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RTD vs Thermocouple

For our purposes, which do you prefer?

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Comments

  • edited January 24

    Without a doubt Rtd. 0.1 degree v 1.0 degree or more Having said that I have a controller with about 14 rtd on it. 3 wire and 4 wire and the temp variation is frustrating. Tried calibration using sous vide and boiling water but still annoying variations over the range in the logs I print out. I also have multiple thermocouple transducers on the same controller - k type and C type up to 1800 C and they good for high temp stuff but not accuracy over a small range.

    If anyone has suggestions on good accuracy over the 70 to 100 C range with repeatability across multiple sensors I can’t wait to hear as I have spent a lot of dollars playing with this problem

  • The biggest part is to confirm what class RTD sensors you have …. A, B or C.

    B is or seems to be standard with +-0.3 to 1 deg C accuracy. Class A which obviously is pricier has an approx. accuracy of +- 0.15 to 0.3 deg C.

  • @GD50 said: Without a doubt Rtd. 0.1 degree v 1.0 degree or more Having said that I have a controller with about 14 rtd on it. 3 wire and 4 wire and the temp variation is frustrating. Tried calibration using sous vide and boiling water but still annoying variations over the range in the logs I print out. I also have multiple thermocouple transducers on the same controller - k type and C type up to 1800 C and they good for high temp stuff but not accuracy over a small range.

    If anyone has suggestions on good accuracy over the 70 to 100 C range with repeatability across multiple sensors I can’t wait to hear as I have spent a lot of dollars playing with this problem

    1. try using some high precision 100 Ohm resistors as dummy probes... order 20 or so of the resistors and then grab 4 or whatever of the ones that read the same with a meter and hook them up... does the controller show the same temp on all the ports with the dummy RTD?
    2. buy a name brand, $$$ probe and compare...

    I think one of the two above solutions should ID the issue...

  • SQD
    edited January 25

    For my HMI and control system temp monitoring I use a combination of digital DS18B20 sensors(3 wire) and RTD's. My 2 RTD's are tied to 2 PID controllers for proportional temperature control of the condensers and the DS18B20 sensors are for SCADA temperature display, safety system, historical trending, etc. I find the DS18B20 to be good for about 0.5 degree accuracy. DS18B20's transmit a digital signal which goes to a ModbusIP gateway which sends it to the HMI and PLC. I've attached a pic to show a bit of what I created(this pic was taken right after I shutdown a run). Been using it for about 3 years now and the system works very well for me. But what works for me might not work for others. :)

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  • I have similar with RTD and 1-wire(DS1820B) the RTD are great if you really want the 0.1 degree, such as vapor temp or even fermentation temp for controlling a chiller... for cooling water and such, the 1-wire probes are great... here is a display of 4 of each being monitored by a $5 ESP32 WiFi Dev board (with a $30 RTD amp board and 4x $5 RTD Amps... (displayed in BruControl, with no fluff at the moment)

    you can see the smoother line of the RTD(Blue), this is overnight ambient temps

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  • Your PID controllers need " Clean " power and that PSU has to be grounded correctly. RTDs are very connection sensitive. Its for sure correct that a good quality brand goes a long way. When they are set up correctly they are less problem than Thermocouples, far less. Wiring terminations have to be rock solid. If you have long runs you may need transmitters. Also you may need to used shielded wire and connect only at one end. They are supposed to be wired up with the correct type of extension wire. Conections have to be flawless.

  • Hello...

    I know Im probably going to get beat up on this, but here is my 2 cents worth from and old electrical engineer that cut my teeth on both RTD and T/C.

    Im an automation/controls/instrumentation integrator in West Palm Beach, Florida with over 40 years experience. To begin with...you can get very good performance from both an RTD or a Thermocouple. I have many customers that prefer one over the other and both swear that their choice is the best. I never argue, I just put in what they request. :) But I can tell you that after going to many of their locations and observing their applications, I understand why both sides make the claims that "their choice" is the best.

    Most comes down to familiarity and maintenance (or lack of) and the initial installation.

    I won't bore you with all the specs of the different types of both (but if you want you can email me with any question you might have on PLC, VFD, HMI, Instrumentation, etc. and I will reply with what has "worked" for me for many years). I will caution you not to get too hung up on the specs tho as these are all acquired in a laboratory environment which is never the real world and can never be duplicated.

    Routing of the device wiring near high voltage wires is a constant concern, and non-grounded or poorly grounded equipment can add to your misreads and frustrations. Cheap controllers and cheap wiring also are a persons worst enemy regardless of the type of device you choose.

    What I will tell you is a good quality Thermocouple installed correctly and maintained, will far outlast any RTD and give you the most consistent repeatability, and is far less prone to noise induced issues that can plague many RTD's. My people install both on a daily basis , but they ALL prefer to install a T/C due to its simplistic installation and no warranty call outs. Mush simpler to understand, maintain and troubleshoot.

    I also have heard people talk about "Calibration" of these devices and just so we are clear, there is NO calibration of any temperature device, you can only VERIFY how close it is and compensate for the amount it is off in your controller.

    I would also recommend that for example IF you are using a type J thermocouple that you verify it with a type K thermocouple. This will give you a more reliable comparison. Also, take advantage of the test terminal blocks that OMEGA offers, it will make your life less frustrating. Make sure you check connections on a regular basis.

    Thermocouples also respond to temperature changes more effectively and will recover from fast changes more quickly.

    I will end my comment that we also offer aircraft part heat treating equipment calibration services for several airlines and this is all an FAA required test using Type K and Type S thermocouples.
    Im not trying to promote OMEGA Engineering but they do have some very good easy to understand and useful charts on thermocouple types, and offer many parts.

    Also, many times when we go out on service calls of people complaining about their RTD or Thermocouple, what we find is that its not the device that's the problem but the configuration setup of their controller that's the problem. SO always make sure your controller is configured and setup correctly for whatever device type you are using and your results will become a lot less frustrating, and invest in a few fairly inexpensive pieces of test meters like Fluke or whatever you like. Process of elimination is still the best method I have found to troubleshoot many issues.

    Well...I know this was a little long, but I hope this helps. IF you want more specific details, or have more specific questions, please feel free to contact me.

    Palm Beach Automation, Inc.

  • Welcome, and thanks for the informative post.

    StillDragon Australia & New Zealand - Your StillDragon® Distributor for Australia & New Zealand

  • Thanks for the feedback @MatrixMerlin

    StillDragon North America - Your StillDragon® Distributor for North America

  • One more thing I forgot to mention about the RTD.

    As you know...the "R" in RTD stands for Resistance...and as we all know any kind of stray electrical noise, voltage, frequencies, grounding and all the other types of electrical interference from all types of things has an adverse effect on the resistance of that wire on an RTD, which results in intermittent and misreads and or non consistent temperature reads. Every electrical formula or law in some way or another reacts with or creates some issue with "resistance" which is the heart of the RTD.

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