Micromeritics AccuPyc II 1340 - C. THEORY; Gas Displacement Theory for 10-, 100-, 350-cm3 Units

AccuPyc II 1340 Appendix C
June 2013 C-1
C. THEORY
This appendix provides theoretical information on the analysis function for the AccuPyc.
10-, 100-, and 350-cm
3
Units
The AccuPyc is a gas displacement pycnometer which measures the volume of solid objects of
irregular or regular shape whether powdered or in one piece. A greatly simplified diagram of
the instrument is shown below.
Assume that both V
CELL
and V
EXP
are at ambient pressure P
a
, are at ambient temperature T
a
,
and that the valve is then closed. V
CELL
is then charged to an elevated pressure P
1
. The mass
balance equation across the sample cell, V
CELL
, is
(1)
where
n
C
= the number of moles of gas in the sample cell
R = the gas constant
T
a
= the ambient temperature
The mass equation for the expansion volume is
(2)
where
P
a
= ambient pressure
n
E
= the number of moles of gas in the expansion volume
When the valve is opened, the pressure falls to an intermediate value, P
2
, and the mass balance
equation becomes
(3)
V
SAMP
Calibrated Sample
Cell Volume
V
CELL
Calibrated Expansion
Volume
V
EXP
Valve
P
1
V
CELL
V
SAMP
–n
C
RT
a
=
P
a
V
EXP
n
E
RT
a
=
P
2
V
CELL
V
SAMP
– V
EXP
+ n
c
RT
a
n
E
RT
a
+=