A UNITED STATES DEPARTMENT OF COMMERCE

PUBLICATION

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NBS TECHNICAL NOTE 764

An Investigation of

the Stability of

and Insulation Leakage in

Some High Temperature

Resistance Thermometers:

An Interim Report

U.S. EPARTMENT

OF

COMMERCE

au

of ds

National

100

US753
NATIONAL BUREAU OF STANDARDS

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too

An Investigation of the Stability of

^and Insulation Leakage in

Some High Temperature Resistance

Thermometers: An Interim Report

Sharrill D. Wood

Heat Division

Institute for Basic Standards

National Bureau of Standards Washington, D.C. 20234

i<f>

NBS Technical Notes are designed to supplement the Bureau's regular publications program. They provide a means for making available scientific data that are of

transient or limited interest. Technical Notes may be

listed or referred to in the open literature.

^*4^.

U.S. DEPARTMENT OF COMMERCE, Frederick B. Dent, Secretary

NATIONAL BUREAU OF STANDARDS, Richard W. Roberts, Director

Issued May 1973
National Bureau of Standards Technical Note 764

Nat. Bur. Stand. (U.S.), Tech. Note 764, 31 pages (May 1973) CODEN: NBTNAE

For sale by the Superintendent of Documents, U.S. Government Printing Office, Washington, D.C. 20402 (Order by SD Catalog No. C13.46:764). Price $0.50 domestic postpaid or $0.35 G.P.O. Bookstore
FOREWORD

In the interest of informing the public on program developments and technical advances within the Division, we are increasing our publications activities, beyond the customary

formal publication of complete pieces of work, through the media of NBS Technical Notes

and NBS Reports. The former provide the means to deal with the subject matter at greater

depth than normally tolerated by today's journal editors and the latter afford an opportunity to describe informally the progress toward current goals where the work is incomplete

.

This is a progress report. The work is incomplete and is continuing. Results and conclusions are not necessarily those that will be included in the final, formal publications . The period covered herein is the calendar year 1972

.

H. H. Plumb, Chief

Temperature Section

National Bureau of Standards
CONTENTS Page

1

.

Introduction 1

2 High Temperature Stability 1

2.1. Thermometer Designs 1

2.2. Experimental Procedure 2

2.3. Results of Stability Tests 3

a. Results of Heat Treatment in the Region of IO65 °C 3

b

.

Results of Heat Treatment at 960 °C and 900 °C 3

3. Stability of Thermometer Resistance at the Freezing Point of Zinc 5

3.1. Equipment and Procedures 5

3.2. Results 6

k . Investigations of Thermometer Insulation Resistance 8

k . 1 . Equipment and Procedures 8

U.2. Results 9

5

.

Summary 9

6 References 12

Figure 1. Details of the silica-glass support for the cross (HTFQ) thermometers.

(a) side view. (b) end view 13

Figure 2. Behavior of four birdcage (HTSS) thermometers and thermometer W-l with

exposure to temperatures in the region of IO65 °C. Open symbols were

not used in computing drift rates . The temperature scale does not apply

strictly to W-l Ik

Figure 3. Behavior of birdcage thermometer HTSS-16 with exposure to temperatures

in the region of 1065 °C . The first point was not used in computing

the drift rate 15

Figure h. Effects of various annealing patterns on birdcage thermometer HTSS-15

after heating in the region of IO65 °C 16

Figure 5. Behavior of cross (HTFQ) thermometers. (a) Exposed to IO65 °C. The

first point in each case was not used in computing the drift rate,

(b ) Exposed to 96O °C IT

Figure 6. Behavior of the tungsten thermometer W-l. (a) Exposed at IO65 °C.

(b) Exposed at 960 °C. Note different scales 18

Figure 7- Drift rate of W-l as a function of time spent at IO65 °C 19

Figure 8. Behavior of birdcage thermometers (HTSS) after exposure at 960 °C and 900 °C20

Figure 9- Details of the construction of the core of the low temperature furnace and of the hermetically sealed freezing-point-of-zinc cell 21

( continued)

iv
CONTENTS Page

Figure 10. Circuit diagram of the apparatus used to measure insulation

resistance at low frequencies. R is the leakage resistance

being measured, e is the point at which the leakage current

is measured 22

Figure 11. Summary of tests of insulation resistance of HTFQ-23 23

Figure 12. Typical sets of data for the insulation resistance of HTFQ-23 at two temperatures and several voltages 2k

An Investigation of the Stability of and Insulation Leakage in Some High Temperature Resistance Thermometers : An Interim Report

Sharrill D. Wood

Data are presented concerning the stability of high temperature

resistance thermometers . Two types of platinum and one type of tungsten thermometer were tested at 1065 °C and 960 °C. Some of the platinum thermometers were also tested at 900 °C and at the freezing point of zinc. Results of tests are also given on the insulation resistance of sensor supports and lead assemblies for

the two types of platinum thermometers . A new design for a zinc-point cell is discussed briefly. Suggestions for future work

are indicated.

