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Metal Freeze Point Cell
Users Guide
2004, Rev. 1, 9/11
© 2004 - 2011 Fluke Corporation. All rights reserved. Specications subject to change without notice.
All product names are trademarks of their respective companies.
Page 2
Limited Warranty & Limitation of Liability
Each Fluke product is warranted to be free from defects in material and workmanship under normal use and service.
The warranty period is one year and begins on the date of shipment. Parts, product repairs, and services are warranted
for 90 days. This warranty extends only to the original buyer or end-user customer of a Fluke authorized reseller, and
does not apply to fuses, disposable batteries, or to any product which, in Fluke’s opinion, has been misused, altered,
neglected, contaminated, or damaged by accident or abnormal conditions of operation or handling. Fluke warrants
that software will operate substantially in accordance with its functional specications for 90 days and that it has been
properly recorded on non-defective media. Fluke does not warrant that software will be error free or operate without
interruption.
Fluke authorized resellers shall extend this warranty on new and unused products to end-user customers only but have
no authority to extend a greater or different warranty on behalf of Fluke. Warranty support is available only if product
is purchased through a Fluke authorized sales outlet or Buyer has paid the applicable international price. Fluke reserves
the right to invoice Buyer for importation costs of repair/replacement parts when product purchased in one country is
submitted for repair in another country.
Fluke’s warranty obligation is limited, at Fluke’s option, to refund of the purchase price, free of charge repair, or
replacement of a defective product which is returned to a Fluke authorized service center within the warranty period.
To obtain warranty service, contact your nearest Fluke authorized service center to obtain return authorization
information, then send the product to that service center, with a description of the difculty, postage and insurance
prepaid (FOB Destination). Fluke assumes no risk for damage in transit. Following warranty repair, the product will
be returned to Buyer, transportation prepaid (FOB Destination). If Fluke determines that failure was caused by neglect,
misuse, contamination, alteration, accident, or abnormal condition of operation or handling, including overvoltage
failures caused by use outside the product’s specied rating, or normal wear and tear of mechanical components, Fluke
will provide an estimate of repair costs and obtain authorization before commencing the work. Following repair, the
product will be returned to the Buyer transportation prepaid and the Buyer will be billed for the repair and return
transportation charges (FOB Shipping Point).
THIS WARRANTY IS BUYER’S SOLE AND EXCLUSIVE REMEDY AND IS IN LIEU OF ALL OTHER
WARRANTIES, EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO ANY IMPLIED WARRANTY OF
MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE. FLUKE SHALL NOT BE LIABLE FOR ANY
SPECIAL, INDIRECT, INCIDENTAL, OR CONSEQUENTIAL DAMAGES OR LOSSES, INCLUDING LOSS OF
DATA, ARISING FROM ANY CAUSE OR THEORY.
Since some countries or states do not allow limitation of the term of an implied warranty, or exclusion or limitation
of incidental or consequential damages, the limitations and exclusions of this warranty may not apply to every buyer.
If any provision of this Warranty is held invalid or unenforceable by a court or other decision-maker of competent
jurisdiction, such holding will not affect the validity or enforceability of any other provision.
Fluke Corporation Fluke Europe B.V.
