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National Institute of Standards and Technology

NIST Technical Note 1411

Reproducibility of the Temperature of the

Ice Point in Routine Measurements

B. W. Mangum

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NIST Technical Note 1411

Reproducibility of the Temperature of the

Ice Point in Routine Measurements

B. W. Mangum

Thermometry Group

Process Measurements Division

National Institute of Standards and Technology

Gaithersburg, MD 20899-0001

June 1995

•^TES O* *

U.S. Department of Commerce

Ronald H. Brown, Secretary

Technology Administration

Mary L. Good, Under Secretary for Technology

National Institute of Standards and Technology

Arati Prabhakar, Director
National Institute of Standards U.S. Government Printing Office For sale by the Superintendent of and Technology Washington: 1995 Documents

Technical Note 1411 U.S. Government Printing Office

Natl. Inst. Stand. Technol. Washington, DC 20402

Tech. Note 1411

24 pages (June 1995)

CODEN: NTNOEF
ACKNOWLEDGMENTS

Acknowledgments are due to those persons who worked in the Liquid-in-Glass Calibration

Laboratory over the years during which most of the data reported here were obtained. Of the eight

persons who participated in taking the data presented here, I am acquainted with only one, J. A. Wise.

Also, Miss Wise was the one who obtained the 1992 data. The more recent data were obtained by

the author.

in

Table of Contents

page

Abstract viii Disclaimer viii Introduction 1 Experimental Details 3

Results and Discussions 6

References 16
List of Tables

page

Table 1. Types of measurements, dates of measurements and instruments

used to obtain data on ice-point baths 14

Table 2. Summary of results from the experiments reported in this paper 15

VI
List of Figures

page

Figure 1. Plot of the SPRT resistance values at the temperatures of the ice

points measured from June 20, 1963 through April 14, 1969

Figure 2. Plot of the SPRT resistance values at the temperatures of the ice

points measured from June 2, 1969 through November 20, 1970 8

Figure 3. Plot of the temperature values obtained in 1992 for ice points 11

VI!
Abstract

This note presents results on the reproducibility and temperature of the ice point of water. The data were obtained over the years 1963 to 1970, in 1992, and in 1995. The data from 1963 to 1970 are

from resistance measurements of a standard platinum resistance thermometer in ice points, made for

the purpose of obtaining the reference value for the thermometer to be used for determining

temperatures of calibration of liquid-in-glass thermometers. In 1992 and in 1995, measurements of

the temperature of the ice point were made, using water (and ice) from different sources. The value

of the temperature determined for the ice point prepared from our highest-purity water is in excellent

agreement with that determined previously. Also, the reproducibility of routine measurements at the

ice point are consistent with what has been generally accepted, but heretofore undocumented.

Key Words

Fixed points, ice point, platinum resistance thermometers, SPRTs, thermometers, thermometry, triple

point, triple point of water.

Disclaimer

Certain commercial equipment, instruments, or materials are identified in this paper in order to

adequately specify the experimental procedure. Such identification does not imply recommendation

or endorsement by the National Institute of Standards and Technology, nor does it imply that the

materials or equipment identified are necessarily the best available for the purpose.

vni
Reproducibility of the Temperature of the Ice Point in

Routine Measurements

B. W. Mangum

Thermometry Group

Process Measurements Division

Gaithersburg, MD 20899

Introduction

Prior to the International Practical Temperature Scale of 1948, amended edition of 1960 [1] (a

textual revision in 1960 of the International Temperature Scale of 1948 [2]), the ice-point was a

defining fixed point of the international temperature scales and was the reference point for standard

platinum resistance thermometers (SPRTs). In the International Temperature Scale of 1927 (ITS-27)

[3], the ice point was defined as the "Temperature of equilibrium between ice and air-saturated water

at normal atmospheric pressure." For the IPTS-48 and later scales (the International Practical

Temperature Scale of 1968 [4] and its revision of 1975 [5], and the International Temperature Scale

of 1990 (ITS-90) [6]), the triple point of water, rather than the ice point of water, was chosen as the

defining fixed point. (The ITS-90 [6] is the first international temperature scale that specifies the

triple-point temperature of water as the reference point for SPRTs.) Nevertheless, the ice point still

is an important reference point for many measurements in which the highest accuracy and precision

are not required. This is the situation for measurements involving the use of liquid-in-glass

thermometers, industrial platinum resistance thermometers, thermocouple thermometers, some

thermistor thermometers, and even SPRTs in some applications. For temperatures below 550 °C,

those types of thermometers are calibrated through the use of ice baths and temperature-controlled

liquid baths. The bath temperatures are determined with a calibrated SPRT. For temperatures above

