A UNITED STATES DEPARTMENT OF COMMERCE

PUBLICATION

NIST

PUBLICATIONS

REFERENCE

NBS MONOGRAPH 133

U.S.

)EPARTMENT

OF

COMMERCE

National

Bureau

of

Standards Wj^k

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100

.U556

#133

1974

Mass and Mass Values

m
NATIONAL BUREAU OF STANDARDS

The National Bureau of Standards" was established by an act of Congress March 3, 1901. The Bureau's overall goal is to strengthen and advance the Nation's science and technology

and facilitate their effective application for public benefit. To this end, the Bureau conducts

research and provides: (!) a basis for the Nation's physical measurement system, (2) scientific and technological services for industry and government, (3) a technical basis for equity in trade, and (4) technical services to promote public safety. The Bureau consists of the Institute for Basic Standards, the Institute for Materials Research, the Institute for Applied Technology,

the Institute for Computer Sciences and Technology, and the Office for Information Programs.

THE INSTITUTE FOR BASIC STANDARDS provides the central basis within the United

States of a complete and consistent system of physical measurement; coordinates that system

with measurement systems of other nations; and furnishes essential services leading to accurate and uniform physical measurements throughout the Nation's scientific community, industry, and commerce. The Institute consists of a Center for Radiation Research, an Office of Measurement Services and the following divisions: Applied Mathematics — Electricity — Mechanics — Heat — Optical Physics — Nuclear

Sciences "

•—• Applied Radiation " -— Quantum Electronics " — Electromagnetics — Time

and Frequency " — Laboratory Astrophysics " — Cryogenics

THE INSTITUTE FOR MATERIALS RESEARCH conducts materials research leading to improved methods of measurement, standards, and data on the properties of well-characterized materials needed by industry, commerce, educational institutions, and Government; provides

advisory and research services to other Government agencies; and develops, produces, and

distributes standard reference materials. The Institute consists of the Office of Standard Reference Materials and the following divisions: Analytical Chemistry — Polymers — Metallurgy — Inorganic Materials — Reactor

Radiation — Physical Chemistry.

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the use of available technology and to facilitate technological innovation in industry and Government; cooperates with public and private organizations leading to the development of

technological standards (including mandatory safety standards), codes and methods of test; and provides technical advice and services to Government agencies upon request. The Institute consists of a Center for Building Technology and the following divisions and offices: Engineering and Product Standards — Weights and Measures — Invention and Innovation — Product Evaluation Technology — Electronic Technology — Technical Analysis — Measurement Engineering — Structures, Materials, and Life Safety * — Building

Environment * — Technical Evaluation and Application ' — Fire Technology.

THE INSTITUTE FOR COMPUTER SCIENCES AND TECHNOLOGY conducts research and provides technical services designed to aid Government agencies in improving cost effec- tiveness in the conduct of their programs through the selection, acquisition, and effective

utilization of automatic data processing equipment; and serves as the principal focus within

the executive branch for the development of Federal standards for automatic data processing

equipment, techniques, and computer languages. The Institute consists of the following

divisions: Computer Services — Systems and Software — Computer Systems Engineering — Information Technology.

THE OFFICE FOR INFORMATION PROGRAMS promotes optimum dissemination and

accessibility of scientific information generated within NBS and other agencies of the Federal Government: promotes the development of the National Standard Reference Data System and

a system of information analysis centers dealing with the broader aspects of the National Measurement System; provides appropriate services to ensure that the NBS staff has optimum

accessibility to the scientific information of the world. The Office consists of the following

organizational units: Office of Standard Reference Data — Office of Information Activities — Office of Technical

Publications — Library — Office of International Relations.

1 Headquarters and Laboratories at Gaithersburg, Maryland, unless otherwise noted; mailing address Washington, D.C. 20234.

3 Part of the Center for Radiation Research.

> Located at Boulder, Colorado 80302.

* Part of the Center for Building Technology.
Mass and Mass Values

Paul E. Pontius

Institute for Basic Standards

National Bureau of Standards

Washington, D.C. 20234

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

NATIONAL BUREAU OF STANDARDS, Richard W. Roberts, Director

issued January 1974
Library of Congress Catalog Number: 73-600054

National Bureau of Standards Monograph 133

Nat. Bur. Stand. (U.S.), Monogr. 133, 39 pages (Jan. 1974) CODEN: NBSMA6

U.S. GOVERNMENT PRINTING OFFICE

WASHINGTON : I97I+

For sale by the Superintendent of Documents, U.S. Government Printing Office,

Washington, D.C. 20402. (Order by SD Catalog No. C13. 44:133). Price 70 cents. Stock Number 0303-01178
Foreword

This monograph has been prepared for the benefit of metrologists and others

concerned with the measurement of mass. It is addressed to two related but

distinct subjects: the history and general philosophy of mass measurement and the

specific problem of the buoyancy correction as appUed to the weighing process. The opening section is a discussion of the role of mass measurement in our

society; its development and its significance. Underlying this section are some observations on the philosophy of measurement as developed by Mr. Pontius during

his years as chief of the NBS section responsible for mass measurement.

In the second section, Mr. Pontius presents in detail the problem of air buoyancy

corrections to the weighing process. The formulae required for the appUcation of

this correction are presented, together with an appendix of helpful tables and some

typical examples.

We hope that you will find these sections thought-provoking with respect to measurement problems in general and helpful in their presentation of the procedures

for correctly applying the correction for air buoyancy in precision weighing.

Dr. K. G. Kessler

Chief, Optical Physics Division

III
Contents

Page Foreword iii Introduction 1 Part 1. The roles of mass metrology in civilization 3

1.1. The role of mass measurement in commerce 3

A. Prior to the metric system of measurement units 3

B. The kilogram and the pound 4

C. In the early United States 6

D. Summary 7

1.2. The role of measurement in technology 8

1.3. The role of measurement in science 10 Part 2. The basis for stating mass values 12

2.1. Statement of the problem 12

2.2. Imphcit and explicit treatment of displacement volumes 13

2.3. Values on the basis of an explicit treatment of displacement volumes 14

2.4. Values on the basis of an implicit treatment of displacement volumes 15

2.5. Conversion, one basis to another 17

2.6. The mass measurement system and the NBS mass measurement service 18 References 20 Appendices

1. Average air densities for a group of selected cities 22

2. Density of air 24

3. Typical weight materials, densities, and coefficients of volumetric expansion 26

4. Variability associated with volume differences and changes in air density 27

5. Displacement volume and the assignment of mass values 28

6. On the selection of an "ideal" weight material 29

7. Examples illustrating the use of apparent mass values 30

IV
Mass and Mass Values

Paul E. Pontius

There are several bases for assigning mass values to weights to be used as mass standards. As a consequence a given weight may have several assigned mass values depending on the basis used. In many eases, the differences between these assigned values, although easily detectable with precise

weighing equipment, are of no practical concern. However, in some instances these differences may be

crucial. The first part of this paper is a historical summary of weighing, standards, and the assignment

of value; and the interfacing of mass measurements with civilization. The second part of this paper

discusses in detail the methods of assigning mass values. Ways to convert from values on one basis to values on another basis are discussed. Sample problems relating to the buoyant effect of the air are presented in the appendices.

Key words: Apparent mass; buoyancy corrections; mass comparison; mass value; true mass; weighing.

Introduction

Measurement — A. specific sequence of operations

which are performed with the intent of estabhshing

a useful ordering.

Ordering differs from counting in that ordering is concerned with the uniqueness of a body in terms of

its properties while in counting, aU objects with

nearly similar properties are treated as being identi- cal [l].* Coins are counted to determine a total money value. One can order coins, however, accord

ing to weight, metal content, color and the hke.' By

ordering a sufficient number of properties, each coin

would be uniquely identified. Ordering in some form

or other has been a part of life from the earUest man. Measurement, a formalized ordering, consists of four

elements: a circumstance which makes ordering

necessary or desirable; a basis for the ordering; a

*Figures in brackets indicate the literature references on page 21.

' Stevens, Ref. [8], suggests counting as pairing a group of objects of a given class with the names of the numbers, the name of the last number in the last pair formed

being the number of the objects in the group. In discussing ordering, he refers to the

postulates of order:

(1) If a # A. then eithera < 6 or5 < a

(2) Ifa<6, thenoT^A

(3) If a < 6 and b < c, then a < c

where a, b and c are some particular attribute, # means is difTerent from and < is not completely defined. < could stand for less than, precedes, older than, etc. In the limit

L (a<b<c), a = b=c

a—c

In this case, if the number A has been assigned to represent the attribute of a, A also compares to the same attribute of b and c. Further, if an additive operation can be

defined for a particular attribute, this same logic can be used to develop an extensive magnitude measurement concept as suggested by Carnap, Ref. [9].

sequence of operations, usually centered on a construct'^ which serves to aid the senses in discerning

otherwise undetectable differences; and, since the

sequence of operations involves a construct, a means

of verifying the usefulness of the end result. The use of common measurements, such as weighing, length measurement and volume measurement, undoubtedly started in the Neolithic Age during a period marking the end of a hunting way of life and the beginning of a full metal-using economy [2]. In the beginning, for those items which could be

compared by these common measurements, relative value, or worth, was established by the physical size of some arbitrary embodiment of a measurement

unit — the heft of a particular stone, the capacity of

a particular pot, or the length of an arm or foot. The

conversion to conventional measurement standards was well estabhshed by the time the written record

begins [10]. In the second millennium, taxes in Egypt were paid in kind, but tax records were on the

basis of equivalent labor [3]. In Mesopotamia, a recognized standard of exchange was one mina of

silver being equivalent to 60 gur of grain [4] , the gur

being a volumetric measure [5]. An early mina

^Construct is used in the sense that a measurement is made with a collection of procedures and instruments which have been accepted more or less by tradition and

which are designed to have optimum response to particular properties or phenomena. The response may, or may not, relate to the desired information. For example, one can

determine the density of an object; however, extrapolating to obtain the density of a

material depends upon the characteristics of the object. Determining a "contained" or "delivered" volume with water will establish product uniformity for a particular type of glassware but the result may not be applicable for other types of fluid.

1
weight (ca. 2400 B.C.) was marked "one mina of wages in wool [6]."

These same common measurements, refined

somewhat, but fundamentally unchanged, remain

with us today. For most of us, our first introduction

to measurement occurred at an early age — learning

simple measurement procedures and the relation- ship of customary measurement units such as the

cup, the foot, and the pound. Understanding, however, requires something more than names for measurement units, a hierarchy of standards and ele- mentary measurement procedures.

Youden [7] observed that people who must rely on measurement data are frequently so absorbed in the broader aspects of a particular problem that they

give little, if any, attention to measurement detail. This observation is true of scientist and layman

alike. Many become highly incensed when chal- lenged on matters concerning measurement and

measurement philosophy. It is a fact, however, that most of our measurement knowledge has been

acquired haphazardly, that prevalent philosophies

have come from a past which was severely limited by

communications problems, and that in more cases than we would care to admit, precise measurements

are specified because we don't know what else to do.

If, in a given undertaking, we are successful, the success blesses all that has gone on before. If trou- bles are encountered, inevitably the procedural

details of the measurements are the first items to be

scrutinized, a fertile area for deception.

When all else fails, the questions which should

have been considered in depth at the very beginning

may come to the fore:

(1) Is the contemplated, or requested, measure- ment relevant?

(2) Is the estimate of the acceptable Hmit of error realistic?

(3) Is the data generated by the measurement

process reaUy what it is supposed to be?

When the stakes are not large, the backwards ap- proach might be rationahzed by accepting the costs

of a certain amount of meaningless measurement ef- fort and an occasional disaster as a part of the cost

of success. However, the scope and cost, both in dollars and resources, of most undertakings today are such that these questions must be considered

thoroughly in the early stages of a project, and decisions must be continuously monitored as the project

develops. There are countless examples of the dire consequences when one fails to do so.

It is not difficult to group into three categories the reasons why most measurements are made. These

categories are:

(1) Measurements to assure equity in trade and

commerce;

(2) Measurements in which the judgment of the

result is related to a particular function rather

than the abiUty to measure, such as in the

development of products or procedures and

the establishment of the uniformity thereof; and

(3) Measurements to assist in understanding

the laws of nature and the universe.

Complexity within and diversity of goals between

the three categories creates almost insurmountable

psychological barriers between the champions of various measurement philosophies. It is hard to separate the operations which are "necessary and

sufficient" for success from the practices that are used for pohtical and economic reasons, or merely

for convenience. Philosophies and the nature of problems within each category change with time.

Opportunities for the enterprising to employ diver- sions to their own benefit have always been present.

The fact remains, however, that one measurement

system must serve all, and that like measurements

generally employ the same equipment and

techniques regardless of the end usage; thus, it is important to understand why each of these categories is unique.

The importance of measurements in today's

society has been influenced by political and

economic factors associated with changes from an

agrarian to a commercial society, the industrial

growth following the dark ages, the scientific and in- dustrial explosion of the last two centuries and the

present electronic era of computers and rapid communications. Ultimately, regardless of the simplicity or complexity of the measurement, the result and its cost must be evaluated relative to its usefulness in accomplishing the task at hand. The work of the

metrologist, if his role is to endure, lies in his contribution toward obtaining proper measurement data

in the most economical way. Removing measure- ment from this supporting role creates a diversion,

directing attention from important matters to what

may well be trivial operational detail.

2
Part 1. The Roles of Mass Metrology in Civilization

1.1. The Role of Mass Measurement in

Commerce

A. Prior to the Metric System of Measurement Units

The existence of deliberate alloys of copper with

lead for small ornaments and alloys of copper with

varying amounts of tin for a wide variety of bronzes

implies an ability to make accurate measurements

with a weighing device ca. 3000 B.C. and perhaps

earher [11]. That trade routes existed between

Babylonia and India, and perhaps the Persian Gulf

and Red Sea countries, at about the same time implies a development of commercial enterprise

beyond barter [12]. Economic records were the

earhest documents and these in turn influenced both

the development of the written language and the

development of numbering systems [13,14]. The

transition between the tradition of an ilUterate craftsman working with metals and a universally ac- cepted commercial practice is largely conjecture.

