timo888 wrote:Your TC reading is almost 3°F hotter than actual. So the water is cooler than the readout. There is no unambiguous evidence that you are burning the coffee. The harshness you're tasting could be the result of insufficient rest. 2 or 3 days post-roast is a relatively short resting period.

Right you are.

When you let the machine come to temperature with the inlet valve open (wait for 20 minutes, say), what does the temp read? If, after 20 minutes, you lift the lever once and only once (or pull 50ml with an empty basket), what does the temp drop to? Does the heating element come on? If so, does the TC overshoot? By how much?

Here's a single dry pump at 60s after 20minutes of idling, with 92.5 C as the set point. Also, while I was at it, some steam recovery curves. Note that the boiler is rapidly cooled because the steam pressure is much less than 20psi, so there is a big inrush of cold water. When the new regulator arrives, I think this will improve since I'll set the feed to 10psi.



enjoy,
raj

Is a dry-pump a flush without the basket in place? How much water is flushed?

(EDIT: Or, now that I recall your earlier posting, is that the pump where you are simply cycling water from the group?)

Have I correctly identified with the highlight the section of the graph that deals with the flush or pump?

It looks as though at around the 5-second mark the flush/pump begins; the temperature drops about 1°C in 10 seconds, reaching its nadir at the ~15 second mark, whereupon it recovers in 5 seconds, overshooting the setpoint by about 0.5°C. Then immediately after the recovery and overshoot, there is another drop of 1°C at about the 20-second mark, and a recovery to the setpoint in the remaining 5 seconds of the 30-second interval.

Also, could you clarify how the transition to steam works? Do you turn the inlet valve off and draw some water to create room for steam, then switch the temp to high?




Regards
Timo

Hi Timo,

Sorry for being unclear: the dry pump means raising the lever until some (tiny) infusion begins and then lowering the lever. This preheats the piston and cylinder. It is as your EDIT suggests. Almost no water is released.

I think your highlighted area is off: I showed 1 minute of idling (which does include some un-pretty cycling), then I pulled the dry shot at 60 seconds. That produces a very large cooling transient that would probably go 2-3C if the heater didn't come on. It does (around 70s) and produce a bit of overshoot (at 80s). By 120s the overshoot has moderated and we are back to brew temp (92.5, I think). The fluctuations at 0-20s are the PID acting poorly.

As per my granita post: I recently pulled 1L of shots using some crap beans from Trader Joes. After 3-4 shots, there was no further need to heat the grouphead. Here's how I conclude that:

1. This series of shots uses a 10sec pre-infusion followed by a 20sec pull. During the pre-infusion, cooled water from behind the piston returns to the chamber and activates the heater. I observed the heater coming on during pre-infusion 2-3 times.

2. Then, the heater started coming on only after the shot was half pulled. This system inlets cold water into the boiler only during shot pulling. Thus, if the head is hot, the heater should only come on during the shot. This is what I observed after the first few shots.

I have on my to-do list to repeat the above experiment with the heater de-activated during the pump. This will give a more accurate # than the 2-3C I cite above. This drop * 800g should tell us how many calories are lost to the head. Is this what you are getting at?

Steam sequence: exactly as you pose. 1. turn off the inlet, 2. open the steam wand with cup or use the group to drain water, 3. turn on steam control (just turns on heater, bypassing PID). In 2-3 minutes (a bit faster if the steam knob is kept closed) there is plenty of steam. I can get to nice, dry steam in 90s if I drain the water from the cold water part of the circuit, so I'll be installing a dump valve there, I guess.

Once done steaming: 1. close steam valve, 2. disable steam override, 3. open inlet. Because the boiler is at <20psi during steaming, inlet water enters and cools the boiler. This is the transient shown three times (once with the regulator sticking a bit). With a little practice, I can get from steam to brew temp in 30s, but sometimes it takes a full minute.

raj

I was taking a break from doing my taxes and my head must have been still reeling. I thought the sixty-second mark was where you were doing the steam tests. Thanks for the clarification. It is surprising that ~45ml of water from the piston cylinder should cause the boiler temperature to drop so precipitously after 20 minutes. Does the temperature drop occur on the upstroke of the lever during the dry pull, or on the downstroke? If on the downstroke, the cooling is analogous to the destabilizing effects of auto-fill, the result, at least in part, of an influx of water from the mains. If on the upstroke, then we have a complicated scenario. Since my head is already mush from reading IRS regulations and instructions, I hope water from the mains is the culprit.

Regards
Timo

For the dry-pulls, the cooling occurs (I think) on the upstroke. Why: on the upstroke, water that has cooled in the head is pushed back into the boiler, and the same quantity of hot water is injected into the inlet supply tubing toward the expansion tank. After 20 min, the head is, say 60C, so a pressure drop of (95-60)C * 45ml/0.8L=1.8C is to be expected.

