I instrumented the machine with a couple of additional sensors: 1) a K thermocouple fastened to the top of the brew boiler (see photo below); and 2) a K thermocouple inserted into the grouphead using one of Eric's E-61 adapters.
The stock machine uses a NTC thermistor, inserted into a thermowell in the brew boiler, for PID control of brew boiler temperatures. The steam boiler is controlled using a traditional pressurestat.
Fresh water is fed into the brew boiler through a heat exchanger that passes through the steam boiler. The fresh water supply to the brew boiler enters through the top. When the steam boiler is on, the supply water to the brew boiler is preheated by the HX. But the steam boiler can be switched on, or off, independently of the main machine switch, allowing operation either with, or without, preheat of the brew boiler supply water.
The objectives of this study were:
- Determine optimum (or nearly optimum) tuning parameters for the Gicar PID
- Examine intrashot temperature behavior
- Examine recovery and intershot stability with a series of closely spaced shots
- Compare behavior with steam boiler on (preheat) vs steam boiler off (no preheat)
Data were collected on three calibrated thermometers:
- Omega HH506RA connected via RS-232 to laptop (for realtime plotting during PID tuning)
- A pair of Fluke 54 II thermometers (data uploaded using Flukeview Forms)
The brew boiler thermocouple sensor consists of a 24 AWG type K bead probe soldered on to a thin brass plate. The plate was punched to allow the M4 stud from the safety thermostat to pass through. The safety thermostat was removed, thermal paste spread thinly on the mating surfaces, the new plate was placed against the boiler surface, and the safety thermostat was threaded back into the boiler, clamping the sensor tightly to the boiler. (I have had poor success using small bare bead probes in similar situations on other projects, and this plate assembly worked much better).