14 



BULLETIN 1022, IT. S. DEPARTMENT OF AGRICULTURE. 



final result. This finding is entirely at variance with the statement 

 of Bitting and Bitting (7, p. 27) that when the container is en- 

 tirely filled the pressure is greatly increased. 



The explanation of the decrease of maximum pressure with decrease 

 of head space is found by a glance at the tables on the distortion of 

 cans and the expansion of water and the theoretical curves of figure 3. 

 By comparison of the expansion of water with the cubical expansion 

 of the can (Tables 1 and 2) it will be seen that the distortion of the 

 can under pressure is always greater than the expansion of water, 

 which results in the creation of a greater head space. Consequently, 

 the ratio between the head space and the distortion of the can is 

 greater the smaller the head space, and therefore with decrease, in 

 head space a slight decrease in maximum pressure is to be expected. 

 The greater distortion of the larger can accounts for the greater 

 variation from the theoretical. 



Distortion of can and increase in volume. — Every change in pres- 

 sure causes more or less distortion of the can, which alters its cubical 

 contents. How great this distortion actually is is shown by Table 2, 

 which gives the experimental findings on the increase in volume in 

 No. 2 and No. 3 tin cans when subjected to various internal pres- 

 sures. These figures present the averages of a number of tests. 



Table 2. 



-Increase in volume of No. 2 and No. 3 tin cans as the result of dis- 

 tortion caused by internal pressures. 



Pressure per square inch in 

 can. 



Increase 



in volume 



(c. 



c). 



No. 2 



No. 3 



cans. 



cans. 



5.4 



28.3 



12.1 



37.1 



18.0 



46.0 



22.6 



54.6 



Pressure per square inch in 

 can. 



Increase in volume 

 (c.c). 



2 \ pounds 

 5 pounds. . 

 10 pounds 

 15 pounds 



20 pounds 

 25 pounds 

 30 pounds 



It will be seen that within the range of canning operations the 

 cubical content of cans may be increased up to 5 per cent or more, 

 depending somewhat upon the size and strength of the individual 

 cans used. 



The increase in volume due to distortion of the can explains the 

 decrease in pressure, and since this distortion increases with the 

 higher temperatures greater variations from the theoretical must take 

 place. Inasmuch as sealing at the higher temperatures causes less 

 strain on the can and consequently less distortion when subsequently 

 processed, the theoretical values are more closely approached. 



