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IS0
O RG AN IZ AT1 ON
I NT ERN AT1 O NA L FOR STA N DARD IZATIO N
IS0 RECOMMENDATION
R 386
PRINCIPLES OF CONSTRUCTION AND ADJUSTMENT
OF LlQU I D-IN-GLASS LABORATORY THERMOMETERS
1st EDITION
September 1964
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 386, Principles of Construction and Adjustment of Liquid-in-
Glass Laboratory Thermometers, was drawn up by Technical Committee ISO/TC 48, Laborutory
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 1951 and led, in 1960, to
the adoption of a Draft IS0 Recommendation.
In April 1960, this Draft IS0 Recommendation (No. 349) was circulated to all the IS0
It was approved, subject to a few modifications of an editorial
Member Bodies for enquiry.
nature, by the following Member Bodies:
Australia Germany Romania
Austria Greece Spain
Belgium
India Sweden
Canada
Israel United Kingdom
Chile Japan U.S.A.
Colombia Netherlands U.S.S.R.
Czechoslovakia New Zealand
France Poland
No Member Body opposed the approval of the Draft.
The Draft IS0 Recommendation was then submitted by correspondence to the IS0 Coun-
cil, which decided, in September 1964, to accept it as an IS0 RECOMMENDATION.
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ISO/R 386 - 1964 (E)
IS0 Recommendation R 386 September 1964
PRINCIPLES OF CONSTRUCTION AND ADJUSTMENT
OF LIQUID-IN-GLASS LABORATORY THERMOMETERS
1. SCOPE
This IS0 Recommendation is intended to provide guidance for drawing up specifications for
liquid-in-glass laboratory thermometers. Each thermometer consists of a glass bulb filled with
liquid and connected with a glass capillary tube. A scale is associated with the tube in such a
way that the temperature can be read from the height of the liquid in the tube.
2. TYPES OF THERMOiMETERS
Two types of liquid-in-glass thermometers are distinguished.
2.1 Solid-stem thermometer
Thermometer having a thick-walled capillary stem, on which the scale is permanently
marked.
2.2 Enclosed-scale thermometer
Thermometer having a capillary stem and a separate strip bearing the scale, both being
enclosed in a protective sheath.
3. ïEMPERATURE SCALE
The temperature scale to which the thermometers refer is the International Practical Scale of
Temperature, adopted by the Conférence Générale des Poids et Mesures in 1960 as the revised
edition of the International Temperature Scale of 1948.
4. IMMERSION
In measuring a temperature, one of the following methods should be used:
(a) either the bulb and the section of the capillary tube filled with liquid should be com-
pletely immersed in the medium whose temperature is to be measured,
(b) or the temperature and the length of that section of the capillary tube filled with liquid
and emerging from the medium should be measured and a correction made for any
difference between the measured value and the specified value of the emergent liquid
column temperature.
The following definitions should be used :
4.1 Total immersion thermometer. Thermometer whose reading should be correct, when the
thermometer is immersed at least to the end of the liquid column in the medium, the tem-
perature of which is to be measured.
NOTE. - If in special instances it is required that the reading should be correct, when the complete thermo-
meter is immersed in the medium, this should be specified.
4.2 Partial immersion thermometer. Thermometer whose reading should be correct, when the
thermometer is immersed to a prescribed depth, and when the emergent liquid column is
under prescribed conditions.
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IÇO/R 386 - 1964 (E)
4.3 Emergent liquid column. That part of the capillary tube filled with the filling liquid of the
thermometer which is not immersed in the medium, the temperature of which is to be
measured.
5. GLASS
The glass should be selected so that the finished thermometer shows the following characteristics :
5.1 Strain in the glass should be reduced to a level sufficient to minimize the possibility of
fracture due to thermal or mechanical shock.
5.2 The bulb glass should be stabilized by suitable heat treatment to ensure that the accuracy
requirements of section 10 can be met.
5.3 The errors in the finished thermometer resulting from the thermometric properties of the
glass used for the bulb should be within the limits specified in section 10.
