Okay, I'll head back over there tomorrow and put the gauge on it again, see what I can suss out.
In the meantime, I'm a little fuzzy on the whole "a tstat is unnecessary" comment. Doesn't the thermostat serve some kind of purpose? Or rather, what purpose, exactly, does the thermostat serve? What am I missing (damaging / destroying / neglecting / etc.) by not having one installed as shown in the drawing?