NATL INST. OF STAND & TECH NI8T

PUaiCATIONS

A11107 osaoao

NBSIR 75-929

A Study of the National Force

Measurement System

Donald E. Marlowe

Engineering Mechanics Section

Mechanics Division

Institute for Basic Standards

National Bureau of Standards

Washington, D. C. 20234

June 1975

8. DEPARTMENT OF COMMERCE

TIONAL BUREAU OF STANDARDS

1^75

NBSIR 75-929

A STUDY OF THE NATIONAL FORCE

MEASUREMENT SYSTEM

Donald E. Marlowe

Engineering Mechanics Section

Mechanics Division

Institute for Basic Standards

National Bureau of Standards

Washington, D. C. 20234

June 1975

U.S. DEPARTMENT OF COMMERCE. Rogers C. B. Morton, Secretary

Edward O. Vetter, Under Secretary

Dr. Betsy Ancker-Johnton. Assistartt Secretary for Science and Technology

NATIONAL BUREAU OF STANDARDS, Errrast AmMer, Acting Director

CONTENTS

Page

1. INTRODUCTION 1

2. STRUCTURE OF THE MEASUREMENT SYSTEM 1 2.1 Conceptual System 3 2.2 Basic Technical Infrastructure 5 2.3 Realized Measurement Capabilities 13

2.4 Dissemination and Enforcement Network 15 2.5 Direct Measurements Transactions Matrix . . 17

3. IMPACT, STATUS AND TRENDS OF THE MEASUREMENT SYSTEM 21 3.1 Impact of Measurements 21 3.2 Status and Trends of the System 22

4. SURVEY OF NBS SERVICES 23

4.1 The Past 23

4.2 The Present 25

4.3 Impact of NBS Services 30

4.4 Evaluation of NBS Program 31 4.5 The Future 31

5. SUMMARY AND CONCLUSIONS 32

APPENDIX A 33

LIST OF TABLES

1. Summary of Principal Domestic Deadweight Force Standards 4

2. Surmary of Foreign Force Standardization Equipment 5

3. Organizations Having Standards, Specifications or Procedures

Involving Force or Strength Measurement 6

4. Tests Performed on Mat'drials and Structures Which Require

Measurement of Forces 5

5. Representative Materials Covered by ASTM Standards Which

Require the Measurement of Force 6

6. Force Instrumentation Manufacturers 12

7. Regulatory Agencies Which Write Standards Specifying

Force Measurement 16

8. Direct Measurements Transactions Matrix for Force 18

9. Torque Instrument Manufacturers 20

i
LIST OF FIGURES

Page

Figure 1 National force measurement system • • • 2

Figure 2 Ranges of forces measured in the measurement system 3

Figure 3 Accuracy levels presently found in the measurement system 4

Figure 4 Whi ttemore-Petrenko proving ring, the first practical transfer

force standard, 1927 4

Figure 5 Schematic of a deadweight calibration machine 7

Figure 6 Proving rings 8

Figure 7 Load cell showing the strain gage installation 9

Figure 8 Multiple load cell calibration setup 10

Figure 9 Impact of force measurements in the standard industrial

classification major groups 17

Figure 10 Impact of force measurements in the standard industrial

classification 2 digit codes 19

Figure 11 Uncertainty of static force calibrations ..... 24

Figure 12 Histogram of force calibrations at NBS 25

Figure 13 Input-output flow chart showing distribution of section effort .... 27

Figure 14 Input-output flow chart for force calibration services 28

Figure 15 Recal ibration interval for force transducers at NBS 29

i i
A STUDY OF THE NATIONAL FORCE MEASUREMENT SYSTEM

Donald E. Marlowe

I Mechanics Division

June 1975

EXECUTIVE SUMMARY

This study outlines the detailed structure

of the National Measurement System as it

pertains to force, and assesses the economic,

social, and technological impacts of force

measurements in the United States.

Force, as a unit of measurement defined by

Newton's Second Law of Motion, is derived

from the basic units of mass, length, and

time. The most precise standards of force

are realized in deadweight calibration

machines such as those maintained by the

National Bureau of Standards, a few government laboratories, one comnercial calibration

laboratory and a few industrial metrology

organizations. The deadweight facilities at

NBS are widely accepted as the national

standards of force.

In practice, deadweight machines are used

to provide a few precisely known forces for

the calibration of high quality force transducers such as load cells and proving rings.

These secondary standards are used to calibrate the multiplicity of devices for field

measurements. Accuracy requirements in the

measurement system range from the 0.002

percent accuracy of primary standards to about 10 percent for rough measurements such

as crane overload indicators.

The principal impact of force measurements

can be classified as both social and economic.

The determination of the strength of a material of the determination of its weight is

often made to guarantee the equity in trade

in a commercial transaction. However, the

safety implications of accurate measurements

is obvious. The measurement is made to guarantee two parties that the product is

delivered in accordance with a contractual

agreement. Very often these agreements refer

to one or more voluntary consensus standard

methods of test. These standards are the

result of the economic needs of the manu- facturing community to agree upon a common

method for making a measurement. If the

standards of force measurement had not existed,

the cost of doing business would have increased to cover the cumulative uncertainties

resulting from inaccurate measurements. In addition, with the current and anticipated

growth in mandatory safety regulations, many

of which require that products be stronger

than ever before, the traditional regulatory

requirements of product purity are being

amended or even supplanted by force measurement. Clearly, however, these regulations

are resulting in increased product cost to the consumer and the government must weigh

the social benefits against the economic

penalties paid.

The history of NBS and of force measurement in this country are closely linked,

beginning in the 1830's with the distribution of mass standards by the Office of

Weights and Measures, and continuing today,

with the development of proving rings, deadweight machines, and load cells and test

methods. In addition NBS has participated

in the formulation of many of the measurement test methods now in use. The role of

NBS at the primary standards level in the

measurement system is to provide adequacy,

stability and consistency to the system. No other organization is so uniquely placed to provide that function.

Our assessment of the National Force

Measurement System is that, while some as- pects of it are stable and adequately maintained, other needed parts of the system

scarcely exist. The historical areas of effort in static force measurement, over the

range from 0.5 newtons (0.1 Ibf) to 5 x 10'' newtons (1 x 10^ Ibf), are fully adequate

with the level or accuracy now provided and

will remain so for the foreseeable future.

They require only continued routine support

and maintenance. In the areas of dynamic

force, very small static forces, information

transfer and metrication, however, the mea- surement system shows serious weaknesses.

The measurement system for dynamic force

does not exist. The standards and test

methoJs that are needed to assure the ac- curacy of dynamic measurements and the reliability, efficiency and safety of load carrying

systems under dynamic conditions such as impact and fatigue are not available. A similar condition exists for the measurement

of very small forces down to 10"'' newtons

(10"^ Ibf). Measurements in this range are
needed in such areas as biomedical research

and ultrasonic, nondestructive testing of

manufactured parts.

The information transfer sector of the

system is quite cumbersome. This is a re- sult of the diversity of groups performing

force measurements. In conjunction with the

adoption of a new method for the calibration

of force measurement devices, the system is now beginning to establish the lines of information return from the end user to the source of primary calibration. NBS is

actively promoting this exchange through

round robin calibration of devices and by

measurement workshops.

The anticipated metrication of force mea- surements has become an area of increasing

concern. In the field of force measurements,

the foreseeable problems of education of

professionals and large, long-lived capital

equipment, have been compounded by the con- fusion about the unit of force which will

evolve in common usage. The SI unit of

force is the newton. However, European

corrmon usage is the kilogram-force. This

usage has evolved in much the same way as the pound-force unit did in the U.S. This

confusion will delay the acceptance of the

SI unit of force and, as a result, the newton may be one of the last SI units which

is accepted in conmon usage.