P.O. Box 9090 P.O. Box 1186
Everett, WA 98206-9090 5602 BD Eindhoven
U.S.A. The Netherlands
11/99
Page 3
Table of Contents
Title Page
Introduction ................................................................................................................... 1
Before You Start ............................................................................................................ 3
Symbols Used ........................................................................................................... 3
Safety Information .................................................................................................... 4
How to Contact Fluke ............................................................................................... 4
Specications ................................................................................................................ 5
Description .................................................................................................................... 5
Care of Your Metal Freezing Point Cell ....................................................................... 6
General Information .................................................................................................. 6
Devitrication of Quartz Glass ................................................................................. 7
Realization of the Fixed Point ...................................................................................... 7
Background Information ............................................................................................... 7
Procedure for Realizing the Freeze (In, Zn, Al, and Ag Fixed Points) ......................... 12
Realization of the Freezing Point of Tin (Sn) ........................................................... 14
SPRT Care At High Temperatures ............................................................................ 14
The Correction for the Pressure Difference .................................................................. 15
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List of Tables
Table Title
Table 1. The Dening Metal Freezing Points of the Resistance Ratios .......................................... 1
Table 2. Subranges of the ITS-90 and Freezing Points Required for Calibration ........................... 2
Table 3. International Electrical Symbols ....................................................................................... 3
Table 4. The Specication of Metal Freezing Point Cells. .............................................................. 5
Table 5. Summary of the First Cryoscopic Constants and the Estimated Effects of Impurities ..... 8
Table 6. The Furnaces for Fixed Points and their Temperature Uniformity ................................... 12
Table 7. Coefcients for the Pressure Difference of Some Dening Fixed Points ......................... 15
Page
List of Figures
Figure Title Page
Figure 1. The Metal Freezing Point Cell .......................................................................................... 2
Figure 2. The Fluke Sealed Metal Freezing Point Cell. .................................................................... 6
Figure 3. Freezing Curve Comparison of One Cell .......................................................................... 8
Figure 4. Two Liquid-solid Interfaces in the Cell ............................................................................. 9
Figure 5. 9114 Furnace Interior with Freeze Point Cell, Cross Sectional View ............................... 10
Figure 6. 9115A/9116A Furnace Interior with Freeze Point Cell, Cross Sectional View ................. 11
Figure 7. The Metal Freezing Point Cell in the Cell Containment Vessel (Basket) ......................... 13
Figure 8. A Typical Freezing Curve for the Zinc Cell ...................................................................... 14
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Page 7
Introduction
The International Temperature Scale of 1990 (ITS-90) is based on a series of dening xed
points. At temperatures above 273.16 K, most of the xed points are the freezing points of
specied pure metals. Pure metals melt and freeze at a unique temperature through a process
involving the absorption or liberation of the latent heat of fusion. A metal freezing point is the
phase equilibrium between the liquid phase and solid phase of a pure metal at a pressure of one
standard atmospheric pressure (101.325 kPa). The freezing points of indium, tin, zinc, aluminum,
silver, gold, and copper are the dening xed points of the ITS-90. The temperature values of
these freezing points assigned by the ITS-90, the pressure effect constants and the resistance
ratios of the ITS-90 reference function (10a) are listed in Table 1.
Fixed Point T
FP In 429.7485 156.5985 4.9 3.3 1.60980185 3.801024
FP Sn 505.078 231.928 3.3 2.2 1.89279768 3.712721
FP Zn 692.677 419.527 4.3 2.7 2.56891730 3.495367
FP Al 933.473 660.323 7.0 1.6 3.37600860 3.204971
FP Ag 1234.93 961.78 6.0 5.4 4.28642053 2.840862
FP Au 1337.33 1064.18 6.1 10 -- --
FP Cu 1357.77 1084.62 3.3 2.6 -- --
[1] Equivalent to millikelvins per standard atmosphere.
Table 1. The Dening Metal Freezing Points of the Resistance Ratios
Assigned Temperature
(K) t90 (°C)
90
Pressure Effect of Fixed
(10-8 K/Pa)
dt/dP
Points
[1]
dt/dh
(10-3 K/m)
Wr (T90)
dWr/dt
( x 0.001)
All of these xed points are intrinsic temperature standards according to the denition of the
ITS-90. Under controlled conditions these freezing points are highly reproducible. The variance
among different realizations of a freezing point should be well within 1.0 mK for the freezing
points of indium, tin and zinc; and within a few millikelven for the freezing points of aluminum,
silver, gold, and copper. For your convenience, Fluke has developed a sealed cell design and new
technique for the realization of the freezing points, which has made it easy to realize these xed
points.
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gpr001.bmp
Figure 1. The Metal Freezing Point Cell
These freezing points are indispensable for the calibration of a standard platinum resistance
thermometer (SPRT). Different sub-ranges require different sets of freezing points, as summarized
in Table 2.
