National Buraau of Standards

Library, N.W. Bldg

FEB 1 2 1964

^ecknlcai ^iote 163

.

THE CONSTRUCTION OF CALORIMETERS

FOR THE MEASUREMENT

OF ABSORBED DOSE

BEN PETREE AND GEORGE WARD

U. S. DEPARTMENT OF COMMERCE

NATIONAL BUREAU OF STANDARDS
THE NATIONAL BUREAU OF STANDARDS

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NATIONAL BUREAU OF STANDARDS

cec/twical ^Ticte

163

NOVEMBER 1962

THE CONSTRUCTION OF CALORIMETERS

FOR THE MEASUREMENT

OF ABSORBED DOSE

B. Petree and G. Ward

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.

by the Superintendent <>l Documents, U.S. Government Printing Office Washington 25, I). CI. - Price 25 cents
Contents

Page

1. Introduction 1

2. Requirements for Calorimeters Used for Absorbed Dose

Measurement 2

3. General Description and Performance 3

3.1. Structure 3

3.2. Performance k

k. Construction of Calorimeters 5

4.1. Core - 6

k.2. Shield -- 11

5. Auxiliary Equipment 15

5.1. Shield Temperature Control Circuit 15

5-2. Core Temperature Indication Circuit l6

5.3. Core Calibration Circuits 17

11
The Construction of Calorimeters for the

Measurement of Absorbed Dose

Ben Petree and George Ward

Direct measurements of energy locally absorbed

in irradiated materials can be made with adiabatic

calorimeters of suitable design. Design criteria

imposed by requirements of accuracy include limitations on size and complexity. Small calorimeters

of simple design with precision better than one percent at dose rates above one rad per second have

been developed. Details of fabrication, auxiliary

equipment and performance are described.

1. Introduction

It is generally conceded that an important parameter to correlate

with the effect of radiation on matter is the energy locally absorbed.

In 1953 the International Commission on Radiological Units and

Measurements called this quantity absorbed dose and its unit the rad

(equal to 100 ergs per gram in the material of interest). Methods for

determining the magnitude of the absorbed dose have been outlined by

this group in later publications-^'^. The ionization method, because

of its high sensitivity and its long usefulness has been treated in some detail by this group. However, the calorimetric method is an alternative and more direct technique, if the dose rate is high enough.

Report of International Commission on Radiological Units and

Measurements, NBS Handb. hf

.

2Recommendations of International Commission on Radiological Units

and Measurements (1953), Am. J. of Roentgenol. Jl, p. 139 (1954).

Report of International Commission on Radiological Units and

Measurements (l956), NBS Handb. 62.

k

Report of International Commission on Radiological Units and

Measurements (l959j, NBS Handb. 78.
Absorbed dose calorimetry measurements have been reported by

several authors-5 ' » ' » but the desired criteria and complete con- struction details are often lacking. The present paper discusses

the criteria that need to be satisfied and the details of construction

of two absorbed dose calorimeters.

This work was supported by the Division of Biology and Medicine

of the United States Atomic Energy Commission.

2. Requirements for Calorimeters Used for

Absorbed Dose Measurement

When calorimetric methods are used for the experimental determination of absorbed dose, certain general features of radiation absorption

must be considered if the measurements are to be accurate, useful and

easily interpreted. The following restrictions on calorimeter design

are thus imposed in addition to those necessary for accurate calorimetry

alone.

Size: Dose rate gradients are generally present in

irradiated media. Consequently small calorimeters

rather than large ones are generally required

for measuring effectively the absorbed dose

at a given point in the medium. This size

limitation appears to be a serious difficulty

in the calorimetric dosimetry of soft X-rays,

where the half-value-layer in aluminum is less

than a few millimeters.

5 Ben Petree, Absorbed Dose Calorimeters for High Dose Rate, Radiation

Res. 9, No. 1, p. 166, July 1958. E. J. Hart, H. W. Koch, B. Petree,

J. H. Schulman, S. I. Taimuty and H. 0. Wyckoff, Measurement Systems

for High-level Dosimetry, Paper No. 1927 , Proceedings of the Second

United Nations International Conference on the Peaceful Uses of Atomic

Energy, Geneva, 1958, 21, p. 188, United Nations, Geneva (1958).

