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Everything You Need to Know About Controlling Your Kiln

Scott Shannon

Thermocouples: Differences Between Thermocouple Types

In order to successfully execute a kiln firing schedule, it’s imperative that your kiln controller is receiving accurate temperature readings from your kiln. This is where thermocouples come into play.

Thermocouples measure temperature and send that information to the kiln controller which then automatically adjusts the power of the kiln according to its preprogrammed firing schedule.

What is a Thermocouple

A thermocouple is a self-powered temperature monitoring device that converts thermal energy into electric current in order to accurately measure the temperature of a heat source. Simple, inexpensive, reliable, durable, and capable of measuring a wide range of temperatures, thermocouples are used in a wide variety of applications. From monitoring the temperature of kilns and ovens, to residential thermostats, automotive and aircraft sensors, industrial and scientific processes, and more, thermocouples are used everywhere.

How Do Thermocouples Work?

Thermocouples consist of two different types of metal (or alloy) wires that run parallel to each other and join together at the tip. When the tip of the thermocouple (also known as the thermocouple measuring junction or the hot junction) is exposed to temperature, the two wires heat up or cool down to different temperatures, generating electromotive force. This phenomenon is known as the Seebeck Effect.

The two metal wires also connect at the reference junction (or the cold junction), which is kept at a constant, known temperature. Historically, this known temperature was created by using an ice bath for a 32° Fahrenheit reference, but today this is accomplished with electronic sensors which allow for thermocouples to be used over a range of ambient temperatures.

The voltage created corresponds with the relative temperature difference between the two junctions, allowing a sensor to calculate the temperature at the measuring junction with an accuracy of within 1° or 2° C.

Factors That Affect Accuracy

While thermocouples are reliable and widely used, there are several factors that can impact the accuracy of their readings at any given point in time:

Size: The physical dimensions of a thermocouple can affect its response time and, therefore, instantaneous accuracy. Smaller thermocouples may exhibit faster response times due to their reduced mass, but they may also be more susceptible to measurement errors caused by thermal gradients or conduction losses.
Location: The placement of a thermocouple within a kiln or oven can influence the accuracy of its readings. Factors such as proximity to heat sources, shielding from external influences, and the ability to measure representative cold junction temperatures can all impact the reliability of the measurements.
Tolerance: Thermocouples have specific tolerances, which define the maximum allowable deviation from their specified temperature-to-voltage relationship. If a thermocouple exceeds its tolerance limits, the readings may become less accurate or unreliable. It is essential to select a thermocouple with an appropriate tolerance for the desired temperature range and application.
Self-heating: The heating of a thermocouple itself can introduce errors in the temperature measurements. Self-heating occurs when the current flowing through the thermocouple generates heat, which can lead to a temperature increase at the sensing junction. This self-heating effect can result in an offset or error in the measured temperature, particularly in low-temperature applications or when high currents are used.
Kiln or Oven State: When a kiln’s elements are on, the air near the elements becomes hotter than the rest of the air inside the kiln. This temperature gradient produces convection currents which swirl warmer and colder air around inside the kiln. As this air moves and passes over the thermocouple, it can cause swings in the temperature reading depending on the response time of the thermocouple. These convection currents are less problematic when the kiln’s elements are off and the internal air temperature becomes more homogenized.
Additional Factors: Other factors that can impact thermocouple readings include electromagnetic interference (commonly produced by switching mechanical relays), oxidation or contamination of the thermocouple junctions, mechanical stress or strain on the thermocouple wires, and the type of reference junction used for cold junction compensation.

To ensure accurate temperature measurements, it is crucial to consider these factors and choose the appropriate thermocouple type, size, location, and tolerance based on the specific application requirements, as well as the type of controller you’re using. Regular calibration and maintenance of the thermocouples are also recommended to verify their accuracy and detect any drift or degradation over time. Some temperature controllers, such as TAP Kiln Controllers by SDS Industries provide diagnostics and preventative maintenance alerts based on usage to ensure thermocouple accuracy.

What are the Different Types of Thermocouples?

