Though I don't see how this is relevant to finding a source for a DS18B20 sensor pre-mounted in NPT threaded housing; I need to measure additional points inside a fluid filled pipe up to 30-50 psig and between 50 and 200 degrees F.
I am fairly mechanically included and have a lot of tools, but trying to make a thermowell is well beyond the tools and materials I have access to. You would need a piece of steel rod 1/2" in diameter with the section that is not threaded, less than 0.350" to pass through the female fitting. Then you would need to drill the 0.350" section out with a 0.250" drill bit without breaking through the side of the 0.050" thick wall (0.350-0.250 =0.1 diameter = 0.050 radius) you are trying to leave behind.
I am pondering if the compression fitting has enough strength to hold the probe in at 50 psig and if the probe would be resuable or if the compression fitting will make it one time use (becuase of damage to the sensor housing).
It is relevent, if I'd known that, I wouldn't have suggested engineering plastic, and jpanhalt probably wouldn't have suggested glueing a sensor inside an adaptor which is what I was thinking too.
A compression fitting might crush the tube on the DS18B20, but if it doesn't it should hold. Mains water is about 40-45 in the UK and compression joints hold that OK.
At 50psi and 200F you don't want it coming out so the olive needs to be tight. That means it won't come off easily. Almost all of the 15mm olives and 22/28mm I've ever undone have "biten into the tube, only very lightly tightened ones can be encouraged to come off.
Ebay(US) have a Chinese 6mm compression to 1/8npt but it's $4, you might get one cheaper at your local gas/plumbing store.
If that doesn't work, I would 2nd Jpanhalt's idea of using an NTC thermistor, they are tiny and would easily fit inside a 1/8 npt.
If you could get hold of a 1/8npt solid plug or threaded rod, you would only need to drill a hole of about 1/16" part way into the plug, then seal it in.
They are quite accurate and the circuit is simple, it needs one ADC.