The working principle of the Pycnomatic ATC gas pycnometer for density measurement of powders and solids.

1. Benelux Scientific B.V.
Solids Density Measurement
by Helium Displacement
The Pycnomatic ATC
(Automatic Temperature Control)

2. • Terms and definitions
• The gas displacement technique: calibration and analysis, procedures and formulas
• Pycnomatic ATC design description
• Factors influencing reproducibility, accuracy and repeatability
• New features of Pycnomatic ATC
• Technical specifications
• Conclusions
Summary

3. The definition of a solid “volume” determines its density
Density refers to the mass contained within a unit volume under specified
conditions. Units are kg/m3 or g/cc. Density is temperature dependent
Envelope or bulk volume
• It is the solid volume comprehensive of the void volumes due to open and closed pores,
cracks and crevices
Real volume
• It is the solid volume comprehensive of the void volumes due to closed pores

4. Envelope or Bulk Density
D < 3.6 nm
Open Porosity
Closed Porosity
Solid material
It is the density referred to the external sample volume.
Can be measured by mercury displacement
under vacuum conditions(mercury cannot penetrate
pores in these conditions)

5. Real density
D < 3.6 nm
Open Porosity
Closed Porosity
Solid material
Can be measured by gas displacement technique
(helium pycnometry). Only closed pores cannot be
filled by helium unless permeation occurs

6. Helium Displacement Method
P
V ref
V ch
Atm
He in
• He is loaded in a calibrated and temperature stabilized reference volume (V ref) at a
pressure of about 2 bar
• Sample is placed in a calibrated analysis chamber (V ch), temperature stabilized, at about
atmospheric pressure
• He is expanded in the analysis chamber, then wasted to atmosphere
• All data are recorded when pressure is stabilized and sample volume is consequently
calculated (see later)

7. The Gas Displacement Principle: The Calibration
• Calibration means to measure the Reference volume and the Sample Chamber volume
• Temperature homogeneity and stability is mandatory for accurate and repeatable results
• If the temperature during a measurement is changing or anyway is different from the one at which
calibration was made, results will be affected by worst accuracy, despite reproducibility can be the
same within the test itself

8. Calibration using certified volume stainless steel spheres
P
V ref V chamber
Atm
He in
P
V ref V chamber
Atm
He in
P
V ref V chamber
Atm
He in
System purging by He
1. Equalize to atm pressure
2. Wait Atm Pressure
stabilization in both
chambers (thermal
stability). Read Patmh
3. Load Helium in reference,
wait stabilization, read Prh
4. Expand gas in the sample
chamber, wait stabilization,
read Peq
5. Repeat all the cycles until
required standard
deviation is achieved
6. h = number of cycles
Step 1: run with spheres

9. Mass balance equation for calibration step 1
P
V ref V ch
Atm
He in
P
V ref V ch
Atm
He in
If temperature is constant
  atmbchrefref PVVPV 
  eqbchref PVVV 
=
Where volumes are:
Vref = reference volume
Vch = analysis chamber volume
Vb = volume of calibrated sphere
Where pressures are:
Pref = loading pressure
Patm = atmospheric pressure
Peq = expansion pressure

10. Calibration procedure: run with empty cell
P
V ref V chamber
He in
P
V ref V chamber
He in
P
V ref V chamber
He in
Atm
Atm
Atm
Step 2: run without spheres
1. System purging by He
2. Equalize to atm pressure
3. Wait Atm Pressure
stabilization in both chambers
due to thermal stability. Read
Patmi
4. Load Helium in reference, wait
stabilization, read Pri
5. Expand gas in the sample
chamber, wait stabilization,
read Peqi
6. Repeat all the cycles until
required standard deviation is
achieved
7. i = number of cycles

11. Mass balance equation for calibration step 2
P
V ref V ch
Atm
He in
P
V ref V ch
Atm
He in
If temperature is constant
atmchrefref PVPV 
  eqchref PVV 
=
Where volumes are:
Vref = reference volume
Vch = analysis chamber volume
Vb = volume of calibrated sphere
Where pressures are:
Pref = loading pressure
Patm = atmospheric pressure
Peq = expansion pressure