This study has served to verify many of

the current concepts of the condition of the

force measurement system. Several strengths

and weaknesses have been discovered and

studied. NBS, through its standards and

calibration activities, will continue to have

a stabilizing and unifying effect on the

system. NBS should increase its efforts in areas of the system where weaknesses have

been found.
1 . INTRODUCTION

A thing of beauty is not always a joy

forever. All too often, stunning fabrics

fray or rip and new appliances fail inexplicably at precisely the wrong time. To help

insure that these failures, which tend to discourage a repeat customer market, do not occur before a reasonable lifetime is achieved from the product, manufacturers

are testing the strengths of their materials

to a greater and greater extent before com- mitting them to production. For example,

a major automobile manufacturer tests up- holstery and trim materials before approval

is granted for their use. Samples of ve- lours, brocades, leathers, vinyls and other

materials are scraped, flexed, torn and

pulled thousands of times to insure that

they will resist wear for the lifetime of

the vehicle. This is a typical example of

physical tes'ing which is brought on by an increasing consumer concern with the durability of products being sold. We are just

now entering an era where the importance of

mechanical testing is coming to the attention of legislators, regulators, and others

making public interest decisions.

What, then, are force measurements? They

are the direct result of the need to know

strength or weight. We are generally

familiar with tne concept of "push or pull"

involving a force and that some measurements

are indeed made to quantify such things as the thrust of a rocket. More often, however,

force measurements are used to provide knowledge of the strength of something, such as

a piece of wire, string or an automobile

bumper or of the weight of apples in a produce oag or the accuracy of the bathroom

scale. In this light, the question of the

adequacy of the standards of physical

measurement in meeting the needs of industry

and the public grows in importance and is

relevant to any examination of the perfor- mance bj' the National Bureau of Standards of

its basic statutory obligations.

NBS management has long recognized the

necessity for analyzing the physical mea- surements performed in this country, with

respect to the quality of measurements being

made and the development of optimum dissemination procedures for the needs of the system.

This paper describes the concept and utilization of force measurement standards and

their everyday use, both implicitly and ex- plicitly, in science, industry, commerce,

health, safety, etc. The network of stan- dards, procedures, and day-to-day measurements at all levels of accuracy is called

the National Force Measurement System.

At first glance, the National Force

Measurement System appears to be fairly

'imited and well defined. This is mis-

"eading and the current study, reported

-pon here, shows that, in our technological

society, lorce measurements play a major

role in the nation's industry and^ corrtTierce and in our daily lives. This report will

not dwell on each of the types of force

measurements which are being made but will

outline the scope of the measurement system

and detail a few of its specific impact

areas

.

2. STRUCTURE OF THE MEASUREMENT SYSTEM

Before we begin any discussion of primary

standards and accuracy requirements of the

force system, it will be useful to outline

its structure. As can be seen from figure

1 the system is stratified according to the

degree of accuracy required in a measurement. Eaci'i block on a level of the system

has approximately the same accuracy requirements and lesser requirements than the level

above it from which it derives its traceability to a primary standard. At the bottom of the system are the two major end user categories of the system; v/eighing system

users and testing machine users.

The major user groups derive their force

standard traceability from several sources. Standards laboratories, such as those

maintained by large industrial firms, the

military services, and commercial calibration services provide a calibration for

force devices by using deadweight machines

or the transfer standard techniques to be

discussed later. Secondary or working

calibration devices are then used to calibrate the testing machines or scale systems.

The entire hierarchy of the measurements

in the force measurement system might be

assembled within a large corporation. A metrology or standards assurance department

which is responsible for all force standards

in the company submits a calibration kit,

consisting of one or more load cells or proving rings, to NBS or another deadweight

facility for a "primary" calibration. After

the kit is returned, all the secondary

and/or v/orking standards are calibrated

against the kit using a back-to-back technique. The secondary standards are used by

the several departments in the company to calibrate testing machines, hardness machines, etc. Load cells are removed from the

scale weighing systems and calibrated

against either the primary or secondary

calibration kits. The testing machines are

used on the production line to assure quality

1

(

\

=T ic

C2: ^

2

control of the product and in the R&D department in the development of new products.

In addition, the company purchases new scales

or testing machines or requires repair of

existing equipment. In these cases, a preliminary calibration is provided by the scale

or testing machine manufacturer. If disputes

arise over the quality of a product between

the company and a buyer, a commercial testing

laboratory is contracted to referee the dispute. The commercial testing laboratory has

derived its traceability in much the same way

the company has. Perhaps a testing machine

calibration service has been employed or the

testing lab may own its own primary calibration kit.

Smaller companies derive traceability in a similar manner with various services of the

system purchased from testing machine manufacturers, calibration services commercial

standards labs, and NBS.

Discussions of the effects of many of these

elements on the system structure will be

presented in later parts of this document.

2.1 Conceptual System

Tradition tells us that the physical con- cept of force occurred to Isaac Newton on a fine fall day in 1686 while he was resting

under the apple tree in his garden. As a measurement quantity, force is defined by the

action of the Third General Conference of the

International Committee on Weights and Measures, October 1901. Force may be defined by

the simplified form of Newton's Second Law of

Motion, F = kma, and can be most precisely

determined from the attraction of the earth's

gravitational field on a known mass with an appropriate correction for the buoyancy of

the displaced air. In this way, the standards

of force are directly traceable to the national standards of mass, length and time. Since

the local value of the acceleration due to gravity and mass values can both be determined to a few parts per million, primary

force standards with good precision are available, e.g., deadweight calibration

machines with uncertainties of less than

20 ppm (0.002 percent) without accounting

for temporal changes in air density. In the

measurement system, force quantities are measured over the range from 5 x 10"^ newtons

the force exerted by a single muscle fiber,

to 5 X 10'' newtons for the Apollo Launch

Vehicle (fig. 2). Over that force range of

14 decades, the accuracy requirement ranges

from the 0.002 percent uncertainty of primary deadweight force standards to the 10 percent needed by metal rolling mills (fig 3).

Early force measurements were made using a combination of weights and multiplying levers.

An early practical portable force measure- ment instrument, the proving ring, was developed by Petrenko and Whittemore at NBS

in 1927 [1] (fig. 4). This was an elastic

steel ring developed for the calibration of

hardness machines. In the next year, the

first deadweight force standards were placed

in service at NBS.

^.MUSCLE

FIBER

O KILOGRAM _DEAOWEIGHT MACHINES

NOT AT NBS.

O 7 4 7 AIRCRAFT

DIESEL LOCOMOTIVE 0

SATURN V THRUST o

SOUND

RETAIL WEIGHING- SYSTEMS

NBS DEADWEIGHT MACHINES

COMMERCIAL ~ WEIGHING SYSTEMS

LOAD CELLS

STATIC TESTING MACHINES

10' 10" 10" 10' 10" 10" 10- 1 lo' FORCE LEVEL, Ibf

10 10^ 10 10 10 10 10

^x'O' 5"'°"' 5»'0'

FORCE LEVEL,

N

5" 103

Figure 2 - Ranges of forces measured in the measurement system

3
Table 1 - Summary of Principal Domestic

Deadweight Force Standards

DWM Organization Capaci ty Accuracy Notes

Ibf percent

National Bureau 1 ,000,000 0.002 of Standards 300,000 .002 112,000 .002 112,000 .002 25,300 .002 11,500 kgf

6,100 .002 3,050 kgf

500 .002

Other US Government Laboratories

US Army Redstone

Arsenal 1,000 .005 White Sands Missile Range 133,000 .005 US Navy Eastern Standards Lab 10,000 .002 Surplus NBS DWM

US Air Force Not The Primary

Edwards AFB 200,000 (a) USAF Standard

NASA Actually 225,000

Marshall SFC 496,000 .005 kqf Lawrence University of Radiation Lab 60,000 (a) Cal i fornia

Commercial Laboratories in the US Transducer

BLH Electronics 125,000 .005 Manufacturer

Interface Inc. 5,000 (a) Transducer Manufacturer

Lebow Associates

,

Transducer

Inc. 5,000 (a) Manufacturer

Morehouse Transducer

Instrument Co. 220 (a) Manufac turer

Nat'l Standards Only Commercial

Testing Lab 12,000 .002 Deadweight Calibration Lat Revere Corp. 100,000 .005 Transducer Manufacturer

Toroid Corp. 12,000 (a) Transducer Manufacturer

»CCUR»CY.

