RapidCoffee wrote: Assumptions:
1) Most home HX machines have a heat exchanger volume of about 100ml (3.5oz). There are exceptions, (e.g., Cimbali), but I believe 100ml is fairly common.
2) The primary purpose of the HX flush in an idling machine is to eliminate overheated water in the heat exchanger.

John -

I agree with your assumption of hx size and would add that an equally important purpose of a flush is to drive the average temperature of the grouphead towards the brew temperature range or value desired. This would be valid for pulling a shot after a long term idle.

I believe it is relatively easy to explain why one "Machine A" requires a longer cooling flush than another "Machine A". This can be attributed to different pstat settings or different locations (altitude) or both. The largest difference would be when the high altitude machine also has the lower pstat setting. In addition to this, I recently measured the boiler pressure on Anita with a 4.5" mechanical gage having a specified accuracy of 1/2% of full scale. There was a disparity between the gages of about 8% (the gage on the machine was reading ~8% higher than the purchased gage). All of the foregoing are quantifiable parameters whereas something like machine cleanliness is difficult to quantify and hence I make no attempt there. Imagine what the situation would be if all of these differences were acting in the same direction?

Now lets take the case of "Machine A" and "Machine B" (different manufacturers). Temperature measurement locations, although similar, are sufficiently different (geometrically) to report different temperatures when, in fact, the overall thermal energy of the two groups is not that far apart. Then you could look at my specific case wherein I am flushing through an empty PF which is always inserted into the machine and that adds a not so inconsequential thermal mass to the grouphead. In the case of the Expobar, design philosophy, in addition to grouphead geometry, plays a role. It APPEARS as though the Expobar is optimized for a speedy recovery (let's have a party ) and that most users pull shots at intervals greater than that originally contemplated by the mfg.

Some HX machines seem to require flushes that are significantly larger than the HX volume. The Expobar appears to be a prototypical example of this phenomenon:

HB wrote:I used a friend's Expobar Lever for a couple months; you aren't kidding - it requires an olympic-sized flush!

jrtatl wrote:No joke here. With the top of my deadband somewhere around 1.14bar, I have to flush 8oz and immediately lock in (or within about 10 sec) for any beans darker than City roast. If I do a few pseudo-shots as I prep my basket, I can sometimes lower the flush volume to 6oz, but this really works better with the lighter coffees.

Could this be due to an overactive thermosyphon that overheats the group? Jon's results support this theory:

RegulatorJohnson wrote:i have used the pulser with and without the thermosyphon restrictor. with it installed and with the pstats running 1.1-1.3 bar. i can flush ~3 ounces from idle and get within brewing temp. without the restrictor it would idle much higher than without the restrictor. i had it working really well was a "flush and go" type of system. i flushed very little water to get to the brewing temp of ~200° .

then i got the vetrano and everything changed. its seems to work well for the "flush and wait". i still only flush very little water. about 3 ounces even from idle. i am at about 4500 feet above sea level in SLC, UT. the vetrano runs between .9- 1.2 bar it idles at about 210.

Sounds reasonable, except for the following:

HB wrote:In all the HXs that I've measured, the temperature beneath the dispersion screen is in the 180-195F range. This is shown nicely in E61 Thermal Analysis Questions. I've excerpted my earlier reply below:

HB wrote:Although you hear lots of discussion of "overheated" groupheads, even the hottest ones I've measured idle a couple degrees below brew temperature at the dispersion screen (e.g., 195F), which seems reasonably consistent with your diagram below. That is, I assume your model only takes into account the solid brass, so my delta temperature between the back "red" of the group and at the screen would be a couple degrees more (yours is 197.5-191.8 = delta of 5.7F; mine would be 7-8F, yielding an effective brew temperature of ~202F).


From E61 Group Espresso Machine: Detailed Interior Schematics and isotherm.blogspot.com

According to this, the E61 grouphead runs cool, not hot. The HX flush warms the group, sure, but enough for a quick 3oz flush to overheat a 9# grouphead? I dunno, maybe it does - and the heck with E61 thermal stability.

Thanks for the responses. Still feeling a bit confused, but what else is new...

i have been shown this picture by a few people.

in my actual experience this is about 15° off.

if i dont use the machine for 4 hours then turn on the TC and read the temp 210-212° degrees. can i not assume that where the TC is placed (erics TC adaptor) that this picture is incorrect. the yellow band would need to be ~210°. so i still feel the same, considering my actual experience.

this graph could be replicated with a lower p-stat setting or a smaller restrictor.

i dont think that you can just point to a chart and say, this applies to any e-61 at any p-stat setting on any machine everywhere.

i also have a infra-red laser thermometer i can point it at the group at various places. it shows me that this graph is too low. at least for me, on my pulser and on my vetrano, in my kitchen, at my altitude, at whatever the barometer says, and whatever the moon phase was.