Table 2. Subranges of the ITS-90 and Freezing Points Required for Calibration
Subrange Freezing Points Required
0 to 961.78 °C FP Sn, FP Zn, FP Al, and FP Ag
0 to 660.323 °C FP Sn, FP Zn, and FP Al
0 to 419.527 °C FP Sn and FP Zn
0 to 231.928 °C FP In and FP Sn
0 to 156.5985 °C FP In
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Before You Start
Symbols Used
Table 3 lists the International Electrical Symbols. Some or all of these symbols may be used on
the instrument or in this manual.
Symbol Description Symbol Description
Metal Freeze Point Cell
Before You Start
Table 3. International Electrical Symbols
X
:
W
B
D
F
T
.
CAT II
Electric Shock
Hot Surface (Burn Hazard)
Read the User’s Manual (Important
Information)
AC (Alternating Current)
AC-DC
DC
Double Insulated
PE Ground
CAT II equipment is designed to protect against transients from energy-consuming
equipment supplied from the xed installation, such as TVs, PCs, portable tools, and
other household appliances.
O
I
M
;
)
P
Off
On
I
Fuse
Battery
C-TICK Australian EMC Mark
Canadian Standards Association
CE Complies with European Union
Directives
The European Waste Electrical
and Electronic Equipment (WEEE)
Directive (2002/96/EC) Mark.
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Users Guide
Safety Information
Use this instrument only as specied in this manual. Otherwise, the protection provided by the
instrument may be impaired.
The following denitions apply to the terms “Warning” and “Caution”.
• “Warning” identies conditions and actions that may pose hazards to the user.
• “Caution” identies conditions and actions that may damage the instrument being used.
To avoid personal injury, follow these guidelines:
• DO NOT use this instrument for any application other than calibration work.
• DO NOT use this instrument in environments other than those listed in the
Users Guide.
• Follow all safety guidelines listed in the Users Guide.
• Avoid leaving a PRT installed for an extended period of time which can cause
the PRT handle to become hot.
• Calibration Equipment should only be used by trained personnel.
• Use the Product only as specied, or the protection supplied by the Product
can be compromised.
• Do not use and disable the Product if it is damaged.
• Use this Product indoors only.
• Have an approved technician repair the Product.
Warning W
To avoid possible damage to the instrument, follow these guidelines:
• Keep the cell clean and avoid contact with bare hands, tap water,
or contaminated PRTs. If there is any chance that the cell has been
contaminated, clean the quartz with reagent grade alcohol before
inserting it into a furnace.
• Use the product in the vertical orientation only.
How to Contact Fluke
To contact Fluke, call one of the following telephone numbers:
• Technical Support USA: 1-877-355-3225
• Calibration/Repair USA: 1-877-355-3225
• Canada: 1-800-36-FLUKE (1-800-363-5853)
• Europe: +31-40-2675-200
• Japan: +81-3-6714-3114
• Singapore: +65-6799-5566
• China: +86-400-810-3435
• Brazil: +55-11-3759-7600
• Anywhere in the world: +1-425-446-6110
To see product information and download the latest manual supplements, visit Fluke Calibration’s
website at www.ukecal.com.
To register your product, visit http://ukecal.com/register-product.
Caution W
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Metal Freeze Point Cell
Specications
Specications
Table 4. The Specication of Metal Freezing Point Cells
Model Number 5904 5905 5906 5907 5908
Fixed Point FP In FP Sn FP Zn FP Al FP Ag FP Au FP Cu
Reproducibility
Expanded
Uncertainty, k = 2
Metal Purity 99.99995 % 99.99995 % 99.9999 % 99.9999 % 99.9999 % 99.9999 % 99.9999 %
Quantity of Metal 0.97 kg 0.96 kg 0.95 kg 0.35 kg 1.35 kg 1.13 kg
Outer Diameter
of the Cell
Overall Height of
the Cell
Inner Diameter of
the Well
Total Immersion
[1]
Depth
[1] The distance from the bottom of the re-entrant well to the upper surface of the metal
(0.15 to 0.3) mK(0.2 to 0.4) mK(0.2 to 0.4) mK(0.6 to 1.0)
mK
0.7 mK 0.5 mK 0.9 mK 1.3 mK 2.4 mK 10.1 mK
48 mm 48 mm 48 mm 48 mm 48 mm 48 mm 48 mm
282 mm 282 mm 282 mm 282 mm 282 mm 282 mm 282 mm
8 mm 8 mm 8 mm 8 mm 8 mm 8 mm 8 mm
195 mm 195 mm 195 mm 195 mm 195 mm 195 mm 195 mm
(1.0 to 2.0)
mK
Contact
Fluke
5909
(2.0 to 4.0)
mK
Description
A typical Fluke Metal Freezing Point Cell is shown in Figure 2. An appropriate quantity of
metal (see Table 4 for detail) with a purity of 99.9999 % is melted into a graphite crucible with
a graphite lid and re-entrant well. Industry sometimes refers to the 99.9999 % purity as “a purity
of 6N”. The impurity in the graphite is less than 3 ppm. All of the graphite parts are subjected to
a high-temperature, high-vacuum treatment before loading the metal sample. It is important to
avoid any possible contamination to the surface of the graphite parts during the manufacturing
process.