I PERCENT

0001

0003 - M»SS ST»ND»ROS

.001 ' DEtDWEIGHT MtCHINES WITH AIR DENSITY CORRECTION

.002 DEADWEIGHT MACHINES WITHOUT AIR DENSITY CORRECTION

.01

OS I PRIMARY ELASTIC FORCE STANDARDS

.1 • COMMERICAl WEIGHING DEVICES

25 - DEVICES USED IN VERIFICATION OF MATERIALS TESTING MACHINES

^ /

to MATERIAL TESTING MACHINES

1

?„ h MOTION WEIGHING. CAR IMPACT TESTS

3 'structural TEST FRAMES, FATIGUE TESTING MACHINES

5 ROLLING MILLS. FORCINGS. IMPACT LOADING

10

Figure 3 - Accuracy levels presently found

in the measurement system

Figure 4 - Whittemore-Petrenko proving ring,

the first practical transfer

force standard, 1927

During World War II, with the rapid increase

in the the number of measurements needed for

aircraft weighings, a force transducer with

an electronic read-out was developed. The

development of these load cells reduced the

physical size of the instruments and the

training of the operator needed to measure

large forces. With this, the number of force

measurements increased significantly.

In the 1960's, deadweight force standards

were installed at several other government

laboratories, at the manufacturing plants of

some transducer suppliers and by at least one commercial calibration laboratory (table 1).

In 1965, six new standards with a capacity up

to 4,500,000 N (1,000,000 Ibf) were installed

at NBS. During this same period, deadweight

capability has also been developed in many

other countries (table 2).

4
Table 2 - Summary of Foreign Force Standardization Equipment

Country DWM Capacity (a)

Total Capacity

Austria

Canada

France

Federal Republic of

Germany

Hungary

Italy

Japan

Nether! ands

Poland

Swi tzerland

United Kingdom

1 Machine, 4500 Ibf

1 Machine, 100,000 Ibf

1 Machine, 56,000 Ibf

14 Machines cover range from

660 Ibf to 450,000 Ibf

1 Machine, 11,240 Ibf

1 Machine, 22,500 Ibf

1 Machine, 2,200 Ibf

2 Machines of 5,500 Ibf

and 125,000 Ibf capacity

1 Machine, 110,000 Ibf

1 Machine, 22,000 Ibf

3 Machines of 500 Ibf,

10,000 Ibf and 100,000

Ibf capacity

3 Machines up to 1,124,000

Ibf with Hydraulic Multiplier

3 Machines up to 56,000 Ibf

with Lever Multiplier

12 Machines up to 3,300,000 Ibf

with Lever or Hydraulic

Mul tipl ier

1 Machine, 225,000 Ibf with

Hydraul ic Mul tipl ier

1 Machine, 225,000 Ibf with

Hydraul ic Mul tipl ier

1 Machine, 1,100,000 Ibf with

Hydraulic Multiplier

2 Machines up to 55,000 Ibf

wi th Lever Mul ti pi i er

1 Machine, 200,000 Ibf

with Hydraulic Multiplier

4 Machines up to 6,756,000 Ibf

with Hydraulic Multiplier

(a) All capacities have been converted to Ibf for comparison with Table 1 One pound-force equals 4.448 newtons.

2.2 Basic Technical Infrastructure

2.2.1 Documentary Specification System

2.2.1.1 Standardization Institutions

Methods for the accurate measurement of

forces and the calibration of force measurement devices have been promulgated by the

American Society for Testing and Materials

(ASTM), the American National Standards

Institute (ANSI), and by the International

Standards Organization (ISO). These standard

methods are referenced by most other standards-writing organizations (table 3).

2.2.1.2 Survey of Documentary Standards

Force measurement standards are generally

incorporated into a contractual agreement

between a purchaser and a vendor. The mea- surement standard is cited as a performance

requirement by the purchaser and tests are performed in accordance with the standard by

the vendor or a mutually agreed upon laboratory. The standards are written by many

organizations and very often are redundant,

i.e., several organizations have standards

for testing the same product. ASTM, for

example, has written voluntary standards for

the performance of more than 20 physical tests requiring force measurement (table 4). These

standards prescribe the test conditions for more than 50 materials and products (table 5).

NBS participates in many of the organizations

which develop voluntary standards.

Procedural specifications for the calibration of force-measuring instruments have

followed an interesting development in this

country. Shortly after the proving ring was invented, a Specification for Proving Rings

was drafted at NBS. The specification included mandatory design features of the instrument as well as calibration procedures

and performance requirements. Although NBS

had no regulatory authority over the manu- facturers of the rings, the policy was that

rings submitted for calibration to NBS were

tested under the document and awarded a certificate of calibration only if they com- plied with the requirements of the specification .

The NBS specification was immediately ac- cepted by both industry and governmental

agencies and, over the next few decades, be- came so thoroughly entrenched as the standard

5
Table 3 - Organizations Having Standards

Specifications or Procedures

Involving Force or Strength

Measurement

Aerospace Industries Association

Air-Condi tioning and Refrigeration Institute

Air Filter Institute

Aluminum Siding Association

American Carpet Institute

American Concrete Institute

American Dental Association

American Merchant Marine Association

American National Standards Institute

American Society of Civil Engineers

American Society of Mechanical Engineers

American Society for Testing and Materials

Asbestos Textile Institute

Asphalt Institute

Association of Iron and Steel Engineers

Clay Products Association

Concrete Reinforcing Steel Institute

Consumer Product Safety Commission

Cordage Institute

Department of Commerce

Department of Defense

Department of Transportation

Elastic Fabric Manufacturers Institute

Galvanized Ware Manufacturers Council

General Services Administration

Gummed Industries Association

Gypsum Association

Gypsum Roof Deck Foundation

Industrial Fasteners Institute

Metal Ladder Manufacturers Association

National Association of Chain Manufacturers

National Association of Corrosion Engineers

National Wooden Pallet Manufacturers Assoc.

