Type K thermocouple wiring & splicing

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asavage
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Type K thermocouple wiring & splicing

#1

Post by asavage »

As ubiquitous as Type K thermocouples are, there's still a lot of misinformation available about them.

This seems a good reference, from what I can determine: https://www.thermometricscorp.com/typek.html


TYPE K THERMOCOUPLE (Chromel / Alumel)


(US) YELLOW jacket: Positive leg, Chromel, is approximately 90% nickel, 10 chromium.

(US) RED jacket: Negative leg, Alumel, is approximately 95% nickel, 2% aluminum, 2% manganese and 1% silicon. MAGNETIC due to the nickel content.

(US color coding is ANSI and is not universal across all geographies and industries)
Type K wiring color identification
Type K wiring color identification
Thermocouple_Type_K_Wiriing_Color_Code_01b.png (142.27 KiB) Viewed 645 times
Type K Thermocouple Characteristics
Type K Thermocouple Characteristics
Thermocouple_Type_K_Characteristics_01b.png (75.67 KiB) Viewed 645 times
Type K Thermocouple Characteristics
Type K Thermocouple Characteristics
Thermocouple_Type_K_Characteristics_02b.png (62.35 KiB) Viewed 645 times

Thermocouple wire is not copper wire, and there is a matching type of thermocouple wire for each type of thermocouple. Type K thermocouple wire is two-conductor (for a single thermocouple) and the each of the two conductors is composed of different metallurgy, to match the thermocouple's native composition.
Thermocouple circuit with splice
Thermocouple circuit with splice
Thermocouple_circuit_01b.png (208.88 KiB) Viewed 645 times


Introducing extra cold junctions opens the potential for those junctions to introduce errors in the signal that cannot be compensated by calibration, especially if there is a temperature gradient across the pair of junctions.

There are a couple of design approaches to extending TC signals. The TC loop (ie the length of the wire run x 2) should not exceed a total resistance of 100 ohms. Thermocouple wire is manufactured in a couple of different grades so you if you want to have a long TC signal wire run you will want to know if you're running standard TC wire, Special Limits TC wire, or Extension TC wire (to name a few) and do the calcs. Generally, Extension grade TC wire has a much lower working temperature range as well.

Best design practice is to either keep the wire run short, or as soon as practical you can convert the signal from millivolt (thermocouple signal) to 4-20ma loop via a converter transmitter. If that isn't practical, using shielded TC wire is a good design element, because when working with millivolt signalling, EMI can be a significant factor.

Besides using TC wire to extend a run, there can be other reasons that you may need to cut/splice/join TC wiring. Sometimes, you need a detachable point in the wiring for serviceability. In this case, quick-disconnects that feature Chomel/Alumel terminals and contacts are widely available.

Type K Thermocouple miniature male connector
Type K Thermocouple miniature male connector
Thermocouple_Type_K_Miniature_Connector_Male_01b.jpg (99.51 KiB) Viewed 645 times
Type K Thermocouple miniature female connector
Type K Thermocouple miniature female connector
Thermocouple_Type_K_Miniature_Connector_Female_01b.jpg (161.57 KiB) Viewed 645 times

However, what if you don't want/desire to have a quick disconnect? Maybe you need to extend the TC wiring, cut & splice for repair. Another approach is to use a TC terminal block, either surface-mount or DIN rail mount.

Omega BS16A Thermocouple terminal strip
Thermocouple terminal strip
Thermocouple terminal strip
Thermocouple_Terminal_Lugs_BS16A_01b.png (191.27 KiB) Viewed 645 times
Omega XBTKK25 DIN rail terminal block
Thermocouple terminal DIN rail
Thermocouple terminal DIN rail
Thermocouple_Terminal_Block_XBTKK25_01b.png (182.24 KiB) Viewed 645 times


One topic is especially contentious: splicing thermocouple (TC) wires. It's difficult to find good information on a plain butt connector for inline splicing of TC wire, but it does exist. Military and aerospace people have this stuff down.

AC 21-99 Aircraft Wiring and Bonding ("THERMOCOUPLE WIRE SOLDERING AND INSTALLATION", Section 2, Chapter 16, begins at page 351)
Advisory-Circular-AC-021c99-AIRCRAFT-WIRING-BONDING-.pdf]AC 21-99 Aircraft Wiring and Bonding, Sect. 2 Chap. 16 Para. 31 Materials wrote:
Qty 1 D-436-133-01 Chromel Splice, Colour Coded Gray.

Qty 1 D-436-133-02 Alumel Splice, Colour Coded Green.

Qty 2 D-436-133-03 Splice Sealing Sleeves.
Thermocouple Wiring Butt Splicing Procedure
Thermocouple Wiring Butt Splicing Procedure
Thermocouple_Wiring_Butt_Splicing_Procedure_01b.png (282.88 KiB) Viewed 645 times

Those butt connectors sort of translate to TE Connectivity numbers.
Thermocouple Wiring Butt Connectors chart
Thermocouple Wiring Butt Connectors chart
Thermocouple_Wiring_Butt_Connectors_01b_ENG_CD_322325_D-1949621-1.png (91.92 KiB) Viewed 645 times
Thermocouple Wiring Butt Connectors chart
Thermocouple Wiring Butt Connectors chart
Thermocouple_Wiring_Butt_Connectors_02b.png (189.7 KiB) Viewed 643 times
AMP has been part of E Connectivity, since 1999, and this product line is their "Strato-Therm" terminals.

The Type K Positive/Chromel/Yellow (Gray) butt connector is AMP/TE Conn. 1-322325-1 (OnlineComponents.com, $4.26 ea, min. qty. (10))

The Type K Negative/Alumel/Red (Green) butt connector is AMP/TE Conn. 1-322325-0 (OnlineComponents.com, $2.32 ea, min. qty. (10))

I just ordered (10) of each for $80 after shipping and tax.


It seems very common for people to try to solder, use twist connectors, or standard (dissimilar metals) screw terminal blocks to splice thermocouple wiring. From what I have read, the most-correct method is to use TC wire butt connectors whose metallurgy matches the TC wires to be spliced. This method avoids creating extra cold junctions in the TC wiring circuit.
Regards,
Al S.

1982 Maxima diesel wagon, 2nd & 4th owner, 165k miles, rusty & burgundy/grey. Purchased 1996, SOLD 16Feb10
1983 Maxima diesel wagon, 199k miles, rusty, light yellow/light brown. SOLD 14Jul07
1981 720 SD22 (scrapped 04Sep07)
1983 Sentra CD17, 255k, bought 06Jul08, gave it away 22Jun10.
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