Thermocouples: You can’t accurately heat treat without them. But how can you choose the best one for your needs? What do current regulations require? Read this helpful explanation, by Víctor Zacarías, managing director of Global Thermal Solutions Mexico, to find out how to choose the right thermocouple.
Keywords: Thermocouple, Heat Treatment, Pyrometry, Temperature Measurement and Control, AMS2750, CQI-9
Read the English version of the article below, or find the Spanish translation when you click the flag above right!
This Technical Tuesday article, first published in English and Spanish translations, is found in Heat Treat Today's February's Air & Atmosphere Furnace Systems print edition.
If you have any facts of your own about thermocouples, our editors would be interested in sharing them online at www.heattreattoday.com. Email Bethany Leone at bethany@heattreattoday.com with your own trivia!
Managing director
Global Thermal Solutions México
The SAE AMS2750 aerospace standard and the AIAG CQI-9, CQI-11, CQI-12, and CQI-29 automotive assessments are the universally accepted standards for temperature control in thermal processing operations. Among many things, they describe the requirements for the use and control of thermocouples used in process ovens and furnaces. In this article you will find the requirements of these regulations so that you can make a correct decision when choosing a thermocouple, and thus have a repeatable measurement that ensures a reliable process.
1. Application
For the appropriate selection of a thermocouple for the control and/or recording of temperature, you must first take into account the type of process. In choosing the right thermocouple, consider some factors that could alter its performance, such as:
- The temperature range at which it will be in use
- The type of atmosphere to which it will be exposed
- Possible electrical interference
- The accuracy required by the applicable specification, etc.
Based on the above, existing regulations refer to a specific classification for thermocouples based on their manufacture and final application. These classifications are:
a) Base thermocouples and noble thermocouples
b) Expendable and non-expendable thermocouples
2. Types of Thermocouples and Their Insulation
2.1 Base Thermocouple or Noble Thermocouple
A base thermocouple is made of basic alloys such as iron, chrome, nickel, copper, etc., and they are the most common types in the industry due to their versatility and cost. Base thermocouples are types K, E, J, N, and T. A good supplier of sensors will recommend a thermocouple based on the application, the temperature range, and your budget (see Table 1).
Source: GTS México
On the other hand, a noble thermocouple is made from metals such as platinum and rhodium: types R, S, and B thermocouples. These thermocouples are more stable at high temperatures and maintain their accuracy for a longer time. However, they have the highest cost since they are made from precious metals. Due to this nature, noble thermocouples are the preferred choice for vacuum heat treatment applications and high temperature processes.
2.2 Expendable or Non-expendable Thermocouples
The second criteria from the regulations are the material which protects the elements of the thermocouple.
Expendable thermocouples are those whose elements are covered by materials such as fiberglass, ceramic fabric, or polymeric coating and are generally provided in the form of a spool. This form allows the user to cut the cable to size and manufacture the thermocouple by joining the two wires by twisting or welding, making them ideal for single use applications such as a TUS test or charging thermocouples, for example (see Figure 1).
Source: Trucal, Inc.
In contrast, a nonexpendable thermocouple is normally protected with ceramic or mineral insulation and covered on the outside by a metallic sheath (the elements are not exposed in this configuration), which gives it a longer useful life. Therefore, it is preferred for use as a control or recording thermocouple (see Figure 2).
Source: GTS México
Whatever the application, when wiring interconnections are required for sensor installation, these connections must be made using standard connectors and terminals such as those shown in Figure 3, as both AMS2750 and CQI-9 prohibit the wiring splice.
Source: GTS México
3. Calibration
According to regulations, all thermocouples used in the heat treatment operation must have been calibrated before being used for the first time. The user of the thermocouple must ensure that they have calibrations traceable to a national laboratory such as the NIST in the United States or its equivalent in Mexico (CENAM).
Pyrometry standards defi ne the acceptable error ranges for thermocouples depending on their final application. These categories for final application include: standard thermocouples, test thermocouples (SAT and TUS), control and recording thermocouples, and load thermocouples (see Table 2). Table 2 describes the maximum errors allowed to be selected depending on the use of the sensor.
Source: GTS México
Once the thermocouple is installed, the person responsible for the heat treatment operation must document the date on which it comes into service, since the regulations establish the life of a sensor based on its application.
When receiving the report/certificate of the thermocouple, the user must review the content of the document, since the standards specifically define the minimum information that shall appear in a calibration report, which includes but is not limited to:
1. Test readings
2. Actual readings
3. Correction factors
4. Data source
5. Laboratory accreditation
6. Calibration method used
The calibration certificate can cover individual thermocouples or a group of thermocouples manufactured from the same lot (spool).
It is very important to note that both AMS2750 and CQI-9 require all calibrations to be conducted by ISO/IEC 17025 accredited organizations, so ensure that you review the accreditation certificate before selecting your supplier.
4. In Summary
If you’ve ever bought the wrong thermocouple, you know how annoying it can be. Therefore, here is a quick guide to select the right sensor for your application in five easy steps:
1. Define the type of thermocouple: base (K, T, J, E, N, and M) or noble (S, R, and B)
2. Define the type of insulation you require: textile fiber, polymer, ceramic, metallic, etc.
3. Specify the exact temperature range in which the sensor will operate
4. Specify the use of the sensor: standard thermocouple, SAT/TUS thermocouple, control/load thermocouple
5. Request the calibration certificate in accordance with the applicable regulations (AMS2750 or CQI-9)
References
ASTM International. ASTM E230, Standard Specification for Temperature-Electromotive Force (emf) Tables for Standardized Thermocouples, Rev. 2017.
Automotive Industry Action Group. CQI-9 Special Process: Heat Treat System Assessment, 4th Edition. June 2020.
International Organization for Standardization. ISO/IEC 17025, General Requirements for the Competence of Testing and Calibration Laboratories, 3rd Edition. 2017.
Nadcap AC7102/8 Audit Criteria for Pyrometry, Rev. A, 2021
SAE Aerospace. Aerospace Material Specifi cation AMS2750: Pyrometry, Rev. G, 2022.
About the Author: Víctor Zacarías is a metallurgical engineer from the University of Queretaro with studies in Strategic Management from Tec de Monterrey. With over 15 years of experience in Heat Treatment Management, he is currently the managing director of Global Thermal Solutions México. He has conducted numerous courses, workshops, and assessments in México, the United States, Brazil, Argentina, and Costa Rica. He has been a member of the AIAG Heat Treat Work Group (CQI-9 committee) and the SAE Aerospace Materials Engineering Committee.
Contact Víctor at victor@globalthermalsolutions.com
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