Optical Manufacturers Association

Outdoor Power Equipment Institute

Prestressed Concrete Institute

Pressure Sensitive Tape Council

Rubber Manufacturers Association

Society of Automotive Engineers

Society of the Plastics Industry

Steel Bar Mills Association

Underwriters Laboratories

Table 4 - Tests Performed on Materials and

Structures which Require

Measurement of Forces

Abrasion Resistance

Bearing Test

Bond Strength

Bursting Strength

Climbing Drum Peel Test of Adhesives

Coefficient of Friction

Compressive Strength

Creep Resistance in Compression

Creep Resistance in Tension

Disk Shear

Fatigue

Flexural Strength

Hardness

High Speed Tensile Testing

Impact Resistance

Puncture Resistance

Residual Strength

Shear Strength

Stress Relaxation in Compression

Stress Relaxation in Tension

Tear Resistance

Tensile Strength

Table 5 - Representative Materials Covered

by ASTM Standards which Require

the Measurement of Force

Adhesives Plastics Film

Air-Condi tioning Hose Plastic Pipe

Aluminum Foil Plywood

Asbestos Resin Mortar

Bituminous Asphalt Rigid Plastics

Brake Hose Rock Core

Brick and Structural Samples

Clay Tile Roofing Shingles

Carbon-Graphite Mat' Is Rubber Belts

Cohesive Soil Rubber Cement

Concrete Paving Mat' Is Rubber to Metal

Construction Mat'ls Bonds

Cooper Magnet Wi re Seals

Corrugated Carboard Shi ppi nq Fiberous Composites Contai ners Glass Fibers Soi 1 -Cement

Glass Fiber Mat Steel Spring Wire

Honeycomb Core Texti le Ion Exchange Resin Timber

Leather Ti re Cord

Lime Mortar and Grout Ti res Metallic Foil Urethane Foam

Metallic Materials Vitrified

Natural Building Stone Ceramic

Package Cushioning Mat'ls Vulcanized

Paint Films Rubber

Paper Whi teware Mat ' 1

s

Paraffin Wax Yarn

Plastics

6
for the nation that it completely overshadowed any other calibration procedures. It maintains that position today, after some forty years of use. In the 1940's an American Society for Testing and Materials document, Method E 74, was adopted following the same general outline as the NBS document,

but without the mandatory design features.

Many of the ASTM specifications for testing

the materials listed above require that all

calibration instruments comply with Method

E 74. In spite of this requirement, only a few calibrations were ever performed under

this Method E 74.

During the 1950's, the difficulties at NBS over possible conflicts of interest resulted

in a policy decision to discontinue all cer- tifications of calibrations except those re- quired by law for State reference standards

of mass, volume, etc. The policy also re- quired that NBS would not issue specifications for proprietary products. The NBS

specification for proving rings was withdrawn at that time and proving ring calibration reports subsequently carried merely the

notation that the instrument complied with

the requirements given in the appendix to NBS Circular 454, an old publication describing the rings. In spite of this, industry still regards the NBS "specification"

as the definitive document in the field.

The NBS policy, while a necessary one, had

the unfortunate effect of making it impossible to update the old specification to take full advantage of new ideas and new

techniques. The design features of the

document were put in to insure certain levels of performance that could not be verified

from a calibration performance analysis in

1930. Today the information can be readily

obtained using computer programs.

Attempts to update the ASTM Method E 74

document have been underway for the past

four or five years but have suffered re- peated set-backs because of the Society's

policy of deferring to any objection. The

feeling of some industry spokesmen that "It's not in the NBS specification" has been a re- peated stumbling block. Recently, a new revision of Method E 74 [2] has been approved by the ASTM committees. NBS has changed

its usual calibration procedures to conform

to the new ASTM Method E 74.

2.2.2 Instrumentation System

2.2.2.1 Primary Standards

Deadweight Machine . A deadweight force

calibration machine consists of a selection

of masses and a means for using the masses

to apply a direct load to the instrument

under calibration (fig. 5). The masses of

the weights are generally adjusted through

multiplication of the mass standard to apply

nominal force values, taking into account

the local value of the acceleration due to gravity and the average buoyant effect of

the displaced air. These nominal values may

be in units of newtons, pounds-force or kilograms-force, but in this country, the U. S. Customary Unit, pound-force, is most commonly used. Deadweight machines do not measure

loads, but supply a limited series of precisely known forces.

JACK

ROOM

FORCE

ROOM

0

HYDRAULIC

JACK

t

LIFTING FRAME

CALIBRATION

DEVICE

WEIGHT

ROOM DEADWEIGHT

STACK —

Figure 5 - Schematic of a deadweight

calibration machine

7
The deadweight standards currently available at NBS (table 1) are undoubtedly the

most important group of force standards in

this country. Comparable standards covering

the same force range are not known to exist

elsewhere, although deadweight force calibration facilities in the lower load ranges

(i.e., up to about 100,000 Ibf or 450,000 N) are available in several countries. Apparently in these countries calibrations by means of combinations of several secondary

standards or force multipliers suffice for

the larger force ranges. In the NBS deadweight machines, the masses of the weights

were adjusted to apply nominal force values,

taking into account the measured local value

of gravity [3] and the average buoyant effect

of the displaced air. The uncertainty of

loads applied with these machines is less

than 0.002 percent and this uncertainty could

be halved by using a value of air density

measured at the time of each calibration.

Standard Weights . State and local Offices

of Weights and Measures indirectly provide

a significant portion of the force standards

calibrations. .This is done through the calibration of masses. The masses are then used

as standards to calibrate scale systems,

testing machines, etc., which actually measure force. The corrections for gravitational acceleration and air buoyancy are seldom required to meet the accuracy requirements imposed on these systems, but such

corrections could be easily made using local

gravity values obtainable from the National

Oceanic and Atmospheric Administration, and

average air density values.

2.2.2.2 Secondary Standards

As was noted above, the development of

high-grade instruments for the accurate

measurement of large forces, i.e., forces

exceeding about 4,500 N (1,000 Ibf), began

shortly after the end of World War I. In 1928 the National Bureau of Standards began

to provide accurate deadweight calibrations

of force measuring instruments up to 450,000 N (100,000 Ibf). A new instrument,

the proving ring, was developed by two Bureau

staff members [1] and provided a reliable

means by which the accuracy of the deadweight

standards could be transferred to other laboratories. Today, nearly half a century

later, the proving ring is still one of the

most reliable instruments available for the

transfer of static force calibrations.

World War II brought a sharp increase in

the demand for force measurements. The

necessity of multiple weighings of heavily

loaded aircraft by relatively unskilled

personnel and the availability of reliable

wire resistancj strain gages prompted the

development of the load cell. The use of

this instrument has grown so that 95 percent

Figure 6 - Proving rings

8
of force measurements are now made with load

cells.

A brief explanation of the characteristics

of the principal types of force measuring

instruments will assist in understanding the manner in which they tend to be used.

Proving Rings . The proving ring is a mechanical device in the form of a hardened

steel ring or loop with well -defined loading

points. Figure 6 shows several rings of the

type now in use. The elastic deflection of

the ring under load is measured with a micrometer screw and these measurements are

then translated into load by use of a table

or graph which summarize the calibration

data. Other mechanical length measuring

devices such as dial indicators can be substituted for the micrometer screw but usually

result in poorer performance. Generally

speaking, the mechanical instruments are

rugged, exhibit excellent qualities of longterm stability, and require only minimal care

and maintenance. They have the disadvantage

of requiring the presence of an operator close

to the point where the load is applied; they

are not direct reading and the values read

from the scale must be recorded manually and

Figure 7 - Load cell showing the strain

gage installation

subjected to a moderate amount of calculation

to determine the force value. In addition

they are not suitable for measuring a rapidly

changing load and require considerable operator skill and training to obtain satisfactory

resul ts

.

Load Cells . The second type of device is

the load cell (fig. 7) in which the elastic

strain in a stressed member is sensed by a fine wire or foil strip cemented to the

strained surface in such a manner that its

electrical resistance changes with strain.

Load cells offer an advantage in versatility

by virtue of having an electrical signal as output. They also offer greater possibility

for compensating for environmental and performance characteristics. They may be read

by an operator located well away from the

point of load application. They may be made

as a direct reading instrument; the output

may be recorded automatically or read manu- ally or even fed directly into an information

storage or computer system. They can follow

rapidly changing or fluctuating loads when

used with high speed recording equipment.

On the debit side, electrical instruments

are more delicate, subject to damage in shipment, exhibit good short-term stability but are less reliable in long-term stability,

and require frequent electrical checking and

maintenance procedures.

Other Force Measurement Devices . Other

instruments which are used as force measurement devices include rings or loops with

differential transformers, a number of instruments using variable reluctance or piezoelectric sensing elements and hydraulic and

pneumatic force capsules. These devices, to some degree, possess most of the advantages

and disadvantages of strain gage type load

eel 1 s

.

