3B Scientific Critical Point Apparatus User Manual

3B SCIENTIFIC

Critical Point Apparatus 1002670

Instruction sheet

02/13 MH/JS

23

22

19
18
17

®

PHYSICS

1 Vernier scale
2 Fixed scale
3 Grease nipple
4 Threaded bush
5 Handwheel
6 Base
7 Frame
8 Threaded axle with piston
9 Piston cover
10 Outlet for thermal medium
11 Inlet for thermal medium
12 End plate
13 Cylinder
14 Conical seal

1220 21

3 4

15 Measuring cell
16 Valve plate
17 Regulating valve
18 Gas connection fittings 1/8"

(for Minican® gas container)
19 Flush valve
20 Hole for thermocouple

5

21 Heat casing
22 Safety valve
23 Manometer (excess pressure indicator)

876

On delivery, the critical point apparatus is filled with
hydraulic fluid. The test gas is not included.

Before filling with the test gas, carry out a volume
calibration, as described in chapter 6, using air as an
approximation of an ideal gas.

Filling with the test gas is described in chapter 7.
Experimental investigations are described in chap-

ter 8.
Important notes on storage of the test gas and

equipment (if not in use for a long period) are stated
in chapter 9.

1516 14

1213

911

1. Contents of instruction manual

Owing to the inevitable diffusion of the test gas
through the conical seal, it is necessary to degas the
hydraulic fluid in the equipment, as described in
chapter 10. This must be done before the equipment
is put away for storage (after removing the test gas) or
if it has been in use for a long time.

The threaded bush in the frame must be lubricated
regularly and also inspected at lengthier intervals.
Refer to section 11 for instructions.

Maintenance work as described in chapter 12 is only
required if the rubber components get worn out and
their functionality is adversely affected.

1

2. Safety instructions

When used properly, the operation of the critical
point apparatus is not dangerous, since both the
experimenter and the equipment are protected by a
safety valve. However, it is extremely important to
observe a few precautionary measures:

• Read the instruction sheet thoroughly and follow

the instructions therein.

• Do not exceed the maximum permissible values

for pressure and temperature (60 bar and 10­60°C).

• Do not operate the equipment without qualified

supervision.

• Always wear safety goggles.

Only increase the temperature at low pressure with
pure gas phase in the measuring cell.

• Before increasing the temperature, wind the

handwheel outwards so that maximum volume is
attained in the measuring cell.

When conducting adjustments, make sure that the
safety valve does not point in the direction of people
who could be injured or objects that could be dam­aged if the valve cover shoots out. When conducting
experiments, pay special attention too to the align­ment of the safety valve.

• When setting up the apparatus, make sure that

the safety valve does not point in the direction of
people who could be injured or objects that could
be damaged.

• When adjusting the safety valve, wrap your arms

around the apparatus to reach the valve at the
back.

If the conical seal is overtaxed, it could get damaged
or even destroyed.

• Never set a pressure above 5 bar if the regulating

valve or the flush valve is open, i.e. if there is no
back pressure from the gas in the measuring cell.

• Never create underpressure by turning the hand

wheel inwards when the valves are shut.

In the frame there is a threaded bush, which is to be
regarded as a safety-related feature (see section 9).

• Lubricate the threaded bush every 100 cycles.

• Inspect the threaded bush annually.

To prevent damage by corrosion inside the instru­ment,

• use a 2:1 mixture of water and anti-freeze fluid as

the thermal medium.

Only as real gas for SF

and nitrogen as ideal gas.

6

3. Description

The critical point apparatus allows us to investigate
the compressibility and liquefaction of a gas. Meas­urements allow determination of the critical point for
the gas as well as the recording of isotherms for an
adiabatic p-V diagram (Clapeyron diagram). The gas
used for testing is sulphur hexafluoride (SF

). SF6 has a

6

critical temperature of 318.6°K (45.5°C) and a critical
pressure of 3.76 MPa (37.6 bar) which makes for a
simple experiment set-up.

The critical point apparatus consists of a transparent
measuring cell of particularly well sealed, pressure­resistant design. The volume of the measuring cell
can be modified by turning a fine-adjustment wheel
and can be read by means of a fixed scale and a rotat­ing vernier scale to an accuracy of one thousandth of
the maximum volume. The pressure is applied via a
hydraulic system using castor oil approved for me­dicinal use. The measuring cell and hydraulic system
are isolated from one another by a conical seal which
rolls up when there is an increase in pressure. This
design means that any pressure difference between
the measuring cell and the oil reservoir is negligible
in practical terms. A manometer measures not the
pressure of the actual gas but that of the oil, thus
eliminating any need for a space within the measur­ing cell. When observing transitions from gas to liquid
or vice versa, the lack of such a dead space means
that the development of the very first drop of liquid
as well as the disappearance of the last bubble of gas
can be observed. The measuring cell is surrounded by
a transparent chamber of water. A circulating thermo­stat arrangement (water bath) means that a constant
temperature can be maintained during the experi­ment with a high degree of accuracy. The tempera­ture can be read and monitored using a thermome­ter.

