Expendable vs. Nonexpendable Thermocouples: A Detailed Look at AMS 2750D
|Fig. 2. Expendable insulated wire|
In order to understand how temperature sensors are applied in AMS 2750D, it is critical to understand their various uses. According to the specification, there are four typical uses for temperature sensors: control sensors, load sensors, system accuracy test (SAT) sensors and temperature uniformity sensors.
Control sensors, as defined in AMS 2750D (8.2.9), are any sensors “connected to the furnace temperature controller, which may or may not be recording.” They are installed within, or as close as possible to, the work zone of thermal-processing equipment. In conjunction with control instrumentation, they control process temperature.
Load sensors (8.2.25), on the other hand, are sensors “attached to the production material or a representation of production material that supply temperature data of the production material to process instrumentation.” They may also be used as control sensors, but they shall not be used to monitor or record any temperature above the maximum processing temperature.
System accuracy test (SAT) sensors (8.2.60) are “calibrated and traceable … with known deviations, if any, used in system accuracy tests.” System accuracy tests are performed at scheduled intervals, as defined by AMS 2750D, to ensure that values being recorded by the process sensors are accurate (within acceptable limits) to the calibrated SAT sensor in each thermal-processing zone.
SAT sensors may be resident, remaining in place between tests (only Type N, R or S thermocouples are allowable) and must be of a different type than the sensor being tested. Conversely, an SAT sensor may also be used as a probe check, or nonresident sensor, temporarily inserted as close as possible to the process thermocouple tip (not to exceed 3 inches between sensors) and can be of like type to the process sensor if desired.
Finally, temperature uniformity sensors (8.2.64) are also “calibrated and traceable … with known deviations that are used for conducting temperature uniformity surveys.” A temperature uniformity survey (TUS) is a test measuring temperature variation in the defined thermal work zone to establish a qualified temperature operating range. The size of the furnace work zone defines the number of sensors required for a TUS and are outlined in detail in AMS 2750D Table 11 (Fig. 1).
|Fig. 3. Beaded nonexpendable thermocouple|
Expendable vs. Nonexpendable Thermocouples
While the preceding introduction gives us a very cursory glance at temperature sensor uses (AMS 2750D provides far greater detail), it is necessary to take a much more in-depth look at how sensors are classified. For the purposes of AMS 2750D, it is essential that we understand the classification of “expendable” versus “nonexpendable” sensors. The scope of use of one as compared to the other varies greatly according to the specification.
Expendable thermocouples (8.2.18) are by definition “made of fabric or plastic-covered wire.” In the industry, these are commonly referred to as insulated wire. Expendable thermocouples are generally supplied on spools, and the insulation is present on each thermocouple wire conductor plus an overall outer insulation jacket. The insulation often consists of either glass braid or ceramic-fiber cloth, though many different insulation materials are commonly available (Fig. 2).
Nonexpendable thermocouples (8.2.32), on the other hand, are defined as thermocouples that are “not covered with fabric or plastic insulations.” Nonexpendable thermocouples consist of various types. One type incorporates bare metal thermocouple conductor wire with ceramic insulators isolating the positive and negative legs. The insulators ensure that the bare-wire conductors remain isolated and that the measuring junction is in contact only at the tip, preventing any false junctions and potential for measurement error. While a commonly used assembly, the ceramic insulators are fragile, which is why many thermal processors are moving toward an alternate style of nonexpendable thermocouple.
A second, more robust version is a mineral-insulated, metal-sheathed (MIMS) thermocouple. Commonly referred to as “MgO thermocouples,” these sensors consist of thermocouple wires, mineral insulation (often magnesium oxide – hence MgO) and a metal sheath. They are available in a wide variety of diameters and materials (Figs. 3 & 4).
MgO thermocouples are commonly available in sizes ranging from 1/16-3/8 inches diameter, though other sizes are available. Popular sizes include 1/16 inch, 1/8 inch, 3/16 inch and 1/4 inch o.d. Sheath materials are offered in 300-series stainless steel for applications below 1700°F and Inconel® for higher temperature ranges. These fully protected assemblies are flexible and, within the temperature limitations of the sheath material, offer enhanced life as compared to both expendable thermocouples and often beaded-ceramic nonexpendable sensors.
|Fig. 4. MgO nonexpendable thermocouples|
Guidelines for Use of Expendable, Nonexpendable Sensors
In a wide variety of thermal-processing applications, base-metal thermocouples (types T, J, K and N) are often applied. In some cases, such as vacuum applications and where process temperatures exceed 2200°F, noble-metal thermocouples are required (types S, R, B and C). Noble-metal thermocouples may also be necessary based upon the class of furnace being used and the temperature uniformity range defined for the thermal-processing equipment.
