First, a disclaimer about the equipment. The brew water temperature is measured with a K-type thermocouple imbedded between a puck of styrofoam and a puck of sponge stuffed inside a basket. The wire is fished out of a hole on the side of the basket then out to the handle. The limitation of this setup is that, the temperature measured is essentially at the sponge, saturated with brew water. There is a definate delay between actual brew temp and recorded temperature. As a result, the 1st half of the temperature curve is to be used in a qualitative manner. 2nd half of the reading, taken while the prob is soaked in brew water, should accurately reflect the brew temp. Also, I took the measurements at fixed volume flow instead of fix time increments due to the test rig we setup. There is a slight error in this method but I believe it's neglectgible. Anyway, it's the best I can do with what I have. If you want to donate a datalogger or a scace device, feel free to write me an email. Much thanks to Jaime for doing the very boring and tedious datalogging with me. I am sure he is REALLY looking forward to another round of this when the PID goes in, haha.
So here is the summary for the stage 1 of the modification project:
1. The original HX loop is very stable at steady state. The shot has less than 1 degree of variation at the tail end. Shot-to-shot variation seemed to be within 1 degree F.
2. The pre-heater loop seemed to further stablized the temperature. Shot-to-shot variation and overall tail temperature is less than 0.5 degree F (the resolution of my meter).
3. With the Isomac Rituale (1400 watt heater), a recovery time of 2.5 min are required to prevent temperature drop-off at the end of the shot (2 min = 1 degree drop off).
4. The HX configuration can be manipilated with cooling flushes (rebound time of 30 sec) to control the brew temperature (within normal espresso brewing range). The pre-heater loop configuration did not like to be flush too much and require much much longer rebound time.
Conclusion:
1. Looking at the numbers, it seemed that the pre-heater loop configuration sets up the machine very nicely for a sucessful PID job. With tighter broiler temperature control, the brew temperature could be really rock solid.
2. While attempting to adjust the pressurestat to obtain a brewing temperature of 201 F, it was discovered that pressurestats do not really like to be set that low and shot to shot variation became very large. This means that, for normal HX operation, a (large) pre-heater loop is NOT adviced as you could not flush the gouphead to proper temperature with reasonable reovery time.
So, that is it for now. I eagerly awaits the arrival of the thermocouple probe, and hopefully can post further results soon.
- Ben