2.2.2.3 Transfer Standards

When deadweight machines are not available,

transfer devices can be used as calibration

standards. Calibrations of this type are accomplished by loading the transfer standard

and the instrument under calibration together,

in series, in a press of suitable design.

Since the conditions are static, the forces

on the tvyo instruments are the same and are measured by the transfer standard. This

technique is frequently referred to as a "back-to-back calibration". A special case of this is the technique employed at the

National Bureau of Standards and elsewhere

for calibration at forces beyond the range

of available deadweight standards (fig. 8).

Here the load on the instrument under

9
Figure 8 - Multiple load cell calibration

setup

calibration is distributed among three or more

transfer standards of lesser capacities.

Uncertainties ranging up to 0.05 percent of

load result from measurements using this

technique.

Other mechanisms have also been used by

standards labs as standards of force. The

hydraulic multiplier is capable of very high

precision and has been used successfully for some time. It has not been adopted to any

extent in this country, even though one manu- facturer has produced prototype models of

very high quality. The multiplier consists

of two free piston assemblies, the smaller

one being essentially of the deadweight pres- sure calibration type. The larger unit con- sists of double piston in a rotating cylinder.

The lower piston forms the base of the unit

and the upper piston exerts the calibration

force on the force-measuring instrument under

test. The two piston-and-cyl inder assemblies

are usually connected through a pressure com- parator. The deadweight load on the smaller

piston is multiplied by the ratio of the areas

of the two assemblies and applied to the instrument under test. Properly used, such a standard can approach the accuracy of

deadweight loading. The method has the advantage that multiplication factor can be

checked by loading the large piston assembly

in a deadweight calibration machine.

Mechanical lever systems have also been

used as transfer standards of force. In Europe, single levers have been used in a number of instances in conjunction with a small deadweight calibration machine. Multiplication of forces by five times is quite

practical and the factor has even been ex- tended to ten in one or two instances. In this country, a few lever systems have been

constructed, mostly designed with a much

higher multiplication factor so that the unit

can operate with relatively small, hand-loaded weights. These designs usually require a multiple lever system and frequently employ

flexure-plate pivots in lieu of knife-edge

pivots because of the greater safety and

stability. Unfortunately, the designers of

these units have given little thought to formal methods of verifying the multiplication ratios as the systems are generally too large and cumbersome to mount in a deadweight

machine. The mechanical lever systems are subject to the disadvantage that the multiplication ratio tends to change under increasing load because of the local deformations of the lever parts.

2.2.2.4 Instrumentation Industry

The instrumentation industry comprises

those companies which supply transducers,

testing machines and weighing scales to the

measurement system.

Force Transducer . The industry which

supplies instruments for the force measurement system is relatively small for the impact which its products have on other industry and commerce. The gross sales were $40

million in 1973 with almost no growth shown over the last several years. As could be

inferred from the two principal transfer

transducers available, the force transducer

industry is divided into two almost exclusive parts. The small proving ring market

is supplied by only two manufacturers

(table 6). About 100 devices per year are marketed with a large majority of the market

concentrated in one of the companies.

The load cell transducer market is supplied

by about 60 manufacturers (table 6) including

at least three importers. Many of these

companies include load cells in their product

line of physical measurement transducers.

In addition to sales of new transducers,

the industry provides a significant

10
recal ibration service either through its own deadweight machines or by comparison with

devices calibrated in deadweight machines

using techniques discussed in Section 2.2.2.3.

Testing Machines . While the testing ma- chine and scale industries appear as customers to the transducer industry, testing

machines and scales are basic tools in the

force measurement system. These industries,

therefore, must be considered a major part

of the instrumentation industry. Equipment

manufactured by this $40 million industry

are the foundation of the measurement of

strength of materials, quality control, and consumer safety section of the technology.

Tables 4 and 5 list some of the interactions

which the testing machine industry has with

society. Mechanical tests are performed on

a wide spectrum of materials from sewing

thread, with a strength of only a few ounces, to high strength steel plates and cable,

with strengths of millions of pounds. Testing machines vary in size and capacity from

those capable of measuring the strength of

a single fiber to the 12 million Ibf capacity

testing machine at NBS.

A partial list of testing machine manu- facturers is also shown in table 6. There

is a strong tie between the industry and the

Office of Weights and Measures. This is especially true for calibration of machines

with capacities less than a few hundred

pounds. Of the eight manufacturers contacted during this study five use weights for

calibration of at least some of the load

ranges of their machines. For recalibration,

however, the majority of users have the ma- chines verified using load cells or proving

rings which are themselves calibrated against

a set of primary standards.

Scale Systems . Load cell scale systems

vary in size from the scale used at the local

supermarket to weigh produce to scale systems

of more than 5 million newtons (1 million

pounds force) capacity, capable of in-motion

weighing of several freight cars simultaneously. Load cell based scale systems are typically used in batch processing operations

to control the flow of materials in a production process and to weigh trucks in motion

on highways for road tax enforcement.

The load cell weighing system market has

grown from sales of 2700 units worth $3.4

million in 1963 to sales of 6400 units worth

$11 million in 1972. It should be noted

here, however, that the total market in scale

systems was $140 million in 1972. This is divided among about 27 manufacturers, over

half of which make load cell weighing sys- tems of some type.

A relatively small number of scale systems

are used by the railroads to determine transportation rates of goods shipped. The 600

railroads which belong to the Association of

American Railroads operate 7000 track scales.

About 500 of these are load cell systems.

Such systems are being installed at ever increasing rates to replace older lever scale

systems. These railroads moved approximately 2.5 billion revenue tons in 1972, worth

about $12.6 billion in income to the railroads [8] . No attempt was made to estimate

the retail value of the goods shipped by

rail, but it includes everything from food

and fuel to automobiles and heavy machine

tool s.

The load cell weighing systems are, for

the most part, relatively large, stationary

installations. Consequently the easiest

way to recalibrate these systems is to bring

a set of standard weights to the scale and

calibrate it in place. Thus, the tie-in to the NBS Office of Weights and Measures is

very strong for this branch of the measurement system.

2.2.3 Reference Data

Force data in the measurement system can also be the result of a mechanical property

measurement and not a weight measurement.

As such the end use of this data is reflected in the type of laboratory which makes the

measurement. Tensile strength, fatigue

strength, tear resistance, etc., measurements are made on materials by research laboratories. The results of these measurements are published in the open literature

for the public use of design engineers. At another level, the corporate research department acquires data which is proprietary

to the company. This is used by the company

to improve existing products or in the planning and design of new products. Finally,

tests are performed as a routine part of the

quality control function on the production

line. The greatest number of measurements

are made at this level, and this data is basically contractual in nature. The buyer

requires a certain performance from the

material. The quality control tests are

used to establish the compliance with the

contract.

11
Table 6 - Force Instrumentation Manufacturers

Proving Rings

Morehouse Instrument Co. York, Pa. 17403 Tinius Olsen Testing Machine Co. Willow Rrove, Pa. 19090

Load Cells

AESA Electric Inc. White Plains, N.Y. 10602 AKO Inc. Enfield, Conn. 06082

Al i nco Cumberland, Md. 21502 AST/Servo Systems Inc Newark, N.J. 07104 Automatic Timing & Controls Co. King of Prussia, Pa. 19406

B&K Instruments Cleveland, Ohio 44101 BLH Electronics, Inc. Waltham, Mass. 02154

Bofors Industries

Linden, N. J. 07036

Brewer Engineering

Marion, Mass. 02738

Cardinal Scale Manufacturing Co. Webb City, Mo. 64870 Celesco Industries Inc. Canoga Park, California 91304

Celtic Industries Inc. Van Nuys, Cal . 91405

John Chatillon & Sons Inc. Kew Gardens, N.Y. 11415 Comptrol Inc. Cleveland, Ohio 44125 Comten Industries Inc.

