2 Basics of capillary viscometry 2.1 Measurement principle Inside the capillary viscometers, the velocity drop re-quired for viscosity measurement is built up in the form of a laminar tu
Trang 1Theory and Praxis
of
Capillary Viscometry
- An Introduction –
Trang 2Authors:
Prof Dr.-Ing habil Jürgen Wilke
Hochschule Anhalt
Food and biotechnology
(Process and environmental Technology Faculty)
Dr.-Ing Holger Kryk
Trang 3Table of contents Page
1 Viscosity – Rheology 2
2 Basics of capillary viscometry 5
2.1 Measurement principle 5
2.2 Designs of glass capillary viscometers 5
3 Measurement of flow time 7
3.1 Manual time measurement 7
3.2 Automatic time measurement 7
3.2.1 Tasks and particularities 7
3.2.2 Detection of the meniscus passage 7
4 Working equation of glass capillary viscometers 9
4.1 Procedure for viscosity determination 9
4.1.1 Neglect of HC correction 9
4.1.2 Calculation of HC correction resp use of given table values 10
4.1.3 Experimental determination of the individual HC correction 12
5 Calibration 14
6 Handling of capillary viscometers 15
6.1 General guidelines for the selection of the measurement system 15
6.2 Cleaning of capillary viscometers 16
6.3 Preparation of the measurement 17
6.4 Performing the measurement 19
7 Causes of errors and special corrections 23
7.1 Correctable errors and corrections 23
7.2 Uncorrectable errors 24
7.3 Frequently occurring error symptoms, possible causes of errors, and ways of elimination 26
8 Special applications 28
8.1 Testing of plastics 28
8.2 Determination of the viscosity of oils and additives 30
8.3 Testing of food 31
9 Formula signs and units used 33
10 Bibliography 35
11 Standards used in capillary viscometry 37
Trang 41 Viscosity - Rheology
Viscosity characterises the flow properties, the
inher-ent friction of liquids and gases
If a fluid is trapped between two plane-parallel plates,
it will require some amount of force to displace the
upper plate
The fluid particles which are directly adjacent to the
plates are firmly bonded to the surface by adhesion
forces In this process the fluid layer neighbouring the
plate being displaced adopts the velocity of the plate
All neighbouring layers stay more and more behind
with the increasing distance to the plate being
moved The cause for this phenomenon can be
found in cohesion forces which counter-act the
recip-rocal dislocation of the individual layers
F
v y
x
Figure 1 Basic model of the shearing operation in
the case of laminar, stationary layer flow
The fluid starts to flow inside the gap A layered flow
builds up (please ref to Figure 1)
The shear strain t (also referred to as s1,2) refers
the quotient of force F and the boundary surface A of
According to Newton's Viscosity Law there is
pro-portionality between the shear strain t and the shear
rate D
The proportionality factor h is referred to as dynamic
viscosity coefficient or, in short, as dynamic
vis-cosity
The unit of measurement is Pa • s, with the indication
being made in mPa • s i.e in numerical conformity
with the former unit cP (Centipoise):
For reasons of convenience, the unit of mm2/s is used which then numerically corresponds to the for-mer cSt (Centistoke) unit
In case of Newtonian liquids h will remain invariant
if the shear rate changes with all other test conditions remaining unchanged
Moving a liquid molecule requires a potential hill to
be surmounted which will lead to the following tionship if Maxwellian Boltzmann velocity distribution
rela-is being applied:
D = k e
E RT
Examples:
n-pentane 0.230 mPa • s (20 °C) Water 1.002 mPa • s (20 °C) Propane triol 1480 mPa • s (20 °C) (Glycerine)
In the case of liquids, and in contrast to gases, h will decrease in a strongly exponential manner with rising temperatures As a rule, the decrease will be the higher, the higher the absolute values of viscosity are and the lower the temperature is, since the inter-molecular interactions are decreasing with the mag-nifying thermal movement of the molecules
This effect indicates the major practical significance
of viscosity, for instance, with regard to lubrication technology, as will be shown below
Trang 5In the case of liquids a complex molecule structure
and an increasing pressure lead to an increase in
viscosity
As regards water, an anomaly occurs owing to the
particular structure If pressure increases, viscosity
will pass through a minimum, since molecule
aggre-gates are being formed the reciprocal friction of
which is lower
In the case of liquid miscible phases h is in general
not made up by the addition of h-values of the
pure components
The viscosity of the miscible phase may be greater or
smaller than h of the isolated components, or may be
in between
The viscosity of the solutions of solid matters is
frequently greater than the one of the pure solvent
The indication is mostly given in terms of relative or
specific viscosity (please refer to chapter 8)
A particular behaviour can be observed with the
con-centration-dependability of viscosity of electrolyte
so-lutions
If the liquid layers are moving at different velocities,
the deformation of the ion cloud will cause the
occur-rence of additional inter-ionic interacting forces which
will affect friction between the individual layers
H Falkenhagen used the theory of inter-ionic
interac-tions, applicable to highly diluted electrolyte, solutions
to derive the Limit Law of Viscosity:
DC = D0 + K c (1.7)
Dc Viscosity at ion concentration c
D0 Viscosity of the pure solvent
Non-Newtonian flow behaviour
Disperse systems, concentrated polymer solutions,
and melts of macro molecules show a marked
non-Newtonian behaviour with increasing shear rates
In their case there is a non-linear dependency
be-tween shear strain and shear rate
Shear-rate dependent flow behaviour:
b - Fluid with dilatant flow behaviour
c - Intrinsic viscous fluid
Plasticity
The flow of the liquids begins only from a minimum
shear strain Below this yielding point the substance
behaves like a solid matter
t = f (D) is linear above the yielding point
CASSON substances:
t = f (D) is non-linear above the yielding point
Pseudo-plasticity (intrinsic viscosity)
These substances are characterised by Newtonian behaviour at low shear rates
At high shear rates h will increase with the shear rate (please refer to Figure 2, curve c)
Trang 6In addition to these effects a shear-time dependent
flow behaviour can be observed with some
non-Newtonian matters:
t = f (D, t)
h = f (D, t)
This means that shear viscosity is influenced by the
duration of the shearing action (please refer to Figure
3)
bac
tsh
Figure 3 Dependency of shear viscosity
on the shear time
a = shear-time independent flow behaviour
b = Rheopexy
c = Thixotropy
The following distinction is made:
Thixotropy
Shear viscosity decreases at constant shear rate with
increasing shear time (typical for sol/gel
transforma-tion)
Rheopexy
Shear viscosity increases at constant shear rate with
increasing shear time
Rheopexy can, for instance, be seen with PVC
plas-tisols They are used for corrosion protection on
