NATL INST. OF STAND & JECH

A111D5 T7M3T3 35

: Publications

NBS TECHNICAL NOTE 1183

U.S. DEPARTMENT OF COMMERCE/National Bureau of Standards

Techniques in HighTemperature Resistance

Thermometry

• Construction of the NBS-Design HighTemperature Platinum Resistance

Thermometer

• Toroidal Resistor for High-Temperature

Platinum Resistance Thermometers
NATIONAL BUREAU OF STANDARDS

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OF STAHDAliDS ..^ NBS TECHNICAL NOTE 1183 -^,;» y/

Techniques in High-Temperature ^^^ Resistance Thermometry up^s-^

I

' (7 ^ Construction of the NBS-Design HighTemperature Platinum Resistance

Thermometer

N. Bass

CSIRO Division of Applied Pliysics

Sydney, Australia 2070

Toroidal Resistor for High-Temperature

Platinum Resistance Thermometers

J. p. Evans and S. B. Tillett

National Bureau of Standards

Washington, D.C. 20234

**''?EAU O^

CO Q

U.S. DEPARTMENT OF COMMERCE, Malcolm Baldrige, Secretary

NATIONAL BUREAU OF STANDARDS, Ernest Ambler, Director

Issued January 1984
National Bureau of Standards Technical Note 1 1 83

Natl. Bur. Stand. (U.S.), Tech. Note 11 83, 18 pages (Jan. 1984)

CODEN: NBTNAE

U.S. Government Printing Office

Washington: 1984

For sale by the Superintendent of Documents, U.S. Government Printing Office, Washington, DC 20402
Foreword

The NBS Technical Note format has been chosen for the two

papers included here because Technical Notes can accommodate more

technical detail than would be appropriate in other publications

emphasizing results and analyses. This presentation of full

detail has at least two distinct benefits. First, detailed

descriptions of apparatus or procedures can aid in the

understanding and analysis of complex behavior, and thus they can

serve as useful background and support references for other

papers. Second, the "how-to-do-it" flavor distilled from

abundant detail can be of assistance to those wishing to duplicate or emulate the effort described.

The two papers in this Technical Note deal with design and

construction aspects of some specific high-temperature platinum

resistance thermometers. Performance experience with these thermometers has been or will be described elsewhere; the papers here

are intended to provide the needed backup. It is also intended

that these papers give enough detail so that workers generally

familiar with the arts and crafts of thermometer making will have

little difficulty in constructing similar devices.

The first paper describes the construction steps of

thermometers whose design evolved at the National Bureau of

Standards during the period 1976-1981. Experiences with many of

these thermometers were presented at the Sixth International

Temperature Symposium in March, 1982 (see reference 3 of the

first paper), along with some information about the details of

construction. This present paper completes and brings up to date

the construction information. It is based on the work done by

N. Bass during his tenure as a guest worker at NBS in 1981, in

which he significantly improved the techniques developed in prior

years by refining, modifying, and adding to them.

The second paper describes a new type of thermometer

resistor. This resistor was also mentioned at the Sixth

Temperature Symposium, but at that time little was known about

its behavior. Subsequently, several new thermometers were made

using the resistor design, and the data obtained with the

thermometers may prove to be useful in future temperature scale

considerations. The second paper is presented in anticipation

that this will be the case.

John P. Evans

111

Construction of the NBS-design High-temperature

Platinum Resistance Thermometer

N. Bass

CSIRO Division of Applied Physics

Sydney, Australia 2070

The construction of a high-temperature platinum resistance thermometer

having a resistance at °C of 2.5 ohm, suitable for use as a defining

instrument up to the freezing point of gold (1064.43 °C, the "gold point"),

is described. The thermometer is made with high-purity fused silica supports

and insulators. The resistor is reference-grade platinum wire wound in a single-layer, bif ilar-helix. The necessity of using the highest purity

materials and of scrupulous cleaning procedures of all component parts, subassemblies and assemblies is stressed. To minimize the effect of electrical

leakage, a fifth or guarding lead is incorporated. The stability and other

characteristics of this type of thermometer have been evaluated at temperatures up to 1064.43 °C.

Key words: Construction; electrical guarding; gold point; high temperature;

IPTS; platinum resistance thermometer; standard interpolating instrument.

1. Introduction

For more than 50 years the National

Bureau of Standards, in order to meet the

needs for improved temperature standards

in science and industry, has been involved in the development of platinum

resistance thermometers [1]. In recent

years, the effort has been directed towards the design and construction of

high- temper atu re platinum resistance

thermometers. Because modern techniques

for resistance measurements often require

the use of a guarded system for highest

accuracy, the NBS design now incorporates

a fifth wire [2] to permit such guarding

of the thermometer leads. The results of

a recent evaluation [3] suggest that a high-temperature thermometer of this design can replace the platinum-10% rhodium/platinum thermocouple as the standard interpolating instrument on the IPTS

for temperatures up to the gold point,

1064.43 °C.

Participation of invited guest

workers in its programs is one of the means by which the National Bureau of

Standards shares its expertise with other

laboratories. The author was fortunate

to be able to take part in the hightemperature resistance thermometer program. As a result of the exchange of

experience through this participation,

techniques used at both the National

Bureau of Standards and the Commonwealth

Scientific and Industrial Research Organization have been developed and

improved.

2. Component Parts

The essential high temperature parts

of the thermometers are made of highpurity platinum and a type of silica

glass known as clear fused quartz. Thermometric quality platinum wire is used

for the thermometer resistor, internal

leads, and guard rings, while clear fused

quartz is used for all protecting and

insulating parts of the thermometers that are to be exposed to temperatures above

room temperature.