12. Reporting the calibration data
•Small volume is calibrated
using 15.5987 cc reference
spheres at 25 °C
•Using only 10 cycles it is
possible to obtain a precision
better than 0.01% on a volume
of about 27 cc
Calibration Ball Volume 15.5987
Number of Measurements 10
Number of Cleaning Cycles 5
Stabilised pressure in bar 0.0001
Stabilised time in seconds 10
Percentile deviation 0.03
Patmh Prh Pch Temp
1.00185 2.00562 1.66355 24.99
1.00188 2.00734 1.66466 24.99
1.00189 2.00704 1.66448 24.99
1.00185 2.00735 1.66465 24.99
1.00185 2.00739 1.66464 24.99
1.00183 2.00706 1.66442 24.99
1.00184 2.00743 1.66466 24.99
1.00181 2.00881 1.66554 24.99
1.00184 2.00749 1.66467 24.99
1.00183 2.00753 1.66468 24.99
Patmi Pri Pci Temp
1.00167 2.00725 1.45378 24.99
1.00172 2.00571 1.45308 24.99
1.00169 2.0057 1.45305 24.99
1.00168 2.00726 1.4537 24.99
1.00165 2.00565 1.45296 24.99
1.00162 2.00795 1.45395 24.99
1.00165 2.00573 1.45299 24.99
1.00168 2.00738 1.45372 24.99
1.00168 2.00443 1.45237 24.99
1.00168 2.00442 1.45237 24.99
Reference volume 22.04569
Standard deviation 0.00402
% Deviation 0.018235
Sample chamber volume 26.99893
Standard deviation 0.00128
% Deviation 0.004741

13. Density analysis first step: Sample and instrument
proper purging
• By helium pulses
User selectable number of pulses to clean the sample under test (suitable for fine powders)
• By helium continuous flow
User selectable flowing time (suitable for solid materials)
• By vacuum (pump is optional)
Vacuum is generated through a restriction to prevent sample dragging
• Combination of the above methods (flow and pulses)

14. Analysis of unknown material volume
•When reference volume and
sample chamber are
calibrated, at the same
temperature, fill the sample
chamber with an unknown
material to test at about 66 %
of the available volume
•Perform sample purge (by
flow, pulses or vacuum)
•Wait temperature stabilization
of the sample
•Perform analytical cycles as
usual
P
Atm
He in
P
V ref V chamber
Atm
He in
P
V ref V chamber
Atm
He in
V ref V chamber

15. Mass balance for sample analysis
P
V ref V ch
Atm
He in
P
V ref V ch
Atm
He in
  atmschrefref PVVPV 
  eqschref PVVV 
=
If temperature is constant and
same as calibration one
Where volumes are:
Vref = reference volume
Vch = analysis chamber volume
Vs = unknown sample volume
Where pressures are:
Pref = loading pressure
Patm = atmospheric pressure
Peq = expansion pressure

16. Pycnomatic for Density Determination
Pycnomatic ATC Main Features
• Built-in real multi reference and (multi) volume
instrument (!)
• Sample chambers: 60, 40 and 20 cc
• Optional sample chambers; 100 and 4 cc
• Integrated fully automatic temperature control
by Peltier device from 14 to 40 °C with utmost
precision of 0,01 °C(!)
• Built-in “glove box” capability for radioactive
material applications (mechanic already
separated from electronic device).
• High precision absolute pressure transducer:
resolution 0.01 mbar (19 bit A/D), stability +/-
0.02 mbar
In addition …
- Xlarge back lighted display
- Alpha-numeric keyboard
- Date and time
- Connections to balance,
printer and computer

17. Pycnomatic diagram
Reference Chamber II
VrII
Reference Chamber I
VrI
Gas vent to
atmosphere or
vacuum
External
Vacuum-Pump
(optional)
InputValve
SampleChamberValve
Output Valves
1
3
2
4
5 6
Restriction
PressureRegulator
Sample Chamber
VcSmall
VcMedium
VcLarge
The following parts are
temperature controlled:
- All Reference Chambers
- The Sample Chamber
- The Pressure Sensor
Gas
Input
Absolute
PressureSensor