met-als If the coating rate is increased the material
be-comes more thick-flowing Rheopex liquids are
char-acterised by a gradual structure formation under
shearing strain
In addition to these viscous properties one can
ob-serve the occurrence of elasticities (1st and 2nd
normal-stress difference) acting perpendicularly to
the flow direction
The combination of viscous and elastic behaviour
leads to the description of viscoelastic fluids
Poly-mer solutions, and recently also biopolyPoly-mers
exhibit-ing molecular-structure dependent viscoelastic
prop-erties of this kind meet with more and more
techno-logical interest, e.g in the production of paints and
coatings, food, cosmetics, and pharmaceutics
The complex nature of this field of work has lead to
the crystallisation of an original term, i.e rheology
(science of flow behaviour)
Rheometry deals with the specific methods and
pro-cedures of determining rheological characteristics Within this nomenclature viscometry is a partial dis-
cipline of rheometry
Principles of viscosity measurement
Rheological measurement procedures are mainly based on mechanical methods, since tension and elongation are mechanical values which are deter-mined on the basis of a defined deformation of the sample
The simultaneous measurement of the electrical, magnetic, and optical properties which may change during the deformation or flow process of the fluids is becoming more and more interesting
Figure 4 shows the major manners of realising the deformation of the sample
v
1 2
v
2
M1
5 6
M2
4
3 2
3 = Coaxial cylinder M 1 , M 2 = Measurement marks
4 = Torque sensor
The present brochure covers the methodological and
metrological particularities of low-pressure capillary
viscometers, the most important of which, in turn,
are the glass capillary viscometers
They are in particular suited for viscosity ments with Newtonian liquids with a kinematic viscos-ity of more than 0.3 mm2/s
measure-Perfection in the manufacture and the sophisticated quality-assurance methods form the basis of stan-dardised measurement systems which are meeting today highest accuracy requirements as to reproduc-tion incertainties and absolute measurement incer-tainty
Trang 72 Basics of capillary viscometry
2.1 Measurement principle
Inside the capillary viscometers, the velocity drop
re-quired for viscosity measurement is built up in the
form of a laminar tube flow within a measurement
capillary
Under idealised conditions
laminar, isothermal flow
stationary flow condition
Newtonian flow behaviour of the liquid
pressure-independence of viscosity
incompressibility of the liquid
wall adherence of the liquid
neglect of the flow influences at the entry and
exit of capillary of sufficient length
the liquid flows in coaxial layers towards the pressure
drop through the capillary A parabolic velocity flow
occurs (please refer to Figure 5)
v
vmax
v = 0
R r
Figure 5 Velocity profile with laminar tube flow
The Hagen-Poiseuille Law is the physical basis of
viscometers working according to the capillary
With regard to viscosity measurement, this results in
two different fundamental measurement principles:
Measurement of the differential pressure at a
con-stant volume flow of the sample through the
cap-illary
Measurement of the volume flow through the
cap-illary at a given differential pressure
The first measurement principle can be used for the design of continuos viscometers the measurement accuracy of which is depending on the achievable measurement incertainty in differential-pressure measurement and the stabilisation of a defined vol-ume flow
This issue is approached in a satisfactory manner the design of device in the form of comparison meas-urement methods
An application of this can be found in solution cometry where the viscosity of the pure solvent is used as a reference liquid The measurement itself is made, inter alia, on the basis of a ”pneumatic Wheat-stone bridge”
vis-Another application of the first measurement principle
is viscosity measurement on plastics melts This process involves short capillaries, frequently gaps of
a predefined geometry (high-pressure capillary
viscometry)
2.2 Designs of capillary viscometers
In the case of low-pressure capillary viscometers
the imaging signal used for viscosity is the time quired by a defined liquid volume to flow through a measurement capillary
re-The driving force is the hydrostatic pressure of the liquid column To achieve higher shear rates, it is possible to use over-pressure
Irrespective of the specific design, the mostly U-shaped glass bodies have ball-shaped extensions the volume of which determines the quantity of the sample
Measurement marks on the glass body, or accurately defined fixed sensors, allow the measurement of the passage time of the boundary layer between the sample and the air (meniscus), a process which en-ables the passage time of a product volume re-stricted in such a manner to be measured with measurement incertainties < 1/10 s
Trang 8Figure 6 shows the two fundamentally different vis-
cometer types after OSTWALD and UBBELOHDE
Figure 6 Glass capillary viscometers after
a) UBBELOHDE and b) OSTWALD
With both viscometers the liquid being examined is
filled through the filling tube (3) into the storage
con-tainer (4)
Considering that the mean pressure height in the
case of the OSTWALD Viscometers depends on the
filling height, the prescribed measurement volumes
have to be observed under any circumstances For
this reason filling is done using a pipette To perform
the measurement, the sample is sucked into the tube
(2) The measurement aims at the time the meniscus
requires to sink from measurement mark M1 to
measurement mark M2 (annular measurement
marks)
In the case of the UBBELOHDE viscometers the
transition point from the capillary (7) to the levelling
bulb (6) has the shape of a ball joint being the end
point of an additional venting tube (1) /32, 33/ After
filling the sample through the tube (3) into the
con-tainer (4), the venting tube is closed
Depending on the operational mode, i.e pressing or
sucking action, the sample is filled by over-pressure
applied to tube (3) or by suction via the tube (2) into
the reference level vessel (6), the capillaries (6), the
measuring sphere (8), and at least up to half of the
pre-run sphere (9)
After venting tube (1), the liquid column in the
level-ling bulb breaks off At the exit of the capillary the
so-called suspended level develops (also refer to
Fig-ure 22) For this reason only a limited sample quantity
- max., min filling marks (10) - may be filled in After
In this way the hydrostatic pressure of the liquid
col-umn is independent of the sample quantity being
filled in
In addition, owing to the geometrical shaping of the levelling bulb (6), the influence of surface tension on the measurement result is almost eliminated
In the case of the UBBELOHDE Viscometer, too, the measurement is aimed at the time required by the liquid meniscus to sink from the annular measure-ment mark M1 down to the annular measurement mark M2.