The quartz components include parts

made of clear fused quartz tubing and

parts made by laser machining. The

tubing is used for thermometer protecting
sheaths and lead insulating tubes. It is

selected from the manufacturer's standard

production material to be round and

straight, and to have the desired inner

and outer diameters. The laser-machined

parts are made by specialists to close

tolerances from flat stock of the same clear fused quartz material. They include cross blades (figure 1) and fourhole disks. The disks can also be made

by slicing sections from a four-bore

quartz tube that has been ground to the

desired diameter.

The manufacturer specifies that the

clear fused quartz has a metallic impurity concentration of less than 50 ppm

(parts per million), mainly aluminum

oxide; the iron content is less than

4 ppm. The material is stated to have a low hydroxyl-ion content, an attribute

believed to enhance its high-temperature

durability. The quartz components are subjected to stringent surface cleaning

procedures throughout the construction

steps. As a final measure, where possible, they are fired at 1100 °C in

flowing O2 to oxidize any remaining

impurities.

Platinum wire of the highest available purity (reference grade) is used for

the resistor and leads. The manufacturer

claims that the metallic impurities are less than 10 ppm in this grade of platinum and that its alpha coefficient is

greater than 0.003926 K"-'-. The wire is

received from the manufacturer in the

hard drawn state.

Room temperature parts of the thermometers contain ordinary glasses, plastics, and metals that are suited to the

intended purpose. These include a borosilicate glass header, epoxy sealant,

external copper lead extensions with

plastic insulators and connector, and an external aluminum head.

3. Resistor Support Cross

The resistor support cross is fabricated from two fused quartz blades

(figure 1), laser-machined from 0.4-mm

thick fused quartz stock. The blades are cleaned by ultrasonic agitation in a 5%

hydrofluoric acid solution to remove

35 mm

jo g

W U U U

1

" ' "

1 E

E ..0 1

Figure 1. Fused silica cross blade. Two

blades assembled together and fused at

ends form resistor support cross.' The

notches are spaced 1 mm apart.

contaminants adhering to or imbedded in

the surface of the silica glass. They

are then washed several times in distilled water, washed in ethyl alcohol,

and allowed to air-dry.

After cleaning, the longitudinally

slotted blades (figure 1) are carefully

fitted together to form a cross. The

assembled blades are held in a four-jaw

pin vise, and one end of the cross is

fused using an oxy-hydrogen flame. The cross is then inverted in the vise and

the other end is fused. All edges,

notches and holes are fire polished to a smooth glazed finish to allow abrasionfree movement of the platinum wire. The

diameter of the finished cross is such

that it fits nicely inside the quartz

protecting sheath, ensuring that the

resistor wires are held close to the

sheath. The notches in the side of the

cross, spaced one millimeter apart, allow

the wire to be wound in a double helix

with a pitch of 2.0 mm. After assembly,

the cross is again etched in the 5% HP

solution and rinsed in distilled water

and ethyl alcohol. It is then fired at

1100 °C for 1 hour in flowing O2.

4. Resistor and Guard Ring Construction

Resistor fabrication begins with

preparation of the platinum resistor

wire. A 510-mm length of reference grade

wire, 0.13 mm in diameter, is washed in

tr ichloroethylene, wiped with a clean

tissue, then washed in acetone and again

wiped with a clean tissue. This removes

any surface lubricant remaining from the

drawing process during manufacture. The

wire is then annealed in air at approximately 750 °C by passing an alternating
electric current through it for five

minutes.

The wire is next cut into two equal

lengths, and a suitable weight (approximately 2 g) is attached to one end of

each of the lengths. The cross is

mounted in a special coil winder (see

below) and the free end of each wire

threaded through appropriate holes in one end of the cross with the weighted end

hanging free. The two coils are wound

simultaneously, and upon completion the

weights are removed and the ends inserted

into holes in the opposite end of the cross. Excess platinum is removed by

flame cutting, and the ends of the coils

are anchored by fusing the wire ends into

small balls that cannot pull back through

the holes. Additional short lengths of

reference-grade platinum wire (the same wire as used for the resistor) are welded

to the upper ends of the resistor to form

a four-lead resistor (figure 2). These

lead branches are anchored in the other

holes in a similar manner to the resistor

wires, and all four leads are extended

beyond the end of the cross with larger

diameter reference-grade wire for later

joining to the leads. A short piece of

resistor wire is used to join the bottom

ends of the coils of the resistor to

complete the circuit (figure 3).

The coil winder, designed at NBS by

L. A. Guildner, facilitates winding of

the platinum resistor wire directly onto

the cross under the tension of two-gram

weight. The cross is supported at both

its ends in the winder. At one end the

support is provided by a four-jaw plati- num chuck, which also serves as a round

mandrel on which the resistor is formed.

As the cross is rotated in the winder,

and as winding progresses, the chuck

withdraws from the cross so that only two

formed turns are in contact with the

chuck at any one time. The winder produces near-perfect round coil turns and

places them precisely in the notches.

The guard rings are made of annealed

reference-grade platinum wire 0.3 mm in

diameter. They are formed on a glass

forming tool (figure 4a) in a clover-leaf

pattern (figure 4b). Each ring consists

of a single full pattern as shown.

Figure 2. Two views of upper end of

resistor showing connection of lead

branch to resistor wire and method of

anchoring wire.

The resistor and guard rings are cleaned in high-purity 2 N HCl for four

hours at 50 '-'C, rinsed several times in

fresh distilled H2O and then in highpurity 6 N HNO3 for four hours at 95 °C.

