RTD Sensor Quality

Looking to buy a bunch of RTDs (15) for various temperature monitoring throughout the distillery. Of course I need a reliable sensor, but they will all be inserted in thermowells, so swapping them out won't be the end of the world if they go bad. When purchasing 15 of something cost savings adds up quickly. I was thinking of going with TCDirect @ $68 each:

RTD Sensor with Standard Head and Process Connection @ TC Direct

But has anybody had an experience with purchasing rtd probes off eBay? These look to be decent quality @ $27.50 each:

RTD(resistance temperature detect)with head Pt100ohm Probe Sensor PT1/2'' 100mm @ eBay

(sorry for the mobile link... I'm at work and they block eBay... thank god they don't block StillDragon)


  • edited March 2015

    RTDs come in different flavors, each flavor represents the accuracy. These are defined standards you can look up:

    • Class C - Pass (you rarely see these anyway, this is where rejects go)
    • Class B - OK (Generally used in CIP or non-critical applications, still better than thermocouples)
    • Class A - GOOD (Pretty much the defacto standard when talking about RTD accuracy)
    • 1/3 DIN or Class AA - Better (These are rarer, as they are costly)
    • 1/10 DIN - Best (Very rare to find these out of specialized applications)

    If they are advertised as Class A RTD, they should meet the standard of Class A, since that's actually an internationally defined standard. Now, coming from a questionable source, off-shore, maybe they aren't quite Class A.

    At a minimum, as long as you are able to perform a calibration, even a Class B sensor will perform well.

    The thing to keep in mind is accuracy and interchangability are not the same. Plug in a different RTD and you'll get a different reading. Also realize that the performance and accuracy of the overall system (sensor and meter) is only as good as the calibration.

    I've been meaning to pick up a few of those Korean units to give them a try, I agree with you that the price is very compelling, especially given they have the connection heads.

    I would highly recommend you do not use thermowells with RTDs, reaction time will be significantly impacted. This is especially so if you are using a very "meaty" RTD sensor (1/4"). A 1/4" sensor in a thermowell might have a 10 second or greater response time. For condenser temperature control, I'd suggest 1/8th max, no thermowell. In the application a heavy duty sensor isn't necessary.

    Would also recommend that at a minimum, you are using a 3-wire RTD setup. This approach eliminates any error being introduced by the wiring.

  • edited March 2015

    Since we generally work between 0-100c, the difference in accuracy between the classes is small, and gets even smaller when you reduce the temperature under 100c. You'll probably also see that paying the premium for a 1/3rd DIN sensor probably isn't going to net you any real benefit. We're still talking about better than 1C.


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  • I think I may pull the trigger on these. I just need to find out from my equipment supplier how deep my thermowells are.

    I've definitely had the same concern about the response time due to using a thermowell above the dephlegmator. My thought is that since I'm using a proportional valve the slow response will "smooth" out the curve and I'll be able to account for it in tuning the PID hopefully.

    If that doesn't work I'm drilling into the still (although I'm trying to avoid this).

  • edited March 2015

    Yeah it'll work just fine, if it's a constraint, just work around it. It's not like these applications have radically swinging/variable temperatures that need to be compensated for. They generally run steady state until the operator makes a change.

  • FWIW - I have a 5mm OD thermocouple in a 1/4" od compression tube thermowell in my dephlegmator, and it woks fine with a proportional valve.

    I have on my list to make a ferrule to eliminate the thermowell and test, but low on the priority list at the moment

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