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)
- 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)