The Relationship Between Water Temperature and Boiler Pressure on a Pre-Millenium La Pavoni

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vze26m98

#1: Post by vze26m98 »

Greetings-

I've read a fair bit about this on HB, but I'm sure I've forgotten some details, and may have missed some critical posts.

I have an early 1980s Pavoni Europiccola: a two-temp switch model, no PSTAT, with a manometer that I recently added. I don't think I'm fully in command of the temperature of the machine, although I'm always enjoying the coffee I pull. My Pavoni idles at ~0.95bar, which seems altogether too hot to me, and so I'm choosing to pull at some chosen (lower) pressure as measured by the manometer. I usually look at a thermometer hooked to the grouphead to understand what's going on there.

So my question is really a general one about the relationship between water and boiler temperature on a machine like this, and how it affects the resulting pull.

Taking an example, if my manometer registers 0.552bar (or 8psi), what temperature is the water in the boiler, and what temperature is the steam? Is the water 183F and the steam 212F or more, or are they some other values?

What I see that confuses me is before a pull, the grouphead might be at 178F, but afterwards, it often is higher than 10 degrees above that mark. I assume that this must be due to the steam coming into the grouphead, which is the design of that era of Europiccola, but given I'm confused about the temperatures of the water and boiler, my analysis is a little poor.

Thanks! Charles

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erics
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#2: Post by erics »

This might help -

Image
Skål,

Eric S.
http://users.rcn.com/erics/
E-mail: erics at rcn dot com

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rpavlis

#3: Post by rpavlis »

The normal way to run these machines is to allow them to heat with both elements turned on until steam begins to be released with some vigour. Raise the handle at this point until just a bit of water is released from the bottom of the group head. Air will be trapped in the top of the group if you fail to do this. The whole idea is for the space ABOVE the piston to be heated by the steam in the boiler with no air in either the boiler or top of the group. (With pressurestat machines you have to bleed the steam wand whilst you raise the piston.) The bottom of the group will be far too low for brewing immediately after you do this, but with the roughly 115-118 degree steam on top of the piston heat will be conducted downward to the bottom part of the group. Before you raise the handle the top of the group will be no more than luke warm. Immediately after this it will be to hot to touch!

(I had severe problems with the metal ball in the pressure relief valve when I first got my machine like this, because the pressure was inconsistent. The ball kept corroding. I put in a stainless one and that did not help too much either, I finally replaced the metal ball with the present polymer one, I think that it is probably PTFE, it does not look like polyphenyl sulphide.) If you have problems with erratic initial pressures you may need to get this very inexpensive fix too.

You need to have your coffee ground and pressed in the filter basket. I think it is better to press it without it in the porta filter, because the problem with the first cup is getting the whole system HOT enough, there is no cooling problem yet. In roughly three minutes the group temperature will probably be near ideal. Different people have somewhat different ideas what this temperature is. The manometer does not let you determine this, only the temperature at the TOP and in the boiler. Many attach thermometers in various ways to the bottom of the group to get an idea when things are right. If you do not use a thermometer you will find that 3 to 4 minutes after you bleed the air from the group you need to pull the shot. It should be good. If too cold the pull will be spongy because the group temperature will be too low for flash vaporisation of water to drive out the air introduced as the handle was pulled up. If too hot your espresso will be over extracted and bitter.

You should pull the second shot as soon as possible, as heat is constantly being transferred to the bottom of the group and it will soon be too hot.

For third and more shots you can turn off the machine and watch the manometer and cycle the power on and off. Perhaps 0.4 bar is okay. A second approach to this is to take a ramekin of cold water and draw this into the group for roughly ten seconds between each shot. You have to have done this a lot of time to know just how long to do this to get perfect shots every time! However, if you take care to be consistent you will do okay either way.

Remember grind must be perfect or nothing else will work right either!

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yakster
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#4: Post by yakster »

An alternate method to using a ramekin of cold water drawn into the group that works quite well for me is to lock in a spouted portafilter that's been in a cold water bath to cool the group between shots. I pull my shots with a bottomless portafilter, since it doesn't require much cleaning, but the mass of the cold spouted portafilter really helps to control the temperature.
-Chris

LMWDP # 272

vze26m98

#5: Post by vze26m98 »

erics wrote:This might help
Eric, thanks for posting this. I'd seen it, but have had trouble interpreting it to my satisfaction.

If I understand it correctly, it correlates the boiler pressure to the boiler temperature. For example, at sea level, at a reading of 0.65bar, the boiler temperature is 238F.

This is because at sea level, we need to add 1.00bar to the manometer reading to get the absolute pressure, correct?

So 0.65bar + 1.00bar = 1.65bar, or 238F, correct?

My follow-on question, then, is what the water temperature is at 0.65 bar? Is the water 238F? 212F? Some other temperature?

I assume that both the boiler and water temperature influence the resulting temperature of the pull, unless they're identical.

(Thanks also to the other respondents. I'll post my method tomorrow after I pull a coffee, and perhaps you might point out the failings of my technique.)

