Dewar seal

Finished multiple seals of this type are shown in Figure 40, 7F and V. The outer tube may be sealed directly to the apparatus for which it is required, or a Dewar seal may first be formed as shown in Figure 50, VII, making the multiple seal re-entrant. The re-entrant form is very useful when, for example, the vnres are directly attached to, and are supporting, electrodes (see p. 157). 

The Dewar seal is important and useful, although a ring seal can sometimes be substituted in simple apparatus—as, for example, in the cold finger liquid air trap shown in Figure 52, III. In the making... 

The work which led to the modern vacuum flask has been described by J. Dewar (1896). Vacuum jacketed flasks are used extensively in scientific laboratories and can be constructed relatively easily tubing is chosen to give a flask of the required size and, after the inside tube has been rounded off at one end, a Dewar seal to the outer tube is made at the other end (Figure 51,7). If a flask with a narrowed neck is required the parts must be prepared as in Figure 51, II, before the Dewar seal is made. 

Cold finger refrigerant traps are often used in vacuum lines as a substitute for the more efficient total immersion traps which, however, tend to cut down the pumping speed of the system. In constructing a cold finger trap a Dewar seal is first made, then, before... 

Figure 52, 111, shows how a quite satisfactory cold finger trap can be made with the Dewar seal replaced by a straightforward internal seal. As before, the side arm must be added immediately the internal seal is completed and the whole annealed together. 

Adiabatic calorimetry. Dewar tests are carried out at atmospheric and elevated pressure. Sealed ampoules, Dewars with mixing, isothermal calorimeters, etc. can be used. Temperature and pressure are measured as a function of time. From these data rates of temperature and pressure rises as well as the adiabatic temperature ri.se may be determined. If the log p versus UT graph is a straight line, this is likely to be the vapour pressure. If the graph is curved, decomposition reactions should be considered. Typical temperature-time curves obtained from Dewar flask experiments are shown in Fig. 5.4-60. The adiabatic induction time can be evaluated as a function of the initial temperature and as a function of the temperature at which the induction time, tmi, exceeds a specified value. 

Certain equipment configurations allow for the use of Dewar flask testing at elevated pressures. Several arrangements have proved successful such as a sealed glass ampoule in the Dewar flask, a steel pressure vessel in the flask, a Dewar flask in an autoclave under inert gas pressure, and a stainless steel Dewar flask. Dewar flasks provided with an addition line can also be used to study chemical reactions. In Figure 2.21, typical temperature-time curves of Dewar flask experiments are shown. 

Figure 11.1 (a) Scheme of an isoperibol titration calorimetry apparatus A Dewar vessel B lid C stirrer D electrical resistance E thermistor F titrant delivery tube G O-ring seal, (b) Vessel for isothermal operation A stainless-steel, platinum, or tantalum cup B water-tight stainless steel container C heater D Peltier thermoelectric cooler E O-ring seal F heater and cooler leads. Adapted from [211],... 

One of the sessions of the Symposium was largely devoted to presentation and discussion on the use of various experimental calorimetric methods for use in assessing possible hazards in chemical processing operations. The methods described covered a wide range of sample sizes and degrees of complexity Grewer, T. Adiabatic small-scale reaction test in Dewar, simple to operate. Janin, R. Measurements of heat release by DSC and of pressure development in sealed microcapsules. 

Seal off offtake 2 and spill metallic zirconium into section 5 via offtake 6. Tightly close offtake 6 with a rubber stopper. Connect the apparatus via offtake 8 to a vacuum system for 15-20 min. Next place test tube 1 into a Dewar vacuum flask with liquid nitrogen and seal off offtake 6. Perform this work wearing eye protection in the presence of your instructor ) After the pressure in the system becomes equal to 10 mmHg (in about 40-60 minutes), seal off offtake 8. Stop cooling test tube 1 and wait until the apparatus acquires room temperature. 

Fio. 9. Sketch of field emission microscope assembly for mobility studies. D, inner Dewar <8, screen A, anode T, tip, TA, tip assembly M, platinum foil mortar, filled with copper wires, oxidized in ssiln electric heating of M produces a controllable flux of oxygen MA, gas emitter assembly V, vacuum lead, sealed off. Outer Dewar and electrical leads are not shown. 

Valve I is opened and the trimethylindium is sublimed, under static vacuum, into the 2-L flask (trap-trap distilled). A hot-air gun is used to assist this sublimation process and to ensure that all the trimethylindium is collected on, or near, the surface of the cold, or frozen, amine/petroleum spirit suspension. When all the trimethylindium is trapped into the 2-L flask, valve D is closed to seal the 2-L flask and the rest of the system is filled with air through valve H (make sure valve D is closed ). The liquid nitrogen Dewar is removed and the 2-L flask is disengaged from the rest of the system by careful disconnection between the adaptor housing valve D and the U tube. The top of this adaptor is plugged with a stopper. 

On completion of the decomposition, the product is sealed, in vacuo, in the collection flask by closing valve M. The oil bath and liquid nitrogen Dewar are removed, and after cooling of the reaction flask to ambient temperature, the remainder of the system is opened cautiously to air through valve L. 

---