St. Petersburg, 111. 33701 Control Process Inc. Plantsville, Conn. 06479

W. C. Di 1 Ion & Co. , Inc. Van Nuys, Cal . 91407

A. H. Emry Co. Mew Canaan, Conn. 06840

Entran Devices, Inc. Little Falls, N.J. 07424

Farr Measurements

East Lake, Cal . 80614

GSE Inc. Farmington Hills, Mich. 48024 Gilmore Industries

Cleveland, Ohio 44122

Gould Inc., Statham Inst. Div. Oxnard, Cal . 93030

S. Himmel stein & Co. Elk Grove Village, 111 . 60007 Houston Scientific Industries Inc. Houston, Texas 77018

Imtra Corooration

Medford, Mass. 02155

Instron Corporation

Canton, Mass. 02021

Intec Inc. Bound Brook, N. J. 08805

Interface Inc. Scottsdale, Ariz. 85260 . . Ki stler-Morse Corp.

Bellevue, Wash. 98005

Kristal Instrument Corp. Grand Island, N. Y. 14072 Lebow Associates , Inc

.

Troy, Mich. 48084

Lion Precision Corp.

Ne-'ton, Mass. 02195 Marcan Products Corp.

S. Norwalk, Conn. 06856 Martin Decker Co. Santa Ana, Cal . 92708 Metrodyne Corp.

Stamford, Conn. 06904 Micros train Inc. Spring City, Pa. 19475 Moxon Inc. Irvine, Cal . 92664 Ormond Inc. Santa Fe Spring, Cal . 90670

PCB Piezotronics Inc. Buffalo, N. Y. 14225 Precision Force Measurement Inc. Cinnamimson, N. J. 08077 Revere Corp. of America

Wal lingford. Conn. 06492

Ruska Instrument Corp.

Houston, Texas 77036 Schaevitz Engineering

Camden, N.J. 08101 Scientech Inc. Boulder, Col . 80303

Sensotec Inc. Columbus, Ohio 43216

Space Corn. Dallas, Texas 75040 Standard Controls Inc. Seattle, Wash. 98144 Strainsert Co. Bryn Mawr, Pa. 19010 Sundstrand Data Control Inc. Redmond, Wash. 98052

T^sk Corn. Anaheim, Cal . 92803 Tensitron Inc. Harvard, Mass. 01451 Testina Machines Inc. Amityville, N.Y. 11701 Thwing-Albert Instrument Co. Philadelnhia, Pa. 19154 Toledo Scale Toledo, Ohio 43612

Torroid Corp.

Huntsville, Ala. 35R04 Transducers Inc. Whi ttier, Cal . 90606 Trans-Sonics Inc. Lexinnton, Mass. 02173 West Coast Research Corn.

Los Angeles, Cal . 90025 Westinghouse Electric Corp.

Pittsburg, Pa. 15238

Testing Machines

ACCOBridgeoort, Conn. 06602

ACG. , Inc. West Haven, Conn. 06516

Ametek

East Moline, 111 . 61244 Applied Test Systems, Inc. Saxonburg, Pa. 16056 John Chatillon & Sons, Inc. Kew Gardens, N.Y. 11415 Comten Industries, Inc.

St. Petersburg, 111 . 33701 Cosa Corp.

Montvale, N.J. 07645 Custom Scientific Instru., Inc. Whippany, N.J. 07981 Detroit Testing Machine Co. Detroit, Mich.

W. C. Dillon & Co. , Inc. Van Nuys , Cal . 91407 Duffers Associates, Inc. Troy, N.Y. 12180 Fatigue Dynamics, Inc. Detroit, Mich.

GCA/Precision Scientific Co. Chicago, 111. 60647 Gilmore Industries

Cleveland, Ohio Claud S. Gordon Co. Richmond, 111 . 60071 Hampden Engineering Corp.

E. Longmeadow, Mass. 01028

Harisonic Laboratories, Inc. Stanford, Conn. 06902 Hunter Snrina

Hatfield, Pa. 19440

Instron Corp.

Canton, Mass. 02021 Interface Inc. Scottsdale, Ariz. 85260

Kinetic Systems, Inc. Waltham, Mass. 02154

Lab Div., Mech. Technology, Inc. Skaneateles, N.Y. 13152 Ladd Research Industries, Inc. Burlington, Vt. 05401 Martin Decker Co. Santa Ana , Cal . 92708 Mausner Eouipment Co., Inc. Carle Place, N.Y. 11514 Mechanical Technology, Inc. Latham, N.Y. 12110 Micro-Strain, Inc. Spring City, Pa. 19475 Monsanto Co. Akron, Ohio 63166

MTS Systems Corp.

Minneapolis, flinn. 55440 Pacific Transducer Corp.

Los Angeles, Cal . 90064 Pegasus Div., Koehring, Inc. Troy, Mich. 48084

Satec Systems, Inc. Grove City, Pa. 16127 Testina Machines, Inc. Amityville, N.Y. 11701 Thwing Albert Instrument Co. Philadelphia, Pa. 19154 Tinius Olsen Testing Machine Co. Wil low Grove, Pa. 19090

12
2.2.4 Reference Materials

There are no system-wide standard materials in force measurement. The only reference materials are those occasionally established and maintained by a corporation. In such a case a wel 1 -characterized specimen

material is used by the lab as a check test

of the calibration of a quality-control

testing machine. This is done once-pershift or once-per-day to verify that a com- plete recal ibration of the machine is not necessary.

2.2.5 Science and People

The education background of the people

in the measurement system, both professional

and technician, is best described as practical training. The differences in training

between professional and technician are only

a matter of degree, not of substance. A professional, usually an engineer, receives

little formal education in physical measurement during his university education. What

formal education he does receive usually

ends with his undergraduate degree. All

later training is gained on-the-job under

the tutelage of his supervisors. Technicians receive almost no formal training, and

training in force measurement may be com- bined with training in several other types

of measurement which are being performed in

the laboratory or on the production line.

The opportunity for formal training in

measurement techniques is generally more available to people working in a research

environment than to production workers.

The science in the measurement system includes the mechanical engineering sciences,

i.e., the measurement of one or more of the

physical properties of materials (table 4),

physics, electronics, statistics, etc. In- formation about this science of physical

measurement is transferred throughout the

system through the journals of the professional societies, technical reports, professional opinions, etc. This mechanism

tends to be limited by the specialized disciplinary nature of the journals and reports.

The interdisciplinary transfer of information

is carried out by the several abstract journals which draw from all available literature. The volume of this compilation task can be appreciated when the number of engineering society journals, university engineering schools, and corporate and government research laboratories are realized.

2.3 Realized Measurement Capabilities

An understanding of the range and accuracy

capabilities of the measurement system can

be derived from a review of figures 2 and 3 above. As might be expected the available

accuracy is often in inverse proportion to the nu. iber of intermediate calibration stages

between the final measurement and the primary

standard

.

The highest levels of accuracy necessary in

the area of force measurements are those for

the deadweight standards. Fortunately, the

nature of a mass determination is such that

the necessary accuracy can easily be obtained.

A deadweight standard with an uncertainty of

0.005 percent of load is adequate for the

calibration of the best elastic force measuring instruments available. The NBS deadweight standards are adjusted to be within

0.002 percent when used without correction.

If corrections for daily variations in air

density and for the measured masses of the

individual weights were to be applied, the

uncertainty of the NBS standard could be less

than 0.001 percent of load. As can be seen from table 1, other deadweight calibration

machines in use in this country are adjusted

to comparable accuracy. All of these ma- chines are considered adequate for present

day needs.

