EN 61868:1999

SLOVENSKI STANDARD
01-december-1999
Mineral insulating oils. - Determination of kinematic viscosity at very low
temperatures (IEC 61868:1998)
Mineral insulating oils - Determination of kinematic viscosity at very low temperatures
Isolieröle auf Mineralölbasis - Bestimmung der kinematischen Viskosität bei sehr
niedrigen Temperaturen
Huiles minérales isolantes - Détermination de la viscosité cinématique à très basse
température
Ta slovenski standard je istoveten z: EN 61868:1999
ICS:
29.040.10 Izolacijska olja Insulating oils
2003-01.Slovenski inštitut za standardizacijo. Razmnoževanje celote ali delov tega standarda ni dovoljeno.


NORME
CEI
INTERNATIONALE
IEC
INTERNATIONAL
Première édition
STANDARD
First edition
1998-11
Huiles minérales isolantes –
Détermination de la viscosité cinématique
à très basse température
Mineral insulating oils –
Determination of kinematic viscosity
at very low temperatures
 IEC 1998 Droits de reproduction réservés  Copyright - all rights reserved
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61868  IEC:1998 – 3 –
CONTENTS
Page
FOREWORD . 5
INTRODUCTION .7
Clause
1 Scope.9
2 Normative references.9
3 Definitions.9
4 Principle.11
5 Apparatus.11
6 Calibration.13
7 Procedure.15
8 Expression of results. 17
Figure 1 – Viscometer bath and accessoires . 21

61868  IEC:1998 – 5 –
INTERNATIONAL ELECTROTECHNICAL COMMISSION
___________
MINERAL INSULATING OILS –
DETERMINATION OF KINEMATIC VISCOSITY
AT VERY LOW TEMPERATURES
FOREWORD
1) The IEC (International Electrotechnical Commission) is a worldwide organization for standardization comprising
all national electrotechnical committees (IEC National Committees). The object of the IEC is to promote
international co-operation on all questions concerning standardization in the electrical and electronic fields. To
this end and in addition to other activities, the IEC publishes International Standards. Their preparation is
entrusted to technical committees; any IEC National Committee interested in the subject dealt with may
participate in this preparatory work. International, governmental and non-governmental organizations liaising
with the IEC also participate in this preparation. The IEC collaborates closely with the International Organization
for Standardization (ISO) in accordance with conditions determined by agreement between the two
organizations.
2) The formal decisions or agreements of the IEC on technical matters express, as nearly as possible, an
international consensus of opinion on the relevant subjects since each technical committee has representation
from all interested National Committees.
3) The documents produced have the form of recommendations for international use and are published in the form
of standards, technical reports or guides and they are accepted by the National Committees in that sense.
4) In order to promote international unification, IEC National Committees undertake to apply IEC International
Standards transparently to the maximum extent possible in their national and regional standards. Any
divergence between the IEC Standard and the corresponding national or regional standard shall be clearly
indicated in the latter.
5) The IEC provides no marking procedure to indicate its approval and cannot be rendered responsible for any
equipment declared to be in conformity with one of its standards.
6) Attention is drawn to the possibility that some of the elements of this International Standard may be the subject
of patent rights. The IEC shall not be held responsible for identifying any or all such patent rights.
International Standard IEC 61868 has been prepared by IEC technical committee 10: Fluids for
electrotechnical applications.
The text of this standard is based on the following documents:
FDIS Report on voting
10/443/FDIS 10/452/RVD
Full information on the voting for the approval of this standard can be found in the report on
voting indicated in the above table.

61868  IEC:1998 – 7 –
INTRODUCTION
Non-Newtonian behaviour and significant increases in viscosity at very low temperatures have
been observed in some mineral insulating oils as a result of microcrystalline growth, with a
potential risk of failure for electrical equipment re-energized under very cold weather
conditions.
Microcrystalline formation is a slow process, which can take relatively long periods of low-
temperature soaking to show its full effects, and which can also be destroyed by inducing
movement in the oil, possibly as a result of heat producing friction or shear forces.
This standard presents the adaptations to be made to ISO 3104 to take into account these
effects and to allow the measurement of kinematic viscosity at very low temperatures, without
subjecting the oil sample to warming or undue movement during measurement, and with the
application of a soaking period, in order to adequately simulate the flow of oil in electrical
equipment, for example in the small cooling passages and timing mechanisms of transformers.