The impartial judgment of the weighing operation was well known ca. 2000 B.C. as evidenced by the

adoption of the balance as a symbol of social justice

[15], a practice which continues today. Then, as now, the weighing operation will dispense equal

value in the form of equal quantities of the same

commodity. It was, and still is, easy to demonstrate

that the comparison, or weighing out, has been ac- complished within the practical limit of plus or minus a small weight or a few suitably small objects

such as grains of wheat or barley. In the beginning

there would have been no requirement that a stan- dard quantity of one commodity should have any

relation to the standard quantity of another commodity. The small weight or object used to verify the exactness of comparison could have been accepted

by custom. Wealthy famiHes, early rulers, or governments may have fostered the development of or- dered weight sets in order to account for and protect

their wealth. Measurement practices associated with

collecting taxes in kind would likely be adopted in all other transactions.

Ordered sets of weights were in use ca. 2000 B.C.

[16]. In these sets, each weight is related to the next

larger weight by some fixed ratio. To develop such a

set was a substantial undertaking. Individual

weights were adjusted by trial and error until both

the one-to-one and summation equahties were

satisfied within the precision of the comparison

process. Ratios between weights varied with

preference to numbers which had many factors [17,18]. For example, if 12B was to be equivalent to

/4, then in addition to intercomparing the 12fi's with

A , the fi's could be intercompared one by one, two by

two, three by three, four by four and six by six. Once

estabhshed, it was not difficult to verify that the

ratios were proper, nor was it difficult to duphcate

the set. Precious metals were used for exchange from the earhest times [24]. "To weigh" meant payment in metal and "to measure" meant payment in grain

[19]. Simple barter had become in essence sales. Goods of one sort being exchanged for goods of

another sort were separately valued to a common

standard, and these values brought to a common

total [20]. Overseas trade involved capitahzation,

letters of credit, consignment, and payment of ac- counts on demand [21]. There is evidence that a mina weight ca. 2100 B.C. was propagated by dupHcation over a period of 1500 years (to ca. 600 B.C.)

[22].

Maspero [23] gives the following description of an

Egyptian market transaction:

"Exchanging commodities for metal necessitated

two or three operations not required in ordinary

barter. The rings or thin bent strips of metal which

formed the "tabnu" and its multiples did not always

contain the regulation amount of gold or silver, and were often of hght weight. They had to be weighed at every fresh transaction in order to estimate their true value, and the interested parties never missed this excellent opportunity for a heated discussion: after having declared for a quarter of an hour that the

scales were out of order, that the weighing had been

carelessly performed, and that it should be done over

again, they at last came to terms, exhausted with

wrangling, and then went their way fairly satisfied with one another. It sometimes happened that a clever and unscrupulous dealer would alloy the rings,

and mix with the precious metal as much of a baser

3
sort as would be possible without danger of detection. The honest merchant who thought he was

receiving in payment for some article, say eight

tabnu of fine gold, and who had handed to him eight

tabnu of some alloy resembling gold, but containing

one-third of silver, lost in a single transaction,

without suspecting it, almost one-third of his goods.

The fear of such counterfeits was instrumental in restraining the use of tabnu for a long time among

the people, and restricted the buying and selling in the markets to exchange in natural products or manufactured objects."

The impact of coinage guaranteed by the government (ca. 500 B.C.) was profound and is still with us today [25,26]. One normally thinks that measure- ments associated with the exchange of goods in com- merce are ordering worth. This is only partly true from the viewpoint of the ultimate consumer. The

establishment of a monetary system permitted a

third party to enter the transaction without the dif- ficulty of physically handhng the material to be

traded. Assigning a money value to a unit measure

of a commodity permitted the establishment of a much broader market which was not generally con- cerned with each local transaction but which,

nonetheless, established in part the money value for each commodity in the local market. The customer,

then as now, must pay the asked price, the measure- ment process merely determining how much the

total transaction wiU be. Commerce thrives on the variation of commodity

values with time and location [27]. This variation,

coupled with confusion and perhaps a willful lack of

communication on matters concerning money value

and measurement units, is a happy situation for the

enterprising entrepreneur. As far as the normal

customer is concerned, the only element he has in common with the seller is the measurement process

and perhaps some preferential treatment associated

with social status, profession, or some other factor

totally unrelated to the value of the commodity.

Emphasis on the exactness of the measurement can mask more important factors such as the quahty of

the product offered for sale. Uniform weights and measures, and common

coinage were introduced throughout the Roman Empire [28,29,30]. Yet, perhaps with the exception of doing business with the government, it was not until the early part of the 18th century that the first real

efforts toward a mandatory usage of uniform measures was started. Many leaders through the ages

have made profound statements relating to the need

for uniform measures. Little, however, was done ex- cept in the control of the quality of the coinage. No

one ruler had been powerful enough to change the

customary measures and practices of his land. This was changed in France with the establishment of the

metric system of measurement units.

B. The Kilogram and the Pound^

It is not generally emphasized that the prime

motivation for establishing the metric system of measurements was the utter chaos of the French

marketplace.^ It was not that the conditions in the

French marketplace were any different than in any

other marketplace, but it was these conditions cou- pled with two other factors which eventually brought

about the reform. These factors were the French

Revolution whose great objective was the elimination of all traces of the feudal system and royalty,

and the influence of the natural philosophers of the

time who realized the international importance of such a forward step in creating a common scientific language. Other powerful influences objected

vigorously to the mandatory standards plan. After

the new standards had been completed they were

not readily accepted. Severe penalties were necessary to enforce their usage in the common measure- ments of the time. On the other hand, the metric

system of measurements almost immediately

became the measurement language of all science.

As with all previous artifacts which eventually

reached the status of measurement standards, the

choice for the basis of the metric standards was ar- bitrary. With the idea of constancy and reproducibility in mind, the choice for the length unit finally came down to either a ten-milhonth part of the length

of a quadrant of the earth's meridian, or the length

of a pendulum with a specified period. The nonconcurrence of most of the important foreign powers

who had been invited to participate in estabUshing

the measurement system left the French to proceed

alone. From the measurements of a segment of a meridi- an between points near Barcelona and Dunkirk, it was determined by computation that the meridianal

distance between the pole and the equator was

^This section is essentially an abstract of two papers. The Moreau paper [32] is an

excellent general paper on the development of the metric standards and the work of the International Bureau of Weights and Measures. The Miller paper [31] is a comprehensive work describing the reconstruction of the Imperial Standard Pound. Reference to specific passages are made in this section.

""At that time there was no shortcoming in the ability to make measurements as evidenced by the use of existing equipment and measurement techniques to estabUsh

the new standards. A comprehensive study of density, hydrometry and hydrostatic weighing had been published in the 12th century [38]. Instructions for adjusting weights for use in assay work pubUshed in 1580 are just outlines, implying that the techniques of weighing and the precision of the equipment are common knowledge amongst assayers [39].
5130740 toises, from which the ten-millionth part, or the meter, was 3 pieds 11.296 lignes. A unit for mass

was defined in terms of length and the density of water. The concept of mass was relatively new to science, and completely new in the history of

weighing which had heretofore been concerned with

quantities of material rather than the properties of matter. With the meter established in customary

units, using hydrostatic weighings of carefully measured cylinders, it was determined that a mass of one

kilogram was 18827.15 grains with respect to the

weights of the Pile of Charlemagne. With these rela- tionships defined in terms of customary units of

measurement, it was then possible to proceed with

the construction and adjustment of new standards

for the metric units. The first task was the construction of provisional

metric standards. The construction of the kilogram

and the meter of the Archives followed, the kilogram

of the Archives no doubt being adjusted^ with the same weights used to adjust the provisional kilo- gram. The kilogram of the Archives, as it was later discovered, had been adjusted prior to a precise

determination of its displacement volume. This important measurement was not made after adjustment

because of the fear that the water in a hydrostatic

weighing would leach out some of the inclusions

which were typical of the platinum of the time. While

the technical developments were going on, the

Treaty of the Meter was consummated, and the

General Conference of Weights and Measures was

established to review and finally accept the work.

Techniques were developed prior to the construction of the prototype standards which resulted in more homogeneous material (introduction of the oil fired furnace and the use of cold working). From a small group of kilograms made from the new material and adjusted in the same manner as the kilogram

of the Archives, the one which was most nearly

identical to the kilogram of the Archives, as deduced

from the data resulting from direct comparisons, was

chosen to be the prototype standard defined to embody a mass of exactly one kilogram. (This standard

is now generally called the international prototype

kilogram, designated by /T, to differentiate it from

other prototype kilograms which are designated by

number or letter-number combinations and used as transfer standards.) The task of manufacturing, ad-

^ Adjusting a weight is adding or removing material from a weight to establish a oneto-one relationship with an accepted standard. In the case of one-piece weights, such as the prototype kilogram, the weight to be adjusted is usually initially heavier than the standard. Material is carefully removed until the one-to-one relationship is estabhshed,

or until the difference is some small part of the on-scale range of the instrument being

used.

justing, and estabhshing the mass values of the

prototype standards for distribution to the nations

who were participating in the metric convention was

long and tedious. The survey to determine the length

of the arc of the meridian had been started in June

1792. The General Conference" formally sanctioned

the prototype meter and kilogram and the standards

for distribution in September of 1889. A second major effort in the construction of stan- dards for measurement was going on within this same period. In 1834 all of England's standards of volumetric measure and weight were either totally

destroyed or damaged by fire in the House of Parliament to such an extent that they were no longer

suitable for use as standards. The Imperial standard

troy pound was never recovered from the ruins. A

commission, appointed to consider the steps to be

taken for the restoration of the standards, concluded

that while the law provided for reconstructing the

standard of length on the basis of the length of a pendulum of specified period and for the reconstruction

of the standard of weight on the basis of the weight

of water, neither method would maintain the continuity of the unit. In the case of length, there were difficulties in car- rying out the specified experiment. In the case of

weight, differences based on the best determinations

of the weight of water by French, Austrian, Swedish

and Russian scientists amounted to a difference on

the order of one-thousandth of the whole weight,

whereas the weighing operation could be performed

with a precision smaller than one-millionth of the

whole weight. Therefore, it was recommended that the reconstruction could best be accomplished by

comparison with other weights and length measures which had previously been carefully compared with

the destroyed standards. It was further recom- mended that the new standard should be the avoir- dupois pound in common usage rather than the

destroyed troy pound. In 1843 a committee was ap- pointed to superintend the construction of the new

standards.

This work resulted in the construction of a platinum avoirdupois pound standard and four copies, the copies to be deposited in such a manner

that it would be unlikely that all of them would be

lost or damaged simultaneously. It was decreed that "the Commissioners of Her Majesty's Treasury may

''The General Conference of Weights and Measures (CGPM) assisted by the International Committee of Weights and Measures (CIPM) and the Consultative Committee for Units (CCU). makes decisions and promulgates resolutions, recommendations and

declarations for the International Bureau of Weights and Measures (BIPM). Reference

[37] reproduces in chronological order the decisions promulgated since 1889.

518-546 0 - 74 -2
cause the same to be restored by reference to or adoption of any of the copies so deposited [33]."

Careful work determined the relationship between

the avoirdupois pound and the kilogram. While it was not until 1959 that the English speaking nations

adopted an exact relation between the pound and the

kilogram, this work provided the basis for coexistence of the two sets of measurement units [34].

The relationship adopted differed only slightly from

that established as a part of the reconstruction pro- gram. (It was in this work that it was discovered that the displacement volume of the kilogram [35] of the

Archives had not been precisely determined before

final adjustment.)

The entire reconstruction was based on the ex- istence of weights RS and SP of known displacement

volume which had been compared with U. The

average air temperature and barometric pressure for several hundred comparisons (used in the above

definition) estabhshed a standard air density po. Knowing the displacement volume of the weight, T,

used to construct the new standard, from comparisons with RS and SP in air of known density, one can compute the weight that T would appear to have

if it were possible to compare it with U in air of density Po without knowing the density of U. In like manner, W above is a fictitious weight of 7000 grains

of the same density as U, the lost Imperial standard;

thus, the displacement volumes of weights must be

known in order to compute values relative to the

commercial pound, W.

This work included the construction and distribution of brass avoirdupois pound standards to approximately 30 countries, including the countries of the

British Empire. Recognizing the practical difficulties which would arise because of the platinum defining

standard and the brass standards for normal use, the

platinum standard was defined to be one pound "in a vacuum"^ and a commercial standard pound was

defined as follows [36]

:

"The commercial standard lb is a brass weight

which in air (temperature 18.7 °C, barometric pres- sure 755.64 mm). . . appears to weigh as much as W.

. . . For in air having the above mentioned temperature and pressure, the apparent weight of such a lb would be 7000/5760 of that of the lost standard."

The density of each of the new standards, both

platinum and brass, was carefully determined. The

^Weighings are not actually made in a vacuum. By properly accounting for the buoyant forces acting un the objects being compared, the data can be adjusted to obtain

the result expected if the weighing had been made in a vacuum. One can also include in the weighing a small weight which is nearly equivalent to the difference in buoyant

forces acting on the objects being compared.

assigned values, as computed from the comparison

data, were expressed in the form of corrections, or deviations from a nominal value of 1, both on the

basis as if compared with PS "in a vacuum," and as

if compared with W in air of the defined density. For

example, the correction for PS in a vacuum was ex- pressed as 0.00000 since under this condition it is defined as 1 pound; however, because of its small

displacement volume, if compared with W in air of

specific gravity log delta= 7.07832 — 10 (air density

approximately 1.1977 g/cm^), it would appear to be

0.63407 grain heavy, thus on this basis the assigned

correction was + 0.63407 grain. This action firmly

established two bases for stating values, one used to verify values assigned to standards with reference to the defining standard, and one to maintain the continuity of established commercial practices.