Key words: Freezing point of zinc; high temperature resistance

thermometer; insulation resistance; platinum resistance thermometer; resistance thermometer; stability; tungsten thermometer; zinc.

1 . Introduction

Two main paths of investigation have been followed recently in the high temperature

platinum resistance thermometry laboratory at the National Bureau of Standards ; one is thermometer stability and the other is thermometer insulation resistance. Drifts and in- stability in thermometer resistance have long been problems for high temperature resistance

thermometers when they are cycled between high temperatures (up to 1100 °C) and room temperature. Typically, a thermometer is subjected to a high temperature for a short time

(e.g., one hour) and then quenched fairly rapidly upon removal from the furnace or the freezing point apparatus. It would be ideal if the resistance of a thermometer were

independent of the thermal treatment it received, but strain and quenching are two well

known mechanisms which can change the thermometer resistance. Researchers have tried to eliminate strain through design. Recent changes in annealing procedures [l] have eliminated

much of the instability due to quenching found in thermometers , but drift and unstable behavior from these and other sources remain problems . In an effort to test the stability

of various configurations and materials , we have conducted some long-range stability tests

on three styles of thermometers

.

2. High Temperature Stability

2.1. Thermometer Designs

We had available six thermometers of the birdcage design [2]. The elements of these

thermometers contained synthetic sapphire insulators and were designated high temperature

synthetic s_apphire or HTSS . The lead wires were strung through small silica glass or alumina tubes and were separated by silica glass, synthetic sapphire, or alumina disks. The insulating disks were more closely spaced near the sensing element of the thermometer to reduce radiation losses. Three of these thermometers (HTSS-lU, HTSS-15 , and HTSS-16) were relatively old (i960), but had not been used for several years. (HTSS-lU was studied in

[2].) The elements and leads were remounted in 800 mm long, high-purity silica-glass

sheaths. This required extending the leads but the elements were not modified. Two of the thermometers (HTSS-21 and HTSS-22) were new; their elements had been made earlier but the

leads had not been attached. The sixth thermometer (HTSS-19) was an old thermometer (1961)

which had been used during the intercomparison of high temperature thermometers and thermocouples [3]. Table 1 contains a summary of the characteristics of the thermometers that were investigated.
Table 1. Thermometer Characteristics

Serial Number

HTSS-lU

HTSS-15 HTSS-16

HTSS-19

HTSS-21

HTSS-22

HTFQ-23

HTFQ-2^

HTFQ-25

W-l

Coil Support

synthetic sapphire

synthetic sapphire

synthetic sapphire

synthetic sapphire

synthetic sapphire

synthetic sapphire

silica glass

silica glass

silica glass

tungsten

Lead Assembly

Insulators ^n'^ alumina 0.26295 alumina 0. 26983 alumina 0.26099 alumina tubes 0.260U8

synthetic sapphire

disks

silica glass 0.25778

alumina 0.25950

silica glass 0.22921

silica glass 0.222U8

silica glass 0.21790 silica glass O.2580

Style

Birdcage

Birdcage

Birdcage

Birdcage

Birdcage

Birdcage

Cross Cross

Cross Steeple

Three resistance thermometers of a new element design were constructed and tested for their stability. Platinum wire, ,k mm diam, was wound in a bifilar helix around a cross shaped silica-glass support (fig. l) made at the National Bureau of Standards. Thermometers

of a similar design have been investigated by Sawada and Mochizuki [h] . The resistance of

the thermometers at the triple point of water is nominally 0.25 fi; these cross thermometers

were designated high temperature fused quartz or HTFQ. Before assembly all silica glass

parts were cleaned by an ultrasonic technique in sequential baths of trichloroethylene

,

methyl alcohol, and distilled water, and then fired in oxygen at 1100 °C for one hour. The

sheath was cleaned by washing with a detergent and then rinsing extensively with distilled

water. It was dried by heating to a low temperature. Finally, it was fired for one hour

at 1100 °C in oxygen. After assembly but before being inserted in the sheath, the thermometer was soaked in aqua regia for three hours, rinsed with distilled water, and baked at temperatures up to 600 °C. After being placed in the silica-glass sheath, the thermometer was evacuated, fired in oxygen for five minutes at 1100 °C , and reevacuated.

The sheath was then sealed after being filled with dry air to a pressure of one atmosphere

at IO65 °C. The same general technique was followed in the construction of the birdcage

thermometers

The resistance element of the third type of thermometer was fabricated from thoriadoped tungsten wire. The element was made by R. L. Anderson while he was at the National

Bureau of Standards and it was built in the style of a "steeple" thermometer [5]. The thermometer leads are W-3%Re alloy wires of 0.37 mm diam and the lead assembly insulators

are silica glass disks and tubes. Initially, the element was annealed at 2000 °C for one hour before the thermometer was assembled and afterwards the fabricated thermometer,

designated W-l, was vacuum baked at 800 °C and filled with argon gas. Only one tungsten

resistance thermometer was tested.