The assembled graphite crucible, with the high-purity metal, is then enclosed in a quartz cell and
connected to a high vacuum system. The cell is drawn down to a proper pressure at a temperature
near the freezing point for several days. During this period the cell is purged with high purity
argon repeatedly to remove any contaminants. Finally, the cell is lled with 99.999 % pure argon
and permanently sealed at the freezing point. The pressure of the argon in the cell at the freezing
point is closely adjusted to 101.325 kPa and the actual value of the pressure recorded. A small
temperature correction for the pressure difference can be made using the information in “The
Correction for the Pressure Difference” section.
In providing the highest quality sealed cells on the market, Fluke’s experts carefully eliminate
possible sources of error. For example, sand-blasting the outer surface of the central re-entrant
quartz well of the sealed cell decreases the radiation losses along the well to a minimum. A long
immersion depth of the thermometer into the liquid metal makes any error due to the thermal
conductivity along the thermometer sheath and leads negligible.
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Argon
Shell
Graphite Crucible
High Purity
Metal Sample
Reentrant Tube
Figure 2. The Fluke Sealed Metal Freezing Point Cell
Care of Your Metal Freezing Point Cell
General Information
The metal freezing point cell is an extremely delicate device. Great care must be taken in
handling, using and transporting the cell. The quartz glass outer shell is easily broken. It is
suggested that the cell be kept in the vertical position for safety, although putting a cool cell in the
horizontal orientation for a short time period will not cause any damage. To prevent damage to
the cell, do not mail or ship the cell with a general freight carrier. The cell should be hand-carried
from place to place. It is extremely important to keep the outer surface of the cell clean to avoid
devitrication of the quartz glass. Never touch the cell with bare hands. Whenever you have to
handle the cell, always wear clean cotton gloves or use clean paper. If there is any chance that the
outside of the cell has been touched with bare hands, clean the quartz glass with alcohol before
inserting it into a furnace.
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Devitrication of Quartz Glass
Devitrication is a natural process with quartz glass. The quartz glass is utilized in a glass
state. The most stable state for quartz is crystalline. Therefore, devitrication is the tendency
of the quartz to return to its most stable state. If the quartz is kept extremely clean and free of
contamination, devitrication will occur only at high temperatures. The process occurs more
rapidly at lower temperatures when the glass has become contaminated by alkaline metals (Na, K,
Mg, and Ca). The alkalis found in normal tap water can cause the process to start.
Removal of the devitrication is not practical as it requires drastic measures and is potentially
dangerous to the instrument and/or the user.
Devitrication starts with a dulling or opacity of the quartz. It develops into a rough and
crumbling surface. Devitrication ultimately weakens the glass/quartz until it breaks or is
otherwise no longer useful.
The best cure for contamination and devitrication is prevention. Being aware of the causes and
signs of contamination can help the user take the steps necessary to control contamination of the
cell. Keep your cell clean and avoid contact with bare hands, tap water, or contaminated SPRTs.
Realization of the Fixed Point
As was mentioned in the Introduction, it is not difcult to realize a freezing point by using
a Fluke sealed metal xed point cell. In order to get the highest possible accuracy, a general
understanding of the freezing process of an ideal pure metal is helpful.