C. In the Early United States

In 1828, the Congress of the United States enacted

legislation to the effect that the troy pound obtained

from England in 1827 be the standard to be used in establishing the conformity of the coinage of the

United States [40]. Apparently it was declared by

Captain Kater, who had made the comparison with

the Imperial pound standard which was later destroyed, to be an "exact" copy [41]. It is assumed

that it was given the assigned value of 1 troy pound,

the uncertainty of the comparison, or the announced

correction, if any, being considered negligible. In

1830, the Senate directed the Secretary of the Treasury to study the weights and measures used at the

principal Customhouses [44]. As a result of this stu- dy, the Treasury Department set out on its own to bring about uniformity in the standards of the

Customhouses.

As a part of this work, Hassler constructed, along

with other standards, a 7,000 grain avoirdupois

pound based on the troy pound of the mint. It was re- ported later [42,43] that Hassler's pound agreed

very well with the c6py of the standard pound

furnished to the United States by England, as mentioned earlier. Eventually, this program was ex- panded by resolution [45] of Congress to include

equipping the states with weights, measures, and

balances. In 1866 the Congress enacted [46] that "no contract or deahng, or pleading in any court

shall be deemed invalid or liable to objection

because the weights or measures expressed or referred to therein are weights or measures of the

metric system." In due course the States were also furnished metric standards.

6
Gross changes in the form of the economy of the

United States have occurred. America has been

profoundly influenced by the nearness of the people

to the soil and the leadership that an agrarian society

develops [52]. As late as 1830 approximately 70 per- cent of the working population of the United States was involved in agriculture and other forms of food

production, and in producing raw materials. Only

about 20 percent were involved in manufacturing.**

In such an environment weights and measures had

a meaning in the value structure somewhat similar

to that of ancient times. Now, something on the order

of 30 percent are all that are involved in the area which includes producing food, raw materials, the

manufacturing of both durable and nondurable

goods, and construction. Thus, the number of items

in which weights and measures have any relation to the value structure is very few, the major cost to the consumer being associated with value added

rather than quantity.

The normal consumer can only choose from those

products offered, selecting on the basis of asking

price. The products offered, because of the high cost associated with estabhshing a large scale production, are only those which have a high probability of being desirable to the buying pubhc. While measure- ment may be necessary to establish the price to the

customer, there is no meaningful relationship

between the weights and measures and the unit price one must pay to acquire the item. One does not weigh automobiles or television sets. Where measurements are a part of the transaction, they are, in essence, merely counting operations similar in nature to counting out a dozen where items are priced

by the dozen. Under these circumstances, the vir- tues of precise measurement and the exactness of the standard do not guarantee equity in the marketplace. D. Summary

In retrospect at this point, it seems clear that both

the construction of the kilogram and the reconstruction of the pound were essentially scientific efforts directed toward assuring the longevity of the respective mass units. Both efforts required precise defini- tions and detail work far beyond that usually as- sociated with the previous history of weighing. Having estabhshed platinum standards, the assignment

of values to weights of other materials (mostly brass)

" The percentages have been estimated from various census reports. Because of the

different classifications used over the years, they are only approximate. They are, however, valid indicators of a shift from an agrarian to an urban society in a very short time

span.

required as much as, if not more, attention to procedural detail. The above two efforts, estabhshment and maintenance of the unit and cahbration, together with

normal usage has, in effect, polarized activities into separate groups: one group which works with defining mass standards, one group which works with

practical everyday weighings, and in the middle a group which ostensibly translates the scientific into the practical. The degree to which such a hierarchy can be effective is related to the extent to which a specific end use can be characterized. If a measure- ment process requirement can be completely

specified, one can devise a plan which will reduce a complex measurement to a simple operational routine. Such an engineered system, however, is not al- ways adequate and may be completely misleading in other areas of usage. The intellectual elegance of the metric system was

lost almost from the start. A careful redetermination

of the density of water created a situation in which,

according to the original definition, the value as- signed to the prototype kilogram would be in error by

about 28 parts in a million. To change the value of

the prototype and all of its copies was unthinkable,

therefore a new "volume" unit was proposed to replace the cubic centimeter. By conference action

in 1901 (3d CGPM, 1901), the unit of volume, for high-accuracy determinations, was defined as the volume occupied by a mass of 1 kilogram of pure

water at its maximum density and at standard pressure, this volume being called the liter. While it is doubtful that the discrepancy was at all significant

in common measurement, the liter has been ac- cepted almost universally. This caused no end of problems concerning both volume and density measurement. The circle has been complete for in 1961

(CIPM, 1961) the cubic decimeter was declared the

unit for precise volume measurement, relegating the

liter to the realm of customary units which still prevail.

Quite apart from the use of weights in commerce,

various technologies over the centuries used weights

as a convenient way to generate forces. The use of suspended or stacked weights to measure the draw

of a bow, the ability of a structure to support a given

load, and to characterize the strength of various

materials has been prevalent throught history and

continues today. This led to an ambiguity in both the names assigned to the units and to the comparison

operations. In 1901 (3d CGPM, 1901), the General

Conference considered it necessary to take action to

7
put an end to the "ambiguity which in current practice still subsists on the meaning of the word weight,

used sometimes for mass and sometimes for mechanical force."

The Conference declared: "The kilogram is the

unit of mass, it is equal to the mass of the international prototype kilogram. The word weight denotes

a quantity of the same nature as force, the weight of a body is the product of its mass and the acceleration

due to gravity, in particular, the standard weight of

a body is the product of its mass and the standard ac- celeration due to gravity."

This did not end the confusion [47,48]. Such a statement made no sense at all to those who were

concerned with commercial weighing. To officially

sanction such a definition of weight is to refuse to recognize that at some time the use of a standard ac- celeration of gravity in Ueu of the appropriate local acceleration of gravity would introduce significant

systematic errors in many measurements [49].

The situation has been rectified by including the

Newton as an accepted unit for force in the supplementary units of the International System of Units,

known as the SI system (11th CGPM, 1960). By this action, the meaning of the words weight and

weighing could revert to more general meanings, for example: weight — an object which embodies a mass

or mass related property of interest; weighing — to make a quantitative comparison.^ While this action

may in time discourage practices such as introducing the term "massing" [51] as meaning to make a mass measurement, universal acceptance may never be achieved because of the natural tendency of the

hterature to propagate what has gone on before.

1.2. The Role of Measurement in Technology

The innovative instinct of the earliest man, which was the basis of his survival, provided the start of technological development. The emergence of the

arts and crafts represents the cumulative experience

resulting from progressive novel action and invention which for the most part was based on perception

rather than thought [53]. Having conceived a novel

action of sufficient functional importance, such as the use of a fhnt blade in Ueu of a sharpened stick,

successive refinements follow, i.e., the bronze blade,

the iron blade and the steel blade, as long as the

^ A facsimile of the first edition of Webster's Dictionary [50] gives the following

definitions:

mass —a tump

weight — a mass by which bodies are weighed

weigh —to try the weight, consider, examine, judge .... etc.

function remains important to society. Fhnt was a trade item in the Neolithic age [54,55]. Most of the

primary crafts were highly developed and widely dif- fused before the development of historical records

[56]. Essential elements in the transformation to an urban civihzation were the conversion of luxuries

into necessities and the increase, and concentration,

of social surplus [57]. While the gourd was functionally satisfactory, the clay pot was nicer; thus the

potter shifted from a part time operation to full time,

and ultimately to the pottery factory and a brisk in- ternational trade in pots, as existed ca. 400 B.C.

[58].

It is difficult to trace the details of the various

crafts. The Sumerians, for example, thought that all knowledge came from the gods, therefore it was

sacred and could not be communicated. The priest

passed on instructions orally being careful to hmit

instructions to the exact steps to be followed [59].

For the craftsman, his knowledge was his livehhood.

Traditions were passed from father to son. FamiUes

became noted for their particular crafts. Later,

where products and trades were concerned, to divulge details was to invite economic disaster from

competition. The impressive state of development

reached, however, can be observed in the artifacts produced and the longevity of some of the

techniques. An example of the latter is the

"touchstone" tests for purity of gold and silver alloys

which made possible the issuance of coinage.

Agricola described in 1556 essentially the same

tests, indicating a longevity in excess of 2000 years

[60]. In terms of the development of the crafts and the

dissemination of the products, the Roman Empire was remarkable. While somewhat short on invention, the Romans perfected masonry, tiling, road

building, surveying, molded pottery, blown glass,

watermill, and a host of others [61]. The use of

glass, for example, in a wide variety of apphcations

including commercial packaging reached a scale un- matched before the 19th century [62]. That these

could not be accomplished without measurement

clearly emphasizes the fact that, where function is the main concern, all measurements are relative. Things work because relative geometry, proportion,

or properties of materials are correct, not because of any particular choice of measurement units. Mortar,

for example, lasts through the ages because the in- gredients have the right properties and are combined

in the right proportions. Machinery works because

each part has the right characteristics and the rela-
tive dimensions are correct. Each craft had to develop its own methods for determining and

describing the parameters which were critical to its particular trade or profession.

Early crafts encompassed the entire operation

from raw material to finished products. As the demand for finished products increased, the time the

craftsmen could afford to spend in making ready raw

materials lessened. In some instances, the materials

in a product came from several distant sources. These situations led to the development of early in- dustries concerned basically with raw materials such

as charcoal and metallic ores, and with quarrying,

lumbering, and weaving. This action was the first breach in the tight security of the craft system. Craft

Guilds appeared during the Medieval Age, and the

resulting "codes" were probably more directed

toward protection from competition than convincing

the possible chents of the perfection of the product.

For example, in 1454 the penalty for divulging the

secrets of Venetian glass was death [64]. Craft

mysteries persisted until the Industrial Revolution

ca. 1750 [65]. The inventions of the 18th and 19th

centuries brought about changes which are considered to be the Industrial Revolution. These

changes can be summarized as follows: (1) a shift from animal and wind power to coal and steam, (2) the effects of this shift on the iron and textile industries [66] , and (3) the change from working for a livehhood to working for a profit [67]

.

The forerunners of industry as we know it today

stem from the miUtary. The first large-scale demand

for standardized goods was the provision of uniforms

for large standing armies [68]. The use of in- terchangeable parts in the assembly of muskets and

rifles was demonstrated by LeBlanc in France, and

Whitney in the United States [69]. Through the

years, the dividing hne between raw material supply

and preprocessing, such as the production of pig

iron, steel and cloth, and product manufacturing has

become more prominent, with the preprocessed

materials becoming more Hke other commercial

commodities. Most items which are procured today,

either by the individual or by the government, are

the results of the combined efforts of many

throughout the world. Industrial subdivision, or compartmentaUzation with its large economic benefits,

has created a special role for measurement. The

material or preprocessed material supplier enlarges

his market by resolving small differences in requirements among his customers. In time, the terminolo- gy of the supplier must be accepted by all who use

his material; hence measurements become wed to marketing requirements rather than functional

requirements.

Subdivision of a task requires a detailed dehneation of what is to be done by each sub-unit. This can

take the form of organization charts, specifications,

detailed drawings, samples, and the hke. Many ways

are used depending upon the nature of the item and

its function in the overall task. If someone else is to provide the service, some limits must be established

for judging that the offered product will perform as intended in the overall endeavor. Determining the

dividing Hne between success and failure is not al- ways easy. These hmits, once estabhshed and re- gardless of whether they were estabhshed by lengthy

experiments, good engineering judgment, or by

sheer guess, become fixed restraints on the next ele- ment of the subdivision. The effect is a dilution of

the abiUty to make function related judgments. In complex situations, no one person knows the fuE

scope of the task, therefore no one can instigate

changes of any sort without fear of jeopardizing the

entire venture.

It is a tendency for tolerances to be tightened by

each organizational element through which the task

must pass. In the procurement-production stage, the

product must comply (within the tolerance) to the

specification or drawing. Comphance is defined by

a set of procedures, usually measurements, which

supposedly will assure the buyer of the suitabihty of

the product for its intended use. The net result is that the most precise measurement processes are frequently used to differentiate between scrap and

acceptable parts in order to consummate a particular

contract, the sorting hmits in many cases having Utile relation to the function the parts must perform.

Troubles are merely transferred to the gage if the

measuremehts are differences between the part in question and a pseudo standard or gage. Difficult problems occur when a specification attempts to describe a complex part completely by dimensions

or specification verbiage.

The mechanism for verifying specification compliance is created for the most part by those who do

not fully understand either the measurement or the

function. Many procedures rely on rituahstic documentation with Uttle attention given to the characteristics of the measurement processes which are used. In many instances the status of the source of

the documentation becomes more important than

problems relating to the environment in which the

required measurements may be valid and the en-
vironment in which the measurements of the product

are to be made. It is not unusual to find that a

prerequisite for doing business is the possession of

such documents and precise measurement facilities which often do not relate to the completion of the

task at hand.

However, in those cases where measurement data

are really critical, the most important measurement

is that on the production floor. The part or assembly

will either operate properly or not regardless of the

supporting hierarchy. The most precise measure- ments could, if necessary, be moved directly to the

production floor to achieve the desired function.

Today, there is little doubt that the solutions of the

most difficult and challenging measurement

problems are being carried out in an environment of

strict industrial security. This is similar to development in the days of the guilds. However, now exter- nal communications are necessary. The present

economic facts of hfe make it necessary to know

what is going on in related science and industry so

that each new task is not a "re-invention of the

wheel." A recent report suggests that innovations

important to one industry may come from a completely nonrelated industry [70]. On the other

hand, to divulge certain information at the developmental level is almost certain to result in an economic setback, perhaps even a catastrophe in the raw materials market, the product market, or in the

capital market, sometimes in all three.

