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NBS rECHNICAL NOTE

483

Construction and Operation of a

Simple High-Precision Copper-Point

Blackbody and Furnace

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U.S. DEPARTMENT OF COMMERCE

National Bureau of Standards

UNITED STATES DEPARTMENT OF COMMERCE

Maurice H. Stans, Secretary

NATIONAL BUREAU OF STANDARDS • A. V. Astin, Director

NBS TECHNICAL NOTE 483

ISSUED MAY 1969

Construction and Operation of a Simple High-Precision Copper-Point

Blackbody ami Furnace

R. D. Lee

Heat Division

Institute for Basic Standards

National Bureau of Standards

Washington, D.C. 20234

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.

For sale by the Superintendent of Documents, Government Printing Office Washington, D.C, 20402 - Price 30 cents

Construction and Operation of a Simple High-Precision

Copper-Point Blackbody and Furnace

R. D. Lee

Detailed instructions for the construction and operation

of an inexpensive copper-point blackbody and furnace are presented. Such a source may he expected to realize the radiance

temperature of 1083.3°C (int. 1+8) with an uncertainty of 0.2°C

and a variation in successive use of less than 0.033°C. The

furnace requires no inert gas or vacuum and utilizes only 225

watts to reach a melt in about an hour after turn on. Melts

and freezes last about five minutes each and at least fifty

melts and freezes are expected during the lifetime of the

furnace

.

Key -words: Blackbody; optical pyrometry; radiometry.

1. Introduction

Copper-point blackbody furnaces have been made which provide sources

of radiance temperature having an uncertainty of 0.2°C at 1083. 3°C (Int.

19^-8). They were designed for use in monitoring the long term stability

of the low range of automatic and visual optical pyrometers but should

also have extensive use in radiometry. Emphasis has been placed on simplicity of design and construction, and the use of inexpensive materials

and components

.

The purpose of this note is to present such detail for the construction and assembly of such a blackbody and furnace that a minimum of time

and thought is necessary to complete the work. One inexperienced in this

type of work should have little difficulty. Even the required machining

has been simplified so that an amateur machinist will do the work adequately. On the other hand, it is believed that the expert in furnace construction will find that a number of the techniques used will save him time in

the assembly. In addition to the construction, the note presents operating

procedures for the blackbody furnace and discusses its performance.

2. Construction

2.1 General Design and Description

Fig. 1 is a cross-section of the copper-point blackbody and furnace.

The inside diameter of the graphite crucible is predetermined by the 1-

inch diameter copper freezing-point standard reference material sample ^5d.

available from the National Bureau of Standards.* The length of the sample

*See NBS Misc. Publ. 260, for sale by the Superintendent of Documents, U.S.

Government Printing Office, Washington, D.C. 20U02, price ^5 cents.

1
is k inches. Of its ^50-gram mass, 200 is sufficient to surround the sight

tube adequately; and consequently the sample is shortened. The sight tube

is no longer than necessary to form an adequate blackbody cavity. This

minimizes the torque on it resulting from the buoyant force of the melted

copper. The cavity has an aperture or target of about 3-5 mm in diameter

and the alumina cone allows a maximum exit cone of over 12 degrees (full

angle)

.

The small projection of the ingot of copper into the back wall of the

crucible lends support to the copper in its solid state. Upon cooling, the

copper will contract concentrically on the sight tube and would otherwise

be wholly supported by it

.

The impervious alumina cone is sealed to the front of the quartz cylinder and the back of the cylinder is also sealed, thus impeding the burning of the graphite. Though air has access to the graphite through the

small aperture of the cone, it has been possible to use such a furnace for over 100 hours (at least 50 freezes) before failure due to burnout.