NOTE. - The maximum temperature up to which a glass may be used depends on the degree of stability
required and is related to the viscosity of the glass at the temperature concerned.
5.4 The accuracy of the reading should not be impaired by devitrifying or clouding during
manufacture.
5.5 The meniscus should be distorted as little as possible by defects or impurities in the glass.
6. LIQUID FILLING
The general requirements for the liquid filling should include the following :
6.1 The filling should remain liquid throughout the temperature range under the conditions
prevailing in the thermometer.
6.2 The liquid should be entirely free from contamination.
6.3 The boiling point of the liquid should be high enough to minimize distillation under the
conditions prevailing in the thermometer.
6.4 For liquids which wet glass, the requirements should also include the following:
6.4.1 The physical properties of the liquid should be such as to ensure that the drainage time
when the thermometer is cooled, is within specified limits.
6.4.2 The liquid should preferably be coloured by means of a light-fast dye which does not
stain the glass.
7. GAS FILLING
When gas filling is employed above the filling liquid, a dry, inert gas should be used, at a pressure
which raises the boiling point of the liquid sufficiently to ensure that reliable readings are obtained
over the complete thermometer scale.
8. CONSTRUCTION
8.1 Shape
The thermometers should generally be straight, and their external cross-section approxi-
mately circular, unless specified to the contrary. For special thermometers, deviation from
the straight shape is allowed. In the case of solid-stem thermometers, an external lens-shaped
deviation from the circular cross-section is allowed, to facilitate reading.
8.2 Top finish
The top of the thermometer may be finished with a glass ring, the diameter of which should
not exceed that of the stem. If a finish other than a ring is provided, its diameter should
also not exceed that of the stem.
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ISO/R 386 - 1964 (E)
8.3 Capillary tube
The inside of the capillary tube should be smooth. The cross-sectional area of the bore
should not show variations from the average greater than 10%. The size of the bore
should be such that the maximum sticking of the liquid does not exceed a specified amount.
8.4 Expansion chamber (safety chamber)
A thermometer should not be heated above its top normal working temperature. To avoid
serious results from momentary accidental overheating, a safety volume should be
provided at the top of the stem." The volume above the scale should be at least equivalent
to that occupied by an interval of 80 degrees Celsius of the scale. If this volume takes the
form of an expansion chamber, this should be pear-shaped with the hemisphere at the top;
an exception is permissible in the case of high-pressure gas filling.
8.5 Contraction chamber
To prevent the liquid from withdrawing into the main bulb at about O "C, or to allow the
inclusion of an auxiliary scale, the bore may be enlarged above the bulb. This contraction
chamber should be elongated in form, and its top should be at least 10 mm below the immer-
sion line, or at least 30 mm for a thermometer with a scale extending above 250 OC.
8.6
Enlargement of bore
No enlargement of the bore should be so located as to produce any variation in the cross-
section of the capillary in the scale portion.
8.7 Specification of dimensions
The following dimensions of a thermometer should preferably be specified (as required) :
8.7.1 Total length (overall length of the thermometer, including bulb and top finish). The
maximum dimension only should be specified.
8.7.2 Bulb length (to be measured from the bottom of the bulb to
the point at which the internal bulb diameter begins to
decrease as the bulb merges into the stem, as shown in Fig. 1).
In general the minimum dimension only should be specified.
_I\
8.7.3 Distance from top of bulb funnel to lower nominal limit of scale.
If the thermometer has more than one scale, this distance should
be to the lower nominal limit of the lowest scale. The minimum
-
dimension only should be specified. This distance should
I
be at least
1
30 mm for thermometers having the lower nominal
limit 3 100 "C
,
13 mm for thermometers having the lower nominal
Measurement
limit < 100°C
of bulb length
8.7.4
Scale length. The minimum dimension only should be specified. If the thermometer
has more than one scale, the minimum dimension should be specified for each scale.
* Overheating is liable to
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