18. Different working principles
P
Reference
Chamber
Sample
Chamber
He in He out
Restriction
Sample
Chamber
Reference
Chamber
He in
P
He out
Filter
Pycnomatic
• No added transducer dead
volume to sample chamber
• No contamination of reference
volumes by powder samples
• No need of filters between
chambers
• Reduced pressure over the
sample (for pressure sensitive
samples as foams)
• Additional outlet restriction for
extra fine powders

19. Examples
Cycle Measured Average Standard Percentage Accuracy on
# Volume (cc) Volume (cc) Deviation (cc) Deviation (%) 15.5985 cc (%)
1 15,60656
2 15,60415 15,6054 0,0017 0,0109 0,043946533
3 15,60298 15,6046 0,0018 0,0117 0,038871259
4 15,60221 15,6040 0,0019 0,0122 0,035099529
5 15,60174 15,6035 0,0019 0,0123 0,032233869
6 15,60226 15,6027 0,0009 0,0060 0,026720518
7 15,59981 15,6018 0,0012 0,0077 0,02115588
8 15,60088 15,6014 0,0010 0,0067 0,018463314
9 15,60004 15,6009 0,0011 0,0068 0,015680995
10 15,59947 15,6005 0,0011 0,0072 0,012770459
11 15,59929 15,5999 0,0006 0,0040 0,0089624
Three full tests on the Measured Measured
same material Volume (cc) Density /g/cc)
Test 1 7,8548 2,6440
Test 2 7,8535 2,6444
Test 3 7,8547 2,6440
Average values on 3 tests 7,8543 2,6442
Standard deviation on 3 tests 0,0007 0,0002
Repeatability% 0,009 0,009

20. Main factors influencing accuracy
Accuracy or precision (absolute error)
• Instrument calibration precision (spheres tolerance, mechanical design, reduced
dead volumes, leaks, etc.)
• Sample volume filling close to 67 % of chamber volume (!)
• Available sample volume (larger volumes provide better accuracy) (!)
• Difference between calibration and analysis instrument temperatures (!)
• Proper thermal stabilization of sample (!)
• Pressure transducer resolution, accuracy, linearity and hysteresis
• Sample drying (for mass determination)

21. Main factors influencing reproducibility (within a single
test for sample volume determination)
Reproducibility (volume or density % standard deviation)
• Instrument (or room) temperature stability during the analysis (!)
• Pressure transducer hysteresis and linearity
• Proper thermal stabilization of the sample (!)
• Sample drying (for water vapor release during the analytical cycles)

22. Main factors influencing repeatability between repeated
tests
Repeatability (standard deviation of density value between multiple repeated measurements)
• Temperature condition of the experiment (!)
• Transducer hysteresis and linearity
• Sample volume under test (!)

23. The effect of sample volume filling
Sample mass 34,474 % Ratio Sphere/Sample = 84%
Volume Deviation Repr. % Density Deviation Dev. % Accuracy %
Test 1 13,0414 0,0013 0,010 2,6434 0,0003 0,010 0,016
Test 2 13,0458 0,0008 0,006 2,6425 0,0002 0,006 0,018
Test 3 13,0423 0,0009 0,007 2,6432 0,0002 0,007 0,009
Test 4 13,0432 0,0006 0,005 2,6431 0,0001 0,005 0,003
Average value 13,0432 0,0009 0,007 2,6431 0,0002 0,007 0,003
Deviation 0,0019
Repeatability% 0,015

24. Volume filling effect: summary of results
Average values on alumina
Ratio % Density Reprod.% Accur. % Repeat. %
84 2,6431 0,007 0,003 0,015
50 2,6472 0,013 0,158 0,026
30 2,6578 0,022 0,561 0,018
88 (w Filler) 2,6442 0,009 0,044 0,009
1. Accuracy is strongly affected by the chamber filling percentage. Best
results are obtained when the sample volume in the cell approaches to
the calibration sphere volume (about 66 % of the analysis chamber
volume) thus accuracy is also related to the sample nature
2. Reproducibility and repeatability don’t suffer too much by reducing the
sample volume in the cell
3. In case the available quantity of sample doesn’t fit the optimum volume it
is possible to reduce the cell volume (multi-volume pycnometers) or
adding a known volume filler.

