Allied Encyclopedia: All About Thermocouple Wire – Part 2

Allied EncyclopediaWelcome to the second half of our “All About Thermocouple Wire” series. If you missed the first half, where we introduced thermocouples, explained their uses, and talked about the differences between thermocouple wire and extension grade thermocouple wire, go read “All About Thermocouple Wire – Part 1” now.

Types of Thermocouple Wire

The thermocouple wire you use will depend on the demands of the application and its environment. Before choosing a thermocouple type, you must consider the expected temperature range, response time and service life required by your application. It is also important to think about whether the thermocouple will be required to operate in a vacuum or an inert, oxidizing or reducing environment, as this will affect the accuracy, stability and service life of the thermocouple wire in use.

Thermocouple wires are broken down into types based on the metal alloys used in their construction. For instance, Thermocouple Wire Type K always has a positively charged Chromel leg and a negatively charged Alumel leg.

  • Type K– Probably the most common general purpose thermocouple type, Type K has the widest operating temperature range. It is constructed with Chromel and Alumel legs.
  • Type J– Made with Iron and Constantan (Copper-Nickel) wires, Type J thermocouples have a more limited temperature range but a higher sensitivity than Type K.
  • Type N– Type N is nickel-based, composed of a Nicrosil and a Nisil leg. Designed to be more stable and resistant to high temperature oxidation, Type N is often considered an “improved” Type K and is gaining popularity.
  • Type T– Stable at extremely low temperatures, Type T thermocouples are often used in cryogenics and ultra-low freezers. This type of thermocouple is made with Copper and Constantan.
  • Type E– Also suited to low-temp applications, Type E has the highest sensitivity of common thermocouples. Thermocouple Wire Type E is constructed with a Chromel leg opposite a Constantan leg.
  • Types S, R and B– The higher costs and lower sensitivities of these noble metal thermocouples restrict them to high temperature measurements for which they are well-suited. They are, however, among the most stable of thermocouples. This makes them ideal for calibration standards because they are reproducible. Type S, for example, is used as the standard calibration for the melting point of gold. Both legs of all three types are composed of varying ratios of Platinum to Rhodium.

Though valuable in extremely high temperature applications, Type B is virtually useless below 50°C because it gives the same output at 0°C and 42°, thanks to the shape of its            temperature/voltage curve.

  • Type C– A Type C thermocouple can be used at extremely high temperatures, but its use is limited to vacuum, hydrogen or inert atmospheres, due to its lack of resistance to oxidation. Tungsten-Rhemium compounds make up both legs of a Type C thermocouple, but the positive leg has a higher percentage of Tungsten than the negative leg.

Type

Positive Conductor

Negative

Conductor

Temperature

Range **

Environment/Uses

K

Chromel

(90% nickel-10% chromium)

 

Alumel

(95% nickel, 2% manganese, 2% aluminum, 1% silicon)

-200°C to 1250°C

  • Vacuum*, oxidizing, inert or reducing*
  • General purpose
  • Testing temperatures during production processes at plants, refineries, etc.
  • Testing heat appliance safety

J

Iron

Constantan

(45% nickel-55% copper)

0 to 750 °C

 

  • Vacuum, oxidizing* inert or reducing
  • General purpose
  • Manufacturing of plastics and resin

N

Nicrosil

(84.6% nickel, 14.2% chromium, 1.4% silicon)

Nisil

(95.5% nickel, 4.4% silicon, 0.1% magnesium)

0 to 1250°C

  • Vacuum*, oxidizing, inert or reducing*
  • General purpose Can be used in applications where Type K stability suffers from high temps,  oxidation and green rot
  • Thermal profiling in ovens and furnaces
  • Temperature measurement of gas turbines and engine exhausts
  • Testing temperatures during production/smelting process in metals industry

T

Copper

Constantan

(45% nickel-55% copper)

-200°C to 350°C

  • Vacuum, mild oxidizing, inert or reducing.
  • Good where moisture is present.
  • Monitoring in food processing
  • Cryogenic applications

E

Chromel

(90% nickel-10% chromium)

Constantan

(45% nickel-55% copper)

-200°C to 900°C

  • Vacuum*, oxidizing, inert or reducing*
  • High sensitivity

S

Platinum-Rhodium

(87% platinum-13% rhodium)

Platinum

0 to 1450°C

  • Inert or oxidizing
  • High temp applications
  • Lab use
  • Used in BioTech and Pharmaceutical industries

R

Platinum-Rhodium

(90% platinum-10% rhodium)

Platinum

0 to 1450°C

  • Inert or oxidizing
  • High temp applications
  • Industry use

B

Platinum-Rhodium

(70% platinum-30% rhodium)

Platinum-Rhodium

(94% platinum-6% rhodium)

870 to 1700°C

  • Inert or oxidizing
  • Extremely high temp applications
  • Used in glass industry

 

C

Tungsten-Rhenium

(95% tungsten-5% rhenium)

Tungsten-Rhemium

(74% tungsten-26% rhenium)

0 to 2300°C

  • Vacuum, inert or hydrogen
  • Extremely high temp applications

*limited use in these conditions

**approximate values for bare conductors

The temperature ranges in the chart are based on the capacities of bare-conductor thermocouple wire. The addition of insulation can increase these ranges. It may also improve the stability and service life of your thermocouple, though its reaction time will likely be reduced. Your insulation choice, too, requires careful consideration; each offers different benefits.

The above chart and descriptions should get you moving in the right direction, but make sure you do your homework. If you need more guidance about what kind of thermocouple grade wire or extension grade wire to use in your thermocouple, view more thermocouple wire specifications and talk to an expert at Allied Wire.

About awcwire
Allied Wire & Cable is a value-added manufacturer and distributor of electrical wire and cable, tubing, and more. We are a family owned and operated company, serving a wide range of industries, including the military, automotive, aerospace, and telecommunications markets. Allied is headquartered in Collegeville, PA. Additional locations can be found across the US, in Merrimack, NH, Tampa, FL, Pewaukee, WI, and Las Vegas, NV. For more information on Allied, visit our main website at www.awcwire.com.

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