I think that maybe you're getting thermostat and thermometer a little confused? Thermostat controls the heat put out by the UTH and the thermometer goes inside the tub and lets you see what the temps are. Have you got a thermostat to control the heat source? This is important because UTH's and other heat sources can become much too hot when left running wide open.

On another note, it is really neat seeing everyone coming together on this one BP.Net at its finest