550 °C, thermocouple thermometers are calibrated either by comparing their electromotive force to
that of reference thermocouples at various temperatures or by measuring their electromotive force

at specified fixed points. Calibration of liquid-in-glass thermometers at the ice-point (using properly

prepared ice baths) is particularly important since, if in addition to its calibration at other

temperatures, the thermometer is calibrated at the ice point, the thermometer need never be returned

to NIST, or any other calibration laboratory, for recalibration. The user can perform the entire

recalibration by simply checking the thermometer at the ice point and then using that value to make

the appropriate correction over the entire calibration range [7].

A properly prepared ice bath is one that consists of a cylindrical Dewar flask (a typical size is one that

is approximately 8 cm in diameter and 36 cm deep), a siphon tube, shaved ice prepared from distilled

water, and distilled water that just fills the voids between the ice particles but does not float the ice. A Dewar flask is the preferred type of container for the ice bath since its insulating properties retard

the melting of the ice. The siphon tube should extend to the bottom of the Dewar for ease in

removing the excess water produced by the melting of the ice. The shaved ice should have the

consistency of snow. If a shaver is not available, the ice may be crushed such that the ice particles

are no larger than 2 or 3 mm in size. In preparing the ice bath, the Dewar flask should be filled to

approximately one-third of its capacity with air-saturated distilled water and then the shaved or

crushed ice should be added until the flask is full. Then the mixture of water and ice should be

compressed until it is packed tightly. Any excess water is removed by the siphon. The bath should

stand for approximately 30 min in order to permit the ice particles and the water to equilibrate before

the bath is used. With use, the ice of the bath will melt, producing excess water. That water should

be removed and the ice replenished so that the Dewar flask is maintained full. Since contamination

of the ice and/or the water will affect the melting-point temperature, it is essential that the necessary

care be taken to prevent contamination. The flask and all other components used with the bath must

be washed thoroughly with distilled water before being placed in the bath. Gloves should be worn

when handling the ice and other items that will be placed in the bath. An adequate support must be

provided above the Dewar flask for thermometers that will be measured in the ice bath. For

photographs of working ice baths and for additional details on their use, see references [8], [9] and

[10].
Although the ice point has been used as a reference temperature for decades and has been investigated

as to its value on a thermodynamic scale [11] and as to its reproducibility when prepared with great

care [12], there are no published data on its reproducibility in routine laboratory measurements. It

has been stated [13] (also, see comments in [12] and [14]), but without any supporting data, that the

temperature of an ice point can be realized with an uncertainty of about 0.002 °C. With great care, Thomas [12] and White [14] reproduced the temperature of ice baths to 0.0001 °C. Unless

corrections for changes in air pressure from one standard atmosphere (and the effects of that on the

amount of carbon dioxide dissolved in the water) are taken into account, some variation in the

temperature of the ice bath is to be expected. One would not expect changes from this source of

irreproducibility to be greater than about 0.4 m°C, however.

At the National Institute of Standards and Technology (NIST), many routine measurements of the

resistance of SPRTs in ice baths have been made over many years in the Liquid-in-Glass Calibration

Laboratory as part of the calibration procedure of liquid-in-glass thermometers. Thus, a history of

the reproducibility of the temperature of the ice point has been obtained over many years during those

routine measurements. Some typical examples of those data will be presented in this paper. As will

be shown, the temperature of the ice point in most of those measurements was not determined; it was

the reference point for the temperature measurements made with the SPRT and it was assumed to

be °C to within the accuracy required. In later experiments, however, the temperatures of the ice

baths were determined and those results will be shown also.

Experimental Details

Most of the data reported here were obtained from measurements made over the period from June

20, 1963, through November 20, 1970. The ice, from which the ice baths were prepared for the

measurements during this period, was obtained from local commercial ice-making companies in 50-

pound or 100-pound blocks. Only ice from the clear parts of the blocks was used in preparing the

ice baths. The ice was shaved in an ice-shaving machine, producing ice with the consistency of snow.