P. Milvy, S. Genna, N. Barr and J. S. Laughlin, Calorimetric Determination of Local Absorbed Dose, Paper No. jkk, Proceedings of the Second

United Nations International Conference on the Peaceful Uses of Atomic

Energy, Geneva, 1958, 21, p. 1*1-2. 7

W. B. Reid and H. E. Johns, Measurement of Absorbed Dose with Calorimeter and Determination of W, Radiation Res. lk, No. 1, p. 1, January

196l.

o J. S. Laughlin and S. Genna, Calorimetric Methods, Chapter 9 in

Radiation Dosimetry , ed. Gerald J. Hine and Gordon L. Brownell,

(Academic Press, New York, N. Y. , 195&).
Homogeneity: The dose rate at a point in a medium is affected

by nearby voids or imbedded masses of a different

atomic composition. Consequently the calorimeter

should be of a simple compact design. Foreign

matter, as for example, thermocouple wire, should

be minimized.

Chemical Stability: Radiation energy absorbed by a molecular

substance generally has noticeable chemical

effects in which energ}^ is absorbed or released.

When this occurs the thermal effect, to which

the calorimeter responds, does not correspond

precisely to the quantity of energy initially

deposited by the radiation. More than 2 percent

of Co energy absorbed in polystyrene fails

to appear as heat and is apparently stored in

the altered chemical structure of some of the

molecules . On the other hand non-molecular

substances, such as metals, store energy only

by atomic dislocations, and this amounts to

only about 0.001 percent of the radiation

energy absorbed. Such material should be used

for calorimeters where possible.

3. General Description and Performance

3.1. Structure

The basic structure of the calorimeters to be described is shown

in figure 1. The calorimeter consists of two principal parts; a calorimetric body or core, in which the absorbed dose is measured,

and a shield that prevents heat exchange with the surroundings by

conduction and radiation. Two thermocouples are attached to the core and one to the shield. They permit the indication of core temperature

and core-shield temperature differences. Electrical heaters are attached to both core and shield. The core's heater permits the

addition of accurately measured quantities of energy to the core for

the purpose of calibrating the core temperature indications in energy

units. The shield's heater permits the adjustment of shield temperature,

as required during a measurement, to prevent heat transfer to the core.
Figures 2-4 of a spherical graphite calorimeter and figures 5

_ 7

of a cylindrical aluminum calorimeter indicate the sizes involved and

show some of the construction details. The cores are made in halves

that are cemented together with a film of electrically conductive

epoxy resin that constitutes the core heating element. The thermocouples are cemented with epoxy resin into small holes drilled in the core and shield. The shields are made in two pieces that fit tightly

together and around their cores. The outer surfaces of the shields are wound with fine resistance heating wire. The graphite calorimeter was mounted in Styrofoam (fig. 4) and, when used to measure the dose rate

in carbon from a Co ^ source, was irradiated from all sides. The

aluminum calorimeter was mounted in an aluminum block (fig. 7/ and,

when used to measure the heat developed in aluminum by high energy

X-rays, was irradiated axially. Both calorimeters were ordinarily

operated in vacua of the order of ten microns of mercury to reduce

conductive and convective heat transfer to the core through air.

3-2. Performance

Examples of the response of these instruments are shown in

figures 8 and 9- Figure 8 is a graph of data obtained from the

graphite calorimeter during a measurement of the energy deposited in

carbon by the gamma rays from a Co irradiator. Figure 9 is a reproduction of a chart recording of the response of the aluminum

calorimeter before, during and after an irradiation in the X-ray beam

of a synchrotron. The steps at 6 minutes and 7-1/2 minutes were

produced by adjustments in the potentiometer with which the thermocouple voltage was measured. The noise on this trace was generated

partly by fluctuating temperature gradients in the laboratory air and

partly by the amplifiers used in the measuring and controlling circuits,

a. Precision

A series of electrical calibrations gave the following results

which are typical of the performance of these simple instruments.

Calorimeter Graphite Aluminum

-3 -6

Power, watts 1.2 x 10 13 x 10

Equivalent dose rate, 130 1 rads per second

Time of runs, seconds 900 900

Number of runs 7 5

Sensitivity, microvolts 0.814 0.710

per 1000 rads

Standard deviation of 0.14 percent 0.5 percent

mean sensitivity
b. Accuracy

Systematic errors are introduced in adiabatic calorimeters when

the temperature gradients in core and shield during calibration runs are not precisely proportional to the gradients during measurement runs. Such differing gradients affect the apparent temperature of core and

shield as registered by the thermocouples, and in consequence, the heat

transfer to the core from the shield during a measurement will in

general not equal that during a calibration. Upper limits of less than

0.24 and 0.15 percent have been calculated for errors of that origin

in the graphite and aluminum calorimeters respectively.

Data in agreement with the calculation for the graphite calorimeter

have been obtained by the ionization method mentioned in the introduction.