In the case of very strongly tinted, opaque liquids, it can be possible that a visual detection of the menis-cus passage through the measurement marks is im-possible owing to the wetting of the tube For manual operation, the Reverse-Flow Viscometer (please re-fer to Figure 7) is used in such cases
hm2
is the time required by the meniscus to flow through the measurement marks M1, M2 and M3 at the re-verse-flow (1)
The standard viscometer introduced was the
CANNON-Master instrument with a capillary
di-ameter of 0.45 mm and a capillary length of 400 mm With the determination of the viscosity of water
h = 1.0019 [cP] ± 0.0003 [cP] 1) (20 °C),
it was possible to define a viscosity scale
The capillaries of viscometers used for industrial
Trang 9ap-3 Measurement of the flow time
3.1 Manual timing
In the most simple case the flow time is taken by an
operator using a stop watch Glass viscometers
manufactured for this purpose have annular
urement marks burnt in above and below the
meas-urement sphere (please refer to Figures 6, 7)
The disadvantages of this method are obvious:
Subjective observation errors or differences in the
reaction time of the operator at the beginning and
end of the timing lead to increasing reproducibility
incertainties and, under certain circumstances, to
systematic errors
In the case of opaque substances the meniscus
cannot be seen One has resort to Reverse-Flow
Viscometers with their more awkward handling and
reduced accuracy
3.2 Automatic timing
3.2.1 Tasks and particularities
In the case of automatic capillary viscometers an
electric signal has to be generated during the
pas-sage of the air/sample or sample/air boundary layer,
respectively, through the measurement marks This
electrical signal is required as
a start and stop signal for the timing process
and as
a status signal for the automatic operation
(filling, emptying of the capillaries)
The detection and transformation of a time signal
does not pose any metrological problems In practical
viscosity measurement the measurement
incertain-ties are determined by the fluid-dynamic
circum-stances and the detection of the meniscus passage
through the measurement marks
The manufacturer of the measurement device has to
ensure by design and production measures that the
viscometer constant will not change even if the
measurement conditions should deviate from the
calibration conditions (e.g measurement and
calibra-tion temperature)
As a result, there would be incidental errors which
would have to be determined and identified for each
device separately Otherwise the user himself would
have to perform calibration And this is the point
where low-pressure capillary viscometry has a
deci-sive advantage over other viscosity measurement
procedures
The well-adapted selection of materials, the
engi-neering-technological mastery of the production
processes, and the sophisticated methods of quality
assurance enable a calibration of the viscometers to
be made
3.2.2 Detection of the meniscus passage
This task requires the use of sensors responding to the difference between the material properties of the air and the product being analysed during the pas-sage of the meniscus through the measurement marks
Optical sensors
During the meniscus passage the optical conditions such as refraction and reflection within the detection plane are changing This leads to a change n the ra-diation intensity of the light arriving from the transmit-ter at the receiver (please refer to Figure 8) For the measurement of time, for instance, the analogous signal provided by a photo diode is transformed into
a pulse used for the start and stop of the time urement Specific threshold values of the analogous signal may be defined for the "filled" or "empty" status
meas-Advantage: Versatile application, simple set-up
Disadvantage: Highly tinted or opaque liquids,
espe-cially those which adhere strongly to the wall, cannot me measured
On the viscometers from SCHOTT-GERÄTE all
opti-cal sensors are accommodated in a measurement tripod made of metal or plastic Within the tripod the fixation rack and the glass viscometer are fastened using a clamping mechanism Figure 8 shows the ar-rangement of the optical sensors within the meas-urement tripod on the viscometer
The light is guided out of the tripod head through bre optics into the tripod legs up to the upper and lower measurement plane The watertight sealing enables the measurement tripods to be placed in liq-uid thermostats
fi-Owing to high precision in the glass-technological and mechanical production as well as through meas-ures of quality assurance it is ensured that the glass bodies and tripods are freely interchangeable, with the certified viscometer constants remaining valid
21
Figure 8 Arrangement of the optical sensors
on the viscometer
1 = Optical fibre input
2 = Optical fibre output
Trang 10Conductivity sensors
Electrolytically conductive measurement liquids
(solu-tions of salts, acids, bases) can be detected using
small-sized electrodes melted into the measurement
plane in the glass wall For signal generation the
electrical resistance is measured
Advantage: Simple set-up; detection of tinted
and opaque liquids
Disadvantage: The sample must be
electrocon-ductive; the supply lines to the sors are to be protected against wa-ter penetration if liquid thermostats are being used
sen-Thermal-conductivity sensors
Small-sized thermistors (NTC resistors), melted in on
the level of the measurement plane, are heated up
As a result to the improved thermal conductivity of
the liquid the thermistor will cool down at the
air/sample transition, and its electrical resistance will
diminish
Advantage: Measurement-signal generation is
inde-pendent of the tint, transparency, and conductivity of
the product being analysed
Disadvantage: More demanding production owing
to the required melting-in of the sensors; incrustation
and contamination hazard in the case of thermally
decomposable samples
Figure 9 shows a TC Viscometer from
SCHOTT-GERÄTE In the tube axis the thermistors with a
di-ameter of < 1 mm in the sealed-in head portion are
clearly visible
lower NTC sensorupper NTC sensor
Figure 9 TC Viscometer from SCHOTT-GERÄTE
The essential factor for safe operation is a good
dy-namic behaviour Figure 10 shows the signal course
resulting developing during filling and run-off
(meas-urement process) through the changing thermal
con-ductivity in the surrounding of the sensor
To compensate the influence of the sample on
dy-namics, the SCHOTT-GERÄTE viscosity
measure-ment devices perform an automatic calibration The
