CHAPTER 16
VACUUM TECHNIQUE
…THE PRODUCTION or VACUA
16—1.
Introduction.—Some pumps operate from atmospheric
pressure,
while others require that a
“ fore pump” reduce the pressure considerably before they Will start pumping.
j
The speed of a pump may be measured by observing the rate
of fall of pressure during the evacuation of a ask of known volume
.
(V, liters).
With a constant leak of gas into the ask, Gaede’s£
equation is
'
'i
»
,
.
S = ——
(lé—l)
°
_
Ï2 _
P2 _ PO
_
where‘ 8 is the speed in liters per second at the mean pressure
(l + 132) /2, 191 is the pressure
in m1lll—
lWater
meters
of mercury
at
the time z‘1 (sec.
ends) , 172 is the pressure
at a later time
‘
(lg) and po is the lowest pressure attamFrom
_““““
'
*
'
'
Vacuum
Bble bY the PBmP'
W1th PumPS “l‘“
.
System
mg a fore… vacuum, the speed depends
.
,
A
on
the vacuum produced by thefore
'
“
{'
'
The speed of evacuati0n of an enclosed
…
.
-
_
system depends not only on thespeed
ÿ
"
B
of the Pump bUt also on the size °fthe
the gaseS
‘
r
,
-
<
ÎÏ.
,OÏ agWen…çoÎbY
l
hencPressure—Thewaær a; www
FIG16—1Thewaterält f
h
‘.
,
16
pt
??” PBmP "S °WBçm €“re ) “,…“ mBV°