s 



BULLETIN 1022, U. S. DEPARTMENT OE AGRICULTURE. 



conductivity of copper at about 1.0, of brass at about 0.25, and of 

 glass at 0.002, the chances of error from heat conduction are hun- 

 dreds of times greater with this thermocouple than with the ther- 

 mometer. 



It must be borne in mind that in experimental work with canned 

 foods small inaccuracies are of minor importance. Because of the 



differences in the con- 

 sistency of the material, 

 of difficulty in prepar- 

 ing uniform packs, of 

 difficulty in adjusting 

 closely the retort tem- 

 peratures throughout 

 the entire process, and 

 of other factors, it is 

 practically impossible 

 to check time-tempera- 

 ture curves closer than 

 0.5° C, and it is doubt- 

 ful whether a difference 

 of 0.5° C, or perhaps 

 even more with mate- 

 rials of this kind, is of 

 any practical signifi- 

 cance. 



PRACTICAL VACUUM TESTS. 



In an attempt to de- 

 termine the optimum 

 temperature for sealing, 

 cans of various mate- 

 rials were prepared in 

 lots which were sealed 

 at the temperatures of 

 20°, 50°, 60°, 70°, 80°, 



90°, and 100° C, respec- 

 tively. In some cases, 

 instead of this proce- 



Fig. 2,-Apparatus used in the concussion tests. ^^ the C|mg ^^ ex _ 



hausted for two, four, or six minutes and then sealed. In each case 

 the lots were divided and one half were tested for vacuum with a 

 common Young's vacuum gauge, and the remainder were subjected 

 to concussion tests to learn to what extent the vacuum affects the 

 denting and collapsing of cans in ordinary handling. 



