Air Sample Pumps - Panasonic PT-DZ12000U Operating Instructions Manual

III AIR SAMPLE PUMPS
Equipment used to generate vacuum is similar to air
compressors. It's even possible to generate compressed air
or vacuum with the same machine, depending on how it is
installed. Vacuum pumps generally can be considered as
compressors in which the discharge rather than the intake is
at atmospheric pressure. The vacuum in a chamber is
created by physically removing air molecules and exhausting
them from the system. Removing air from the enclosed
system progressively decreases air density within the
confined space, thus causing the absolute pressure of the
remaining gas to drop and a vacuum is created. Because the
absolute maximum pressure difference that can be produced
is equal to atmospheric pressure (nominally 29.92"Hg at sea
level), it is important to know this value at the work site.
For example, a pump with a maximum vacuum capability
of 24"Hg cannot generate a 24" vacuum when the
atmospheric pressure is 22" Hg (as in Mexico City, for
instance). The proportion of the air evacuated will be the
same, however. This pump therefore will pull
22 x 24/29.92 = 17.6"Hg vacuum in Mexico City.
• The maximum pressure difference produced by pump action
can never be higher than 29.92"Hg (14.7 psi), since this
represents a perfect vacuum.
• The mass of air drawn into the pump on each suction stroke,
and hence the absolute pressure change, decreases as the
vacuum level increases.
• At high vacuum levels, there is significantly less air passing
through the pump. Therefore, virtually all the heat generated
by pump operation will have to be absorbed and dissipated by
the pump structure itself.
INTRODUCTION