They are then agitated several times in

fresh distilled H2O, boiled in distilled

H2O for four hours, rinsed in ethyl alcohol, and finally air-dried.

5. Preparation of Lead Insulating Tubes

and Protection Sheath

The lead insulating tubes and the

protection sheath require special preparation procedures because of their dimensions. The lead insulating tubes are made of fused quartz 1.5 mm diameter X 0.5 mm bore x 760 mm long. To

minimize heat losses caused by radiation

piping, the outer surface of the tubes is
•««(% <««% «% ilil^

Figure 3. Lower end of resistor showing

platinum link that joins the two coils to

form a continuous resistor circuit.

80 mm ^

(a)

(b)

GUARD RING FORMING TOOL

PLATINUM GUARD RING

Figure 4. Guard ring and forming tool.

roughened by blasting with AI2O3 powder

over a length of 500 mm, leaving about

10 mm at the bottom end clear. The protection sheath is made from fused quartz

tubing 7.25 mm diam. x 5.00 mm bore x 800 mm long, closed round at one end.

Since both types of tubes are too long to

be cleaned conveniently by ultrasonic

agitation, two alternative methods have

been developed.

In the first method, the lead insulating tubes are placed in a burette-type

tube clamped in a vertical position. To

prevent the tubes from falling into the

stop-cock hole, a small perforated glass

disc is placed at the bottom of the burette. The following cleaning steps

are then taken:

(i) The burette is filled with a 5% HF solution to cover the tubes. Air

bubbles are displaced from the tubes by

heating the burette with a hot-air gun

and tapping the side of the burette. The

tubes are allowed to etch for several

minutes.

(ii) The HF solution is drained

off and the top of the burette is connected to a cold-water tap with flexible

tubing. The system is thoroughly flushed

with the stop-cock open. Tap water is

used in this step for convenience.

(iii) The burette is allowed to

drain and the tubes are rinsed with distilled water by filling and draining the

burette. The rinsing is repeated several

t ime s

.

(iv) The tubes are rinsed with

ethyl alcohol, then removed from the

burette and allowed to air-dry.

The second method requires the use

of a "peristaltic" type pump. The lead

insulating tubes are placed in a horizontally-mounted glass tube about 1 cm in

diameter and several cm longer than the

lead tubes. The glass tube is connected

to the pump (and to liquid reservoirs)

with flexible plastic tubing that will

withstand the acids and solvents to be

circulated through it. Liquids are

forced through the tube, in the sequence

described above, with enough pressure to ensure that they flow through the small

bores of the lead tubes; the flowing is

easy to detect by observing the movement

of small air bubbles in the bores.

The inside of the protection sheath

is chemically cleaned in a 5% HF solution

for several minutes, and it is then

rinsed with tap water, with distilled H2O

several times, and with ethyl alcohol.

The sheath may also be cleaned by an adaptation of the above pump method,

either before it is closed at one end, or afterward, or both.

After the lead insulating tubes have

been dried, the upper end of each tube is

fire polished. The purpose of this is to

provide a smooth entrance and exit for

the lead wires as they expand and contract. By directing a small oxy-hydrogen

flame directly into the bore, the inner
edge is polished without shrinking the

bore of the tubeAs a final preparation step, both

the lead insulating tubes and the sheath are fired at 1100 °C in flowing Oj for one hour. Oxygen is introduced into the

closed-end sheath through a small diameter quartz tube extending nearly to the

bottom of the sheath. This firing is

intended to oxidize any remaining impurities and to flush away the volatile

oxides,

6. Lead Construction

The internal leads are made from

0.25-mm diameter unannealed referencegrade platinum wire. Four wires, each

800 mm long, are cleaned with trichloroe thylene-soaked tissue then acetonesoaked tissue, straightened by stretching

gently, and inserted into the cleaned

lead insulating tubes, A 10-mm length of

0,3-mm reference-grade platinum wire is

welded to the bottom end of each platinum

lead, the weld bead being small enough to

fit nicely inside the bore of the tube.

The small bead is pulled about 2 mm into

the tube, and then the end of the tube is

heated enough to fuse the quartz, but not

enough to melt the platinum. This con- stricts the end of the tube bore so that

the small bead cannot be pulled back out.

The four lead tubes are held in

place by the platinum guard rings so that

the tubes cannot touch each other directly nor touch the wall of the protecting sheath. Three of the clover-leaf

shaped rings are spaced closely together

at the resistor end to provide axial

radiation shielding and radial thermal

coupling of the lead tubes to the sheath.

Three additional single rings are spaced

along the tubes at about 200, 400, and

600 mm from the top of the resistor. The

rings are all connected electrically by a wire welded to them. This fifth guard

lead is made of the same cleaned reference-grade platinum wire as is used for

the thermometer leads. It is uninsulated, and it exits from the thermometer

at the borosilicate glass header. The

fifth wire can be connected to an active

electric guard circuit in the external

measuring system.

The lead assembly is mechanically

terminated at the resistor end with a fused quartz spacer disk, 4.8 mm diam and

1.0 mm thick, containing four equally

spaced holes. The four thermometer lead

wires can protrude through the holes, but

the lead tubes cannot. The protruding

ends of the lead wires are fused into

balls, locking the leads and the disk

together (figure 2). A four-bore glass

spacer is placed temporarily over the

upper ends of the leads, and the lead

wires are welded together at their upper

end to form a loop. This loop, later

removed, is used to suspend the lead

assembly in a water refluxing washer.

