ASTM E1750-95
(Guide)Standard Guide for Use of Water Triple Point Cells
Standard Guide for Use of Water Triple Point Cells
SCOPE
1.1 This guide covers the nature of two commercial water triple-point cells (types A and B, see Fig. 1) and provides a method for preparing the cell to realize the water triple-point and calibrate thermometers. Tests for assuring the integrity of a qualified cell and of cells yet to be qualified are given. Precautions for handling the cell to avoid breakage are also described.
1.2 The effect of hydrostatic pressure on the temperature of a water triple-point cell is discussed.
1.3 Procedures for adjusting the observed SPRT resistance readings for the effects of self-heating and hydrostatic pressure are described in Appendixes X1 and X2.
1.4 This standard does not purport to address all of the safety concerns, if any, associated with its use. It is the responsibility of the user of this standard to establish appropriate safety and health practices and determine the applicability of regulatory limitations prior to use.
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Designation: E 1750 – 95
Standard Guide for
Use of Water Triple Point Cells
This standard is issued under the fixed designation E 1750; the number immediately following the designation indicates the year of
original adoption or, in the case of revision, the year of last revision. A number in parentheses indicates the year of last reapproval. A
superscript epsilon (e) indicates an editorial change since the last revision or reapproval.
INTRODUCTION
The triple point of water is an important thermometric fixed point common to the definition of two
temperature scales of science and technology, the Kelvin Thermodynamic Temperature Scale (KTTS)
and the International Temperature Scale of 1990 (ITS-90). The ITS-90 was designed to be as close to
the KTTS as the experimental data available at the time of the adoption of the ITS-90 would permit.
The temperatures (T) on the KTTS are defined by assigning the value 273.16 K to the triple point of
water, thus defining the thermodynamic unit of temperature, kelvin (K), as 1/273.16 of the
thermodynamic temperature of the triple point of water (1, 2). The triple point of water, one of the
fixed points used to define the ITS-90, is the temperature to which the resistance ratios W(T) 5 R(T)/
R(273.16 K) of the standard platinum resistance thermometer (SPRT) calibrations are referred.
The triple points of various materials (where three distinct phases, for example, their solid, liquid,
and vapor phases, coexist in a state of thermal equilibrium) have fixed pressures and temperatures and
are highly reproducible. Of the ITS-90 fixed points, six are triple points. The water triple point is one
of the most accurately realizable of the defining fixed points of the ITS-90. When properly used, the
water triple point can be realized with an uncertainty between + 0.0 K and − 0.00015 K. In
comparison, it is difficult to prepare and use an ice bath with an uncertainty less than 0.002 K (3).
The triple point of water also provides a useful check point for verifying the condition of
thermometers. For example, measurements at the water triple point made just before and immediately
upon the return of an SPRT from calibration can reveal any shift that might have occurred during
transportation. Also, a valuable trend history of the stability of a thermometer is documented by
maintaining a systematic record of the thermometer readings using a qualified water triple-point cell
(see 4.3).
1. Scope responsibility of the user of this standard to establish appro-
priate safety and health practices and determine the applica-
1.1 This guide covers the nature of two commercial water
bility of regulatory limitations prior to use.
triple-point cells (types A and B, see Fig. 1) and provides a
method for preparing the cell to realize the water triple-point
2. Referenced Documents
and calibrate thermometers. Tests for assuring the integrity of
2.1 ASTM Standards:
a qualified cell and of cells yet to be qualified are given.
E 344 Terminology Relating to Thermometry and Hydrom-
Precautions for handling the cell to avoid breakage are also
etry
described.
E 1594 Guide for Expression of Temperature
1.2 The effect of hydrostatic pressure on the temperature of
a water triple-point cell is discussed.
3. Terminology
1.3 Procedures for adjusting the observed SPRT resistance
3.1 Definitions—The definitions given in Terminology
readings for the effects of self-heating and hydrostatic pressure
E 344 apply to terms used in this guide.
are described in Appendix X1 and Appendix X2.
3.2 Definitions of Terms Specific to This Standard:
1.4 This standard does not purport to address all of the
3.2.1 inner melt, n—a thin continuous layer of water be-
safety concerns, if any, associated with its use. It is the
tween the thermometer well and the ice mantle of a water
triple-point cell.
3.2.2 reference temperature, n—the temperature of a phase
This guide is under the jurisdiction of ASTM Committee E-20 on Temperature
Measurement and is the direct responsibility of Subcommittee E20.07on Funda-
equilibrium state of a pure substance at a specified pressure, for
mentals in Thermometry.
example, the assigned temperature of a fixed point.
Current edition approved Oct. 10, 1995. Published November 1995.