Measurements with an ion chamber were made of exposure rate from a Co °

source, and at the same position measurements with the graphite calorimeter were made of the rate of heat evolution. After allowing for

attenuation of the gamma rays in the graphite, the dose rate predicted

from the exposure rate could be compared with the dose rate measured

directly by the calorimeter as follows:

Exposure Dose Rate Roentgens per Second

Measured 157*5 ± 1 percent

Effective (157-5 x .974) 153.4 ± 1 percent

Heating in Carbon p Milliwatts per Gram

Predicted (0.868 x 153-4 x 10 ) 1.332 ± 1 percent

Measured (calorimeter) 1.325

(Predicted) - (Measured) 0.009 (0.7 percent)

c. Resistance to Radiation Damage

The graphite calorimeter was not affected in its electrical or thermal characteristics by a total absorbed dose of about 10' rads

accumulated during test and use. However, some darkening of the

epoxy resin was observed, and possibly a slight tendency to become

brittle.

5 The aluminum calorimeter received a total dose of about 10 rads

No signs of damage resulted.

4. Construction of Calorimeters

The various parts will be taken up in order in the following

sections

:
k.l. Core

a. Dimensions and Composition

There appear to be four principal considerations in choosing the

size of the core:

Stability of core temperature

Homogeneity of core temperature

Homogeneity of core composition

Degree of space resolution

The first two affect the accuracy of the calorimetric determinations

made by the instrument; the last two affect the usefulness of those

determinations to the particular dosimetric measurement that is

desired. These four requirements are conflicting in the matter of core size, as the first and third lead to large cores while the second

and last lead to small cores.

(1) Stability of Core Temperature

The rate at which the core temperature, 9, drifts in response to

a unit core-shield temperature differential is:

d0 =

h

dt C

where h is the heat transfer coefficient in watts °C of all heat

paths from core to shield, including air, wires, spacers and thermal

radiation , and C is the heat capacity of the core in watt-seconds

°C . Since it is desirable to minimize the drifts in temperature

caused by unavoidable differentials, the largest core mass compatible

with other requirements is desirable to give the largest value of C.

Increasing the mass of the core does not affect the sensitivity of the

calorimeter if, as is most frequent, it is completely immersed in the

radiation field. In designs where thermal radiation accounts for an important fraction of h, it is also desirable to adopt a spherical or nearly spherical shape to minimize the surface area of the core relative to the volume. The quantity ^ is approximately the time

constant of the relaxation of core-shield temperature differences,

and is readily measured in an assembled calorimeter. For both

calorimeters the calculated and measured time constants are about 1000

seconds.

(2) Homogeneity of Core Temperature

It is desirable to minimize by design the temperature inhomogeneity

of the core so as to minimize systematic changes in heat transfer to

the core which accompany core temperature gradients. An upper limit to

the fractional error, 6, caused by inhomogeneities of core and shield
temperature is given by:

6 = d + i)

T T

/ c + s

\ T

o

) whe re

T and T are the time constants for the relaxation of temperature

c s gradients in core and shield, respectively,

T is the time constant for the relaxation of temperature differences

between core and shield, and

N = heat capacity of shield 4 heat capacity of core. (N=4 for graphite calorimeter; N=l for aluminum calorimeter)

The time constants in this expression can be calculated with fair

accuracy for simple designs, and can be measured directly for greater

accuracy.

The value of T is related to the core dimensions as follows,

c provided the core is homogeneous and convex:

T < ^ £f where

c k

N jr

it = 3.1k

p = density, g cm

3

1 -1

c = specific heat, joules °C g

k = thermal conductivity, watt cm °C

L = largest dimension of core, cm

T = core time constant, sec.

c Larger values of T are to be expected for the split cores, but the

increase was slight for the two calorimeters described. For both

aluminum and graphite cores the calculated and measured time constants were about one second.

Since the value of T was about 1000 seconds for both instruments,

the errors attributable to temperature gradients in the cores do not

exceed 0.08 percent and 0.05 percent for the graphite and aluminum

calorimeters respectively. The corresponding contributions of the

shields to the systematic error are treated in a later section where

it is found that they do not exceed 0.l6 percent and 0.1 percent

respectively.

(3) Homogeneity of Core Composition

The thermocouple, heating element, and connecting wires which

are fastened to the core may receive a different dose than the core material proper because of their differing atomic numbers. The extent

to which the calorimeter response is affected by the presence of those
foreign materials depends upon the energy of the radiation, and is

least for photon energies near 1 Mev where the Compton process

predominates. At lower energy where the photoelectric process is

important and at higher energy where pair-production is important,

the effect is more pronounced. Corrections may not be very precise,

depending as they do upon a knowledge of the radiation spectrum. It

is therefore desirable to minimize the effect by utilizing the largest

possible core and the smallest possible mass of foreign material.