The furnace heating element is #l6 B. & S. gauge nichrome (80% nickel,

20% chromium) wire. This size wire is easy to bend, yet large enough to

withstand early burnout due to corrosion. The coil has a resistance of

about 5 ohms, making a medium-power variable auto-transformer adequate for

supplying the required 225 watts of power to the furnace. Longer windings

have been tried, but the additional heating caused the cone to expand and

crack the quartz in the vicinity of the front seal. With the shortened

overhang of the winding at the front of the crucible, the radiometric performance is optimized with the overhang at the back of the crucible as shown in Fig. 1.

Quartz wool is chosen for insulation within the cylinder, chiefly

because of its availability in small quantities. Externally the cylinder

is wrapped with l/l6 inch thick high-temperature insulating paper to an outside diameter of k inches. Variations of the amount of wrapping varying from 3 to 5 inches outside diameter were tried with no appreciable

difference in radiometric performance. With k inches the outer surface

is cool enough to be briefly touched by hand. The wrapping is bound with

a small size {#2k B. & S. gauge) nichrome wire.

2.2 Crucible and Sight Tube

Dimensions of the crucible and sight tube are given in Fig. 2. Dimensions not shown are dependent upon fitting with the particular quartz

cylinder and ceramic cone that are purchased. The crucible is machined on

a lathe from a 1-1/2 inch diameter high-purity graphite rod. Use of pure

graphite reduces the possibility of contamination of the copper. Since

the manufacturer claims that the impurities in the graphite do not exceed

20 parts per million, care should be taken not to contaminate the graphite

while working it. In particular, tools and hands should be oil free.

Inside cuts on the crucible and sight tube are made with drills. An

inner diameter of the crucible of 26 mm allows for the expansion of the

2
1-inch (25.^ mm) copper ingot upon heating. If a 26-mm drill is not readily available, the more common 1-Inch drill can be used and the resulting

hole enlarged, or the copper can he machined to a smaller diameter. The

outer diameter of the crucible should be 0.02 inch less than the inner

diameter of the quartz cylinder to allow for the unequal expansion of the

quartz and graphite. The cylinder, nominally 1-3/8 inches inner diameter,

may be slightly tapered, so an average of measurements taken at both ends

should be used as the inner dimension. Because the expansion coefficients

of the various materials, particularly the graphite, are not sufficiently

veil known to achieve a close fit at 1083°C, the allowances made for both

the expansion of the graphite within the quartz and the copper within the

graphite are liberal.

After the outer diameter of the crucible is machined and its hole

drilled, the crucible can be cut off by using a hack saw held against the

turning work. This rough cut can be finished later to the specified length

in Fig. 2. Meanwhile, it is convenient to fabricate the lid from the

graphite remaining in the chuck. It is suggested that the central hole in

the lid be drilled before the lid is machined to fit the crucible. Otherwise, strains relieved on drilling might destroy the fit desired. Graphite

pieces fitted together loosen somewhat under high temperature for long

periods. Therefore, the fit should be quite snug initially.

The sight tube is fabricated from a high-purity 5/8-inch diameter rod

with the inner diameter drilled out before the outer surface is machined.

It is advisable initially to use a drill l/6h inch smaller than the indicated 13/32 inch for most of the cut . The lid should be tried for a snug

fit onto the tube during the final cuts on the outer diameter. While the

back wall of the tube is being machined to a thickness of a little over

1/32 inch, a vernier caliper indexed to 1/128 inch and a low power eye

loupe are helpful in locating the start of the cut inward parallel to the

drill taper. The work is illustrated in Fig. 3. The cut should start

3/128 inches beyond the inner depth. This depth can be found by measuring

the length of the drill (excluding the end taper) and subtracting the protrusion of the shank of the drill when inserted in the sight tube. Parallax should be avoided in using the eye loupe. If there is uncertainty in

locating the start, another 1/128 inch may be added to the outer length.

The work may be cut off by using a fine-tooth hacksaw blade, and the cut

finished by careful grinding or filing.

The graphite sight tube diaphragm is also made from the 5/8" graphite

stock. It is drilled out to about the same size as the small end of the

cone. Without removing the partly finished diaphragm from the lathe, it can be tested for a snug fit as shown in Fig. k. The test should be made

with the diaphragm at about the expected recessed position because the

inner diameter of the tube may have a slight taper.