1.3. The Role of Measurement in Science

In sharp contrast to both previous areas of discussion, the advancement of science depends completely upon a free and open exchange of information

[71]. Thus, having agreed to accept an arbitrary set of measurement units, it is imperative that the continuity of the units be maintained. By constructing a minimal set of units and constants from which all measurement quantities of interest can be derived,

ambiguities are removed. By defining a means to reahze each unit, in principle one can construct the

units he needs without introducing ambiguity into the measurement system. What happens in practice

is, of course, another story.

Most defining experiments are complex and tedi- ous and not always related to the problems of measuring things or describing phenomena. Having

established a definition of the unit of time based on

an atomic phenomenon, and having constructed the

hardware to reahze the unit, the ease by which the

unit can be disseminated by broadcast makes it highly unlikely that more than a few would seriously

consider dupUcating the effort. Mass, on the other

hand, is and will no doubt for some time be embodied in a prototype standard to be disseminated by

methods which are in essence many thousands of

years old. By international agreement, the Sl-defined measurement units together with a substantial group of auxiliary units have replaced and augmented the

original three — length, mass and volume — of the

metric system. Having accepted the structure of the

SI, the definition, or redefinition, of the measure- ment units, insofar as possible, must maintain the

continuity of the original arbitrary units. Further, the

uncertainty of the unit as realized must be compatible with the exploratory experiments in which the

unit may be used.

One requirement for a phenomenon to be con- sidered in redefining a unit is that under the contemplated definition, the newly defined unit would be more stable than the unit under the current defini- tion [72]. Having verified that this would be the case, the next task is to determine the unit in terms

of the new phenomenon to a degree such that the uncertainty of the unit as expressed by the new

phenomenon is within the uncertainty Kmits as- sociated with the unit as expressed by the old phenomenon. The important point is this action re- lates only to the definition of the unit, and may not be

extendible in any form to the manner in which the

unit is used to make other kinds of measurement.

Because all units are candidates for redefinition, and

because one is now able to evaluate the performance

characteristics of a wide variety of measurement

processes [73], a new definition for the "best" measurement process must be established. In the distant past, a weight was attested, or certified, to be an exact copy of another by the reputation or position of the person making the comparison

and by his stamp on the weight. Having obtained

such a verification, one was free to use the marked

weight as he wished. The report of calibration from

a currently existing measurement facihty is in es- sence no different. Throughout history, the status of

the standard with which the unknown was compared

and the status of the facility doing the comparison

established the quality of the work. Since all methods of comparison were essentially the same, to refute all criticism one might decide to pay more and

wait longer in order to utihze the highest status
facility of the land. Little attention was given to the

consistency of the measurements at operating level because there was no way to manipulate the masses

of data required to evaluate a single measurement

process, let alone a whole series of interconnected

processes. One was paying for a judgment.

It has been well known from the beginning of precise measurement that repeated measurements

often produce different numbers. The man who put

his mark on the weight was in effect saying that it is close enough to some standard to be considered as an exact replica. The report of caUbration says "call

it this number," the number sometimes being ac- companied by an uncertainty which is ridiculously

small with reference to any practical usage, or when

stated as a deviation from some nominal value, the

deviation or the number being so small that the user may consider the item as exactly the nominal value.

It is now possible to look in detail at the per- formance characteristics of a measurement process

[74] and at the consistency of measurement at any

point in the entire system [75]. Further, the cost of

relating a measurement to the manner in which the

unit is defined may be prohibitive if indeed it is at all possible. Under these circumstances, the definition of the best process must start from the end use

rather than the defining standard. Having first established that a particular measurement is necessary to the success of the venture at hand, the best

process is that which produces these results in the

most economical manner, based on verification by

demonstration. This applies equally to the most complex scientific study or the simplest measurement.

As a point of departure, it is necessary to make it clear to all the basis on which certain mass values

are stated.

11
Part 2. The Basis for Stating Mass Values

2.1. Statement of the Problem

The nonrelativistic physical laws concerning the mass properties of objects were formulated by Newton [76], thus relative to the history of weighing, the

concept of mass as a property of an object is a recent

one. Because of the size of the solar system, Newton

could consider the planets and their satellites as mass points in the vacuum of space [77]. On the

other hand, most weighing devices are force comparators which operate in the normal environment of

the surface of the earth. The typical weighing instrument responds to a

vertical force vector. The principal component of

this vector is gravitational force. Next in order of

magnitude is the buoyant force of the atmosphere.

There are other vertical components which affect the instrument indication such as might be as- sociated with air currents, electrostatic forces, magnetic forces, etc. [78]. However, if the object under

study is sufficiently stable in mass, these other

forces can usually be controlled with careful design

and operation so that their effect on the weighing in- strument performance is insignificant.

The buoyant force of the atmosphere precludes a mass point assumption (a mass with no displacement

volume). This force is proportional to the displacement volume of the object under study and the density of the immediate environment. If ignored, it is frequently the source of the largest measurement

discrepancy in precise mass measurement. If not understood, much time can be wasted on detailed computation with no tangible benefits to the work at hand [79].

The response of the weighing instrument to an ap- pUed force system is a number indication or observation, Oi, expressed symboUcaUy as: gi.^-pVy)oOr (1)

where J/^'" represents the mass property of one object; p the density of the surrounding environment;

Script letters are used to denote the mass property embodied in the objects of in- terest. Italic letters are used to denote the numbers assigned to express the mass property quantitatively relative to a standard of defined mass. For example, given an object with mass defined as one unit, yK'= 1 = A', and an unknown mass^, the mass properties .V and determine in part the mass difference as observed. The number assigned to the unknown,^, is deduced from the data produced by the measurement process.

Vy the displaced volume; g the local acceleration of

gravity; and Oi the observation. The symbol ^

means that the observation Oi results from the

operation of putting the object whose mass is on

the balance pan. (The symbol Oj, j=l to n, represents the number reading produced by the in- strument. The sequence is repeated for each group

of interrelated comparisons.)

One would hope that there would be an exact oneto-one relationship between the observation, Oi, and

the mass of the object, S^, so that no further effort need be spent on the measurement. That this is not

the case can be shown easily by following the same

procedure with a second object which has the mass

M'^ to obtain observation O-z: - pV.) o O2 (2)

Subtracting (2) from (1), and introducing a factor,

to convert the observed numbers to mass units.

{^-pV^) - -pV^)=K{0r-02)

or {y-^')=KiOr-02)+p{V^-V^) (3)

In a normal environment, equality in observation,

(Oi — 02) = O, means equaUty in mass, if, and only

if, the displaced volumes are equal. In most cases a true estimate of {^ — ^) cannot be determined

directly from the observations. In order to illustrate more clearly the nature of the difficulties this situa- tion causes, one must examine the traditional method of number assignment.

If there exists a "master" artifact with a defined mass value A'^ (the named or nominal value), and if our first weight is considered to be a copy of this artifact, the value S is usually expressed by two numbers; the "defined" value of the artifact, N, and a correction Cts such that S = + Cr,,. The value, X, to be assigned to a second weight,

would also be expressed as a nominal value, A'^, plus a correction Cr^, where Cr^ would be computed from

"The operations required to establish a numerical value for intioducegon the

right hand side of the equation. As shown, K includes the factor Later this factor becomes {bIs) or 1 if ail of the weighings required in the sequence have been done under

the influence of the same ^ The details of this operation are not discussed in this paper. 12
Crs and the measured mass difference, {9' — ^)^

as determined in (3) above.

= N+ Crj.

where

Corrections can be used in two ways. A number,

say 0.98, can be expressed with reference to a nominal number, say 1, by the relation (nominal +

correction) =(1 - 0.02)= 0.98. On the other hand, one might view the absolute value of the correction,

|— 0.02|, as a means of judging the closeness of 0.98

to 1. The latter usage is by far the most prevalent. In many cases it is assumed that {S^ — ) - K{Oi —

O2) , thus adjustments of df*" to obtain a small Crx

may be illusory if the magnitude of p(Vy — V.^) is large. The fact that every object displaces a volume

of air must be treated in some way in aU practical mass measurements.

2.2. Implicit and Explicit Treatment of

Displacement Volumes

The term implicit, as applied to the treatment of

displacement, is used to characterize one method for determining a number to represent the mass of an

object. The mass values assigned by this method,

while somewhat arbitrary, are consistent within

prescribed limits and are suitable for many end use

requirements. Detailed treatment of displacement is not generally required in comparing weights since it

is considered in formulating the method. To illus- trate, given an object that has an assigned mass

value S, consider the task of assigning a value, A^*, to another object.

The implicit treatment of displacement volumes

accepts the observed difference, K(Oi — O2), as an

estimate of S — X. Thus

X*^S - K(Oi - O2) (4)

In such a procedure, X* is the mass that the un- known, would appear to have when compared

with an accepted standard. Such a procedure would

certainly be convenient, and is indeed the case when

S is a valid estimate of y\ and when the displacement volumes for y and # are identical. When

these conditions do not apply, as is the case in many

mass measurements, X* is not necessarily a valid estimate of the mass, at, in terms f)f the taws (»f physics.'^ While the consistency of the numbers as- signed on this basis can be demonstrated, \X* — X\

may be large compared to the precision of the measurement process. The explicit treatment of displacement volumes

strives to establish X as a. valid estimate of / . In such a system the number assignments are consistent and the values are valid estimates in terms of

the laws of physics. In this case, from (3) X = S-{^-^)

X = S-K(Or- O2) - p(V, - V, ) (5) One must determine the air density p, at the time

of each weighing, and one must have reasonable esti- mates for the displacement volumes, V<f and V

Historically, values have been assigned by a procedure in which displacement volumes have been

treated implicitly. For weighings which are made

primarily to establish equahty of goods in trade, the

buoyant force is, in many cases, substantially

smaller than other accepted variabilities inherent to the material being weighed, i.e., least count,

evaporation, smallest accounting unit, etc. On the

other hand, faith in the system by the populace depends largely on the existence of good stable stan- dards and the ability to demonstrate a consistency

beyond that needed under normal circumstances.

The demonstration should be simple, direct and

within the capabilities of most practical weighing

processes. With the advent of the metric system and its stan- dards, it became necessary to treat the problem of

displacement volume explicitly — at least to the point

of establishing the kilogram and the mass of the

customary standards then in use relative to the kilo- gram. In this process, the customary standard was

not changed — only the ratio of mass between it and

the kilogram was established. This permitted a conversion between customary units and the kilogram

but left the practical measurement system

unchanged.

While on the surface the assignment of mass

values appears to be perfectly straightforward, in fact it is not. There are many subtleties involved.

Troubles occur in several ways. For example, the

characteristics of the defining standard used in the

In force measurements, for example, the use of "apparent mass" values w ilhoul modification can introduce significant systematic error. Using an apparent mass value and an assumed density of 8.4g/cni^ can leave a residual systematic ern.>r which may be

significant in the most precise force and pressure measurement processes [82].

518-546 0 - 74 -3
implicit volume treatment are different in various

countries. In such a situation, the value assignment

for a given object is not unique, but differs by measurable amounts depending on whose standard is used. Further, the value assignment may differ from

the actual mass by measurable amounts.

The implicit volume basis for stating the mass

value, while specifically defined at the level of the

highest calibration laboratories, is not generally un- derstood by all who must make precise mass measurements. Detailed adjustment of values stated on one basis to values appropriate to another basis is frequently tedious. "Apparent mass" or "weight in air" are frequently used in situations where an explicit treatment would be more appropriate. These

points are not necessarily faults of the impUcit treat- ment but they emphasize a need to understand the

details of such a system and the Limiting boundary

conditions associated with its general usage.

2.3. Values on the Basis of an Explicit

Treatment of Displacement Volumes

The explicit treatment of displacement volumes

requires a detailed treatment of the term p(Vy— V,f ) in eq (3). The magnitude of this term can be illus- trated by considering a kilogram weight made from

platinum-iridium alloy (a material far too dense and

far too expensive for everyday use) and a kilogram

stainless steel weight. Inserting approximate values:

p(Vss - V^)=\.2 mg/cm3(125 - 46.5)cm3 = 94.2 mg

For a mass measurement process which has a standard deviation on the order of .050 mg (not un- common), it is apparent that daily fluctuations in air density on the order of 0.1 percent can be a signifi- cant source of variability in a sequence of repeated

measurements. While not treated in this paper, it should also be noted that the buoyant effect must be

considered in establishing the constant used to re- late instrument indication to mass units, as shown in many of the equations.

The density of the environment, p, is a function of

the composition and of the pressure, temperature,

and relative humidity at any given instant. The composition does not change rapidly, but the other three

continually change in a cycUc manner. This causes

a local variation in air density of perhaps as much as 3 percent. Variability in air density between various

localities, because of changes in pressure with

change in elevation, may be as much as 20 percent.

Tables in appendix 1 show the approximate average

air density in various locations throughout the

United States. Data on atmospheric pressure, temperature and

relative humidity are usually taken before and after a given series of weighings. The National Bureau of

Standards mass laboratory uses the formula given

in appendix 2 to compute the average air density over the elapsed time interval. In less precise measurement processes the average air density from ap- pendix 1 may be adequate.

The displacement volume depends upon the

characteristics of the material and the manner in which the weight or object has been constructed. It

is also a temperature dependent quantity. In critical cases, the displacement volume at a specific temperature, fo, can usually be determined by a separate

experiment in which the object is weighed in two

mediums to obtain two relations:

from which

(.^ - p,VJ) -(y- pfV,^) = K(0, - 0;)

or, reduced to temperature to; K{0,-0;) yo ^

{pf — Pa)

(6)

where p i is the known density of a light medium

such as air, and Pf is the density of a heavy medium

such as water.'^ Sometimes a suitable estimate for displacement

volumes can be computed from published values for the density of material used in the construction of the weights. In some cases, particularly where large

weights are involved, displacement volume esti- mates can be computed from physical measurements. A stated volume, VJ^, is appropriate at only one

temperature, to, thus to determine the volume, fy, at another temperature, t, the coefficient of expansion,

OLj, must be known or estimated.

vAi + ccjt-t,)]

From equations 1 and 2, with the iexmK(0\ — O2)

expressed in mass units, the value X, based on a "known'' vakie S for the standard, X, can be comIn the case of determining displacement volumes for materials which cannot be submerged in a fluid, reasonable estimates can be obtained from sequences of weighings at various locations where the average air density is markedly different. 14
puted from one of the f(»llowing two formulas or combinations thereof:

S - pVJ>[l + a^(t - to)] - X + pV/[l-\-aJt - to)] = K{0,-0,) (7) or

1 p[l + aj,t - to)]'

X

p[l + ajit - to)]

= K{0r-02] (8)

Detailed calculations to obtain X can range from

simple hand computations to extensive computer

solutions of incomplete block comparison designs.

