressures to see if I can taste the difference.

Everyone has different tasting abilities (so called super tasters have a genetic trait for it) so I guess some will be able to taste the difference, I'm not so sure, but it will be fun trying...

You are exactly right there Jimbo! 90C is way to hot, if you're aiming at 92C on the coffee puck. 80C is bang on the money! I think people forget that with a single boiler machine like a La Pav, the water in the boiler is really really hot (superheated to 115C) so the group wants to be quite a lot less than 92C.


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## jimbojohn55

interested to see the setup of probes/ sensors your using ?- I like the scientific approach coupled with the taste test, may have to try a few different beans as well over a few weeks and log it alongside the grind adjustment to get the best results.


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## rich987

jimbojohn55 said:


> interested to see the setup of probes/ sensors your using ?- I like the scientific approach coupled with the taste test, may have to try a few different beans as well over a few weeks and log it alongside the grind adjustment to get the best results.


The temp logging I've done is using my own homebrew logger (Ardunio) which can have an many channels as I want.

For puck temp, I drilled a 2mm hole in the basket and put the thermocouple up inside the middle of the puck. No good for day to day to use, but all I wanted was to compare group temp with puck temp, to try and calibrate the temp of the group. Not easy.

I don't have a pressure logger (yet), just a gauge and a custom made piston rod.


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## Aikendrum105

Very cool mod for the group pressure gauge !


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## rich987

Aikendrum105 said:


> Very cool mod for the group pressure gauge !


Thanks, it was quite tricky to make.

Once you get a feel for lever force vs pressure, it becomes a little redundant, but it shows pre-infusion pressure and also prevents portafilter sneezes!

It helped me a lot with learning the machine.


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## Northern_Monkey

Apologies for the thread necro, I've also fitted a brew pressure gauge on my Europiccola.

Looking like 5 bar on my gauge is the max in terms of what my mechanical sympathy and fear from the unit flexing will allow.

Is everyone typically using about 4.5 - 5.5, I couldn't see people going much lower or higher really?


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## MediumRoastSteam

Northern_Monkey said:


> Apologies for the thread necro, I've also fitted a brew pressure gauge on my Europiccola.
> Looking like 5 bar on my gauge is the max in terms of what my mechanical sympathy and fear from the unit flexing will allow.
> Is everyone typically using about 4.5 - 5.5, I couldn't see people going much lower or higher really?


At the lever day, coffeechap brought a Pavoni fully pimped. I had no problems pulling shots at 6,7,8 or even 9 bars. To me, it felt that 6 or 7 was what I'm used to. 9 is indeed more than I would normally apply at home, however, my Pavoni has no gauges or thermometers (so I don't worry about it)


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## Northern_Monkey

MediumRoastSteam said:


> At the lever day, coffeechap brought a Pavoni fully pimped. I had no problems pulling shots at 6,7,8 or even 9 bars. To me, it felt that 6 or 7 was what I'm used to. 9 is indeed more than I would normally apply at home, however, my Pavoni has no gauges or thermometers (so I don't worry about it)


 You are braver than I! ? Really felt like I would get a steam leak at the boiler/grouphead interface if I pulled much harder.

Glad it worked out at higher pressures though, I might try a bit closer to 6 and see how it goes. Swapping the higher brew gauge onto the sight glass thread adapter could check the calibration at a roughly known pressure (no shots though), sure it is pretty close but low readings from a systematic error could make things a bit interesting trying to hit 9 bars...


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