Temperature thermocouple tables

Type B thermocouples (Table 11.56) offer distinct advantages of improved stability, increased mechanical strength, and higher possible operating temperatures. They have the unique advantage that the reference junction potential is almost immaterial, as long as it is between 0°C and 40°C. Type B is virtually useless below 50°C because it exhibits a double-value ambiguity from 0°C to 42°C. 

Type J thermocouples (Table 11.58) are one of the most common types of industrial thermocouples because of the relatively high Seebeck coefficient and low cost. They are recommended for use in the temperature range from 0 to 760°C (but never above 760°C due to an abrupt magnetic transformation that can cause decalibration even when returned to lower temperatures). Use is permitted in vacuum and in oxidizing, reducing, or inert atmospheres, with the exception of sulfurous atmospheres above 500°C. For extended use above 500°C, heavy-gauge wires are recommended. They are not recommended for subzero temperatures. These thermocouples are subject to poor conformance characteristics because of impurities in the iron. 

The Type K thermocouple (Table 11.59) is more resistant to oxidation at elevated temperatures than the Type E, J, or T thermocouple, and consequently finds wide application at temperatures above 500°C. It is recommended for continuous use at temperatures within the range — 250 to 1260°C in inert or oxidizing atmospheres. It should not be used in sulfurous or reducing atmospheres, or in vacuum at high temperatures for extended times. 

The Type N thermocouple (Table 11.60) is similar to Type K but it has been designed to minimize some of the instabilities in the conventional Chromel-Alumel combination. Changes in the alloy content have improved the order/disorder h ansformations occurring at 500°C and a higher silicon content of the positive element improves the oxidation resistance at elevated temperatures. 

The Type T thermocouple (Table 11.63) is popular for the temperature region below 0°C (but see under Type E). It can be used in vacuum, or in oxidizing, reducing, or inert atmospheres. 

Instruments based on the contact principle can further be divided into two classes mechanical thermometers and electrical thermometers. Mechanical thermometers are based on the thermal expansion of a gas, a liquid, or a solid material. They are simple, robust, and do not normally require power to operate. Electrical resistance thermometers utilize the connection between the electrical resistance and the sensor temperature. Thermocouples are based on the phenomenon, where a temperature-dependent voltage is created in a circuit of two different metals. Semiconductor thermometers have a diode or transistor probe, or a more advanced integrated circuit, where the voltage of the semiconductor junctions is temperature dependent. All electrical meters are easy to incorporate with modern data acquisition systems. A summary of contact thermometer properties is shown in Table 12.3. 

At relatively high temperatures thermocouple thermometers are most commonly used to measure temperature. The thermoelectric power of three frequently used thermocouples is compared in Figure 10.2. The choice of thermocouple depends on the temperature range, the chemistry of the problem in question, sensitivity requirements and resistance towards thermal cycling. The temperature range and typical uncertainty of some of the most commonly used thermocouple thermometers are given in Table 10.2. 

The experiments were carried out in a Netzsch STA 409 C (Simultaneous Thermal Analysis - STA) in the TGA/DSC configuration. The STA has a vertical san le carrier with a reference and a sample crucible, and in order to account for buoyancy effects, a correction curve with empty crucibles was first conducted and then subtracted from the actual experiments. Platinum/Rhodium crucibles were used in order to get the best possible heat transfer. The thermocouple for each crucible was positioned Just below and in contact with the crucible. The ten rerature obtained from the measurement is the temperature in the reference side. This temperature is converted to the temperature in the sample side by using the DSC-signal in pV and a temperature-voltage table for the thermocouple. The product gases were swept away by lOO Nml/inin nitrogen which exited the top of the STA, The STA was calibrated for temperature and sensitivity (DSC) with metal standards at each heating rate. 

A thermocouple generates an electrical potential which is roughly proportional to the difference in temperature between the two junctions (Seebeck effect), and is well suited for differential temperature measurements (10). It may also be used for absolute and relative temperature measurements by keeping one junction, the reference junction, at constant temperature. Thermocouples normally used in DTA instruments are shown in Table 6.1. The temperature limits listed are for relatively accurate measurements with... 