2.2. Experimental Procedure

The procedure followed in testing the long-term stability was the same for the three

types of thermometers. Resistance readings were first taken at the triple point of water. The thermometers were then placed in a pre-heated furnace where they remained for a specified period of time after which they were annealed. Then triple-point resistance

readings were taken again. These tests differ from normal use of the thermometers in two

important respects. First, the thermometers are exposed to high temperatures for a longer

time than in normal usage. Second, the thermometers are not quenched before annealing.

Repeated measurements at the triple point of water without intervening heat treatment

show that the average difference in 20 successive pairs of readings is 0.0*+ mK and the standard deviation is 0.28 mK. This represents the reproducibility of our measuring system

and thermometers. One cross and four birdcage thermometers were used to obtain these data.
2.3. Results of Stability Tests

a. Results of Heat Treatment in the Region of IO65 °C

Figures 2, 3 and k summarize the results of tests for thermometer W-l at IO65 °C, and

for five birdcage thermometers at 1100 °C, at IO65 °C, and at the freezing point of gold.

Thermometer HTSS-16 (fig. 3) was tested much longer than the others because of its unstable

behavior. Thermometer HTSS-21 was resheathed twice in the course of the experiment and data

is given for the most recent sheath only.

There was a relatively large initial shift in the ice-point resistance for all five birdcage thermometers as a result of the heat treatment. This is probably caused by the annealing of cold working done during construction. Thermometer HTSS-1^ continued to drift

rapidly for a fairly long period of time but it had been dropped while out of its sheath prior to these tests and had thereby suffered a large amount of cold working and possibly

slight contamination. Thermometer HTSS-22 was the only one that exhibited an initial in- crease in the ice-point resistance. Thermometer HTSS-21, after the initial decrease in the ice-point resistance, continued to increase in resistance with additional exposure to high

temperatures

.

Figure k is essentially an enlargement of a part of figure 2. From it, we can see the effects of various annealing patterns. The first four points represent the resistance at the ice point following an anneal of rapid cooling from IO65 °C to 650 °C (1.5 h) , one hour

at 65O °C , and slow cooling to room temperature . The thermometer was not quenched from

IO65 °C prior to annealing. The next six points were taken after a quench from the gold

point to room temperature, one hour at 650 °C, and slow cooling to room temperature over

night. The last eight points were taken after a new annealing pattern which is based on the work of Berry [l] . The thermometer was placed in a furnace at 1065 °C (following a quench

from the gold point to room temperature) and the furnace temperature was slowly lowered to U50 °C at a rate of 90 °C/h. For some of these last points, the thermometer was not quenched but had been in the annealing furnace at IO65 °C for several hours when the anneal began. The shift in the ice-point resistance after the first gold point was not removed by an anneal of one hour at 650 °C followed by slow cooling. The new annealing

procedure was removed most of the quenched-in shift and brought the ice-point resistance

closer to its original curve.

Figure 5a gives the results of the heat treatment of the three cross thermometers at IO65 °C. These thermometers were found to be generally more stable than the birdcage

thermometers and to behave remarkably similarly. They were constructed at nearly the same time from the same lots of materials and HTFQ-2U and HTFQ-25 had the same thermal history.

Thermometer HTFQ-23 had been used in the insulation resistance tests prior to the long-term

stability tests and was subsequently repaired and resheathed. However, this treatment does not seem to have affected it to any noticeable extent.

Figures 6a and 2 give the results of testing the tungsten thermometer, W-l, at IO65 °C

.

We can see from figure 6a that the thermometer had not been well stabilized when the tests began. From figure 7, we see that after approximately i+00 hours at IO65 °C and above, the drift rate reached 1 pfi/h. The rate of change of the ice-point resistance seems to be an exponential function of the time at elevated temperatures. In figure 2, it is seen that the drift rate of W-l is approaching that of some of the birdcage thermometers

.

b. Results of Heat Treatment at 960 °C and 900 °C

The results of heat treatment at 960 °C for the birdcage thermometers are given in figure 8. These data were taken after the work at IO65 °C. As might be expected, the stability is better than the stability at IO65 °C. Thermometer HTSS-16 is still relatively

unstable and HTSS-lU is still drifting. This may possibly be due to its gross cold working

during construction but is more likely an "equilibrium" drift rate.

From figure 5b we see that the cross thermometers are well behaved at 960 °C. Two measurements at the zinc point were taken with each thermometer between the work at IO65 °C and these data. Thermometer HTFQ-2U was found to be more stable than the others but the
range of drift rates is only 0.02 mK/h. Only HTSS-16 and HTSS-21 had such small drift rates

at 960 °C and they showed greater variability.