Metal Freeze Point Cell
Realization of the Fixed Point
Background Information
Theoretically the melting and freezing temperatures for an ideal pure metal are identical.
However, with the introduction of impurities in the metal, the melting and freezing equilibrium
points are usually slightly lower. The freezing plateau of an ideal pure metal is conceptually at.
The only exception is during the supercool. Impurities in the metal generally introduce a slightly
negative slope to the plateau. Most of the different types of impurities will cause a drop in the
freezing plateau. For example, gallium impurities in tin will cause a drop in the freezing plateau.
A few of the types of impurities can cause an increase in the plateau. For example, gold impurities
in silver will cause the freezing plateau to increase. An extremely high purity metal, 99.9999 %
or higher, behaves very closely to an ideal pure metal. Figure 3 shows the difference between a
freeze of an ideal pure metal and a high-purity metal. The approximate effect of the impurity on
the equilibrium point can be calculated using the rst cryoscopic constant. This calculation is
discussed in the Guidelines for Realizing the International Temperature Scale of 1990 (ITS-90).
For general uncertainty comparisons, the rst cryoscopic constant, the metal purity requirement,
and the difference in the liquidus point are outlined in Table 5. In a modern temperature standard
laboratory using a SPRT, a temperature change as low as 0.01 mK (0.00001 °C) can be detected.
Therefore, the best technique for realizing the freezing point with a real sample is one that
measures a temperature nearest to the freezing point of the ideal pure metal. The beginning of the
freezing curve of a high purity metal is the closest temperature to the ideal freezing point which
can be obtained in a modern temperature standard laboratory. A slow induced freezing technique
was found to t the purpose best (details of the technique are described in the section Procedure
for Realizing the Freeze). A very slow freeze allows enough time to calibrate a number of SPRTs
in the beginning part of a single freeze.
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Temperature (° C )
Users Guide
Table 5. Summary of the First Cryoscopic Constants and the Estimated Effects of Impurities
Substance
Indium 0.00732/K 99.99999% -0.01 mK
Tin 0.00329/K 99.9999% -0.3 mK
Zinc 0.00185/K 99.9999% -0.5 mK
Aluminum 0.00149/K 99.9999% -0.7 mK
Silver 0.000891/K 99.9999% -1.1 mK
Gold 0.000831/K 99.9999% -1.2 mK
Copper 0.000857/K 99.9999% -1.2 mK
660.38
660.36
660.34
660.32
660.30
1st Cryoscopic
Constant
Impurity Level
Deviation from Pure
Liquidus Point
660.28
660.26
660.24
660.22
0246 8101214161820
Time (hours)
Theoretical freezing curveofanideal pure metalwithout supercool
Freezing curve of an ideal pure metalwithsupercool
Freezing curve of a realhigh-purity metal
gpr003.eps
Figure 3. Freezing Curve Comparison of One Cell
The induced technique generates two liquid-solid interfaces in the cell. A continuous liquid-solid
interface that, as nearly as is practical, encloses the sensor of the SPRT being calibrated. Another
liquid-solid interface is formed on the wall of the graphite crucible. In such a situation, the outer
interface advances slowly as the liquid continues to solidify. Ideally this generates a shell that
continues to be of uniform thickness completely surrounding the liquid, which itself surrounds the
inner liquid-solid interface that is adjacent to the thermometer well (Figure 4). The inner interface
is essentially static except when a specic heat-extraction process takes place. For example,
the insertion of a cool replacement thermometer. It is the temperature of the inner liquid-solid
interface that is measured by the thermometer. Sometimes the inner liquid-solid interface is called
the dening temperature interface.
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Shell
eF
Graphite Crucible
Liquid Sample
Metal Freeze Point Cell
Background Information
Melting Stat
Solid Sample
Reentrant Tube
reezing State
gpr004.eps
Figure 4. Two Liquid-solid Interfaces in the Cell
It is extremely important for the process described here that there is a uniform, stable and
controlled temperature environment enclosing the xed-point cell. Fluke has developed several
designs of xed-point furnaces to satisfy these requirements. The Model 9114 furnace has three
independent heaters and controllers designed to be used for a temperature range up to 680 °C as
shown in Figure 5.
