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  • thermocouples

    some k-elements have the "yellow plugs" and some have spade terminals. how do you connect the spades to an instrument for the plugs? the same for (r and s).

    and btw, why does polarity matter? its voltage that gets measured, no?

    connected like this i get 1000°c in a flame, another premade "yellow plug" k-element shows 1300°c. (im aware the values would be erratic in this range.)
    Attached Files

  • #2
    A thermalcouple is like a battery cell, kinda.
    So it initiates current flow, so that is DC so to speak,
    so I guess polarity matters, only in the sense that the meter
    is taking the current and multiplying the voltage to something that
    is calibrated to read temp in degrees.

    -D
    DZER

    Comment


    • #3
      How to connect spades: cut the wires and attach suitable thermocouple connector. In this case male Mini K-type.

      Polarity: reverse polarity would be negative temperatures (referenced to meter internal reference junction)
      Boiling water would read something like -50Cel if you reverse the wires.
      Location: Helsinki, Finland, Europe

      Comment


      • #4
        Doozer is kinda right. What a thermocouple does is generate a DC voltage proportional to heat. The different types of thermocouples (K,R,S,T and more) have differing response curves and temperature limits. Type K and J are probably the most common. The voltage generated is quite small and the voltage per degree is absolutely tiny. Because of this the entire system must be proper and stable if accuracy is to be expected. Now for a bit of technical explanation. A thermocouple is simply two different metals joined at the probe tip. The metals are specific to the type of thermocouple and polarity. When thermocouple probes are connected to standard (non thermocouple) extension wire and/or non thermocouple specific connectors you create another thermocouple junction in series with the measurement junction (probe). While this added junction is not an "actual" thermocouple, it still acts that way and generates a voltage. This voltage either adds or subtracts from the actual measurement voltage and therefore adds error to the reading. Depending on the application the error may or may not be enough to matter. Another thing about thermocouple meters and inputs is they are designed to be floating, differential inputs. This means they are not grounded and only measure the voltage between the thermocouple wires. This helps make them resistant to noise on the cables.
        In your example, I can see 300 C of error caused by the crimp connections and other outside influences. Put a Type K connector on the probe and try it again. Remember that polarity is important on the connector! I think you will be surprised at the result 😊.

        Below is a cut and paste from a company that makes thermocouple probes. I have no attachment to them, they just had a good block of data that was easy to copy and explains the Type K thermocouple....



        Type K Thermocouple


        Type K Thermocouple (Nickel-Chromium / Nickel-Alumel): The type K is the most common type of thermocouple. It’s inexpensive, accurate, reliable, and has a wide temperature range. The type K is commonly found in nuclear applications because of its relative radiation hardness. Maximum continuous temperature is around 1,100C.

        Type K Temperature Range:
        • Thermocouple grade wire, –454 to 2,300F (–270 to 1260C)
        • Extension wire, 32 to 392F (0 to 200C)

        Type K Accuracy (whichever is greater):
        • Standard: +/- 2.2C or +/- .75%
        • Special Limits of Error: +/- 1.1C or 0.4%

        Type of K Thermocouple
        • Consideration for bare wire type K thermocouple applications:

        Type K Thermocouple Reference Table:







        Last edited by rdfeil; 02-04-2021, 05:36 PM.
        Robin

        Happily working on my second million Gave up on the first

        Comment


        • #5
          Sebeck effect popped in, you don’t want back asswards, it will be obvious! It’s just a 2 pin plug, don’t recall crimps in there, just 2 pins, found some with bootlace ferrules once that were reading off whack, replaced plug with a new one, ok after
          mark

          Comment


          • #6
            The more junctions you put in, the more variables and errors.

            If you look at the connections for thermocouples to equipment, they will often be chunks of material, with the two connections for one thermocouple lead close together, sometimes pushed together with an insulator between. That keeps both junctions at the same temperature, so that the "net junction" adds nothing...

