Just wondering why didn't you go with the Tri-Clamp Liquid tight RTD Sensor
listed on Auberins website? It would have saved you the hassle of an add'l NPT fitting that may leak and it only costs about $5-10 more than the NPT liquid tight RTDs. It also saves you the $14.95+shipping for the triclamp to 1/2" NPT fitting that you needed.
Update: I just built my temp. controller in a Home Depot toolbox and I'm currently using it for my keezer temps. However, I'll soon be building a RIMS tube similar to yours, so I appreciate all the pics and parts list that you took the time to post. I had a 1/4" NPT RTD probe lying around at work and went for it, but I ended up accidentally burning it out by immersing it in a small Thai food container filled with water in my keezer. <smacks forehead> Stupid! Luckily, I had a cheap air thermocouple lying around as a cheap alternative in the interim for keezer control.
I wanted more accurate liquid temp measurements for potentially better control, but all I found out was; A) don't expose the back end of a "liquid tight" probe to water, because it's only liquid tight on the business side of the fitting; B) liquid temp measurement causes greater swings in keezer temps than air temp measurement because the freezer coils are still super-chilled for a little while after the SSR kicks off. Thus, they impart a much lower interior air temp to the keg until the water catches up. Air temp. measurement takes a little longer to chill down the kegs when you first fire it up or on a newly inserted keg, but once chilled to temp, it tends to keep a tighter control band on the system temp of the keg. Just my two cents on that.