YMMV.

jon

Well, that would help explain the long flushes. The thermocouple/thermometer is reading the thermosyphon temp, which I'd expect to be hotter than the idling grouphead temp in an E61. Remember, the grouphead is radiating heat and cooling the thermosyphon. But your IR thermometer reading is interesting. What sort of temps do you get? Any chance that someone with a stock Expobar (or other machine that runs "hot") could report back on this?

Thanks - John

i dont know if you can see this or not...

the infrared temp was switching between 211 & 212° and i had the infrared pointed just below erics TC.

the top number is the infrared reading.

the bottom number is the probe inside.

the machine has been idle for about 5 hours.



jon

RapidCoffee wrote:


This is your 2-group Astoria, right? I would expect a 250ml heat exchanger to require a 250ml flush on an idling machine.

At least, yeah. For quite a few blends the dance is rather complicated, flushing up to twelve ounces to get the thing back to where I want it by the time the cup is dry and the timer starts!

RapidCoffee wrote:The thermocouple/thermometer is reading the thermosyphon temp, which I'd expect to be hotter than the idling grouphead temp in an E61.

No, at idle Eric's TC is reading the grouphead temperature. The thermosyphon is circulating through the lower part of the "mushroom" area.

• 
  Eric's annotated picture from E61 Group Espresso Machine: Detailed Interior Schematics, original image copyright Verna Design, Inc.

I'm surprised that someone out there with access to nifty hi-tech tools and better math skills than most hasn't already analyzed this. How hard would it be to measure the temp of water in the thermosyphon as well as the GH temp and come up with some sort of calculation regarding heat loss? Then couldn't you also use some of that data and some known facts about specific heat and water flow, etc. to figure out just how the physics of the whole thing works?

Personally, I suspect that most unrestricted e61 groups idle higher than desired brew temp and that water from the HX first adds a little heat to the group and subsequently cools it back down as the overheated water is exhausted while flushing. Between the cooler water that runs through the group while the flush is taking place and the stalled thermosyphon action (when the HX water is cooler than the group temp) you get the combined effect of bringing the group to proper temperature. It's interesting to note that (at least on my machine) the temperature I see on Eric's thermometer adapter starts out showing the idle GH temp, then jumps up to a higher number when I start the flush, comes back down to around the idle temp and stays there for a bit, then starts rapidly declining until I stop the flush, it then bounces back up to some lower (than idle) number for a second, then gradually comes back down for the next 45 sec - 1 min and bottoms out, holding there for at least 30 seconds or so before it starts climbing again.

Another thing I find interesting is that I assumed the Quickmill machines would all behave the same, but maybe with the Vetrano or machines newer than mine they've modified a bit so it doesn't run as hot(?). Of course, all of this is just for fun as I'm happy with the shots I get using my flush regimen and I'm sure John, Jon, Jeff, Eric, Dan and all the rest are as well, no matter how dissimilar our techniques might be.

(I have to give a lot of credit to Eric for his education via e-mail about the temps the thermometer reads and GH temperature behavior.)

Justin

The ORIGINAL intent of the thermocouple adaptor (and also digital thermometer adaptor) was to measure the temperature of the water as it approached the puck.

As knowledge was gained via this forum's contributors and thinking caps properly adorned, it became (to me) obvious. A temperature measuring device at or slightly below the recommended depth of the thermocouple is measuring a temperature (at idle) that is VERY REPRESENTATIVE of mean grouphead temperature.

jesawdy wrote:No, at idle Eric's TC is reading the grouphead temperature.

erics wrote:A temperature measuring device at or slightly below the recommended depth of the thermocouple is measuring a temperature (at idle) that is VERY REPRESENTATIVE of mean grouphead temperature.

Oops, of course you're correct. Pardon the brain f@rt.

Jon's temp measurements are in good agreement (but are at odds with the diagram, and what Dan reported). It would be interesting to get temp measurements from a "long flush" E61 machine (like a stock Expobar). But this goes a long way towards explaining things. Thanks!

jgriff wrote:Personally, I suspect that most unrestricted e61 groups idle higher than desired brew temp and that water from the HX first adds a little heat to the group and subsequently cools it back down as the overheated water is exhausted while flushing. Between the cooler water that runs through the group while the flush is taking place and the stalled thermosyphon action (when the HX water is cooler than the group temp) you get the combined effect of bringing the group to proper temperature.
...
Another thing I find interesting is that I assumed the Quickmill machines would all behave the same, but maybe with the Vetrano or machines newer than mine they've modified a bit so it doesn't run as hot(?). Of course, all of this is just for fun as I'm happy with the shots I get using my flush regimen and I'm sure John, Jon, Jeff, Eric, Dan and all the rest are as well, no matter how dissimilar our techniques might be.

I'm becoming convinced this is the correct explanation. Some speculation: flush differences in otherwise-similar QuickMill machines could be due to the higher flow rate of the Vetrano's rotary pump. For a given volume flush, there would be less time for heat transfer to the grouphead.