Thermometer W-l (fig. 6b) was found to drift considerably less as a result of heating

at 960 °C than from heating at IO65 °C. These measurements followed the work at IO65 °C. Its drift rate is still the largest of all the thermometers and it is difficult to tell if the drift rate is decreasing exponentially as was the case at IO65 °C.

The ice-point resistances of the birdcage thermometers resulting from heating at 900 °C are shown in figure 8. These experiments were preceded by the work at 960 °C and by measurements at the zinc point for some of the thermometers. These six thermometers, when heated

at this temperature, exhibited drift rates that were equivalent to the drift rates of the

cross thermometers at 960 °C and were less variable than the cross thermometer drift rates

at 96O °C.

It should be noted that the heat treatments given the birdcage and cross thermometers

differed in two respects . The birdcage thermometers were never continuously at a high

temperature for more than twenty hours while the cross thermometers were often at a high

temperature for one hundred hours at one time. Any change in the ice-point resistance

which was not time dependent would be expected to have a larger effect on the drift rate of the birdcage thermometers because they were cycled more frequently than the cross thermometers . The second difference is that the cross thermometers were never given any short

cycles in the gold-point cell followed by quenching. Any quenching effects that were not removed during annealing would affect only the birdcage thermometers. However, we feel

that our current annealing procedure is removing noticeable quenched-in defects

Drift rates given in this paper are the change in the ice-point resistance (calculated

from the triple point resistance and corrected to zero current) per hour of exposure at elevated temperatures. The drift rates that were obtained are summarized in table 2 for the three types of thermometers at the temperatures at which they were tested. These drift

rates were computed by fitting a straight line, using the least squares procedure in OMNITAB [6], to the observed values of the ice point resistances as a function of the time

of exposure to high temperatures. Also, given in table 2 are the residual standard de- viations and their associated degrees of freedom. The residual standard deviation is a measure of the scatter of the data points about the straight line fitted through them; it

is not a measure of the variability of the y-intercept or of the slope of the line. These variabilities are given by the standard deviations of the coefficients found for the straight line. The standard deviation of the slope (drift rate) was in all cases 30% or less of the drift rate, and in all but three cases it was 15$ or less.

Table 2. Dri ft Rat'ss

HTSS HTSS HTSS HTSS HTSS HTSS HTFQ HTFQ HTFQ

Serial Number lU 15 16 19 21 22 23 2k 25 W-l

1065 °C Drift rate V ^mK

-0.16 -0.03 0.21 0.11 -0.13 0.07 0.06 0.0U 0.30

-0.15 -0.03 0.20 0.11 -0.12 0.08 0.07 0.05 Residual standard

deviation, mK 2.35 1.05 6.16 3.11 2.77 2.73 2.85 3.11 5.82 Degrees of freedom 18 21 39 15 21 6 6 6 8

960 °C

r, --cm. V® -O.OU -0.03 -0.02 0.07 0.0U -0.02 -0.01 0.10 Drift rate „mK -o.oit -0.03 -0.02 0.07 0.0U -0.02 -0.01 Residual standard

deviation, mK 0.61+ 0.3T 1.07 0.8U 0.68 1.35 0.24 0.15 0.15 2.61 Degrees of freedom 10 10 10 10 10 10 6 6 6 It

(continued)
Table 2 (continued) HTSS HTSS HTSS HTSS HTSS HTSS HTFQ HTFQ HTFQ

Serial Number 111 15 16 19 21 22 23 2\ 25 900 °C Drift rate U"mK

-0.02 -0.02-0.02

-0.02

-0.01

-0.01

0.02

0.02

0.03 0.03 Residual standard

deviation, mK 0.ll4 0.3 1* 0.52 o.ia 0.36 0.27

W-l

Degrees of freedom 12 12 12 12 12 12

It has been pointed out by Curtis [7] that drift rates for very long-term (>1000 hours)

experiments often differ substantially from the drift rates for shorter-term experiments

.

Time did not permit us to test these thermometers beyond a few hundred hours but it would

seem unlikely that there would be much difference in the drift rates for long and short- term heat treatments because most of the thermometers seemed relatively stable. Grain

growth could cause a change in the drift rate with time. However, several of the birdcage

thermometers were old and three had been at 960 °C and above for at least 300 hours before

these tests .

It would appear from the results that with careful monitoring and sufficient annealing,

the long-term stability of the cross thermometers and of four birdcage thermometers is satisfactory, but the stabilities of thermometers HTSS-19 and HTSS-16 are not good. One can say little at this time about the tungsten resistance thermometer, except that better initial

stabilization procedures must be found and that with proper stabilization, its performance

could approach the birdcage thermometers

.

