How long can a thermocouple be used? Will the measurement accuracy decrease over a long period of time?

Time and temperature are both measures of quality.

In daily industrial production, the accuracy of temperature measurement is crucial. As one of the commonly used temperature sensors, the long-term stability of thermocouples is a very important technical indicator, especially for enterprises with high maintenance difficulty (such as coal-fired power plants).

What is the long-term stability of a thermocouple?

Simply put, long-term stability refers to how long a thermocouple can be used after being installed in equipment without changing its accuracy. As users, we naturally hope that this time can be as long as possible.

Why do thermocouples experience measurement deviations?

After long-term use, thermocouples can indeed experience measurement deviations. This is mainly due to the following factors:

1. Material aging and contamination

Thermocouple wires may be contaminated by the environmental atmosphere or impurities in the protective tube during long-term use. Different levels of contamination will produce different additional potentials, thus changing the original calibration characteristics. For example, when using a platinum-rhodium 10-platinum thermocouple with a ceramic tube, if the ceramic tube contains iron impurities, the platinum-rhodium wire will be contaminated by iron, affecting its thermoelectric characteristics.

2. Changes at High Temperatures

At high temperatures, thermocouple materials may undergo changes such as grain growth and oxidation, leading to gradual alterations in thermoelectric properties. After prolonged use, the thermoelectric potential-temperature relationship of some thermocouples may drift, affecting measurement accuracy. Thermocouple wires are often made of alloys; due to differences in vapor pressure among the components, the alloy composition ratio changes over time at high temperatures, causing significant changes in thermoelectric potential.

3. Oxidation and Reduction Reactions

Many thermocouple instabilities are caused by the oxidation of the thermocouple wires. Copper-constantan, iron-constantan, and nickel-chromium-nickel-silicon thermocouples are all susceptible to oxidation. If the thermocouple is uniformly oxidized, the impact may be minor; however, if preferential oxidation occurs (e.g., chromium in a nickel-chromium electrode undergoes preferential oxidation under low oxygen partial pressure), it will alter the wire composition, causing severe effects.

4. Mechanical Stress and Embrittlement

During use, thermocouples may experience internal stress due to external forces. These stresses can lead to physical inhomogeneities. In addition, embrittlement is a common factor leading to thermocouple failure, mainly caused by contamination, grain growth, oxidation, and recrystallization at long-term high temperatures.

What factors affect the lifespan of a thermocouple?

The lifespan of a thermocouple is limited by several factors:

Material quality and characteristics: High-quality thermocouple electrode materials and protective sheath materials can effectively extend the lifespan. For example, platinum-rhodium thermocouples, due to the high stability and oxidation resistance of the platinum-rhodium alloy, can be used for a relatively long time under suitable conditions.

The severity of the operating environment: In extreme environments such as high temperature, high pressure, strong corrosion, and high flow rate, the wear rate of thermocouples will accelerate.

Installation and maintenance methods: Improper installation may lead to mechanical stress concentration or poor heat conduction, while lack of regular maintenance will cause the thermocouple performance to gradually decline.

How to improve the long-term stability of a thermocouple?

Material selection is key

One of the most important factors determining the long-term stability of a thermocouple temperature sensing element is the selection of materials. For example, the wire produced in Germany incorporates other rare elements in its chemical composition, giving it excellent accuracy and long-term stability.

The Importance of Manufacturing Processes

The formulation and implementation of manufacturing processes also significantly impact the long-term stability of thermocouples. This includes:

How to properly weld the thermocouple wires and ensure adequate insulation protection

Maintaining a stable temperature and humidity environment in the production process

Each product must undergo at least three tests

Proper Installation and Regular Maintenance

Proper installation and regular maintenance are crucial for maintaining the measurement accuracy of thermocouples. This includes:

Installing in a location that accurately reflects the temperature of the measured medium

Avoiding strong mechanical vibrations

Using within the specified temperature range

Perform regular inspections and cleaning

Replacing damaged sensing elements promptly