7. Assembly of the Resistance

Thermometer

The resistor is joined to the lead

assembly by fusing its lead extensions to

the balled ends of the leads. The assem- bly is then suspended in the water refluxing unit and washed for a minimum of

100 hours. The purpose of this step is

to remove unwanted materials inadvertently introduced during assembly. After

washing, the resistor and lead assembly

is placed in a clean glass drying tube

and dried in flowing O2 in a furnace at

300 °C.

The thermometer is next inserted

into the cleaned protection sheath, and

the temporary glass disk is removed.

The thermometer is positioned in the

sheath so that there is a gap of a few

millimeters between the end of the resistor and bottom of the sheath, and so that

the upper end of the lead tubes protrude

slightly out the sheath. The lead tubes

are then fused to the end of the sheath,

or to a small quartz ring the same diameter as the sheath. Care is taken to

space the lead tubes equally around the

sheath or ring, and to avoid constricting

the bore of the lead tubes. The result

of this procedure is that the lead tubes

and sheath become a connected fused

quartz structure, and that the platinum

leads are fixed relative to the structure

at the resistor end of the leads, permitting little, if any, movement of the

resistor upon expansion and contraction

of the lead wires.
The expansion of the leads is accom- modated by platinum expansion coils located at the upper end of the thermometer. The four expansion coils are made

by close winding 8-1/2 turns of 0.25-mm

diameter platinum wire on a 4-mm diameter

mandrel, leaving a straight tail 40 mm

long on one end of the coil. The platinum need not be reference grade, since

the expansion coils remain at room temperature, and it is advantageous to keep

the coils in the unannealed, hard drawn

state. The coils are stretched to a length of 25 mm, and they are then welded

to the trimmed platinum thermometer leads as close to the lead tubes as

practicable.

32

MAIL. - BOROSILICATE GLASS

DIMENSIONS IN mm

Figure 5. Body of glass thermometer

header. The small end of the header is

joined to the thermometer protection

sheath.

The expansion coils are housed in a borosilicate glass header (figure 5),

which is attached to the upper end of the

protection sheath with low vapor-pressure

epoxy resin. A cross-shaped glass divider (figure 6) is placed inside the

header to keep the coils separated. The

coil tails are threaded through holes in

the borosilicate glass header cap (figure

7) where they are sealed with the same

epoxy, and a glass tubulation on the cap

serves as an evacuating and filling tube.

The guard lead emerges from the thermometer through the epoxy seal between the

header and sheath.

FUSED AT ENDS ONLY

Figure 6. Cross-shaped divider. The

divider, made from clear fused quartz

microscope slides, is inserted into the

header to keep the platinum expansion

coils separated.

organic material, the system is flushed

and filled to atmospheric pressure with

O2 and the furnace temperature raised to

1100 °C for 20 minutes. The temperature

is then reduced to 800 °C and the thermometer evacuated. The furnace is maintained at this temperature and the

pumping continued until the pressure

becomes less than 5 x 10~° Pa, but for at

least 50 hours. The temperature is then

reduced to ambient, and the thermometer

is filled to a pressure of 40 kPa with

pure argon containing 10% O2 and sealed

at the constriction in the glass tube.

Outside the glass header the four

platinum thermometer leads are trimmed to

the same length and then welded to copper

CONSTRICTION

,1 - Fh J^_ , »f 5

5 15

'- .v

80

^\

MATL. - BOROSILICATE GLASS

DIMENSIONS IN mm

The assembled thermometer is now placed in a horizontal bake-out furnace

and attached to a suitable oil-free

vacuum system via the glass tube (see

figure 7). To oxidize any remaining

Figure 7. Header cap. The header cap is

inserted into the large end of the header

body and sealed with epoxy. Thermometer

leads exit through the four holes. The

tube is used for evacuating, filling, and

sealing the thermometer.
Figure 8. Resistor end of completed

thermometer. Oil has been placed on the

roughened protection sheath so that the

guard rings may be seen in the

photograph.

extension leads using a plasma needle arc welder. The leads are insulated with

PTFE tubing, and the four platinum-copper

junctions are clamped together with a heat shrinkable plastic tubing to minimize thermal emfs. A copper lead extension is also welded to the guard lead,

and all five copper extensions are then

soft-soldered to a multi-contact electrical connector. A thin-wall aluminum head

(20 mm dia. x 125 mm long) , attached to

the sheath at its upper end with silicone

rubber, completes the construction. This

protects the glass header and supports

the connector. As a final step, the

outside of the sheath is roughened by

blasting with AI2O3 powder for 500 mm

above the resistor position. The resistor end of the completed thermometer is

shown in figure 8.

The author wishes to thank the Executive of the Commonwealth Scientific

and Industrial Research Organization for

granting him a study award, the National

Bureau of Standards for guest worker

status and hospitality during the tenure

of this award, and Dr. J. F. Schooley,

whose efforts made it all possible. Special thanks are due to J. P. Evans, L. A. Guildner, S. Tillett and R. E. Edsinger

for their invaluable assistance with the

project, and to all the staff at NBS for

the friendship and courtesy they extended

to the author during the period of his

visit.

8. References

[1] Meyers, C. H. Coiled filament re- sistance thermometers. J. Res. Nat.

Bur. Stand. (U.S.). 9 : 807-813;

1932 December.

[2] Cutkosky, R. D. Guarding techniques

for resistance thermometers. IEEE

Trans. Instrum. Meas. IM-30 (3) : 217-220; 1981 September.

[3] Evans, J. P. Experiences with high

temperature platinum resistance

thermometers, chapter in Temperature, its measurement and control in

science and industry. Vol. 5. J. F. Schooley, ed. New York: American

Institute of Physics; 1982. 771-781.