The fact that volume, pressure and temperature can
all be read with a high degree of accuracy means that
accurate p-V diagrams or pV-p diagrams can be re­corded without much difficulty. Pressure and tem­perature-dependent volume correction enable us to
achieve accurate quantitative results which are well in
agreement with published values.

4. Contents

1 Critical point apparatus, filled with hydraulic fluid

(castor oil). With attached gas connection fittings
for MINICAN® gas container and protection for gas
supply connections. Test gas (SF

) not included.

6

1 Oil filling device
1 Allen key, 1.3 mm (for grub screw on the vernier

scale)
1 Plastic tubing, 3 mm diameter
1 1/8" tube fitting (wrench width 11 mm)
1 Grease gun

2

J

Δ⋅=

Δ

5. Technical data

Sulphur hexafluoride:

Critical temperature: 318.6 K (45.5°C)
Critical pressure: 3.76 MPa (37.6 bar)
Critical volume: 197.4 cm

3

/mol

Critical density: 0.74 g/mol

Maximum values:

Temperature range: 10-60°C
Maximum pressure: 6.0 MPa (60 bar)
Threshold value for

safety valve: 6.3 MPa (63 bar)
Theoretical long-term

pressure: 7.0 MPa (70 bar)
Theoretical rupture

pressure: >20.0 MPa (200 bar)

Materials:

Test gas: Sulphur hexafluoride (SF

)

6

Hydraulic fluid: Castor oil
Measuring cell: Transparent acrylic
Temperature coating: Transparent acrylic
Recommended

thermal medium: mixture of water and anti­ freeze in the ratio 2:1

Determination of volume:

Piston diameter: 20.0 mm
Piston surface: 3.14 cm

Displaced volume: 3.14 cm
Maximum volume: 15.7 cm

2

2

× displacement

3

Scale division for
displacement: 0.05 mm

Maximum displacement: 50 mm

Determination of pressure:

Manometer: Class 1.0 (max. 1% deviation

from full scale value)
Measured quantity: Excess pressure
Indicator: 60 bar max.
Manometer diameter: 160 mm

Connections:

Hole for
temperature sensor: 6 mm dia.

Connections for
thermal medium: 7 mm dia.

Connection for
regulating valve: 1/8’’ dia.

Gas connection: 1/8’’ (3.17 mm) dia. (as

supplied)

General specifications:

Dimensions: 380 x 200 x 400 mm

3

Weight: 7 kg approx.

6. Volume calibration

6.1 Preliminary notes:

NOP M

Q

L

K

R

S

A

B C E F

Fig. 1: Cross-section of apparatus with measuring cell (A),

conical seal (B), oil chamber (C), piston (D), cylin­der (E), heat casing (F), silicone seal (G), end
plate (H), square grommet (I), piston cover (J),
threaded axle (K), gasket (L), manometer connec­tion (M), guide tube (N), spring (O), sleeve (P), hole
for temperature sensor (Q), circular grommet (R) and
valve plate (S)

D

I

H

G

One turn of the handwheel winds the piston into/out
of the cylinder by means of a threaded axle. This
leads to a change of volume in the oil chamber (see
Fig. 1). Since oil is practically incompressible and all
the other components other than the conical seal are
almost rigid, a change in volume in the oil chamber
causes the conical seal to deform, thereby creating an

almost equal change in volume ΔV
cell. As a first approximation for ΔV

sAV

G

where

(1)

2

cm143.A= and Δs = displacement of piston.

in the measuring

G

, we can assume:

G

The piston displacement is shown in divisions of
2 mm on the fixed scale. Intermediate values are read
on the vernier scale in divisions of 0.05 mm.

The fixed scale can be moved by loosening the two
knurled screws. The vernier scale can be repositioned
and turned around the threaded axle on loosening
the grub screw (between scale positions 0 9 and 1 0).

6.2 Zero point calibration:

The zero point for the volume scale must be deter­mined by conducting a calibration.

For this, we take advantage of the fact that in a pres­sure range of 1-50 bar and in a temperature range of
270-340 K, air acts as a near-ideal gas (the real gas
factor has a deviation of less than 1% from 1). There­fore, at a constant temperature (e.g. room tempera­ture) for two piston displacements s

and s1 and for

0

3

the corresponding pressures p0 and p1 of the trapped

T

⋅

⋅

=

air, we get:

spsp ⋅=⋅ (2)

1100

Substituting

p

s Δ⋅

1

0

=

pp

−

s

01

sss Δ+=

10

and rearranging gives:

(3)

Rough calibration of scales:

• Open the regulating valve wide.

• Loosen the grub screw for the vernier scale by

half a turn (it is now possible to turn the scale
easily on the threaded axle without moving the
handwheel, although a counterpressure acts
against this independent movement).

• Wind the handwheel out till you detect a notice-

able resistance.

• Without turning the handwheel, turn the vernier

scale on the threaded axle till the 0.0 mark is on
the top and the fixed scale shows approx. 48 mm.