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SPRT Equilibration
Bloc k
Top Zone
Heater
Top Zone
Controller
RM
OP1
A/M
PAR
OP2
SPRT
Thermal Guard Assembly
PRT Sensor
Main Controller
HART
SCIENTIFI C
SETDOWNUPEXIT
TOP ZONE
Main Heater
Dierential TCs
Dierential TCs
Bottom Zone
Controller
RM
A/M
OP1
OP2
Bottom Zone
Heater
Freeze-Point
Cell
PRIMARY ZONE
Cell Support
Container
Ther mal Block
(3 zone subdivision)
PAR
BOTTOM ZONE
Water Cooling
Coils
10
Supercooling
Gas Supply
(Argon)
gpr005.eps
Figure 5. 9114 Furnace Interior with Freeze Point Cell, Cross Sectional View
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Metal Freeze Point Cell
Background Information
Top Support
Block
Retaining Plate
SPRT
Thermal Guard
Assemb
ly
Top Cover
Cutout Thermocouple
Cooling Coils
Thermal Shunt Disk
Heating Element
Control and Cutout
Thermocouple
Metal Freeze-Point Cell
Basket and Cover
Metal Freeze-Point Cell
Heat Pipe
Ceramic Fiber
Insulation
(Circulating Air)
Bottom Support
Figure 6. 9115A/9116A Furnace Interior with Freeze Point Cell, Cross Sectional View
Air Gap
Block
gpr006.eps
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The Model 9115A furnace with a sodium-in-Inconel heat pipe is designed for a temperature range
from 500 °C to 1000 °C. Although the 9115A furnace can be used up to 1100 °C, the longevity of
the heat pipe may be curtailed if used above 1000 °C for an extended period of time. The Model
9116A furnace (Figure 6) is designed for extended use above 1000 °C, or more specically, the
freezing point of copper (1084.62 °C). The furnaces and their temperature uniformities are listed
in Table 6.
Table 6. The Furnaces for Fixed Points and their Temperature Uniformity
Fixed Point The Equipment Used Temperature Uniformity
The freezing point of indium Model 9114 furnace, three zones ±0.02 °C
The freezing point of tin Model 9114 furnace, three zones ±0.02 °C
The freezing point of zinc Model 9114 furnace, three zones ±0.02 °C
The freezing point of aluminum Model 9114 furnace, three zones ±0.03 °C
The freezing point of aluminum Model 9115A furnace, heat pipe ±0.03 °C
The freezing point of silver Model 9115A furnace, heat pipe ±0.1 °C
The freezing point of copper Model 9116A furnace, heat pipe ±0.05 °C
The cell should be placed into the cell containment vessel before insertion into any furnace.
Ideally each cell would be kept in its own unique vessel. The cell containment vessel (basket)
for the Model 9114 furnace is shown in Figure 7. A fused silica glass (quartz) basket is used to
support and enclose the freezing point cell for the Model 9115A/9116A. Fiber ceramic insulation
is placed in the bottom of the cell basket to protect the cell. Insulation is also placed on top of the
cell for protection and to reduce heat loss.
Procedure for Realizing the Freeze (In, Zn, Al, and Ag Fixed Points)
This is the procedure used in the Fluke metrology lab with the Fluke Sealed Fixed Point Cells.
Other procedures are sometimes employed in industry.
All of the freezing points except tin are realized in a similar way.
1. Insert the cell with the cell containment vessel carefully into the furnace.
2. Set the temperature of the furnace about 10 °C higher than the freezing point. Allow all of the
metal to melt completely.
3. After all metal is completely melted, the furnace is set at a stable temperature 1 °C or 1.5 °C
higher than the freezing point over night.
