Cryostats consumption

Cryostats have been much used in small-molecule crystallography because they allow rapid cooling to very low temperatures with minimal cold gas consumption and offer great advantages in the area of frost prevention. Unfortunately, most designs employ beryllium shrouds or other nontransparent material, and are of a size which does not lend itself to crystals mounted in capillary tubes and flow cells. A recent advance is the description of a Mylar cryostat specifically designed for... 

The temperature of the reactor content (Tj.) is controlled by varying the power of a compensation heater inserted directly into the reactor content. As with an electrical heater, cooling is not possible, so the compensation heater always maintains a constant temperature difference between the reactor jacket and the reactor content. Thus cooling is achieved by reducing the power of the compensation heater. The heat-flow rate from the reactor content through the wall into the cooling liquid ( Fiow) is typically not determined because the heat-flow rate of the reaction is directly visible in the power consumption of the compensation heater. The temperature of the cooling liquid (Tj) is maintained at a constant temperature by an external cryostat. The power-compensation principle was first implemented by Andersen and... 

Tensile tests of extremely fine wires can be successfully performed in the temperature range of 4.2° to 300°K using the basic apparatus previously developed [ ], a new modification of a universal type of test chamber, and specially developed gripping techniques. If fine wires were the only type of specimen of interest or a large volume of these tests was anticipated, it would appear desirable to construct a similar, but smaller, test chamber to minimize liquid-helium consumption. The particular cryostat used in this investigation is of a universal type and is capable of accommodating various types of mechanical tests. By inserting appropriate fixtures between the pull rods, it is possible to perform compression, bend, and tensile tests of specimens of assorted sizes and shapes. 

Brief descriptions of other tensile cryostats have been given in the literature [8,9,10] from a machine of very small size for temperatures down to 20 K [8] to a large machine of 60,000 lb force capacity down to 4,2 K [9], No mention of liquid consumption was given for these machines. 

The virtues of the present design are simplicity, portability (static vacuum insulation is used), safety with the use of liquid hydrogen, low liquid hydrogen or helium consumption when these liquids are used, the ability to recover all helium gas if desired and the ability to obtain any temperature in the range between 4 and 300 K without the use of external heat exchangers. The cryostat was designed to withstand forces of up to 5000 lb. These features will be discussed in detail or will become obvious in the succeeding sections. 

CONTAINER. The basis of the cryostat, which is shown in Fig. 1, is a standard commercial stainless steel dewar vessel 16 in. high and 5 in. OD (A). These cans are sturdy units, readily available, and their small size contributes greatly to the portability of the completed cryostat as well as to the low consumption of liquid hydrogen and helium. The single dewar is used in preference to the double dewar arrangement often employed with liquid helium in order to escape complexity and the associated vacuum problems. The sacrifice in thermal performance is more than adequately compensated by convenience and simplicity. 

LIQUID CONSUMPTION, When used to maintain temperatures at 20 K, for each tensile experiment the cryostat uses about 2 liters of liquid nitrogen for pre-cooling the equipment and about 2 liters of liquid hydrogen of which 1.8 liters is accounted for by the capacity of the cryostat and the remainder is vaporized in transfer from the storage dewar andin cooling the cryostat to 20°K. 

The schematic of a standard on-top bath cryostat [8] is shown in Figure 3.5.3. Radiation shields and a fN2 tank surround the inner fHe tank. At the bottom of the IHe tank, the microscope is suspended by springs and radiation cooled by two concentric radiation shields that are connected to the f He and fN2 tanks, respectively. For fast conductive cooling, the microscope is pressed against the base plate of the f He reservoir. After cooling down, the microscope is released from the base plate for better vibration isolation [8j. The helium consumption of such a cryostat is about 0.11 h [12]. 

Flow cryostats can be operated at various temperatures up to room temperature [9]. These cryostats allow for a faster cooling down than bath cryostats. However, the lowest temperature is usually a few degrees higher than that achieved with a bath cryostat and also the helium consumption rate is considerably higher, namely about 1.3 lh [12]. 

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