The same thing starts to happen during a pull: on the upstroke, piston water goes to boiler; and boiler water goes toward expansion tank. But when the infusion happens at the top of the stroke, the boiler water that was displaced toward the expansion tank returns to the boiler as 45ml enters the piston. No net water from the mains has entered yet. Now, during the pull, the back of the piston is sucking water in, and 45ml of cold water is drawn from mains. Assuming the tubing is either warm, insulated, and non-mixing, things are fine. To the extent that there is cooling of the boiler water in the expansion tubing, some instability will happen.

The expansion tubing is fairly narrow, so while mixing is no likely, cooling definitely is. In practice, assuming I've got the group hot, the cooling of the boiler occurs on the down stroke, after the piston is already filled. Thus, it seems that the draw-water instability is not an issue.

r

ps: while I have your attention, Timo, I wanted to ask about pressure control/gauge mods. Alchemist's mod seems great, but I'm not too handy on the lathe. I have something much simpler in mind--a torque wrench. Given dimensions, we can convert bar in the piston to torque (I get 1 bar = 34.5 inch-# = 40cm-kg for my pre-M 1.74" dia piston with 1.0" lever arm between the pivots). So, with a torque wrench applied to the lever arm--treating the lever arm as a torque extender--will allow me to control/measure the piston pressure. Drawbacks compared to Alchemist's mod are errors due to friction, general coolness (always a factor), and the fact that I'll learn pressure with different angle/grip/hand position since I'll be using the torque wrench, not the normal lever. One advantage is that pressure meters are notoriously inaccurate, while a torque wrench can easily be 4% (mine) or 1% (for $$$). The simplest way to do this is to tap a 6M screw thread on the lever fork. Then I can put a screw in with a nice hex head to which I can attach my dial torque wrench. Any comments/suggestions?

matadero210 wrote:I wanted to ask about pressure control/gauge mods. Alchemist's mod seems great, but I'm not too handy on the lathe. I have something much simpler in mind--a torque wrench. Given dimensions, we can convert bar in the piston to torque (I get 1 bar = 34.5 inch-# = 40cm-kg for my pre-M 1.74" dia piston with 1.0" lever arm between the pivots). So, with a torque wrench applied to the lever arm--treating the lever arm as a torque extender--will allow me to control/measure the piston pressure. Drawbacks compared to Alchemist's mod are errors due to friction, general coolness (always a factor), and the fact that I'll learn pressure with different angle/grip/hand position since I'll be using the torque wrench, not the normal lever. One advantage is that pressure meters are notoriously inaccurate, while a torque wrench can easily be 4% (mine) or 1% (for $$$). The simplest way to do this is to tap a 6M screw thread on the lever fork. Then I can put a screw in with a nice hex head to which I can attach my dial torque wrench. Any comments/suggestions?

I do not know by how much the torque wrench arm's contact with the fulcrum pin would distort the torque reading on the dial.

Regards
Timo

Hi All,

The torque wrench idea initially failed for one reason: an M6 screw hasn't the strength to stand up to 200-400 cm-Kg. I spoke to a machinist friend who suggested that a larger screw should work, maybe M10 or bigger. But then he suggested that JB welding a small piece of tubing into a port drilled into the base of the cylinder would be his suggestion (ie: install a gauge). I'm going to try a 1/8" hole through the bell housing that should end at the place where the screen meets the bell, outside of the piston. Because the big O-ring sits just outside of that, if the hole is correctly placed (it will taper to 1/16 or less at this end) it should get cylinder pressure without compromising the basket-screen seal.

What I really need to get this right is a good cross section of the pavoni head, with dimensions. Anyone seen such a beast?

The new regulator arrived. I had gotten quite used to 20psi boiler pressure. I think 15psi is better, but it makes very little difference. The new reg also has a pressure gauge on it, so I can see how the larger expansion tank works (while the smaller one doesn't). The next step in plumbing is to try a back-pressure relief valve. I'll put this just after the regulator. This will do the job of the OPV and might allow discarding the expansion tank. Will let you know. As it is, the initial design is still the best (hence no new pictures).

r

All,

I succeeded today in milling a 1/8" hole into the side of the Pavoni housing, diagonally, down to the place where the screen meets the housing (and the portafilter o-ring seals everything). I drilled a 0.040" hole straight up into the housing from the portafilter/screen side, so that the two holes meet. The plan is to JBWeld a 1/8" Copper tube into the hole and compression fit the other end to a gauge.

Will let you know how it comes out--should have photos of the complete system in the next few days.

raj

Wow, you really get things done! I'm looking forward to the results!

Henry

matadero210 wrote:The next step in plumbing is to try a back-pressure relief valve. I'll put this just after the regulator. This will do the job of the OPV and might allow discarding the expansion tank.

You might want to check out this thread- variable-brew-pressure-via-steam-wand
if you have an extra steam valve I think you could put it before the boiler and use it to regulate the pressure during pre-infusion.

Henry