Dewars pumping

All of the preceding material has been theory - generalities. Now we address the steps needed to actually operate a new FPA type. Step 1 is to setup the electronics to generate specific biases to the desired LCCC pads, and create new timing files. Step 2 - done before we mount an FPA in the dewar - is to verify by measurement that the proper stimuli appear at the desired LCCC pads. Only when we know that all is correct at the LCCC pads will we perform Step 3 place an engineering grade FPA in the dewar, pump and cool it, and apply power. Finally, Step 4 is to evaluate the results, note unexpected results, and attempt to understand possible causes and correct them. 

If the pump is a filter pump off a high-pressure water supply, its performance will be limited by the temperature of the water because the vapour pressure of water at 10°, 15°, 20° and 25° is 9.2, 12.8, 17.5 and 23.8 mm Hg respectively. The pressure can be measured with an ordinary manometer. For vacuums in the range lO" mm Hg to 10 mm Hg, rotary mechanical pumps (oil pumps) are used and the pressure can be measured with a Vacustat McLeod type gauge. If still higher vacuums are required, for example for high vacuum sublimations, a mercury diffusion pump is suitable. Such a pump can provide a vacuum up to 10" mm Hg. For better efficiencies, the pump can be backed up by a mechanical pump. In all cases, the mercury pump is connected to the distillation apparatus through several traps to remove mercury vapours. These traps may operate by chemical action, for example the use of sodium hydroxide pellets to react with acids, or by condensation, in which case empty tubes cooled in solid carbon dioxide-ethanol or liquid nitrogen (contained in wide-mouthed Dewar flasks) are used. 

When 5.00 mL of ether has been delivered by the syringe pump, the pump is shut off The reactor is allowed to run an additional 15 min before the fluorine and the mercury arcs ate shut off. The preaerosol furnace, the evaporator heater unit, and the coolant pump are shut off. Once the system approaches ambient conditions, all the helium carriers are shut off and the product trap valves are closed The product trap and its Dewar flask filled with liquid nitrogen are removed to the vacuum line where the trap is evacuated... 

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... 

Preparation of a Typical Au-Acetone Colloid. The metal atom reactor has been described previously. (39,59, 60) As a typical example, a W-A1 0 crucible was charged with 0.5Qg Au metal (one piece). Acetone (300 mL, dried over K2C0 ) was placed in a ligand inlet tube and freeze-pump-thaw degassed with several cycles. The reactor was pumped down to 1 x 10 Torr while the crucible was warmed to red heat. A liquid N2 filled Dewar was placed around the vessel and Au (0.2g) and acetone (80g) were codeposited over a 1.0 hr period. The matrix was a dark purple color at the end of the deposition. The matrix was allowed to warm slowly under vacuum by removal of the liquid N2 from the Dewar and placing the cold Dewar around the reactor. 

It is important to note that some types of dewar necks are made of plastic materials which are permeable to gases and in particular to He. The permeation phenomenon has a strong dependence on temperature and is negligible at 4K (see e.g. ref. [6]). If the dewar remains for a long time at room temperature in an atmosphere containing He gas, the vacuum space is slowly filled with He which must be pumped before the filling with cryogenic liquids. 

Fig. 5.4. Evaporation cryostat for temperatures T > 4.2 K. A mixture of gaseous and liquid 4He is transferred from the storage dewar to the cryostat by means of a transfer tube. The mixture is pumped through a spiral tube which is first thermally anchored to the experiment chamber. The temperature of the experiment can be...

If there are no leaks, the evacuated traps are immersed slowly in a Dewar vessel full of liquid nitrogen and then the diffusion pump is switched on with and open, 7, and closed. The residual glow from the Tesla discharge should disappear rapidly from the space between the diffusion pump and T. Then 7 is opened and the traps pumped down, then 7 is opened and the space bounded by 7, 7, Tjj and Tjj, and finally the main trunk line and the subsidiary trunk line T -T are connected for hard pumping. 

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.

In all cases, the pump is connected to the still through several traps to remove vapours. These traps may operate by chemical action, for example the use of sodium hydroxide pellets to react with acids, or by condensation, in which case empty tubes cooled in solid carbon dioxide-ethanol or liquid nitrogen (contained in wide-mouthed Dewar flasks) are used. 

Fig. 12. Metal dewar liquid nitrogen cell. (A) Nitrogen fill, (B) nitrogen vent, (C) liquid nitrogen, (D) main pump out, (E) detachable tail section, (F) sample, (G) aluminum foils, (H) Mylar window, and (I) thermocouple. Reproduced from Herber and Hazony (87) with permission.

---