working point of the start/stop timing is adaptively set
by the device software during the filling process of
the capillaries on the basis of a respectively
1 0
2
s
Figure 10 TC sensor signal
a during filling and b during emptying
S - switch point of the timer device
Ultrasonic sensors
The propagation of sound waves in the frequency range > 20 kHz is different in gases and liquids, and owing to the changing sound impedance (product of sonic speed and specific weight) the waves are re-flected from boundary layers
In the case of the echo process (reflection) a sound head, attached to one side of the measurement mark and acting both as emitter and receiver, detects whether gas or liquid is present in the measurement plane
The radiation process uses separate emitting and ceiving modulators located at opposite tube posi-tions
re-Advantage: The signal formation is independent
of other sample properties, i.e the application of the process is versa-tile; no sealing in the glass required
Disadvantage: Coupling of the sound heads bears
production-technological difficulties, especially in the case of an applica-tion in liquid thermostats; greater signal-processing efforts required; higher price
Gas-ionisation spark-discharge detection
The electrodes melted in on the level of the detection planes are connected to a high-voltage generator If the liquid, acting as an electrical insulator, uncovers the electrodes a spark discharge will occur in the gas chamber if a sufficiently high breakdown voltage is selected The electrical pulse is used as a control signal
Advantage: Detection is possible in dull, opaque
liquids
Disadvantage: The process cannot be used in the
presence of least traces of water in the product being analysed (water
Trang 114 Working equation of glass capillary viscometers
In the metrological sense, the working equation
represents the statistical characteristic of capillary
viscometers The user uses them for the
determina-tion of viscosity on the basis of the flow time
The starting point is formed by the flow model in the
form of the Hagen-Poiseuille Law (equation 2.1) The
driving force is the hydrostatic pressure of the liquid
column in the form of the mean pressure height hm
(please refer to Figures 6, 7)
Considering that the volume flow V is recorded via
the measurement of the flow time t, the following
equation results for kinematic viscosity n:
In addition to the flow time, equation (4.1) contains
only constants and geometric details
For a given viscometer they can be summarised into
one characteristic magnitude, the so-called
viscome-ter constant K:
In order to take into account the tolerances which are
inevitable in the manufacture of the devices, K is
de-termined for each individual viscometer by way of a
calibration (please refer to Chapter 5)
According to equation (4.2) there is a linear
correla-tion between kinematic viscosity and flow time
Fig-ure 11 shows this correlation in the form of a
charac-teristic (curve a)
Figure 11 a ideal and
b real viscometer characteristic
When applying the flow model in the form of the
Hagen-Poiseuille Law, additional pressure losses
oc-curring at the capillary ends are not taken into
account Owing to the finite capillary length, however,
the pressure losses occurring at the in- and outflow
affect measurement accuracy As a consequence of
these additional pressure losses the measured flow
time tg is greater than the time t resulting from
Hagen-Poiseuille Law
The basic hydrodynamic process was first examined
by Hagenbach /5/ and Couette /6/
The difference between the measured and cal flow time tH is therefore referred to as Hagen- bach-Couette Correction Time (or, in short, HC correction or Hagenbach correction):
This results in the following corrected working tion for glass capillary viscometers:
equa-n = K • (tg - tH) (4.4) The smaller the flow time is, the greater becomes the Hagenbach-Couette Correction Time Curve b in Figure 11 shows the real course of the characteristic
In practical viscosity measurement there are in ciple three ways to take into account the Hagenbach-Couette Correction and thus to deter-mine the kinematic viscosity of the product being analysed
prin-4.1 Methods of viscosity determination
4.1.1 Neglect of HC correction
The selection of a capillary with a small diameter, adapted to the viscosity of the product being ana-lysed, involves long flow times In this case HC cor-rection takes such a small value that a correction may be omitted within the framework of the required accuracy
The flow times to be observed if HC correction is glected in order not to exceed a relative error e can
ne-be calculated according to equation (4.5) or equation (4.6), respectively:
tg³ 19.95 m V
L KÎ
æ
èç öø÷
1 2
m = empirical coefficient of HC correction
m = 1.12 (Re > 100) /N10/
Equation (4.5) is applicable to viscometers with sharp-edged capillary ends When using viscometers with funnel-shaped capillary ends, equation (4.6) should be used /N10/
Trang 124.1.2 Calculation of HC correction resp
use of given table values
The manufacturer calculates HC correction times on
the basis of the geometrical dimensions as a function
of the flow time and states them in the device
de-scriptions
Understanding the calculation algorithm requires first
an explanation of the theoretical basics of
Hagen-bach-Couette Correction
Figure 12 shows the true march of pressure in the
capillary /7/
The deviations from the ideal march result from
hy-drodynamic processes in the in- and outflow zone of
the capillary They are taken into account in the flow
model (please refer to Figure 13) in the form of
addi-tional terms
L
le
l ,p
Figure 12 Axial march of pressure in the capillary
Figure 13 Flow model with correction terms
The mean flow rate v in the capillary results from:
v = V
In this way the following corrected Hagen-Poiseuille
Law for the determination of viscosity results from
equation (4.7), Figure 13:
F
F = p R
(4.9) Couette did already take into account the pressure
loss Dpc by way of adding a fictitious length n · R to
the capillary length L in equation (4.10)
This correlation was confirmed by Kerstin, Solokov, and Wakeham /8/ by a numerical solution of the Na-vier Stokes' equations
of equation (4.10) is considered concurrently in the form of the empirically determined parameter m Therefore this correction is often briefly referred to as Hagenbach correction in literature /12, N6 N10/
Hagenbach-Couette Correction Dr
Viscous portion Pressure loss owing to the
increase in the kinematicenergy of the liquid when flowing into the capillary
Pressure loss owing
