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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.

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    Swagelock brand T may work they have ferrules that crimp down and seal. But they are expensive a brass t with ferrule cones may also work Parker makes these and the also make one with a plastic ferrule that I have used in the past for temp and PH probes. Basically I would use compression for the strength to hold up to vibration and pressure.
    – Ed Beal
    Mar 7, 2017 at 19:43
  • OR a small hose clamp, the screw kind.
    – Trevor_G
    Mar 7, 2017 at 19:46
  • The temperature of the outside of a metal fitting, when covered in insulation, will be a very good measure of the temperature of the fluid inside (except for short-term fluctuations). You don't need to break the seal.
    – Chris H
    Mar 7, 2017 at 20:38
  • "expensive in quantity". how many do you need for a 6' loop? there are mechanical thermometers instead of electronic. Sharkbite makes an inline fitting with temp gauge for 1/2 pipe. sharkbite.com/product/temperature-gauges
    – Alaska Man
    Mar 8, 2017 at 11:19
  • @ChrisH My biggest concern with a fitting temperature, besides accuracy, is speed. If the temperature of the liquid drops from 44C to 42C, how quickly would the external temperature change to match that? And how accurate would the reading be?
    – Nicholas
    Mar 8, 2017 at 14:04

4 Answers 4

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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.

enter image description here

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.

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I think what you are looking for are called 'instrumentation fittings'.

However, it doesn't sound like you have a temperature sensor yet, why not just go for one with threads (eg, search "temperature sensor MPT")? Easy to install with any type of plumbing using a tee and otherwise standard parts (bushings and maybe a FPT adapter).

enter image description here

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  • This is a good solution, but I can find few under $20 each, and those that I do find in that price range have very poor reviews. Are these really necessary over a cheaper, DIY solution?
    – Nicholas
    Mar 8, 2017 at 14:07
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If you want it to last for 3 years, choose a thermowell, that's what they are made for.

One problem with your proposed fitting is that only the very tip of the sensor will be exposed to the flow. Depending on the sensor (good luck finding details on that from your source) this may make the response very slow & inaccurate. A better fitting might have a 1/2" bore with the thermowell running along it, and 3/8 fittings at either end, so the fluid passes by most of the sensing element.

There is a fitting the name of which escapes me at the moment that has two half-grooves with a screw thread to push them together, which squeeze an O-ring that seals a rod or rod-like item that fits through the center of the fitting and o-ring. They are mostly used for things that have to be moved in and out (or something inside moved by the rod being attached to it.) Similar to a cable gland.

For a typical electronic sensor I would agree that lag time for one bonded to a metal fitting (such as a simple coupler) or the exterior of metallic tubing with the outside of the area around the sensor insulated would be minimal, particularly for 3/8" diameter.

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The other answers are correct, you need a well. If you don't want to buy one browse the plumbing aisle and see if you can figure something out. Start with a Tee fitting that has as large a female side port as possible (perhaps 1/2" or 3/4"). Find a small prethreaded stub pipe about 1 or 2" long that is 1/4 or 3/8" diameter and a cap to plug one end of it. The cap needs to fit into the side port and be positioned in the middle of the liquid to be measured flowing through the end ports of the Tee. Now find a bushing that has a female inside thread that matches your stub pipe, and a male thread on the outside that matches the side port of the T. This allows the cap to fit into the side port of the T and fills the gap between the inside of the side port and the outside of the stub pipe. Done right you can thermal epoxy your sensor down the center of the stub pipe against the cap and get good coupling with the liquid. Metal fittings are best. Hope the horrible ASCII art helps.

T = T Fitting (threads inside), C=Cap (threads inside), P=Stub pipe (threads outside), B=Bushing (threads inside and outside)

T  ^   T
T  |   T
T      T
T      TTTTTTT
T  CC     BBBBB
T  C PPPPPPPPPPPPP
T  C              
T  C PPPPPPPPPPPPP
T  CC     BBBBB
T      TTTTTTT
T      T
T  ^   T
T  |   T
T Flow T
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