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Metal Freeze Point Cell
Procedure for Realizing the Freeze (In, Zn, Al, and Ag Fixed Points)
Cap
Fiber Ceramic Insulation
Cell Containment Vessel or “Basket”
Fiber Ceramic Paper to Center Cell in Basket
Metal Freeze Point Cell
(cell interior construction shown)
Fiber Ceramic Cushion
gpr008.eps
Figure 7. The Metal Freezing Point Cell in the Cell Containment Vessel (Basket)
4. The next morning, the furnace temperature is decreased slowly (0.1 °C to 0.15 °C). In order
to monitor the metal sample temperature, a SPRT is inserted into the cell. The temperature of
the metal sample decreases to less than the freezing point before recalescence. The amounts of
supercool are different from metal-to-metal.
5. After recalescence the thermometer is removed from the furnace immediately and two cold
(room temperature) quartz glass rods are inserted into the xed point cell one by one, each for
about ve minutes.
6. The preheated SPRT to be calibrated is introduced into the cell, while the furnace is kept at a
stable temperature of 0.5 °C below the freezing point.
This procedure provides a very stable, long freezing plateau that typically lasts for more
than ten hours. The changes in temperature in the rst half of the plateau are usually within
±0.2 to 0.3 mK. A typical freezing curve is shown in Figure 9.
A number of SPRTs can be calibrated in a single freezing plateau. When multiple SPRTs are to be
calibrated from a single freeze, we suggest that the SPRTs be preheated to a temperature slightly
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Users Guide
higher than the freezing point before inserting the SPRT into the furnace. As was mentioned
earlier, the cold quartz glass rods inserted into the cell will generate a liquid-solid interface
adjacent to the thermometer well (see Figure 4).
0.6552574
0.6552569
0.6552564
0.6552559
0.6552554
0.6552549
0.6552544
0.6552539
F18 Bridge reading (Rt/Rs)
0.6552534
0.6552529
10/8
10:00
10/8
13:00
10/8
16:00
10/8
19:00
10/8
22:00
10/9
1:00
Date and time
10/9
4:00
10/9
7:00
0.5mK
10/9
10:00
10/9
13:00
Figure 8. A Typical Freezing Curve for the Zinc Cell
Realization of the Freezing Point of Tin (Sn)
Since tin requires a 25 °C or more drop in temperature to achieve supercool, nucleation is
achieved by additional cooling supplied by a cold gas ow. The procedure for the freezing point
of tin is similar to that of the other xed points, except the need to compensate for the large
temperature difference required for supercool.
Follow Steps 1 through 4 in the “Procedure for Realizing the Freeze (In, Zn, Al, and Ag Fixed
Points)”.
When the temperature indicated by a thermometer immersed in the tin sample reaches the freezing
point, using the Model 9114 furnace introduce a cold gas ow upward around the outer surface of
the cell until recalescence. “Cold gas ow” means compressed air at an approximate rate of 5 to
20 liter/min. (0.2 to 0.7 CFM) and roughly 200 kPa (29 psia). After recalescence, shut off the cold
gas ow. The furnace is kept at a stable temperature of 0.5 °C below the freezing point as with
other metals. The Model 9114 furnace as shown in Figure 5 has a specially designed core for the
realization of the freezing point of tin.
SPRT Care At High Temperatures
Each SPRT calibrated at temperatures above 500 °C is subjected to quenched-in vacancy defect
effect when the SPRT is removed from the furnace. This quenched in lattice vacancy defect
effect must be removed before calibration at the triple point of water. Therefore, when the SPRT
is removed from the cell, place it in an auxiliary furnace set at the same temperature as the xed
point. Slowly cool the SPRT at a rate of roughly 100 °C/hour above 500 °C. Once the SPRT has
reached 500 °C, it may be removed directly to room temperature.
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The Correction for the Pressure Difference
This is the procedure used in the Fluke lab with the Fluke Sealed Fixed-point Cells. Other
procedures are sometimes employed in industry.