Except for the blades, the ice-shaving machine was tin coated.
Some measurements to determine the reproducibility of the temperature of ice baths, as well as for

the determination of the value of the temperature of those baths, were made also in 1992. In that

case, however, the ice baths were prepared with ice made from water distilled in the laboratory with

a simple commercial water still. The water used to fill the voids between the ice particles of the ice

baths was from that same laboratory still. That still was not of a particularly high quality, as

evidenced by the results on the ice baths. Except for using a different source of ice, and possibly for

the distilled water as well, the baths were prepared in the same way as described above for the earlier

measurements. A different bridge, described below, was used for measuring the resistance of the

SPRT. The SPRT was measured at the triple point of water as the reference point to determine the

temperature of the ice bath. The triple point of water, rather than the ice point, has been used as the

reference point for SPRTs used in the calibration of the various types of customer thermometers

named above since November 16, 1971.

Some additional measurements of the temperature of the ice point were made in 1995. For the ice

baths for the first set of these measurements, the source of the distilled water used to fill the voids

between the ice particles, and that used for making the ice itself, was a new laboratory water still. The results indicate that the water produced by this still is more highly contaminated than that

produced by the still employed for the water and ice used for the ice baths investigated in 1992.

Since the temperature of the ice points prepared from water (and ice) from the new water still

indicated that the water from this source was not very pure, a set of measurements was made on some

ice points prepared from ultra-pure water obtained from P. A. Berezansky of the Analytical Chemistry

Division of NIST. That water had been distilled and de-ionized. In preparing ice points with this

ultra-pure water, care was taken to ensure that all parts of the ice-point system were clean and had

been rinsed with the ultra-pure water. The parts were washed thoroughly, rinsed at least ten times

with tap water, then rinsed ten times with distilled water, and then rinsed ten times with the ultra-pure

water. The bath was prepared and its temperature measured, referenced to the triple point of water.

Results presented later in the results section show that this water had the highest purity of any used

in these experiments.
In a separate experiment in 1995, the temperature of an ice point prepared from tap water was

measured. The purity of the tap water is probably similar to that available in most large cities, and

it contains a fairly high concentration of soluble salt.

The data from June 20, 1963, through November 20, 1970, were obtained from measurements with

a calibrated G2 Mueller bridge. That bridge continued to be used through June 24, 1975. With that

bridge, the smallest-resistance dial had steps of 0.0001 ohms. Thus, the resistance of the SPRT could

be measured directly to 0.0001 ohms and estimated to 0.000 01 ohms, i.e., estimated to the equivalent

of 0.0001 °C for the 25.5-ohm SPRT.

The data from June 25, 1975, through March 13, 1995 were obtained with a 400 Hz Cutkosky bridge

[15], which is capable of resolving 0.000 001 ohms, equivalent to 0.000 01 °C, when a current of

1 mA, or greater, flows through the SPRT. With the Cutkosky bridge, readings for the SPRT in the

ice point and in the triple point of water cell were repeatable to 10 u°C to 20 u°C.

Except for some of the measurements in 1995, the same SPRT (a Meyers' thermometer [16]) was

used to obtain the results presented here on the ice point. Prior to any measurement at the ice point,

sufficient time was allowed for the SPRT to reach equilibrium with the ice bath. This also applied

to measurements in the triple point of water cell in the experiments of 1992 and 1995 in which the

temperature of the ice point was measured with an SPRT. Of course, the SPRT was first cooled in

ice water before being placed in the triple point of water cell.

Unfortunately, the barometric pressure was not recorded (and presumably no correction made for

differences from one standard atmosphere) for the data from June 1963 through November 1970

reported here. Since the minimum uncertainty of liquid-in-glass thermometers is approximately

0.02 °C and since a change in atmospheric pressure of 1333 Pa (10 Torr) changes the melting point

of ice only by approximately 0.0001 °C, in the past it has not been deemed necessary to make

corrections for changes in the atmospheric pressure. Although the barometric pressure was not

recorded during measurements in 1992 either, records of the average daily barometric pressure during
that time were obtained from the Northeast Regional Climate Center and approximate corrections

were made to the data. For only five of the 50 datum points was the average daily barometric

pressure sufficiently different from one standard atmosphere to give a correction of as much as

0.1 m°C. None was as large as 0.2 m°C. The barometric pressure was recorded for the

measurements made in 1995 and the data were corrected to one standard atmosphere; for those

measurements, the corrections were negligible.

