The testing of the SerialCouple prototypes continue (Part 1, part 2, part 3). For those not following along, the SerialCouple board is a low cost thermocouple adapter, meant to be used in conjunction with a standard FTDI USB adapter to enable you to monitor and log a single thermocouple channel to a computer using standard serial port monitoring software. Production boards are still a few months out, but while they're in the works, I've been playing around with different things to do with the SerialCouple. This demonstration is taking advantage of the extra IO pins afforded on the serial port connector to control a heater as a sous-vide cooker.
Water Temperature Performance
I played with implementing a PID algorithm on the SerialCouple a little bit, but I eventually came to the conclusion that the simplicity of a basic thermostat (heater on if below set point) algorithm, coupled with a correctly sized heater, have adequate performance for my needs. In the initial trials, I only used 2 cups of water in the pot to see how well the SerialCouple could maintain a set temperature. Unfortunately, the hot plate was powerful enough that the transfer delay between the hot plate and the pot caused the system to oscillate, but turning down the power, and later adding more water, significantly improved performance.
As I said, work is progressing slowly on revising the SerialCouple boards for production. I'm planning on fixing all of the routing errors, and adding the option of a PCB thermocouple jack instead of the screw terminals I'm currently using. Current expected cost is going to be about $30 (plus an FTDI cable if you don't have one already). Email me if you're interested in possibly buying one when they become available (sometime around January 2012, I expect) and would like to be kept updated.