Contents
Ten notes, one question each.
- How to choose a thermocouple type: K vs E vs J vs T
- Constantan vs manganin – when to pick one over the other
- Cu-Ni (NC) vs constantan – low resistance vs low temperature coefficient
- Pure nickel N4 vs N6 – which purity grade for your application
- Monel 400 vs copper-nickel (NC) – corrosion-first vs value-first
- NiCr 80/20 vs NiCr 60/15 – the type K positive leg
- Choosing the temper: soft (M) vs half-hard (Y2) vs hard (Y)
- Shunt resistor material: Cu-Ni (NC), constantan or manganin
- Choosing wire for a mineral-insulated (MI) thermocouple cable
- What to specify before you quote precision alloy wire – five fields
Note 01
How to choose a thermocouple type: K vs E vs J vs T
The four types differ in range, sensitivity and the atmosphere they tolerate. Pick the type by whichever constraint is binding for your process - not by habit.
| Type | Positive leg | Negative leg | Range | Sensitivity | Best-fit duty |
|---|---|---|---|---|---|
| K | NiCr | NiSi | -200 … +1350 °C | Medium | General industrial, widest range |
| E | NiCu | Constantan | -200 … +900 °C | Highest | Low-to-mid temperature where sensitivity is critical |
| J | Iron | Constantan | -210 … +760 °C | Low-medium | Reducing atmospheres, low-to-mid temperature |
| T | Copper | Constantan | -250 … +350 °C | Medium | Lowest temperature, highest precision of the four |
Decision rule - pick the type by the binding constraint
- Range is the constraint -> K. It is the only one of the four rated to about 1350 °C.
- Sensitivity at low-to-mid temperature is the constraint -> E. Highest EMF output, so the smallest signal error when the instrument is limited.
- A reducing or acid-fume atmosphere is the constraint -> J. The iron leg is the only one that tolerates it.
- Lowest temperature and highest accuracy is the constraint -> T. The copper leg is the most precise at the bottom of the range.
Common cross-choices
- K vs E around 800 °C: K for durability and range; E when the instrument needs maximum signal and the range can stop at 900 °C.
- K vs J: J wins only in a reducing atmosphere; otherwise K is broader.
- E vs T below 350 °C: T for the lowest, most stable reading; E when you need sensitivity up to 900 °C.
- K vs T above 350 °C: K - the copper leg of T cannot reach the upper range.
Note 02
Constantan vs manganin - when to pick one over the other
| Property | Constantan 6J40 (Cu-Ni) | Manganin 6J8 / 6J12 / 6J13 (Cu-Mn-Ni) |
|---|---|---|
| Composition | Cu approx. 55 %, Ni approx. 45 % | Cu base with Mn and Ni |
| Resistivity @ 20 °C | High, similar magnitude - per batch report | High, similar magnitude - per batch report |
| TCR @ 20 °C | Near zero | Very low (slightly lower than constantan) |
| Thermal EMF vs copper | Low | Very low; 6J13 the lowest |
| Standard role | E / J / T thermocouple negative leg; wire-wound and sampling resistors | Standard resistors, precision shunts, ratio bridges, metering |
Decision rule
- A thermocouple negative leg (type E, J or T) -> constantan. It is the specified negative-leg metal. Do not substitute manganin or the reference table no longer matches.
- A standard resistor, precision shunt, metering element or ratio bridge, with the lowest TCR and thermal EMF required -> manganin, and 6J13 where thermal EMF against copper must be as small as possible.
- A wire-wound or sampling resistor -> constantan, or a Cu-Ni NC grade when the target resistivity sits far from the 0.5 µΩ·m region.
- Cost matters and the duty is a general power or sampling resistor -> constantan; keep manganin for precision metrology.
The one rule that ends most debates: constantan is chosen by application, manganin is chosen by a precision requirement. No precision metering requirement? Default to constantan.
Note 03
Cu-Ni (NC) vs constantan - low resistance vs low temperature coefficient
Both are copper-nickel alloys, but they solve different problems: one hits a resistance value, the other holds it steady.
| Property | Cu-Ni NC003 - NC050 | Constantan 6J40 (Cu55Ni45) |
|---|---|---|
| Composition family | Graded Cu-Ni series | Fixed approx. 55 Cu / 45 Ni |
| Resistivity @ 20 °C | approx. 0.03 - 0.50 µΩ·m (graded, per GB/T 6454-2008) | Confirmed on the batch inspection report |
| Primary trait | Low, controllable resistance value | Low temperature coefficient (TCR) |
| Role | Shunt / sampling / power / braking resistor, load bank | Thermocouple leg, sampling and wire-wound resistor, metering |
Decision rule
- A specific low resistance value, with normal TCR acceptable -> NC series. Pick the grade whose resistivity hits your target; more nickel means higher resistivity.