            The "net junction" is the sum of wire 1 to it's terminal, and wire 2's terminal to wire 2. Ideally, if the terminals are at the same temperature, those two junctions cancel out, so it is the same as having the two wires connected together directly. If you have random connections in various places, they will likely be at different temps and probably will produce voltages that cause errors.

            Remember, the measurement is relative to some extent.... if you put junctions at each end of a length of thermocouple wire, the thermocouple current would be related to the difference in temp between the two ends. If one was at -100C, the current would be different than if it were at +100C. Each junction produces voltage related to its temperature, and the cold end opposes the voltage at the hot end.

            So you want to use compatible materials, ideally the same as the wire, to make connections. Otherwise you add junctions, each with its little voltage.
            CNC machines only go through the motions.

            Ideas expressed may be mine, or from anyone else in the universe.
            Not responsible for clerical errors. Or those made by lay people either.
            Number formats and units may be chosen at random depending on what day it is.
            I reserve the right to use a number system with any integer base without prior notice.
            Generalizations are understood to be "often" true, but not true in every case.

            Comment


            • #7
              Good answers above so I won't try to repeat them except on the polarity thing which bears some emphasis. The thermocouple produces a DC Voltage and the meter uses a sensitive circuit to measure it. If you reverse that DC Voltage, then the meter can not do it's job properly. So you do need to observe correct polarity. Reversing the polarity will, at best produce no reading. At worst, it can produce an erroneous reading.

              Now, every connection between wires, from the thermocouple to the meter IS ANOTHER THERMOCOUPLE. And each of these thermocouples introduces another DC Voltage into the circuit. The two metals chosen for the actual, measuring junction are picked for, among other reasons, maximizing the Voltage that they generate. And hopefully, the other wire junctions in the circuit will have smaller generated Voltages.

              Due to these additional connections which each form another thermocouple, the thermocouple really measures the DIFFERENCE between it's tip and the area where those primary thermocouple wires are attached to the cable that runs to the meter. What this means is if you are measuring the temperature inside an oven, it is not only the oven's temperature that governs the Voltage generated, but also the temperature where those connections to the cable are. This is usually room temperature, but it can be higher if the oven is poorly insulated and leaking a lot of heat.

              So some working principles for connecting a thermocouple to it's meter and using it are to use the SAME metals on each wire for each step in the circuit and to make the connections between them as physically close together as possible so they are at the same (room) temperature.

              The connections at each point (like theromcouple to the cable) should be made the same way. If you have one that uses crimped connector(s) and the cable breaks there, a soldered connection on just one wire would not be a good idea. Even using a different brand of crimp connector may introduce different metals and errors. Both connections at each point should be made in the same way, using the same materials and techniques. Even the plating on a conductor introduces another junction and another generated Voltage. Bare copper in a steel terminal will produce one junction but plated copper in a plated steel terminal will have THREE junctions. For best accuracy, all of these should be the same for both conductors of the thermocouple.

              The final thing to look out for is keeping that first connection point as far removed from the heat of the furnace as possible. Use as much insulation and as much physical distance from the interior heat of the furnace as possible. Most thermocouple-meter combinations probably assume a room temperature around 20°C or 68° F. So that is the temperature that the first connection point for both wires should be at (YMMV).

              Keeping the two connections of each of the connection points as close together as possible along with using the exact same metals will ensure that these additional junctions will all produce the same DC Voltages, but with opposite polarities so they will cancel out in the overall circuit.
              Paul A.
              SE Texas

              And if you look REAL close at an analog signal,
              You will find that it has discrete steps.

              Comment


              • #8
                thanks every body, but the main question remains: many thermocouples come with the spades. how do you use them? what are they supposed to be connected to? if they work on some connection block of an instrument they should work as i have done, no?

                (i have to admit they are "ebay purchases", so maybe the chinese have screwed up putting regular crimp terminals on there?)

                edit: what is the melting point of the junction? cant find anything on it.
                Last edited by dian; 02-05-2021, 12:37 AM.