In any case, the basic relations are as shown in (7) and (8) above. Appendix 3 lists the material densities

and coefficients of expansion used by the National

Bureau of Standards in those cases where more

specific data are not available. Explicit treatment of displacement volumes, while

detailed, has an advantage in that any time a more

appropriate estimate of displaced volume or density

is obtained, previous data can be reanalysed using

the new volume estimates. This, in turn, changes mass measurements from a series of isolated comparisons from which one can only accept the last value into a related sequence of comparisons each of

which contributes to estabhshing the appropriate

value estimate.

An assessment of the adequacy of an exphcit treatment of displacement volumes cannot be made

on the basis of one measurement. Verification of the

validity of the numbers assigned is in the consistency of the results of experiments which involve mass

along with other measurement parameters, and in the ability to demonstrate that the values assigned

over a wide range of conditions agree within the

statistical limits for the measurement process.

2.4. Values on the Basis of an Implicit

Treatment of Displacement Volumes

Stating values on the basis of an implicit treatment

of displacement volumes stems from a practice dat

ing far back in history. The adjustment of one weight

relative to another to obtain an "exact" copy implies

a "zero" observed difference, or at least a difference

within some prescribed hmits. If the adjusting

process reduces the magnitude of the term K(Oi —

O2) from eq (3) to the point that it can be ignored, the

value assigned to the unknown could be assumed to be the same as the accepted value for the known.

That is, if: K(0, - O2) O

then, from eq (3),

Ignoring the term p(Fy^— Fy), it would seem that {y — ,^) 0 and the apparent mass of^ would be: ^* = S + € = S- [(^-.2) - p(V^-V,)]

Expressed in terms of material density:

X'

Estabhshing an "ideal" material of density d* to replace d'y, and limiting the density, d/, of materials

to be used in the manufacturing of weights, e can be

held to predictable limits over some range of air density, p. The apparent mass can be stated another way.

The apparent mass, X*, is the amount of any

specified material which will "exactly" balance the

unknown in a specified atmosphere, po=1.2

mg/cm'', at the specified temperature, f = 20 °C. For a standard with defined mass value A', made

from material of density d^, the computed apparent mass value is: N 1 ds

1 - pn

(9)

where po is the defined air density and f/* is the density of the appropriate "ideal" material.

One can designate a practical mass standard to be one or more weights which are made from material

which is essentially identical to the "ideal" material

and which are closely adjusted to nominal value on

the basis of an explicit treatment of displacement

volumes. From this point on, other weights can be

adjusted, or one can establish values for other

weights, directly from the comparisons with the

practical mass standard. Following a definite procedure, the mass values are assigned to weights

without detailed corrections for the buoyant effect of the environment and these values can be verified within an impressively small Umit. The simphcity of such an approach is lost, however, when, in the next

15
step, the density of the object in question is not within the range normally specified for weights.

It has been traditional in the United States to use "normal brass" as the "ideal" material. NBS Circular 3 [87], in 1918, estabhshed the precise basis for stating values as:

". . . it is necessary to establish the values of commercial weights on a definite basis, and to this end

all corrections and tolerances apply to the apparent mass as determined in air having a density of 1.2 mg

per milliliter, against (brass) standards having a density of 8.4 g per cubic centimeter at 0 °C, whose coefficient of cubical expansion is 0.000 054 per degree

centigrade and whose values are based on their true mass or weight in vacuo."

Circular 547 issued in 1954 [87], no doubt for- malizing a prior practice, expanded the definition as

follows:

" 'Apparent mass vs brass' values are those that the

weights would be assigned on the basis of a comparison at 20 °C in normal air against normal brass

standards ... no corrections would be applied for the

buoyant effect of the 'normal air' (defined as having

a density of 1.2 mg/cm^), . . . (normal brass standards

being defined as standards composed of brass having a density of 8.4 gjcm^ at 0 °C and a coefficient of

cubical expansion of 0.000 054 per degree Celsius

(centigrade))."

Circular 547 also required for each new set of

weights a manufacturer's statement as to the density

and composition of the material used to make the

weights, the nature of the surface protection if used,

and the type of construction.

The above specification has been the basis for consistent practical mass measurements in the

United States for some time. These statements imply

a direct comparison with a brass standard, a procedure which is suitable for many purposes.

While the apparent mass value assigned is normally

used in a manner which treats the displacement

volume implicitly, many laboratories, including the

National Bureau of Standards, assign such values on

the basis of exphcit treatment of displacement

volumes.

Material density, like displacement volume, is temperature dependent. At the present time, most

density data is given at i= 20 °C, the specified temperature at the time of "comparison." The density of

the "ideal" material, 8.4 g/cm^ is stated at 0 °C.

While this "ideal" material is called density 8.4 g/cm^ the effective density at 20 °C is 8.3909

g/cm'','* the above equation being

N( 1 N* = -

0.0012

dN

1- 0.0012

8.3909

(10)

"Apparent mass" values have interesting properties. Consider two weights with mass S8 and

and with assigned apparent mass values^* andfl*.

The observed mass difference between these

weights when compared in an environment of density pi is: ^,0,-p.)=^(l-^)-^(l-|j)

The assigned number difference, however, is:

A*-B*=

A[ 1-^\-b{ 1-^

po

Subtracting the one equation from the other and simphfying:

K{0,-02)-{A*-B*)

If the two weights are well adjusted and made from

the same material, the observed difference and the

assigned number difference in essence agree exactly. Minor variations, due to slight differences in the density of material and in the adjustment could

easily be smaller than the claimed level of instru- ment performance.

If d^ and d^s are approximately equal to dg^ as is the case for most normal brasses or stainless steels,

then:

(po-P.)(4-|-)=ApAr

From the definition of density as mass per unit volume:

r <iK„ <i„F„ Km K„[l+a(f-/)]

For brass, defined to be of density 8.4 g/cm-* at 0*C, with a defined coefficient of cubical expansion 0.000 054:

8.4 [1+0.000054(20)] - ^^'^ 8/cm»

16
The magnitude of Ap is not hkely to exceed the range

of air density over the geographic area in which ap- parent mass values are used. The magnitude of AF

is restricted by the range of material densities per- mitted in the construction of weights. The product,

ApAF=€, is usually small relative to use requirements so that the observed difference is a reasonable

estimate of an assigned number difference based on

apparent mass, thus: K(0^ - 02) = (A* - B*) + € (11)

It is not a universal practice to state apparent mass values with reference to an 8.4 g/cm^ material.

The choice of "8.4" material as an ideal material un- doubtedly dates far back in history. It is now a practice in some areas to state apparent mass values with

l eference to a material of density 8.0 g/cm^ at 20 °C

[80. 81, 91]. (There may also be other bases for stat- ing values.) The use of material of density 8.0 g/cm^

as the basis for stating values is particularly attractive to those who are interested in the refinement of

the intercomparison of modern weights without the

bother of added computational work. This can be

achieved if good, stable artifact standards can be obtained in which the density is the same as that of the

ideal material. A further treatment of the comparison between "8.4" and "8.0" material is presented in section 2.5 and appendix 6. While the practice of treating displacement

volumes implicitly is rather complex, it is possible,

as has been demonstrated over a number of years, to obtain a high degree of consistency. The ability to demonstrate with ease the consistency of assigned

values to well designed weights which have been

carefully calibrated with proper treatment of dis- placed volume is undoubtedly an important factor in the development of modern, precise, direct indicating weighing equipment. It has, however, created a

situation in which different numbers are assigned to represent the same property:

X= the mass of the object, or X* = the apparent mass versus brass of an object, or X** = the apparent mass of the object versus

density 8.0 gjcm^ material.

The differences between these numbers can be

significant in certain situations.

2.5. Conversion, One Basis to Another

In many practical weighings the small inconsistencies between mass values assigned on different

bases are of academic interest only. In precise

weighing, however, it may be unexpected to find that a mass value assigned to a particular weight can be

verified on one direct reading instrument, but on

another instrument of the same type, the assigned

value appears to be in error. Such discrepancies are

typical when weights with mass values assigned on

different bases are intercompared. This section

presents some of the methods for converting values on one basis to equivalent values on another basis. A. Given the mass X of weight find the

defined apparent mass versus brass value, A^*, and

the defined apparent mass versus density 8.0 g/cm^

material, Z**. The density of the weight at 20°C,

dx°, must be known.

XI

x* =

0.0012\

1 - 0.0012\

8.3909/

(12)

X** — XI- 0.0012 \ dl )

I 0.0012\ (13)

8.0

B. Given the apparent mass value, A^* orA'**,find

the mass value of the = weight. The density of the

£^ weight, rf^, must be known.

V 8.3909j _

^ V 8.0 ) _ 0.0012 \ / _ 0.0012\

dl I V dl I

C. Given the apparent mass value A'* find A^**, or given apparent mass valueA'** findX*

X*\\

0.0012

8.3909/

0.0012 \ 8.0 /

Z*(l + 0.000 007) (15)

1 0.0012 \ 8.0 / 0.0012\

8.3909/

Z**(l -0.000 007)

(16)

The above formulas apply to summations of weights

as well as to single weights provided the material

density is the same for all weights in the summation.

(Other forms of these formulas appear in the htera- ture [83,84].) Where weights of different material

densities are included in a summation, for exactness

17
a separate computation must be made for each

material density. It is prudent however to make

some preliminary computations to verify that the ef- fort is worthwhile. Gross material density changes

usually occur with the smaller weights, in which

case, the difference between the values computed by

the various methods may not be detectable in the

measurement process.

D. The observation on a direct reading weighing

instrument is an estimate of the apparent mass of the

object under test. The success of these devices depends in part on careful material control of the built- in weight materials, and a careful adjustment of the

built-in weights. In normal operation, the weighing can be accomplished in a minimum time. In simplified form, the comparison can be shown as: 2 '^J-pyia] + [ 2 ^-pVib)^O,^0 (17)

j=0

so that, subtracting (17) from (18)

(18)

(19)

Initially m = OandX^ includes everything which

is suspended from the beam and internal to the in- strument. Dialing off the weights constructs S,^

which cannot differ from S in excess of the on- scale range of the instrument. The unknown, is compared with S^, the dial reading, /?*, being the

nominal summation of nominal values assigned to the weights on some specific basis and the observed

difference being a subdivision of some small weight

internal to the balance. Under the defining conditions of the basis for the values. A' can be computed

as follows, provided the density of the ^ weight,

c?j°, is known at the defining conditions.

X

0.0012\

+ e (20)

or

0.0012 \

+ € (21)

where € represents the sum of the deviations from

nominal values of the weights in X^.

If the density, dx'^, is very nearly the same as the

internal weights and the ideal material density on which the values are based, the following can be as- sumed in many cases with neghgible error:

or

X* = R* + K(02) + €

X** = R** + K(02) + €

(22)

(23)

It is possible to use more sophisticated procedures

to establish the mass of an unknown relative to the

balance indications, however these procedures are tedious and time consuming unless the balance

manufacturer can furnish a table of mass

equivalents for each dial position. In some instances

it may be expedient to use the instrument as a comparator, comparing the unknown with weights for which all the appropriate parameters are known.

Other forms of these relations appear in the litera- ture [41,42], depending upon how the problem has

been formulated and the number of terms retained

in series expansion. Many of these relations have

been established for specific purposes, thus a careful evaluation would be in order before using in other

circumstances. In all cases, one must first evaluate

the task at hand to verify that a consideration beyond

the implicit treatment of displacement volumes is really justified. If, in fact, exphcit treatment of dis- placement volumes is required, one must start with

a set of weights with assigned values, the uncertainty

of the values being comparable with the precision of

the process. One must know the characteristics of

the material to be weighed and the characteristics of

the measurement process. One must compute the air density for each weighing. If any step is omitted,

confidence in the end result is illusory.

2.6. The Mass Measurement System and the

I\BS Mass Measurement Service

In order for the mass measurement system to function effectively, one must be able to make consistent mass measurements which are compatible

with a variety of requirements [85]. One service of

the Bureau of Standards provides in part a means by

which all mass measurements within the system can

be related. This service emphasizes the characterization of the object to be used as a mass stan- dard, e.g., stabihty, volume, coefficient of expansion,

etc. and the characterization of the measurement

process used to relate the value assigned to an un- known to the accepted value of a standard.

18
The NBS report is in most cases a comprehensive

report which includes a complete description of the

measurement process and methods, all of the data,

and the analysis of this data including that concerning the monitoring of the state of control of the NBS

measurement process. The mass value, the uncertainty of the value, and the displacement volume at 20 °C is reported. The uncertainty figure is an ex- pression of the overall uncertainty using three stan- dard deviations as a limit to the effect of random er- rors of measurement associated with the NBS measurement processes. The magnitude of systematic

errors from sources other than the use of accepted

values for certain starting standards are considered

neghgible on the basis of separate studies of the NBS

process. The errors associated with the use of certain NBS standards other than at the 1 kg level are included in the uncertainty statement. It should be

noted that the magnitude of the uncertainty reflects the performance of an NBS measurement process. The mass unit, as realized in another measurement

process, will be uncertain by an amount which is a combination of the uncertainties of the two

processes, the NBS process and the process in which the calibrated standards are used.