Except for a few triple points, the values of temperature assigned to the dening xed points by
ITS-90 correspond to the temperatures at the standard atmospheric pressure: 101.325 kPa. The
actual pressure in a cell may be not exactly the standard value. During the course of manufacture
of a xed-point cell, it is easier to seal the cell if the pressure in the cell is a slightly lower than
the room pressure. The actual pressure in the cell exactly at the xed point was measured at
Fluke. This actual argon pressure in the cell at the freezing point is provided on the Report of
Test, or Certication, enabling calculation of correction for the difference in pressure. During
measurement at a xed point, the sensor of a SPRT is usually placed at a height which is “h”
meters lower than the upper surface of the pure metal and where the pressure is higher than that at
the surface due to the static head. ITS-90 gives all of the necessary coefcients for the calculation
of the correction caused by the pressure difference, which are summarized in Table 7.
Table 7. Coefcients for the Pressure Difference of Some Dening Fixed Points
Assigned Value
Substance
Argon (T) 83.8058 25 3.3 0.004342
Mercury (T) 234.3156 5.4 7.1 0.004037
Water (T) 273.16 –7.5 –0.73 0.003989
Gallium (M) 302.9146 –2.0 –1.2 0.003952
rin0Indium (F) 429.7485 4.9 3.3 0.003801
Tin (F) 505.078 3.3 2.2 0.003713
Zinc (F) 692.677 4.3 2.7 0.003495
Aluminum (F) 933.473 7.0 1.6 0.003205
Silver (F) 1234.93 6.0 5.4 0.002841
Gold (F) 1337.33 6.1 10 —
Copper (F) 1357.77 3.3 2.6 —
(T) – Triple Point
(M) – Melting Point
(F) – Freezing Point
of Equilibrium
Temperature
T Kelvin (K)
Temperature
with Pressure,
p K1; dT/dp
-5
(10
mK/Pa)
Metal Freeze Point Cell
The Correction for the Pressure Difference
Variation with
Depth K2 : dT/dh
(mK/m)
Approximate
dW/dt
The correction of temperature caused by the difference in pressure can be calculated by using the
following equation:
Equation 1: Pressure Dependent Temperature Correction
Δt = (P − P
) × k1 + h × k
0
2
P: the actual pressure of argon in the cell exactly at the xed point temperature
P
: the standard atmospheric pressure. For example, 101.325 kPa
0
dT
k1 =
dP
dT
k2 =
dh
h: the immersion depth of the midpoint of the sensor of a SPRT into the metal used for the xed
point
The immersion depth of the midpoint of a SPRT sensor in Fluke metal freezing point cell is
approximately 0.17 m (the distance from the bottom of the central well to the surface of liquid
metal is about 0.195 m). The actual pressure of the argon at the freezing point in the cell,
15
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5904/5905/5906/5907/5908/5909
Users Guide
p, is provided in the Report of Test. The temperature correction, Δt, can be calculated using
Equation 1.
Example:
The pressure of argon at the freezing point in the aluminum freezing point cell S/N 5907-5AL004
is 80,817 Pa as given in the Report of Test. k1 and k2 for the freezing point of aluminum can be
found in Table 5, k1 = 7.0 * 10
is 0.17 m for most of standard platinum resistance thermometers. Therefore, use Equation 1 to
calculate Δt.
Substituting values into Equation 1:
(80,817Pa – 101,325 Pa)
Consequently:
Δt = −1.164 mK
Hence, the actual temperature of a sensor of a SPRT at the point of total immersion during a
freezing plateau in the cell is calculated using Equation 2.
Equation 2: Calculation of the Actual Temperature, t
t1 = t + Δt
Therefore:
–5
7.0 x 10
Pa
mK / Pa and k2 = 1.6 mK / m. The average immersion depth
-5
mK
+ 0.17 m
1.6 mK
m
1
= –1.44 mK + 0.27 mK
t
= 660.323 °C − 0.00117 °C = 660.32184 °C
1
where t is the dening xed point temperature. For example, 660.323 °C for the freezing point of
aluminum.
The resistance ratio, W
, for the particular cell exactly at the freezing point of aluminum can be
Al
calculated using the following equation. The value for dW/dt is taken from Table 7.
Equation : Calculation of W
W
Al
= W(
t
) − [Δt]
1
dW
dt
for the exact dening xed point temperature.
Al
Substituting values:
3.37600860 – (–0.001164) x (3.204971 x 10
Thus the W
W
= 3.37601233
Al
for the cell is:
Al
-3
)
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