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I would like to read the temperature of oil in a metallic vessel. However I'd like to route the thermocouple so it stays out of the way. Is it OK to just insert the "naked" thermocouple "bead" in a bendy piece of stainless to protect it from the oil? "Stainless" thermocouple probes are extremely common but I'm not sure what is the internal construction. The temperatures I'm interested in are between room temperature to a maximum of 400C.

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  • \$\begingroup\$ Why should it need protecting and from what? \$\endgroup\$
    – Andy aka
    Commented Feb 25, 2022 at 17:09
  • \$\begingroup\$ Hello @Andyaka. I am not sure how to further explain: my assumption is that the thermocouple comes with a braided sheath that would completely get soaked in the oil, making it almost impossible to keep clean. Now, can the thermocouple "head" junction be in contact with a third metal like stainless? \$\endgroup\$ Commented Feb 25, 2022 at 17:42
  • \$\begingroup\$ @AlessioSangalli How is the vessel heated, exactly? And have you considered fiber optic phosphor thermometry? (Which can cover the range from about -250 C to about +450 C, as a practical matter, with good accuracy and repeatability between instruments.) \$\endgroup\$
    – jonk
    Commented Feb 25, 2022 at 17:46
  • \$\begingroup\$ @jonk it is just a common induction heater, however the oil is subject to impurities and I want a way to keep it clean. \$\endgroup\$ Commented Feb 25, 2022 at 19:03
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    \$\begingroup\$ Omega has a large range of products that meet your description. Usually the construction is mineral insulation (MgO) inside the SS tube. Transitions to regular wiring are a bit of a pain (best to order them terminated in a connector/pigtail). \$\endgroup\$
    – W5VO
    Commented Feb 25, 2022 at 19:51

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I definitely see no reason why this can't be done. There are a few considerations electrical, thermal, and mechanical in nature...

Some good information here:

Because the bead of the thermocouple is exposed, there are several application limitations. The beaded wire thermocouple should not be used with liquids that could corrode or oxidize the thermocouple alloy. Metal surfaces can also be problematic. Often metal surfaces, especially pipes are used to ground electrical systems The indirect connection to an electrical system could impact the thermocouple measurement.

So you need to think about three things:

  1. Dependant on the type of thermocouple, the "bead" may not be electrically isolated. You will likely want to isolate it somehow (fibreglass fabric, silicone, or polyimide/kapton should all tolerate the heat) from the stainless tube to not cause signal problems. (Note, I can't say what grounding the bead would do exactly regarding noise or absolute measurement error, as it depends on how the measurements circuitry reads the thermocouple voltage. Ex: one side of the thermocouple could be grounded at the amplifier, which would cause problems. Even if the sensing side only provides some impedance to ground, different ground potentials could cause a current flow which would distort the measurement)
  2. Thermal mass and conductivity will impact the reaction time and absolute measurements of your sensor. The extra metal of the tube will slow the rise and fall time of the sensor reading relative to the oil. The electrical isolation material could also act as thermal insulation, further slowing the rise and fall time. Also, the thermal conductivity of the metal may cause error depending on how you mount it. Picture a tube immersed only slightly, while having a long portion exposed to the air above the oil. The heat will run up the tube and dissipate in the cooler air above, causing the tube to be slightly cooler than the surrounding oil. Your exact requirements for reaction time and temperature accuracy will decide how careful you need to be of these effects.
  3. Finally mechanical considerations. Mounting your stainless tube, sealing one end, sealing the location it passes through a wall below the oil surface if that's the case, etc, etc. This affects the thermal considerations as discussed above, as well as the electrical, since if you mount the tube so it is completely electrically isolated, you don't have to worry about isolating the sensor. You may have to worry about ESD however.
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There are steel tubes with threaded unions designed to protrude into tanks to hold temperature sensors - have 4 on my solar tank. Usually called sensor pockets or similar.

Only issue may be how quick temperature change is detected, but not a problem on the solar tank, may need to be considered for your application.

Also known as Thermowell pockets, see

https://www.sontay.com/en-gb/products/temperature-sensors/tt-po-immersion-sensor-pockets/

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  • \$\begingroup\$ That would be a wonderful solution but unfortunately I don't have such threaded hole and the sensor must be submerged from above. \$\endgroup\$ Commented Feb 25, 2022 at 21:24
  • \$\begingroup\$ @AlessioSangalli so weld or silver solder the boss onto the vessel - you said metallic… copper, steel etc \$\endgroup\$
    – Solar Mike
    Commented Feb 25, 2022 at 21:34

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