Toroidal Resistor for High Temperature Platinum

Resistance Thermometers

J. P. Evans and S. B. Tillett

National Bureau of Standards

Washington, D. C. 20234

A new type of resistor for high temperature platinum resistance thermometers, called the "toroidal" resistor, is described. It is designed to be

easy to make from readily available materials, and it features robustness,

small size, and freedom from strain. Limited experience indicates that the

resistor performance meets these design expectations, and that the resistor

may be a useful alternative for high temperature thermometers.

Key words: Construction; design alternatives; high temperature; platinum

resistance thermometer; practical temperature scale; resistor; silica glass;

stability

1. Introduction

We have designed a new type of thermometer resistor as part of a continuing

effort to develop high temperature plati- num resistance thermometry. Our purpose

has been to explore resistor design alternatives that can foster and enhance

the use of such thermometry to define a practical temperature scale.

The use of a platinum resistance

thermometer as a scale defining instrument at high temperatures imposes certain

requirements on the temperature sensing

platinum resistor. The platinum must be

very pure and it must be in a "strain

free" condition at all temperatures. It must be capable of reaching the temperature of the environment to be measured as closely as possible. It must be protected

from chemical change and permanent dimensional change. And it must be configured

to permit precise determination of its

electrical resistance.

In designing a thermometer resistor one attempts to meet the requirements

cited above. The customary design approach is to use platinum in the form of

wire, to support the wire on an electrically insulating structure of metal

oxide, to equip the resulting wire-wound

resistor with four lead terminations, to

encapsulate the resistor in a closefitting envelope, and to fill the envelope with a benign gas (containing some oxygen). The resistor design must take

into account the properties of the materials used, such as potential sources of

contamination for the platinum and the

electrical characteristics of the insulators at high temperatures. Ease and cost

of construction and use must also be

considered. The various requirements are

not always compatible, and design details

are therefore often the result of compromises between conflicting requirements.

A number of resistor designs have

evolved from efforts to embody the con- ceptual requirements into useful high

temperature platinum resistance thermometers. Most of the designs incorporate

circular coils of platinum wire in one form or another. Tightly wound coils

have been supported on notched cruciform

cadres (in helical spirals), on twisted

silica-glass ribbons, and around small

insulating tubes. In other designs,

single-layered helices of wire have been

mounted on notched crosses, on grooved

rods, or, if the wire diameter is large

enough, in virtually unsupported free

suspension. A recent design has a

s i ng 1 e - 1 ay e r e d bifilar helix of

relatively heavy platinum wire supported on a flat notched blade [1, 2]. In
another design, the "birdcage" resistor

[3], the platinum wires are kept straight

and parallel to the axis of the resistor

over most of their length.

In this paper we describe the design

details and construction of the new re- sistor, which we call the "toroidal"

resistor because of its shape. Our goal

has been to design a resistor that is

relatively easy to make from readily

available material. Design compromises

have favored robustness, small size, and

freedom from strain at the possible ex- pense of optimum thermal communication

between the resistor wires and their

surroundings. Limited experience with

the resistor indicates that it does indeed meet the design objectives, but

considerably more experience would be

needed to evaluate fully its efficacy in

a variety of temperature measuring

situations.

2. Design Approach

The toroidal resistor is shown in

figure 1. It consists of a silica-glass

former on which is wound the platinum

resistor wire. The former is a piece of

tubing slotted at both ends, and the

resistor wire, wound in and out of and

around the tube, is held in position by

the slots. The resulting coil is an

elongated, flattened toroid -- hence the

resistor's name. A four-hole silica

glass disk fused to one end of the tube serves to support and separate the four

resistor lead branches.

The use of silica-glass tubing for

the former has considerable advantage.

High-purity silica glass is readily

available as clear fused quartz or as synthetic fused silica, and it is relatively inexpensive. Its electrical,

thermal, and mechanical properties are well suited for high temperature resistance thermometers. The material can be

cut and ground by conventional techniques, it is easily flame-worked, and it can be cleaned by simple treatment.

Silica-glass tubing is manufactured in a wide range of sizes; suitable diameters

can be obtained from stock without

resorting to special fabrication, and

Figure 1. The toroidal resistor (supported on a rod for illustration). The

silica-glass former is 15 mm long and

4-mm diameter. The resistor wire is

0.25-mm diameter. Protruding from the

four-hole silica-glass disk at the top of

the resistor are balls of platinum that

secure the four lead branches in place.

formers of any practicable length can be

cut from ordinary production tubing.

The tools needed for construction of

the toroidal resistor are often found in

a well equipped laboratory. All cutting

of the silica glass, including the end

slots, can be done with a type of lowspeed diamond saw commonly used for the

preparation of metallurgical specimens.

Disks cut from silica-glass rods can be

drilled by means of an air-propelled

abrasive device that is also useful in

other stages of thermometer construction.

Alternatively, disks can be cut from

four-bore silica-glass tubing of the

proper diameter, a product that is some- times available commercially. Flame

working of the glass and welding of the

platinum wire are easily accomplished

with miniature oxy-hydrogen or oxy-gas

torches. The platinum wire is wound on the former by hand; only small

electronic-type hand tools are required

for final adjustment and positioning of

the wires. Finally, a small ultrasonic

bath provides a convenient and effective means of cleaning the various parts.