3. Stability of Thermometer Resistance at the Freezing Point of Zinc

3.1. Equipment and Procedures

We have recently installed a new type of freezing-point-of-zinc cell. Because of earlier problems with contamination of cells [3], it was decided that hermetically sealed

cells were necessary. The cell (fig. 9) was made from borosilicate glass and the crucible

and thermometer well were made from high-purity graphite. In addition to being shorter in overall length and offering a greater depth of immersion than the old cell, the new cell has

a double thermometer well. Next to the zinc is a graphite well, and between that well and

the thermometer sheath is a borosilicate-glass well which is an integral part of the cell. We have not noticed any particular problems of inadequate immersion or heat transfer because

of this double well arrangement . Melting curves indicate that the zinc in the new cell is not as pure as the zinc in the old cell (which is not hermetically sealed)

.

In addition to protecting the zinc from contamination, the new cell design has allowed

us to modify our low temperature furnace. Because of the small size of the cell, the aluminum moderating block in the furnace could be removed and replaced with an aluminum

"can" (fig. 9)- The zinc cell was placed inside this can. In this way, the cell was surrounded on all sides by aluminum. The furnace windings were simplified and now only a single winding is used. The old furnace and zinc cell are described in [3] . Initial tests with a 25-fi standard platinum resistance thermometer indicated that the thermometer can track the temperature gradient due to the hydrostatic pressure of the liquid metal over 7 cm

to within 20 uK.

In the course of testing the new zinc cell, we also gathered some data on the performance of the thermometers. Initial resistance measurements at the zinc point were taken with the birdcage thermometers after their stability tests at 960 °C, and the final measurements at the zinc point were taken after the tests at 900 °C. The resistances of the cross thermometers were studied at the zinc point before and after exposure at 96O °C. Occasionally, consecutive measurements were taken during one freeze. During this procedure,

the thermometer was allowed to attain thermal equilibrium with the freezing zinc, resistance

readings were taken, and the thermometer was removed for five minutes. The thermometer was
then reinserted and the process repeated. Up to three sets of readings were taken during a single freeze. Sometimes, several thermometers were tested during one freeze. The same procedure was followed with the exception that a different thermometer was introduced into the zinc-point cell after the five minute wait.

3.2. Results

The changes in thermometer resistances at the zinc point, in mK equivalents, are summarized in table 3. The birdcage thermometers had an initial shift in their zinc-point

resistances of -1.2 mK to 9-9 mK. These values are based on an average of the observed

changes in terms of R freezing point of zincZn ' Znand D

Zn' where R_ is the thermometer resistance at the

'Zn

and DZnZn

RZn/R0>

= R

Zn V

R. is the average of the ice-point resistances read before and after the zinc point

some cases, the ice-point resistance changed more than the zinc-point resistance but no pattern emerged for the four birdcage thermometers . After the shift , the thermometers

stabilized but they were not quite as stable as the cross thermometers

Table 3. Zinc Point Data

Difference from preceding zinc point, X. , - X. , mK l+l l

In

RT AR„ AW^ AD„ AR„ Zn Zn Zn

HTSS-lU 0* 0* 0* 0*

-U.

11.9 1.8 -5.7

1.7) 1.9 1.8 -0.1

0> (a) -0.8) - 0.8 -0.8

HTSS-16 0* 0* 0* 0* k.k 16.3 9.0 -l+.l

0.1+) 6.1+ 2.7 -2.1 -0.3? - 0.3 -0.2

-O.k) - 0.1+ -0.5

HTSS-21 0* 0* 0* 0* 8.3 9.7 8.8 -0.1+ 0.6) 0.6 0.6 0.7? 0.7 0.7 -0.3) - 0.3 -0.2

HTSS-22 0* 0* 0* 0* 9.9) -11+.2 0.5 -8.2

0.3 0.1+ 0.1+ 0.1

)

0.1

HTFQ-23 0** 0** 0** 0** 1.0** 1.1** 1.0** 0** 17.1 31+.2 23.8 -5.8

0.5 0.9 0.7 -0.1

- 2.1+ - 2.3 - 2.1+ 3.0 2.6 2.9 0.2

0.2 0.6 0.3 -0.1

0.2) 0.1 0.2 0.3? 0.3 0.3 -0.2) - 0.2 -0.2
Table 3 (continued)

RT

HTFQ-2U

HTFQ-25

AR„ AW„ AD^ ARn Zn Zn Zn

x* 0** 0** 0** 0.5** 0.1** 0.3** 0.1**

26.9 30.0 28.1 -1.0

3.0 - 2.9 - 2.9

3.8 k.O 3.9 -0.1 0.1 0.3 0.2 -0.1

0.2 0.3 -0.2

0.6 0.7 0.6

0.2) - 0.5 -0.3 0.1 0.2> 0.2 0.2

0)

0** 0** 0** 0** 1.3** 1.1** 1.2** 0** 17.9 32.0 23. k -U.

