Thermocouple shielding

Fiber-optic thermometers can be applied up to 300°C, but are too fragile for real industrial applications. In turn, optical pyrometers and thermocouples can be used, but pyrometers measure only surface temperatures which in fact can be lower than the interior temperatures in reaction mixtures. Application of thermocouples which in case of microwaves are metallic probes, screened against microwaves, can result in arcing between the thermocouple shield and the cavity walls leading to failures in thermocouple performance. 

Figure 5. Assembly drawing (a) and general view before assembly (b) of M-H container 1 -case, 2 - heat exchanger, 3 - filter with gas input/output fitting, 4 - point for metal-hydride loading, 5 - thermocouple shield, 6 - position for electric heater, - welded seams.

A thermocouple shielded by aluminum foil of eniis-sivily 0.15 is used to measure the temperature of hot gases flowing in a duct whose walls ate maintained at 380 K. The thermometer shows a temperature reading of = 530 K. Assuming the emissivity of the thermocouple junction to be c = 0.7 and the convection heat itansfer coefficient to be h = 120 W/m "C, determine the actual temperature of the gas. What would the Ihermomcler reading be if no radiation shield was used ... 

Sulfur desorption fiom the poisoned nickel catalysts was studied by temperature programmed hydrogenation (TPH). The poisoned catalyst beds were powdered to homogenize them. Approximately 0.5 g of powdered catalyst was placed in an atmospheric quartz tube reactor which had a thermocouple shield. The reaction gas was argon/hydrogen (70% Ar/30% Hj). The content of hydrogen in the gas was about the same as in the sulfurpoisoning tests. 

Tempering ring Leads with heaters Tube thermocouples Shield with heaters Calorimeter heater Thermometer Shield thermocouples... 

The transient isotope-switching apparatus has been described previously. The reactor was a plug-flow, fixed-bed quartz microreactor, 8 mm in diameter, with a total volume of 0.8 cc. Approximately 0.4 g of catalyst was supported in the middle of the reactor on a fused quartz frit. Temperature in the catalyst bed was measured with a chromel-alumel thermocouple shielded in a quartz jacket. The reactant gas stream was controlled by a mass flow control switching system capable of switching one or more of the reactants to its isotopically labelled counterpart in approximately 1 s. The partial pressures and flow rates of the reactants are not altered by this switch so that the steady-state concentrations of reactants and products in the gas phase, and on the catalyst surface, are not disturbed. 

FIGURE 47.9 Schematic plan view and detailed side view of an adjustable oven characterization fixture (oven diagnostic tool) composed of pairs of thermocouples, shielded from one another by a rigid, heat-resistant insulator. The thermocouple positions are adjustable in width and length for investigating temperature distribution along the tunnel width. 

Standard commercial iastmmentation and control devices are used ia fluorine systems. Pressure is measured usiag Bourdon-type gauges or pressure transducers. Stainless steel or Monel constmction is recommended for parts ia contact with fluoriae. Standard thermocouples are used for all fluorine temperature-measuriag equipment, such as the stainless-steel shielded type, iaserted through a threaded compression fitting welded iato the line. For high temperature service, nickel-shielded thermocouples should be used. 

In industrial appHcations it is not uncommon that the thermocouple must be coupled to the readout instmment or controUer by a long length of wire, perhaps hundreds of feet. It is obvious from the differential nature of the thermocouple that, to avoid unwanted junctions, extension wine be of the same type, eg, for a J thermocouple the extension must be type J. Where the thermocouple is of a noble or exotic material, the cost of identical lead wine may be prohibitive manufacturers of extension wine may suggest compromises which are less costiy. Junctions between the thermocouple leads and the extension wine should be made in an isothermal environment. The wine and junctions must have the same electrical integrity as the thermocouple junction. Because the emf is low, enclosure in a shield or grounded conduit should be considered. 

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. 

Figure 7. Ion source and reaction chamber for the study of ion-molecule reactions at different temperatures. Notation used is same as in Figure 4 except CB = copper block, EL = electrode attached to pressure reducing capillary, TC = thermocouple, TS = thermal shield, ISP = evacuated space reduces thermal conductivity from CB to flange. From Klassen, J. S. Blades, A. T. Kebarle, P. ). Am. Chem. Soc. 1996, with permission.

