Thermocouple Stability - Understanding Drift

Base-metal thermocouples, ANSI standard Types J, K, T, E, have innate thermoelectric instability related to time or temperature-dependent instabilities in many of their chemical, physical,  and electronic properties. 

When a thermocouple is used to measure the temperature of a particular environment, it can be expected that the measured voltage does not change if the temperature of that environment remains constant.  Actually, the voltage can change over time, even though the temperature of the environment remains constant:  this phenomenon is called DRIFT.  Drift is a source of error in thermocouple measurement. 

Drift occurs because of metallurgical changes in the conductors during the operation of the thermocouple.  Because these changes are time dependent, the voltage change from the expected value, called drift, is also time dependent.  An example of drift is shown below, in Figure 10, for a 1.5mm bare wire Type ‘K’ thermocouple exposed at 500ºC.  The change in voltage is reported as a function of the exposure time. 

Metallurgically, drift can be distinguished in the following:

• Surface modifications, which are related to changes in the conductors because of interactions between the conductors and the environment around the them.
• Bulk modifications, which are related to changes in the volume of the conductors.

Some examples of surface modifications can be identified below: 

• Oxidation (bare wire configurations)
• Depletion of elements from the conductors [bare wire/mineral insulated metal sheathed(MIMS)]
• Contamination from the environment (bare wire/MIMS configuration)
• Interaction with the insulator (MIMS configuration)
• Interaction with the sheath (MIMS configuration)

In relation to bulk modifications, the following phenomena can occur: 

• Phase transformations
• Short/long range ordering transformations
• Grain growth
• Residual strain and dislocations annihilation
• Recrystallization

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References 

{1} R.E. Bentley, “Long-term drift in mineral-insulated Nicrosil-sheath type K thermocouple”. Sensor and Actuators A, 24(1990) 21-26

Balancing Creativity, Design, Engineering, Cost, and Quality to Meet a Customer’s Unique Specifications Is the Ultimate Challenge

Custom temperature sensor production is the design, engineering, and manufacturing process of goods based on the unique specifications of a customer, including build-to-order parts, one-offs, short production runs, as well as mass customization. If you can't produce it, developing an idea for a product is meaningless. An understanding of design, materials, application and budget is required to manufacture your product correctly. 

There are, however, many unknowns that can drive up expenses and create confusion. Between design and engineering, materials and applications, production and installation, there is often a knowledge gap. At Duro-Sense, we add a knowledge layer that fills these crucial gaps because we bring a unique blend of design, application engineering, product expertise, and teamwork. By making it easier for engineers, designers, manufacturers, and clients to communicate and work together towards project success, we deliver enhanced value to any project.

Duro-Sense Corporation
310-533-6877
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Fast Selection Criteria between Thermocouples and RTDs

Both RTD and thermocouple probes monitor temperature, but which one is right for your application?

What temperature range are you trying to monitor?

Generally, if the temperature is above a hundred and fifty degrees Celsius, a thermocouple would be used. For anything below a hundred and fifty degrees Celsius, choose an RTD.

What is the required sensor accuracy? 

RTDs provide more accurate readings with repeatable results. Choose RTDs when temperature accuracy and repeatability are critical. 

What is the purchase budget?

Thermocouples can be up to three times less expensive than RTD sensors, making thermocouples the right choice when purchasing large quantities or requirements on tight budgets.

Use these three criteria to narrow down your selection process. There are many other differences between thermocouples and RTDs you need to understand before selection and application.

Always consult a temperature sensor application expert before installing or specifying a thermocouple or RTD where failure can cause harm or personal injury.

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100-Ohm Platinum RTD Temperature Sensors

RTDs (short for Resistance Temperature Detectors) are temperature sensing devices that change resistance value as its temperature changes. The most popular RTD is the 100-ohm platinum sensor, used for many years to measure temperature in laboratory and industrial process applications. 100-ohm platinum RTDs (PT100) have a reputation for accuracy, repeatability, and stability.