8

0.2 o.k 0.3 -0.1

• 0.5 - 0.2 - O.k 0.1

0.6 0.8 0.7 -0.1

0.1) - 0.1 -0.1

0.1 0.1 0.1 0.1

0.1I 0.1 0.1

* data taken before exposure to 900 °C

** data taken before exposure to 960 °C

(a) bracketed runs were taken during the same freeze.

The cross thermometers showed a large initial increase of 20 mK to 30 mK in the zincpoint resistance. Because the ice-point readings remained relatively steady over the entire

time that zinc-point readings were taken, strain and improper annealing do not seem to be the source of all the instability. It is particularly noteworthy that the ice-point re- sistance decreased while the zinc-point resistance increased. This would definitely point

away from strain as the source of the change . Part of the increase may be due to the increasing devitrification of the initially clear silica glass sheath which results in the reduction of the radiation "piped" up the sheath. McLaren [8] has shown that errors equal

to or exceeding 16 mK may be due to radiation losses up the stem of a standard 25-S2 thermometer. However, we might expect to find less than this because the sheaths devitrified only slightly during testing at 960 °C.

Another source of the observed change might be a change in the pressure in the zinc

cell. If a leak had developed, the cell pressure would tend to approach one atmosphere at room temperature and 2.3 atmospheres at U20 °C. However, a pressure change of 1.3 atmospheres would cause the zinc point to change by 5.6 mK and this does not explain our large shift

.

Perhaps the zinc had somehow become contaminated between the last measurements with the birdcage thermometers and the last measurements with the cross thermometers . It was not removed from the furnace during the course of these measurements but let us postulate that something unusual happened. A 25-fl thermometer was used to monitor the zinc point over the eight month period in question, and it showed a total change in the zinc point of 0.7 mK. Therefore, it seems clear that the thermometers have changed. We do not know whether they

changed gradually during the 300 hours at 960 °C or suddenly. In either case, we conclude

that it may be necessary to monitor the zinc point or some other suitable fixed point, as
well as the triple point of water, during long-term stability tests.

After the shift in the resistance at the zinc point had occurred, the thermometers

behaved in a stable manner. The fifth zinc point on HTFQ-23 and the fourth on HTFQ-2U

(table 3) were measured consecutively during a continuous freeze. The plateau began to fall off shortly after HTFQ-2U was inserted. Previously, plateaux had usually lasted two

hours but this one began to decrease after only one hour. All the following zinc-point measurements agreed with the first zinc points after the initial shift , and it was felt that

the freeze had not progressed properly. These two points met the Dixon criterion [9] for rejection for a = 0.01 and so the runs were discounted.

Values of the zinc-point resistance of the cross thermometers following the large

initial shift (with the exception of the two measurements noted above) were quite well

behaved. The maximum observed change within this stable subset of readings was 1.1 mK and. most of the changes were considerably smaller and positive . This is in keeping with our observations of the birdca.ge thermometers. The average changes in the resistance at the

zinc point (based on an average of R„ , W„ , and D„ ) from one freeze to the next were 0.02,

0.2 and 0.1 mK for the three cross thermometers with standard deviations of 0.3, Q.h, and

0.5 mK, respectively.

We do not know whether the changes in the zinc-point resistances of the cross thermometers following the work at 960 °C were larger than the changes in the zinc-point re- sistances of the birdcage thermometers because the intermediate temperature was 960 °C instead of 900 °C in the case of the birdcage thermometers or because there is some inherent

characteristic in the cross thermometers which caused the- change . One way to check this

would be to perform the experiment using 900 °C as the intermediate temperature for the

cross thermometers or using 960 °C for the birdcage thermometers.

h. Investigations of Thermometer Insulation Resistance

The birdcage thermometer was designed to provide a strain-free element and a small resistance which would reduce the effects of the electrical insulation leakage that shunts the sensor. A 25-fi thermometer must have insulation resistance in the lead assembly and

coil support of at least 25 Mfi at the triple point of water if the effect of the shunting

is to be less than 1 in 10 6

. A 0.25-fi thermometer needs only 1 MQ. of insulation resistance

at the gold point to keep the effects of shunting below 1 in 10°

.

U.l. Equipment and Procedures

To check the insulation resistance of the cross thermometers , a special thermometer was

constructed. Its construction was similar to the other cross thermometers, except that the thermometer element was cut at its mid-point. In this way we could measure, under realistic

conditions, the insulation resistance of the element support and lead assembly as a function

of temperature, frequency, and voltage. A less extensive investigation was carried out with

a birdcage thermometer (HTSS-22) , which had a fifth "lead" wire running from just above

(but not touching) the sensing element to outside the thermometer head. The measurements,

though not as ideal as measurements with the cross thermometer, were useful.