The composition of the graphite and aluminum cores and their

intended applications were as follows:

Graphite Aluminum

Total Mass

Percent composition:

core proper

foreign materials

Radiation spectrum

0.9 g

97

3

1.25 Mev (Co

60

)

l.k g

99-9

0.1

X-rays Ep>50 Mev

(h) Degree of Resolution

The dose values determined by the calorimeter are averages over

the volume of the core, and as such do not give directly the precise

value at any given point. The expected dose gradients in the medium

and the required accuracy of measurement should both be considered

in deciding upon the largest tolerable core dimensions. The gradients

depend greatly upon the configuration and energy spectrum of the source

as well as upon the depth in and atomic number of the medium. Therefore the maximum core dimensions will depend very specifically upon

the measurement that is to be made

.

b. Core Heaters

Conducting film heaters were used in both graphite and aluminum

cores. The mass of foreign material thus added to the cores was

considerably smaller than if a separate heating element had been

installed inside of or attached to the core. The film heater has the

advantage also of distributing well the generated heat, a desirable

method of increasing the uniformity of core temperature during

calibrations

.

(l) Choice of Resistance

There appear to be two principal considerations affecting the

choice of core heater resistance:

The value should be large enough compared to the resistance

of the heater lead-in wires that the measured calibrating

power can be accounted for precisely; and small enough to

8
permit the required power to be developed with a reasonably

small applied voltage.

The values shown below were satisfactory.

Core Graphite Aluminum

Core resistance, ohms 250 8000

Lead-in resistance, ohms O.S^ 0.5

-5 S Calibration power, watts 10 -^ 10"^

Potential, volts 0.5 0.3

(2) Film Preparation

9 A resin or ink with the following composition by weight was used for the core heaters

:

Epoxy Resin 100

Hardener (BF^, piperdidine) 7-5

Carbon Blacic 5-10

The ink is a paste at room temperature and has a shelf life of about

six months. Hardening is induced by baking for about one hour at about

200 °C. The cores were assembled with ink and spacers as shown in

figure 10. They were clamped to prevent slipping, baked, and allowed

to cool. The clamps were then removed and the spacers were pulled out.

The cores thus prepared were very strongly united but the resistance

values obtained show considerable spread. Resistances ranging from

about 40 ohms to 1300 ohms were obtained. for a dozen graphite cores. Half of those were above 200 ohms and were considered acceptable.

The aluminum cores were less satisfactory, the resistances ranging

from less than an ohm to more than 1000 megohms. Although the values

were quite unpredictable, several acceptable cores were obtained having

resistances between one and ten kilohms. It seems likely that the

aluminum tends to remain insulated from the ink, perhaps because of its

oxidized surface. Thus, the heating in such films may be concentrated

at a small spot rather than well distributed.

The ink was obtained from the Diamond Ordnance Fuze Laboratories,

Washington, D.C., where it is ordinarily used to form resistors on printed circuits. We are indebted to Dr. Norman Doctor for suggesting

its use and to Miss Marian Davies and Miss Mary Lee Hebb for advice

concerning its properties and for the preparation of the graphite core heaters. The preparation of the ink requires special equipment and

techniques.
(3) Power Lead-in

The heating current was brought to the core through enameled

copper wires of 0.00175 in. diameter. Connection was made to the

aluminum core by wedging the wires into holes with aluminum pegs, and

to the graphite core by soldering the wires to electro-deposited

copper spots at the poles of the core. Wood's alloy was used for the

solder in an effort to avoid overheating. Figure 11 shows the details

These or other simple methods of connection are possible for cores of

any material having sufficient electrical conductivity. Connections

for non-conducting cores would require different and possibly more

elaborate construction.

The use of some material other than copper for the power lead-in

will reduce heat transfer to the core, and is therefore desirable

provided the increased electrical resistance does not prevent accurate

calibration.

c. Thermocouples

(1) Choice of Wire

Chromel P-constantan thermocouples were used in both calorimeters,

That combination is well suited to this application for two reasons:

the thermoelectric power, 60 microvolts/°C, is the largest available

from common thermocouple materials; the thermal conductivity of both

metals is small. The wires are available in many sizes and with

various kinds of insulation. Glass fiber insulation with silicone

binder was used in both calorimeters, and appears to be quite

satisfactory.