                Comment


                • #9
                  Originally posted by dian View Post
                  thanks every body, but the main question remains: many thermocouples come with the spades. how do you use them? what are they supposed to be connected to? if they work on some connection block of an instrument they should work as i have done, no?

                  (i have to admit they are "ebay purchases", so maybe the chinese have screwed up putting regular crimp terminals on there?)

                  edit: what is the melting point of the junction? cant find anything on it.
                  Those are intended for use with PID temperature controllers like these-

                  https://www.amazon.com/Inkbird-Tempe...dDbGljaz10cnVl

                  Also analog pyrometers-

                  https://www.axner.com/analogue-pyrometer.aspx
                  I just need one more tool,just one!

                  Comment


                  • #10
                    Originally posted by dian View Post
                    thanks every body, but the main question remains: many thermocouples come with the spades. how do you use them? what are they supposed to be connected to? if they work on some connection block of an instrument they should work as i have done, no?

                    (i have to admit they are "ebay purchases", so maybe the chinese have screwed up putting regular crimp terminals on there?)

                    edit: what is the melting point of the junction? cant find anything on it.
                    As wierdscience mentioned those spades are supposed to be used with panel mounted equipment.

                    Your "contraption" should also work, error is probably max 10 degrees if meter&connectors are at relatively stable indoor temps.
                    BUT The few Chinese Ebay thermocouples I have seen have been all over the place. 10% or 20% errors(or more), I wonder if they are made from wrong type of alloy.
                    (J, K, N, T types are all ballpark similar.)

                    Melting point of junction for K-type is somewhere around 1400 Cel but standard covers temperature "only" to 1370c
                    Above that meter probably reads OL

                    K-type thermocouple is very short-lived at extreme temperatures. Most often they use really thick (2mm or so) bare wire insulated with ceramic rings. These are typically good for something like 1260 Cel
                    Location: Helsinki, Finland, Europe

                    Comment


                    • #11
                      https://www.thermometricscorp.com/thertypk.html
                      Location: Helsinki, Finland, Europe

                      Comment


                      • #12
                        so are the spades made from special, compatible alloys?

                        good link above, first time i see a b-type. iv been looking for a ceramic r/s-probe for a while, cant find anything afordable (around 40euros, because above 55 euros incl. shipping i pay customs and vat).

                        ill be checking my thermocouples with a heat gun then, to see how they work

                        Comment


                        • #13
                          Originally posted by dian View Post
                          so are the spades made from special, compatible alloys?

                          good link above, first time i see a b-type. iv been looking for a ceramic r/s-probe for a while, cant find anything afordable (around 40euros, because above 55 euros incl. shipping i pay customs and vat).

                          ill be checking my thermocouples with a heat gun then, to see how they work
                          Spade material is not critical if you don't need super accurate results. Proper thermocouple connectors (the green and yellow ones) are made from same material as the thermocouple itself.

                          R and S (and B) type are super expensive because of the material(platinum-rhodium). Around here industrial ones typically have 0.5mm diameter wires inside them and cost about 1000 euros per meter.

                          TCdirect is one of the cheapest sources for good thermocouples (not ebay specialities)
                          https://tcdirect.co.uk/product_2_190_1

                          3mm OD type-N with Nicrobell-D sheath will probably survive the rated 1260C for quite a while. For anything above that you really need type S/R/B
                          Location: Helsinki, Finland, Europe

                          Comment


                          • #14
                            Cheap S-type in Germany:
                            https://www.ebay.de/itm/Thermoelemen...QAAOSwLYBgBX1e
                            Location: Helsinki, Finland, Europe

                            Comment


                            • #15
                              There is also really cheap chinese S-types like this https://www.ebay.de/itm/S-Typ-Platin...k/333753728627

                              Wires on those are so thin that if you sneeze in the same room they break!
                              Location: Helsinki, Finland, Europe

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