- Resistance that must stay constant across a temperature swing -> constantan. Its TCR is near zero, so the value barely moves.
- A thermocouple negative leg -> constantan. NC grades are not used as thermocouple legs.
In one line: NC means "I need a target resistance value". Constantan means "I need a temperature-stable resistance".
Note 04
Pure nickel N4 vs N6 - which purity grade for your application
| Property | N4 · Ni201 | N6 · Ni200 |
|---|---|---|
| Nickel content | ≥ 99.0 % | ≥ 99.5 % |
| Resistivity @ 20 °C | Low - per batch inspection report | Low - per batch inspection report (the lowest of the two) |
| Oxidation resistance | Good | Excellent |
| Sealing and joint integrity | Good | Best, to glass and ceramic |
| Relative cost | Lower | Higher (higher purity) |
Decision rule
- Conductivity, low-loss leads, or sealing to glass or ceramic is the priority -> N6. The extra purity lowers resistivity and gives the most reliable seal.
- General nickel duty at lower cost (mesh, woven cloth, ordinary corrosion parts, ordinary leads) -> N4.
- A thermocouple leg is needed -> use the thermocouple family. Pure nickel is a lead material, not a thermocouple metal.
Note 05
Monel 400 vs copper-nickel (NC) - corrosion-first vs value-first
| Property | Monel 400 | Cu-Ni NC series |
|---|---|---|
| Composition | approx. 70 % Ni - 30 % Cu | Graded, Cu-rich through Ni-rich |
| Resistivity @ 20 °C | High - per batch inspection report | approx. 0.03 - 0.50 µΩ·m (graded) |
| Primary trait | Corrosion resistance in aggressive service | Low, controllable resistance value |
| Role | Structural and fastening wire, marine and nuclear, specialty welding | Shunts, sampling and power resistors, load banks |
Decision rule
- Corrosion resistance is the binding constraint (seawater, hydrofluoric acid, alkalis, chemical or nuclear-adjacent service) -> Monel 400. NACE MR0175 by arrangement for sour service.
- A specific low resistance value or a resistor function is needed -> NC series. Monel's resistivity is too high for most low-resistance shunt work.
- Strength at moderate temperature in a corrosive environment, with no resistance function -> Monel. A copper-rich NC grade would corrode faster.
In one line: pick Monel when the environment is the specification; pick an NC grade when the electrical value is the specification.
Note 06
NiCr 80/20 vs NiCr 60/15 - the type K positive leg
These are composition variants of the type K positive-leg alloy, not two different thermocouple types. The numbers refer to the nominal nickel and chromium portions of the positive metal; the balance is the fixed silicon and manganese content that gives type K its EMF behaviour.
| Variant | Nominal positive-leg make-up | Typical use |
|---|---|---|
| NiCr 80/20 | Approx. 80 % Ni / 20 % Cr | Standard type K; widest availability; the conventional instrument and probe wire |
| NiCr 60/15 | Lower nickel, higher-alloy positive-leg variant | Duty where the alloy must be shifted - a higher-chromium window for hot corrosion, or a specific range and EMF trade-off |
Decision rule
- Standard industrial type K, and the published reference tables (IEC 60584-1 / ASTM E230) must match -> NiCr 80/20. The published EMF tables assume the conventional positive leg.
- A positive leg tuned for a specific high-temperature or hot-corrosion window -> NiCr 60/15.
- The reading must be correct against the published type K tables -> stay on 80/20. A variant leg changes the EMF curve and must be calibrated, not treated as a drop-in.
In one line: default to NiCr 80/20 for standard type K; move to 60/15 only when the operating window demands the alloy shift, and re-pair the negative leg and re-verify against the reference table, because a composition change is not a calibration-neutral swap.
Note 07
Choosing the temper: soft (M) vs half-hard (Y2) vs hard (Y)
| Temper | Ductility | Tensile and spring | Best for |
|---|---|---|---|
| M (soft / annealed) | Maximum | Lowest | Winding, braiding, mesh, most resistance elements |
| Y2 (half-hard) | Moderate | Moderate | Parts that need some stiffness but must still bend or wind |
| Y (hard / as-drawn) | Low | Highest | Structural wire, electrodes, parts that must hold their own shape |
Decision rule
- You will wind it (mesh, cloth, resistor, coil) -> M. Maximum ductility means the most passes without fracture.
- It must keep a formed shape but still be worked a little -> Y2.