• Loosen the knurled screws of the fixed scale and

shift the scale to the side till the 48-mm bar is ex­actly above the centre line of the vernier scale
(see Fig. 2).

• Tighten the knurled screws again. In doing so,

make sure that the fixed scale does not press
against the vernier scale.

100 20304050mm

00
19

18

17

• Calculate the zero corrected piston position s

1, corr

using Equation 3.

• Adjust the vernier scale to the corrected value

and, if necessary, move the scale again.

• If required, wind the handwheel out a little and

secure the vernier scale with the grub screw.

Measurement example:

= 1 bar, p1 = 16 bar, p1 – p0 = 15 bar

p

0

s

= 48.0 mm, s1 = 3.5 mm, Δs = 44.5 mm

0

Therefore,

s

= 2.97 mm.

1, corr

The vernier scale must therefore be adjusted so that
now only 2.97 mm are shown instead of 3.50 mm.

Note:

After calibrating the zero point, it is possible to obtain
qualitatively accurate measured values. With regard
to temperature

T and pressure p, it is also possible to

obtain quantitatively accurate measurements of the
isotherms in range around to the critical point where
the two phases exist simultaneously. However, espe­cially in the liquid phase, the measured isotherms are
rather too widely separated.

6.3 Detailed calibration:

The exact relation between the volume VG in the
measuring cell and the scale reading

s is dependent

on the volume of oil in the oil chamber. The oil
chamber also expands marginally in proportion to the
pressure as a result of the spring in the manometer
tube. Additionally, when the temperature is in­creased, the castor oil expands to a greater extent
than the rest of the equipment. This means that the
pressure rises at a slightly greater rate at higher tem­peratures. All of these phenomena can be calculated
if appropriate calibration has been effected using air
as an ideal gas.

The ideal gas equation would thus be:

Vp

Rn

(4)

⋅=

16

with

J

3148.R =

molK

Fig. 2: Piston position reading at 48.0 mm

Zero correction:

• Shut the regulating valve (the pressure in the

measuring cell now corresponds to the ambient
pressure

p

= 1 bar. To within the accuracy of the

0

measurement, the manometer should display an
excess pressure of 0 bar).

• Wind the handwheel in till an excess pressure of

15 bar has been reached (absolute pressure

p

= 16 bar).

1

• Read the piston position s

displacement

Δs = s

– s1.

0

and calculate the

1

After taking the overpressure reading
pressure can be calculated from:

p = p

+ 1 bar (6)

e

The absolute temperature is given by:

T = ϑ + ϑ

where ϑ0 = 273.15°C (7)

0

The volume is given by:

sAV

G

where

(8)

2

cm143,A= and s is the “effective” piston

displacement.

From the measured displacement

p

, the absolute

e

s

, it is possible to

e

calculate the effective piston displacement as follows:

4

(9)

(

)

ϑ⋅−⋅++=

CpCsss

ϑ

pe 0

By substituting in equation 4, we get:

⋅ϑ⋅β−⋅β++⋅

0

pe

ϑ+ϑ

0

Apssp

ϑ

(10)

0

=⋅−

Rn

If we take several readings at various temperatures
and pressures, we can calculate the term:

n

()

⎛
⎜

Q

=

∑

⎜

=

1i

⎝

p

ϑ+ϑ

0

Apssp

⋅ϑ⋅β−⋅β++⋅

ϑ

ii0ii

2

⎞
⎟

Rn

(11)

⋅−

⎟
⎠

Sample measurements:
Table 1: Measured values for calibration

i s

/ mm

e

ϑ

p / bar

1 40.0 20.0°C 6.6

2 20.0 20.0°C 12.4

3 10.0 20.0°C 23.3

4 5.0 20.0°C 41.8

5 3.5 20.0°C 53.9

The free parameters s

, βP, βϑ and n should be appro-

0

priately selected so that the value of Q is reduced to a
minimum.

Additionally required (see also chapter 8):

1 Compressor or bicycle pump and valve
1 Bath/circulating thermostat 1008653/1008654
1 Dig. quick-response pocket thermometer 1002803
1 Type K NiCr-Ni immersion sensor, -65°C-550°C

1002804
2 Silicone tubes, 1 m 1002622
1 l Anti-freeze fluid with corrosion-inhibiting additive

for aluminium engines (e.g., Glysantin® G30 ma-

nufactured by BASF)

Conducting the calibration:

• Connect the circulation thermostat as described

in chapter 8 and fill it with the water/anti-freeze
mixture.

• Connect the plastic tube (3-mm internal diameter)

to the 1/8" gas connection fittings.

• Open the regulating valve.

• Wind the handwheel outwards, making the piston

move till it reaches say the 46.0 mm position.

• Use a compressor or a bicycle pump to create an

excess air pressure of approx. 3-8 bar in the
measuring cell.

• Shut the regulating valve.

• To record measurements, vary the volume in the

measuring cell or the temperature of the thermo­stat and wait till a stationary equilibrium has
been attained. Then take a pressure reading.

