By: Dave Dlugos, Product Marketing Leader, Temperature Products on May 1st, 2024

What is Thermocouple Calibration?

This article was originally published on July 24, 2023, by Rick Zerafin and was updated on April 29, 2024, by Dave Dlugos.

Maintenance, reliability, process and Health Safety Environment (HSE) teams rely on accurate and stable temperature measurements to meet their goals. Thermocouples, new out of the box, are a critical tool for these measurements that help plant personnel maintain safe and efficient operations.

Today, instrument designers use a wide array of thermocouple types, such as Type K, Type E and Type T to measure temperatures in applications ranging from cryogenic temperatures of -196 °C to the extremes of 1700 °C.

This article will discuss how thermocouple calibration offers added benefits that promote process improvements and support compliance requirements. When you are done reading, you will also find additional related resources that can help you gain more insight into these important instruments.

What is Thermocouple Calibration?

The most common type of calibration is a comparison between the sensor to be installed into a process at the plant and a reference sensor with a known error range. In critical sensor applications involving hazardous materials or extreme conditions, a certification is used to ensure an instrument is properly calibrated.

The reference sensor, typically a Platinum Resistance Thermometer (PRT), is certified through accredited laboratories to ISO 17025, and traceable through national laboratories such as NIST to the International Standard, ITS-90.

The comparison analysis between both the reference sensor and test sensor is completed by lab engineers and technicians. Calibration certificates are issued with the results and contain pertinent information on all test equipment and reference sensors used in the calibration process (see Figure 1).

Figure 1: Ashcroft® Sample Thermocouple Calibration Report.

certified calibration chart

UUT Readings Vs. Tolerance

The calibration report will indicate the error for the test sensor (UUT, Unit Under Test) at each temperature point as compared to the measurement of the reference PRT sensor.

For more information, please see our article: “How does a thermocouple work?”

Why Should I Calibrate My New Thermocouples?

So, why should new thermocouples be purchased with calibration certificates?

For most applications, products like the Ashcroft® S50 Industrial Thermocouple are manufactured to (ASTM) and (IEC) accuracy tolerance standards, which provide sufficient accuracy without the need to calibrate.

ASTM E230 provides two accuracy levels:

Standard Limits of Error
Special Limits of Error

IEC 6584 designates three accuracy tolerance classes:

Class 1
Class 2
Class 3

Industrial plants can have temperature points ranging from five hundred to over two thousand for large petrochemical complexes. Calibrating even a small percentage of the sensors involved in this kind of operation – plus managing the associated documentation – can be cost-prohibitive if it’s not approached strategically.

A careful review can help maintenance and reliability teams select temperature points where calibration costs can return significant, long-term benefits. With a strategic process for temperature point calibrations, quality and safety managers can expect benefits that include increased process safety, product quality, and equipment longevity. Preventive maintenance personnel should ensure temperature sensors operate at the specified level for each application.

The decision to take the additional step and calibrate new thermocouples depends on several factors including the application type. Critical applications have elevated risk levels associated with out-of-tolerance process variable measurements, plus the potential for environmental impact, damage to equipment or operators, or significant financial losses due to regulatory fines.

What industries and applications benefit most from thermocouple calibration?

The goal of calibration is to minimize temperature measurement uncertainty and reduce error to acceptable levels based on requirements for specific applications and industries. Industrial plants typically establish corporate specifications with variations at the local plant level to help meet industry guidelines, mitigate risk, avoid hazardous results, prevent product losses, reduce emissions, and increase safety for personnel and the environment.

Engineering, Procurement and Construction (EPC): Most new projects through EPCs will request calibration on instrumentation to ensure instruments are accurate and operating correctly. This is critical for a safe plant startup and benchmarking the instrument performance.

Afterward, instrumentation planners will limit calibration instrumentation installs to critical applications where a calibration certificate provides a high return on the associated costs.

Food & Pharmaceutical/Life Sciences: These industries rely on instrument calibration to meet federal agency requirements and maintain FDA Approvals.

Batch Pharmaceutical Applications: These applications use calibration programs to ensure measurements are within specified accuracies. Out-of-tolerance events, which can result in the rejection of a product, can be prevented with calibration programs that include appropriate calibration intervals. The correct calibration frequency will ensure that installed temperature sensors do not fall out of specification and provide inaccurate measurements.

Petrochemical, Chemical, Refining, and Energy Industries: These industries document and archive Process Variable Temperature measurements through control systems to comply with regulatory agencies like the Environmental Protection Agency (EPA).

For example, High Volume Thermal Incinerators in petrochemical and chemical applications must ensure temperatures are high enough to ignite critical gases like Carbon Monoxide (CO), Volatile Organic Compounds (VOCs), and other toxic compounds in their combustion chambers. Combustion chamber ignition temperatures can range from 980 °C to 1200 °C. Calibration reports are proof of a calibration process and help confirm temperature measurements are valid and supplied by a verified sensor.

Safety Shutdown Systems with Fast Time Response Control Loops: These are used in applications with harsh and demanding conditions, including processes involving cryogenic or elevated temperatures as well as those with corrosive media, to safely shut down operations in the event of a hazardous situation. Calibration is used to standardize thermocouples within a redundancy and voting logic package.

What are the benefits of calibration?

Strategic thermocouple calibration is all about maximizing the resources you put into calibrating your instruments to avoid operational errors, system malfunctions or costly compliance issues.

Here are a few more specific benefits:

Accuracy and precision

Calibration reports provide additional information that control systems personnel can use to increase the accuracy and precision of temperature measurements via correction factors. Tighter measurement accuracies help designers scale processes, maximize yields, and advance clean energy solutions.

Thermal oxidizers are an example of a system that recovers heat to reduce fuel intake as well as reroute heat transfer for steam generation. Accurate temperature measurements are necessary to monitor changes in the process and open or close valves to maintain efficient, safe operations.

Compliance requirements

Meeting these requirements ensures temperature sensors are manufactured to the quality needed to meet industry and regulatory standards. These processes are often audited by a third-party agency, so it’s not enough to simply think you comply.

Peace of mind

Both sensor and certificate are delivered to the plant, providing process operations, maintenance, reliability, and HSE personnel with an elevated level of confidence in a thermocouple that meets all pertinent national and international standards.

Final thoughts on thermocouple calibration.

Now that you know what thermocouple calibration is and its benefits, you can research the solution that’s best suited for your application. Keeping these factors in mind can help ensure that you avoid problems and keep your process running with reliable and accurate temperature measurements.

You deserve to feel confident in your measurement equipment. At Ashcroft, we understand that not all applications are the same and requirements can vary. We provide support for your everyday operational needs, MRO and turnarounds, working with engineering procurement construction (EPC), and corporate engineering firms on large capital projects with a dedicated support team.

If you want to learn more about thermocouples and other temperature sensors, check out some of our other blog posts:

Feel free to contact us today to talk to one of our industry experts and get your questions answered.

And if you’d like to learn more about thermocouples and temperature sensors, download our brochure:

About Dave Dlugos, Product Marketing Leader, Temperature Products

Dave Dlugos has a BSEE degree and 40 years of experience in the measurement industry performing design engineering and product management. He has earned 4 U.S. patents and joined Ashcroft in 2007, currently as the Product Marketing Leader for Temperature products. He is a senior member of the International Society of Automation (ISA), past ISA District 1, Vice President, ISA water and wastewater division board member and the President of CT Valley ISA Section.