The Conceptual Heat Table
Take soft copper tubing and bend it into a coil. The coil should be pretty tight, and flat as a pancake. Squish the copper coil between a piece of aluminum plate and some plywood. Use a pump to push hot water through the coil. Make sure the water is a consistent temperature and you are good to go. In the schematic below, the copper coil sandwich is called the "Heat Table Assembly."
Keep it simple. And don't forget cheap. Oh yeah "safe" too. Using hot water means we cannot get the table any hotter than 212°F. We can get cheap hot water by using an electric tea kettle. The water temperature of the tea kettle can be maintained by just turning the power to the kettle on/off based on the water temperature. This could be done with a temperature switch... but then we would not need a computer... which simply is not fun. So the micro controller monitors the temperature and runs the power control for the kettle. Using the kettle seems lame but is actually a great idea: 1) the original kettle design cannot run dry, 2) the kettle will turn itself off after 30 minutes of continuous running, and 3) the kettle costs $20 at Target.
An easy way to attach 3/8" hose to 3/8" soft copper tubing is to use a brass 3/8" FIP to 3/8" barb adapter. The FIP side of the adapter is *almost* large enough to fit around the outside of 3/8" soft copper tubing. So drill it out. After widening it using a 1/2"bit, it can be coaxed over the end of copper tubing (use twisty motion). Then solder.
The Reality
The focal point of the project is a coil of soft copper tubing to transfer the heat from the heated water to the top plate. Finding suitable tubing may be a problem. A trip to the local auto store fixed me up with 1/2" radiator hose (black). Which, as it turns out, has an inner diameter that fits perfectly on the outside of the 3/8" copper tubing we used. That means we just used hose clamps... no barb adapters needed.
An indicator light was added to the power control box to show when the outlets had been energized. Had there been time, we should add two indicators. One showing power to the power control box, and one showing when the outlets are energized. After an unfortunate water spill, the UBW board was enclosed in a small project box and attached to the power control chassis with velcro.