All About Thermocouples (part 2)
Our discussion of thermocouples continues this month with more practical advice and a deeper understanding into how to install and maintain thermocouples so as to control the heart of any thermal-processing system. Let’s learn more.
Correct installation techniques and good maintenance procedures are a must. The most common operational problem with furnaces today is inaccurate temperature control due, in large measure, to the fact that the thermocouples have been in the furnace longer than their normal life expectancy.
How to Install Thermocouples
The one common feature of all furnaces is the fact that within their insulated chambers there are horizontal planes of even heat flow (i.e. areas of equal temperature called isotherms). Factors that cause these temperature planes or gradients to occur include uneven heating, inadequate circulation, uneven distribution of the workload within the furnace, improper location of the heating source and the like. In order for a thermocouple to properly sense furnace temperature it must be oriented parallel to these isotherms and installed into the furnace a minimum of 20 thermocouple diameters. For example, a 3-mm (1/8-inch) diameter thermocouple will need to be inserted 63.5 mm (2.5 inches) into the chamber furnace.
What is the maximum-use length of a thermocouple?
The two main factors in determining useable thermocouple length are total loop resistance and prevention of electrical signal noise. Since thermocouples are made from dissimilar wire, the resistance will vary based on the type of wire as well as its diameter and length. The allowable loop resistance (typically under 100 ohms) is affected by the input resistance of the amplifier circuit to which it is attached. As a general guideline, 20 AWG or thicker wire is adequate for runs up to 30 meters (100 feet). Thermocouple wire creates a low voltage signal and should not be run near power wires, motors, etc. To help minimize noise pickup, thermocouples used in industrial furnace applications are almost always run in separate metal conduit.
There are three basic types of construction for thermocouples (Table 1). Each has advantages and disadvantages. For example, ceramic-beaded thermocouples (Fig. 1) lack flexibility when compared to base-metal thermocouples, which can have various types of insulation placed directly on the wire, making them extremely flexible. By contrast, ceramic-beaded thermocouples can often be used at higher operating temperatures than many of their base-metal counterparts.
Mineral-insulated, metal-sheathed thermocouples (Fig. 2) provide excellent performance, and they can be used as replacement elements to ceramic-beaded, base-metal elements in many cases. Metal-sheathed cable is offered in many sizes and materials and can be optimized to help provide long-lasting, stable temperature measurements.
Why protection tubes?
A thermocouple is often inserted into a protection tube (Fig. 3). Protection tubes (Table 2, online) are used to shield thermocouples from contaminants and/or mechanical damage. The heat treater must be aware that all types of thermocouple protection tubes can crack or distort, potentially damaging or exposing the thermocouple to the environment they were intended to protect it from. In addition, contamination from handling/touching the wires, oils and dirt inside the protection tube and contamination buildup on the outside of the protection tube are some of the many factors that can cause thermocouple error.
Attaching Thermocouples in Workloads
For stationary loads, one of the best ways to determine part temperature is to use one or more workload thermocouples. Ideally, you would like to have access to an internal blind hole that would allow you to measure the core temperature of the thickest cross section of the part. Unfortunately, we do not always have this luxury. The next best choice is to use dummy blocks that are predrilled and representative of the maximum cross section of the parts being run. In lieu of a blind hole or dummy block, wiring the thermocouple in intimate contact with the surface of the part is the next best option, using the rule of thumb of one hour per inch of cross-sectional area for soaking the part.
Be aware that some people suggest tack welding a thermocouple tip, but this practice is highly questionable because it can change the millivolt signal and provide inaccurate results. Epoxy or other adhesives fail prematurely in service and are not a good choice. Running a test load under simulated production conditions is often the best way to determine heat-up and soak times for moving loads or in furnaces with internal transfer mechanisms.
Thermocouples in the Heat-Treat Industry
The heat treater, not the original equipment manufacturer or maintenance department, should be responsible for the selection of the type of thermocouple as well as making sure that they are in the correct position within the furnace (both location and insertion depth) to accurately sense and control furnace temperature. Thermocouples located too close to heating or insulation sources or too close to the workload itself will not represent true furnace temperature. Temperature uniformity checks of the workload areas must factor in deviation from the control thermocouple. Although temperature offsets are allowed in some instances, this practice is highly discouraged.
The majority of heat-processing applications in the metals industry, including virtually all heat-treating processes, occurs in the range of -185 to 1650?C (-300 to 3000?F). No one type of thermocouple can span this entire range, and quite often we attempt to use a particular thermocouple well beyond its normal temperature range simply because “it’s available.” This practice should be avoided since it affects both accuracy and, once exposed to abnormal conditions, may alter its life expectancy. As strange as it seems, it is not uncommon to find the wrong type of thermocouple being used to control a critical process.
Thermocouples must be checked regularly for accuracy against a known certified standard (probe) thermocouple, and this procedure should be done (at least) annually. This calibration must take place while the thermocouple is installed in its normal operating location for reliable measurements. The operating life of a thermocouple depends on its operating temperature, time at operating temperature, ambient temperature, cyclic range (high to low temperature variation) and, most often overlooked in heat-treating applications, the influence of contaminants either on an exposed thermocouple or on the protection tube itself. Many thermocouples are replaced on a periodic basis, typically every six to nine months depending on the severity of the end-use application.
Thermocouple wire is supplied according to various industry standards (Table 3).
Thermocouples are not, as many people believe, a “set it and forget it” technology. They require constant monitoring and confirmation of accuracy to ensure that the temperature being sensed and controlled is precise. Remember, the quality of your products is highly dependent on these simple and relatively inexpensive devices, which makes their selection, care and replacement critical to your success. IH
1. Mr. James T. LaFollette, GeoCorp, Inc. (www.geocorpinc.com), technical and editorial review.
2. Herring, Daniel H., “What is a Thermocouple?” Heat Treating Progress, March 2003.
3. ASTM E230/E230M-12 (Standard Specification and Temperature-Electromotive Force (EMF) Tables for Standardized Thermocouples), ASTM International.
4. The Right Thermocouple Makes A World of Difference, The Cleveland Electric Laboratories, white paper.
5. Volume 4: Heat Treating, Metals Handbook, 10th Edition, ASM International.
6. Wang, T. P., Thermocouples for Special Applications, Proceedings of the International Conference: Equipment and Processes, 1994, ASM International.
7. Nanigian, J., Improving Accuracy and Response of Thermocouples in Ovens and Furnaces, Proceedings of the International Conference: Equipment and Processes, 1994, ASM International.
8. Omega Electric Company (www. omega.com)
9. Kanthal Corporation (www.kathal.com)