Although the barometric pressure was not recorded, and consequently no corrections made, for the

data obtained from June 20, 1963 through November 20, 1970, those measurements do represent

typical measurements at the ice-point. Thus, those data are probably more representative of the

reproducibility to be expected in routine measurements by the average user of ice-point baths than

if corrections had been made for the barometric pressure.

Results and Discussions

The data obtained over the period from June 20, 1963 through November 20, 1970 are divided into

two segments because the SPRT was annealed and recalibrated on April 16, 1969, the annealing

resulting in a decrease in the resistance of the SPRT at the ice point by the equivalent of about 7 m°C.

The data obtained over the period from June 20, 1963 through April 14, 1969 are shown in figure

1 and the data obtained from June 2, 1969 through November 20, 1970 are shown in figure 2.

The gradual drift to larger resistance values that is evident in these graphs (equivalent to about

15 m°C in figure 1) is most likely due to the drift of the SPRT. It is unlikely that changes in the

Mueller bridge could contribute to the drift since the bridge was calibrated annually. Thus, it is

thought that the drift reflected changes in the SPRT and that the changes were caused by handling

of the SPRT by the various persons performing the calibrations, of which there were eight over the

time period of these data, some of whom were undergoing training for work in the laboratory. Since

the changes in the resistance of the SPRT with time was rather large, it is obvious that the SPRT was

not handled carefully. The annealing at the time of calibration (April 16, 1969) removed strains
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compared to changes observed for SPRTs in industrial laboratory use.

From figures 1 and 2, one can see that, with the exception of a few outliers, the data in each figure

lie within a 2 m°C band along either side of an (imaginary) upward-sloping line (sloping because of

the drift in the resistance value of the SPRT due to handling). If each of the ice-point values had been

referenced to the temperature of the triple point of water, the effect of the drift of the SPRT could

have been removed and the scatter of the data of each figure would have been about a line of zero

slope.

There are four very obvious outliers in figure 1, at reading numbers 51, 135, 285 and 393. The last two of these measurements were made by the same person but the other two measurements were made by two other persons. It appears that the reason for these outliers is measurement error or error

in recording of data, most likely in recording the bridge balance, e.g., recording a value of the dial that

was k radians from the correct dial reading. It is highly unlikely that the ice could have been

contaminated to such an extent to cause such a large depression of the melting-point temperature.

One reason for this assumption is that the ice baths corresponding to the outliers would have been

prepared from the same block of ice for either the preceding and/or the following measurement and

there were no apparent problems with either of those measurements. This rules out contamination

of the ice, as received. Distilled water from the same laboratory source would have been used for

each of the hundreds of ice baths reported, so it too could not have been the source of a possibly

large amount of contamination. Carelessness in preparation of the ice baths is unlikely also, since the

persons preparing the ice baths would have had to contaminate the baths with approximately a mole

fraction of 5 x 10"5 impurity in order to have depressed the melting-point temperature by the amount

ofthe outliers. Also, it is unlikely that the changes in atmospheric pressure could account for more

than about 0.6 m°C depression; even that amount would have required an increase in barometric

pressure of about 8000 Pa (60 Torr), a very large change in barometric pressure.
Figure 3 represents the data obtained in 1992. These data also were obtained during routine

calibrations of liquid-in-glass thermometers. They were obtained in order to determine not only the

reproducibility of measurement of the ice-bath temperature, but also to determine the value of that

temperature. The water used to prepare the ice and to fill the voids between the particles of ice was

obtained from a small commercial laboratory water still. As indicated by the relatively depressed

readings, the water was not of particularly high purity. The average value of the 50 points is -

0.22 m°C and the sample standard deviation is 0. 1 1 m°C. The range of the data is from +0.02 m°C

to -0.54 m°C, with only three points being lower than -0.4 m°C and only one point being lower than

-0.5 m°C.

The scatter of the data shown in figure 3 is about an order of magnitude less than that of figures 1

and 2. Although the smallest dial reading of the Cutkosky bridge corresponds to 1 uQ (equivalent

to 0.01 m°C) and the smallest dial reading of the Mueller bridge corresponds to 100 uQ (equivalent

to 1 m°C), the number of digits of the bridge readings recorded (including interpolation between dial

reading ofthe Mueller bridge) differ only by a factor of 10. The reason for this is that the sensitivity

ofthe Cutkosky bridge does not permit interpolation to better than 1 uQ at a thermometer current

of 1 mA, unless the output of the bridge is connected to a strip-chart recorder. If a strip-chart

recorder had been used, then a factor of 100 could have been obtained. A strip-chart recorder was

not used for the measurements reported here, however. In any case, the scatter of the data of figures

1 and 2 cannot be attributed to bridge resolution.