Two measurement systems were used. For the higher frequencies (50 to 1000 hz) a capacitance bridge was used and resistance values were read from the bridge. For the lower frequencies (l0-3 to 10_1 hz) , the circuit shown in figure 10 was used. R_ is the shunting

insulation resistance, and e is the measured voltage across a 100 °. resistor. The leakage

current through the thermometer is supplied by a dry cell and is i. = e/100. Hence, the leakage resistance, R , is given by

RT

= V/i

L

= lOOV/e.

Measurements of e were made by connecting a strip chart recorder across the 100 °. resistor.

The voltage V, applied across the thermometer, was established using a resistor of known

value (10^ or 10° 0.) in place of R and adjusting the slidewire. The current was reversed

by manual switching. All measurements at 600 °C and above were made in the gold-point cell
to approximate the conditions of actual use. The resistances at °C were determined

from measurements taken in a triple-point-of-water cell.

k.2. Results

Figures 11 and 12 give the results of these tests for the cross thermometer. It would

seem that an insulation resistance of 1 MR is not as easily achieved as we had initiallythought [3]. Below 850 °C, the insulation resistance exceeded 1 MR. There was very little

dependence on frequency or voltage until the temperature dropped below TOO °C . Thus, the frequency dependence of the insulation resistance is a problem with ac resistance bridges

.

These results indicate that researchers should make measurements whenever possible to determine the magnitude of shunting errors under their operating procedures

.

Figure 12 presents typical sets of data on insulation resistance for two temperatures

at several frequencies and voltages . For the lower temperatures , there is a larger

difference between the 10 V and 1 V readings than is observed at the higher temperatures

.

Although higher voltages yielded higher insulation resistances, voltages above 1 V should

not be used. Since the insulators do not obey Ohm's law, voltages should be kept as small

as is practical. Although the actual value used in thermometry is less than 1 V, we see that the change between 1 V and 0.1 V is small so the value chosen below 1 V is not critical.

The insulation resistances measured in the birdcage thermometer were of the same order

of magnitude as the cross thermometer resistances. The resistance at IO65 °C was . 5 MR

at frequencies between 50 and 1000 hz . The insulation resistance at °C is given in table

h. There is less than an order of magnitude difference between the two types of thermometers,

Table k. Insulation Resistance of HTSS-22 at °C

Frequency, hz Resistance, MR

50 8.1+ x 10 3

100 2.1 x 10 4

200 1.0 x lO 4

U00 U. 7 x 10 3

1000 1.8 x 10 3

We observed considerable polarization of the insulators at the low frequencies; the initial resistance after the switch was reversed changed exponentially with time. The effects of polarization were not examined at higher frequencies (^50 hz) but some polarization may have occurred.

Additional work is planned with different configurations and materials for thermometer

elements. The frequency dependence of the insulation resistance is expected to be a problem. Therefore, more data on other insulators, e.g., BeO , are needed before ac resistance bridges can become standard laboratory instruments. More data must be taken in the mid-frequency range (0.01 to 50 hz) because the frequency-resistance curve appears to break in this region.

5 . Summary

The results of the stability tests were, for the most part, encouraging. All drift

rates of three types of thermometer as a result of heat treatments at the temperatures

discussed were equal to or less than 0.3 mK/h and most were less than 0.1 mK/h. The residual
standard deviations of most of the thermometers about the straight line fit to the data were

small. It appears that the cross thermometers were slightly more stable and less subject to drift than the birdcage thermometers . The stability of thermometer HTSS-15 was outstanding

among the birdcage thermometers (even among the cross thermometers). The results of the

heat treatments of thermometers HTSS-16 at IO65 °C and HTSS-19 at 96O °C showed them to be

the least stable and the least satisfactory.

The reproducibility of the cross thermometers at the high temperature fixed points

(silver and gold) will have to be tested. They have not been cycled "rapidly" (>20 hours of exposure to high temperatures) nor have they been quenched before annealing with the ex- ception of two or three sets of measurements at the silver point taken with each thermometer

after the second set of measurements at the zinc point. The results of these measurements

are given in table 5

.

Table 5- Silver Point Data

Difference from preceding silver point, X - X. , mK

RT S/K AV Ag Ag

AE

(

HTFQ-23 °\

1.9(

(a) 1.8 1.9 -0.8 -1.5 -1.0 0.1

2.1 2.3 2.1

HTFQ-21+ 0.6 O.k 0.6

HTFQ-25

-0.5 -0.lt -0.3 -0.1

(a) bracketed runs were taken during the same freeze.

The birdcage thermometers were not as steady at the zinc point as the cross thermometers

but they did not exhibit the large initial shift found with the cross thermometers . Further

research will have to be carried out to probe the mechanism of the shift. Thermometer

resistances at the zinc point (and perhaps the tin point) should be monitored more frequently

during exploratory stability tests at 960 °C and above.

