FAQ (Frequently Asked Questions)

What is the unavoidable error (SRO: Short Range Ordering) of a K thermometer?

What is the unavoidable error (SRO: Short Range Ordering) of a K thermometer? If a type K thermocouple is inspected/calibrated for temperature after being used for some time at from around 250 to 550℃, a large error to the positive side may be generated. What are the cause and countermeasures?

■ Phenomenon
When a type K thermocouple is exposed to a temperature range of approx. 250 to 550℃, the thermo-electromotive force gradually increases at the exposed part, and when the insertion length is changed, a temperature higher than actual temperature is indicated higher by several ℃; however, the thermocouple returns to the original status when the temperature becomes approx. 650℃ or higher. This temperature range slightly differs depending on references. This phenomenon remarkably appears when the insertation deapth in the actual use differs from the insertion depth under calibration. If the insertion length is not changed, a large change in the measurement value does not appear.

■ Cause
Generally, the phenomenon occurs when the Seebeck coefficient reversibly changes due to SRO (Short Range Ordering) of the crystal structure. The part exposed to the temperature becomes heterogeneous due to an increase in the Seebeck coefficient because the arrangement of Ni and Cr, which are the elements of the type K thermocouple, becomes sequent at from 250℃ to 550℃. Providing a temperature gradient for the thermocouple by varying the insertion length to the heterogeneous part increases the thermo-electromotive force.
The increased amount of the thermos-electromotive force greatly depends on the temperature and time, causing the largest impact at the range of 350 to 450℃, and relatively large errors may be generated for a short period of time; for example, 0.9℃ for five min. for 350℃, 1.9℃ for 45 min., 3.7℃ for 7 hours, and 5℃ for 30 days. It is said that the ruled array of Cr is dominant; however, as for this phenomenon, heating at more than approx. 650℃ deletes the regularity, and the original status returns. The change in the temperature and the Seebeck coefficient is shown in the figure below.

■ Countermeasure
Since this phenomenon is a physical phenomenon of the type K thermocouple, it is fundamentally difficult to prevent this; however, providing heat treatment (Aging) allows the phenomenon to be less likely to be caused. In advance, a heat treatment is provided for a thermocouple with lower thermo-electromotive force near the temperature used in the actual site (e.g., 450℃) in order to provide SRO beforehand so that the thermocouple becomes stable within the tolerance of the temperature range. With this method, even if the thermocouple is used in a temperature range of 250 to 550℃, the thermo-electromotive force increase will be minor. However, since the original thermo-electromotive force is low, when a thermocouple is used at a temperature (approx. 650℃ or higher), which is above the SRO range, the original characteristics return, and the tolerance is deviated toward the negative side. However, it is difficult to obtain the wires of the thermocouple element with such characteristics.
Besides, when a thermocouple with SRO provided is inserted more deeply than the previous use, a normal value will be indicated.

■ Notes on Heat Treatment
The heat treatment is provided to alleviate SRO; however, attention needs to be paid since the sheathed thermocouple may be damaged due to the tempertature of the heat treatment and the materials of the thermocouple.
When austenitic stainless steel is heated in the range of 500 to 850℃ and cooled down, carbon (C) is extracted into the particle boundary organization, and chrome (Cr) is taken into the organization, resulting in the creation of chromium carbide, which easily causes particle boundary to be corroded. This treatment is called sensitization, which proceeds relatively quickly into a corrosive environment, and metals become fragile and easily damaged. Therefore, it is appropriate to select low carbon steel (SUS316L), stabilized austenitic stainless steel (SUS347 and SUS321), etc. for metal sheaths, which require heat treatment to prevent SRO.

■ References

• 1)

  A.W.Fenton : The travelling gradient approach to thermocouple research, Temperature Vol.4 -1972

• 2)

  3)Yoichi Tamura: Actual Usage “Thermoelectric Pyrometer” Measurement Technology-May, 1995

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