When the sight tube, diaphragm, and lid of the crucible are assembled,

the diaphragm should first be inserted most of the way. To press the sight

tube into the lid, it should be grasped where it is reinforced by the

diaphragm. Then, with the rim of the sight tube reinforced by the lid, the

diaphragm can be pushed with the cone as far as the cone will go. The

3
cone, abutted against the graphite diaphragm and rim of the sight tube,

forms a seal which reduces the circulation of air.

2.3 Copper Ingot

The copper ingot should approximate the inner configuration of the

crucible to minimize voids. The end of the ingot is tapered to the 26-mm

drill taper at the end of the crucible. However, considering the added

difficulty, it is not worthwhile to machine the small 3/l6-inch configuration to fit into the back wall (Fig. 2). The copper ingot should be

shorter than the depth of the crucible by l/l6 inch to allow for the differences of expansion of the copper and graphite.

A 1/2-inch diameter hole is drilled in the ingot for the sight tube.

The ingot is not likely to be straight, ana in order to center the hole

fairly well the ingot should be positioned in the chuck so that the drill

progresses within the chuck jaws. A depth 1/32 inch more than that re- quired to seat the sight tube provides a safe clearance between the copper

and the fragile end of the tube. A sharp drill is required and a simple

way to insure this is to start with a new one. Cutting oil will ease the

drilling.

The ingot is reversed in the lathe and, as with the crucible, the

excess length can be cut off with a hack saw preparatory to machining the

back taper.

The copper should be washed clean of oil before any checks of clearances of the ingot with the crucible or sight tube are made. As a final

precaution against contamination, the finished copper ingot can be immersed

for a minute in nitric acid diluted to half strength, or until surface

oxides are removed.

2.k Cylinder and Cone

The quartz cylinder has a nominal inner diameter of 35 mm (1-3/8

inches) and an outer diameter of 38 mm (1-1/2 inches). However, cylinders

and also cones, that are available at a reasonable cost do not have close

dimensional tolerances. If the cone obtained fits loosely into the cylinder, no difficulty arises. If the cone is too large, it can be allowed

to protrude from the cylinder by as much as a quarter of an inch without

any detrimental effect on the furnace performance.

The specifications of the cone for this size cylinder are as follows:

Impervious alumina

Taper, 12 degrees

Small end, 1/U inch OD

Large end, 1-3/8 inch OD

Wall thickness, less than l/l6 inch

Tolerances, average production.

Thin-walled cones of the type desired are difficult to make; therefore

the thickness of the wall is not specifically given. Cones received have

h
had apertures of from 3 to h mm diameter, and these were satis factory,

A high temperature cement is required to seal the cone into the cylinder. A cement that hardens "by chemical set will have the necessary

strength and adherence at room temperature. The rims of "both the cone and the quartz cylinder are coated with a mixture (of filler and hinder)

of low consistency, ahout that of heavy cream. The crucible is placed in

the cylinder and used to position and support the small end of the cone while the seal hardens. The relative position of the crucihle and cylinder should he marked so that the crucible can be replaced in the same position later. The time required for hardening may be determined by cementing together test pieces of ceramics at the same time the seal is made.

A clear quartz cylinder allows a visual check of the packing of quartz

wool around the cone (crucihle removed). A metal strip rounded on the end

to ahout the inner diameter of the cylinder will be helpful in packing the

insulation. The packing should be fairly firm. The space is too narrow

to pack closer to the front end than about 1 inch. The small aperture of

the cone should he reamed with a wet artist brush to clear out any shreds

of wool.

2.5 Heater Winding

In forming the heater winding one starts with 2.\ feet of #l6 B. & S. gauge ni chrome wire. This is ahout two feet more than is required. The

wire is straightened by anchoring it at one end and pulling it sharply

such that a slight elongation is felt. A strong rod or a large screw

driver can he used as a crosswise handle for this purpose.

