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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 1

Theory and Praxis

of

Capillary Viscometry

- An Introduction –

Trang 2

Authors:

Prof Dr.-Ing habil Jürgen Wilke

Hochschule Anhalt

Food and biotechnology

(Process and environmental Technology Faculty)

Dr.-Ing Holger Kryk

Trang 3

Table 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 4

1 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 5

In 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)

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In 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

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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 tube flow within a measurement

capillary

Under idealised conditions

Ÿ laminar, isothermal flow

Ÿ stationary flow condition

Ÿ Newtonian flow behaviour of the liquid

Ÿ pressure-independence of viscosity

Ÿ incompressibility of the liquid

Ÿ wall adherence of the liquid

Ÿ 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:

Ÿ Measurement of the differential pressure at a

con-stant volume flow of the sample through the

cap-illary

Ÿ 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

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Figure 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

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ap-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:

Ÿ 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

Ÿ 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

Ÿ a start and stop signal for the timing process

and as

Ÿ 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

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Conductivity 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

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4 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/

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4.1.2 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 13

For 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 14

The 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 15

Examples 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 16

di-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 17

6 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:

Ÿ UBBELOHDE Viscometer

Ÿ OSTWALD Viscometer

Ÿ 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 18

Table 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:

Ÿ 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-Ÿ 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 19

Initial 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:

Ÿ Carefully clean and dry all parts coming in contact with the substance to be measured,

Ÿ 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 20

Filling 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 21

Suspending 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:

Ÿ 10 min: low-viscosity substances;

Ÿ 20 min: high-viscosity substances,

low-viscosity substances in the case of Reverse-Flow Viscometers;

Ÿ 30 min: high-viscosity substances in the case of

Reverse-Flow Viscometers

Trang 22

Manual 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:

Ÿ Viscometer type

Ÿ 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

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