Thermocouples temperature measurements

Uncertainty of the temperature of reaction in stopped-fiow reaction-rate measurements can be a serious source of error. There is an advantage in having the temperature-measuring thermocouple positioned at the observation cell. 

FIG. I—Cross view of the reactor and water pressure reserve. B exhaust vah-e, S specimen, U umbrella D,. D, stainless steel disks P pipe sustaining the sample holder, it is a pipe that contains the temperature measurement thermocouple T cylinder containing a specimen F isolated heating resistor and R furnace control thermocouple. 

American National Standard for Temperature Measurement Thermocouples, ANSI-MC96-1 1982, Instrument Society of America (sponsor), 1982. 

An identification of the apparatus used by type and commercial name, together with details of the location of the temperature-measuring thermocouple and the interface between the systems for sample heating and detecting or measuring evolved gases. 

Identification of the thermobalance, including the location of the temperature-measuring thermocouple. 

Identification of the apparatus, including the geometry and materials of the thermocouples and the locations of the differential and temperature-measuring thermocouples. 

For electric temperature measurements thermocouples and resistance thermometers are universally applicable. They are commercially avaUable in a variety of designs [18]. 

Temperature Measurement. Thermocouples are by far the most reliable devices for monitoring temperature in DTA. A typical arrangement is shown in Figure 17.5. 

Until recently, temperature measurements in microfluidic systems were limited to measures of bulk fluid temperature at the inlet and outlet of microfluidic sections or simply measurement of the substrate temperature. With regard to local temperature measurements, thermocouple probes provide highly accurate measures of fluid and/or substrate temperature with excellent temporal response. However, thermocouples can often be physically intrusive and generally suffer from poor spatial resolution since most probes have a characteristic size of several micrOTs or more. Alternatively, microfluidic devices can be fabricated with integrated microscale resistance temperature detectors (RTDs s) embedded in the substrate with spatial extents on the order of a few microns [13]. Micro-RTDs overcome the intrusiveness issues of thermocouples however, their fabrication can be quite complex and RTDs still suffer from poor spatial resolution which limits their ability to resolve local thermal... 

Temperature measurement Thermocouples Bimetal thermometer Resistance thermometer Radiation pyrometer Infrared measurement Conductivity pH Viscosity Infrared analyzer Mass spectrometer Radiation analyzer Chromatographic analyzer Ultraviolet analyzer Photonic measurement... 

Figure 3 A schematic diagram of the experimental setup d = density measurement (vibrating U-tube) p = pressure drop measurement (piezo-electric) t = temperature measurement (thermocouple) hc= hydrocyclone mv= manual valve...

Control Devices. Control devices have advanced from manual control to sophisticated computet-assisted operation. Radiation pyrometers in conjunction with thermocouples monitor furnace temperatures at several locations (see Temperature measurement). Batch tilting is usually automatically controlled. Combustion air and fuel are metered and controlled for optimum efficiency. For regeneration-type units, furnace reversal also operates on a timed program. Data acquisition and digital display of operating parameters are part of a supervisory control system. The grouping of display information at the control center is typical of modem furnaces. 

Temperature measurements ranging from 760 to 1760°C are made usiag iron—constantan or chromel—alumel thermocouples and optical or surface pyrometers. Temperature measuriag devices are placed ia multiple locations and protected to allow replacement without iaciaerator shutdown (see... 

The Ferranti-Shidey viscometer was the first commercial general-purpose cone—plate viscometer many of the instmments stiU remain in use in the 1990s. Viscosities of 20 to 3 x 10 mPa-s can be measured over a shear rate range of 1.8-18, 000 and at up to 200°C with special ceramic cones. Its features include accurate temperature measurement and good temperature control (thermocouples are embedded in the water-jacketed plate), electrical sensing of cone—plate contact, and a means of adjusting and locking the position of the cone and the plate in such a way that these two just touch. Many of the instmments have been interfaced with computers or microprocessors. 

The temperature of the gas leaving the sulfur burner is a good indication of SO2 concentration, even though the thermocouples employed for temperature measurement (qv) frequently read somewhat lower than the tme temperatures, because of radiation and convection errors. A temperature of 970°C corresponds to about 10.0 vol % SO2, 1050°C to 11.0 vol % SO2, and 1130°C to 12.0 vol % SO2. Other temperatures and concentrations are in similar proportion. 

