I have a loop consisting of approximately 6' of 3/8" ID tubing, pressurized with a pump running at approximately 1 GPM. I would like to add a few temperature sensors into the loop to read the water temperature at varying points. Reading the temperature of the exterior of the tube will provide sub-optimal accuracy. Fittings with built in thermometers are available, but expensive in quantity and would require the addition of a regulator to step down voltage in my application adding complexity and more cost.
The best way I can figure to do this is to buy a fitting like this, and then insert the sensor of a thermometer (such as this) into the reduced opening and then seal around it. However, I suspect quite a bit of wiggle room around the sensor and fear that silicone will blister and eventually leak under the pressure. And since there is a sensor line going into the opening I cannot simply tape over the silicone to resist the blistering.
This application requires reliability in the seal for a minimum of 3 years. The seal can be permanent; there will never be a need to remove the thermometer from the fitting or line. The loop will be filled with distilled water at temperatures ranging from 5C to 60C and the seal must hold at both extremes.
Best Answer
Many years ago at the beginning of my engineering career we made sealed temperature sensors using silicon diodes bonded onto the ends of Teflon insulated wire. The wires were then fed through two holes drilled through a short length of 0.25" diameter Teflon rod. The rod was the inserted into a standard brass compression fitting using a standard brass compression ring that tightly sealed the Teflon rod and formed it very tightly around the pass through wires.
What you see above is a heater assembly that produced a temperature controlled gas stream in the range of -55C to +125C. The two diode assemblies on the heater were for redundant critical over temperature safety cutouts and the one at the pipe TEE was used to monitor the gas stream temperature for closed loop control of the heater assembly. Thermal mass of of the system was part of maintaining a stable gas stream temperature.
Note that silicon diodes biased at a constant current of 1mA produce excellent temperature sensors that are linear over the whole operating range of -55to+135C.