Types of Thermocouples and Their Use
This article explains what a thermocouple is, the types of thermocouples, and their use in measuring temperature in a variety of industries.
If Thomas Johann Seebeck could see us now, he would be wearing a broad smile. His 1821 discovery, the Seebeck Effect (thermoelectric effect), marked the beginning of industrial thermocouples. Seebeck would be pleased to see how thermocouple technology is advancing to meet the needs of the process industries. This article contains several new products that are in use today.
Why have thermocouples been in use for nearly 200 years, and why haven’t they been replaced by another measurement tool? The reasons are many. Thermocouples are still the most versatile temperature sensors available. A variety of thermocouple calibrations, including Type J, T, K, E, N, S, R, B and C, cover temperature ranges from -450 to 4000°F. Thermocouples can be used in many different atmospheres – deep freeze, liquids, solids, surface temperatures, air and gas furnaces, vacuum furnaces and hot isostatic pressure vessels. Thermocouples are also adaptable. Depending upon the application, they can be bent, welded to parts and inserted into small areas. They can be short (less than one inch) and very long (up to 200 feet). A basic thermocouple can be made by simply welding or crimping two dissimilar wires necessary for each calibration together, creating a measuring junction. They can also be manufactured to the very exacting specifications needed for high reliability – required in aerospace and nuclear applications.
Thermocouples Becoming Lighter and Faster
Thermocouple designers and manufacturers are in the “time and temperature” business. We need to achieve the most accurate temperature reading in the least amount of time. When rapid response is a mandate, the temperature change in the environment being monitored must reach the measuring junction rapidly. For this to happen, the temperature or steps in changing temperature must penetrate the sheath material and insulating material. Consequently, the smaller the diameter (less material to penetrate), the faster the response time.
Mineral-insulated cable as small as 0.010-inch diameter is preferred for many applications for two reasons. The first reason is speed. When high temperatures are reached quickly, a conventional, less-responsive thermocouple could miss rapid temperature increases that must be measured. The second reason for small-diameter, mineral-insulated cable is high flexibility for ease of routing.
In addition, higher-quality thermocouple materials are being used to control thermocouple drift. Today, many customers are looking for superior accuracy and stability. A recent industry trend is the replacement of the more traditional K calibration with N, which exhibits better stability characteristics at elevated temperatures.
Precious-Metal Sheath Options Save Money
Is this a typo? Platinum is still expensive, yet advanced noble-metal element/sheath designs are saving money. In many cases, costs can be reduced through the use of composite sheath materials. A platinum sheath can be seamlessly transitioned to lower-cost materials such as inconel or stainless steel. Platinum may be a requirement as a protective sheath material in many high-temperature applications. It is generally only necessary, however, in the higher-temperature work zone from 2000–2800°F. When a continuous alloy sheath is required, the remainder of the sheath can be seamlessly transitioned to a lower-cost alloy sheath material, thus reducing material cost. For example, a thermocouple that was formerly a 96-inch-long platinum sheath with a hot-zone requirement of only 10 inches can now be transitioned to 86 inches of a lower-cost material. Platinum sheath lengths can range from 2 inches to several feet and can be as small as 0.020 inches in diameter. Where conditions are more severe, diameters can be increased.
New System Accuracy Tests - AMS 2750D
Aerospace Material Specification (AMS 2750D) covers pyrometric requirements for thermal-processing equipment used for heat treatment. When impractical or impossible to perform a System Accuracy Test (SAT) in a traditional manner, a resident probe may be employed. SATs have specific requirements to qualify control thermocouples. Above 500°F, the resident SAT probe must be Type N, R or S. Above 1000°F, a non-expendable sheath must be used. Furthermore, a resident SAT thermocouple must be of a different calibration than the sensor being used to control the furnace.
These requirements have led to new designs. Dual-calibration MgO thermocouples are now available for this application requirement. The dual thermocouple will incorporate the control thermocouple and the resident probe thermocouple in a common sheath. Dual calibrations such as K and N, J and N, and K and S are some of the available combinations in sheath diameters as small as 1/8 inch.
