Dewars design

One of the consequences of Dewar s work was his invention of the vacuum flask to minimize heat loss. It was expensive and time-consuming to liquefy gases hence, Dewar designed a container where, once liquefied, gases could be kept for as long as possible. Still known as the Dewar flask among chemists, it is more widely known as the Thermos, named after the company that obtained the patent for the flask and to whom Dewar lost an ensuing court case. 

Fig. 6.5 A cross section of the Dewar and alternate Dewar designs.

Experimental Dewars Developed by the National Bureau of Standards (1) 49 Breathing Oxygen Storage Dewars (2) 54 Small Liquid Hydrogen Dewars, Design and Use (2) 71... 

In the case of the photometer the complete instrument is cooled below 2K by liquid helium. The heat loads and cooled volumes have been found to be similar enough in the three cases however to consider development of a single dewar design compatible with these and presumably other Instruments. Preliminary sizing of the dewar is compatible with at least two of the model instruments being flown on any one mission. The masses, thermal control, data handling and control of these instruments present no major problems. 

For the lowest practical source setting, our choices of source, spectral bandpass, and field of view must yield irradiances equal to or less than the required background. We make the initial calculations with theoretical formulas described in Chapter 2. There are so many variables that this aspect of dewar design is a bit iterative it is discussed in Section 9.2.5. 

The rudiments of dewar design and use of cryogens are discussed in Chapter 12. Vacuum practices are discussed in Chapter 13. Windows and spectral filters are discussed briefly in the Chapter 14. 

TABLE 12.6 Heat Load Calculations for Helium Dewar Design... 

The importance of these effects is illustrated by comparing the heat load components for two hypothetical dewar designs (Table 12.6). The dewars and the calculations are clearly simplified, and some features are exaggerated Dewar A is clearly... 

Bamford and Dewar have adapted the latter method to the deduction of values for r, and hence to the determination of the individual rate constants kp and kt. They chose to observe the rate of polymerization by measuring the increase in viscosity with time, using for this purpose a specially designed reaction cell equipped with a viscometer. Having established by separate experiments the relation-... 

The typical volume of a dewar is 1001. Figure 5.1 shows the design of a commercial vacuum-superinsulated dewar for liquid 4He. Such dewars are made from aluminium or stainless steel. The evaporation rate of a good dewar is about 1% per day. 

Very practical for measurement between 4.2 K and room temperature are the so-called dipstick cryostats, designed for insertion into storage dewars (typically 5 cm neck diameter). Construction details of an insertable cryostat are reported in ref. [7],... 

Other types of equipment available to investigate the gas evolution are various autoclave tests (Section 2.3.3.2), isoperibolic autoclave tests (Section 2.3.1.2), and closed Dewar tests (Section 2.3.2.2). Mass flux data are also required in designing any vent facilities (Chapter 3). 

Modem Dewar flask equipment includes an adiabatic shield, a compensation heater, and a computer to provide for control and for data acquisition and analysis. An example of the application of an advanced design is the adiabatic storage test (AST) [120,121]. In the AST, the heat generated at nearly adiabatic conditions by the reacting or decomposing substances is determined as a function of time. 

Techniques such as adiabatic calorimetry (Dewar calorimetry) were by then well established [2, 118, 119]. All these techniques can be used for obtaining data to design for the prevention of runaway reactions, that is, to design for inherent plant safety. 

As stated at the beginning of this Introduction, (homo)aromaticity refers to a special (thermodynamic) stability relative to some hypothetical reference state. It is therefore most attractive to use a thermochemical discriminator for the designation of homoaromaticity. However, such thermochemical methods suffer the same disadvantages when applied to homoaromaticity as they do in the case of aromaticity (see for example Garratt, 1986 Storer and Houk, 1992). There have been several recent studies using the heats of hydrogenation of potential homoaromatics in an attempt to classify these species (vide infra). Due, in the main, to the hypothetical nature of the localized model reference states there is some debate regarding these results (see Dewar and Holder, 1989 Storer and Houk, 1992). 

A disadvantage of such an STM is that there is no method to treat the tip and the sample in the cryostat. The sample and the tip have to be prepared in air, the relative position in air adjusted, then the setup immersed to a dewar. The samples to be smdied are limited. However, Hess et al. (1989, 1991) have made startling progress using such an STM design in the study of superconductors. The secret of their success is to use an extremely inert superconducting material, NbSe2. 

Fig. 9.4.10 Apparatus for the gas flow-arc plasma method. The apparatus is composed of two components. The upper part is a glass Dewar, which accumulates small particles in a cryogenic matrix on the trim cooled with liquid nitrogen (LN). Sorv, inlet of organic vapor Syr, syringe for transferring produced colloids under anaerobic conditions RP, rotary pump S, target sample. Lower part is for plasma discharge. A BN furnace has gas inlets (G) and is specially designed for Ar gas to flow in screwed stream hence the plasma is emitted in a jet flame due to a plasma pinch effect. The black parts are copper electrodes cooled by water. In order to maintain a constant spacing between the surface of sample and tbe upper electrode, the sample position can move vertically so that the current through the sample to the upper electrode is precisely controlled and constant. This is very important to produce powders with a narrow size distribution.

Standard optical metal or metal-glass cryostats are commonly used in low-temperature studies of TSL and TSC. Less expensive facilities for liquid N2 temperatures have been designed, and closed cycle cryotips may be employed as well. TSR requires dynamic temperature cycles. Therefore, it is generally advantageous to design the sample holder-dewar arrangement as small as possible to enable quick turnaround times. 

---