[Instrumentation Interview Question] What is Thermocouples? Standard Thermocouples

Thermocouple


A thermocouple consists of a pair of different metal or alloys joined together at both ends. One end, measuring junction, is placed where temperature is to be measured. The two conductors extend out of the measurement area to the reference junction. An electromotive force (emp or mV) is produced which is function of temperature difference between the two junctions and material of the wires.
E = k (T1 – T2)
 
Thermocouple working principle
Since the mV generated by the thermocouple is the function of temperature difference between the two junctions, control or compensation of reference junction temperature is required.

Standard Thermocouples

Seven types of thermocouples (TC) have been given letter designation by ISA (Instrument Society of America). These are listed in table along with normal working temperature measurement range. A brief description of important thermocouples, which are extensively used in steel industries, is produced in the following paragraphs.

                                                           TABLE

TYPE              COMPOSITION        RANGE          EMF                WIRE
                                                            (Deg C)           (mV)                GAUGE
B                     Pt-6% Rh vs.               0 to                  0 to                  24
                        Pt-30% Rh                  1820                13.81
R                     Pt vs    .                       -50 to               -0.226 to          24
                        Pt-13% Rh                  1768                21.10
S                      Pt vs.                           -50 to               -0.236 to          24
                        Pt-10% Rh                  1768                18.69
J                       Iron vs.                        -210 to             -8.096 to          8
                        constantan                   760                  42.92
K                     Chromel vs.                 -270 to             -6.458 to          8
                        Alumel                                    1372                54.87
T                      Copper vs.                   -270 to             -6.258 to          14
                        constantan                   400                  20.869
E                      Chromel                      -270 to             -9.835 to          8
                        constantan                   1000                76.358


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4 comments:

  1. Give me the RTD detail like this Thermocouple

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  2. There are some more types of thermocouples like W-Type, N-Type, C*-Type with there different ranges. Where W-Type Thermocouple made up of Tungsten/Rhenium with a temperature range of 32 to 4200 DegF and N-Type made up of Nicrosil/Nisil with a range of 32 to 2300 DegF.

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  3. Very informative article on Thermocouples and RTDs. Thank you Sebastian for sharing it! I prefer using thermocouples than RTDs, because thermocouples have a fast response time than RTDs do. Also, thermocouples do not self heat, and RTDs do. Now, that isn't to say RTDs are less useful than thermocouples. They have their own benefits over thermocouples. For instance, RTDs have a better interchangeability, long-term stability, and a higher accuracy than thermocouples. What it ultimately comes down to for me is cost. I use thermocouples, because they are less expensive than RTDs. However, that doesn't necessarily mean that thermocouples are better than RTDs. This is merely one person's preference. Transitionally, I really enjoyed reading your article, and I look forward to reading more articles from you.


    Thanks again! :-)
    Germaine K. | Thermo/Probes Inc.

    ReplyDelete
  4. Hi Germaine Koziarski,
    I am an Instrumentation Engineering student wants to know about your comment briefly. Can you briefly explain in which cases thermocouples are used and in which cases RTD's are used.

    ReplyDelete