• Use appropriate adjustment software to set the s

β

, βϑ and n parameters so that the quadratic

P

,

0

equation for the errors Q is reduced to a mini­mum (see equation 11).

• If you like, you can adjust the vernier scale

around s

so that this correction is not necessary.

0

With the set parameters, it is possible to calculate the
“effective” piston displacement s from the measured
displacement s

using Equation 9 and then to calcu-

e

late the calibrated measuring cell volume using Equa­tion 8.

6 5.0 20.0°C 41.8

7 5.0 10.0°C 38.9

8 5.0 30.0°C 45.3

9 5.0 40.0°C 49.0

10 5.0 50.0°C 53.5

The following parameter values are obtained:

s

= 0.19 mm,

0

P

mm

0230

.=β

bar

n = 0.00288 mol.

7. Filling with test gas

7.1 Handling of sulphur hexafluoride:

Sulphur hexafluoride (SF6) is a non-toxic gas and is
absolutely safe for humans. The MAC value for danger
of suffocation on account of oxygen deprivation is
1000 ppm. That is equivalent to 6 filled measuring
cells per 1 m

However, SF

3

of air.

is extremely harmful to the environment

6

and can give rise to a greenhouse effect 24,000 times
stronger than CO

. Therefore, do not allow large quan-

2

tities to be released into the environment.

7.2 Gas connection via fixed pipes:

Additionally required:

1 SF

gas cylinder with manufacturer’s/supplier’s rec-

6

ommended gas fittings/valves, e.g. SH ILB gas cylinder
and Y11 L215DLB180 regulating valve from Airgas
(www.airgas.com).

1 Pipes with outer diameter of 1/8" and, if necessary,
adapters, e.g. from Swagelok (www.swagelok.com).

1 open-end spanner (13 mm), 1 open-end spanner
(11 mm)

According to the principles of “good laboratory prac­tice”, it is recommended to utilise a gas supply via
fixed pipes, especially if the equipment is regularly in
operation.

Filling begins with several flush cycles in which the air
is flushed out of the pipe. The number of cycles re­quired to flush out the air depends on the length of

mm

,

0340.=β

ϑ

grd

and

5

the pipe (more precisely, on the ratio of the pipe

m

m

length to the volume of the measuring cell). In the
process, care should be taken that the quantity of the
greenhouse gas SF

released in the environment is

6

reduced to a minimum.

Connecting a fixed pipe:

100 20304050m

00

19

18

17

16

15

ab

Fig. 3: Connecting a fixed pipe

(a) flush valve, (b) regulating valve

• If necessary, pull out the protection for the gas

connection and loosen the valve nut (11 mm) to
remove the 1/8" gas connection fittings.

• Connect the pipe (if necessary with adapters) to

the gas fitting.

• Beginning with the valve nut, slide the supplied

screw joints onto the tubing. (See Fig. 3: follow
the sequence and alignment specified along with
the cable binder)

• Insert the pipe into the regulating valve and

tighten the valve nut till the point is reached
where it is no longer possible to move the pipe
any further using only your fingers.

• Hold the regulating valve still with an open-end

spanner (13 mm) and tighten the valve nut by a
further 270°.

Now, the connection is gas-tight. When loosening the
valve nut afterwards, the regulating valve also needs
to be held still with a spanner.

Flushing out air:

• Use the handwheel to set the piston position to

10 mm.

• Slowly open the regulating valve and let in the SF

6

till a pressure of approx. 10 bar has been at­tained.

• Shut the regulating valve.

• Open the flush valve slightly till the pressure has

dropped to almost 0 bar.

• Shut the flush valve.

Filling with test gas:

• After at least four flush cycles, open the regulat-

ing valve till the pressure attained is once again
10 bar.

• Shut the regulating valve.

• Turn the handwheel in the reverse direction till

the piston reaches a position of say 46 mm.

• Slowly open the regulating valve and shut it again

when a pressure of 10 bar has been attained.

7.3 Filling with gas from a MINICAN®:

Additionally required:

1 MINICAN® gas container with SF

, e.g. from the

6

company Westfalen (www.westfalen-ag.de
If the equipment is used only occasionally, it is more

practical to draw the test gas from a MINICAN® gas
container. The gas connection of a MINICAN® con­tainer is similar in design to a commercial spray can,
i.e. it opens when the MINICAN® container is pressed
directly onto the gas connection fittings.

Here too, filling begins with several rinsing cycles for
flushing out the air.

SF

6

ab

Fig. 4: Filling with test gas from a MINICAN® gas container

(a) flush valve, (b) regulating valve

Flushing out air:

• If necessary, pull off the protection for the gas

connection.

• Use the handwheel to set the piston position to

10 mm.

• After removing the protective cap, position the

MINICAN® container with SF
tion fittings.

onto the gas connec-

6

• Press the MINICAN® container onto the gas con-

nection fittings, slowly open regulating valve (b)
and let in SF

till a pressure of approx. 10 bar has

6

been attained.