The resistor shown in figure 1 is

only about 15 mm long. It is wound with

platinum wire 0.25-mm diameter, and it

has nominal resistance at C of

10
0.37 ohm. The resistor former is made of

4.0-mm diameter tubing; the eight

"fingers" at each end, formed by the

slots, are flared out uniformly so that

the former fits nicely in the center of a protecting sheath (the envelope) having

an inner diameter of 5.0 mm. Since there

is enough radial clearance between the

former and the sheath wall, and since end

slots are wide enough and flame polished,

the resistor wire is free to move without

constraint (if the resistor axis is vertical) as it expands and contracts upon

temperature change, and the only tension on the resistor wire is due to its own

mass. Furthermore, expansion and con- traction of the wire are not likely to

alter permanently the resistor wire

geometry in any way. These features

constitute the main performance merits of

the resistor design.

3. Preparation of Silica-glass Parts

Preparation of the silica-glass

parts begins with the selection of tubing

and rod. The former tubing is selected

to be round and straight, and to be free

of cracks, striations, air lines, or other flaws that might weaken the tubing

and cause it to break during the fabrication steps. Inner and outer diameters

are not critical so long as the finished

former will fit inside the protecting

sheath with sufficient clearance on its

outside and enough room on its inside to

accommodate the resistor wires. The disk

rod or four-bore tubing must be small

enough in diameter to fit inside the

protecting sheath but large enough to be

attachable to the former. The former for

the resistor shown in figure 1 utilizes

nominally 4.0-mm o.d. x 2.0-mm i.d. clear

fused quartz tubing selected from the

manufacturer's standard tolerance production. The four-hole disk is cut from

four-bore silica glass tubing ground to a diameter of 4.8 mm.

The next step is to support the

silica-glass stock for sawing. This is

done by "potting" or "blocking" the

tubing or rod in a larger supporting

glass tube with a suitable compound in

order to minimize breaking, cracking, or chipping of the fragile parts. Wax and

shellac based products are commercially

available for this purpose. We have used

a hard wax compound made of rosin, beeswax, and carnauba wax (in the weight

ratio 16:2:1); it melts at about 150 °C

and it is easy to remove with solvents.

The glass parts are potted either by

immersing them in a container of molten

wax, or by filling the support tube with

molten wax. After the potting wax

hardens, the material on the outside of

the support tube is removed by scraping

and wiping. Several disk rods or tubes

may be potted in the same large size

support tube for efficient production,

but each piece of former tubing must be

supported by its own close-fitting glass

support tube (see figure 2). This is

because the former, after it has been cut

to length, must be slotted at each end

precisely on diameters, a step more

easily accomplished if the axes of the

former tubing and the support tube

coincide. The 4.0-mm former tubing used

here fits nicely inside selected 1/4-inch

standard wall borosilicate-glass tubing.

Tube lengths of about 50 mm are convenient for potting and cutting into the

finished former length.

All glass cutting is done with a thin circular diamond saw, operated at

low speed, and cooled and lubricated with

distilled water. The width of the saw

blade is determined by the desired width

of the former end slots, which in turn is

Figure 2. Silica-glass former tubing

potted in close-fitting borosilicate

glass holding tube with hard wax

compound.

11
Figure 3. Silica-glass former in holding

tube mounted on saw for cutting end

slots. The circular disk is an indexing

device.

related to size of the resistor wire.

The saw used here produces a kerf of

about 0.3 mm. Disks are cut by slicing

thin sections from the potted multi-bore

tubing or rod perpendicular to its axis.

The disks should be thick enough to avoid

distortion when they are fused to the

former end -- a thickness of 1.0 mm is

about right. The former tube is cut to

length (15 mm for the resistor shown in

figure 1), and it is then mounted in a holding fixture to cut the end slots (see

figure 3). The fixture must be capable

of producing centered, uniformly-indexed,

controlled-depth slots. Each end of the

resistor has eight peripheral slots

2.5 mm deep produced by four cuts.

After the glass parts have been cut,

they must be removed from their temporary

supports, freed of potting wax, and

thoroughly cleaned. The disks may be

separated by simply dissolving away or melting away the supporting wax. The

former should be removed by melting the

wax, carefully separating the former from

the holding tube, and then draining off

the molten wax. The remaining wax film

is removed from the parts by gentle agitation in appropriate solvents; care

should be taken because the former is

especially fragile at this stage. After

the parts are rinsed in ethyl alcohol and

distilled water to remove the solvents,

they are lightly etched by agitation for

10 minutes in a 5% solution of HF, and

then rinsed again in distilled water.

The etching is intended to remove surface

contaminants, but it also seems to lessen

fragility of the parts, probably by re- lieving strains at sharp corners and

microscopic cracks. The parts are airdried in preparation for flame polishing,

and they are not touched again by bare

hands until that step has been completed.

Flame polishing of the glass parts

produces smooth, glazed edges, and it

relieves any strains remaining from the

cutting operation. The polishing should

be carried out using a torch flame only

big enough to glaze edges and fuse corners without distorting the part, and it

should be observed with enough optical

magnification to assure the desired result. The former is conveniently supported on a holder made of silica-glass

tubing or rod clamped in a maneuverable

vise (see figure 4). During flame

working of the former slots, the eight

"fingers" formed by the slots must be

flared out uniformly to the diameter of

the inside of the protecting sheath.

This can be done by tilting the former

slightly and applying the torch flame to

the proper spot near the base of the

finger. The procedure is not difficult,

but it does require a little practice.

When all fingers have been so worked, and

the flare appears to be symmetric, the

former should be gaged for proper fit in

Figure 4. Silica-glass former on holding

jig for flame working. Some of the

"fingers" have been flame polished and

flared.

12
Figure 5. Stainless steel hole locating

jig (supported on a rod for illustration). A silica-glass disk is clamped

between the two halves of the jig, and

holes are drilled with abrasive powder

propelled by air from a nozzle inserted

into the locating holes in the jig. The

rectangular openings are exhaust ports

for the spent abrasive.

the sheath. The four-hole disk may be

flame polished at this stage, or it may

be polished later when the disk is

attached to the former.