Measurement of the hotness or coldness of a body or fluid is commonplace in the process industries. Temperature-measuring devices utilize systems with properties that vaiy with temperature in a simple, reproducible manner and thus can be cahbrated against known references (sometimes called secondaiy thermometers). The three dominant measurement devices used in automatic control are thermocouples, resistance thermometers, and pyrometers and are applicable over different temperature regimes. 

Thermocouples Temperature measurements using thermocouples are based on the discovery by Seebeck in 1821 that an electric current flows in a continuous circuit of two different metalhc wires if the two junctions are at different temperatures. The thermocouple may be represented diagrammaticaUy as shown in Fig. 8-60. A and B are the two metals, and T and To are the temperatures of the junctions. Let T and To be the reference junction (cold junction) and the measuring junc tion, respectively. If the thermoelectric current i flows in the direc tion indicated in Fig. 8-60, metal A is customarily referred to as thermoelectricaUy positive to metal B. Metal pairs used for thermocouples include platinum-rhodium (the most popular and accurate), cmromel-alumel, copper-constantan, and iron-constantan. The thermal emf is a measure of the difference in temperature between To and T. In control systems the reference junction is usually located at... 

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

Total Radiation Pyrometers In total radiation pyrometers, the thermal radiation is detec ted over a large range of wavelengths from the objec t at high temperature. The detector is normally a thermopile, which is built by connec ting several thermocouples in series to increase the temperature measurement range. The pyrometer is calibrated for black bodies, so the indicated temperature Tp should be converted for non-black body temperature. 

Because indirect-heat calciners frequently require close-fitting gas seals, it is customaiy to support aU parts on a selFcontained frame, for sizes up to approximately 2 m in diameter. The furnace can employ elec tric heating elements or oil and/or gas burners as the heat source for the process. The hardware would be zoned down the length of the furnace to match the heat requirements of the process. Process control is normaUy by shell temperature, measured by thermocouples or radiation pyrometers. When a special gas atmosphere must be maintained inside the cyhnder, positive rotaiy gas se s, with one or more pressurized and purged annular chambers, are employed. The diaphragm-type seal ABB Raymond (Bartlett-Snow TM) is suitable for pressures up to 5 cm of water, with no detectable leakage. 

Analysts should review the technical basis for uncertainties in the measurements. They should develop judgments for the uncertainties based on the plant experience and statistical interpretation of plant measurements. The most difficult aspect of establishing the measurement errors is estabhshing that the measurements are representative of what they purport to oe. Internal reactor CSTR conditions are rarely the same as the effluent flow. Thermocouples in catalyst beds may be representative of near-waU instead of bulk conditions. Heat leakage around thermowells results in lower than actual temperature measurements. 

Temperature Measurement shift. Measurement not representative of process. Indicator reading varies second to second. Ambient temperature change. Fast changing process temperature. Electrical power wires near thermocouple extension wires. Increase immersion length. Insulate surface. Use quick response or low thermal time constant device. Use shielded, twisted pair thermocouple extension wire, and/or install in conduit. 

Thermocouples are primarily based on the Seebeck effect In an open circuit, consisting of two wires of different materials joined together at one end, an electromotive force (voltage) is generated between the free wire ends when subject to a temperature gradient. Because the voltage is dependent on the temperature difference between the wires (measurement) junction and the free (reference) ends, the system can be used for temperature measurement. Before modern electronic developments, a real reference temperature, for example, a water-ice bath, was used for the reference end of the thermocouple circuit. This is not necessary today, as the reference can be obtained electronically. Thermocouple material pairs, their temperature-electromotive forces, and tolerances are standardized. The standards are close to each other but not identical. The most common base-metal pairs are iron-constantan (type J), chomel-alumel (type K), and copper-constantan (type T). Noble-metal thermocouples (types S, R, and B) are made of platinum and rhodium in different mixing ratios. 

Manual on the Use of Thermocouples in Temperature Measurement. ASTM Manual MNL 12. American Society for Testing and Materials, 1993. 

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