The boldface numbers in parentheses refer to the list of references at the end of
this standard. Annual Book of ASTM Standards, Vol 14.03.
Copyright © ASTM, 100 Barr Harbor Drive, West Conshohocken, PA 19428-2959, United States.
E 1750
fundamental state (see 4.2) by comparison with a bank of
similar qualified cells of known history, and it may be so
qualified and the qualification documented by its manufacturer.
4.4 The temperature to be attributed to a qualified water
triple-point cell is exactly 273.16 K on the ITS-90.
4.5 Continued accuracy of a qualified cell depends upon
sustained physical integrity. This may be verified by techniques
described in Section 6.
4.6 The triple point of water cell is capable of high accuracy,
that is, uncertainty levels of 60.1 mK or less. Specified
measurement procedures shall be followed to achieve this high
accuracy.
5. Apparatus (4-6)
5.1 The essential features of type A and type B water
triple-point cells are shown in Fig. 1. A transparent glass flask
free of soluble material is filled with pure, air-free water and
then is permanently sealed, air-free, at the vapor pressure of the
water. A reentrant well on the axis of the flask receives
thermometers that are to be exposed to the reference tempera-
ture.
5.2 The water used as the reference medium shall be very
pure. Often it is distilled directly into the cell. The isotopic
composition of cells filled with “rain water” is expected not to
vary enough to cause more than 0.05 mK difference in their
triple points. Extreme variations in isotopic composition, such
as between ocean water and water from old polar ice, can affect
the realized temperature by as much as 0.25 mK.
5.3 For use, a portion of the water is frozen within the cell
to form a mantle of ice that surrounds the well and controls its
temperature.
5.4 The temperature of the triple point of water realized in
a cell is independent of the environment outside the cell;
however, to reduce heat transfer and keep the ice mantle from
melting quickly, it is necessary to minimize heat flow between
the cell and its immediate environment. This may be done by
immersing the cell in an ice bath that maintains the full length
of the outer cell wall at or near the melting point of ice.
Alternatively, commercial automatic maintenance baths, built
specifically for this purpose, are available. In such baths, the
triple point of water equilibrium of the cell, once established,
FIG. 1 Configurations of two commonly used triple point of water
can be maintained for many months of continual use. To avoid
cells, Type A and Type B, with ice mantle prepared for
radiation heat transfer to the cell and to the thermometer, the
measurement at the ice/water equilibrium temperature. The cells
outer surface of the maintenance bath is made opaque to
are used immersed in an ice bath or water bath controlled close
to 0.01°C (see 5.4) radiation.
6. Assurance of Integrity
3.2.2.1 Discussion—at an equilibrium state of three phases
6.1 The temperature attained within a water triple-point cell
of a substance, that is, at the triple point, both the temperature
is an intrinsic property of the solid and liquid phases of water
and pressure are fixed.
under its own vapor pressure. If the water triple-point condi-
4. Significance and Use
tions are satisfied, the temperature attained within the cell is
4.1 This guide describes a procedure for placing a water more reproducible than any measurements that can be made of
triple-point cell in service and for using it as a reference it.
temperature in thermometer calibration. 6.2 The accuracy of realization of the water triple-point
4.2 The reference temperature attained is that of a funda- temperature with a qualified cell depends on the physical
mental state of pure water, the equilibrium between coexisting integrity of the seal and of the walls of the glass cell and on
solid, liquid, and vapor phases. their ability to exclude environmental air and contaminants.
4.3 The cell is subject to qualification but not to calibration. 6.3 Initial and continued physical integrity is confirmed by
The cell may be qualified as capable of representing the the following procedures:
E 1750
6.3.1 Test for the Presence of Air: even though previously qualified, is no longer qualified for use
6.3.1.1 Remove all objects from the thermometer well. as a water triple-point cell.
6.3.1.2 The solubility and the pressure of air at 101 325 Pa
7. Realization of the Water Triple-Point Temperature
lower the ice/water equilibrium temperature 0.01°C below the
(4-6)
triple-point temperature. Since air is more soluble in water at
7.1 The ice mantle that is required to realize the triple-point
lower temperatures, the test for air shall be done at room
temperature of water can be prepared in a number of ways.
temperature. The test is less definitive when performed on a
They produce essentially the same result. A common procedure
chilled cell. At room temperature, with the cell initially upright
is as follows:
and the well opening upward, slowly invert the cell. As the axis
7.1.1 Empty the well of any solids or liquids. Wipe the well
of the cell passes through horizontal and as the water within the
clean and dry, and seal the well opening with a rubber stopper.
cell strikes the end of the cell, a sharp “glassy clink” sound
7.1.2 If the triple point of water cell has not already been
should be heard. The distinctive sound results from the sudden
tested for the presence of air, perform the tests indicated in
collapse of water vapor and the “water hammer” striking the
6.3.1 for presence of air.
glass cell. The smaller the amount of air, the sharper the clink
7.1.3 To obtain an ice mantle of fairly uniform thickness
sound; a large amount of air cushions the water-hammer action
that extends to the top, immerse the cell completely in an ice
and the sound is duller.
bath, and chill the cell to near 0°C.
