Temperature measurement and control

Occurrence of the temperature inhomogeneity along a film during its heating was reported recently (45f). Different results were obtained when temperature measurement and control were effected only in one point, and in three points of the desorption cell, which illustrates that caution should be observed in this respect when studying thermodesorption from the films. This caution is obviously yet more essential when working with powdered materials. 

These instruments, designed by CSIRO and Milestone, include, in addition to pressure and temperature measurement and control, a number of other features allowing for greater safety and reproducibility of reaction conditions, such as stirring to minimize temperature gradients, rapid cool-down at the end of the heating period and energy shut-down if temperatures or pressures exceed safe levels. 

Laboratory furnaces. Several types of furnaces are used in the laboratory these are often available as commercial rigs, generally equipped with more or less sophisticated temperature measurement and control devices. As an alternative, a lab-made or commercial furnace and its temperature measuring devices may be connected to a multi-channel data acquisition/actuator/switch unit, to be programmed by a personal computer, in order to plan and carry out thermal treatments, to collect and retrieve measured thermal data, etc. 

Temperature measurements. It is well-known that several generally commercially available instruments, based on different principles, may be used for temperature measurement and control. We will take a look at a few types, but a little more attention will be devoted to the thermocouple devices. 

J. S. Johnston, Editorial of special issues on temperature, Measurement and Control 20, 5 (1987). 

J. R. Leigh, Temperature Measurement and Control, INSPEC, Edison, NJ (1988). 

G. K. McMillan, Advanced Temperature Measurement and Control, ISA, Research Triangle Park, NC(1995). 

Chapters XVI-XX deal with basic experimental methods of broad value in many types of experimental work—electronic measurements, temperature measurement and control, vacuum techniques, diverse instruments that are widely used, and miscellaneous laboratory procedures. These chapters have been revised and updated in various ways. In the case of Chapters XVI and XVIII, the text has also been shortened from that which appeared in the seventh edition. Finally, Chapter XXI presents a thorough discussion of least-squares fitting procedures. 

G. W. Oetjen, W. Ehlers, U. Hackenberg, J. Moll, and K. Neumann, Temperature measurements and control of freeze-drying processes, in Freeze-Drying of Foods, Natl. Acad, of Sciences, Natl. Res. Council, ed., Washington, D.C., 1962, pp. 25-42. 

According to Maciel [22] "The availability of high-resolution NMR techniques for solids permits the use of NMR as a structure-determination bridge between the solid and liquid states of matter". Structural information is provided which is not available from X-ray diffraction. For decomposition studies, variable temperature facilities are required. Haw [20] has described the problems associated with such measurements, including rotor design and also temperature measurement and control. 

Foxboro Bulletin F-20B, Temperature Measurement and Control Systems, (My, 1982)... 

The measurement of temperature is one of the most common physical measurements routinely made. It is so common that it is often overlooked as a variable when complex biochemical reactions are being studied. This is unfortunate, because an error in the temperature of a reaction may produce a large error in the results that becomes apparent when the results are compared with those of known standard reactions. For example, if the rate of reaction of an unknown enzyme is being studied at a temperature that is different by 0.1°C from the temperature at which the standard reaction was measured, an error as large as 2-5% in the observed rate of reaction can occur. The experimental data would not correlate then with the known enzyme reaction rates. Such errors lead to confusion in determining mechanisms and to the large variations that occur even in normal values from one clinical laboratory to another. This article seeks to bring the importance of accurate temperature measurements to the attention of biomedical scientists. We will identify the latest methods of temperature measurement and control as well as new temperature fixed-point standards that are or will shortly become available. 

Figure 1A is a schematic of the temperature measurement and control circuit. A type T thermocouple (see Note 2), inserted in a glass capillary, measures the temperature of a capillary with similar characteristics to capillaries holding the reaction mixture. The thermocouple is connected to an A/D channel of the data acquisition system (DAS) in a differential configuration (rrrNote 3). The DAS system is also used to communicate output commands (D/A channels) from the computer to the thermocycler. The DAS system employed in this study has eight channels available that can be used as either inputs or outputs. The fans and the heater are connected to the output channels of the board via solid state relays. 