• Shut the regulating valve.

10020304050m

00

19

18

17

16

15

6

• Open the flush valve slightly till the pressure has

dropped to almost 0 bar.

• Shut the flush valve.

Filling with test gas:

• After at least four flush cycles, press the MINI-

CAN® gas container against the gas connection fit­tings. Slowly open the regulating valve and let in

till a pressure of approx. 10 bar has been at-

SF

6

tained.

• Shut the regulating valve.

• Wind the handwheel in the opposite direction till

the piston reaches a position of say 46 mm.

• Press the MINICAN® gas container against the gas

connection fittings, slowly open the regulating
valve and shut it again when a pressure of 10 bar
has been attained.

7.4 Recommendation for storage lasting for short

periods of time:

One gas filling can remain in the measuring cell for
several days.

If no experiments are being conducted, wind the
handwheel back till the piston is in a position where
it is subjected to the lowest possible pressure – say,
for instance, 46 mm.

If possible the apparatus should always be kept filled
with the thermal medium.

8. Experiments

8.1 Experiment set-up:

Additionally required:

1 Bath/circulating thermostat 1008653/1008654
1 Dig. quick-response pocket thermometer 1002803
1 Type K NiCr-Ni immersion sensor, -65°C-550°C

1002804
2 Silicone tubes, 1 m 1002622

1 l Anti-freeze fluid with corrosion-inhibiting additive

for aluminium engines (e.g., Glysantin® G30 ma-

nufactured by BASF)

• Place the equipment at a suitable height so that it

is convenient to observe the measuring cell. Posi­tion it so that the safety valve does not point in
the direction of any people who could be injured
or objects that could be damaged.

• Connect the silicone tubing from the outlet of the

circulation thermostat to the inlet of the heat cas­ing and from the outlet of the heat casing to the
inlet of the circulation thermostat.

• Prepare the thermal medium consisting of 2 parts

water to 1 part anti-freeze by volume.

• Fill the circulated thermostat bath.

8.2 Qualitative observations:

Liquid and gaseous states, dynamic state during phase
transformation, transition points occurring at differ­ent temperatures.

• Vary the volume by turning the handwheel and

the temperature by means of the thermostat. Ob­serve the safety instructions while doing so.

• Carefully shake the set-up to conduct simple

observations on the boundary between liquid and
gas.

In the vicinity of the critical point, it is also possible to
observe the critical opalescence. Owing to the con­stant changing of state between liquid and gaseous
states in small regions of the measuring cell, a kind of
“mist” develops and the sulphur hexafluoride appears
to be turbid.

8.3 Measuring isotherms in a p-V diagram:

• At maximum volume, set the desired temperature

on the circulation thermostat.

• Gradually reduce the volume in the measuring

cell (in steps down to a position of 10 mm). Wait
till a stationary equilibrium has been attained be­fore taking pressure readings.

• Then, beginning with the minimum volume,

gradually increase the volume till the piston posi­tion is once again at 10 mm. Wait till a stationary
equilibrium has been attained before taking pres­sure readings.

• Convert the excess pressure readings into abso-

lute pressure and the piston positions into vol­ume, as described in chapter 6.

In the low-volume region, stationary equilibrium is
attained more quickly during transition from higher
to lower pressure – i.e. from a lower volume to a
greater volume – since the phase boundary layer for
the phase transition from liquid to gas is created by
vapour bubbles present throughout the liquid. Sta­tionary equilibrium then takes around 1 to 5 minutes
to attain, whereby the measurements on the fringe of
the region where both phases exist take longest.

The recommended threshold value of 10 mm refers to
a filling pressure of 10 bar. Above this value, there
will certainly be no occurrence of a liquid phase in
the permissible temperature range. The threshold
value shifts to the “right” if the filling pressure is
higher.

8.4 Measuring isochores in a p-T diagram:

• Set the desired initial temperature. Subsequently

set the desired volume.

• Gradually allow the temperature to decrease.

• Wait till a stationary equilibrium has been at-

tained then take the pressure reading.

Measurements where both phases are present can be
plotted to generate a vapour-pressure curve.

Attainment of equilibrium takes up to 20 minutes
after each change of temperature due to the fact that

7

the water bath and the measuring cell must attain the
desired temperature first.

8.5 Determining the mass of gas:

Blow the gas out of the measuring cell into a gas-tight
plastic bag and then weigh it:

• If necessary, remove the gas supply pipe and

attach gas connection fittings.

• Wind out the handwheel, say to 46 mm.

• Open the regulating valve a little and release the

gas through the gas connection fittings into the
plastic bag.

• Shut the regulating valve.

• Determine the mass of the released gas. In doing

this, take into consideration the empty weight of
the bag and the buoyancy of air.

• Reduce the volume of the measuring cell till the

pressure in the measuring cell has reached its
original value.

• Calculate the original mass of gas from the vol-

ume difference before and after emptying the
measuring cell and the volume which is still pre­sent in the measuring cell.