The coil is formed on a metal tube 1-1/8 inches diameter, chucked in

a lathe geared down to its lowest speed (if possihle, to U8 rpm) . The wire

is attached to the tuhe hy hooking into a small hole. It should be wound

with no spacing, letting the wire slip through the hands with enough tension to form a tight coil on the tuhe. A lathe run in reverse will draw

the wire over the top so that the coiling can be watched. The coil when

released will spring open to a diameter somewhat less than 1-1/2 inches.

To facilitate expanding the wire onto the cylinder, a piece of graphite "beveled as shown in Fig. 5 is useful. The coil is fed on, rung hy rung,

while the cylinder is hand rotated.

The coil, in its final position, should overhang the crucihle hy 3 A

inches in the front and 1-1/2 inches in the back. Variations of a quarter

of an inch will not significantly affect furnace performance.

To space the rungs evenly, a soft-solder wire, l/l6 inch in diameter,

is hand-wound "between the nichrome rungs. The coils should be tightened

by twisting with the direction of rotation. After the pliahle solder wire

is removed, the winding should he tightened and any rung that has moved

should he repositioned.
The excess wire can now "be cut off, leaving enough for the leads.

After the lead wire is straightened tangentially to the cylinder and a check is made to insure the alumina protection tubing (inner bore l/l6

inch diam. ) will slip on easily, the wire should he "bent out along the

cylinder toward the ends. After the alimina tubing is slipped on it can be fastened in place, temporarily, with some of the #2U gauge nichrome

wire while each end of the tubing is spot cemented with the high temperature cement. "When the cement is sufficiently dry, the windings should be

brushed with a soapy solution and rinsed in preparation for complete ce- menting.

2.6 Completion of the Furnace

The cement used to cover the winding should be one specified for this

purpose because it is more likely that it will be free of components that

may unduly corrode the wire. A mixture of cement thin enough to flow in

around the wires should be brushed on first; then a mixture thick enough

to stay molded on the winding should be applied either with a brush or spatula. It will be helpful if the open end of the cylinder is slipped

onto a large horizontal rod or wooden dowel and rotated while the cement

is applied. The cement is built up about 1/8 inch from the cylinder surface and overextended on either end of the winding. An uneven surface of

cement can be smoothed with a wide strip of sheet metal, lightly applied

along the length of the cement while the cylinder is rotated. An occa- sional wetting of the strip will prevent the cement from sticking and

pulling away.

Drying can be accelerated by applying 20 to 25 watts of power to the

winding. When the cement is partially dry and can be cut with a knife

without sticking, the ends should be trimmed back evenly leaving about

1/8 inch to overlap the winding. To insure a thorough drying of the ce- ment, the 25 watts is then maintained overnight.

For proper insulation, end sections of the cylinder not covered with

wire or cement should be covered with pieces of insulation paper to a thickness about the same as that of the cement. The seams of the pieces

should abutt the alumina lead tubes . The paper can be bound in place with

small nichrome wire. The entire length of the cylinder may then be wrapped

with insulating paper and the furnace completed with the insertion of the

crucible, packing of the back insulation, and sealing with about a 3/8-

inch thickness of the chemical-setting cement.

3. Operation and Performance

A few accessories are needed in the operation of the furnace besides

a variable auto-transformer. An ammeter is convenient although not essential. The maximum exit cone of the radiation is over 12 degrees and provides a considerable margin for the presently available automatic and

visual pyrometers. Nevertheless, a method for aligning the pyrometer axis

with the axis of the exit cone will give added confidence that the aperture stop is in the pyrometer. Crosswires have been mounted in the smaller

6
end of a short hollow cone made of graphite, which fits into the furnace

cone and centers the wires in front of the aperture. Alignment is achieved

when the "blackbody aperture and cross-wires are superimposed on the optic

axis of the pyrometer. Although the precision of alignment is greater the

farther apart the cross-wires and aperture, in some automatic pyrometers

with short object distances the range of the focus may be a limitation.