to the increased wall friction inside the flow-in path Ie
+ +
Trang 13For a given glass capillary viscometer
Parameter m mainly depends on the shape of the
capillary ends and the Reynolds number (Re)
The Reynolds number is an important
non-dimensional similitude characteristic for fluidic
de-scription of incompressible fluids:
It characterises the flow shape, i.e laminar or
turbu-lent, conditioned by inertia and friction (viscosity)
Depending on the production technology the capillary
ends of viscometers may be sharp-edged or
funnel-shaped (please refer to Figure 14)
Figure 14 Capillary ends of viscometers
a - sharp-edged b - funnel-shaped
With regard to sharp-edged capillary ends a constant
value of m = 1.12 was calculated on a theoretical
ba-sis /9, 10, 11/ This value is also contained as a
maximum guidance value in /N10/ For reasons of
production technology, however, ideally sharply cut
capillary ends are not realisable
For Re > 100 the value as calculated was confirmed
in experiments In the case of Re numbers below 100
m will drop sharply and retain only approx 30 - 40 %
of its initial value at Re = 25/12/ With Re < 10, m is
so small that it can be neglected /13/
If the capillary ends are funnel-shaped, m will be a function of the Reynolds number all across the me-trologically utilised flow-time range
Cannon, Manning, and Bell /14/ arrived at the ing functional correlation:
follow-Re0.037
=
Equation (4.15) forms the basis of the calculation of
HC correction according to the applicable standards /N6, N8, N10/
In this way the following working equations result for viscometers with sharp-edged or funnel-shaped cap-illary ends:
sharp-edged capillary ends
n = K t - B
tg
×
(4.16) L
8
V1.12
= B
FUBBELOHDE Viscometer: B = 2.5 /N6/
tH = B
funnel-shaped capillary ends
If Hagenbach-Couette correction in the form of a time correction according to equation (4.4) is used, the
HC correction time is calculated as follows:
2 1 2 3
2 g
R)
K (2L
V 1.66
= E
t
E-
K
= ×n
(4.18)
tH = E
K t g2
(4.19)
Trang 14The E / K correction terms for UBBELOHDE and
Micro UBBELOHDE Viscometers can also be taken
from the relevant DIN standards /N6, N7/
For reasons of production technology, capillary
vis-cometers from SCHOTT-GERÄTE have
funnel-shaped capillary ends The correction times tH are
given in the operation instructions
4.1.3 Experimental determination of the
individual HC correction
In the case of small flow times HC correction will
have a increased influence on the measurement
re-sult In addition, owing to after-flow effects of the
liq-uid and the beginning of the deformation of the
sus-pended level, the viscometer characteristic of
UBBELOHDE Viscometers is affected
If falling short of the measurement range as
recom-mended in the operating instructions is inevitable, an
individual HC correction for the respective viscometer
has to be determined in experiments
To do so, two standard liquids of a known viscosity
are to be used, with the viscosity of the product being
analysed lying between the viscosities of the
stan-dard liquids The smaller the difference between the
viscosities, the more accurate the result of the
cor-rection procedure
Realisation of the correction procedure:
1 Determination of individual values for the
Hagenbach correction with the standard liquids:
1und tH2:
t
1 - t
1 K - t
= t
1 2 1
2
g g 12 H
ö ç
ç è
æ
(4.21)
2 1
H H 12
t
1 - t 1
t - t
Figure 15 individual Hagenbach correction /N9/
a Hagenbach curve according to equation (4.19)
b real course of the individual Hagenbach- correction
c interpolation straight line
b
a
c
Trang 15Examples of viscosity determination
Viscosity measurement of n-decane at J = 23 °C (n » 1.21 mm2/s) with UBBELOHDE Viscometers
mean measured flow time: tg = 1234.57 s
HC correction time is approx 0.3 s This corresponds
to approx 0.024% of the flow time This means that
neglecting the HC correction time would not cause
any significant change of the measurement result
mean measured flow time: tg = 116.05 s
Calculated HC correction time
3 30 mm2/s mean measured flow time: tg = 39.95 s Calculated HC correction
according to equation (4.19):
tH = 1.03 s The measurement range of the viscometer was fallen short of Furthermore, the HC correction time is above the max correction time of tH = 0.66 s as indi-cated in /16/ for precision measurements
In this case a viscometer with a smaller capillary ameter should be resorted to If this is impossible, the individual HC correction time for precision meas-urements has to be determined in an experimental manner
Trang 16di-5 Calibration
The viscometer constant K is determined individually
for each glass capillary viscometer by way of
calibra-tion
By careful calibration in combination with the use of
high-quality measurement and testing means and
close-tolerance reference standard sources the
manufacturer guarantees a reproducible calibration
of highest precision Measurement and
reproducibil-ity incertainties of calibration have a direct influence
on the measurement incertainty of the viscometers
Measurement principle
The determination of the constants is done by a
si-multaneous flow-time measurement in the
viscome-ters to be calibrated (test specimens) and in the
ref-erence standard sources the constants of which were
determined by the "Physikalisch-Technische
Bunde-sanstalt (PTB)" (Federal Physico-Technical Institute)
in Brunswick
Realisation
In a thermostat bath with a constant temperature of
± 0.01 K the flow time of a test liquid through a
multi-tude of glass capillary viscometers is measured
Test liquids are no reference standard sources Their
viscosity is only known within a tolerance range of
± 10 % around a guidance value The test liquids
used are mono-substances or mineral-oil products
with narrow boiling profiles
Two of the viscometers are reference standard
sources the flow times of which is used to calculate
the kinematic viscosity of the test liquid Owing to the
use of two Reference Viscometers a functional test is
carried out automatically during calibration
The constants of the test specimens are determined
on the basis of kinematic viscosity of the test liquid and the flow time (please refer to Figure 16)
To ensure a high statistical certainty, two ment cycles involving seven flow-time measurements each are run, with the first measurement of the re-spective measurement cycle being considered as preliminary test
measure-The measurement temperature is 23 °C ± 0.01 K It
is verified using at least two officially gauged mercury capillary-column thermometers with a resolution of 0.01 K