At the next level of performance are the

elastic force-measuring instruments such as proving rings and load cells and force-multiplying devices such as mechanical levers and

hydraulic multipliers used as transfer force

standards where an accuracy of 0.05 percent

of load is adequate. These instruments and

devices exhibit error characteristics that

are somewhat different from those found in

a deadweight system. The weights of a deadweight machine can be adjusted to be within

a certain percentage of their desired values

which is usually the same for all of the

weights in a given machine. Indeed, the

smaller weights may be adjusted to a some- what closer percentage tolerance than the

larger ones. As a consequence, the usable

range of a deadweight calibration machine

may be quite large. For instance, the

112,500 N (25,300 Ibf) deadweight machine

at the National Bureau of Standards may be

used at a load as low as 900 N (200 Ibf),

less than 1/100 of its capacity. An elastic

force-measuring instrument, on the other

hand, has an inherent error band of nearly

constant force amplitude throughout its

entire range such that the error in percent

of load increases markedly in the lower portion of its range. A force-measuring instrument used as a 0.05 percent reference

13
standard may be useable only down to 1/3 of

its capacity load. The same effect is present in lever systems and hydraulic multipliers where the constant-force error band

is contributed by friction. As a consequence, a laboratory that depends on transfer force standards is obliged to maintain

a substantial number of force measuring instruments in order to cover a sufficient

range of loads to satisfy the needs of its

customers. The initial cost of such an array of instruments and the annual cost of

keeping them in calibration by deadweight

standards appears to have discouraged the

growth of commercial calibration laboratories

Only one such laboratory is known to be

operating at this time.

Outside of the calibration laboratories,

there is a wide range of accuracy levels

for general force measurements. Performance

testing of aircraft engines and static weighing of aircraft, railway cars, and batch

materials generally require accuracies com- parable to the adjustment and maintenance

standards for commercial weighing devices,

about 1 part in 1000 or 0.1 percent of load.

These needs are met very largely through the use of load cells. Load cell weighing

scales of moderate capacity are usually

calibrated in situ by the conventional means

of scale calibration, using known masses,

but many of the modern weighing applications

require verification of the scale at loads

well beyond the range of these techniques.

The two NBS railway track scale test cars,

for example, are limited to a maximum test

load of 100,000 lb each. Modern railway

weighing needs, on the other hand, frequently range up to loads as great as 3.3 MN (750,000 lb). A similar situation exists

for many other weighing applications. As a result, there is a tendency to construct,

high capacity scales with load cell weighing

elements, often predicated on the concept of

calibrating the cells at some distant calibration laboratory before installing them

in place.

At the next lower level of accuracy are

the large number of measurements made to verify materials testing machines to an accuracy of one percent. The commonly ac- cepted accuracy for verification devices is

0.25 percent of load. Currently, the bulk

of these machine verifications are done with

proving rings which seem to be preferred

because of their ruggedness and long-term

stability under adverse conditions.

Materials testing machines themselves re- present the next level of accuracy in force

measurement. Here the maximum uncertainty

is 1 percent of load although some new testing machines are sold with a guarantee of

0.5 percent. The number of laboratories

equipped with some form of compression or tension testing machine is very large, and

many laboratories will have several such

machines in full use throughout a working

day. In a large steel mill, for example,

five to ten machines may be in use, with

tensile specimens being tested as fast as the operators can mount the specimens and remove the broken pieces.

Certain other types of materials or structural testing equipment can be operated at slightly lower levels of accuracy. Fre- quently portions of structures are tested in

a loading frame, i.e., a rigid frame that

constrains tiie external forces exerted on

the structure by hydraulic jacks or similar

mechanisms. Here the loads are measured

with individual load cells or even elastic

members equipped with dial indicators coupled in series with the hydraulic jacks.

In this type of test there are usually no stated requirements for accuracy and an un- certainty of two to three percent of load

is generally considered satisfactory and in line with the other parameters of the test.

A markedly different situation exists with

regard to fatigue testing equipment. In fatigue tests as well as other dynamic tests

the problem of inertial loading can be of

great significance. The instrument used to verify the accuracy of the fatigue testing

machine, usually a load cell, is customarily

calibrated und^^r static conditions, but is

subject to dynamic loading during the verification of a fatigue machine. Because of

interactions between machine components,

the need to measure forces remote from the

specimen under test, and possible changes

in instrument performance under dynamic

conditions, a dynamic uncertainty of two to three percent of load is about the best that one can be assured of in a fatigue testing

machine.

In the field of motion weighing of railway

cars, highway vehicles, and belt conveyers

for industrial materials, forces are mea- sured with uncertainties which range from

one to ten percent. Again the problem is one of dynamic versus static force measurement. The frequencies of force fluctuation

are low and probably the measurement made

by the force transducer is little biased by

the dynamic response of the instrument itself, but the jrror is due to the dynamic

behavior of the scale platform structure

and the dynamics of the vehicle being

weighed.

14
At the lowest level of accuracy are the

force measurements of such rough industrial

quantities as forces between the rolls of a rolling mill, forces applied in forging

presses or extrusion operations and other

types of impact or non-repetitive loading.

In these cases, repeatability of a measurement is more important than its accuracy and

an uncertainty of five to ten percent is

usually acceptable. Probably the future

will require an improvement in this level of

accuracy. The task of improving accuracy in all of the dynamic applications includes not only providing more accurate instrumentation,

but also understanding the dynamic responses

of the total system that is in motion.

2.4 Dissemination and Enforcement Network

2.4.1 Central Standards Authorities

The Force Measurement System has no internal regulatory functions. As a result, there

is no absolute central standards authority,

other than those required to disseminate the

three base units which define force. In a sense, all the deadweight machines such as those at NBS and the Office of Weights and

Measures constitute the central authority.

The exceptions are the Department of Defense

and the recently established Energy Research

Development Administration, which require

specific traceability to NBS for force mea- surement devices or material testing machines.

The requirement is not for traceability to the deadweight machines, but to NBS. Many

contractors provide acceptable traceability

through the Department of Defense calibration laboratories or through weight sets calibrated by state and local Offices of

Weights and Measures. The case of the ERDA

is somewhat better defined. ERDA has inherited a well-established enforcement net- work which was established by AEC. All

standards are traceable to a single set of

transducers maintained by a contractor who

provides the necessary calibrations to ERDA's suppliers and contractors.

In the most general case, however, the

dissemination of force standards takes place

through the voluntary recal i bration of force

transducers, scales, testing machines, etc. Because there is no central authority, jissemination of force standards is initiated at the user level and not from higher levels in the structure (fig. 1). A small study was made of the results of this voluntary en- forcement network and this will be discussed

later in section 4.2.

2.4.2 State and Local Offices of Weights

and Measures

The position which the Office of Weights

and Measures has in the Force Measurement

System has been cited several times in this

document. Their possession of one of the two types of primary standards for forces

less than about 10,000 Ibf, gives the OWM

a co-equal position to those laboratories

having the deadweight machine. As such, it

is a major link in the dissemination network.

The various state Offices of Weights and

Measures also perform a regulatory function

where retail conmodities are sold by weight.

It is through this mechanism that the

general public interacts most often with the

force measurement system.

2.4.3 Standards Laboratories, Testing

Laboratories and their Services

The role of standards and testing laboratories has also been discussed above, and

their relationship to other parts of the

system can be seen in figure 1.

With the exception of the few laboratories

who have a deadweight capability (table 1),

the role of the standards laboratories is

limited to a back-to-back calibration of

force instruments using secondary standards

and the calibration of standard weight sets,

a role similar to that of the Office of

Weights and Measures cited above. In their

case, however, their lack of statutory re- sponsibility Tor equity in retail sales

limits their impact with the general public.