The disk used in the resistor shown

in figure 1 was made from commercially

available four-bore tubing, but a fourhole disk may be made in other ways.

Disks cut from silica-glass rod of the

correct diameter can be drilled by air

propelled abrasive powder (e.g. aluminum

oxide) with the aid of a simple holelocating jig (see figure 5). The holes

may be somewhat tapered, but they serve

the intended purpose adequately. Of

course, a disk so drilled would have to

cleaned after the drilling. Disks may

also be drilled by ultrasonic cavitation

or by laser, and commercial establishments can provide these services.

4. Assembly of the Resistor

The assembly of the prepared parts

into a finished resistor is a hand operation involving only the use of hand

tools, holding jigs, and the torch.

Handling of the parts is kept to a minimum, but it is not possible to eliminate all contact between the parts and

hands or tools. If an operation has the

potential for contaminating the silica

glass or platinum, it is conducted so that the contamination may be removed

after assembly.

The platinum wire is made ready for

assembly by cleaning its surface with

solvents and annealing it electrically

until it is soft enough to work easily.

Two different wire sizes may be used -- one diameter for the resistor and another

for the lead connections; or the same

size wire may be used for both purposes.

In the resistor shown, 0.25-mm diameter

wire is used for both the resistor and

the lead connections. In any case, all

of the wire must be reference grade of

the highest available purity.

The first step is to install plati- num lead connections in the four-hole

disk (see figure 6). Two platinum "U's" are fashioned from the lead connection

wire to fit into disk holes by pairs. At

the middle of each "U" bottom is welded a piece of resistor wire several millimeters longer than the length of the

former. Both "U's" are then inserted

fully into the holes from the same side

of the disk, and the ends of the platinum

wire on the other side of the disk are trimmed to equal lengths. The trimmed

ends are then fused with the torch to

form a platinum ball on each of the four

lead connections that cannot be pulled

back through the hole. The lead

Figure 6. Lead connection branches before (left) and after installation in

four-hole silica-glass disk. The

branches are secured to the disk by the

fused platinum balls. The wires ex- tending down from the branches are part

of the resistor wire.

13
connections are thereby locked onto the

disk and can move very little with respect to it. The length of platinum wire

required to form the balls depends on wire and hole size; it is readily determined by trial.

Next the four-hole disk is attached

to the former. The former is positioned

vertically in a holding fixture, and the

disk is placed on top of it with one resistor wire running down through the

center of the former and the other wire

extending out through one of the slots at

the top. The tip of the "fingers" are then fused to the disk one at a time with

the torch, taking care not to melt the

platinum, not to close the slots, and to

keep the structure undistorted and symmetric. Sometimes it is necessary to

pull a finger, when it is hot enough to

flow, into contact with the disk with a small silica-glass puller rod. After all

of the fingers are fused to the disk, the

former is again gaged to make sure that

it will fit properly in the sheath.

The former is now ready to accept

the resistor winding. To one end of an appropriate length of cleaned resistor

wire is welded a piece of scrap platinum

wire, later removed, that serves as a "leader". The other end of the resistor

wire is welded to the wire emerging from

the lead connections through a slot at

the top of the former. This must be a butt weld, and, in order to avoid problems such as shorting or binding, the

weld diameter must be not much larger

than the wire diameter. The leader is

threaded up the center of the former and

out through an adjacent upper slot creating a loop around the former wall. The

wire is pulled until the loop bottom is

positioned in the proper slot at the

bottom of the former. The loop is then

flattened, straightened, and adjusted

with tweezers or other suitable tools

until the wires lie flat against the

inner and outer walls of the former. The

sharp bend at the bottom of the loop must

not touch the former, but also the loop

must not be so long that it will come out

of the notch upon maximum expected thermal expansion. At the top of the former

the resistor wire is bent down sharply to

start the next loop, and the process is

continued around the former until all

slots have been filled. The leader is

removed, and the resistor wire is welded

to the other piece of wire extending down

from the lead connections through the

center of the former. A final adjustment

of all wires to make sure that they cannot touch each other completes the resistor assembly.

As a final step, the resistor must

be cleaned. It is first agitated in

ethyl alcohol and distilled water to remove surface soils that accumulated

during assembly, and then it is etched by

agitation for 10 minutes in a 5% HF solution. This etching accomplishes two

important things: (1) it removes from the

surface of the platinum wire finely divided silicon dioxide that is vaporized

onto the wire during the fusing of the

silica-glass, and (2) it removes a minute

layer of silica from the glass parts and

along with it any remaining surface contaminants. The cleaning is completed by

thorough rinsing in distilled water and

an alcohol rinse to aid drying.

5. Evaluation of the Resistor

Toroidal resistors have been incorporated in several high temperature

resistance thermometers. These thermometers employ guarded leads and they are sheathed in long silica-glass tubes

7.25-mm o.d. x 5.00-mm i.d. The assembly

of thermometers of this type is reported

elsewhere [4]

.

Three thermometers were built with

the 0.37-ohm resistor described here and

tested at high temperature. All three

proved to be just as stable during a 500-hour exposure at 1100 °C as any other

thermometers made at NBS, and they re- tained values for the alpha coefficient

greater than 0.0039269 K~^, indicating

that the resistor platinum remained pure

and strain free during and after exposure

to the high temperature. Measurements at

thermometric fixed points show that the

thermometers exhibit the same resistancetemperature relation as other thermometers, and that they can be used to make

precise temperature measurements. In

contrast, the self-heating of the

toroidal resistor due to its measuring

14
current is somewhat greater than for some other resistors, and immersion characteristics of thermometers using the

toroidal resistor may be somewhat less

favorable than those of thermometers

using some other types of resistors.

