I thought this might be generally helpful. It is only
for open boiler machines (La Peppina, Caravel, Mini Gaggia, etc.).
I use a router speed control. (You can get them anywhere, but I got mine at Amazon http://www.amazon.com/MLCS-9400-Standard-Router-Control/dp/B001JHQ3G8/
). Less than $30.00
I removed the clip from the back. I also shortened the cord, but neither modification is necessary. The beauty of the device is its simplicity and reliability. Just plug it in between your machine and the outlet.
To calibrate, I measure, not at the group, but in the boiler. I just use a Thermapen digital thermometer. Any good thermometer will work.
1. Turn the rocker to "FULL". Let it come to a full boil.
2. Switch the rocker to "VAR". Measure the temperature at each number and make yourself a little table of equivalents. ) eg. 4=199 degrees. You could also make a new dial, mark the old dial, put a piece of tape on the dial, etc.
To use, just turn the rocker to FULL. When it boils, turn to VAR and turn the dial to the number you want. In the La Peppina the change of temperature is almost immediate. Nothing could be easier, no wires hanging out and the device has its own internal fuse, adding a layer of protection.
It also makes it possible to keep the machine at a controlled temperature for some time without furious boiling, unplugging etc.
When done, turn the rocker to "OFF" and power is off. No need to turn the switch on your machine to "OFF".
PS. I've now used this with my La Peppina for almost a year on an almost daily basis and have had no issues. In practice, I don't worry much about the numerical temperature. I taste the first shot. If it needs more heat, I turn the knob a bit to increase the temp. And so on. The response to temp rise or fall is faster than it takes for me to grind, pack and pull another shot, so it works very well. Frankly, something in me rebels against using a numerical readout/computer with an open boiler machine. This solves that.