7
I. INTRODUCTION
“Pycnometer” is derived from the Greek word pyknos which has long been identified with volume
measurements. The MULTIPYCNOMETER is an instrument specifically designed to measure the
true volume of a variety of solid materials by employing Archimedes’ principle of fluid
displacement and Boyle’s Law of gas expansion. The displaced fluid is a gas which can penetrate the
finest pores to assure maximum accuracy. For this reason helium is recommended since its small
atomic dimension assures penetration into crevices and pores approaching two Ångströms (2 x 10
-
10
m). Its behavior as an ideal gas is also desirable. Other gases such as nitrogen can also be used,
often with no measurable difference.
It is used to determine the true volume of solid or powder samples by measuring the pressure
difference when a known quantity of gas under pressure is allowed to expand from a precisely
known reference volume into a sample cell holder, also of known volume, containing the sample cell
with sample.
Figure 1 is a flow diagram of the MULTIPYCNOMETER. The shaded area represents the known
reference volume(s) V
R
. After the system is purged with analysis gas, the valve that admits gas in to
the system is closed, the selector valve between V
R
and the cell holder V
C
is turned to connect them,
and the vent valves are opened. The system is now at ambient pressure P
a
and the state of the sample
cell with sample is defined by
(
)
RT
n
=
V
V
P
a
a
S
C
a
−
(1)
where n
a
is the number of moles of gas occupying the calibrated cell volume (V
C
) with sample
present of volume (V
s
), R is the gas constant and T
a
is ambient temperature in kelvin.
When the reference volume alone is pressurized above ambient (after isolating it from the sample
cell holder), the state of the reference volume (V
R
) can be expressed as
RT
n
=
V
P
a
1
R
1
(2)
where P
1
represents a pressure above ambient (17 psig, ~120kPa, for example) and n
1
is the total
number of moles of gas in the reference volume (V
R
).
When the selector valve is turned again to connect the reference volume to the sample cell holder,
the pressure will fall to a lower pressure P
2
, given by
(
)
RT
n
+
RT
n
=
V
+
V
V
P
a
1
a
a
R
S
C
2
−
(3)