In addition, corrections to nominal values are re- ported in which the displacement volume has been

treated implicitly. Corrections are furnished on the

basis of "apparent mass versus brass," as previously

defined, and on the basis of "apparent mass" versus

a matericd of density 8.0 g/cm^ in air of density 1.2 mg/cm^ at 20 °C. An example of the use of the "apparent mass versus brass" corrections in the adjustment of weights of different materials is presented

in appendix 7. The uncertainties associated with the assigned

values are descriptive of the NBS mass measure- ment process and in no way reflect the effectiveness

of the transfer of the value from NBS to another

facility, or the output of another faciUty. A complete

assessment of the uncertainty at any point in the

system requires a characterization of all of the measurement processes involved.

On the other hand, the requirements within the mass measurement system are primarily functional,

that is, the variability associated with mass measure- ment is only one of many sources of variability which

must be controlled to accomplish a particular mission. In many cases, the variabihty of the mass measurement is by far the smallest in magnitude. In these cases, the number produced by the measure- ment process is considered to be "exact" as long as

the area of doubt associated with that number is small with respect to the magnitude of the effects from other sources of variabihty. If an acceptable

area of doubt is still large relative to the ability to measure, simplified practices should be used. For

example, the details of the "basis for the value" and

corrections for the buoyant effect of the air have no meaning relative to bulk weighing of highway materials or weighings on a pricing scale in which an ac- ceptable variability is 2 cents on 5 dollars at unit prices of $0.26 per pound through $1.25 per pound

[86].

Relating to a functional requirement does not negate the need for measurement process characterization. Appropriate characterization of any measurement process is fundamental to verifying that the results are produced in the most economical way

and are consistent with the end use requirements. In addition, the specific characteristics of a particular

process determine the need for further interest in the details of measurement. In the absence of a study of the characteristics of a particular process

relative to its functional requirement, one can

frequently become involved in details which are of no real concern.

It is important to note, in conclusion, that the

preciseness of any defined basis for stating mass

values cannot be reahzed beyond the performance

characteristics of the measurement process in which

the mass standards are being utihzed. The use of National Bureau of Standards reports of assignment of mass values to document the vahdity of the output of an uncharacterized measurement process is illusory.

A paper such as this is largely a review of material

from many sources and conversations with many

people. Devoting a large portion of the paper to histo- ry is an attempt to provide a background which will be helpful in the interpretation of a rather complex

existing situation. I want to thank those who contributed their thoughts and ideas. In particular I want to thank Dr. J. A. Simpson and Mr. J. M.

Cameron for their many constructive comments. Dr. C. Eisenhart for his help in assembling the reference

material, and the staff of the iMass, Length and

Volume Section including Mr. H. E. Aimer and Mr. Lloyd Macurdy, both now retired, who reviewed the many drafts. A portion of a paper by Mr. H. A. Bow- man and Mr. R. M. Schoonover, Reference 90. is reprinted in part as Appendix 2.
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Co., New York, 1946). [68] Mumford, Lewis, Agents of Mechanization, see [67] , p. 92. [69] Usher, Abbott P., see [53] , pp. 378-380.

[70] Technological Innovation: Its Environment and Management, Dept. Commerce (U.S.), Panel Report, p. 7 (Jan. 1967). [71] Astin, Allen V., Standards of Measurement, Scientific American, 218. No 6, p. 5, (June 1968). [72] Huntoon, R. D., Status of the National Standards for Physical Measurement, Science, 50, p. 169, (Oct. 1965). [73] Eisenhart, Churchill, Reahstic Evaluation of the Precision and Accuracy of Instrument Calibration Systems, J. Res.

Nat. Bur. Stand. (U.S.), 67C (Eng. and Instr.), No. 2, 161-187

(Apr.-June 1962). [74] Pontius, P. E. and Cameron, J. M., Realistic Uncertainties and the Mass Measurement Process, Nat. Bur. Stand.

(U.S.), Monogr. 103, 17 pages (1967). [75] Pontius, P. E., Measurement Philosophy of the Pilot Pro- gram for Mass Calibration, Nat. Bur. Stand. (U.S.), Tech. Note 288, 39 pages (1968). [76] Cajori, Florian, Translation and explanatory appendix. Sir Isaac Newton's Mathematical Principles of Natural Philosophy and His System of the World, pp. 38-639 (University of California Press, Berkeley, 1947).

[77] Rutherford, D. E., Classical Mechanics, Ch. 2, (Interscience Publishers, Inc., New York, 1951). [78] Macurdy, L. B., Response of Highly Precise Balances to Thermal Gradients, J. Res. Nat. Bur. Stand. (U.S.), 67C, No.

3, 135-138 (July-Sept. 1964). [79] Campbell, Norman R., An Account of the Principles of .Mea- surement and Calculation (Longmans, Green and Co. Ltd., New York 1928). [80] Smirnova, N. A. and Moskvin, N. E., Single Nominal Density

for Weights, Izmeritel'naya Tekhnika, No. 2, pp. 25-28 (USSR), (Feb. 1963). [81] Bigg, P. H., Weight-in-Air Basis of Adjustment of Precision Weights, J. Sci. Instr., 36, 359-361 (Aug. 1959). [82] Cross, J. L., Reduction of Data for Piston Gage Pressure Measurements, Nat. Bur. Stand. (U.S.), Monogr. 65, p. 3, 9 pages (1963). [83] Macurdy, L. B., Measurement of Mass, Ch. 72, Treatise on

Analytical Chemistry, Vol. 7, Part I, pp. 4247-4277 (John Wiley & Sons, Inc., New York, 1967) Mettler Reprint No. 11 (Mettler Instrument Corp., Hyattstown, N.J.). [84] Benedetti-Pichler, A. A., Essentials of Quantitative Analysis

(Ronald Press, New York, 1956). [85] Astin, A. V., Some Problems of Improving Accuracy of Measurement, IRE International Convention Record, Part

9, 1962. [86] Specifications, Tolerances and Other Technical Require- ments for Commercial Weighing and Measuring Devices,

Nat. Bur. Stand. (U.S.), Handb. 44, 4th Edition, p. 3 (1971). [87] Macurdy, L. B. and Lashof, T. W., Precision Laboratory Standards of Mass and Laboratory Weights, Nat. Bur. Stand. (U.S.), Circ. 547 (superceding NBS Circ. 3) (1954). This circular is out of print but its contents have been incor- porated in NBS Handbook 77, Precision Measurement and

Calibration, Volume III, Optics, Metrology and Radiation,

pp. 588-706. This handbook is available for reference or cir- culation in most Government Depository Libraries throughout the United States. [88] Standard Methods of Testing Single-Arm Balances, ASTM

E319-68, Book of ASTM Standards, Part 30 (American

Society for Testing and Materials, Philadelphia, 1968). [89] Stabler, Thomas M., Specifications and Tolerances for Field Standard Weights, Nat. Bur. Stand. (U.S.), Handb. 105-1. 23 pages (1972). [90] Bowman, H. A. and Schoonover, R. M. with Appendix by

Jones, M. W., Procedure for High Precision Density Deter- minations by Hydrostatic Weighing, J. Res. Nat. Bur. Stand. (U.S.), 71C (Eng. and Instr.), No. 3, pp. 179-198 (July- Aug. 1967). [91] Weights of Precision Class EiE.FiFiM, from 50 kg to 1 mg.

International Recommendation No. 20. International Organization of Legal Metrology, Bureau International de

Metrologie Legale. Paris, France (1973).

21
Appendix 1 . Average Air Densities for a Group of Selected Cities f

Listed below are the approximate average Winter

(January) and Summer (June-July) air densities for a group of selected cities throughout the continental

United States. These densities are for an assumed

temperature of 23 °C and are based on information

about the barometric pressure and relative humidity

supplied by the Weather Bureau.

It should be remembered that these are average

air densities and that the actual air density at a given

time and place may differ from that given by as much as 3 percent in either direction.

Place

State

Alabama.

Arizona.

Arkansas

.

California.

Colorado ..

Connecticut

.

District of Columbia.

Florida

Georgia

.

Place

City

Birmingham.

Montgomery.

Flagstaff. Phoenix.. Tucson... Yuma

Little Rock .

Los Angeles.... Oakland San Diego San Francisco.

Denver Grand Junction. Pueblo

Hartford

Washington

.

Jacksonville. Key West.... Miami

Tampa

Atlanta Augusta

(Aiken, S.C.) Savannah

Air density

Winter

1.16

Sur

(mgjcm^)

L16

1.18 1 18

0.92 0 93

1.15 1 14

1.08 1 08

1.19 1 18

1.18 1 18

1.17 1 16

1.19 1 16

1.18 1 18

1.18 1 18

0.98 0 98

1.00 1 00

1.00 1 00

1.18 1 18

L19 1 18

1.19 1 18

1.19 1 18

1.19 1 18

1.19 1 18

1.15 1 14

1.18 1 18

1.19 1 18

State

Idaho...

Illinois.

Indiana

Kansas.

Kentucky.

Louisiana.

Maine

Massachusetts.

Michigan

Minnesota

.

Mississippi

.

Missouri

Montana.

City

Boise

Cairo Chicago

Moline

Springfield

Fort Wayne

Indianapolis

Burlington Des Moines

Dubuque

Sioux City

Concordia Dodge City

Topeka

Wichita ,

Louisville

New Orleans

Shreveport

Eastport

Boston

Alpena

Detroit Grand Rapids

Marquette

Sault Ste. Marie

.

Duluth

Minneapolis

St. Paul (Airport)

Vicksburg

Kansas City

St. Louis

Springfield

Havre

Helena

Kalispell

Air density

Winter

(mgjcm^)

1.07

1.18

1.16

1.17 1.16

1.15

1.15

1.16

1.15 1.16

1.15

1.13 1.08

1.15

1.14

1.17

1.19

1.18

1.18

1.18

1.16

1.16

1.15

1.15

1.16

1.14

1.15 1.15

1.18

1.15

1.17 1.14

1.08 1.02

1.03 22
Place

State

Nebraska

Nevada

New Hampshire

New Jersey New Mexico

New York

North Carolina.

North Dakota...

Ohio

Oklahoma

Oregon

Pennsylvania..

Rhode Island... South Carolina

City

Lincoln North Platte.

Ely Las Vegas....

Concord Newark

Albuquerque.

Albany

Buffalo

Hatteras.

Raleigh..

Bismarck

Devils Lake. Fargo

Williston

Cincinnati

.

Dayton

Cleveland..

Oklahoma City

.

Baker

Medford...

Portland.. Roseburg.

Philadelphia. Providence...

Charleston.... Columbia

Air density Place

Winter

(mglcnv^)

1.14

1.07

0.95

1.11

1.17

1.19 1.00

1.18

1.16

1.18

1.18

1.11 1.12 1.15

1.11

1.16

1.15

1.15

1.14

1.04

1.13 1.18

1.17

1.18

1.18 1.19

1.18

Summer

(mpjcm^)

1.14

1.07

0.96

1.11

1.17

1.19

1.00

1.18

1.15

1.17

1.17

1.11 1.12

1.15

1.11

1.16

1.15 1.15

1.14

1.04

1.14

1.19

1.17

1.18

1.18

1.18

1.17

State

South Dakota.

Tennessee.

Texas.

Utah Vermont

Virginia

Washington .

.

West Virginia

Wisconsin

Wyoming

City

Huron

Rapid City.

Knoxville (Oak

Ridge). Memphis

Nashville

Abilene

Amarillo

Austin Brownsville .. Fort Worth..

El Paso Houston San Antonio.

Salt Lake City. Burlington Norfolk Richmond

Seattle Spokane

Walla Walla.

Elkins Parkersburg.

Green Bay. Madison

Milwaukee.

Casper

Cheyenne..

Sheridan...

Air density"

Winter

{mglcm')

1.14 1.13

1.05 1.06

1.16 1.15

1.18 1.17 1.17 1.17

1.12 1.11 1.04 1.04

1.17 1.16 1.19 1.18 1.16 1.16

1.04 1.04

1.18 1.18

1.17 1.17

1 09

1.17 1.17 1.19 1.18

1.18 1.18

1.18 1.17

1.19 t.l8

1.15 1.13

1.11 1.11 1.16 1.16

1.16 1.16

1.15 1.15

1.16 1.15

0.97 0.97 0.94 0.96

1.03 1.04

23
Appendix 2. Density of Air^

Major changes in the density of air, p.4, occur due

to variation in the temperature, pressure, and relative humidity, and each of the parameters should be

carefully observed. The instrumentation used for monitoring these ambient parameters need not be

state-of-the-art type apparatus because it is necessary only to assure that the standard deviation in p.4 is held to a few micrograms per cubic centimeter. Apparatus should be chosen which is convenient to use and maintain. A. Temperature. A mercury-in-glass thermometer

divided into 1/10 °C intervals over the range of 20 to 30 °C would be adequate.

B. Relative humidity can be measured on an electric hygrometer of the type discussed by Wexler^

with its sensing element placed in the weighing

chamber of the balance. This instrument usually

contains a built-in thermistor type thermometer

which is used to monitor the temperature in the immediate vicinity of the relative humidity sensing ele- ment. This should not be used as a replacement for the mercury thermometer. Although it is quite

adequate for the use intended by the manufacturer

(and it should be used in connection with relative humidity measurements) it will introduce serious uncertainties if used as a replacement for the mercury

instrument.

Shng and aspiration psychrometers are unsatisfactory in this application. Their required air- flow would cause unacceptable disturbances inside a balance, and they lack the required sensitivity and

speed of response. Wexler discusses electric hygrometers in detail and we beheve that their con- venience, speed of response and sensitivity are such

that no other type instrument currently available

should be considered. Wexler estimates their drift at about 2 percent per year, and feels that semi-annual

cahbration is adequate.