On January 24, 1995 and January 27, 1995, two additional measurements of the temperature of ice

baths were made. Different SPRTs from different instrument companies were used on the different

days. One was designed for high temperature applications, using fused-silica glass for the coil and

lead supports. Both sets of measurements gave the same temperature for the ice baths, which was

-0.60 m°C. This is significantly lower than the average value of -0.22 m°C obtained from the

measurements in 1992. A new commercial water still for producing the distilled water used for the

ice and the water filling the voids between the ice particles of the ice baths was purchased and

installed in mid- 1994, however, and the water from that still represents the only known difference in

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the ice baths of 1995 from those used for the measurements made in 1992. Consequently, it is

suspected that the distilled water from the new still contains more soluble impurities than that

produced with the old still.

On March 3, 1995, the temperature of an ice point prepared using ice made from tap water and using

tap water to fill the voids between the ice particles was measured. The value was -15.8 m°C.

A determination ofthe temperature of an ice point made from the ultra-pure water obtained from P.

A. Berezansky of the Analytical Chemistry Division ofNIST was made on March 10, 1995 (a Friday).

It was thought that this water would be of the highest purity of any used in these experiments and,

consequently, yield the best value for the temperature of the ice point. After correcting for the

hydrostatic head and the difference of the barometric pressure from one standard atmosphere, the

value obtained for the ice point was +0.089 m°C. This value for the ice point is in good agreement

with that obtained by Thomas [12].

After completion of the measurements on the ice point prepared from the ultra-pure water, the Dewar

containing the ice bath was covered loosely with Saran Wrap to prevent the water therein from

becoming contaminated over the weekend. Also, more ice was prepared over the weekend from the

high-purity water. On March 13, 1995 (the following Monday), the experiment was repeated, except

that the water left in the Dewar over the weekend was used again for filling the voids between the

ice particles in the ice bath. The bridge readings for the SPRT in the ice point were repeatable to

15 u°C and those in the triple point of water cell were repeatable to 3 u°C. After correcting for the

hydrostatic head and the difference of the barometric pressure from one standard atmosphere, the

value obtained for the ice point was -0. 108 m°C. This was somewhat of a surprising result since the

only difference between the experiments of March 10 and March 13 was the use of the water that had

been standing in the Dewar over the weekend instead of using water from the original container to

fill the voids between the particles of ice. Since the Dewar is constructed of glass, it could not have

contaminated the water. Consequently, the contamination must have come from the atmosphere, and

specifically from the air entering the laboratory through the air ducts.

12
Table 1 summarizes the types of measurements, dates of measurements, bridge used, resolution, and

source of ice and water used in the experiments reported here. Table 2 summarizes the results from

the experiments.

Although the data obtained in 1992 and in 1995 were much more reproducible that those obtained

in the earlier measurements, the scatter of the data of the earlier measurements supports the

previously-unsubstantiated, but widely-held, view that the reproducibility of the ice-point temperature

of properly prepared ice baths is about 0.002 °C.

13
Table 1. Types of measurements, dates of measurements and instruments used to obtain data

on ice-point baths

SPRT

mfr/SN

Bridge Resolution Meas. type Source of ice for ice bathSource of ice water for ice bath

Date

Meyers/44

5

G2 Mueller 0.1 m°C SPRT

resistance,

(ref), reproducibil

ity

commercial

ice house

lab. still 6/20/63 to 4/14/69

Meyers/445 G2 Mueller 0.1 m°C SPRT

resistance,

(ref.), reproducibil

itv

commercial

ice house

lab. still 6/2/69 to 11/20/70

Meyers/445 Cutkosky,

400 Hz

0.01 m°C temp, of ice bath,

reproducibil

ity

from

distilled water from

lab. still

lab. still 8/19/92 to 11/19/92

Meyers/445 Cutkosky,

400 Hz

0.01 m°C temp, of ice bath,

reproducibil

ity

from

distilled water from

lab still

lab. still 1/24/95

Chino

RS92X-4

Cutkosky

400 Hz

0.01 m°C temp, of ice bath,

reproducibil

ity

from

distilled water from

lab. still

lab. still 1/27/95

Chino

RS87A-6

Cutkosky

400 Hz

0.01 m°C temp, of ice bathfrom water

from the tap

from the tap 3/03/95

Chino

RS87A-6

Cutkosky

400 Hz

0.01 m°C temp, of ice bathfrom

distilled &

de-ionized

water

distilled &

de-ionized

3/10/95

Chino

RS87A-6

Cutkosky

400 Hz

0.01 m°C temp, of ice bathfrom

distilled &

de-ionized

water

distilled &

de-ionized

3/13/95

14
Table 2. Summary of results from the experiments reported in this paper

Dates of

measurements

Measurement type Reproducibility

(expanded

uncertainty, k=2)

Value

6/20/63 to 4/14/69 resistance 2m°C

6/2/69 to 1 1/20/70 resistance 2m°C

8/19/92 to 11/19/92 temperature 0.23 m°C -0.22 m°C

1/24/95 to 1/27/95 temperature -0.60 m°C

3/03/95 temperature (tap

water)

-15.8 m°C

3/10/95 temperature (high

purity water)

+0.089 m°C

3/13/95 temperature (high

purity water, left standing for 3 days)

-0.108 m°C

15
References

[I] The International Practical Temperature Scale of 1948, amended edition of 1960, Comptes

Rendus des Seances de l'Onzieme Conference Generale des Poids et Mesures, 124-133.

[2] The International Temperature Scale of 1948, Comptes Rendus des Seances de la Neuvieme Conference Generale des Poids et Mesures, 89-100.

[3] The International Temperature Scale of 1927, Comptes Rendus des Seances de la Septieme

Conference Generale des Poids et Mesures, 94-99.

[4] The International Practical Temperature Scale of 1968, Comptes Rendus des Seances de la Treizieme Conference Generale des Poids et Mesures, A1-A24.

[5] The International Practical Temperature Scale of 1968, amended edition of 1975, Comptes

Rendus des Seances de la Quinzieme Conference Generale des Poids et Mesures, A1-A21;

Also, Preston-Thomas, H., The International Practical Temperature Scale of 1968,

Amended Edition of 1975, Metrologia 12, 7-17 (1976).

[6] Preston-Thomas, H., The International Temperature Scale of 1990 (ITS-90), Metrologia

27, 3-10 (1990); ibid. p. 107.

[7] Wise, J. A., A Procedure for the Effective Recalibration of Liquid-in-Glass Thermometers,

NIST Special Publication 819, U.S. Government Printing Office, Washington, DC 20402,

August 1991.

[8] Wise, J. A., Liquid-in-Glass Thermometry, NBS Monograph 150, U.S. Government

Printing Office, Washington, DC 20402, January 1976.

[9] Wise, J. A. and Soulen, R. J. Jr., Thermometer Calibration: A Model for State Calibration

Laboratories, NBS Monograph 174, U.S. Government Printing Office, Washington, DC

20402, January 1986.

[10] Busse, J., Liquid-in-Glass Thermometers, Temperature, Its Measurement and Control in

Science and Industry, Vol. 1, American Institute of Physics, (Reinhold Publishing

Corporation, New York, 1941), pp. 228-255.

[II] Beattie, J. A., The Thermodynamic Temperature of the Ice Point, Temperature, Its Measurement and Control in Science and Industry, Vol. 1, American Institute of Physics,

(Reinhold Publishing Corporation, New York, 1941), pp. 74-88.

16
[12] Thomas, J. L., Reproducibility of the Ice Point, Temperature, Its Measurement and Control

in Science and Industry, Vol. 1, American Institute of Physics, (Reinhold Publishing

Corporation, New York, 1941), pp. 159-161.

[13] Gray, G. N. and Chandon, H. C , Development of a Comparison Temperature Calibration

Capability, Temperature, Its Measurement and Control in Science and Industry, Vol. 4, H.

H. Plumb, Editor-in-Chief, (Instrument Society of America, Pittsburgh, 1972), pp. 1369-

1378.

[14] White, W. P., Freezing Points and Triple Points with Water, J. Am. Chem. Soc, 56, 20-24

(1934).

[15] Cutkosky, R. D., An ac Resistance Thermometer Bridge, J. Res. Natl. Bur. Stand. (U.S.)

74C, 15-18(1970).

[16] Mtyers, C. H., Coiled Filament Resistance Thermometers, Bur. Standards J. Research 9,

807-817 (1932) (RP 508).

17



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