Each calibration can guarantee the metrological rectness of the viscometer constants only for a lim-ited period of time It is therefore recommended to check the constants on a regular basis or to have them checked by the manufacturer, respectively The check may be done either by comparison measure-ments using reference standard sources (please see above) or with calibrating oils from the ”Deutsche Ka-librierdienst (DFD)” (German Calibration Service) However, if regular oils are being used, the limitation
cor-of the accuracy cor-of the test procedure caused by the incertainty of the regular-oil viscosity indication should be noted Considering that this incertainty is in general above the measurement incertainties stated for glass capillary viscometers, this calibration method is not recommended for precision measure-ments
Please refer also to DIN 51 561 - 4, Part 4: ter calibration and determination of measurement in-certainty, taking into account the user note /N9/
1
Hg
P
t - t
=
) t - (t K
) t - (t R 2 R22
K
=2
n
Trang 176 Handling of glass capillary viscometers
6.1 General guidelines on the selection
of the measurement system
Selection of the viscometer type
The following viscometers from SCHOTT-GERÄTE
can be used for viscosity measurement with
trans-parent liquids:
UBBELOHDE Viscometer
OSTWALD Viscometer
CANNON-FENSKE-Routine Viscometer
These include devices for both manual or automatic
measurements involving optoelectronic detection of
the meniscus passage In addition it is possible to
use TC-UBBELOHDE Viscometers equipped with
thermistor sensors Owing to the advantages
re-ferred to in chapter 2, UBBELOHDE Viscometers
should be preferred over the other types in most
ap-plications
In the case of measurements involving low-foaming
or bubbling liquids one should use OSTWALD or
TC-UBBELOHDE Viscometers, since foam of bubbles
affect the functioning of the photoelectric barriers In
the case of highly foaming liquids, however,
TC-UBBELOHDE Viscometers should not be used, since
the thermistors' function may be affected by adhering
foam particles In addition, no clear detection of the
meniscus passage is possible in the presence of
in-tense formation of foam
For determining the viscosity of mixed substances
containing highly volatile components and matters
reacting with the ambient air, the use of OSTWALD
or CANNON-FENSKE Routine Viscometer is
rec-ommended
If only sample or solvent small quantities are
avail-able, the use of Micro UBBELOHDE or Micro
OSTWALD Viscometers is favourable
For reason of thermally caused volume changes of
the product being analysed, high- or low-temperature
measurements should always be performed using
UBBELOHDE Viscometers
CANNON-FENSKE Reverse-Flow Viscometers for
manual measurement of the viscosity of opaque
liq-uids, or BS/IP/RF U-Tube Reverse Flow Viscometers
(from approx 6000 mm2/s) for highly viscous
sub-stances are available
For automatic viscosity determination, to be used in particular with opaque oils and emulsions, TC-UBBELOHDE Viscometers are the choice Ow-ing to the fact that the thermistor sensors are glass-sealed and melted hermetically tight in the viscome-ters, it is, for instance, also possible to measure con-ductive and highly aggressive liquids
Capillary selection
The measurement range of the viscometers is termined by the capillary diameter (0.25 10 mm) Each capillary diameter has a capillary number and a viscometer type number assigned which is indicated
de-on a test certificate
To select a viscometer, the viscosity of the substance
to be analysed has to be estimated
The selection as such is based on a rough tion of the flow time exclusive of the HC correction according to equation (4.2)
calcula-In accordance with the DIN standard, the min flow time to be sought after should be 200 s /N10/ for most viscometers However, trials have shown that it
is also possible to realise shorter flow times without impairing the measurement accuracy
When using micro viscometers the flow time can be reduced to 30 s According to the most recent re-search results, even flow times down to approx 10 s /15, 34, 35/ are possible if individual Hagenbach-Couette Correction with automatic flow-time meas-urement is applied
In the operating instructions of the viscometers the min flow times are stated as a function of the capil-laries
Table 1 shows as an example the measurement ranges as a function of the capillary diameter for UBBELOHDE Viscometers
Trang 18Table 1 Measurement ranges of UBBELOHDE Viscometers /16/
Measurement range[mm2/s]
6.2 Cleaning of capillary viscometers
Careful cleaning of viscometers is an essential
pre-requisite for an exact and reproducible measurement
value Practical experience has shown that increased
scattering of the flow times is in most cases caused
by contamination In this context even smallest
quan-tities of microscopically small particles of dust within
the viscometer may lead to standard deviations of up
to several per cent
Particles which adhere firmly to the capillary wall and
are frequently almost invisible are often the cause of
systematic measurement errors Errors of this type,
leading to an increase of the flow times, can hardly
be told from the individual values of a measurement
series The larger the capillary diameter, the smaller
is the danger of contamination
In addition to solid particles, oil or fat films adhering
to the internal wall of the viscometer may affect the
flow times In particular when measuring substances
with a higher surface tension (e.g aqueous media)
droplets, adhering to the wall and affecting the
measurement result, may occur during the start-up
process This is why it is recommendable to measure
only substances with similar properties in one and
the same viscometer If this is impossible, a
particu-larly careful cleaning process has to be carried out
As a principle, all cleaning agents should be filtered
prior to use using glass frits with a corresponding
pore width Paper filters have a tendency of losing
fi-bres and are thus not recommendable
Initial cleaning
Especially as a result of transportation and storage, severe contamination may occur so that a thorough initial cleaning is inevitable
The following cleaning agents have proven
to be suitable:
concentrated sulphuric acid with an addition of tassium dichromate (chromic-sulphuric acid mix-ture); when working with chromic-sulphuric acid mixture, extreme care has to be taken; chromium-(VI) compounds are toxic
po- a solution consisting of 15 % hydrochloric acid and
3 Rinse the viscometer using distilled water
4 Rinse with a filtered, miscible, highly volatile solvent, e.g with acetone
