Helium consumption

To obtain temperatures above 4.2K, one can use the main 4He bath and regulate the experiment temperature by means of a thermal impedance and a heater. The method is expensive, in terms of helium consumption, especially for T > > 4K. 

The enthalpy of copper at nitrogen temperature is H77K = 6 J/g, so the total entropy of the sphere will be about 6 x 106 J. The time needed to cool from 77 K down to 4K is of the order of 4h. The total helium consumption from room temperature to 4.2 K would be about 6001. The temperatures reached in a test run are reported in Table 16.2. The expected final sphere temperature is about 20 mK. A comparison of MiniGRAIL and Nautilus cool down is made in Table 16.2. The high power leak on the sphere has been attributed to a time-dependent heat leak caused by the ortho-para conversion (see Section 2.2) of molecular hydrogen present in the copper of the sphere (see Fig. 16.5) (the Nautilus bar instead is made by Al). A similar problem has been found in the cool down of the CUORICINO Frame (see Section 16.6). 

Basically, no special devices have been developed to handle mobile phases in nano-HPLC. However, our experience dictates that reservoirs small in volume and of high quality glass are preferred for this purpose. Solvent containers should be air tight and free from any contamination. The use of helium gas, through a sparging device, may be beneficial for degasification of solvents in nano-HPLC, which can improve check valve reliabilities, especially at nano flow, and diminish baseline noise in UV detection. Besides, each reservoir should be equipped with a shutoff valve for efficient helium consumption [9]. 

At temperatures of about 2 x 10 °C the so-called a-process also becomes possible. Under these conditions the y-rays produced in the helium consumption are sufficiently energetic to bring about the change... 

Table I Typical annual helium consumption and associated cost for a superconducting magnet system between 1967 and 1980. Underlying assumption typical refill volume 30 liters average price per liter = 5.00.

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. 

The pump occupies a volume of approximately 1 ft , with more specific dimensions as shown in Fig. 2. In order to minimize helium consumption, every effort has been made to shield the cold plate and its supply lines from thermal radiation of over 100°K. The cold plate is totally enclosed by chevrons and a conductance-cooled frame. The helium supply line is shielded from atmospheric temperature inside as well as outside of the chamber. This shield is cooled by liquid nitrogen lines. Outside of the chamber the shield also acts as a vacuum barrier. 

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. 

Liquid helium is also used to attain the temperatures needed to study the low-temperature properties of matter, e.g. superconductivity. Use within low-temperature physics and cryogenics accounts for 30% of helium consumption. 

With the cost of helium (at least from a UK perspective) also looking likely to increase in the following years, the choice of magnet system is becoming increasingly important. There are considerable trade-offs relating to field stability, homogeneity, helium consumption and sweep hysteresis. 

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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