Comparison with quoted values:

Using tabulated values, e.g. Clegg et al. [4], it is alter­natively possible to calculate the mass of gas in the

measuring cell from the measurements of ϑ, p, and V.

8.6 Evaluation:

We can clearly see from Fig. 5 that, despite the rela­tively simple equipment, it is possible to achieve
measurements which match closely to the reference
values plotted on the graph.

8.7 Bibliography:

[1, 2] Sulphur Hexafluoride, in-house publication,
pp. 27 [1], 30 [2], Solvay Fluor und Derivate GmbH,
Hannover, Germany, 2000

[3] Otto and Thomas: Landolt-Börnstein – Numerical
Data and Functional Relationships in Science and
Technology, Vol. II, Section 1, Springer-Verlag, Berlin,
1971

[4] Clegg et al.: Landolt-Börnstein – Numerical Data
and Functional Relationships in Science and Technol­ogy, Vol. II, Section 1, Springer-Verlag, Berlin, 1971.

[5] Din, F.: Thermodynamic Functions of Gases, Vol. 2,
Butterworths Scientific Publications, London, 1956

[6] Vargaftik, N.B.: Handbook of Physical Properties of
Liquids and Gases, 2

nd

ed., Hemisphere Publishing

Corporation, Washington, 1983
[7] Nelder, J. and Mead, R.: Comp. J., Vol. 7, p. 308,

1965

9. Storage for long periods without use

If no experiments are to be conducted over a long
period, the test gas should be released and the piston
should be turned to its rest position where the conical
seal is only very slightly curled and does not press
against the walls of the measuring cell.

• If necessary, allow the equipment to cool. Wind

the handwheel back till the lowest possible pres­sure is present.

• Release the test gas through the flush valve.

• Turn the handwheel to move the piston to its

“rest position”, at approx. 5 mm.

• Shut the flush valve again.

• Before storing away the equipment, the hydraulic

fluid needs to be degassed (as described in chap­ter 10) if the equipment has been in use over a
long period of time.

• Store the equipment in a safe place where it is

not exposed to direct sunlight.

• The thermal medium should be kept in the appa-

ratus during storage, as the additives inhibit cor­rosion and efflorescence caused by electrochemi­cal potentials between the different materials. Al­ternatively, the apparatus can be flushed with de­ionised water and then dried using compressed
air (oil-free, max. 1.1 bar).

8

1

p

/ MPa

5

4

3

2

1

0

0

Fig. 5: p-V diagram of SF6, measured with the critical point apparatus:

Readings taken at 10°C (
(
Reference values from [2] for pressure of liquid at 10°C (
and 50°C (

246 81012

V

/ ml g

), 20°C ( ), 30°C ( ), 40°C ( ), 45°C ( ) and 50°C ( ),

) threshold value of liquid-gas mixture, ( ) Reference values from [1] for vapour pressure,

), 20°C ( ), 30°C ( ), 40°C ( )

)

9

-

10. Degassing the hydraulic fluid

Owing to the inevitable diffusion of the test gas
through the conical seal, the pressure in the measur­ing cell slowly decreases over a long period. The gas
diffusing through the conical seal first dissolves in the
hydraulic fluid but does not have any significant in­fluence on the measurements.

However, if the test gas is removed from the equip­ment (for storage of the equipment) and the pressure
of the hydraulic fluid consequently falls to the ambi­ent pressure, then the test gas will escape from the
hydraulic fluid due to Henry's law. This leads to a
gradual increase in pressure in the oil chamber which
must be avoided at all costs as there is no back pres­sure in the measuring cell. On account of this, it is
necessary to cleanse the hydraulic fluid of all gas
before storing the equipment.

To degas the hydraulic fluid, the oil is made to boil in
a vacuum. Since the pressure difference on both sides
of the conical seal should not exceed a particular
limit, it is necessary to maintain, as best as possible,
the existing underpressure constant on the gas side.

Additionally required:

1 Castor oil approved for medicinal use e.g. 1002671
1 Vacuum tube, 6 mm internal diameter

1 Stopcock (or variable-leak valve)
1 Vane-type rotary pump

1 Open-end spanner (14 mm), 1 pair of tweezers
Absorbent paper, cardboard box

Storage of the equipment:

• If necessary, allow the equipment to cool. Wind

the handwheel back till the lowest possible is pre­sent.

• Release the test gas through the flush valve and

shut the flush valve thereafter.

• If necessary, remove the gas supply pipe and

attach the gas connection fittings.

• Unscrew the vernier scale.

• Open the regulating valve.

• Wind the handwheel so that piston moves in till

an excess pressure of 1 bar has been attained.

• Shut the regulating valve.

• Wind the handwheel back by two turns.

• Place the equipment with the manometer facing

downwards towards the ground). The manometer
should rest on a support approx. 6-cm-thick (see
Fig. 6).

Caution: the piston should never be wound out to
more than 25 mm, since the guide tube may slip out
during subsequent operations.