In addition to these defining and secondary temperature standards, a thermocouple wire (SRM 733, a silver-28 at.% gold alloy) has been certified, which serves to compare manufactured wire to standard reference thermocouple tables between 4 and 273 K. SRM 767, a superconductive thermometric fixed-point device, provides temperature calibration in the range 0.5 to 7.2 K This device incorporates five high-purity elements (lead, indium, aluminum, zinc, and cadmium) in long, thin cylinders whose superconductive transition temperatures are certified to be reproducible within 1 mK. 

The pdf files in the Internet version of the Handbook mimic the new layout of the pages in the print edition. Furthermore, the number of interactive tables has been increased from 76 to 103. Among the topics included for the first time in interactive form are vapor pressure of the elements at high temperature heat of dilution and molar conductivity of acids electron and proton affinities atomic transition probabilities speed of sound in various media the NIST thermocouple tables and many others. The database from which these interactive tables are generated now has over 106,000 records which can be searched, manipulated, and displayed in a variety of ways. 

The calibration period for thermocouples. Table 17.5, showed a wide spread in practice. The frequency should be governed by the operating temperature and type of thermocouple therefore it is not surprising that there is a wide spread in the reported calibration periods. Best practice is, of course, to calibrate before each test but experience may be sufficient for laboratories to relax this procedure as data on thermal drift are recorded and analysed. It is somewhat worrying that several laboratories did not specify any calibration period for their thermocouples, all thermocouples require... 

To avoid or minimize unwanted thermal EMFs, thermocouple junctions are used in pairs as shown in the bottom portion of Figure 12.10. One of the junctions is maintained at a known temperature - often an ice bath. The temperature of the other junction can then be inferred from the resulting total EMF using standard thermocouple tables or special calibration tables prepared for your specific thermocouple. 

For general instruction on the use of thermocouples, see ASTM Manual on the Use of Thermocouples in Temperature Measurement (ASTM, 1981). The National Bureau of Standards publishes a complete set of thermocouple tables (Powell et al., 1974), and White (1987) provides a table of output voltage versus temperature for copper-constantan and two gold-iron thermocouple materials for cryogenic work. 

The thermocouple thermometer operates on the principle that an electric current will flow in a closed circuit of two dissimilar metals when the junctions of the metals are at two different temperatures. Thermocouple materials are available for use within the approximate limits of —300 to 3200°F. Platinum is the generally accepted standard material to which llie thermoelectric characteristics of other materials are referred. The emf-temperature relations of conventional thermoelements versus platiniun are shown in Fig. M-2. Reference tables of... 

The thermocouple reference data in Tables 11.55 to 11.63 give the thermoelectric voltage in millivolts with the reference junction at 0°C. Note that the temperature for a given entry is obtained by adding the corresponding temperature in the top row to that in the left-hand column, regardless of whether the latter is positive or negative. 

Whereas it is no longer an iaterpolation standard of the scale, the thermoelectric principle is one of the most common ways to transduce temperature, although it is challenged ia some disciplines by small iadustrial platinum resistance thermometers (PRTs) and thermistors. Thermocouple junctions can be made very small and ia almost infinite variety, and for base metal thermocouples the component materials are very cheap. Properties of various types of working thermocouple are shown in Table 3 additional properties are given in Reference 5. 

TABLE 8-7 Recommended Temperature Measurement Ranges for RTDs and Thermocouples... 

ASTM E230-96el. Standard Specification for Temperature-Electromotive Force (EMF) Tables for Standardized Thermocouples. American Society for Testing and Materials, 1996. 

The pellet charge was about 0.0065 g (0.0060 to 0.0068) into a cell with a 4.0 mm diameter. The holder was placed in the center of a quartz tube, equipped with gas and thermocouple ports and Kapton windows. The amount of sample used was optimized for the Fe K edge, considering the absorption by Si of the catalyst. The quartz tube was placed in a clamshell furnace mounted on the positioning table. Each sample cell was positioned relative to the beam by finely adjusting the position of the table to an accuracy of 20 pm (for repeated scans). Once the positions were fine-tuned, the samples were heated to about 120°C in 5% CO/He at 10°C/min. Then the samples were heated to about 270°C over a 3-hour period. They were held at this temperature for 4 h and then cooled. 

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