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U DC 531.75
IS0
O RG A N I ZAT I O N FOR STA ND AR D I2 AT I O N
I N T E R N AT I O N A L
I SO R E CO M M E N DATI O N
R 650
RELATIVE DENSITY 60/60 OF HYDROMETERS
FOR GENERAL PURPOSES
1st EDITION
February 1968
COPYRIGHT RESERVED
The copyright of IS0 Recommendations and IS0 Standards
belongs to IS0 Member Bodies. Reproduction of these
documents, in any country, may be authorized therefore only
by the national standards organization of that country, being
a member of ISO.
For each individual country the only valid standard is the national standard of that country.
Printed in Switzerland
Also issued in French and Russian. Copies to be obtained through the national standards organizations.
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BRIEF HISTORY
The IS0 Recommendation R 650, Relative density 60160 "F hydrometers for general
purposes, was drawn up by Technical Committee ISO/TC 48, Laboratory glassware and
related apparatus, the Secretariat of which is held by the British Standards Institution (BSI).
Work on this question by the Technical Committee began in 1954 and led, in 1959,
to the adoption of a Draft IS0 Recommendation.
In November 1963, this Draft IS0 Recommendation (No. 687) was circulated to all
the IS0 Member Bodies for enquiry. It was approved, subject to a few modifications of an
editorial nature, by the following Member Bodies :
Argentina Greece Switzerland
Australia Hungary Turkey
Austria India U.A.R.
Be 1 g i u m Israel United Kingdom
Korea, Rep. of U.S.A.
Brazil
Bulgaria Netherlands U.S.S.R.
Chile New Zealand Yugoslavia
Colombia Poland
Czechoslovakia Spain
Two Member Bodies opposed the approval of the Draft :
France
Germany
The Draft IS0 Recommendation was then submitted by correspondence to the IS0
Council, which decided, in February 1968, to accept it as an IS0 RECOMMENDATION.
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IS0 / R 650 - 1968 (E)
IS0 Recommendation R 650 February 1968
RELATIVE DENSITY 60/60 OF HYDROMETERS
FOR GENERAL PURPOSES
1. SCOPE
This IS0 Recommendation specifies requirements for five basic series of glass hydrometers of
constant mass which indicate relative density 60160 OF with reference to water and comply with
IS0 Recommendation R 387, Principles of construction and adjustment of hydrometers. The
dimensions of the hydrometers have been chosen in such a way as to lead to convenience in use
and economy in manufacture.
2. BASIS OF SCALE
L 2.1 The scale should indicate relative density * 60160 OF with reference to water.
NOTE. -The relative density 60/60 "F of a liquid with reference to water is the ratio
density of the liquid at 60 "F
density of water at 60 "F
2.2 The use of a scale other than one based on density (mass per unit volume) is not in general
recommended but, in view of its importance in trade between various countries, the scale
based on relative density with reference to water is accepted.
3. REFERENCE TEMPERATURE
The standard reference temperature for the hydrometers should be 60 OF. When used in a liquid
at this temperature, the hydrometer should indicate the relative density of the liquid at 60 OF with
reference to water at 60 OF.
NOTE. - For the purposes of this IS0 Recommendation, the standard reference temperature 60 OF can be taken
as equal to 15.56 "C.
4. SURFACE TENSION
The adjustment should be related to specific capillary conditions as follows :
4.1 When the hydrometer is slightly displaced from its equilibrium position in a liquid, the
stem should pass through the liquid surface without causing any apparent alteration in the
shape of the meniscus.
4.2 The hydrometer should be adjusted with regard to surface tension. Except where the highest
precision is required, one of the standard categories of surface tension given in Annex A
should be used.
For hydrometers of the highest precision, intended for use in particular liquids (e.g. alcohol
solutions), the surface tension values appropriate to clean surfaces of these liquids and to
the actual indications of the hydrometer should be used (see clause 13 (b) (iii)).
* The expression " relative density " complies with IS0 Recommmdation R 31, Part III, Quantities and units
of mechanics. The term '' specific gravity " is often used in English instead of '' relative density ", when
the reference substance is water.