Fig. 6: Storage of the equipment for oil filling

c, d

e

d

c

Fig. 7: Dismantling the safety valve

(c) counter nut, (d) valve cap, (e) compression spring,
(f) hexagonal piston, (g) steel ball bearing

f

g

Dismantling the safety valve:

• Loosen the counter nut (14 mm) and use a screw-

driver to remove the valve cap (see Fig. 7).

• Remove the compression spring, the hexagonal

piston and the steel ball bearing in succession
with a pair of tweezers and store them in a safe
place, for instance in a cardboard box.

Assembly of the oil filling device:

• Loosen the valve nut of the oil filling device, re-

move the cover and place the valve nut above the
safety valve (see Fig. 8).

• Do not screw the oil filling device on too tight (the

gasket ring should not be squeezed out).

• Open the regulating valve.

• Wind the handwheel inwards to its end position

up to the frame (if necessary, loosen the vernier
scale). Subsequently wind the handwheel out by 3
turns.

• Place absorbent paper underneath and fill the oil

container with castor oil to no more than half
way.

• Screw on the cover of the oil filling device with

the valve nut.

Connection of vacuum pump:

• Connect a plastic hose with 3 mm internal diame-

ter to the gas connection fittings of the equip­ment and the smaller connector of the oil filling
device.

10

k

l

• In order to connect the vacuum pump, take a

vacuum hose with 6 mm internal diameter and
connect it via a stopcock or preferably via a three­way valve to the larger connector of the oil filling
device.

i

h

Fig. 8: Assembly of the oil filling device and connection of

vacuum pump (h) oil container, (i) valve nut,
(k) cover, (l) stopcock (or variable-leak valve)

Degassing:

• Check whether the regulating valve is open and

the flush valve is shut.

• Switch on the vacuum pump. Open the stopcock a

little and observe the formation of bubbles in the
castor oil.

Close the stopcock to interrupt the evacuation process
if the formation of bubbles is so strong that they can
reach the filter that is mounted on the cover. The
stopcock may be opened only after the bubbling has
subsided.

After several minutes (depending on the suction ca­pacity of the connected vacuum pump), the vaporis­ing pressure of the castor oil is attained and the oil
begins to boil. This can be noticed when vapour bub­bles begin to form “out of the blue” and rapidly be­come larger in size as they move through the oil.

The oil is now is sufficiently degassed.

• Shut the regulating valve and the stopcock.

Dismantling:

• Pull out the vacuum hose from the stopcock (the

hose fitting with the stopcock continues to re­main on the oil filling device).

• To avoid any surges, slowly open the stopcock

and wait for the pressure to even out.

• Pull out the hoses from both of the connectors on

the oil filling device.

• Unscrew the container from the safety valve.

Since castor oil is relatively viscous, it trickles out of
the container very slowly. Thus, this step can be con­ducted easily. A cleaning cloth (or kitchen paper)
which is held below the container immediately after
unscrewing it prevents any drops forming.

• With a cleaning cloth, remove excess oil from the

safety valve and subsequently wind the hand­wheel inwards very slightly till the oil level in the
valve is exactly at the same level as the edge
where the steel ball bearing sits.

• Insert the steel ball bearing, position the hexago-

nal piston with the short bore onto the ball bear­ing (use tweezers for this) and insert the compres­sion spring into the longer bore.

• Carefully screw the valve cap on in its end posi-

tion (not too tight) and loosen it by two turns.

Positioning the safety valve:

• Set-up the equipment and place it in a way that

the safety valve does not point in the direction of
people who could get injured or objects which
could get damaged.

• Open the regulating valve. Wind the handwheel

fully out and shut the regulating valve again.

• Turn the handwheel in till an excess pressure of

approx. 65 bar has been attained.

• From the front, wrap your arms around the appa-

ratus to reach the safety valve located at the back.
Slowly unscrew the valve cap of the safety valve
till the pressure drops to approx. 63 bar.

• Tighten the counter nut (14 mm).

Rest position:

• Wind the handwheel back till the pressure has

dropped to max. 10 bar.

• Open the regulating valve and turn the hand-

wheel to its “rest position” at approx. 5 mm.

• Shut the regulating valve.

After completing these steps, the equipment can
either be stored or refilled with test gas.

11. Upkeep and maintenance of threaded bush

11.1 Lubricating the threaded bush

To minimise wear, the threaded bush in the frame
should be lubricated approximately every 100 cycles
(one cycle = a pressure increase from 10 to 60 bar and
the subsequent reduction to 10 bar), or once weekly.
Lubrication only takes about 1 min and extends the
service life of the bush significantly. For lubrication, a
light-coloured multi-purpose grease with no graphite
or similar additives is recommended.

Procedure:

• Inject one full stroke of lubricant from a conven-

tional grease gun into the threaded bush through
the nipple at the frame.

• Wipe up any surplus lubricant emerging from the

bush.

When it emerges, the lubricant will also pick up any
traces of plastic that might have worn off during op­eration, so that will be flushed out too.

11

11.2 Examine threaded bush.

The threaded bush in the frame is subject to slow but
constant wear, and therefore the axial play must be
checked once a year:

• Release the pressure from the measuring cell and

adjust the piston to the 10 mm position.