Fig. 12.3 MultiPREP 36/P rotor used in an ETHOS multimode microwave reactor. Temperature measurement with a shielded thermocouple in one reference vessel (Milestone, Inc.) [80].

Figure 10.4 Adiabatic high-temperature calorimeter [15], 1 Calorimeter proper 2 Silver guard 3 Silver shield 4 Shield heater 5 Thermocouple sleeve 6 Silica glass container 7 Sample 8 Calorimeter heater 9 Pt resistance thermometer 10 Silica ring spacer 11 Type S thermocouple 12 Guardheater 13 Removable bottom. Reproduced by permission of F. Grpnvold.

The sample of desorbed tritide is placed inside a quartz tube that is connected to a gas-handling manifold by a TorrSeal . A quartz sleeve with Silicon Carbide (SiC) in the annular space is placed around the end of the quartz tube, surrounding the sample with microwave susceptor. The quartz tube and susceptor sleeve are thermally insulated from the rest of the microwave cavity. An internal thermocouple measures the temperature of the sample and provides the temperature signal for process control of the desired temperature. A shine block (alumina foam), attached to the thermocouple, blocks radiant heating of the TorrSeal and the upper area of the quartz tube and manifold. An IR pyrometer is used as a secondary measure of the temperature of the susceptor, and therefore of the sample. A stainless steel shield reflects microwaves from the quartz tube not in the susceptor sleeve, eliminating the production of a plasma at low pressure in the quartz tube. 

Figure 7.2.2 Schematic diagram of the flow probe developed by Dorn and co-workers and used for the direct coupling of SFC to NMR (a) insulated glass transfer line (b) glass insert (c) Cu/constantin thermocouple (d) stainless steel equilibrium coil (e) brass shield (f) Helmholtz coil (g) ceramic flow cell (h) brass Swagelok fitting. Reprinted with permission from Allen, L. A., Glass, T. E. and Dorn, H. C., Anal. Chem., 60, 390-394 (1988). Copyright (1988) American Chemical Society...

Electrolytic Procedure. The cell is placed inside a vertical-column, Hevi-Duty, multiple furnace and is surrounded by an Inconel heat shield (Fig. 13). The temperature-control thermocouple (Pt vs. 90% Pt — 10% Rh) is located between the crucible and the heat shield and is protected by an outer ceramic tube. A calibration run is initially made without a charge in the crucible. A second thermocouple is inserted through the heat shield and centered on the cover of the electrolytic cell. This thermocouple is read directly by means of a potentiometer the temperature of this thermocouple corresponds closely to that of the melt. The temperature controller is adjusted until the second thermocouple indicates the desired temperature for the electrolysis. ... 

An electrolysis can be carried out by removing the second thermocouple, filling the crucible with a charge, and connecting the leads from the power supply to the cell. An Inconel plug is used to close the thermocouple hole in the heat shield. The d.-c. constant-current power supply is turned on to warm up without any current fed to the cell. The temperature is then raised to the operating temperature of 780°C. at a rate of 200°C. per hour. When the temperature has equilibrated, the voltage is applied, and the current is adjusted to 45 ma. The electrolysis is carried out for 5 days. 

Type MK-2012, Fisher Scientific Co., 711 Forbes Ave., Pittsburgh, Pa. 15219. t This calibration must be done since a thermocouple cannot be placed directly into the melt, and the heat shield prevents the control thermocouple from giving an accurate temperature reading. 

Temperature calibration of a thermogravimetric analyzer is more complicated than with other thermoanalytical devices, since in most designs, the thermocouple junction cannot be in contact with the specimen or its container. Beyond gas flow shielding problems, temperature differences between the specimen and thermocouple junction can be exacerbated by a vacuum atmosphere in which there is no conductive medium for heat transfer and thus temperature equilibration. Even if both the specimen and thermocouple junction are exposed to the same heat flow at a given time, the specimen has a much higher total heat capacity hence, the specimen will lag the thermocouple junction in temperature. 

The thermocouple of Prob. 8-127 is placed horizontally in an air-conditioned room. The walls of the room are at 32°C, and the air temperature in the room is 20°C. What temperature is indicated by the thermocouple What would be the effect on the reading if the thermocouple were enclosed by a polished-aluminum radiation shield ... 

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