BULLETIN 1022, U. S. DEPARTMENT OF AGRICULTURE. 



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METHODS AND APPARATUS. 



DETERMINATION OF PRESSURE. 



In the pressure and vacuum studies, preliminary experiments soon 

 showed that the pressure and vacuum gauges commonly used were 



too coarse and inaccu- 

 rate for careful work. 

 Because of their me- 

 chanical construction 

 they are always open 

 to the danger of be- 

 coming weakened, 

 which makes frequent 

 standardization neces- 

 sary. They also seem 

 abnormally sensitive 

 to barometric pressure, 

 for which corrections 

 are difficult. Their use 

 in this work was there- 

 fore abandoned except 

 in certain rough tests 

 mentioned below, and 

 in their place a special 

 mercury manometer 

 was substituted. Fig- 

 ure 1 illustrates the 

 manometer. 



This manometer 

 consists of two glass 

 arms, A and B, which 

 are connected by a flex- 

 ible tube (C) of rubber 

 pressure tubing rein- 

 forced with closely 

 wound copper wire to 

 minimize expansion. 

 A scale Z>, so gradu- 

 ated as to allow read- 

 ings in both directions 

 from 0, is so placed 

 that the zero mark 

 stands at x of the glass 

 arm A, which repre- 

 sents the constant level 

 of the mercury in that arm. The flexible tube E, which is 

 identical in structure with (7, connects with an air-tight i unction 



EXHAUST 



Fig. 1. — The manometer used in the pressure-vacuum 

 tests. A, Fixed glass arm of the manometer ; B, 

 movable glass arm ; C, flexible rubber tube wound 

 with copper wire connecting glass arms A and B ; 

 D, scale, graduated in centimeters or inches ; E, 

 flexible rubber tube wound with copper wire connect- 

 ing manometer with the can-sealing device ; F, test 

 can ; R, steam retort ; T, thermometer ; 0, zero mark 

 on the graduated scale ; X, mark on glass arm A 

 corresponding with the zero mark on the scale D, at 

 which the mercury column in A is held constant. 

 The plus and minus signs on the scale indicate the 

 portions of the scale above and below the zero mark, 

 respectively. 