• Using a vernier caliper, determine the minimum

and maximum distance between the handwheel
flange and frame; to do so, first wind in the
handwheel and then wind it out.

If the two distances differ by more than 0.3 mm, then
the bush needs to be replaced.

11.3 Replacing the threaded bush

Additionally required:
1 Threaded bush from set of seals (1002672)
The threaded bush is to be replaced no later than

every ten years even if the limit of wear has not been
reached (tests on a rig failed to produce any measur­able wear [<0.05 mm] after 1000 cycles), because
reliable data on the long-term stability of the plastic
used (POM-C) are not yet available.

• Depressurise the measuring cell.

• Unscrew the fixed scale.

• Undo the grub screw of the handwheel flange and

remove handwheel.

• Loosen the four screws in the cross piece of the

frame and remove it along with the threaded
bush by winding it down the axle.

• Unscrew the lubricating nipple (size SW 7) and use

a 3-mm Allen key to loosen the threaded pin
screwed in across the threaded bush by 4 turns.

• Knock the threaded bush out from the side of the

handwheel using a suitable mandrel. Alterna­tively insert an M14 screw loosely into the bush
and force the bush by hitting the head of the
screw.

• Fit the new bush such that the cross piece is

aligned with the lubrication nipple.

• Clamp the bush in a vice (with flat jaws or suit-

able insert).

• Screw back in the threaded pin (min. 6.0 mm

countersunk) and the lubricating nipple.

Bush material: POM-C = Polyoxymethylene copolymer
Oversize (press fit): 0.05 – 0.1 mm.

12. Changing the seals

Additionally required:

1 Allen key (6 mm)
1 Set of seals for critical point apparatus 1002672

consisting of
1 Conical seal,
1 Circular grommet,
1 Grommet 78x78 mm

2

,
4 Copper gasket washers
1 Threaded bush

After a certain period of time, it may be necessary to
replace the conical seal or other seals, especially if the
equipment has been exposed to direct sunlight.

12.1 Dismantling the equipment:

• If necessary, allow the equipment to cool and

wind the handwheel back till the lowest possible
pressure is present.

• Release the test gas through the flush valve and

shut the flush valve.

• If necessary, dismantle the tubing.

• Open the regulating valve.

• Wind the handwheel back till it has come to a

position of 25 mm.

• Tilt the equipment to the right and place it in an

upright position on a suitable surface resting on
the handwheel and the edge of the equipment
base.

• Use the Allen key (6 mm) to uniformly loosen

each of the four screws in the valve plate by 1/8
of a turn till the tension has been reduced.

• Unscrew and remove the screws.

• Also remove the copper gasket washers.

• With increasing force, twist the valve plate to the

left and right till the seals have been loosened.
Do not twist the regulating valve.

• Remove the valve plate (the measuring cell might

still be sticking to the plate).

• Twisting the equipment some more to loosen the

remaining seals between the measuring cell and
the cylinder and between the measuring cell and
the valve plate.

• Twist the guide tube to remove it from the coni-

cal seal.

12.2 Cleaning the dismantled equipment:

Castor oil can be removed quite easily by using white
spirit. However, white spirit attacks the acrylic of the
casing and measuring cell. Use a (mild) washing-up
liquid solution to remove greasy finger marks and
other impurities. New seals too should be cleaned
with white spirit and a washing-up liquid.

12

12.3 Assembling the equipment:

In case castor oil had been removed from the oil
chamber:

• Pour a fresh quantity of castor oil in up to about

5 mm below the upper edge of the cylinder (at
the beginning of the depression).

• Insert both of the silicone seals.

• Turn the conical seal inside out and dampen the

stud with some castor oil then screw it into the
guide tube.

• Unfold the conical seal back to its original shape,

position the spring on the piston and insert the
guide tube into the piston.

• Mount the measuring cell and position it flush

along the edges of the cylinder.

• Place the heat casing at the centre of the lower

silicone seal.

• Fit the circular grommet and, with the help of a

ruler placed on the heat casing, position it paral­lel to the cylinder (see Fig. 9, the semicircular
holes should then be below the valve openings).

Fig. 9: Positioning the circular grommet

• Place the valve plate at the centre and position it

parallel to the end plate.

• Fit the M8×40 screws with new copper gasket

washers and loosely screw them in.

• Tighten the screws. Take care to ensure that there

is uniform pressure on the circular grommet (if
the pressure is too high, the grommet makes a
greyish mark on the transparent acrylic, whereas
if the pressure is lower the surface looks milky).

12.4 Recommissioning:

• Degas the hydraulic fluid and pour the oil into

the equipment (see chapter 10).

• Position the safety valve (see chapter 10).

• Conduct a fresh volume calibration (see chap-

ter 6).

3B Scientific GmbH • Rudorffweg 8 • 21031 Hamburg • Germany • www.3bscientific.com

Subject to technical amendments

© Copyright 2013 3B Scientific GmbH