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IS0 / R 650 - 1968 (E)
5. REFERENCE LEVELS FOR ADJUSTMENT AND READING
5.1 Hydrometers should preferably be adjusted for readings taken at the level of the horizontal
liquid surface. If a hydrometer so adjusted is used in an opaque liquid, readings may be
taken at the top of the meniscus where it appears to meet the stem, but appropriate correc-
tion to the level of the horizontal liquid surface should then be made.
To avoid the necessity for making such corrections, hydrometers intended for use in opaque
liquids may alternatively be adjusted for readings taken at the top of the meniscus where
it appears to meet the stem. If a hydrometer is so adjusted, this should be clearly indicated
on the scale (see clause 13 (c)).
NOTE. - Appropriate corrections are given in Annex C.
5.2 The middle of the thickness of a scale line should be taken as its definitive position.
6. IMMERSION
In order that the readings of the hydrometer should be correct, the emergent stem should be dry
except in the immediate vicinity of the meniscus.
7. MATERIALS AND WORKMANSHIP
7.1 The bulb and the stem should be made of transparent glass as free as possible from strain
and visible defects.
NOTE. - Various measurement tables for liquids have been drawn up on the basis of using hydrometers
made of glass having a particular thermal expansion coefficient. When the actual expansion of the hydrometer
departs significantly from the particular value which underlies the measurement tables, an appropriate
correction should be made.
7.2 Where the loading material is solid, it should be fixed in the bottom part of the hydrometer
and should not in general soften if heated to 80°C; if, however, a hydrometer is likely to
be used at a temperature higher than 70 OC, the loading material should have a softening
point which is higher than 80 OC. The material should not deteriorate in use. Where mercury
is the loading material, it should be confined in the bottom part of the hydrometer.
7.3 There should be no loose material elsewhere in the instrument.
7.4 The scale lines and inscriptions should preferably be black and should be clearly and per-
manently marked.
7.5 The strip on which the scale and inscriptions are marked should have a smooth surface.
The scale should show no evidence of charring. When the stem is exposed to a temperature
of 80 OC, or such higher temperature at which the hydrometer will be used, for the time
necessary to carry out a determination, the strip bearing the scale should not become
discoloured or distorted.
8. FORM
8.1 The outer surface should be symmetrical about the main axis.
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IS0 / R 650 - 1968 (E)
I
8.2 There should be no abrupt changes in cross-section. The tapered design shown in the
Figure below is preferred, but any design which does not permit air bubbles to be trapped
is acceptable.
FIGURE. - Preferred design of hydrometer bulb
c
8.3 The hydrometer should float with its axis essentially vertical. 1.5" is suggested as the maxi-
mum permissible deviation.
8.4 A thermometer should not form part of a hydrometer of high sensitivity.
9. SCALE
9.1 General
9.1.1 The strip on which the scale and inscriptions are marked should remain securely fastened
in place at the temperature of use (see clause 7.5).
9.1.2 Appropriate means should be incorporated for ensuring that any displacement of the
scale or of the strip bearing the scale is readily apparent. No displacement should be
tolerated.
9.1.3 No hydrometer should have more than one type of scale. If a hydrometer has two
scales of the same type, the values indicated by them shouId not differ.
9.2 Graduation lines
9.2.1 The graduation lines should be distinct and of uniform thickness not exceeding 0.2 mm.
9.2.2 There should be no evident local irregularities in the spacing of the graduation lines.
9.2.3 The graduation lines should be perpendicular to the axis of the hydrometer.
9.2.4 The scale should be straight and without twist.
9.2.5 A line parallel to the axis of the instrument and indicating the front of the scale is per-
mitted.
9.2.6 The highest and lowest graduation lines indicating the nominal limits of the scale should
be long lines (see clauses 9.3.1 (a), 9.3.2 (a) and 9.3.3 (a)).
9.2.7 The shortest scale lines should extend at least one-fifth of the way round the circum-
ference of the stem.
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IS0 / R 650 - 1968 (E)
9.3 Sequence of graduation lines
9.3.1 On the hydrometers whose smallest scale division is 0.001 relative density:
(a) Every tenth graduation line should be a long line.
(b) There should be a medium line between two consecutive long lines.
(c) There s
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