6 
BULLETIN 1022, U. S. DEPARTMENT OF AGRICULTURE. 
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 
of the mercury in that arm. 
_——————— SS 
ExHaust7 
Fic. 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 ecan-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. 
The 
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 be a flex- 
ible tube (C’) of rubber 
pressure tubing rein- 
forced with closely 
wound copper wire to 
minimize expansion. 
A scale D, so gradu- 
ated as to allow read- 
ings in both directions 
from 90, is so placed 
that the zero mark 
stands at x of the glass 
arm A, which repre- 
sents the constant level 
fee oe tube #, which is 
identical in structure with C, connects with an air-tight junction 
