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UDC 536.51 : 542.2
IS0
I NT ERN AT I ON AL ORGANIZATION FOR STANDARD I2 AT I ON
IS0 RECOMMENDATION
R 655
LONG ENCLOSED-SCALE THERMOMETERS
FOR PRECISION USE
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 R655, Long enclosed-scale thermorneters .for precision
use, was drawn up by Technical Committe lSO/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 October 1963, this Draft IS0 Recommendation (No. 673) 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
Hungary Poland
Australia India Spain
Austria Israel Switzerland
Belgium Italy U.A.R.
Chile
Japan United Kingdom
Czechoslovakia Korea, Rep. of U.S.S.R.
France Netherlands Yugoslavia
Greece New Zealand
Two Member Bodies opposed the approval of the Draft:
Germany
U.S.A.
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 655 - 1968 (E]
IS0 Recommendation R 655 February 1968
LONG ENCLOSED-SCALE THERMOMETERS
FOR PRECISION USE
INTRODUCTION
This IS0 Recommendation is based on IS0 Recommendation R 386, Principles of construction
and adjustment of liquid-in-glass Labotarory thermometers. It is one of four IS0 Recom-
mendations specifying requirements for basic series of long and short solid-stem and enclosed-
scale thermometers, intended for general use for precision work.
For ease of reference, each thermometer of the series has been allocated a combination of letters
and figures indicating the type of thermometer, the value of the smallest graduation interval, and
the upper and lower limits of the nominal scale range. The letter abbreviations given below have
been selected, after taking into account the descriptions in various languages usually given to
these types of thermometer:
STL . Long solid-stem thermometers,
STC . Short solid-stem thermometers,
EL . Long enclosed-scale thermometers,
EC . Short enclosed-scale thermometers.
1. SCOPE
This IS0 Recommendation specifies requirements for a basic series of long enclosed-scale
thermometers, not exceeding 375 mm in length, for general use in precision work.
NOTE. - There are in existence many different specifications for thermometers of the general types covered by
this series. It is intended that this series should replace all such specifications, except those for which there is a
well-established justification.
2. TYPE OF THERMOMETER
The thermometer should be of the liquid-in-glass enclosed-scale type.
3. TEMPERATURE 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
The thermometers should be adjusted for use at total immersion (i.e., the reading should be
correct when the thermometer is immersed at least to the end of the liquid column in the medium
whose temperature is required to be measured).
NOTE. - If thermometers divided to 0.1 or 0.2 degree Celsius are required to read correctly when the comp lete
thermometer is immersed in the medium, this should be stated in the purchasing contract and the thermometer
should be specially marked.
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IS0 / R 655 - 1968 (E)
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
Mercury should be used as liquid filling, except for thermometers with a scale extending below
-38 OC. For temperatures down to -55 OC, the eutectic alloy of mercury and thallium (8.5%
thallium by mass) should be used.
7. GAS FILLING
Only a dry, inert gas may be used as gas filling. It is essential that thermometers intended for use
above 100 "C should be gas-filled. The pressure of the gas should be high enough to raise the
boiling point of the mercury sufficiently to ensure that reliable readings are obtained over the
complete thermometer scale.
8. CONSTRUCTION
8.1 Shape
The thermometers should be straight and their external cross-section approximately circular.
8.2 Top finish
The top of the thermometer should be finished with a spherical end separated by a con-
stricted neck, or with a glass ring.
8.3 Strip bearing the scale
The strip bearing the scale should be of translucent material suitable for the temperature to be
measured. It should be placed tightly against the capillary tube inside the sheath, in such a
way that it can freely expand in length; a suitable method of fixing is by fusing a glass tube
to the sheath and to the upper end of the strip bearing the scale.
8.4 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 lO%, and the bore should be wide
enough to ensure that jumping of the meniscus does not exceed one-fifth of the graduation
interval.
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IS0 / R 655 - 1968 (E)
8.5 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 capillary tube.* 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, except for
thermometers EL/l/ 180/420 and EL/2/0/600. For thermometers in which the upper limit
of the scale is below room temperature, the lower end of the " pear " should be consider-
ably elongated, to avoid risk of a break in the liquid column at room temperature.
8.6 Contraction chamber
A thermometer in which the lower limit of the main scale is above O
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