It is desirable, for small heat leakage, to use small size thermocouple wires. However, if excessively fine wires with consequent high

electrical resistance are used, then the signal-to-noise ratio at the

detector will be poor. The choice of wire size should be influenced

by the total thermocouple circuit resistance, which in turn depends

upon the length of the leads. For the graphite calorimeter, the

experimental arrangement required leads about lk ft long and wires

of 0.005 in. diameter were used. In the aluminum calorimeter, for

which leads only 6 ft long were necessary, wires of 0.003 in. diameter

were used.

(2) Welding

Couples were produced by a capacitor discharge process, the

apparatus for which is shown in figure 12. The wires to be welded

were held in place on the bottom block with tape so as to cross on

the electrode, 1. The top block was closed down and held with about

150 grams weight. S was closed to charge C to the desired voltage,

10
10

usually about 20 volts, then S„ was closed to produce the weld

The couples so formed were trimmed and tied and given a narrow hairpin

shape by pulling with a wire looped through the couple and pressing

between thumb and finger as shown in figure 1JA.

(3) Embedding

The thermocouples need to be embedded in such a way as to be

thermally attached to, but electrically insulated from the core. Our

efforts to insulate the thermocouples by painting on resins and enamels

were not very satisfactory, as all of those tried tended to form beads

before hardening and left the wires largely bare. Several coatings

were necessary to insulate the couples used in the graphite cores.

More reliable results were obtained by another process pictured

in figures 13B-D. An epoxy resin plug was cast around the couple for

insulation (fig. 13B). This plug was then pressed out of the Teflon

mold, its insulation tested in mercury, and embedded with more resin

in the core. The liquid resin was inserted into the holes with the

help of a hypodermic syringe with a ground-down needle. It is desirable

for the hole diameter to be small so as to minimize the proportion of

foreign material in the core, and when the resin conducts heat less

readily than the core, to improve the speed and accuracy of temperature

indication.

k.2. Shield

a. Dimensions and Composition

It is desirable, as for the core, to minimize the quantity of

foreign material contained in or attached to the shield in order that

the distribution of radiation that would exist in a homogeneous medium

may be perturbed as little as possible. For the same reason, the

spacing allowed between core and shield should also be minimized. The

shield should be limited in thickness to a value sufficient to accomodate

Sp should be of a type having low resistive losses under the conditions

of use so that the energy available for welding cannot fluctuate from

one weld to the next. An ordinary copper single-pole knife switch was

used. It was closed rapidly by striking it with a light wooden stick.

The electrodes of figure 10 were number 12 copper wires secured to the

hinged plates with epoxy resin. They were flattened initially by sandpapering them while the blocks were held closed. Sandpapering was also

done to clean the electrodes before each weld, and is apparently needed

for consistent results.

11
thermocouple and heater wires and to satisfy special requirements of

the particular measurement. Unnecessary thickness is not desirable

because the time lag for the propagation of temperature changes from

the heater on the outside of the core to the thermocouple inside, is

detrimental to accurate control of shield temperature. The time lag

varies as the thickness squared.

The dimensions of the shields were as follows:

Graphite Aluminum

cm in.

Wall thickness - -- O.k 0.040

Diameter (outer) 2.2 0.518

Height (spherical) 0.3^6

Spacing, core to shield:

Radial 0.2 0.020

Top and bottom (spherical) 0.008

b. Isothermal Requirements

It was mentioned in the preceding section on core dimensions

that systematic errors result from temperature inhomogeneities in the

shield. These errors can be minimized by the following means without

complicating the calorimeter structure:

(l) Reduce the size of the shield.

The value of T is proportional to the area of thin

shields of a fixed shape. For example on a spherical

shield:

T =££ g! where

s k o

p = density, g cm

c =* specific heat, joules °C g

k = thermal conductivity, watts cm °C

D ** diameter of shield, cm

T * shield time constant, sec.

s The values of T for both calorimeters were calculated to be about

two seconds. Thus, the fractional errors attributable to

As an example, the thickness of the graphite shield, O.k cm, was selected to prevent electrons originating outside the calorimeter in

materials other than carbon from reaching the core.

12
temperature gradients in the shield do not exceed 0.16

percent and 0. 1 percent for the graphite and aluminum

calorimeters respectively.

(2J Apply heater windings with uniformly spaced turns over the whole exterior surface of the shield. That will

give uniform average surface density of heating power.

Uniformly good thermal contact for the whole length of

heater is equally important.

(3) Space the heater windings closely. Periodic temperature

variations on the inner surface set up by the heater wires

outside can be reduced to any arbitrarily small amplitude

by the choice of small enough spacing. The amplitude will

not exceed A degrees C where:

a4^ —l 2 k . ,

-—T smh 2n—

I = heater current, amperes

v

p = resistance of heater wire, ohm cm

k = thermal conductivity of shield, watts cm °C

T = thickness of shield, heater to thermocouple, cm

L = heater winding spacing, center- to-center , cm

This expression also shows that A is proportional to the

total shield heating power, which is in turn approximately

proportional to the number of degrees above ambient temperature

that the shield is operating. Hence,

(k) Operate the shield as close to ambient temperature

as the experiment permits.