6.3.1.3 With a type A cell, continue to tilt the cell to make
7.1.4 Remove the cell from the bath and mount it upright on
a McLeod-gage type test until the vapor bubble is entirely
a plastic foam cushion. Wipe the cell dry around the rubber
captured in the space provided in the handle. The vapor bubble
stopper before removing the rubber stopper.
should be compressed to a volume no larger than about 0.03
7.1.5 Remove the rubber stopper and place about 1 cm of
cm (4 mm diameter). It may even vanish as it is compressed
dry alcohol in the well to serve as a heat-transfer medium while
by the weight of the water column. As in the tilt test, the bubble
forming an ice mantle around the well within the sealed cell.
test is more definitive when the cell is at room temperature (see
7.1.6 Place a small amount of crushed dry ice at the bottom
6.3.1.2). Since type B cells do not have a space to capture the
of the well, maintaining the height of the dry ice at about 1 cm
vapor, the amount of air in the cell is estimated by comparing
for a period of 2 to 3 min. In repeated use of the cell, the ice
the sharpness of the clink sound with type A cells.
mantle melts mostly at the bottom; hence, it is desirable that
6.3.2 Test for the Presence of Water Soluble Impurities:
the ice mantle be thicker at the bottom. Crushed dry ice may be
6.3.2.1 When ice is slowly formed around the thermometer
prepared from a block or by expansion from a siphon-tube tank
well, impurities are rejected into the remaining unfrozen water.
of liquid CO .
Therefore, the impurity concentration of the unfrozen water
7.1.7 At the interface of the well, the water is initially
increases as the ice mantle thickens. The ice is purer than the
supercooled, and the well becomes abruptly coated with fine
unfrozen water. Consequently, the inner melt (see section
needles of ice frozen from the supercooled water.
7.1.3) that is formed from the ice mantle is purer than the
7.1.8 After a layer of ice forms around the bottom of the
unfrozen water.
well, fill the well with crushed dry ice up to the vapor/liquid
6.3.2.2 Prepare a relatively thick ice mantle, according to
interface.
Section 7, by maintaining the dry ice level full for about 20 min
7.1.9 Replenish the dry ice as it sublimes, maintaining the
instead of 15 min. Make certain that the ice does not bridge to
well filled to the liquid surface, until a continuous ice mantle as
the cell wall (Note 2).
thick as desired forms on the surface of the well within the
6.3.2.3 Prepare an inner melt according to 7.1.13. Make
water (usually 4 to 8 mm thick). The mantle will appear thicker
measurements on the cell and determine the zero-power
than its actual thickness because of the lenticular shape of the
resistance according to Section 8 and Appendix X1.
cell and the refractive index of water. The actual thickness may
6.3.2.4 After 6.3.2.3, gently invert the water triple-point cell
be best estimated by viewing from the bottom of the cell while
and then return it to the upright position several times to
it is inverted or by immersing the cell in a large glass container
exchange the unfrozen water on the outside of the ice mantle
of water.
with the inner melt water.
NOTE 2—Caution: During preparation, the mantle should never be
NOTE 1—Caution: When inverting the cell, do not allow the floating
allowed to grow at any place to completely bridge the space between the
ice mantle to severely strike the bottom of the water triple-point cell.
well and the inner wall of the cell, as the expansion of the ice may break
6.3.2.5 Make measurements on the cell and determine the the cell. In particular, if bridging occurs at the surface of the water at the
top of the cell under the vapor space, melt the ice bridge by warming the
zero-power resistance, according to Section 8 and Appendix
cell locally with heat from the hand, while gently shaking the cell.
X1, of the same thermometer that was measured in 6.3.2.3.
6.3.2.6 Typically, for high quality water triple-point cells, 7.1.10 When the mantle attains nearly the desired thickness
the results of 6.3.2.3 and 6.3.2.5 will not differ by more than or after maintaining the dry ice level in the well at the water
0.03 mK. surface for about 15 min, return the cell to the ice bath with the
6.4 Any cell that had previously been qualified by compari- entrance to the thermometer well slightly above the ice bath
son with cells of known integrity (as in 4.3), that has not surface and allow the dry ice to sublime completely. By
thereafter been modified, and which currently passes the tests allo
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