That said, the type and classification of thermocouples chosen – as defined in AMS 2750D – have a dramatic effect on efficiency, thermocouple life and performance. We know that AMS 2750D makes a distinction between expendable and nonexpendable sensors. In order to best understand the distinction, we need to look at sensor use and the parameters set by the specification.
When examining the use of expendable versus nonexpendable thermocouples, the categories of control sensors, load sensors, SAT sensors and TUS sensors each have very specific parameters as defined by AMS 2750D. Understanding these parameters as set forth in the specification is essential to compliance.
Control Sensors Control sensors, like all sensors, fall into the two previously defined categories – expendable and nonexpendable. As set forth in AMS 2750D (184.108.40.206), an expendable thermocouple (insulated wire) used as a control sensor is limited to a single use. According to the specification, “use” consists of one cycle of heating and cooling of a thermocouple.
Nonexpendable thermocouples (ceramic-beaded or MgO thermocouples), on the other hand, can be used until the sensor fails or drifts out of the calibration error limits as outlined in Table 1 of AMS 2750D (Fig. 5). However, one important variable related to drift is that these limits do vary based on the furnace class.
All calibration for control thermocouples must be performed within the temperature operating range of the thermal-processing equipment and shall not exceed 250°F between points. Recalibration of type-K and type-E thermocouples used above 500°F is prohibited regardless of whether the sensors are expendable or nonexpendable.
The use of expendable and nonexpendable thermocouples as load sensors is very specifically defined under AMS 2750D. When using expendable thermocouples as load sensors, a maximum 30 uses are permitted at temperatures of 1200°F or lower, while temperatures above 1200°F allow for only a single use, per AMS 2750D (220.127.116.11).
Nonexpendable thermocouples have a more delineated specification according to AMS (18.104.22.168). The operating temperature of the furnace specifically determines the life of a nonexpendable load sensor. When exposed to temperatures 2300°F or greater, nonexpendable thermocouples have a single use. As temperatures decrease, however, the useful life of nonexpendable load-sensor thermocouples increases substantially.
Operating between 2200°F and 2299°F, nonexpendable thermocouples have 10 uses. Operating between 1801°F and 2199°F, nonexpendable load sensors are good for 30 days or 90 uses. From 1200°F to 1800°F, 90 days or 180 uses are permissible. Finally, nonexpendable load sensors used below 1200°F allow for 90 days or 270 uses (22.214.171.124).
It is important to remember that the highest temperature a load sensor sees in operation does become the governing usage criterion, and this must be applied when the load sensors are used across multiple ranges. Without a doubt, however, it is clear that nonexpendable load sensors have significantly greater life than expendable load sensors in the majority of temperature operating ranges.
System Accuracy Test (SAT) Sensors and Temperature Uniformity Survey (TUS) Sensors
For the purpose of usage as test sensors, AMS 2750D treats SAT and TUS sensors in a similar fashion. When dealing with expendable thermocouples used in either SATs or TUSs, life of the sensors are based upon the “U” formula outlined in paragraph 126.96.36.199. While expendable thermocouples cannot be recalibrated under the provisions of AMS 2750D, their reuse is allowable as long as “U” is not greater than 30. Any use above 1800°F constitutes the sensors’ one and only use. The “U” formula is as follows:
U = (number of uses below 1200°F) + 2 x (number of uses between 1200°F and 1800°F) £ 30.
In other words, expendable thermocouples count as one use under 1200°F, two uses for every cycle between 1200°F and 1800°F, and can only be used once above 1800°F before they must be discarded. Below 1800°F, the number of uses is tallied under the formula until U=30, at which point the thermocouples can no longer be used as test sensors.
By contrast, nonexpendable thermocouples, when used as test sensors, can be used (and reused) for a period of 90 days regardless of the number of thermocouples or the thermal-processing operating temperature range. Also, nonexpendable type-E and type-K thermocouples (used below 500°F), type-J, type-N, and noble-metal thermocouples (types S, R and B) can be recalibrated as long as they meet the limits of error set forth in Table 1 (Fig. 5).
While expendable test sensors do at times provide the convenience of manufacturing thermocouples in house quickly, nonexpendable sensors offer many advantages. Insulated wire serves the needs of many applications, but a detailed analysis of the benefits of MgO-style thermocouples often proves to enhance efficiencies in the world of thermal processing. The attributes of longer life, greater temperature range and robust design as compared to wire and less susceptibility to atmospheric conditions warrants a close look at applying nonexpendable thermocouples in your thermal-processing equipment (Fig. 6). IH
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