There are a wide variety of thermocouple types that are suitable for different types of applications. Each type of type of thermocouple has different characteristics for temperature range, sensitivity, durability, vibration resistance, chemical resistance, and application capability depending on the physical properties of its metals.

Thermocouple types are named according to a lettering system. Thermocouple Types C, E, J, N, K, and T are composed of base metals and Types B, R, S, and P are composed of noble metals. Below we’ll be exploring the characteristics and applications of the different thermocouple types.

Base Metal Thermocouples

Base metal thermocouples are the most common types of thermocouple. These thermocouples are composed of base metals or alloys, such as iron, copper, nickel, and chromel.

Type C Thermocouples

Material: Tungsten-Rhenium (+ and -)
Temperature Range: 32 – 4208° F (0 -2320° C)
Accuracy/Limit of Errors: Standard: ± 1% or 4.5° C
Physical Properties: Type C thermocouples are capable of accurately measuring extremely high temperatures. However, they have no oxidation resistance, meaning they are only suitable for applications with vacuum, hydrogen, or inert atmospheres.
Applications: High Temperature Materials Manufacturing, Power Generation, Aerospace, Semiconductor Processing and Equipment, Military and Defense Testing

Type E Thermocouples

Material: Chromel (+) and Constantan (-)
Temperature Range: -454 – 1832° F (-270 – 1000° C)
Accuracy/Limit of Errors: Standard: ± .5% or ± 1.7° C, Special: ± .4% or ± 1° C
Physical Properties: Type E thermocouples are highly accurate with a fast response, even in sub-zero applications. They have the strongest signal and highest output of the base metal thermocouples and aren’t subject to corrosion at cryogenic temperatures (-238 – 460° F).
Applications: Gas Temperature Measurement, Cryogenics, Aerospace Industry, Applications with Magnetic Fields

Type J Thermocouples

Material: Iron (+) and Constantan (-)
Temperature Range: 32 – 1382° F (0 – 750°C)
Accuracy/Limit of Errors: Standard: ± .75% or ± 2.2° C, Special: ± .4% or ± 1.1° C
Physical Properties: Type J thermocouples are capable of accurate temperature monitoring in a vacuum or for inert materials. However, they are susceptible to oxidation at low temperatures or moist environments. While they are the least expensive general-purpose thermocouples, Type J thermocouples also have the shortest lifespan, and their accuracy will be permanently impaired if exposed to temperatures greater than 1400° F.
Applications: Plastic Manufacturing, Laboratory Processes, Ovens, Kilns, Furnaces

Type N Thermocouples

Material: Nicrosil (+) and Nisil (-)
Temperature Range: -450 – 2372° F (-270 – 1300°C)
Accuracy/Limit of Errors: Standard: ± .75% or ± 2.2° C, Special: ± .4% or ± 1.1° C
Physical Properties: Type N thermocouples have superior corrosion resistance and are capable of measuring high temperatures compared to other base metal thermocouples. However, they also have a slower response and lower sensitivity.
Applications: Refineries, Petrochemical Industry, Ovens, Kilns, Furnaces

Type K Thermocouples

Material: Chromel (+) and Alumel (-)
Temperature Range: -328 – 2282° F (-200 – 1250°C)
Accuracy/Limit of Errors: Standard: ± .75% or ± 2.2° C, Special: ± .4% or ± 1.1° C
Physical Properties: Reliable, accurate, inexpensive, with fast response across a wide rand of temperatures, Type K thermocouples are the most commonly used type of thermocouple. With oxidation resistance and radiation hardness, Type K thermocouples are extremely versatile. However, they shouldn’t be used in vacuum applications, low oxygen, or sulphuric environments.
Applications: Steel and Iron Industry, Petroleum Refineries, Nuclear Applications, Chemical Production, Ovens, Kilns, Furnaces

Type T Thermocouples

Material: Copper (+) and Constantan (-)
Temperature Range: -328 – 662° F (-250 – 350° C)
Accuracy/Limit of Errors: Standard: ± .75% or ± 1° C, Special: ± .4% or ± .5° C
Physical Properties: Type T thermocouples are extremely stable and capable of operating at extremely cold temperatures. However, it has a narrow temperature range compared to other thermocouple types.
Applications: Food Production, Cryogenics, Deep Freezing, Laboratory Processes

Noble Metal Thermocouples

Noble metal thermocouples use platinum alloys, making them accurate at extremely high temperatures – but also significantly more expensive!

