What Effects Does Green Rot Have on Type K Thermocouples

When it comes to precision temperature measurements, there is no substitute for the accuracy and reliability of Type K Thermocouples. But what happens when green rot sets in? In this blog, we will explore how green rot affects Type K Thermocouples and what can be done to prevent it.

Green rot is a severe issue of type K thermocouples, as it can lead to negative drifts, inaccurate measurements, and even open circuits due to embrittlement when exposed to temperatures between 800°C and 1,260°C (1,472°F to 2,300°F) in low oxygen environments.   However, in applications with abundant oxygen supply or no oxygen at all, green rot is not a problem. Therefore, to ensure accurate measurements and reliable results, it is essential to consider these factors when selecting a suitable thermocouple for the job.

Inexpensive, fast-responding, robust, and self-powered industrial thermocouples are ideal for temperature measurements in various applications. Constructed of two alloys, thermocouples comprise two wires connected at one end to create the measuring point (hot junction). Their fast response time and robust design make them suitable for industrial settings, while their self-powering ability eliminates the need for an external power source.   As a result, investing in industrial thermocouples is a cost-effective way to ensure accurate temperature measurements.

Thermocouples are sensors used to measure temperature and come in various types, each designated by a letter. Each type features a distinct temperature-versus-EMF (electromotive force) characteristic, depending on the particular combination of conductors. Some thermocouples are better suited to low-temperature environments, while others may not register an EMF until higher temperatures are reached. Knowing which type of thermocouple is best for your needs is critical to getting accurate temperature readings.

The most popular thermocouple for a wide array of applications is Type K, which is capable of accurate temperature readings from -200°C to 1,260°C (-328°F to 2,300°F).   This is because the positive leg of the thermocouple is composed of a nickel-chromium (NiCr) alloy, while the negative leg is made of a nickel-aluminum (NiAl) alloy (ferromagnetic).   This ensures reliable temperature readings, even in varying conditions.

Although type K thermocouples are rugged and long-lasting, they – like all thermocouples – can lose accuracy over time and in certain operating conditions. One of the issues that can affect the accuracy of these general-purpose temperature measurement devices is “green rot. This occurs when the insulation covering the thermocouple wiring begins to degrade, resulting in reduced accuracy and reliability. To prevent green rot and keep your thermocouples in optimal working condition, it’s crucial to ensure they are regularly inspected and maintained.

What is Type K Thermocouple?  

The Type K thermocouple consists of Chromel and Alumel conductors that meets the output necessities as said in ANSI/ASTM E230 or IEC 60584 for Type K thermocouples.  This may be an immersion sensor, a surface sensor, a wire, or another style of sensor or cable.

Type K wire is primarily available in two color codes, yellow connectors and yellow and red conductors per ANSI/ASTM E230, or green Connectors and green and white conductors per IEC 60584.

Pros

• To measure temperature, it provides good linearity of emf.
• It provides good resistance against oxidation below 1000 °C(1600°F).· Powerful output.
• Comparitively cost-effective than other thermocouples.

Cons

• Not suitable for reducing atmosphere but can withstand metallic.
• Aging of the emf characteristic when compared to noble metal thermocouples (B, R, and S).
• Not suitable for vacuum packaging as the chromium will evaporate within the excellent element.
• Green-Rotis phenomenon may occur due to low oxygen levels for the thermocouples, which are used between 815°C to 1040°C (1500 to to1900°F).
• Type K thermocouples should not be used in a sulphur environment. This is because both elements will corrode rapidly and the negative element will eventually fail mechanically by becoming brittle.

What is Green Rot?

Green rot, also known as chromium oxidation, is a quick process that occurs in high-temperature environments, usually between 800°C and 1,260°C (1,472°F and 2,300°F).   In low-oxygen environments, chromium oxidation can be accelerated further by hydrogen or other reducing agents, resulting in a layer of scaly green corrosion on the positive leg.   An oxide layer on the surface of the NiCr leg typically protects it from this oxidation process. As a professional, it is crucial to be aware of the conditions that lead to green rot to protect chromium-containing components from corrosion.

As oxidation corrodes the chromium, the usually non-magnetic NiCr leg becomes magnetic, decreasing the thermoelectric voltage at the reference junction (cold junction). The greater the corrosion, the larger the negative voltage drift, leading to substantial temperature measurement errors. Green rot also causes the NiCr leg to become more brittle. If not checked, the corrosion can cause the positive leg to break, opening the circuit and rendering the device inoperable. Professional maintenance and inspection are vital to avoiding these issues and ensuring the device continues to operate correctly.

What to Do About Green Rot

To minimize the formation of green rot and extend the lifetime of a type K thermocouple, it is essential to take preventive measures.

• The thermocouple’s protection tube or thermowell must be cleaned and removed from reducing compounds.
• Add additional oxygen to the thermowell.
• Injecting an inert gas into the thermowell.
• Incorporating a titanium conductor that absorbs oxygen.
• Cables with mineral-insulated sheaths (MI) should be used.

These precautions will minimise – but not eliminate – the formation of chromium oxide. This is why Type K thermocouples are best suited for applications in oxidising or inert gas environments. To ensure accurate temperature measurement in a low-oxygen environment, it is recommended to use another thermocouple type. For example, a type N thermocouple has a similar range as type K. Its positive leg is made of a nickel-silicon alloy, which provides an additional layer of protection from corrosion. As a professional, it is vital to consider the environment and thermocouple type to ensure accurate temperature measurement.

Thermocouples are a reliable and efficient method of temperature measurement. To ensure you get the most out of your thermocouple for your application, it’s essential to select the right one. Green rot and other issues can occur when the wrong thermocouple is chosen. At BCST, our experts can advise which thermocouple is the ideal choice for your application. Contact us today to discuss your needs.