So, I have tried several combinations of solenoid valves and finally settled on 12v DC gravity type valves.  They require very low pressure to activate, as opposed to sprinkler and even other allegedly "low-pressure" valves.  The Iwaki impeller type pumps that I am using do not supply enough pressure in this system to activate anything but the gravity type solenoids.  Not a big deal, but I would have saved some money had I done a little bit of homework and read the specs on the other solenoids.  My bad.

Initially the system used ice cubes and hot water from a tap to provide the hot and cold water needed for the test.  This works, but it is less easy to control temperature as a variable and it requires ice cubes and hot water access for repeated tests.  I am scavenging some wine chillers for their Peltier devices and aluminum carafes.  I hope to take advantage of these to more systematically control the hot and cold water.

I drilled holes in threaded plugs for the temperature sensors.  I simply epoxy in the sensor and voila!  This seems to work just fine, but I have placed epoxy on the outside of the plug and it tends to be too thin in spots and flake off.  I believe it to be cosmetic only, but who doesn't want their pain-inducing medical device to be as pretty as possible?  I will be looking for a solution to this.

I purchased pipe fittings from a variety of sources and have found some significant quality differences along the way.  Some, but not all, of the less expensive fittings require a much tighter fit to not leak.  Some leak, regardless of the hose clamp tightness.  This isn't a big deal, but I would be careful when sourcing parts, if I were planning to build many of these.

As far as hose clamps go, the automotive style clamps really seem to work well compared to plastic type clamps.  They are around $1 each at the local auto parts store, but have seemed to be worth the expense.

Hope this provides you with some visibility to some of the obstacles that I have encountered!