Should I PID my Heat Exchanger? This is a question I am asked almost every day, each time I venture outdoors
I was often at a loss to know how to answer this very important question. Although I'd PID'd my current espresso machine (with Jim Schulman and Barry Jarrett's help) early in its life, I'd grown accustomed to living with it over time and not given it much additional thought. I have done some Scace Device datalogging on this, my Cimbali Junior D1 Rotary machine, but I haven't had much "before" data to compare with the "after."
Enter my former primary machine, the Cimbali Junior S1 Pourover Vibe machine which was reposing in my basement, awaiting duty at a moments notice should the rotary fail. I've always had a soft spot in my heart for this machine, my first *real* espresso machine, the one I had special ordered for me 10 years ago (and waited 3 months to receive) because I "needed" to have it in brass, from the now defunct Boise office of the now defunct Portland Oregon Cimbali dealer. But I digress.
I'd entertained the possibility of PID'ing this machine for a couple of years but could never justify it. Recently, I decided I didn't need to justify it, so W-T-F, just do it. I begged Barry J. to make me a special length probe, purchased a Fuji PXR3 from an ebay auction, bought a project box and other minor supplies, and had all the other stuff I needed (an SSR and wiring) already sitting around in the parts drawers.
I should add that this entire project, although really fairly simple, took longer at every turn than I'd expected. Each time I devoted 45 minutes to do some simple thing, it ended up taking 3 hours, most often due to my own silly screwups, such as machining the project box upside down from what I needed in order to have access to the wiring connectors used on the Fuji. I've probably got around 25 hours in this job, but if I had it to do over could probably do another one in a fifth the time. But I don't want to do another one, I'd like to encourage you to consider doing something similar on your heat exchanger machine, by showing what kind of results I've been able to get on the first day with my revitalized machine.
Before starting the PID installation, I did some baseline thermometry with the Scace device and my Fluke datalogger. I intended originally to post some of the "before" graphs but in the interest of time and space I'll simply say the obvious; they were quite variable and limited by the hysteresis of the deadband of the pstat that was used. The machine made a lot of tasty shots in its life as a pstat controlled machine, but it was not practical to control shot temperatures other than by differential flushing, a technique familiar to many HEX machine users. Differential flushing does offer some range in shot temperatures but allows little or no certainty about what one is getting nor any assurance that it can be repeated reliably at a later time. Fiddling around with pstat adjustments when one changes coffee blends would be enough to make most people tear their hair out, so I doubt many do this at all often.
I don't claim to have figured out all the ins and outs of how best to use this PID'd machine, certainly not at this point. I autotuned the PID and did a slight adjustment to the "D" parameter, about doubling it; that's the extent of what I have tried so far. What I have done is some preliminary study with sequential shot and "walk up" shot temp curves, trying to judge control over temperature in both repeated shot series and for individual "walk-up" shots. The graphs below will show that I have not yet defined either the upper or the lower range of shot temperatures which will work; I think I can probably get a useful degree on each side of the graphs I have to show now. Not all flush volumes were tested, either, but 50mls used uniformally seemed to work fairly well.
Anyway, herewith the shot data in graph form:
At the low end, shooting for a shot temperature around 199F, and using a boiler temperature of 239.5F:
Next, a boiler temperature of 241F yields a shot temperature around 201F:
"Walk Up" shot testing at this temperature shows pretty good temperature stability across 4 random shots:
A shot temperature of around 202F is produced with a boiler temperature around 242.5F:
which has decent "walk up" shot temperature stability:
A boiler temperature of 245F produces a little bit hotter shot in the range of 203F:
Clearly, the result of pushing up the boiler temperature is non-linear as it relates to shot temperature; further work would be needed to fully define the usable shot temperature range derived from the boiler temperature setting.
Several points to make about these results and then I'll let the data speak for itself. The shot frequency on shot series was that each shot was followed by 2 minute idle period. This is different than I have done before with results from my PID'd rotary machine. The interval was determined empirically when I found that shot temps declined a little sequentially with a shorter interval, in spite of the boiler temp increasing as shots were pulled. This machine was sold as a low volume catering machine; it is, after all, a pourover. The rotary machine appears to have faster thermal transfer between the boiler and the HEX; I know not why, but there were 7 years separating the manufacture of the two machines and presumably Cimbali made some internal design and or materials changes.
This slower recovery in the HEX has no bearing on ability to froth; with the PID the machine froths as well and recovers as quickly as the rotary machine, just the temps in the HEX lag a little. Every machine design is going to respond differently to electronic temperature control, and only by experimenting will you discover what works best for your particular piece of equipment.
If you are considering modifying your HEX machine by putting in a electronic temperature control with a PID, I hope this post will encourage you to do so. Every machine is going to respond to PID'ing differently, but I think it will be a rare HEX machine that does not have better inter and intrashot temperature consistency after the modification, plus you will be able to actually control the temperature of your straight shots with not a lot less accuracy than some of the mega expensive double boilers that are either now being sold or will soon be released on the market. When the occasion calls for frothing, just push up the boiler temp and froth away; temperature stability of shots going into milk drinks is not all that important, at least in my opinion.
(this is crossposted on alt.coffee