Two other thermometers were built

with resistors having formers 30 mm long.

One resistor was wound with 0.125-mm

diameter wire, giving a resistance at

°C of about 2.90 ohms, and the other

resistor was wound with 0.25-mm diameter

wire, giving a resistance of about

0.71 ohms. The resistor made with the

heavier wire was nearly as stable as the ones described above upon exposure for

several hundred hours at 1100 °C, and its

alpha value remained greater than

0.0039272 K" The resistor made with

the smaller wire, however, exhibited a continuing increase in resistance during

the same exposure, though its alpha value

stayed relatively constant at

0.0039257 K-1

The experience cited above is con- sistent with design expectations. The

toroidal resistor appears to be "strain

free" when used in a vertical orientation

(as was done), and there has been no evidence of a tendency for the resistor

turns to become distorted or shorted

together. The continuing resistance in- crease observed in the resistor made with

fine wire has been attributed to creep of

the platinum wire at high temperature;

evidently resistor stability is favored

by the use of a short former and large

diameter wire. The greater heating effect and increased required depth of

immersion are to be expected since half

of the resistor wire is remote from the

wall of the thermometer sheath. However,

there is partial compensation because its

short length lets the resistor be immersed farther into the measured

environment.

This preliminary evaluation of the

toroidal resistor has established the

simplicity and flexibility of its design

and some of its favorable performance

characteristics. These features make it an attractive possible alternative for

use in precise high temperature platinum

resistance thermometers. However, a com- plete evaluation of the resistor will

require additional work. It should be

tested at high temperatures in a horizontal orientation, its intrinsic electrical

leakage across the former should be determined, investigations should be made

of its reactance and electrical coupling

to its environment, and more should be

learned about its immersion characteristics. An important design enhancement

would be the inclusion of means for electrical guarding. When such work is done,

thermometer makers desiring to construct

resistors by hand with simple tools will

have a solid basis for assessing the

merits of the toroidal resistor.

6. References

[1] Li Xumo; Zhang Jinde; Su Jinrong;

Chen Deming. A new high-temperature

platinum resistance thermometer.

Metrologia 18(4): 203-208; 1982

December.

[2] Long Guang; Tao Hongtu. Stability

of precision high temperature

platinum resistance thermometers,

chapter in Temperature, its measure- ment and control in science and

industry. Vol. 5. J. F. Schooley,

ed. New York: American Institute of

Physics; 1982. 783-787.

[3] Evans, J. P.; Burns, G. W. A study

of the stability of high temperature

platinum resistance thermometers,

chapter 34 in Temperature, its mea- surement and control in science and

industry. Vol. 3, part 1. C. M. Herzfeld, e d

. - i n-c h i e f ; F. G.

Brickwedde, ed. New York: Reinhold

Publishing Corporation; 1962.

313-318.

[4] Bass, N. Construction of the NBSdesign high-temperature platinum

resistance thermometer, part 1 of

Techniques in high temperature re- sistance thermometry. Nat. Bur.

Stand. (U.S.) Tech. Note 1183; 1*984 January.

15
NBS-n4A REV. 2-8C1 U.S. DEPT. OF COMM. BIBLIOGRAPHIC DATA

SHEET (See instructions)

1. PUBLICATION OR

REPORT NO. NBS TN 1183

2. Performing Organ. Report No 3. Publ ication Date January 1984

4. TITLE AND SUBTITLE

Techniques in h ig h- t empe r a tu r e resistance thermometry:

1. Construction of the NBS-design high-temperature platinum

resistance thermometer. 2. Toroidal resistor for hightemperature platinum resistance thermometers.

5. AUTHOR(S)

(1) N. Bass; (2) J. P. Evans and S. B. Tillett

6. PERFORMING ORGANIZATION (I f joint or other than N B5, see instructions;

NATIONAL BUREAU OF STANDARDS

DEPARTMENT OF COMMERCE

WASHINGTON, D.C. 20234

7. Contract/Grant No.

8. Type of Report & Period Covered

Final

9. SPONSORING ORGANIZATION NAME AND COMPLETE ADDRESS (Street. City, State, ZIP)

Same as item 6.

10. SUPPLEMENTARY NOTES

|~ Document describes a computer program; SF-185, FlPS Software Summary, is attached.

11. ABSTRACT (A 200-word or less factual summary of most significant information. If document includes a si gnificant

bi bl iography or ! iteratu re surve/, mention it here) This Technical Note consists of two papers dealing with design and

construction aspects of high-temperature platinum resistance therm.ometers intended for use as standard interpolating instruments up

to the gold point (1064 °C). Performance experience with the thermometers has been or will be presented elsewhere.

The first paper describes the construction of a thermometer utilizing a resistor of reference-grade platinum wire wound in a single-layer, bifilar helix. The supports and insulators are made

of high-purity fused silica, and the thermometer incorporates an

electrical guard system to minimize the effects of electrical

leakage.

The second paper describes a new type of resistor for high-temperature platinum resistance thermometers, the "toroidal" resistor. The

resistor is designed to be easy to make from readily available

materials, and it features robustness, small size, and freedom from

strain.

12. KEY WORDS (Six to twelve entries; alphabeti cal order; capitalize only proper names; and separate key words by semicolon Sj Construction; design alternatives; electrical guarding; gold point; high temperature; IPTS; platinum resistance thermometer; practical temperature scale; resistor; silica glass; stability; standard

Interpol at i ng i n<:;1-rnmp-n4-

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