(5) Design the shield so that its separate parts fit

together tightly for good thermal connection.

c. Thermocouple

(l) Location

The shield's thermocouple should indicate the temperature of the

inside surface of the shield, since it is with that surface that the core exchanges heat. Therefore the shield's thermocouple should be

embedded as close as possible to the inner surface of the shield.

13
(2) Orientation

The differential thermocouple circuit may givetf false indications

of temperature difference if the two opposing couples differ in thermoelectric power. We have generally found that effect to be least if the

pair of couples is made from adjacent segments of wire as shown in 10

figure lk, and not from widely separated or totally unrelated segments.

d. Heaters

The shield heaters of both calorimeters consisted of 4h~5 nyloninsulated resistance wire wound on the outer surfaces of the shields

and held in place with epoxy resin. Figures 3 and k show the graphite

shield heater. The winding was applied to the ungrooved surface and

secured by tape until the resin holding it was hard. Figures 5 and 6

show the spiral winding grooves on the aluminum shield, and figure "J, a portion of the completed heater winding. The center-to-center

spacing of turns was 0.028 in. The groove was square and just large

enough for the wire to slip in.

The heater resistance of both shields was about 250 ohms. The

power requirements of the heaters were a few milliwatts, the precise

value depending upon both ambient and core temperatures.

e

.

Core Spacers

The core of the graphite calorimeter was supported on tapered

polystyrene pegs that were glued to the inner surface of the shield

with epoxy resin. The manufacture of these small pegs was avoided

when the aluminum calorimeter was constructed by substituting polystyrene

spheres. Spheres of the required sizes were obtained by sieving

polystyrene Expandible Beads, ^ a molding resin consisting of remarkably

accurate spheres with a distribution of diameters from a few thousandths

to about one tenth inch. Spheres of that kind were also used to support

the calorimeter in its aluminum mounting block.

f

.

Wire Arrangement

The wires entering the calorimeters were bonded to the shields

with epoxy resin to improve the thermal contact, so as to prevent or

Methods for testing thermocouple wire for homogeneity are given by

Wm. F. Roeser and H. T. Wensel, Methods of Testing Thermocouples and

Thermocouple Materials, p. 284, Temperature, its Measurement and Control

in Science and Industry (Reinhold Publ. Corp., New York, N. Y. , 19^-l).

13Koppers Corporation, Pittsburgh, Pennsylvania.

14
reduce the exchange of heat between the core and the outside by way of

the wires. The graphite shield was sufficiently thick to permit the

machining of a slot in which the wires were embedded around a fraction

of the circumference of the shield. The aluminum shield was not thick

enough to permit that construction, however, and the bundle of wires

entering the shield was simply cemented in place in the entry hole.

As a further step to reduce conduction of heat to the core along

the wires, considerable excess length of the wires was allowed in the

gap between the core and shield. Even so, conduction along the copper

wires was nearly one half of the total heat leakage to the cores in

both calorimeters.

5- Auxiliary Equipment

Three separate circuits shown on figure 15 are required for the

operation of these simple calorimeters. They permit, A, control of

shield temperature, B, indication of core temperature, C, calibration

of core temperature indication in units of energy. The functioning of

these circuits will be described and their components discussed in the

following sections.

5.1. Shield Temperature Control Circuit (fig. 15A)

For an adiabatic condition of the core, the shield and core temperatures are equal and the voltage V"n generated by the opposed

thermocouples C and C" is zero. If a temperature difference develops,

it is indicated in magnitude and polarity by a corresponding V . V"n

is amplified, and, acting through a controller, changes the shield

heater current so as to reduce Vn and restore the adiabatic condition.

The entire calorimeter is operated above the temperature of the

surroundings, so that heat can be lost from the shield when necessary.

a. Thermocouple Leads

The following precautions may be necessary when a relatively high

degree of stability is required, as in a calorimeter for small dose

rates. In both circuits the thermocouple wires were as short as possible

and were protected by thermal insulation from the randomly changing

temperature gradients of laboratory air. The thermocouple wires were

handled carefully to avoid producing inhomogeneities by mechanical

stress. Mechanical vibration during use was guarded against, and the

entire thermocouple circuit was carefully shielded against electrical

disturbances.

b. Reference Junctions

A melting ice reference temperature was used with the graphite

calorimeter, which operated at 15 °C. It was necessary to use a

15
desiccant to prevent moisture condensation on the cold wires.