5 Dry by way of purging with dry, dust-free air or in a drying cabinet
They use of highly alkaline solvents may lead to versible leaching in the glasses which may even
irre-In addition, SCHOTT-GERÄTE offers a KPG utility pipette for determining the optimally suited capillary number for the respec-tive measurement task
Trang 19Initial cleaning
Immediately after each measurement, the viscometer
has to be cleaned using suitable solvents The use of
a vacuum pump has proven suitable for this purpose
Cleaning method when using a vacuum pump:
1 Connect the vacuum pump via a liquid trap to the
capillary tube
2 Fill the cleaning liquid into the filling tube and the
venting tube
(in the case of UBBELOHDE Viscometer)
3 Periodically close the filling and the venting tube
while the liquid is being sucked off
A pulsating liquid column will occur, dissolving
even set-in contamination
4 Repeat the cleaning process two or three times
5 Rinse with a highly volatile solvent
6 Dry by way of sucking dry, dust-free air
through the assembly
Cleaning method without a vacuum pump:
1 Fill the cleaning liquid into the filling tube
2 Suck the liquid several times into
the measurements sphere
3 Clean the remaining viscometer parts
by shaking the viscometer
4 Empty the viscometer
5 Repeat the cleaning process two to three times
6 Rinse using a filtered, highly volatile solvent
7 Dry by purging with a dry, dust-free air
or in the drier
In particular when cleaning without a vacuum pump,
it is furthermore recommended to wait for an
addi-tional 20 to 30 minutes prior to the beginning of the
cleaning cycle If measurements are not made
im-mediately subsequent one to another, the cleaned
viscometers are to be stored in a dust-free
environ-ment Immediately prior to the next measurements,
the glass body is to be rinsed and dried once again
If the viscometer was not in use for several weeks,
cleaning should be done using one of the substances
suitable for initial cleaning after an action time of at
least one hour The same cleaning process should
also be performed if scattering of the measurements
values above the repeatability limit specified for the
viscometer, or systematic measurement errors, occur
during operation, with such errors not being
elimi-nated by cleaning using one of the correspondent
solvents
In order to minimise the likelihood of the occurrence
of such errors from the onset, regular cleaning of the
viscometers using the liquid specified for initial
clean-ing is also recommended at larger timely intervals
Automatic cleaning
Especially for examinations of mineral oils in UBBELOHDE or CANNON-FENSKE Routine Vis-cometers, SCHOTT-GERÄTE is offering the AVS 26 Viscometer Cleaner Using this device it is possible
to clean viscometers without having to take them out
of the thermostat baths This process requires cial viscometers with an attached rinsing tube The AVS 26 Viscometer Cleaner works in combina-tion with the automatic viscosity-measurement de-vices of the AVS series Several rinsing programs are available During the rinsing process the vis-cometer cleaner pumps solvent alternately through all tubes of the viscometer The device is intended for use with up to two solvents The rinsing process may
spe-be followed by a drying cycle For automatic ing, the maximum viscosity limit of the product being analysed is approx 8000 mm2
clean-/s at 25 °C
The use of an automatic rinser, however, does not release the user from a periodical, careful manual cleaning
6.3 Preparation of the measurement
Preparation of the sample
Solid particles contained in the sample to be ined have a similar effect on the measurement result
exam-as contamination in the viscometer For this reason, you should immediately prior to per-forming the measurement:
Carefully clean and dry all parts coming in contact with the substance to be measured,
filter the samples
- low-viscosity samples:
glass filter, porosity 2 to 4 (10 - 100 mm)
- highly viscous samples:
sieve, mesh width 0.3 mm
Paraffin or resin-containing products as well as stances with a pour-point of less than 30°C below the testing temperature are to be treated thermally prior
sub-to performing the measurement The measurement temperature must be at least 20°C higher than the pour-point
Trang 20Filling UBBELOHDE and OSTWALD
Viscometers
The substance to be examined is to be filled into the
liquid reservoir via the filling tube
Considering that the average pressure height of the
OSTWALD Viscometer depends on the filling
quan-tity, the sample volumes for OSTWALD and Micro
OSTWALD Viscometers indicated in table 2 are to be
adhered to in any case For this reason, a pipette is
to be used for filling
UBBELOHDE Viscometers have two division marks
on the reservoir vessel showing the minimum and
maximum filling quantity In case of Micro
UBBELOHDE Viscometers there is only one mark
which is to be adhered to within a tolerance range of
about ± 1 mm This means that more accurate
dos-ing is not required It should only be ensured that the
opening of the venting pipe on the reference level
vessel is above the liquid level
Considering that air bubbles occurring during the
measurement process may lead to scattering of the
measurements values, it has to be ensured that no
bubbles occur during the filling of the viscometers
For this purpose, the viscometer is held in a slightly
inclined position, and the liquid is filled in such a
manner that it will float down into the reservoir vessel
along the filling tube without any bubbles occurring
Best results when filling UBBELOHDE Viscometers
were achieved using throw-away syringes with an
at-tached glass-tip filter When using syringe filters, prior
filtration is not necessary
Especially when filling in substances of a higher
vis-cosity into OSTWALD Viscometers the pipette should
be immersed deeply into the filling tube in order to
prevent after-flow errors
Table 2 Filling quantities for
various viscometer types
Filling CANNON-FENSKE Routine Viscometers
CANNON-FENSKE Routine Viscometers (please fer to Figure 17) are held upside down for filling The capillary tube (1) immerses into the liquid to be measured, while suction is upheld at the other tube until the liquid has reached the annular mark M2 Af-ter filling, the viscometer is restored to normal meas-uring position
re-Considering that filling a Reverse Flow Viscometer is somewhat more complex, reference is made at this point to the standards /N5, N28, N47/ as well as to the operating instructions
Figure 17 CANNON-FENSKE Routine Viscometer
1 tube with capillary
2 venting tube
3 reservoir