- It is structural, an electrode or a spring -> Y. Highest tensile; use where the wire must not creep.
Practical notes
- Finer diameters fracture more easily in hard temper, so at the ultra-fine end (0.018 mm) most duty stays in M or Y2.
- A single order can mix tempers per spool if the application varies - state the split on the drawing.
- Temper affects spring-back in forming and therefore final part dimensions; call this out with the tolerance when a fitted part is involved.
Note 08
Shunt resistor material: Cu-Ni (NC), constantan or manganin
| Material | Resistivity @ 20 °C | TCR | When it wins |
|---|---|---|---|
| Cu-Ni NC grade | 0.03 - 0.50 µΩ·m (graded) | Normal | A specific low resistance value; power, braking and load-bank duty |
| Constantan 6J40 | Per batch inspection report | Near zero | A temperature-stable shunt in a metering range; general sampling resistor |
| Manganin 6J12 / 6J13 | Per batch inspection report | Very low | Standard resistors, precision shunts, lowest thermal EMF against copper |
Decision rule
- The shunt must hit a target resistance value and carry power -> a Cu-Ni NC grade. More nickel means higher resistivity.
- The shunt is in a metering range and its value must stay constant as temperature swings -> constantan 6J40. Near-zero TCR means the reading does not drift with self-heating.
- A standard or precision shunt, or a metering bridge, where thermal EMF against the copper connecting wire must be minimised -> manganin, and 6J13 for the lowest thermal EMF.
In one line: value-driven (power, load bank) -> Cu-Ni NC. Temperature-stability-driven (metering range) -> constantan. Precision or lowest thermal EMF (ratio bridge) -> manganin 6J13. Confirm TCR against the self-heating limit before locking the grade.
Note 09
Choosing wire for a mineral-insulated (MI) thermocouple cable
An MI cable pulls bare thermocouple legs through a sheathed conductor. The wire has to meet three requirements at once: a thermocouple type matched to the duty temperature, a fine enough diameter for the sheath, and consistent thermal EMF along the whole cable.
Decision rule
- The duty temperature range. General industrial up to about 1350 °C -> type K, the default MI cable core. High sensitivity at low-to-mid temperature -> type E. Reducing atmosphere or acid fumes -> type J. Lowest temperature and highest precision -> type T.
- The conductor diameter. Small-diameter sheaths need the fine end of the 0.10 - 6.00 mm range; larger sheaths take a mid-to-large gauge. Confirm the gauge against the sheath inner diameter so the legs and filler compress cleanly.
- The temper. MI conductors are drawn into the sheath, so soft temper (M) is the correct condition - maximum drawability, least fracture risk.
In one line: pick the type from the temperature range and atmosphere, use the soft temper, match the gauge to the sheath inner diameter, and order the legs as a matched pair so the thermal-EMF behaviour of the whole cable is consistent.
Note 10
What to specify before you quote precision alloy wire - five fields
Most late-stage failures are not the wire. They are a specification that was never pinned down: a tolerance that silently widened, a document the receiving QA asks for that was never quoted, or a paired thermocouple shipped as two unmatched spools. Fix these five in writing before quotation.
- Grade or family. Which of the six families, and the exact grade - 6J40 constantan, N6 pure nickel, NC025, type K legs. If you only know the application (a shunt, a thermocouple, a heating lead), state that and we will propose grade and gauge.
- Diameter and tolerance. Nominal diameter within 0.018 - 6.00 mm, and the tolerance window. The tighter the window, the more draw passes and the lower the yield - a tighter window costs. State the window you actually need.
- Temper. M (soft) for winding and drawing, Y2 for stiffness-with-work, Y for structural duty that must hold shape.
- Quantity and entry point. A 100 g sample supplied at cost, minimum order 1 kg, production pricing from 5 kg. There is no container minimum; freight cost per kilogram is what scales, so consolidating the first order beats splitting it.
- Document requirement. Material test certificate (standard with every shipment), EN 10204 3.1 inspection certificate, third-party (CNAS) test report, thermal-EMF pairing data, RoHS and REACH declarations. Set the exact standard, edition and certificate type before the order - it affects lead time and price, not just paperwork.
In one line: send grade, diameter and tolerance, temper, quantity, and the exact document list - in writing. If you only know the application, send that and let us propose the grade and gauge, then confirm the spec in writing before anyone commits.
Next step
Send the grade, diameter, temper and quantity.
Tell us what the wire has to do and the date your line needs it. We come back with a confirmed lead time, the achievable tolerance and the documents we can issue. Sample 100 g at cost; orders from 1 kg; production pricing from 5 kg.