A re-entrant glass cell containing solidifying diphenyl ether,

which provides a temperature of 26.8 °C was used for the aluminum

calorimeter's reference junctions. The calorimeter was operated

at a temperature of 29-3 °C. The basis for choosing the reference

temperature is mentioned in a following section on core temperature

indication.

c. Amplifiers

The amplifiers of figure 15 were high gain contact modulated

d.c. amplifiers with low equivalent input noise and drift. Each

contains an internal adjustable voltage source in series with the

input. The voltage can be used to control the drift rate of the core temperature if, as invariably occurs, the drift rate is not precisely

zero when Vn = 0.

d. Controller

The block marked CONTROL on figure 15 consisted of a recording

millivoltmeter followed by a three-mode current adjusting controller

equipped with a magnetic amplifier. That equipment, which was used

with the aluminum calorimeter, provided a continuous controlling action

and a pen-and-ink graph of the core-shield temperature differences.

The graphite calorimeter was operated with manual control of the

shield temperature, an operator adjusting a multiple turn rheostat

as required to keep V equal to zero. Manual control is not recommended

because it requires the constant and exacting attention of an operator.

5.2. Core Temperature Indication Circuit (fig. 15^)

A single couple C

11 ' produces a signal V corresponding to the core temperature which is measured by comparison to a potentiometer.

An amplifier is used to secure the necessary accuracy of comparison.

Changes in V correspond to energy deposited in or removed from the core

.

a. Reference Junctions

In principle the junctions J of the thermocouple wires to copper

can be maintained at any steady reference temperature. However, there

is an advantage to the use of a reference temperature close to the core temperature. The total voltage V

p

will then be relatively small, and

excessive stability will not be required of the potentiometer with

which changes in V are measured. When changes in V are very small

1^Furnished by Dr. Chas. P. Say lor and Mr. Delmo Enagonio of N.B.S

16
that advantage may be important. If experimental conditions permit,

the core and reference temperatures should both approximate the room

temperature, as gradients along the wires will then be small and the

effect of wire inhomogeneity correspondingly reduced.

b. Potentiometer

The potentiometers used in this circuit need not be calibrated,

but instead are required only to furnish a set of stable and re- producible potentials. The Lindeck constant resistance potentiometer

circuit was used, as it is well suited to meet these requirements.

It can be made relatively free from parasitic thermal emf ' s

x>» xu which

are of serious concern at microvolt levels.

5.3. Core Calibration Circuits (fig. 15C)

The purpose of these circuits is to permit the addition of

accurately measured quantities of energy to the core so that the

indications of core temperature will be calibrated directly in absolute

energy units. If that procedure is followed then the quantities of

energy deposited in the core by radiation are obtained directly from

the core temperature indications with no dependence whatever upon

knowledge of the heat capacity of the core, or upon the thermoelectric

power of the measuring junction.

Accurate energy measurement was complicated by the tendency of the core heater resistance to drift slightly with temperature and also,

in the aluminum calorimeter, to fluctuate slightly. The following

methods were used to suppress the resulting power fluctuations.

a. Constant Voltage Circuit

17

In the circuit of figure l6A the power to the core is supplied

by a battery through a known resistor equal to the initial or nominal

core resistance, R. Measurements of battery voltage, E, and core

voltage, V, with an ordinary potentiometer then permit the calculation

of power:

p - vi - v Sf

K

Since V is nearly equal to E/2, put

V r E/2 + v

Forest K. Harris, Electrical Measurements, p. 176 (John Wiley and

Sons, New York, N. Y. , 1952).

1 Walter H. Wood, Rev. Sci. Instr. 28, No. 3, 202 (1957)-

17 Harold Hoge , Rev. Sci. Instr. 20, No. 1, 59 (19^9)

17
Then P = (E/2+v)

^. < E/g ) -v

so that fractional changes in core power are equal to the squares of

the fractional changes observed in core voltage. The battery voltage,

E, is not disturbed by the changing core resistance, and can therefore

be measured precisely; hence the power determination is accurate in

spite of fluctuating core voltage. The fractional changes in core voltage observed for either calorimeter including drift and fluctuations

were less than 0.01, so. the fractional power uncertainty from that cause was less than 10" .

b. Constant Current Circuit

An adjustable constant power source that is more stable is shown

in figure l6B. The basis for that circuit is the constancy of the

collector current, J, of the transistor. The current is approximately

equal to E, /r, and is only very slightly affected by changes in the

main battery voltage, Ec, or by changes in load resistance. The core

is shunted by a known resistor that is equal to the initial or nominal

core resistance R; for convenience, the series resistor is adjusted

to half that value. Measured values of E and V permit the power to be

calculated:

P ~ VI = V

Since V is very nearly equal to E, put

V E + v.

