THERMOCOUPLES
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Whether inserted into a patient's body to monitor tissue temperature or used as a measurement and control device on external patient-support apparatus, thermocouples usually can be depended upon to give trouble-free service and long life as components of medical devices. Occasionally, however, performance difficulties may be encountered. These result from improper application or operation of the devices.
The information presented in this article is meant to serve as a short guide for medical device developers who use thermocouples. It is intended to help them obtain the benefits of accuracy and economy for which the thermocouple alloys are known.
Selection
Both service temperature and initial accuracy can vary greatly depending on the combinations of metals used to form thermocouple pairs. A type T thermocouple might be best for meeting low-temperature, high-accuracy application requirements, whereas in high-temperature metals processing and heat treatment, device designers might prefer to use a platinum-metal type S thermocouple. The most commonly used combinations are those designated as J and K types.
Letters of the alphabet and color codes are conventions for differentiating these metal combinations and are referred to as calibrations. Color coding may be used with wire and connectors as defined by standards of the American Society for Testing and Materials (ASTM) and the International Electrotechnical Commission (IEC). The designer's obligation is to choose a type that matches the operational temperature of the application.
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If properly applied, thermocouples can provide long life to medical products. They can be inserted into the body or can be used in external devices.
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Size
Heavy-gauge thermocouples are generally more stable at high temperatures than finer-gauge versions. In many applications, however, such as temperature measurement during cryogenic ablation procedures carried out in patient-support devices, a heavy-gauge thermocouple is not appropriate. The heavy-gauge version cannot satisfy requirements for flexibility, rapid response, equipment geometry, and other parameters. In such cases, designers must find a balance between the long-term stability of heavy sizes and the greater versatility of smaller thermocouples. But where high-temperature stability is critical, designers should use the largest practical size that still satisfies other requirements.
Protection and Performance
It is essential to look at factors that affect thermocouple performance. These factors include evaporation, diffusion, oxidation, corrosion, and contamination. All can induce drift in electromotive force because of their effect on the composition of thermocouple alloys. Keeping in mind that these environmental factors are destructive to all common thermocouple materials, designers must ensure that the thermocouple is protected in adverse operating conditions. The use of sheathed unit construction can address this issue in many applications.
Proper installation of the thermoelements in suitable protection tubes is essential when bare-wire thermocouples are used. When the insides of such tubes are clean and free of contaminants such as sulfur-bearing oils and refractories, they can defend against the harmful effects of corrosive atmosphere. The right diameter-to-length ratios also allow adequate interior ventilation.
Installation
A thermocouple should be installed in a location that ensures that the temperatures measured are representative of the equipment or medium. Direct flame impingement on the thermocouple, for example, produces an inaccurate temperature reading. Proper placement of the sensor in relation to the workload and heat source can compensate for various types of energy demands from the workload. Sensor placement can also be a means to limit the effects of thermal lags in the heat transfer process. The controller can respond only to the temperature changes it perceives through feedback from the sensor location. Therefore, sensor placement will influence the ability of the controller to regulate the temperature around a desired set point.
The placement of the sensor cannot compensate for inefficiencies in the system caused by long delays in thermal transfer. Inside most thermal systems, temperature will vary from point to point. This raises tissue compatibility and sterilization issues in medical devices.
Depending on the medical device application, one of several thermocouple sensor placement concerns may arise, as follows.
Sensor in a Static System. A system is static when it is characterized by slow thermal response from the heat source, slow thermal transfer, and minimal change in the workload. When the system is static, placing the sensor closer to the heat source keeps the heat fairly constant throughout the process. In this type of system, the distance between the heat source and the sensor is small, so thermal lag is minimal. Therefore, the heat source cycles frequently, reducing the potential for overshoot and undershoot at the workload.
A sensor placed at or near a heat source can quickly sense temperature changes, thereby enabling tight control to be maintained.
Sensor in a Dynamic System. The system is dynamic when it features rapid thermal response from the heat source, rapid thermal transfer, and frequent changes in the workload. When the system is dynamic, placing the sensor closer to the workload enables it to detect the load temperature change more rapidly and allows the controller to take the appropriate output action more quickly. However, in this type of system the distance between the heat source and the sensor is notable, causing thermal lag, or delay. The heat source cycles therefore are longer, causing a wider swing between the maximum (overshoot) and minimum (undershoot) temperatures at the workload.
The electronic controller selected for this situation should include the characteristic photoionization detection (PID) features of anticipation and offset ability to compensate for these conditions. Being situated at or near the workload, the sensor can quickly sense temperature increases and decreases.