If cement is used to mount the cross-wires, it can be refractory cement,

and alignment can be made on a hot furnace. A high-intensity microscope

illuminator and a small mirror are useful in illuminating the aperture and

wires for viewing. However, a flashlight is adequate.

Experience has shown that the optic axis of the furnace does not shift

significantly upon heating and therefore can be aligned while cold. The

edge of the diaphragm is readily visible, when the blackbody is at 1083°C,

in a visual pyrometer and it can usually be seen in automatic pyrometers,

though in some cases the contrast is poor. The accurate centering of the

pyrometer axis in the blackbody aperture is not essential since the radiance temperature over the area of the aperture is uniform to within 0.01°C.

Fig. 6 is a typical melting and cooling curve of the copper-point

blackbody furnace with power inputs of 225 watts and 110 watts respectively. At 225 watts the melt begins in about TO minutes. When the melt has

been completed the copper is allowed to overheat 5°C as determined by the

pyrometer before the power is turned down. Longer melts and freezes can be obtained through appropriate power adjustments. However, the beginning

and the termination of the plateaus then become less distinct.

Attempts were made to improve the curves, particularly the melting

curve. On the assumption that the need for a more uniform heating of the

copper along its length was indicated, the windings were lengthened or shifted, and the amount of insulation around the cylinder varied, as was mentioned previously. No significant improvement was made by variations

within the restrictions of a simple design. However, the performance indicated in Fig. 6 is adequate for monitoring the best present day automatic pyrometers whose sensitivity is about 0.1°C.

For the most precise work the freezes, which have more definitive

plateaus, are used. The radiance temperatures corresponding to the

freezing plateau of four different copper-point blackbody furnaces are compared in the following table:

Furnace No,

1

2

3

h

Table 1

Mean radiance

temp. (Int.U8)

No. of

freezes

1083.291°C

1083.320

1083. 271* 1083.3^8

h

19

9

5

Standard deviation

of a freeze

0.010°C

.011

.012

.017

Average 10 8 3- 30

8
Furnace Nos . 2 and 3 are of the construction herein described. Furnace-e No. 1 was made of commercially available heating elements with windings

embedded in semi-cylinders. It failed after eight freezes (of which four were used with uncalibrated pyrometers ) through burnout of the crucible

which was evidently inadequately protected by the porous ceramic. Furnace

No. k was constructed of a cylinder of impervious mullite and a mullite

cone with the heater element extended from end to end.

Measurements were made with a photoelectric pyrometer similar to the

NBS photoelectric pyrometer which has been previously described [l] 1. The

pyrometer used was given a one-point calibration at 1083°C. In this calibration the sum of the absolute values of the systematic errors was 0.0^°C

and the standard error [2] was 0.015°C. The standard error of the pyrometer when determining a temperature is 0.006°C. Over the six month period

of the work, the pyrometer was corrected for pyrometer lamp instability

(discussed in ref. l) through gold-point (l063°C) measurements on each

day that measurements of the copper-point furnaces were made. The goldpoint furnace was compared to an infrequently used gold-point furnace on four different occasions throughout the period with a standard deviation

of the four comparisons being 0.006°C. Since the standard error of such

comparisons has been determined to be 0.008°C, it was concluded that the

gold-point blackbody and furnace had not deteriorated in its frequent use.

The uncertainty of the mean radiance temperature of each furnace was computed according to the recommendations of Eisenhart [2] of NBS. The

standard error of the calibration process (0.015°C) was combined in quadrature with the standard error of the copper-point (0.006°C) and the goldpoint (0.006°C) measurements. This value, 017°C , was tripled and added

to the estimated systematic error (0.0U°C) for an overall uncertainty of

0.091°C. For the purpose of predicting the uncertainty of a furnace con- structed independently, variations of the design of the furnaces listed

in the table are viewed as adequately bracketing any other versions of the

described design including the possibility of using materials from different manufacturers. The sample standard deviation of a furnace about the

average of 1083.308°C is 0.033°C*. This value, when tripled and added to

the 0.091°C uncertainty, results in a rounded off uncertainty of 0.2°C.