Then P » (E+v)

2E-V

R

2 2

E-ytr E -v R R

so that fractional changes in power are equal to the squares of the

fractional changes observed in core voltage. The values E and V are nearly equal for this circuit, so that the measurements of potential

need not require range changes on the potentiometer. The power level can be continuously adjusted by varying r.

18
THERMOCOUPLES

HEATERS

Fig. 1. Basic structure of adiabatic calorimeters

19
20
Fig. 3- Graphite calorimeter. Outside surface

of shield showing heater wires.

21
Fig. k. Graphite calorimeter mounted in Styrofoam holder.

22
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S110A0H0IIAI '1VN9IS

26
MINUTES

15

10

<*'

X-RAY

OFF

ON

OFF

2I0'7 VOLTS

Fig. 9- Response of aluminum calorimeter to heating by X-rays (90 Mev

peak). Dose rate, 0.86 rad/sec. The steps at 6 minutes and

7-1/2 minutes were produced by adjustments of the potentiometer

with which the thermocouple voltage was measured.

27
^ I V////V7TI

A B

Fig. 10. Preparation of cores. A, graphite core: 1, Teflon washer

0.002" thick. 2, ink (10 parts carbon). B, aluminum core:

1, Mylar strips 0.0004" thick wiped with silicone grease.

2, ink (5 parts carbon).

28
Fig. 11. Heater connections. A, graphite core: 1, graphite. 2, copper

spot. 3, Wood's alloy. h, copper power lead. B, aluminum

core. 1, aluminum. 2, aluminum peg. 3, epoxy resin used to

strengthen power lead. k, copper power lead.

29
Fig. 12. Thermocouple welder. A, holder: 1-2, flat copper electrodes

cemented to hinged blocks ^>-h. 5-6, thermocouple wires. B,

welding circuit 1-2, electrodes. S^, charging switch. S2,

welding switch.

R, 100,000 ohms

C, 200 |if oil capacitor

E, 22 1/2 volts

V, voltmeter.

30
^

o.oi35"dia

B

0.016" dia. CORE

V

D

Fig. 13. Shaping and embedding thermocouples. A, shaping. B, method

of preparing insulation. C, couple with insulation. D,

insulated couple embedded in core.

31
Y' Y' Y'

©• €>

J2

X Z

A

X z

B

X z

C

Constanta n

Chromel P

Fig. 1^. Orientation of differential thermocouple wires. A, wire from

spool long enough for both leads. B, wire cut in half. C, wire

made into thermocouple pair with original ends, x-^y, still free.

32
^ CALIBRATION ^^ POWER c

SUPPLY

i^^TT^N

If a V 1 • hH '" X Cl\C C /l

\ < —\ V' • x / —»—• AMP DET B

J

POT

(

< )J

*vD >

-• • - "• •" "* CURRENT TO * AMP CONTROL SHIELD HEATER A

Fig. 15. Absorbed dose calorimeter with auxiliary circuits. A,

shield temperature control. B, core temperature indication. C, calibration, a, core, b, shield, C'-C"-

C

1 ' 1 , thermocouples. J, constant temperature reference

junctions. AMP, dc breaker type amplifier. CONTROL,

three-mode current adjusting control. POT, Lindeck potentiometer. DET, chart recorder. CALIBRATION POWER SUPPLY,

constant power source.

33
A/WWV

DUMMY

LOAD

v

v E3core A

i i

-A/WWV

<—E—

v E3 core B

^-vvwD-

+ +0|D|Db

Fig. l6. Stabilized power sources. A, constant voltage type: R,

stabilizing series resistance equal to core resistance. B,

constant current type: TR, transistor type 2N^04. E

b

, 1.5

volt reference cell. Ec, K.*j volt collector battery. R,

stabilizing shunt resistor equal to core resistance, r, 50,000

ohm ten-turn rheostat.

* U.S. GOVERNMENT PRINTING OFFICE : 1962 0—663192

3^
DEPARTMENT OF COMMERCE

Luther H. Hodges, Secretary

riONAL BUREAU OF STANDARDS

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

e scope of activities of the National Bureau of Standards at its major laboratories in Washington, D.C., and

lder, Colorado, is suggested in the following listing of the divisions and sections engaged in technical work,

eneral, each section carries out specialized research, development, and engineering in the field indicated by

itle. A brief description of the activities, and of the resultant publications, appears on the inside of the

ASHINGTON, U. C.

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