Variable temperature accessories are now available with commercial spectrometers. The nature of restricted space in a magnet gap and the necessity of constant temperature at the pole faces renders temperature measurement and control a much more difficult problem than in most other kinetic and equilibrium measurements. The space requirement necessitates a flow system for heat transfer with good Dewar vessels for leads and N.M.R. probe attachments. The heat-transfer medium has... 

The apparently simple task of precise temperature measurement (and control) is really not so at all, since tolerable temperature ranges become small, particularly at higher temperature levels. The Arrhenius definition ... 

Thermogravimetric instrumentation should include several basic components in order to provide the flexibility necessary for the production of useful analytical data. These components are (a) a balance (b) a heating device (c) a unit for temperature measurement and control (d) a means for automatically recording the mass and temperature changes and (e) a system to control the atmosphere around the sample. 

Temperature Measurement and Control. Temperature-sensing devices are usually thermocouples placed as close to the sample as possible. Thermocouples are inexpensive, rugged, and fairly linear in their response to temperature changes. Platinum resistance thermometers are also used in this application. The emf generated by the thermocouple may be used to drive one axis of an x-y recorder, or a feedback circuit to the heater may be used to obtain a programmed linear heating rate. In the latter case, the time axis of a strip-chart recorder is proportional to temperature. Instruments that depend upon a linear increase in power to the heater often have severely nonlinear temperature increases because of heat losses to the environment. 

The measurement and control of the temperature of experimental apparatus in cryogenic environments has been widely explored p]. Problems in such measurement and control by thermoelectric and thermal resistance effects are receiving constant attention. However, the application of Chromel-P vs. constantan thermocouples to cryogenic temperature measurement and control has not become widespread. The reason for this limited usage is not clear, especially since the sensitivity and potential 2. 3] fQj. his thermocouple system are higher than for the more popular copper vs. constantan thermocouple system. Furthermore, the use of low-thermal-conductivity Chromel-P P] wire, instead of copper wire, would reduce heat leaks into cryogenic systems. 

In addition to the two-point temperature control, other temperature measurements and control loops in each zone can be added to act as control monitors. Through low select devices on the output signal, these monitors can automatically take control of energy input to prevent hot spots. With sufficient monitors, overshooting of product temperature can be eliminated. 

If a line stop occurs, the 2200 F (1200 C) zone temperature can cause strip thinning or separation. Therefore, a protective control scheme is needed. (See temperature measurement and control discussions that follow.)... 

The major requirements for a successful ebulliometry experiment are diermal stability, equilibration of both concentration and temperatnre, temperature measurement and control and pressure measurement and control. It is an advantage of ebulhomelry to know very exactly die constant pressure applied since pressure constancy is a prerequisite of any... 

In the cryoscopic method, the freezing temperature of a solution is compared with that of the pure solvent. The polymer must be solvable in the solvent at flie freezing temperature and must not react with the solvent either ehemieally or physically. Difficulties may arise from limited solubility and from the formation of solid solutions on freezing. Application of cryoscopy to polymer solutions is not widespread in literature despite the simplicity of the required equipment. Cryoscopy was reviewed by Glover, who also discussed technical details and problems in concern with application to polymer solutions. A detailed review on cryometers and cryoscopic measurements for low-molar mass systems was recently made by Doucet. Cryometers are sold commercially, e.g., Knauer. Measurements of thermodynamic data are infrequent. Applications usually determine molar masses. Accurate data require precise temperature measurement and control as well as caution with the initiation of the crystallization process and the subsequent establishment of equilibrium (or steady state) conditions. High purity is required for the solvent and also for the solute. 

Temperature measurement and control. To understand the effect of temperature on gas solubility measurements, there are three factors to consider ... 

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