Staggered Junctions and Multiple Sensors
The need for multi-sensor, staggered-junction or combinations of both within a common sheath is accelerating. An example of a process calling for simultaneous temperature measurements at different levels is the glass container industry, wherein the temperatures are often measured at the top, midpoint and bottom of the molten-glass stream to confirm uniformity. Applications calling for as many as 12 thermocouples can now be configured inside one common sheath as small as 0.125-inch OD, with or without staggered junctions.
Meeting Engine Test-Sensor Requirements
Turbine engine manufacturers that supply the truck, earthmover, power generation or aircraft markets work tirelessly to increase horsepower, decrease emissions and reduce weight in their engines. Since the mid-‘90s, engines have become smaller, far more fuel efficient and have increased in power.
Turbines can reach temperatures of up to 2800°F in seconds. High temperatures and rapid increases have dictated advanced engine materials and sensors. For example, test engines with single-crystal vanes are smaller and hotter and generate higher performance. Ceramic engine parts can exceed 2800°F and require advanced thermocouples to monitor higher temperatures. New, faster, lighter and smaller thermocouples help provide the most accurate reading possible. Engine manufacturers test performance and emissions at very high temperature. Precision temperature data is critical because a test engine that runs a few degrees too cool could fail to meet performance requirements.
Control systems for higher-technology engines require faster and faster response times. Thermocouples must respond in fractions of a second versus seconds. Transient temperatures, melt points and indications of surge must be sensed immediately, so the faster the response time, the more time the digital controls have to react and correct undesirable conditions and perhaps prevent catastrophic events.
Now, diagnostic rakes and probes must contain more sensors, but they are installed in smaller and smaller spaces. In the past, 20 sensors were considered a high number on a test engine. Today, 1,000 sensors can be an insufficient number. In the past, 0.040-inch diameter was considered the smallest. Now, even 0.010-inch diameter, while commonly used, can be too large.
Test engines are carefully monitored for fuel consumption and performance. Every ounce of fuel consumed by an engine increases its operating costs. “Power by the hour” is the critical measure of engine performance. Engines that run hotter may be more efficient, but the trade-off may be lower engine life. Rotating engine components are measured for strain as well as temperature. They have higher creep and fatigue at high temperatures, but at higher RPM, more horsepower and thrust can be generated.
Poor Engine Test Data Can be Fatal
Rakes and probes require state-of-the-art engineering and design. Craftsmanship is critical for maximum accuracy. Poor data causes two problems:
- Indicates that a problem is present when not there, thus forcing unnecessary hardware and material changes.
- Missing a problem that could later cause a field failure.
Correct thermocouple design and material selection for specific engine applications is nearly impossible without the assistance of a qualified research and development instrumentation-engineering group.
Fiber-Optic Sensors Gaining Momentum
Fiber-optics technology is raising the standards for sensing in many applications. Engine testing requires that many points on an engine be measured for temperature, vibration and strain. Fiber-optic sensors are now an alternative because they are corrosion resistant, immune to EMI and RFI, and are effective in hostile environments. Additionally, fiber-optic sensor systems can relay data hundreds of times faster than traditional sensors.
Routine engine tests may require hundreds of sensors. Traditional thermocouple sensors must be individually manufactured, tagged and then installed into the data-acquisition system. It can take hundreds of hours to prepare for an engine test. Unlike traditional sensors, up to 80 fiber-optic sensors can be multiplexed on a single fiber and installed on a single-channel input. Traditional sensors would require 80 separate connections. Fiber-optic sensors reduce installation time and expense plus reduce weight and space requirements. Fiber-optic sensors are leading the way into sophisticated health monitoring systems that will be installed on all new aircraft designs in the near future. IH
For more information: Contact Rodger Shepherd, national sales manager for Cleveland Electric Laboratories, 1776 Enterprise Parkway, Twinsburg, OH 44080; tel: 330-425-4747; e-mail: sales@thermo couple.cc; web: www.Clevelandelectriclabs.com. Author Don Lieske is vice president, quality and inside sales, and Don Way is engineering manager at the Test Cell Instrumentation R&D Facility in Tempe, Ariz.
Additional related information may be found by searching for these (and other) key words/terms via BNP Media SEARCH at www.industrialheating.com: Seebeck Effect, thermocouple, AMS 2750, system accuracy test, fiber-optic sensor