Insulation leakage is a problem. The frequency dependence of the leakage must be carefully investigated in light of the increasing popularity of ac resistance bridges. Work is planned to investigate both silica glass and sapphire in both the birdcage and cross configurations. Efforts will be made to determine the effect of the lead assembly configuration

on frequency dependence. The drop in insulation resistance below 1 MR at high temperatures

is disturbing. New insulators should be tested. An effort should be made to determine how

much leakage is through the leads and how much is through the element support

.

At this stage, we can really say little about the suitability of our tungsten

thermometer. It seems to be approaching the stability of the birdcage thermometers and could surpass their stability. Certainly, further work is merited on the investigation of initial stabilization and coil configurations.

10
It is clear from our results that several projects should be continued or started. Stability testing should continue with more frequent monitoring of the thermometers at low temperature fixed points. New insulator and element configuration combinations should be tested for stability and insulation resistance. Work should continue with tungsten and new materials for thermometer elements should be investigated.

11
6. References

[l] Berry, R. J., The Influence of Crystal Defects in Platinum on Platinum Resistance

Thermometry, Vol. h, Temperature, Its measurement and Control in Science and

Industry (instrument Society of America, Pittsburgh, Pennsylvania, in press).

[2] Evans, J. P. and Burns, G. W. , A Study of Stability of High Temperature Platinum

Resistance Thermometers, Vol. 3, Part 1, 313, Temperature, Its Measurement and

Control in Science and Industry (Reinhold Publishing Corporation, New York, 1962).

[3] Evans, J. P. and Wood, S. D., An Intercomparison of High Temperature Platinum

Resistance Thermometers and Standard Thermocouples, Metrologia J_, Wo. 3, 108

(July, 1971).

[k] Sawada, S. and Mochizuki, T., Stability of 25 Ohm Platinum Thermometer up to 1100 °C

,

Vol. h , Temperature, Its Measurement and Control in Science and Industry

(Instrument Society of America, Pittsburgh, Pennsylvania, in press).

[5] Anderson, R. L. , The Stability of Platinum Resistance Thermometers, Vol. h, Temperature, Its Measurement and Control in Science and Industry (instrument

Society of America, Pittsburgh, Pennsylvania, in press).

[6] Hogben, D., Peavy, S. T., and Varner, R. N., OMNITAB II, User's Reference Manual,

Natl. Bur. Stand. (U.S.), Tech. Note 552, 26^ pages (October, 1971).

[7] Curtis, D. J. and Thomas, G. J., Long Term Stability and Performance of Platinum

Resistance Thermometers for Use to 1063 °C, Metrologia h, No. k, 18U (October, 1968)

[8] McLaren, E. H., and Murdock, E. G. , Radiation Effects in Precision Resistance

Thermometry: I Radiation Losses in Transparent Thermometer Sheaths, Can. J. Phys. kk, 2631 (1966) .-

[9] Natrella, M. G., The Treatment of Outliers, Chapter 17, Natl. Bur. Stand. (U.S.),

Handbook 91, 2nd edition (October, 1966)

.

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2k U.S. GOVERNMENT PRINTING OFFICE : 1973 51 1 -325/229
FORM NBS-114A (1-71) U.S. DEPT. OF COMM. BIBLIOGRAPHIC DATA

SHEETS

1. PUBLICATION OR REPORT NO. NBS TN-7642. Gov't Accession No.

3. Recipient's Accession No.

4. TITLE AND SUBTITLE An Investigation of the Stability of and Insulation Leakage

in Some High Temperature Resistance Thermometers: An Interim

Report.

5. Publication Date May 1973

6. Performing Organization Code

7. AUTHOR(S) 8. Performing Organization Sharr-m T). Wnnrl

9. PERFORMING ORGANIZATION NAME AND ADDRESS 10. Project/Taslc/Work Unit No. NATIONAL BUREAU OF STANDARDS DEPARTMENT OF COMMERCE

WASHINGTON, D.C. 20234

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15. SUPPLEMENTARY NOTES

16. ABSTRACT (A 200-word or less factual summaty of most significant information. If document includes a significant bibliography or literature survey, mention it here.) Data are presented concerning the stability of high temperature resistance thermometer

Two types of platinum and one type of tungsten thermometer were tested at 1065 °C and

960 °C. Some of the platinum thermometers were also tested at 900 °C and at the

freezing point of zinc. Results of tests are also given on the insulation resistance

of sensor supports and lead assemblies for the two types of platinum thermometers. A new design for a zinc-point cell is discussed briefly. Suggestions for future work

are indicated.

17. key words (Alphabetical order, separated by semicolons) Freezing point of zinc ; high temperature

resistance thermometer; insulation resistance; platinum resistance thermometer;

resistance thermometer; stability; tungsten thermometer; zinc

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