Most RTD element designs include a length of finely coiled wire wound around a ceramic or glass bobbin. The inherent system is fragile, so it is typically placed inside a sheathed metallic tube to protect from shock and vibration. RTD sensing elements are made from a material with a very predictable and repeatable change in resistance. This predictability and stability is the basis for its widespread application.

The 100-ohm platinum RTD provides accurate temperature readings with reasonable accuracy, excellent stability, and repeatability. They are also significantly immune to generated electrical noise, and as such, they are well suited for temperature measurement in industrial plants, near motors, generators, and high voltage equipment.

There are two 100-ohm platinum RTD standards, the American and the European (known as the DIN or IEC standard), with the IEC standard considered the default for PT100. The IEC751 standard requires the RTD to have an electrical resistance of 100.00 O at 0°C and a TCR (temperature coefficient of resistance) of 0.00385 O/O/°C between 0 and 100°C.

Because 100-ohm platinum RTDs use resistance to measure temperature, the lead wires, connectors, and measuring devices introduce additional resistance. These must be compensated for by configuring the RTD circuit to null out these outside resistances by incorporating a third or fourth lead wire to offset the introduced error.

For more information about 100-ohm platinum RTD temperature sensors, contact Duro-Sense Corporation. Call them at 310-533-6877 or visit their website at https://duro-sense.com.

Temperature Sensors Used for Power Generation

In an electrical generating plant, most temperature measurements are performed with RTDs (resistance temperature detectors) and thermocouples (T/Cs). 

RTD's are sensors that produce a measurable change in electrical resistance, while thermocouples have a change in mV signal in response to temperature change. 

RTD's consist of a thin conductor (nickel, platinum, copper) wrapped around a glass or ceramic bobbin, placed into a protective sheath, and backfilled with an electrically inert material but thermally conductive. 

Power plants use 100-ohm platinum, 100-ohm nickel, 120-ohm nickel, and 10-ohm copper RTDs. Though offering excellent accuracy and long-term reliability, RTDs are vulnerable to mechanical shock and vibration in a generating plant. They are more costly than thermocouples and are generally limited to about 1110 ° F. A very appealing feature for RTDs is their electrical noise immunity, a significant advantage over thermocouples. Finally, inexpensive instrument wire is all that is required to connect the RTD to the measuring instrumentation. 

A thermocouple consists of two wires made of dissimilar alloys, joined at both ends. One junction is coined the "hot junction," the other is the "cold junction" (or reference junction). When the hot junction experiences temperature change, a voltage is produced proportional to the temperature difference between hot and cold junctions. 

T/Cs are made of various alloy combinations and "calibrations" for different temperature ranges. Type J, K and N are the most common thermocouples for power generation applications below 1800 ° F; R and S types are common for applications above 1800 ° F. Besides the evident higher temperature capacity, thermocouples have a quicker response and greater endurance to shock and vibration. However, thermocouples are more susceptible to conducted and radiated electrical noise due to the minute signals generated. Another problem with thermocouples is their deterioration over time when used at high temperatures, hence being less stable than RTDs. One final concern is running an expensive thermocouple extension wire of the same type as the sensor-measuring instrument thermocouple.

Duro-Sense Corporation
310-533-6877
https://duro-sense.com

Temperature Sensors for Aerospace and Space Exploration

Duro-Sense Corporation provides the precision temperature sensors to the aerospace, aviation, and space industries and has a reputation for manufacturing the finest quality temperature sensors available. 

Robert J. Collier Trophy

Duro-Sense engineers bring proven solutions to your most difficult problems. The company has a long history of supplying thermocouples and RTDs to the top manufacturers in the aerospace industry. The Duro-Sense R&D department is staffed with some of the industry's most qualified people, working in the most modern facilities to help advance the state of the art in temperature measurement. 

Duro-Sense products are time-tested and proven to be ultra-reliable, accurate, and of extremely high value. 

Contact Duro-Sense Corp. today with you aerospace or space exploration temperature sensing requirement. Call them at 310-533-6877 or visit their website at https://duro-sense.com.