Thermocouple Reliability at high Ambient Temperatures

Hello. Are there any issues with using the Rio in high ambient temperatures with thermocouples? I have a Rio with four type K thermocouples connected to channels 0-3. I am using the Rio to collect data from the TC’s and periodically save the data to a database using Node-Red. My issue is that the first few times it worked great (ambient temp 75°F), but the last two times I tried the TC’s went nuts. The TC’s should have been around 1,500°F, but I would get readings that fluctuated greatly (±200°F within a few seconds, negative values, then finally steady state around 1,200°F. I could see them glowing, so I knew they were hotter. When connected to my Fluke 54 II, it read as I would have expected. As I said, this happened twice when the ambient tempo was about 105°F. The TC’s are Type K, and I used some Ø1/16" Inconel sheath grounded as well as Ø1/8 sheathed ungrounded with the same results. I was thinking that the sheath was grounding out to the machine and causing some interferences. Some systems put a resistor between the negative TC wire (red in this case) and ground.

I ran the system for 12 hours overnight in my office collecting data every 30 seconds with no problems, where again, it is nice and comfortable.

Has anybody else had this experience? Thanks.


The RIO specifications are all in a table on the product page.

Thank you Beno. This is what is bothering me. I am well below the 70°C threshold. Any other thoughts?

Sorry, I misread your units…

Yes, if you’re using fiberglass insulated TC wire/cable, I found this to be a real problem. Wat happens is that the fiberglass is so loosely woven that is will not stay together and ends up either shorting to ground or to each other. Problem is PFA only handles up to 500 DegF. If you can use pre-assembled TC’s with a conduit head, then use PFA insulation from there. You can also use mineral insulated but that is worse I think.

I ended up using RTDs but they typically do not go up to ranges of TCs.

Also, don’t use really small probe tubes because the wire size inside the tube is tiny and very delicate, I found that the wire would break all the time. If you are measuring air temp, you can use exposed tip of air tip probes, but in the higher ranges, that would possibly be a problem.

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Thanks. I’m using all Inconel sheathed, mineral insulated wires. I use these all the time for furnace monitoring or for profiling time & temp through the furnaces. These in particular are long (30’ / 9m) and small diameter for flexibility. I can only use them a few times before the insulation breaks down and you get multi junctions. The very small ones (Ø1/16" sheath) cannot be provided ungrounded, which I now realize is giving me some problems when I run two at a time. What really confuses me is that the first time I had problems I was using a verified ungrounded thermocouple in a Ø1/8" sheath and much shorter, only 8’ long.

I think I need to do some detailed tests to verify what is going on. I’m sure it is all Engineer Induced faults (me) and not the Rio, but I just want to make sure as I didn’t see much on the forums.

I can’t say regarding RIO, but I have had those problems on the snap line which all turned out to be cable and wire problems.
I get why you are using 1/8" but I found that the hassle wasn’t worth it in terms of reliability. However you probably need to get as fast a reading as possible.
I was speaking previously about the thermocouple wire coming back to the control panel, the typical fiberglass extension cable, which I found to be incapable of staying insulated and would ground out to the conduit it was running in.
If you have a decent process meter, I would use it to verify the problem when you see it in opto.