61868  IEC:1998 – 9 –
MINERAL INSULATING OILS –
DETERMINATION OF KINEMATIC VISCOSITY
AT VERY LOW TEMPERATURES
1 Scope
This International Standard specifies a procedure for the determination of the kinematic
viscosity of mineral insulating oils, both transparent and opaque, at very low temperatures,
after a cold soaking period of at least 20 h, by measuring the time for a volume of liquid to flow
under gravity through a calibrated glass capillary viscometer.
It is applicable at all temperatures to both Newtonian and non-Newtonian liquids having
2 –1
viscosities of up to 20 000 mm × s . It is particularly suitable for the measurement of the
kinematic viscosity of liquids for use in cold climates, at very low temperatures (–40 °C) or at
temperatures between the cloud and pour-point temperatures (typically –20 °C) where some
liquids may develop unexpectedly high viscosities under cold soak conditions.
2 Normative references
The following normative documents contain provisions which, through reference in this text,
constitute provisions of this International Standard. At the time of publication, the editions
indicated were valid. All normative documents are subject to revision, and parties to
agreements based on this International Standard are encouraged to investigate the possibility
of applying the most recent editions of the normative documents indicated below. Members of
IEC and ISO maintain registers of currently valid International Standards.
ISO 3104:1994, Petroleum products – Transparent and opaque liquids – Determination of
kinematic viscosity and calculation of dynamic viscosity
ISO 3105:1994, Glass capillary kinematic viscometers – Specification and operating
instructions
3 Definitions
For the purpose of this International Standard, the following definitions apply:
3.1
kinematic viscosity
the ratio between the viscosity and the density of the liquid. It is a measure of the resistance to
flow of a liquid under gravity
2 –1
NOTE – In the SI, the unit of kinematic viscosity is the square metre per second (m × s ).
3.2
newtonian liquid
a liquid having a viscosity that is independent of the shear stress or shear rate. If the ratio of
shear stress to shear rate is not constant, the liquid is non-Newtonian

61868  IEC:1998 – 11 –
4 Principle
The time is measured, in seconds, for a fixed volume of liquid to flow under gravity through the
capillary of a calibrated viscometer under a reproducible driving head and at a closely
controlled temperature. The kinematic viscosity is the product of the measured flow time and
the calibration constant of the viscometer derived by direct calibration with a certified reference
oil according to ISO 3104.
5 Apparatus
5.1 Viscometers
Viscometers of the glass capillary type, calibrated and capable of measuring kinematic
viscosity within the limits of precision given in 8.2, are acceptable (see note 1). The visco-
meters listed in ISO 3105 meet these requirements (see note 2). Use only calibrated
viscometers with constants measured and expressed to the nearest 0,1 % of their respective
values. It is not intended to restrict this standard either to the use of only those viscometers or
to the use of U tube viscometers; however, glass capillary viscometers of the Cannon Fenske
2 –2 2 –2
Routine type (series 500 and 600), with constants between 8 mm × s and 20 mm × s ,
have been found particularly suitable at very low temperatures.
NOTE 1 – Procedures for the calibration of viscometers are given in ISO Standard 3105.
NOTE 2 – The calibration constant C is dependent upon the gravitational acceleration at the place of calibration and
this shall, therefore, be supplied by the calibration laboratory together with the instrument constant. Where the
acceleration of gravity g in the two locations differs by more than 0,1 %, correct the calibration constant as follows:
g
C=× C
2 1
g
where the subscripts 1 and 2 indicate respectively the calibration laboratory and the testing laboratory.
5.2 Viscometer holders
The holder shall allow the viscometer to be suspended in a position similar to that adopted for
calibration. The proper alignment of a vertical datum part may be confirmed by using a plumb
line.
5.3 Viscometer bath and accessories
A suitable set-up, placed in a freezer tank, is described in figure 1. Any transparent liquid bath
may be used provided that it is of sufficient depth that at no time during the measurement will
any portion of the sample in the viscometer be less than 20 mm below the surface of the bath
liquid or less than 20 mm above the bottom of the bath. Methanol has been found appropriate
as a bath medium. However, any liquid which is clear, does not freeze at operating
temperature, and does no harm to the silicone-based caulking material used for the glass bath
construction, can be used instead of methanol, for example 2-propanol.
The temperature control shall be such that the temperature of the bath medium does not vary
by more than 0,03 °C over the length of the viscometers, or between the position of each
viscometer, or at the location of the thermometer.
5.4 Temperature measuring device
Calibrated liquid-in-glass thermometers of a minimum accuracy after correction of 0,02 °C may
be used, or any other thermometric or battery-operated device of equal accuracy. All readings
and corrections shall be made at least to the nearest 0,02 °C. IP Standard thermometer 68C or
ASTM standard thermometer 73C (corresponding to ASTM specification E1), containing a

61868  IEC:1998 – 13 –
suitable mercury thallium alloy, have been found appropriate. These thermometers have
0,05 °C graduations, but allow interpolations to be made by good operators down to 0,01 °C.
5.5 Timing device
Any timing device may be used provided that the readings can be taken by the operator with a
discrimination of 0,2 s or better, and that it has an accuracy of less than 0,01 % when tested
over intervals of 15 min.
Electrical timing devices may be used if the current frequency is continuously controlled to an
accuracy of 0,01 % or better. Alternating currents, as provided by some public power systems,
are intermittently rather than continuously controlled. When used to activate electrical timing
devices, such control can cause large errors in viscosity flow measurements.
Synchronous clocks, dependent on power frequency, should meet this accuracy requirement,
as should a high quality mechanical stopwatch, although a quartz-based stopwatch, checked
against a broadcast time standard, or a synchronous clock that is driven by a large inter-
connected power system would be preferred.
6 Calibration
6.1 Viscometers
Use only calibrated viscometers with constants measured and expressed to the nearest 0,1 %
of their respective values.
6.2 Thermometers
Calibrated liquid-in-glass thermometers shall be checked to the nearest 0,02 °C by direct
comparison with a suitable certified reference thermometer.
6.2.1 Calibrated thermometers shall be checked for accuracy at total immersion, which
means immersion to the top of the mercury-thallium alloy column, with the remainder of the
stem and the expansion chamber at the top of the thermometer exposed to the freezer's
temperature; the expansion bulb sh
...