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I currently have a Class A Pt100 RTD connected via an Adafruit RTD amplifier to an Arduino using SPI. It has good thermal contact with the warm surface after immersion in a thermally conductive (and electrically isolating) paste. There is no additional liquid protection for the sensor.

After a few days of temperature cycling from 10 deg C to 90 deg C, four out of six of the RTDs have failed and read as open circuits. The leads which are the same colour (and shorted at the sensor terminals for 4-wire sensing) read short-circuit, indicating that the leads are undamaged.

Has anyone else experienced this issue? If so, how did you fix it?

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    \$\begingroup\$ Is there any mechanical load on the system. Like vibration or other forms of movment? How are the RTD fixed? Where is thermal expansion accounted for? \$\endgroup\$
    – kruemi
    Jun 22 at 8:25
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    \$\begingroup\$ You mention "liquid protection" which implies that this is immersed in some sort of liquid. Are you depending on the thermal grease to provide electrical isolation between the liquid and sensor? If the liquid makes contact with any of the metal parts on the sensor, you may be running in to electrolytic corrosion which will dissolve one of the conductors on your sensor. \$\endgroup\$
    – qrk
    Jun 22 at 22:50
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    \$\begingroup\$ @kruemi No mechanical load on the system (vibrations, etc). RTDs are secured using polyimide tape to prevent movement. Aside from that, there is no other thermal expansion that has been accounted for. \$\endgroup\$
    – Jeremy G
    Jun 23 at 2:13
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    \$\begingroup\$ @Finbarr when you mention hazardous - do you mean to human health or the process? If it's the process - what is hazardous about the paste? Does it allow for electrical contact between the pins or some other failure mode? \$\endgroup\$
    – Jeremy G
    Jun 23 at 2:15
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    \$\begingroup\$ @JeremyG: It's in the safety datasheet. "H410: Very toxic to aquatic life with long lasting effects." \$\endgroup\$
    – Oskar Skog
    Jun 23 at 7:48

3 Answers 3

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These are "naked" RTD and are exceptionally fragile. Look at them in the wrong way and they break. Essentially, there's the platinum coated chip and the wires are simply bonded to it. No strain relief at all.

They are meant to be put in some kind of enclosure (like a probe) with strain relief on the cable and an adequate mechanical fixture (often a blob of thermal epoxy)

Unless you need an exceptionally fast response (but then I'd consider a thermocouple if the precision is acceptable) you would have fewer issues using a ready to use probe (usually in a stainless steel can, but there are many kinds of thermal probes)

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    \$\begingroup\$ Agree about the stainless steel can. However, I'm looking to measure something in a small space (2-3 mm diameter). The cans tend to be 4+ mm and don't fit. \$\endgroup\$
    – Jeremy G
    Jun 23 at 2:16
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    \$\begingroup\$ thermal epoxy the whole sensor unit then \$\endgroup\$ Jun 23 at 8:11
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Seems pretty benign. Zinc oxide (the usual component in thermal paste) in a silicone grease carrier. The element itself in those bare sensors is passivated except where the connections are, and platinum is not really susceptible to corrosion unless there is some aqua regia you didn't mention. Suggest you clean them off and have a close look.

I suspect whatever you are using to get the "good thermal contact" is fracturing the brittle alumina substrate. You need some kind of spring action when you have this kind of setup or the forces can get very high due to differential coefficients of thermal expansion.

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  • \$\begingroup\$ The good thermal contact is, hopefully, supplied by the thermal paste - not something mechanical like a fixture. I can't imagine the alumina and Pt are separating due to differences in thermal conductivity. I've cleaned the sensors and can't see any mechanical issues (without a microscope, anyway). If it's damaged, it must be under the hard blue coating. \$\endgroup\$
    – Jeremy G
    Jun 23 at 2:20
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It sounds like maybe the paste hardens near 10°C and places mechanical stresses on the Pt100

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    \$\begingroup\$ Can you expand this answer? It reads more like a comment \$\endgroup\$
    – Voltage Spike
    Jun 22 at 15:57

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