25. A real Innovation: a built-in temperature control
• Sample and reference chambers are temperature controlled by a Peltier Device
• Selectable temperature range from 14 to 40 °C, steps of 1 °C
• 3 sensors to calculate the “real” sample temperature
• Temperature stability +/- 0.01 °C (in the above range)

26. Temperature effects
Temperature variation effects can be:
1. Sample temperature stabilization during a test
2. Room temperature variations in non-temperature controlled instruments
3. Effect of temperature on density
4. Calibration and analysis performed at different temperature conditions (for non-
temperature controlled instruments)

27. Best accuracy by a special pressure transducer
• Pressure range from vacuum to 3 bar absolute
• Resolution (Displayed): 0.01 mbar
• Accuracy: +/- 0.02 mbar
• Temperature compensated
• Fast response time
• No hysteresis effect

28. High precision mechanics design
• Reference volumes and sample chamber made in a single temperature controlled
aluminum block
• Reduced dead volumes
• Mechanics can be completely separated from electronics for nuclear applications
(longer connecting cables are required)
• Special design prevents risk of fine powders dragging during gas expansion

29. Technical Specifications
Sample cell volumes About 20, 40 and 60 cc (indicative
values)
Optional sample cell volumes Extra small: about 4 cc
Extra large: about 100 cc
Reference (expansion) chamber
volumes
About 20, 40 and 60 cc
(indicative values)
Temperature control range From 14 °C to 40 °C,
steps of 0.01 °C
Temperature sensors Three, PT100 type
Temperature resolution 0.01 °C
Temperature stabilization +/- 0.01 °C at 20 °C

30. Technical Specifications
Pressure transducer range From vacuum to 250 Kpa absolute,
T compensated, linearized
Pressure displayed resolution 0.01 mbar (19 bit A/D converter)
Pressure reading accuracy +/- 0.02 mbar
Purging procedures By He pulses, He flow or vacuum
Maximum number of cycles (user’s
selectable)
100
Calibration procedures Integrated, storing up to 3
reference and sample chamber
volumes
Calibration method By calibrated stainless steel
spheres

31. Technical Specifications
Memory capacity (analysis) Up to 2 complete runs (max of 100
cycles each)
Memory capacity (calibration) Up to 3 reference volumes and 3
sample chambers (with relevant
raw data)
External gas connections 1 inlet for Helium
2 outlets (one direct and one via
restriction)
Communication ports 1 serial port to PC
1 serial port to balance
1 parallel port to printer
User’s interface Back-lighted display (40 characters
x 4 lines)
Alpha-numeric keyboard

32. Technical Specifications
Power supply 100-240 VAC, 50-60 Hz, 240VA
Dimensions cm (w x d x h) 25 x 33 x 45
Weight (preliminary) 17 kg with Peltier device
11 kg without T control
Reproducibility (preliminary) Typically 0.01% on sample volume (evaluation
on dry and thermally equilibrated samples,
sample real volume filling at about 66% of
nominal vessel volume)
Accuracy (preliminary) Typically 0.01% on sample volume
(evaluation on dry and thermally equilibrated
samples, sample real volume filling at about
66% of nominal vessel volume)

33. Conclusions: is density test an easy analytical
technique?
• Multi volume and multi reference assure constant accuracy and repeatability almost
independently from the available sample volume
• Precise and accurate temperature control provides utmost reproducibility and accuracy
independently from environment temperature variations, thus avoiding the need of
continuous re-calibration for reliable results
• Pressure transducer precision, accurate temperature control and precise mechanical
design assure optimal performances also testing very small volumes of sample

34. Thank you for your attention !