Type B Thermocouples

Material: Platinum-Rhodium (+ and -)
Temperature Range: 32 – 3092°F (0 – 700° C)
Accuracy/Limit of Errors: Standard: ± 5% or ± .5° C
Physical Properties: Type B thermocouples are extremely accurate and stable at extremely high temperatures. They are corrosion-resistant and are suitable for oxidizing environments. However, they are susceptible to contamination and require appropriate protection.
Applications: Industrial Glass, Metal Melting and Pouring, Analytical Instrument Calibration, Nuclear Reactor Temperature Regulation, Semiconductor Processing and Equipment

Type R Thermocouples

Material: Platinum-Rhodium (+) and Platinum (-)
Temperature Range: 32 – 2642°F (0 – 1450° C)
Accuracy/Limit of Errors: Standard: ± .25% or ± 1.5° C, Special: ± .1% or ± .6° C
Physical Properties: With a higher percentage of rhodium, Type R thermocouples are more expensive than other noble metal thermocouple types, but also have a higher output and improved stability. They are resistant to oxidation as well as chemically aggressive material. However, they are susceptible to contamination and require appropriate protection.
Applications: Industrial Glass, Power Generation, Mining, Laboratory Processes, Temperature Sensors, Ovens, Kilns, Furnaces

Type S Thermocouples

Material: Platinum-Rhodium (+) and Platinum (-)
Temperature Range: 32 – 2642°F (0 – 1450° C)
Accuracy/Limit of Errors: Standard: ± .25% or ± 1.5° C, Special: ± .1% or ± .6° C
Physical Properties: Type S thermocouples are extremely similar to Type R thermocouples in regard to physical properties. However, a slightly lower percentage of rhodium makes Type S thermocouples slightly less stable.
Applications: Industrial Glass, Power Generation, Mining, Laboratory Processes, Temperature Sensors, Ovens, Kilns, Furnaces

Type P Thermocouples (Platinel II)

Material: Palladium-Platinum-Gold (+) and Gold-Palladium (-)
Temperature Range: 32 – 2543°F (0 – 1395° C)
Accuracy/Limit of Errors: Standard: ± .2mV (up to 1200° C)
Physical Properties: Type P thermocouples are designed to approximate the same curve as Type K thermocouples. They are extremely accurate and stable. Type P oxidation-resistant and can be used in inert atmospheres, but they are susceptible to contamination and require appropriate protection.
Applications: Regulating Gas Turbine Engines, Temperature Sensors, Ovens, Kilns, Furnaces

What Thermocouple Type Should I Use for my Kiln or Heat Treat Oven?

The type of thermocouple you should use for your kiln or heat treat oven largely depends on the temperature requirements of your firing schedules. Due to its durability, reliability, and accuracy across an extensive range, a Type K thermocouple is a popular choice for most kilns and heat treat ovens. Type K thermocouples are suitable for most heat treat applications.

However, some materials, such as porcelain, have temperature requirements that exceed the range for Type K thermocouples, in which case you may consider using a Type R or Type S thermocouple. The original TAP Kiln Controller by SDS Industries supports Type K, Type R, and Type S thermocouples. The TAP II, along with every other TAP Product by SDS industries, supports Type K thermocouples.

Pair Your Thermocouple with the Right Temperature Controller

The TAP and TAP II Controllers by SDS Industries are the most advanced, precise, and easy-to-use temperature controllers on the market today. With responsive touchscreen controls, an intuitive graphical UI, and integration with the TAP Kiln Controller Mobile App, TAP Kiln Controllers can pair with any relay-controlled kiln or oven to allow you to easily manage and execute your kiln firing schedules.

We invite you to explore our selection of digital kiln controllers, standalones, and conversion kits on our online store. You can also purchase TAP Digital Controllers or TAP-Controlled Kilns and Heat Treat Ovens through one of the following distributors: