NBSIR 74-586

Calibration of Platinum Resistance

Thermometers Using an

Intercomparison Scheme

R. M. Schoonover and H. H. Ku

Institute for Basic Standards

National Bureau of Standards

Washington, D. C. 20234

December 1974

Final

Prepared for Optical Physics Division and

Applied Mathematics Division

Institute for Basic Standards

Washington, D. C. 20234

NBSIR 74-586

CALIBRATION OF PLATINUM RESISTANCE

THERMOMETERS USING AN

INTERCOMPARISON SCHEME

R. M. Schoonover and H. H. Ku

Institute for Basic Standards

National Bureau of Standards

Washington, D. C. 20234

December 1974

Final

Prepared for Optical Physics Division and

Applied Mathematics Division

institute for Basic Standards

Washington, D.C. 20234

U. S. DEPARTMENT OF COMMERCE, Secretary

NATIONAL BUREAU OF STANDARDS, Richard W. Roberts. Director

Calibration of Platinum Resistance Thermometers

Using an Intercomparison Scheme

by

R. M. Schoonover and H. H. Ku

Institute for Basic Standards

1.0 Introduction

The International Practical Temperature Scale of 1968 (IPTS 68)

is based on a number of defining fixed points, and the standard

platinum resistance thermometer (SPRT) is used as an interpolation instrxment between these fixed points. From 0 *C

to 630. 7A *C, these SPRTCs) are calibrated at the National Bureau

of Standards using the triple point of water (0.01 "C) , the tinpoint (231.9681 "O, and the zinc-point (419.58 'C) . A thorough

discussion of platinum resistance thermometry and IPTS 68 is

given in NBS Monograph 126 [1].*

Our current hydrostatic weighing experiment procedures (e.g., see

[2]) require that we relate working temperatures (10 - 25 "C) to

IPTS 68 with an uncertainty no larger than a few millldegrees

Celsius. This requirement is not stringent and is easily

obtainable with an SPRT calibrated in the fixed point manner. However, in density measurements made by hydrostatic weighing the

long stem SPRT is not practical because of its excessive length

and sheath fragility. The commercially available capsule type

PRT(s) with flexible leads are most useful in this application,

but when sealed for direct immersion in water and other liquids,

it can no longer be calibrated over the extreme temperature range

of the fixed points.

We have managed this problem by potting the electrical leads exit area of the capsule thermometer in epoxy suitable for direct

immersion in our working liquids and the development of an appropriate calibration procedure that relates the thermometer to

IPTS 68.

The calibration procedure reported herein we refer to as a wireto-wire calibration. That is, the platinum resistor of one or more SPRT(s) calibrated by the fixed point method are used as standards to calibrate one or more platinum resistance

thermometers (PRT(s)) whose properties of Interest are unknown.

This comparison at several temperatures results in the

calibration of the PRT(8).

* Numbers in brackets refer to similarly numbered references at

the end of this paper.

- 1 -
2.0 Calibration Setup

SPRT(s) calibrated In accordance with IPTS 68 and for use in

range of 0 - 630.74 "C are assigned constants alpha (a) and delta

(6). Also determined, but not considered a constant, is the

value of resistance at 0 °C called R(0) in absolute ohms. To

meet IPTS 68 requirements alpha for any given SPRT must not be

less than 0.003925.

To assign IPTS 68 temperatures based on an SPRT, the resistance

R(t) at temperature t must also be measured. Likewise, for us to

calibrate any PRT by the wire^to-wire method, we must measure

R(0) and a sufficient number of selected R(t)s for a meaningful

determination of alpha and delta.

The essential elements of our calibration setup are a triple

point of water cell, a means of controlling temperature, a bridge

for measuring resistance and a switch to expeditiously exchange

thermometers in the electrical circuit. A layout of the

calibration setup is shown in figure 1.

2.1 Triple Point of Water Cell

One of the defining points of IPTS 68 is the triple

point of water (0.01 **C) . The care and use of the cell

is thoroughly described in literature [1]. It has been

used here to measure thermometer resistance at the

triple point R(tp) from which R(0) can be calculated.

This calibration is discussed in section 3.1.

2.2 Controlled Temperature Bath

The controlled temperature bath is a commercially

available unit and requires only minor modification for

this calibration procedure. The water bath portion of

this unit shares a common chassis with the proportional

electronic temperature controller. The bath capacity is

about 20 liters with cube-like dimensions.

The range of temperature control is stated to be from - 20 to 70 °C, a range far greater than our needs. Our

experience shows the bath stability to be about 0.005 °C

per hour over a range of 5 - 35 "C. The performance

beyond this range is unknown.

Initially the thermostat bath was supplied with a cover plate to enclose the free thermally controlled water

surface. However, this left a 10 cm air space above the

maximum fill level after filling and for this reason the cover was abandoned.

- 2 -
We supplied a copper cover plate suspended in contact

with the free water surface. All of the thermometer

leads were clamped to this plate by means of plastic

tape to minimize thermal emf's and to provide thermal

lagging.

A hole was cut through the plate Just large enough to

receive a glass tube which contained the thermometers

for thermal soaking at various selected temperatures.

Contained inside the glass tube is an aluminum cylinder

(thermal mass) with six holes, one for e^ich thermometer,

in close proximity to each other.

Once the thermometers were inserted into the tube and

cylinder, a fluorocarbon liquid with good heat

dissipation properties was added to insure good thermal

contact. The fluorocarbon level was kept slightly below

the level of thermostated water surrounding the outside

walls of the glass tube. The area above the

fluorocarbon was packed with cotton in and around the

protruding thermometer stems and lead wires.

The entire area above the copper plate including

thermometer stems and lead wires was completely covered

with styrofoam pellets for thermal insulation from the

surrounding air. In addition, a black photographic

cloth was placed above the thermometer stems that

protruded to prevent thermal effects from light piping

and stray light.

2.3 Resistance Measuring Bridge

The values of R(t) for each of our thermometers at the

various temperatures were determined by a Mueller-type

bridge (calibrated twice in the last ten years)

.

Detailed instructions for the use of this bridge with

SPRT(b) are set forth in reference [1]. This particular

bridge is thermostated for stability and the smallest

resistance increment is 0.0001 ohms (without

interpolation) . It should also be noted that the bridge

had been very carefully calibrated.

Provisions are made for an external null detector and a wire suspension type galvanometer was used. The

galvanometer optical path was increased from 1 to 2

meters by the insertion of an additional lens in the

light path. This merely provided more sensitivity

making small displacements more visible to the eye.

- 3 -
2.4 Switch > ,

Normally a switch is not used in precise laboratory

thermometry. In this experiment, however, it is

desirable to switch from one thermometer to another

rapidly when taking data. Any switch used in this

application will be more convenient with a low and

stable contact resistance.

Such a switch was recommended to us by the Electricity

Division of NBS. This type of switch is commercially

available for use as a high quality instrument switch.

The contact resistance is specified to be less than

0.001 ohm and stable to 0.0005 ohm and emfs less than 1

microvolt. There were more switch positions and poles

available than used.

We had no problem with switch stability over the very

short time span needed for an R(t) measurement,

approximately 1 minute. (This does not eliminate

thermal soaking the PRTCs) for hours.) The switch was mounted to a heavy brass plate and the leads were

clamped to it to minimize thermal emfs. The entire

assembly was placed in a glass fiber insulated box to

maintain quiescent thermal conditions.

3.0 Wire-to-Wire Calibration

3.1 Calibration Procedure

Two standards, SPRT 1 and 2, and four test thermometers,

PRT 3, 4, 5, and 6, are used in this experiment. SPRT 1

has a long history of stability and is used as the

primary standard. SPRT 2 was intended to be used as a check standard, i.e. a thermometer the calibrated values

of which, derived from this experiment, can be checked

against its fixed point calibrations. Unfortunately, we

noted a shift in the triple point value of SPRT 2 from

its original value. Subsequently, SPRT 2 was recalibrated by our Thermometry Section as it was used

and again after annealng.

Resistance for the triple point of water was determined

twice for each thermometer, each run consisting of readings in the normal-reverse-normal positions*, using a reversing switch in the battery circuit. The average of

the two normal readings is averaged with the reverse reading and the 12 resistance values are given under

column OHMS in table 1. The R(0) values are extrapolated from the resistance at triple point by

subtracting 0.00101 and are given in the next column.

A stable switch and temperature bath make it possible to

shorten the NRRN measurement order to NRN not possible

in a fast drifting experiment.

- 4 _
Next, the six thermometers were Inserted In the constant

temperature bath and resistance readings were taken in

accordance with a designed schedule for each of 9

temperatures nominally 10, 15, 19, 2A, 29, 34, 24, 5 and

15 °C. Each reading, as was in the case of triple point,

is the average of the normal-reverse-normal readings.

Typical data are given in table 2 for temperature

nominally 15 °C.

For each temperature, a sequence of 24 resistance

measurements were made in the following time sequence,

with every two measurements considered to be within the same time period:

Set 1

TIME PERIOD THERMOMETERS

1 3-5

2 1-3

3 1-4

4 2-5

5 2-6

6 4-6

Set 2

78

9

10

11

12

- 5 -
The sequence in the second set is the reverse of that of

the first set and with 3 and 5, and 4 and 6

interchanged. The original intention was to use thermometer 1 as standard in set 1 and thermometer 2 as standard in set 2 and compare the calibrated values

resulting from the two sets. Since thermometer 2

changed its triple point value since its last fixed

point calibration and its stability was open to

question, thermometer 1 was the only one used as a standard. Nonetheless, the design has the advantage

that any drift in the constant temperature bath over the

period of measurements can be detected and compensated

for, if necessary. For other designs see for examples

NBS Technical Note 844 [4].

3.2 Adjustment for Trend in Constant Temperature Bath

From the 24 measured resistance values for each temperature, estimates of the resistance value for each

thermometer were obtained by fitting the following

equation to the data, using the method of least squares.

The equation takes the form:

y^j - R + + e..

where y^ • resistance reading of the ith thermometer

on the jth occasion (order)

R ' mean of all resistance readings

tj : effect of the ith thermometer, and

e jj : random error of measurement with mean

zero and variance o^.

The OMNITAB command FIT [3] was used for the calculation

with the constraint Etj « 0, i.e. the effect of each

thermometer is measured from the average of all

thermometers. The result using this model was, however,

unsatisfactory. A plot of the residuals (observed value

- fitted valee) against the order in which the

measurements were taken showed clearly trends in

temperature over the time period required for taking the

24 measurements. This trend was more pronounced in some

teiiq>erature8 than others. A plot of these residuals at

nominally 15 "C is shown in figure 2. Standard

deviations computed from those fits for the nine bath

temperatures varied from .000044 to .00045 ohms.

- 6 -
Next the model was modified to Include the time order

effect using:

Yij - R + tj + Oj+

where the additional term 0 represents the jth order

effect, j - 1, 2, 12, with constraint EO-0. The

resulting plot of residuals now exhibit the random

pattern desired and the plot Is shown In the bottom half

of figure 2 for comparison. The adjusted resistance

values for the six thermometers and the residual

standard deviations are given in table 3 for each

temperature. All standard deviations computed from

these fits were now reduced to about .00005 ohms.

3.3 Calibration Equations for PRT(s) and Tables

Using the adjusted resistance values of thermometer 1,

the tgg values of the constant temperature baths were

calculated from the fixed point calibration. The

temperatures in tgg scale were changed to t' scale by

the relationship (p. 22 of reference [1]):

tgg - t' + M(t')

with the M(t') values interpolated from Appendix D of

the same publication. Table 4 gives the tgg , M(t'),

and t' values for the 9 constant temperature baths. The

t ' values are needed for the reason that the

relationship

R(t')/R(0) - 1 + At' + Bt'2

holds for t', but not for t^g. Least squares estimates

of A and B were computed for each thermometer using the

measured R(t') and R(0) values for the more general

equation

1^ - W(0) + At' + Bt'2 (1)

Then a and 5 can be calculated through the relationship:

a - A + lOOB

6 - -10-'*B/(A + lOOB)

and the use of the thermometer can proceed in the manner

as described in section 6.1 of reference [1]. The value

of W(0) is very near to unity (dlf ferenciB< .0005 "C) and

- 7 -
can be corrected If deemed necessary. We note that a and i obtained here have the same meaning and can be

used In the same manner as those given In reference [1].

Alternately, we found that the equation

can be fitted to the data to give adequate t' values

given R(t)/R(0). To check this, a table of values of

R(t)/R(0) versus value of t for Integral values of t at

one-degree Intervals was generated by the following

procedure

(1) Fit equation (1) using calibration data to obtain A

and B. Fit equation (2) using calibration data to

obtain a, b, and c.

(2) Using t' values corresponding to tgg from 0° to 35

"C at 1 degree Intervals, compute the corresponding

ratio R(t)/R(0) using equation (1).

(3) Using the ratio R(t)/R(0) thus calculated, compute

t' values using equation (2) with a, b, c estimated

from the data.

The largest bias noted between t' entered and t' computed was of the order of .0002 *C at about 0 'C.

Table 5 gives t' (entered), R(t)/R(0) values, and t' (computed) . Apparently the discrepancy between the use

of equation (2) and the solution of the quadratic

equation (1) Is tolerable.

4.0 Uncertainty of Calibrated Temperature Values

The following possible sources of errors are discussed separately

below, based on data from this experiment and on knowledge

of the histories of the two standard platinum resistance

thermometers

.

(a) Method of Computation . Since the tgg and t' temperatures used in the constant temperature baths were

based on that of SPRT #1, adjusted for trend, the

resulting adequacy of fit of R(t)/R(0) values to

equation (1) for SPRT //I would Indicate first, the

accuracy of the computation, and secondly, the adequacy

of the adjustments. The standard deviation computed

from the fit of the nine values to equation (1) was 2.6

X 10"® in R(t)/R(0), giving assurance that rounding

error in computation was negligible and the adjusted

- 8 -
I

values did agree with values obtained from fixed point

calibrations. SPRT 01 was annealed and recalibrated by

fixed point method after our experiment and no change

was detected.

(b) Bias In Fitting t' as a Quadratic of R(t)/R(0) . This reverse fitting procedure by equation (2) Is an approximation to solving a quadratic equation In t' for

given t' over the range 0-35 "C, and as such Introduces

certain biases In the results. Comparison of t' and tgg

values confirmed the existence of this bias, although

negligible In the use of these PRlCs). For SPRT itl, the

maxlmiun bias was +.00023 **€ at 0 "C, and changed signs

at about 8 "C, 21 and 31 *€. These biases were noted

on all PRT(s) with the same signs and about the same magnitudes

.

(c) Failure of the Check Standard . SPRT #2, which was

Intended as a check standard, changed Its R(0) value

considerably between fixed point calibrations before and

after our experiment. The R(0) value measured during

our experiment was 25.A5AA7, measured after the

experiment by our Temperature Section was 25.A541, and

after annealing was 25.4336. The standard deviations

computed from the residuals of the fit to equations (1)

and (2) were also largest among the six PRT(s), and

there was also a bias In the calibrated values.

(d) Uncertainty of the Calibrated Capsule PRT(s) 03.

//5« and 06 . Based on the results obtained for SPRT #1, we are of the opinion that the calibrations of the

capsule type thermometers by this procedure are of

sufficient precision for our present work. We have

computed that the standard deviation of a predicted

value of t, given the ratio R(t)/R(0), Is less than

0.00035 'C between 5-30 "C, and hence a 3-slgma limit

may be taken as 0.001 'C. Below 5 °C and above 30 *C,

the standard deviations and 3-slgma limits would be

about double this magnitude, or 0.002 "C. The

uncertainty In the fixed point calibration of SPRT 01 Is

taken to be 0.0005 'C, Interpolated from figure 17 of

reference [1], and based on histories of several such

calibrations of this stable SPRT. The uncertainty of

the calibration. In the range of 5-30 •C, Is therefore

0.0015 °C.

(e) Errors In Using the Calibrated PRT(8) . In using

these PRTCs), an R(t)/R(0) value Is measured. The

measurement error In R(t)/R(0) values Is In addition to

the uncertainty of the calibrated values. In our case,

- 9 -
for example, the standard deviation of a measured value

of R(t)/R(0) (average of four In the design) Is calculated to be 0.000003, equivalent to 0.0007 *€. A 3-

sigma limit on this source of error Is therefore 0.002

•c.

5.0 Summary

In this experiment we have demonstrated the feasibility of a wire-to-wire calibration procedure using one or more standard

platinum resistance thermometers as standard. Within the range of

temperature of 0-35 'C, the uncertainty of the calibration, apart

from the uncertainty of the reference standard, Is estimated to be

less than 2 mlllldegrees.

*****************************

REFERENCES

[1] Riddle, J.L., Furukawa, G.T. and Plumb, H.H., Platinum

Resistance Thermometry, Nat. Bur. Stand. (U.S.), Monogr.

126 (1973).

[2] Bowman, H.A. , Schoonover, R.M. and Carroll, C.L., A

Density Scale Based on Solid Objects, J. Res. Nat. Bur.

Stand. (U.S.), 78A (Phys. and Chem.), No. 1, pp. 13-40

(Oct. 1973).

[3] Hogben, D. , Peavy, S.T. and Vamer, R.N., OMNITAB II

User's Manual, Nat. Bur. Stand. (U.S.), Tech. Note 552

(1971).

[4] Cameron, J.M. and Halles, G.E., Designs for the

Calibration of Small Groups of Standards In the Presence

of Drift, Nat. Bur. Stand. (U.S.) Tech. Note 844 (1974).

- 10 -

+Q.0006

• • •

0 —I

M

MU

w

o

w

ft

-0.0006 °C-

+0.0001 -

• . Unadjusted

A — Adjusted

-0,0001

FIGURE 2: Plot of residuals illustrating behavior of bath at 15 "C during

50 minute interval required for the 24 observation series.

Data shovm before and after adjustment for trend.

- 12 -
Resistance Measurements

Thermometer

1

1

2

2

3

3

4

4

5

5

6

6

Normal

25.47277

25.47277

25.45538

25.45538

25.55339

25.55339

25.59119

25.59109

25.62159

25.62139

25.54780

25.54770

Reverse

25.47077

25.47077

25.45558

25.45558

25.60209

25.60209

25.57608

25.57608

25.60890

25.60910

25.55529

25.55529

Normal

25.47277

25.47277

25.45538

25.45538

25.55339

25.55349

25.59119

25.59109

25.62149

25.62149

25.54760

25.54760

25.47177

25.47177

25.45548

25.45548

25.57774

25.57777

25.58363

25.58358

25.61522

25.61527

25.55150

25.55147

RlOl

25.47076

25.45447

25.57676

25.58260

25.61424

25.55048

TABLE 1: Resistance measurements at triple point of water

and 0 "C for the six thermometers.

- 13 -
Resistance Measurements

Thermometer Normal Reverse Normal R(;i5)

3 27.11835 27.16194 27.11845 27.14017

5 27.17874 27.17874 27.17884 27.17876

1 27.02593 27.03093 27.02613 27.02848

' 3 27.11885 27.16234 27.11905 27.14064

^ 1 27.02634 27.03123 27.02644 27.02881

4 27.14364 27.14964 27.14374 27.14667

2 27.01143 27.01113 27.01153 27.01130

5 27.17974 27.17964 27.17984 27.17971

2 27.01153 27.01153 27.01173 27.01158

6 27.11094 27.11524 27.11104 27.11311

4 27.14404 27.14994 27.14434 27.14707

6 27.11094 27.11534 27.11104 27.11317

4 27.14414 27.15004 27.14434 27.14714

6 27.11104 27.11534 27.11114 27.11322

2 27.01163 27.01153 27.01183 27.01163

4 27.14414 27.15004 27.14424 27.14711

2 27.01183 27.01163 27.01203 27.01178

3 27.11994 27.16334 27.11994 27.14164

1 27.02724 27.03203 27.02724 27.02964

6 27.11114 27.11554 27.11114 27.11334

1X

0 7 n9 71 A z0 7 mi Q1 / . U JX?

J

97 fl971A Z / . UZ / X*f Z97/ . nOQ^L UZ7JH

c. J Z / . XOV/XH 27.18014 27.18009

3 27.11984 27.16324 27.11984 27.14154

5 27.18014 27.18004 27.18014 27.18009

TABLE 2: Resistance measurements at temperature

nominally 15 "C for the six thermometers.

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15 -
R(t)/R(0) M(t') t'

1.0418116

1.0611844

1.0782584

1.0962757

1.1166101

1.1351945

1.0962589

1.0237095

1.0612091

10.50307

15.38107

19.68631

24.23561

29.37759

34.08415

24.23137

5.95161

15.38730

0.004054

0.005531

0.006568

0.007485

0.008277

0.008780

0.007484

0.002459

0.005505

10.50712

15.38657

19.69288

24.24310

29.38587

34.09294

24.23886

5.95407

15.39281

Table 4. Bath temperatures in t^g and t' as determined from adjusted values of

R(t)/R(0) for thermometer #1.
R(t)/R(0) t'

t' (computed (computed

68

(entered) usxng tiCj , jlj using r*q

0 »0000000 .9999966 .0002217

1 1 1.0039813 1.0006204

2 2 » 0008750 1.00796*^7 2.0010117

3 3 » on 1 294n 1.0119470 3.0013989

4

11T » om 70 1 n 1.0159281 4.0017782

5 5 0020950 1.0199079 5.0021448

6 6 »002^779 1 .0238866 6.0025088

7 f * 002o'|90 1.0278640 7.0028619

8 • 003^^0 /9 1 .0318402 8,0032074

f\V Q UU JJj'+y 1

.

u3do 1 V . U U ODH 1 f

10 10 <0038910 1.0397890 10.0038723

11 1 1 X JL »' nniL? UU*rC-X1 tiQ r7 1.0437616 11.00^1907

12 1 ? » U U H » 7 1.0477329 12.0045007

13 i o » U U 1+ Ot; W 1.0517031 13.0047983

14 1.0556720 14.0050929

15 15 • 005^^009 1.0596398 15.0053766

16 16 •0056698 1.0636063 16.0056467

17 1

1 7» » U U O 7<C / o 1.0675716 17.0059090

18 1 a 1.0715357 18.0061638

19 19 1 n"7c:u.QftA

20 20 0066388 1.0794604 20.0066385

21 ?1 1.0834209 21,0068638

22 cc 1.0873801 22.0070722

23 CO . nn7Pc;7Q 1.0913382 23.0072732

24 Oil » U U / 1.0952951 24.0074649

25 25 .0076199 1 .0992508 25.0076468

26 26 .0077870 1.1032053 26.0078149

27 t » . U U i^'^j^ 1.1071586 27,0079682

28 , nnAfiQi fi 1 .1111106 28.0081193

.

• nnoPoftQ U Uo'-ilOo 1 . llDUoXD

30 30 •0083568 1.1190112 30,0083728

31 • UUO^ /Do 1.1229596 31.0084827

32 32 •0085859 1.1269069 32.0085845

33 33 •0086865 1.1308530 33,0086713

34 3^+ •0037795 1.1347979 34.0087452

35 35 •0088630 1.1387415 35.0088115

Table 5. Comparison of t' (entered) and t' (computed) for

thermometer //5 using:

(1) R(t)/R(0) = W(0) + A t' + B t'^, and then

(2) t' = a + b[R(t)/R(0)] + c[R(t)/R(0)]^,

where W(o), A, B, and a, b, and c are calculated from calibration data.

USCOMM NBS LH - 17 -
NBS-IKA (REV 7-7-i)

U.S. DEPT. OF COMM.

BIBLIOGRAPHIC DATA

SHEET

1. I'lm.li .A 1 10\ OK Kl I'OK 1 NO. NBSIR 74-586

2. Cuiv 't .Ac c (• S S lufl No.

3. Rcc ipicni 's Acce-'ision No.

4. Tn Li-, AND siin i

n

Calibration of Platinum Resistance Thermometers Using

an Intercomparison Scheme

5. Publication Date December 1974

6. Performing Organization ( bdc

7. AUTHOR(.S)

R. M. Schoonover and H. H„ Ku

8. l^rrlorrm^^Or^^.

9. F'KRF0RM1N(. OR(, AN IZATION NAMI- AND ADDRH.S.S NATIONAL BUREAU OF STANDARDS

DEPARTMENT OF COMMERCE

WASHINGTON, D.C. 20234

10. Fro)( cr/Ta,sk/Work Unit No.

11. ( ontract /Grant No.

12. ^pcKi s. )r in^' ( .in 1 / at lull N.inu ami ( nniplctt- Adctrcs.'. (Stret-I, Cily. >'IcjIv, ZIP)

SAME

13. 1 ype of Report & Period

• overed Final

14. Sp(,)nsoring Agency Codf

15. si;ppi.i' m1';n l AKV noi h.s

16. AMSI 1<A( I (A JOO-word or less factual summary of most sif^tficant information. It doLiimt-nt includes a significant

bibliography or literaturt^ survey, mention il hfrf.)

In this report we describe a procedure for the calibration of

capsule type platinum resistance thermometers, PRTCs), using one

or more standard platinum resistance thermometers, SPRT(s), in

the tenqjerature range of 0 - 35 °C. These PRT(s) were designed

to be used in density work through hydrostatic weighing vrhere

SPRT(s) cannot be used because of opace and other limitations,

but the procedure is thought to be generally useful in other

applications as well.

The schedule of intercomparisons was designed to eliminate

possible trends in temperature variations of the constant

temperature bath setup. Results of calibration can be expressed

either in terms of the two constants alpha (a) and delta (6), or

in a table relating R(t)/R(0) to tge within the range of

calibrations. ^ ,

. '

The uncertainty of the values of t^g calibrated by this procedure

is believed to be within 2 millidegrees Celsius, not including

the uncertainty of the SPRT that is used as standard.

17. KEY WORDS f.s;\ to twelve i-ntrii-s, alphabetical order; capitalize only the first letter of the first key word unless a proper name; separated by semicolons ) Adjustment for trend; calibration; capsule type thermometers; intercomparison;

IPTS-68; platinum resistance thermometers; wire-to-wire calibration.

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(THIS RFF^ORT)

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