4 lower timing mark M2
5 upper timing mark M1
6 pre-run sphere
7 capillary
8 measuring sphere
9 tube extension
Trang 21Suspending the viscometers in the racks
SCHOTT-GERÄTE offers for all viscometer types
fixation racks or holders, respectively, which ensure a
stable, vertical suspension of the viscometers in the
thermostat bath In addition, they protect the
vis-cometers from breaking
Prior to the measurement, UBBELOHDE
Viscome-ters are to be suspended in the racks provided for
this purpose, and fixed in position by pressing the
spring downwards
Figure 18 UBBELOHDE Viscometer
with fixation rack
6.4 Performing the measurement
Thermostat treatment
Viscosity is highly depending on the temperature
/24/ For this reason, the viscometers have to be
treated in a thermostat during each measurement
The thermostats used are automatically controlled
liquid viewing thermostats The viscometer has to be
immersed until the bath liquid is at least 2 cm higher
than the liquid meniscus in the viscometer in its
Out-The liquid bath and in particular the thermometer are
to be protected from direct exposure to light sources There recommended bath liquids are
below 0 °C: antifreezers, e.g
The viewing thermostats of the CT series, developed
by a SCHOTT-GERÄTE especially for capillary cometry, meet the requirements with regard to the timely and local constancy of the temperature of the bath liquid They are equipped with openings or in-serts, respectively, for two (CT 52/2, CT 1650/2) or four capillary viscometers (CT 1650/4)
vis-Once filled and placed in the fixation rack or the holder, respectively, the capillary viscometers are hung into the thermostat bath the temperature of which was pre-adjusted When using viewing ther-mostats of the CT series, special viscometer-rack in-serts for manual measurement are available
Subsequently, the sample is exposed to thermostat treatment in the viscometer
When performing measurements using UBBELOHDE, OSTWALD or CANNON-FENSKE Routine Viscometers it is recommended to suck the liquids at least three times into the measurement sphere in order to speed up the heat transfer This procedure is not possible the case of Reverse-Flow Viscometers Their temperature adjustment should therefore be correspondingly longer
The following temperature-adjustment times are recommended:
10 min: low-viscosity substances;
20 min: high-viscosity substances,
low-viscosity substances in the case of Reverse-Flow Viscometers;
30 min: high-viscosity substances in the case of
Reverse-Flow Viscometers
Trang 22Manual measurement
For the measurement of the flow times, the liquid is
sucked into the measurement sphere by applying a
vacuum to the capillary tube When using
viscome-ters with a feeder sphere, the latter should be filled at
least up to its half
Viscometers without a pre-run sphere are filled until
the liquid meniscus is approximately 20 mm above
the upper annular mark If UBBELOHDE
Viscome-ters are used, the venting tube should be closed with
a finger tip prior to starting sucking in Upon
comple-tion of the filling process the succomple-tion hose is removed
from the capillary tube and, in the case of the
UBBELOHDE Viscometer, the venting tube is
re-leased
When measuring highly viscous samples it is
rec-ommendable to keep the capillary tube closed after
releasing the venting tube until the levelling bulb has
run empty and the suspended level has built up
When examining highly volatile substances it is
rec-ommended to perform the filling of the measurement
sphere by applying an over-pressure to the filling
tube, if no bubbles occur in the liquid Closing and
opening the venting tube in the case of
UBBELOHDE Viscometers should be done
analo-gously
The measurement involves the period of time over
which the lower for vertex of the meniscus sinks from
the upper edge of the upper annular mark down to
the upper edge of the lower annular mark The stop
watch used for timing should have a dissolution of at
least 0.1 s When the meniscus passage is detected,
it has to be made sure that the annual mark is at eye
level
Figure 19 (c) shows the proper detection of
the meniscus passage
Figure 19 Detection of the meniscus passage
in the case if manual measurement
(a), (b) - wrong (c) – correct
In order to make the measurement values available for statistical evaluation the measurement process should
be repeated several times Especially in the case of UBBELOHDE Viscometers, on order to avoid any formation of bubbles, it should be noted that a re-newed sucking or pressing up of the measurement substance must only begin when the drainage of the liquid from the capillary is completed
When using Reverse-Flow Viscometers, sucking the liquid into the measurements sphere is not applica-ble To perform the measurement, the tube which was closed after filling, is opened on the side of the measurement sphere, and subsequently one meas-ures the time over which the liquid rises from the lower to the upper annular mark The CANNON-FENSKE Reverse-Flow Viscometer is equipped with two measurement spheres one on top of the other, i.e two measurement values are available after just one liquid passage To repeat a measurement when using Reverse-Flow Viscometers, they have to be emptied, cleaned, and refilled after each measure-ment
If the repeatability limit of a measurement series (2.8 times the standard deviation) exceeds the re-
producibility limit indicated for the specific cometer, one has to assume the presence of exter-
vis-nal influences In this case the measurements have
to be repeated on a new part of the filtered sample after the viscometer has been cleaned If only a
”maverick” is present it may be deleted or, as a better alternative, be replaced by an additional measure-ment value If necessary, a check for runaway values
of this kind is to be performed /17/
The calculation of viscosity is done on the basis of the mean value of the flow times
Automatic measurement
For automatic viscosity measurement using UBBELOHDE, OSTWALD, and CANNON-FENSKE Routine Viscometers, SCHOTT-GERÄTE offers the automatic viscosity measurement devices of the AVS series
Table 3 will give you an overview of the device gram
pro-The selection of the AVS/S, AVS-SK, and AVS/S-CF measurement tripods for automatic viscosity meas-urement with optical detection is determined by:
Viscometer type
Bath liquid of the thermostats (metal tripod for non-aqueous media, PVDF measurement tripod as a corrosion-free option)
For measurement using the TC-UBBELOHDE cometer no measurement tripod is required The vis-cometer is clamped into a special fixation rack and