It can be seen from the standard deviation of a freeze in Table 1

the ability to reproduce a radiance temperature with a given furnace is

considerably better than the overall uncertainty. A plot of the freezes

of furnace No. 2, taken over a period of five months, is shown in Fig. 7-

1 Figures in brackets indicate literature references in this paper,

*Any variation in the copper samples is included. However, a previous

study [3] of l6 samples of the current batch of copper showed no significant evidence of nonuniformity , with a standard error for the mean of

0.011°C.
For the first. 20 hours the furnace was on no more than necessary to com- plete the freezes and was allowed to cool down to room temperature after

each freeze. Thereafter, the furnace was frequently left on for about

8 hours at a time at 1070°C or above to accelerate any deterioration or contamination. Up to 100 hours the furnace had undergone Ul freezes. All

of the temperatures of the freezes are not plotted in Fig. 7; excluded are the measurements made with uncalibrated instruments. It is estimated that

the deterioration of the above furnace when left on at 1070°C is equivalent to that in obtaining 10 more freezes.

The blackbody represented in Fig. 7 failed by burnout of the sight

tube after 5 more freezes and at about 108 hours at high temperature. In

the last k freezing points prior to the burnout the melting curve had

shifted about 0.3°C higher than the freezing curve. The freezes had maintained their reproducibility. It might well be that a growing disagreement between the melts and freezes is a good indication the furnace should

be retired.

The blackbody of furnace No. k gave way after 70 hours. In this case copper poured into the cone, but it solidified without dropping out of the

furnace. The sight tube probably had a slight fault and was finally crushed

upon cooling by the greater contraction of the copper relative to graphite.

The emissivity of the blackbody is not significantly different for

variations of from 3 mm to h mm diameter that may be expected of the small

end of the ceramic cone. A test was made in which an aperture 2 mm in diameter was, shortly after taking a freezing point, drilled out to 3.5 mm

(the limits of the ceramic cone), and another freezing point taken. The

measured difference was 0.022°C.

h. Summary and Conclusions

Copper-point blackbodies and furnaces of simple design have been developed and evaluated with respect to the uncertainty and reproducibility

of their radiance temperature. Those constructed according to the details

given may be assigned a radiance temperature of 1083. 3°C (int. U8) with an uncertainty of 0.2°C,* and a variation in successive freezes of less than

0.033°C.**

*The quoted uncertainty refers to the radiance temperature of the blackbody and not necessarily to the freezing point of the NBS standard reference material copper. The melting range of approximately 0.1°C shown in

Fig. 6 and inherent in the design precluded the possibility of reducing

the uncertainty of 0.5°C reported on the certificate accompanying the

standard sample copper.

**Three times the standard deviation for the variation of repeated freezes

on the same furnace (based on 18 degrees of freedom).
The cost of materials purchased in the minimum allowable quantities

can be expected to be between $100 and $150. There will be an excess of

some materials which may be used for additional furnaces at no further ex- pense .

5. References

[l] "The NBS Photoelectric Pyrometer and its Use in Realizing the International Practical Temperature Scale Above 1063°C", Metrologia 2_, 150 (October 1966).

[2] Eisenhart, C. "Expression of Uncertainties of Final Results",

Science l60 , 1201 (1968).

[3] NBS Test Report on Determination and the Freezing Point of Standard

Sample Copper Cu ^5d, 21 November 6l.

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Figure k. Checking the fit of the unfinished diaphragm in the sight

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CYLINDER BEVEL

Figure 5. Bevel for expanding the nichrome coil onto the cylinder.

Left-over graphite, which is easy to machine, can be used.

13
MELT

(225 WATTS)

FREEZE

(110 WATTS)

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