D4F Senior Member Joined: 15 Mar 2012 Posts: 1,811 Location: USA Expertise: I like coffee
Espresso: Gaggia Classic PID Grinder: Preciso
Posted Tue Dec 17, 2013, 3:16pm Subject: Re: Gaggia Classic SBDU Preheat PID Temperature Recovery and Stability, Steam
Brew temperature is probably best guided by using a Thermo filter. Another method that may help is use of boiler wall temperature. There is a central zone about two thirds up from the bottom of the boiler and up over the top of the boiler where the temperature is relatively stable. A thermistor, sensor, or thermocouple may be placed in this area. They may be adhered to the boiler. I have had reasonable readings by sliding a thermistor or thermocouple under the Thermo fuse clip. Since the Thermo fuse is in cylindrical silicone there is a little space between the silicone and the clip.
In the first pages of this thread Andy used a Polder thermometer with the bead cemented on the boiler. I happened to note his Polder temperature and the 235° F range, while adding heat during the brew. That temperature was approximated using PID controller alarms. Eventually 214° F was selected as SV on the PID controller and the temperature rise to about 235° F was controlled by selected alarm temperatures. It is interesting to note that there should be about a 20° F temperature rise of the upper boiler during the brew of a double to blunt intra-shot drop. Perhaps a couple of degrees to allow a minimal drop. As noted in other postings there still needs to be a 3 to 4 seconds steam switch blip of heat followed by rest so that the boiler shell is hot as the cold water enters. In the 7 to 8 seconds described above, the boiler shell temperature as measured on the upper central shell will not start to rise. My PID controller temperature will be rapidly rising but not the boiler wall temperature. Heat should be added to approximate about one third of the brew time or perhaps 8 to 9 seconds total of switch on time. The approximate 20° temperature rise of the boiler wall should be fairly smooth over the brew time. The heat on cycle will be weighted a bit toward the front of the 25 seconds. That approximate 20° rise may occur over a total time of 25 to 35 seconds depending on whether you time from brew switch on or from first drip. It will take the approximate 8 seconds of switch on time to offset the cold calories in the incoming water. Jonr noted a similar boiler wall temperature using his computer board controlled temperature, a few posts above. That suggests that a 20° F temperature rise during the brew will help stabilize the intra-shot drop whether using steam switch, PID, or computer based control.
If you stick to the approximate 20F rise, volume of brew will be accounted for. More volume through the boiler will take more heat, or on time to get the boiler wall temperature to rise 20F, and likewise a small volume will decrease heat requirement. I have tried 50 60 ml of brew water and 25 30 ml and had to adjust the amount of heat with brew accordingly to keep control of the intrashot drop. I also used a thermofilter, but could get close just using boiler wall temperature.
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