Having wasted about three whole days datalogging shots with the Scace Thermofilter on my Cimbali Juniors, I had nothing to show for it but a bunch of horrible looking graphs that were beginning to convince me that one could never get temperature stability out of a HEX machine and that the PID Jim Schulman and I put in (with some assistance from Barry Jarrett) a couple of years ago had accomplished nothing. I was getting ready to call my banker to see about a loan on a new LM GS3:-)
I've had a number of emails going back and forth with such local luminaries as Jim Schulman, Barry J., even Andy S., and I was getting ready to jump off the roof or hang myself in the closet. Jim kept saying I didn't have the flush down low enough, I should try 110ml, Barry said, hey, temperature stability, HEX, no can do, which was more or less what I inferred from my emails with Andy as well. I seemed to have my choice of good first shots but overheated seconds and thirds and so forth, or hot first shots followed by good later shots.
And then----- I got LUCKY. I started to do some datalogging while playing around with the boiler temperature and the flush volumes. And here is what I found: by reducing the boiler temperature to well below anything I've used before, 231F, which equates to about 0.6 bar on my front panel gauge, and markedly reducing the cooling flush down to 45ml (an ounce and a half, about a third of what I've used before), I struck PAYDIRT.
It would be nice to compare these results to what I can get on the same machine under pstat control, down at the same temperature range and with the same flush; I don't have time to do that now since I'm headed off on a trip soon, but I'll try to get to it in January. I don't think that pstat control can match these results, as the gentleness of the curves and the consistency of them would not easily be obtained with the ~6 degree hysteresis present in my boiler with the pstat calling the shots.
I think these results, which I have now duplicated in 3 separate series this afternoon, show that this works, and validates the approach of reducing HEX boiler temperature, reducing flush size, and replacing the Pressurestat with electronic temperature control. If anyone else out there has a PID'd HEX machine, it sure would be nice to see this duplicated on another type of machine.
Just a note about methodology before I post the links to the graphs: The idea was to start off from an idle period, which is the hardest for any sort of espresso machine as this tends to produce either overheated or cool groups depending on the machine. I pulled one thermofilter shot about 15-30 seconds after the small cooling flush, then waited 1min 30 seconds to pull the next one, and so forth. Although on my earlier testing I'd removed the Scace PF in between shots, I've been convinced by my own testing that this accomplishes nothing because the Thermofilter just measures the water as it flows through and any impact of the PF temperature (it is a chopped PF and the probe is far away from metal or other heat sinks) is eliminated after 1 second into the shot. Therefore, after the small cooling flush I returned the Scace device to the group and left it there throughout the 10 or 12 minutes it took to do each series. One other reason for doing this is that the basket they are using on the Thermofilter doesn't seal well with Cimbali group gaskets and once I have a good seal in there I don't want to risk a poor seal on later shots which would make it hard to trust the measured results.
Have a lookie at these graphs of shots, pulled as above, on a lowly Heat Exchanger machine:
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