Calibration can be performed by the manufacturer, NBS, or the experimenter himself using vari- ous techniques leading to a cahbration accuracy of

' Adapted from "Procedure for High Precision Density Determinations by Hydrostatic Weighing" [90]. The nomenclature of the original article has been retained, p.4, the density of air, is identical top as used in this paper.

' Wexler, A., NBS Circ. 586 (1957).

about 1 1/2 percent using the methods of Wexler and

his associates^. C. Barometric pressure is most easily observed on

an aneroid barometer. Such an instrument should be

checked against a mercury barometer. We recom- mend specifying an antiparallax mirror scale for in- struments used in this work.

D. The basis for air density calculations. If we

have a mass, mc, of dry gas at pressure p and absolute temperature 6, whose molecular weight is Mg,

then from the ideal gas law

pV^nRd--=

and

and

mo '

Ma

Re

me

V

ma

Wg

pVMg

Re

1p

Re

PG

P

(2)

where n is the number of moles involved and pc the

density of the dry gas.

If we have a mass, of water vapor at pressure

e and temperature e occupying volume V, then a

similar argument gives

mw —

Re

If the above masses of gas and vapor are mixed in volume V, the barometric pressure B, will be (by Dalton's Law of partial pressures) equal to p + e and p =

B — e. The density, p,4, of the mixture is ma + mw A/ft Re B-e 1-

Mr

Using MaiRe from eq (2)

pA

pa

P

B-[e\ Mw

Ma

If the density of dry gas, pa, is measured to be po,

at some reference temperature ^o, and reference

' Wexler, A., Brombacher, W. G., NBS Circ. 512 (1951). 24
pressure po, then its density at any temperature d

and pressure p is

and using this value of pc, in the above equation for pA gives

pA

PoOo

dpo

B-e 1

On the basis of the natural scale of atomic weights

M,,= 18.0160 and Mr. = 28.966.^ If po= 1.29304 x

10-=^ g/cm^ when 0o= 273.16 °K and po= 760 mmHg,^

then

^B- (0.0037803) (e,)(//)^

px = 0.46475

f4 + 273.16

where tA is temperature in °C, the tabulated saturation vapor pressure of water at tA and H the relative humidity expressed as a percentage. This for- mula assumes moist air behaves in accordance with

the ideal gas law, under which circumstances compressibility (defined as pVjRd) is unity. Goff and

Gratch^ have measured the compressibility of moist

air and have found that over the range of temperature, pressure and relative humidity ordinarily exist- ing in the laboratory it varies between 0.9995 and

0.9997, so the above formula provides values of Pa

too high by this factor. We feel justified in multiplying its right member by an average value of 0.9996,

so that

Pa = 0.46456

B- (0.0037803) (es)(//)

f.4 + 273.16

(3)

' Harrison, L, Pt. 1. pp. 15.16. Humidity and Moisture. 3. edited by Waxier, A. and

Wildhack, W. (Reinhold, New York, 1965.)

" NBS Circular 564, table 2A, p. 25 (1955). ®Goff, J. A. and Cratch, S., Smithsonian Meterological Tables. 6th Edition, Smithsonian Institution (1958).

Equation (3) may be used by the experimenter for calculation of air density based upon observed

values of temperature, barometric pressure, relative humidity and tabulated values of e«, the vapor pres- sure of water at saturation. In this equation pA is a very weak function of e.,, changing by only 0.24 X

lO"** g/cm^ for a 1 mmHg change in e.,. Therefore it is possible to eliminate the inconvenience of using ta- bles by making a very crude approximation for eg in terms of the observed temperature without causing

unacceptably large errors in Pa. Most weighings are conducted between 20 and 30 °C, and within this range we may assume

es=1.435t^ - 11.72

under which circumstances

0.464554fi - H (0.00252f., - 0.020582)

+ 273. 16

(4)

where Pa is in milligrams per cubic centimeter with

adequately small error. On the rare occasions when it is necessary to per- form weighings outside of this temperature range, some adjustment in the approximation is called for. Over the temperature of 15 to 50 °C, air density may

be approximated by

p.4 =

464.565 - //(0.085594f2 - 1.8504?.4 + 34.47)

«.4 + 273.16

(4A)

where Pa is in micrograms per cubic centimeter.

Although these two approximations result in very

large errors in Cs, errors in p.4 associated with their use do not exceed one microgram per cubic centimeter which is quite adequate for weighing work.

25
Appendix 3. Typical Weight Materials, Densities, and Coefficients of

Volumetric Expansion

Where specific statements of displacement

volume or material density and coefficient of cubical

expansion are missing, the appropriate data from the

following table is assumed to apply. The data is ap- proximate, having been taken from various handbooks, or having been obtained from the weight

manufacturers. The use of this data provides a

specific basis for determining mass values, presuming that the manufacturers exercise some form of

material control. For the most precise work, it may

be necessary to determine the volume or density of the weight material by other means.

\ Material Density

at 20 °C

Coefficient

of cubical

n 1 r c 1 c 1 /\nierican oalancc VjO. otainless oteel... 7 09 8.3909 0000^4, Stainless Steel 7.8 .000045 Platinum 21.5 .000026 Tantalum 16.6 .000020 Aluminum 2.7 .000069 Nichrome V 8.5 .000039 Nichrome 8.39 .000039 '"Brunton" Metal (Ainsworth Stainless

Steel) 7.89 .000045 Ainsworth Stainless Steel 7.85 .000045 Naval Brass (Ainsworth) 8.4 .000054 (W. & L. E. Gurley Stainless Steel) 7.916 .000045 Fisher "Permas" Stainless Steel 7.8 .000045

7.84 .000045

7.8 .000045 "Beckerloy" Stainless Steel Torsion Balance Co 8.0 .000045 Gold 18.0 .000043 Quartz (Crystal) 2.65 .000033

Mettler Instrument Corp. Stainless Steel 7.76 .000045 Check Weight 8 "State Weight 8"

Stainless Steel 8.0 .000045

26
Appendix 4. Variability Associated with Volume Differences and Changes

in Air Density

Equation (3) can be shown as: K(0i-02)=(y-Jt)-p{Vy- V^)

K(Oi —O-zjha number which is computed from the

indications of the weighing device as y and Jt' are compared. Since the air density, p, is continuously

variable, no unique number will be obtained. The air density, p, changes in a cycUc manner, therefore the

average observed number, KiOi — Oz), stable and

offset from — ^) by the amount p{V.9 — Vr).

It is of interest to know the magnitude of the variability associated with p(V/ — V,/) and the magnitude

of the offset in order to determine the necessity for a detailed adjustment of the observed data. Normally

it is not difficult to determine the appropriate air density, but it may not be possible to determine

precise displacement volumes in all cases. Every measurement process has an inherent

variability. For example, a sequence of comparisons

of ^ with itself would produce a sequence of numbers, Nk, as follows:

With 6^ = ^ a.x\dVy= Vy, the variability of the ob- served difference is identical to the variabihty of the

measurement process, which is represented by the

random variable, e,, from a probability distribution

with mean zero and standard deviation, (To. Considering the air density to act like a random

variable, such that:

Pj = P + Vi where p is some average air density and the 17; have standard deviation cri, then, in the case where

obs diffi= 5^ - r - p(V, -v,)+€i- -miVy - vj

Thus the variability of the observed difference is a combination of two random variables.

One does not normally have long sequences' of

' Long sequences are necessary to detect small changes in the standard deviation. For

example, to be reasonably sure (probability of 0.95) of detecting a difference of 20 per- cent in the ratio (TiifTz with the usual statistical test (0.05 level of the F test) one would need 165 measurements in each group; for a 10 percent difference. 60; and for a 50 per- cent change, 35 measurementscomparisons to estabHsh estimates of both cr\ and cto. One can, however, compute the appropriate air density for each comparison, and test for correlation

between the observed difference and the air density.

If the air density determination has been done carefully, correlation indicates a significant volume dif- ference which in turn implies that one or both of the

assigned displacement volumes are in error. The

absence of correlation does not mean that the

volumes have been assigned correctly but that the

variability from this source is much smaller than

other variabilities which affect the process.

In a given location, with a 1 kg process of precision

5 = 0.050 mg, and a normal air density variation over

a range of 0.05 mg/cm^, one would be hard pressed

to detect a volume difference error of 1 cm^. On the

other hand, if comparisons were made at different lo- cations, subject to a range of air densities of 0.25

mg/cm^, correlation would be evident with a much

smaller number of intercomparisons.

The significance of the magnitude of offset,

p{Vy — Vij- ) , is also established relative to the proc- ess precision. For the above example, with p=1.2

mg/cm'^ and {Vy~Vt) — \ cm\ the offset is sig- nificant with respect to the process precision. An adjustment of the observed data is required, particularly if the weights are to be used in other locations. The same logic applies when volume differences are so large that data adjustment must be made. In

this case

Ni + p/Fy - v^) = - cf) + ei - ^p^v

where Ap is the difference between the actual air density and the computed air density pj. AF is, in

Like manner, the difference between the actual

[Vy — V^) and the number assigned to be (Vy — V./).

If, after corrective action has been taken, a correlation is still evident, further refinements are necessary. It still remains, however, that one can only

verify that the magnitude of the variabiHty from this source has been reduced to some fraction of the residual variability from other sources.
Appendix 5. Displacement Volume and the Assignment of Mass Values

The implicit treatment of displacement volumes

in essence lumps the gravitational effect and the

buoyant effect of the air to produce an effective mass

value. The explicit treatment separates the two ef- fects, that is, the stated mass value must always be

used with the appropriate displacement volume.

Two quantities are involved, the density of the air and the volume, or material density, of the weight

under study. The air density, determined by measured parameters and formula, can be sufficiently accurate for most appUcations. The displacement

volume, however, must frequently be determined

from approximate data or rough estimates of material density. It is of interest to know how this condition

affects the assignment of mass values. The following

hypothetical situation will illustrate the problem.

A well characterized weight, ct/, of known density,

f/g/. and with assigned value. A, is used to establish a value for weight i^. It is assumed that weight ^

has a density of rf^. Having assigned a value B

relative to A, weight ^ is, in turn, used to establish

a value, C, for weight which is also a well characterized weight with density d-^. After some

time, it is discovered that the assumed density, dss,

is in error. New evidence establishes a more appropriate value for d^. One must now decide whether

the current value, C, is still appropriate or whether

the sequence of measurements must be repeated to establish a new C. Assuming and to be nominally 1 kg

weights with rf.i^' known to be 7.8 g/cm'' and d^

assumed to be 8.4 g/cnr*, the first comparison —

^) might indicate an apparent difference of 5 nig.

The value fi would be computed as follows:

fi8.. = ^-5mg-p,(^-^)

The second comparison (^ — ^), in air of density

P2, might indicate an apparent difference of —7 mg.

The value C would be computed as follows:

C = fiB.. + 7mg-p,(^-£)

At this point it is discovered that c?^ should be 8.0 g/cm^ l ather than 8.4 g/cm^. Recomputation gives a new value, fi:

fis.o = ^-5mg-p,(:^-^)

from which:

BsA~ Bs.o — P]

B_

8.4 B_

8.0

For 5 ~ 1 kg and pi = 1.2 mg/cm^

fiH.4-5H.o- (1.2)(119-125)mg - - 7.2 mg

This is indeed a significant change in value assignment and perhaps would be cause for great concern. However, a mass value is never used alone but al- ways in conjunction with a specific displacement

volume, or density. A recomputation of C, based on

the new mass value and density for gives:

P2 8.0 d^,

so that B

8.4

If pi = p2, the value assignment, C, is not affected

by the change in value B. If fpi — P2) is sufficiently

small, (Cold — Cneiv) wiU also be small, in which case

it would be immaterial as to which value is retained.

If all terms are large, and the measurements suffi- ciently documented, a new value for C could be computed from the existing measurement data. If, however, sufficient documentation has not been

established, the feature of being able to update by

computations based on current data is lost and the

whole procedure must be repeated.

28
Appendix 6. On the Selection of an **Ideal" Weight Material

Selecting an ideal weight material is of interest

where the implicit treatment of displacement volume

method is used to determine mass values. A judicious choice of "ideal" material, together with

restrictions on the materials to be used in the construction of weights, results in a system whereby the

consistency of the numbers assigned can be verified by routine procedures. The relation for the discrepancy, D, between the observed difference between

two weights and the number assignment difference

as developed in Part 2, Section 2.4, can be written

as:

]_ 1 B

\ \_ Pi A__B^

(1,1 (li6j

Examining the above relation, D = 0 i{ A = B and

if all of the weights are made from the same material,

da = db = d*. These conditions were close to reality

in the past when most weights were made from brass

or bronze. It may be the case in the future when most

weights will be made from stainless steel. The largest term in the relation is

A_

The magnitude of this term is a function of the local

air density, pi, which cannot be specified. In this country f) varies from approximately 1.2 mg/cm^ in the coastal regions to approximately 0.9 mg/cm'^ in the mountain regions. Since it is not hard to adjust so

that A ~B, clearly the biggest gain in reducing the

discrepancy is in the selection of materials so that da ~ db. The magnitude of the first term is determined by

the selection of a "standard" air density, po, and a material of density d* for the "ideal" weight. The

use of po = 0.0012 g/cm'' seems to be widely ac- cepted, lid* is selected to be about the average density of normal weight material, the range of magnitude is not changed but the sign is sometimes positive and sometimes negative — a situation suggesting

a percentage improvement which may be largely il- lusory relative to the normal precision of measure- ment or the permitted deviation from nominal value

allowed in the adjustment tolerance.'

' The limits for the systematic error Pi{Va — Vb) are established by (1) mass oiA and B and (2) the range of densities of materials which are used in constructing A and B. This Hmit is not affected by the choice of "ideal" material. Selecting an "ideal" material

at either extreme is like establishing a unidirectional tolerance (i.e., 1. 000 —

O.0OO/-H).OO5) which expressed as a percentage with respect to the nominal value 1.000,

is twice as large as the percentage error associated with a bidirectional tolerance (i.e., 1.000 +0.0025/-0.0025). The choice is academic when the acceptable tolerance limits are large (i.e., 1.00+0. 10/-0. 10) or when the process precision is large (i.e., o'=0.05 >

0.005 > 0.0025).