Thanks!

okmed

#6: Post by okmed »

When water and steam occupy the same space they are at the same temperature. Water boils at 212 at atmospheric pressure and the steam coming off of it is 212 also. If you go to the top of a mountain the pressure decreases and so does the boiling point of water. In a boiler pressure vessel as the water boils and steam is created this increases the pressure in the vessel which increases the boiling point. Those values that Eric gave you are the different boiling points of water at the different pressures. So to answer your question the water and the steam are at the same temperature. The only time the temperature will be higher is when all the water is boiled off (never do this with an espresso machine) and all you have is steam, continuing to heat the steam will then raise it above the boiling point for that given pressure. This is called superheated steam. Hope this helps.

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homeburrero
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#7: Post by homeburrero »

vze26m98 wrote:If I understand it correctly, it correlates the boiler pressure to the boiler temperature. For example, at sea level, at a reading of 0.65bar, the boiler temperature is 238F.
Yes. Eric's graph is gauge pressure.
vze26m98 wrote:This is because at sea level, we need to add 1.00bar to the manometer reading to get the absolute pressure, correct?
Yes, you'd add appx 1.0 bar at sea level to the gauge pressure to get absolute pressure. Espresso machine discussions rarely use absolute pressure. Sometimes people use the label "barg" to make it clear they are talking about gauge pressure, but usually just use "bar" - we all know they are referring to a gauge reading.
vze26m98 wrote:So 0.65bar + 1.00bar = 1.65bar, or 238F, correct?
Yes, at sea level.
vze26m98 wrote:My follow-on question, then, is what the water temperature is at 0.65 bar? Is the water 238F? 212F? Some other temperature?
As okmed pointed out, at that pressure the water and the steam in the boiler is at 238F.

When the first few ml of superheated water enters the brew chamber it will be coming out of the siphon tube at a temp above 212F and will flash boil to steam. This helps drive out the pocket of air that is above the puck right after lifting he lever. (You see discussion of this in the rpavlis post above.) As the pressure builds in the brew chamber, all water above the puck is back to the liquid state, and has been cooled considerably by the brass chamber and portafilter. Ideally to a brew temp in the 195F - 205F neighborhood. The group is cooling the water, and therefore the water is heating the group - thus your observation that the group temp goes up during the pull.
Pat
nínádiishʼnahgo gohwééh náshdlį́į́h

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erics
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#8: Post by erics »

Eric, thanks for posting this. I'd seen it, but have had trouble interpreting it to my satisfaction.
I can understand that as the correlation between temperature and pressure within a heated pressure vessel (most espresso machine boilers) can be complex. For example, this correlation depends not only on altitude but also local barometric pressure as reported by your favorite news source. That can change temperatures about +/- 0.75 degrees at the extremes. The chart is based on a reported barometric pressure of 29.92" Hg and, of course, makes the assumption that your pressure gage is "cat's meow" accurate.

The pressure within the boiler also varies with the action of the pressurestat (if so equipped) - hence the terminology "mean effective pressure".

But, if you know your altitude, put a little faith in your pressure gage, and your news service reports 29.92 "Hg, it provides a convenient and reasonably accurate correlation.
This is because at sea level, we need to add 1.00 bar to the manometer reading to get the absolute pressure, correct?
That's close. To get the true absolute pressure based on a specific gage reading, you would add the current atmospheric pressure expressed in bar. 1.0 bar is equal to 14.50377 PSI (14.5 among friends) :) which is equal to 29.592999 "Hg (29.593 among friends). But the chart already takes all of these unit conversions into account but based on a reported atmospheric pressure of 29.92 "Hg.

As previously stated when this chart was first published, the MS Excel file is gladly available for the asking - obviously gratis.
Skål,

Eric S.
http://users.rcn.com/erics/
E-mail: erics at rcn dot com

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drgary
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#9: Post by drgary »

Charles, I think this may help. It's less numbers focused and more about how you toggle the switching to start with the group temperature slightly low and then pump it up to starting brew temperature. I also like to control how quickly the temperature is rising in the group to adjust the range between starting temp at the group and finishing temp. This is done by not maxxing out the boiler pressure.

Reaching the Zen Zone with a Two Switch La Pavoni
Gary
LMWDP#308

What I WOULD do for a good cup of coffee!

vze26m98

#10: Post by vze26m98 »

First off, thanks to everyone who clarified the relationship between pressure and temperature for me. I had mistakenly assumed that the water would be at a lower temperature than the steam, and hadn't factored in absolute as opposed to gauge readings. So, with a 0.5bar reading on the gauge, I believed that the water was ~183F instead of ~233F.

Thanks also to rpavlis and yakster for the clarifying methods, and drgary for linking to a thread of his that I hadn't seen.

I had thought I would post a sketch of how I've been pulling with the Europiccola, but since starting this thread, I haven't been very happy with my espresso or technique. :-( The lower temps that might have been fine for Kimbo or Passalqua at 100% ristretto, aren't so great for Howell's Calabria Daterra, which might also be a bit long in the tooth, having been roasted two weeks ago. I'm also experiencing channeling for the first time, I think as a by-product of my rectification of the tremendous static that the Kimbo produced when ground in my Pharos.

The manometer I added to my Pavoni doesn't have the green/red indications that the Pavoni Professional gauge has. This thread had me looking at pictures of it, and I see the green region runs between 0.5-1.1bar. So I assume that this is the range where there's sufficient super-heated steam to purge the air from the grouphead, but not so hot as to produce unacceptable coffee?

Best wishes, Charles