8 PROCEEDINGS OF THE NATIONAL MUSEUM vol. 119 



Due to the difference in the coefficients of expansion of the two 

 materials at operating temperatures, however, fracturing of the steel- 

 to-copper attachments developed. To overcome this difficulty, 

 the refrigeration fine was soldered to the outer siu-face of a copper 

 sleeve, negating the need for attachment between the copper tubing 

 and the steel wall (fig. 4). This innovation not only solved the frac- 

 tm-ing problem but also established the refrigeration line as a de- 

 tachable unit within the system, allowing easy removal from the 

 chamber for maintenance and repair. 



Since the refrigeration line entered and left the chamber through 

 two small openings in the rear of the tank, the openings around the 

 tubing were sealed with soft solder. When installation of the re- 

 frigeration line was completed, the chamber was tested thoroughly 

 for vacuum leaks. 



Refrigeration of the specimen chamber. — Withm an evacuated 

 system, heat is transferred principally by radiation. Radiation 

 travels in a Ime-of-sight path between a specimen and surroimding 

 surfaces. There is no heat produced within the specimen chamber; 

 therefore, the only appreciable load on the refrigeration system is 

 ambient heat that leaks in through the insulation. This load can be 

 calculated in BTU's per hour from the information given (p. 17). 



Any refrigeration compressor (q.v.) serving the specimen chamber 

 shoidd be capable of producing controlled temperatures at the chamber 

 walls of —15° to —30° C. (after heat leak has been taken into account). 



When selecting a refrigeration compressor, it is desirable to double 

 the calcidated heat load (primarily heat leak) to allow a sufficient 

 margin for error (table 1). 



Table 1. — Refrigeration compressor capacity in BTU's per hour (based on a 90° 

 F. ambient temperature; r-12 (freon-12)=Dichlorodifluoromethane; F-22 

 (freon-22)=Chlorotrifluoromethane) 



Condensing chamber.— The physical requirements for the condens- 

 ing chamber are similar to those of the specimen chamber. 