Sensor in a Combination Static and Dynamic System. When the heat demand fluctuates and creates a system ranging between static and dynamic, the designer should place the sensor halfway between the heat source and the workload to divide the heat-transfer lag times equally. Because this system can produce some overshoot and some undershoot, the electronic controller selected for it should include the PID features to compensate for these conditions. This sensor location is the most practical one in most thermal systems.
Immersion Depth
Heat conducted away from the hot junction causes the thermocouple to indicate a lower temperature. Therefore, designers should provide for an immersion depth of the thermocouple into the medium being measured that is sufficient to minimize heat transfer along the protection tube. As a general rule, the immersion depth should be a minimum of 10 times the outside diameter of the protection tube.
Oxidation, corrosion, evaporation, contamination, or metallurgical changes can cause inhomogeneities to develop in a pair of thermocouple wires. Changes in depth of immersion shift inhomogeneous wire into a zone with a steep temperature gradient. This shift can alter the thermocouple output and thus produce erroneous readings. These changes are usually gradual, so it is essential to avoid conditions that can cause such inhomogeneities.
Whenever possible, thermocouples should be checked in place. If they must be removed for inspection, then they should be reinserted at the same depth or deeper to prevent errors caused by an inhomogeneous segment of wire in a steep temperature gradient.
Heating Cycles
A thermocouple should be used to control a single temperature, or successively higher temperatures, only. Although this procedure ensures maximum accuracy, it cannot always be followed, for various reasons. In many cases, thermocouples can continually traverse a broad range of temperatures providing adequate results. Errors that arise out of cyclic heating are similar to those generated by changes in immersion depth. Changes may range from 2° or 3° for thermocouples in good condition to a variance of many degrees when the thermocouples are badly corroded. Therefore, both the heating cycle and the condition of the thermocouple affect the accuracy. The use of top-condition thermocouples in applications where cyclic heating cannot be avoided is recommended as an approach for attaining the maximum accuracy possible.
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Thermocouple life expectancy varies greatly, and many factors contribute to determining that life span.
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Preventive Maintenance
Establishment of a reliable preventive maintenance program is advisable. Thermocouples, protection tubes, and extensionwire circuits should be checked periodically. Inspection frequency depends on a number of factors, but once a month is usually sufficient.
Both ceramic and metal protection tubes should be visually examined for excessive corrosion, wear, oxidation, or physical damage. Tubes that exhibit signs of damage or excessive corrosion should be replaced. Wiring should be examined for possible damaged insulation. It should also be checked to ensure that connection points are tight.
Wiring Components
The type of thermocouple determines the extension wire and connector wire used. Designers must avoid using the incorrect wiring components between the control and the process thermocouple. Incorrect wiring can result in erroneous readings. Connector and extension wires are usually color coded to match the thermocouple calibration. However, it is important to note that different color-coding standards are used worldwide. For example, green is used as the IEC color for a type K thermocouple and as the ASTM color for type S.
Accuracy versus Repeatability
Designers must determine whether it is more important to repeat a process day after day or to know the exact temperature attained by a material being processed. A case in which accuracy would be paramount is the heat treatment of metal to obtain specified strength properties. Repeatability is the virtue sought when the temperature of the outside of a pipe is measured at intervals and used as an indicator of the temperature of the liquid flowing through the conduit.
Limit Sensing
All systems should be examined for potential thermocouple failure. Designers—and users—need to know how the system would react if the thermocouple circuit were to open or form a secondary junction. The use of limit sensors can provide for safe system operation. These sensors and their controls should be kept separate from the system control.
Electrical Noise
The sensor input and power output lines, as well as the power source line, have the potential to couple, or link, the control circuit to a noise source. Depending on its intensity, noise can be coupled to the sensor circuit by any one of several ways, or by a combination of them. One avenue is common impedance coupling. Common impedance coupling occurs when two circuits share a common conductor or impedance—even common power sources.
Magnetic inductive coupling is another possibility. This form of coupling generally appears where there are wires running in parallel or in close proximity to each other, as happens when the wires from several different circuits are bundled together in order to make the system wiring appear neat.
Electrostatic capacitive coupling also appears where wires run parallel to each other, but the similarity to magnetic inductive coupling ends there. Electrostatic, or simply capacitive, coupling is a function of the distance for which the wires run in parallel, the distance between the wires, and the wires' diameters.
The final possible means of linkage to a noise source is electromagnetic radiation coupling, which occurs when the sensor is very close to a high-energy source such as television or radio broadcasting towers.
Conclusion
Thermocouple life expectancy varies greatly, ranging from just a few hours to many years. The many factors that contribute to determining life expectancy include calibration type, the environment, temperature, sensor design, and thermal cycling. As designers gain experience with a particular process, they will get a sense of when to check and when to replace thermocouples. Attention to the good thermocouple practices outlined here will ensure a long medical device service life at the highest achievable levels of performance and accuracy.