29
Appendix 7. Examples Illustrating the Use of Apparent Mass Values

There is a generally widespread belief that the use

of apparent mass values eliminates the need for detailed attention to the buoyant effect of the air. In

certain circumstances, this belief is well founded;

however, it may not be true in all cases. In the examples that follow, the conditions have been chosen to show that a confidence based on such a belief may

be illusory. If apparent mass values are to be used in a particular situation, it is important to establish the

magnitude of the discrepancy introduced by so doing, and to verify that such usage will not affect the

desired result. The methods used in these sample

problems can be used for such an evaluation. The

examples are applied to all types of weighing

devices, the difference from one to another being

only in the determination of the term K(0\ — O2).

1. Test for, or adjust to obtain, compliance with

specified weight adjustment tolerances.

There are established tolerances, or limits, for the

permitted deviation from nominal value for various

kinds of weights.' There are no universally accepted

rules for judging a weight to be out of tolerance. This

does not cause difficulty when the tolerance limits are large and when the weight is adjusted to be very

close to a nominal value. The situation can be quite

confusing when the tolerance limits are small or when the values of the weights are close to these

limits. Two factors are involved in the judgment to accept or reject: the random error of the measure- ment process, that is, the standard deviation; and

the offset or systematic error introduced by failure

to consider certain variables which affect the

process output.

Rules for interpretation with respect to random

error must be agreed upon by the parties concerned.

These rules should reflect an assessment of the risk of accepting an unsatisfactory weight or rejecting a

satisfactory weight, either with respect to a contractual requirement or an end use requirement. This

aspect of the problem is not considered in the examples which follow.

' Adjustment tolerances for various types of weights were established in NBS Circular No. 3 [87]. Revised and expanded tables were given in NBS Circular 547 [87]. A

specification for field standards. NBS Handbook 105-1 [89] , was prepared in 1969. Current tables for adjustment tolerances are being prepared by Subcommittee E31 of the American Society for Testing Materials.

The systematic error offsets the results of one measurement process from those obtained by other

measurement processes. Failure to consider the factors contributing to systematic error can create a

situation in which one facility, acting according to the mutually agreed upon rules mentioned above,

would judge an item to be satisfactory, but a second

facihty would judge the same item to be unsatisfactory. The former would accept, but the latter would

reject. The example presented is illustrative of this aspect of the problem.

Testing for comphance with adjustment tolerance can be conducted in different ways. The role of the

balance, or weighing device, is to estabUsh an observed mass difference between the standard and

the test object under local environmental conditions.

The observed difference is a number which varies as

a function of the actual difference and the standard

deviation of the process. The role of the known stan- dard is to provide a point of departure for the test, or adjustment. The adjustment process is a continuation of the test. If, in the test, it is judged that the

weight is "out of tolerance," material is added or removed by some appropriate procedure until, on

retest, an "in tolerance" condition is obtained.

Both the test weight and the standard must be well

characterized, that is, one must know the effective

material density, and the coefficient of expansion.

The characteristics of the local environment must

also be known. The degree to which this information

must be known depends upon the size of the

tolerance band. Certainly one would want the uncertainty of the computed corrective terms to be some

small fraction of the tolerance. In the limit, uncertainties in these terms smaller than about one stan- dard deviation of the measurement process are not

significant.

Given: A known platinum standard, density 21.5 g/cm^ at 20 °C, with the following assigned values:

Apparent Mass Coefficient of versus Brass Mass Cubical Expansion

Ig- 0.003 mg lg-0.090 mg 0.000 026
and a test weight of density 8.3909 g/cm^ at 20 °C,

coefficient of cubical expansion 0.000 054. Test, or

adjust, the weight to comply with tolerance specification based on apparent mass versus brass values.

(All current tolerance specifications for weights in the United States are based on apparent mass versus

brass values.) The weighings are to be made in an environment of 1.02 mg/cm^ density at a temperature of 23 °C. Since the test material is the same as the material

used to define apparent mass, from eq (9) in section

2.4, the mass value and the apparent mass value will be identical. The local observed difference between

the platinum standard and a hypothetical test object

of value exactly one, with characteristics the same

as the real test object, can be computed from eq (8) of section 2.3:

p[l+Q^^(f-fo)] p[l + ar{t-to)]

'

d1 = K{0,-0^)

0.99991 1-

-1 1-

0.00102 [1 + 0.000 026(23-20)]

21.5

0.00102 [1 + 0.000 054(23-20)]

8.3909

or = K{0,-02)

KiOx-Ot) =-0.000 016g

This means that, in this specific weighing environment, the test weight, which appears to be heavier

than the platinum standard by the amount 0.016 mg,

would have a mass value, X, or an apparent mass

value, X*, of 1 gram. Having established this, there

are several courses of action which could produce

useful results. For example, the test weight should

be heavier than the standard by 0.016 mg, so, with

the standard on the balance, the balance could be set

to read (1 g - 0.016 mg) or 0.999984 g. Under this condition, any similar test weight with a value, or adjusted to give a value, within a reading of 1 g plus or minus the tolerance hmits would be acceptable.

In comparison, from the assigned apparent mass

value of the platinum standard S* = 1 g — 0.003 mg,

one might erroneously conclude that a test weight

with value 0.003 heavier than the standard would be

acceptable. The difference in result between the two

approaches is 0.013 mg, an amount which is signifi- cant with a typical process precision of 0.002 mg.

Weights with values near the tolerance limit would

be accepted in one case and rejected in the other. However, if the process precision and the tolerance

hmits are large compared to the 0.016 mg systematic

error, routinely applying a correction of this magnitude may not be significant.

If, in this example, the weight being tested, or adjusted, had been made from material of density 7.8 g/cm^ at 20 °C, the coefficient of expansion being

0.000045, an additional calculation is required. The

mass of test material which will satisfy the definition of apparent mass versus brass at the specified conditions can be computed from eq (14) of section 2.4:

0.0012

8.3909/

1- 0.0012

8.3909

0.0012\

dl )

0.0012 \ 7.8 /

This value must be substituted in eq (8) of section

2.3:

p[l + ay(f-^o)]

] dl J

, o.oom

' 8.3909/

0.0012 \ 7.8 j J

p[\ + aAt-to)^

d'.

0 QQQ 01 \^ - Q-Q01Q2 [1 + 0-000 026(23-20)]

L 21.5

0.0012\^

8.3909/

0.0012

1-

7.8 /_

0.00102 [1 + 0.000 045(23-20)]

7.8 K{Or-0,)

or KiOi-Oz) =-.000 017 g

Having determined K(0^, — O-z), the interpretation is the same as in the previous example.

2. Establishing the mass of an object with a direct

reading instrument.

As before, one is concerned with the order of magnitude of the systematic error introduced when the

value as given by the balance is assumed to be the mass of the test object. If the magnitude of this 31
systematic error is large with reference to the precision of the process, a detailed accounting may be

necessary.

Given: A balance indication of 97.875465 g for a

test object; find the mass of the test object. The

weights internal to the balance are of material density 7.78 glcrn^ at 20 °C. The density of the test object

is 1.5 g/cm^ at 20 °C. The weighing has been made in an air density of 1.15 mg/cm^ at 20 °C. Part of the success of the direct reading balance can be attributed to careful control of weight material and careful adjustment of the internal weights.

The internal weights are adjusted on an apparent mass basis, mostly with reference to material of density 8.3909 at 20 °C, but sometimes with reference to material of density 8.0 g/cm^ at 20 °C (see sec. 2.4). The internal weights are usually adjusted with

respect to standards made from the same material,

the standards having been assigned appropriate ap- parent mass values. Since the material of the stan- dard and of the internal weight are the same, no

systematic error, other than that associated with

measurement and adjustment process, is introduced

in the transfer of value. The mass equivalent of the

indicated value can be computed directly from equation (14) of section 2.5, considering the total reading,

that is, the dial settings plus the indicated scale

reading, asS*.

0.0012\

8.3909/

^ 0.0012 \ 7.78

Equation (8) of section 2.3 can be used with the fol- lowing reasoning. Referring to eqs (17) and (18) of section 2.5, if the test weight # in equation (18) is identical in aU respects to the indication, Oz,

would also be identically zero. In the problem under

consideration, the # of eq (18), which is the dial readings plus the scale reading, since the scale reading is merely a subdivision of the smallest dial weight, becomes a well characterized standard S. Equation (8) can now be used with K(Oi — O2) = 0. Hence:

p[l + a^{t - to)]

-X

p[l + «r( M^-^o)] 1_i

di J

97.875465

0.0012

8.3909

0.0012

L 7.78 J

0.00115

7.78

-X 1- 0.00115

1.5 = 0

so that X= 97.937182 grams.

The systematic error introduced by using the

number indicated by the balance as an estimate of

the mass of the test object is:

(97.875465 - 97.937182)^= - 0.061717g

This is a large error relative to the expected precision for a measurement process at the 100 g level. For example, such an error might be significant in the preparation of titrating solutions by gravimetric

methods. The error, however, is a proportional one; thus in the dispensing of such a solution, as from a

plastic "squeeze" bottle, the error amounts to only

about 0.61 mg per gram of solution dispensed. Again,

while the error may be large relative to the available

measurement precision, it may not be significant

relative to a particular end use requirement.

3. Checking the indication of a precise direct reading balance with a set of calibrated weights.

In general, this task cannot be done with a set of

weights known only to comply with some specified

adjustment tolerance. A well characterized set of

standards is required with assigned values such as shown in table 1. The determination of the uncertainty of the summation of values for the group of

standards used is not treated in this discussion. It must be emphasized that the results of such a check

must be evaluated with reference to the uncertainty

of the values of the standards, the precision of the process, and the hmits stated by the manufacturer.

In some instances, it may be desirable to estabhsh

precise values for particular dial settings of the in- strument, or for the weights internal to the instru- ment. Such service may be available from the instru- ment manufacturer. Detailed procedures are not discussed in this paper.

Given: A direct reading balance with internal

weights adjusted on the basis of apparent mass ver- sus brass, determine if the balance indicates the ap- propriate value for a test weight of mass 53.512 g. The weights internal to the balance are made from

material of density 7.8 g/cm^ at 20 °C and a coeffi- cient of expansion 0.000045. The balance is located

32
in an environment of air density 1.17 mg/cm^ at a temperature of 22.5 °C.

Table 1

Mass (g) Vol. at 20 °C Coefficient of (cm') expansion

50.000 395 6.35329 0.000 045

3.000 016 0.38120 .000 045

0.499 951 .03012 .000 020

.010 004 .00371 .000 069

.002 007 .00074 .000 069

As in the previous example, a balance indication

of 53.512 g would be observed for a test weight made

from the same material as the internal weights of the

instrument, and adjusted to have an apparent mass

versus brass of 53.512 g. One can compute S as before: 53.512^1 -2:^^)

S = - 8.3909/

1 Q.0012 \ 7.8 /

Equations (7) and (8) of section 2.3 can be combined

in part to obtain the following:

53.512

1 0.0012

8.3909

0.0012

1- 0.001 17 [1 + 0.000045 (22.5 - 20)

]

7.8 - [53.000411 - (0.00117) (6.73449) [1 + 0.000045

(22.5-20)]] - [.499951- (0.00117) (.03012)

[1 + 0.000020(22.5-20)]] - [.012011 - (0.00117) (.00445) [1 + 0.000069(22.5 -20)]] = KiOi-0,) -0.000100 g

This result can be used in the following way. With

the balance "zeroed" at some particular number,

and with the appropriate dial settings for 53.512 g, the summation of the test weights should give an in- dication 0.100 mg less than the "zero" settings. If this is the case, and subject to a consideration of the

factors mentioned above, one would conclude the

balance indication is appropriate.

In the above three examples it is presumed that the controllable sources of systematic error have

been minimized. These include temperature dif- ferentials, operator techniques, and instrument malfunction, as well as the constancy of the standards

and built-in weights. Surveillance tests should be

used to monitor the constancy of the standards.

Details for such tests are available on request.

Balance tests should be performed in accordance

with ASTM E319-68 [88].

33

FORM NBS-114A (1-7U

U.S. DEPT. OF COMM. BIBLIOGRAPHIC DATA

SHEET

1. PUBLICATION OR REPORT NO. NBS MN-133

2. Gov't Accession No.

3. Recipient's Accession No.

4. TITLE AND SUBTITLE

MASS AND MASS VALUES

5. Publication Date January 1974

6. Performing Organization Code

7. AUTHOR(S) „ , „ „ Paul E . Pontius 8. Performing Organization

9. PERFORMING ORGANIZATION NAME AND ADDRESS

NATIONAL BUREAU OF STANDARDS

DEPARTMENT OF COMMERCE

WASHINGTON, D.C. 20234

10. Project/Task/Work Unit No.

1 1 C~ c\ntTCi ft / errant Mr»

12. Sponsoring Organization Name and Address

Sane as No. 9.

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15. SUPPLEMENTARY NOTES

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

bibliography or literature survey, mention it here.)

Theire are several bases for assigning mass values to weights to be used as mass

standards. As a consequence a given weight may have several assigned mass values

depending on the basis used. In many cases, the differences between these assigned

values, although easily detectable with precise weighing equipment, are of no

practical concern. However, in some instances these differences may be crucial.

The first part of this p^^er is a historical summary of weighing, standards, and

the assignment of value; and the interfacing of mass measurements with civilization.

The second part of this paper discusses in detail the methods of assigning mass

values. Ways to convert from values on one basis to values on another basis are discussed. Sanple problems relating to the buoyant effect of the air are presented

in the appendices.

17. KEY WORDS (Alphabetical order, separated by semicolons)

Apparent mass; buoyancy corrections; mass corparison; mass value;

true mass; weighing.

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