Testing laboratories are divided into two sub-categories: industrial research and

development laboratories and commercial testing laboratories. These categories are not absolute, as will be shown, but the titles

of the two sub-groups are fairly descriptive

of their functions.

Research and development laboratories are generally those operated by industrial or- ganizations for product development purposes.

In 1967, there were 1437 of these laboratories with a total funding of about $1.1

billion [5]. Research performed by a sta- tistical sample (388) of these laboratories

is broken down as follows [6]:

R & D on government contract - $884 million

Miscellaneous R&D - 554 million

Commercial testing - 2.9 million

Other testing - 51 million

Total - 993 million

15
Testing laboratories are those set up and

run for profit in the testing services performed. Seldom are they affiliated with a large corporation, but they often test products for such corporations and perform as referees in disputes between corporations.

At the time of the 1967 industrial census,

there were 1243 testing laboratories with

gross revenues of $215 million [5]. Again,

a representative sample (551) of these laboratories have revenues broken down as

fol lows [6]

R & D on government contract - $ 5 million

Other R & C - 3 million

Commercial testing - 143 million

Other testing - 5.5 million

Total - 156 million

As can be seen from figure 1 , the DoD and

NASA laboratories are almost a self-contained

measurement system. This situation has

evolved over the years primarily because of

the size of the government's needs. It is

instructive to trace the path of the measurement system in a government organization,

namely the U. S. Navy as an illustration.

The Navy standards laboratory system is

nominally headed by the Type 1 Laboratory,

represented by the Eastern and Western Standards Laboratories. These labs maintain the

U. S. Navy's force standards, in the form of

deadweights or transfer standards which have

been calibrated at NBS. Load cell kits are calibrated by the Type 1 Laboratories for

six Type 2 Laboratories. These secondary

standard, load cell kits are used by the

Type 2 Laboratories to calibrate load cell

kits for the 7 shipyards, 15 floating laboratories, 3 missile tenders and 12 Naval Air

Stations of the fleet. An estimate of the

level of effort involved is provided by the

following statistics. A single Type 2 Laboratory calibrates about 1000 force devices

per year with a staff of 3 to 4 people. A shipyard lab, one of the direct users of the

Type 2 Laboratory services, submits devices

to the Type 2 Laboratory to be calibrated.

Of these devices, a load cell kit containing

8 cells ranging from 12,000 N (3,000 Ibf) to 1,200,000 N (300,000 Ibf) is used to calibrate 4 tertiary sets of standards of 8 cells

each. The 4 tertiary sets of standards are

used to calibrate 50 ring dynamometers, 12 testing machines, 50 spring dynamometers,

shipyard crane dynamometers and special test

set-ups as required.

As noted above, extensive calibration facilities are maintained by the three military

services and NASA. In addition, the NBS

manages a testing machine calibration ser- vice for laboratories doing concrete and

cement testing for the state highway departments. A staff of six are employed to calibrate testing machines used in about 250

state highway departments, concrete producer

and contract testing laboratories per year.

These calibrations are performed in accordance

with AST! Designation E329.

2.4.4 Regulatory Agencies

The regulatory control which the Department

of Defense and tne Energy Research Development Administration exercise over the force

measurements made by their industrial con- tractors has been discussed above. These

agencies explicitly require traceability to NjS.

The function of a regulatory agency is to prepare and enforce standards, regulations

or codes to insure equity in trade and public

safety. In the performances of this function

several federal agencies are involved in the

regulation of force measurements. The role

of tiie Office of Weights and Measures in the

regulation of the accuracies of retail scale

systems has been discussed. Table 7 shows

a list of several agencies involved in the

enforcement of oc>-jpational and public safety

codes which involve force measurement, generally involving the strength and safety of

products and struccures. In many cases,

there exists a strong tie between these

codes and the documentary standards discussed in section 2.2.1.2.

Table 7 - Regulatory Agencies Which Write

Standards Specifying Force

Measurements.

Consumer Product Safety Commission

Federal Aviation Administration

Federal Highway Administration

Federal Railroad Administration

Federal Trade Commission

Food and Drug Administration

Maritime Administration

Mining Enforcement and Safety Administration

National Highway Traffic Safety

Admi ni stration

U. S. Coast Guard

16
2.5 Direct Measurements Transactions Matrix

2.5.1 Analysis of Suppliers and Users

The Force Measurement System encompasses

the entire goods fraction of the domestic

economy to some degree. As can be seen in figure 9, the impacts on manufacturing are much greater than those on trade or services.

A more detailed picture of the impact areas

can be seen through a breakdown of the

Standard Industrial Classification (SIC)

groups into the two digit SIC codes (fig.

10). Figures 9 and 10 are a direct result

of the early attempts to develop the relationships between the various elements in the system, as shown in figure 1. An outline of the method used to develop these

figures is given in the appendix to this

report. These figures show that the greatest

utilization of force measurements is in the

primary metals, metal products, machinery,

communication and transportation industries

(SIC codes 33, 34, 35, 36, 37). In addition,

impacts are felt in the chemical and measurement equipment industries and at government

and other testing and research laboratories.

Of the industries identified during this

survey as users of force measurements, over

half were represented in SIC two digit codes

35, 36, and 37. Most force measurements are made as part of quality control procedures.

An understanding of the importance of the

measurement to these industries can be gathered from an analysis of the Direct Measurements Transactions Matrix for Force (table 8).

Significant correlation of information can

be made between the structure of the measurement system, (fig. 1) and the transactions

in the system. The principal suppliers of

force measurements have a very loose con- nection to the end user. Similarly, the

principal suppliers of commercial goods

supply few measurements to the industries

outside their own. In most cases, they do not supply force measurements outside their

own corporation. The force measurements

which are supplied by these groups are generally strength of materials measurements

and are required by the purchase contract.

Progression left to right and top to bottom

across the Input-Output chart is tending to- ward lower levels of accuracy and less dependency on parallel parts of the measurement system. The matrix shows adequacy and

FORCE SYSTEM USER PROFILE BY MAJOR SIC GROUP

GROUP B MINING

C CONSTRUCTION

0 MANUFACTURING

E TRANSPORTATION

F WHOLESALE TRADE

G RETAIL TRADE

H FINANCE

1 SERVICES

J PUBLIC ADMINISTRATION

jMmm mmm r-—^ „ r^^^ rxTOxi mmm

C D E F G H I J

SIC MAJOR GROUPS

Figure 9 - Impact of force measurements in the standard industrial

classification major groups

17
DIRECT MEASUREMENTS

TRANSACTIONS

MATRIX FOR

FORCE

SUPPLIERS

J o y> ig

50 X

~i o ^

o o OlC? <-o (-> J. g ^ 3 £

n 12 13

^ S 5 -I CD —

? = g

^8

1 PROFESSIONAL SCIENTIFIC & ACADEM IC iOmUNITY

2 INTERNATIONAL METROLOGICAL ORGANIZATIONS

3 DOCUMENTARY SPECIFICATION AND STANDARDS ORG

3 2

Z J 3 4 4

2

1 J

?—rr

3 ' 3

3 J

4

2

r 3

2

TRANSDUCER MANUFACTURERS

TESTING MACHINE MANUFACTURERS

SCALE AND WEIGHT MANyFACTURlRS_

2 2 4 I 3 —ti—

1 r

2—7

3 I 3

? h

3 : 2 ?—rn

3 1

7

J

3

2 3

3

2 372

3 1 3

2 3

3

1

4 2,3 3

1—3l5 T

3 ! 3 I 1

T—

8 STATE OFFICE

OF WEIGHTS AND MEASURES,

X I X

3 3 4

1 3

3

1 3

3 7 13

STANDARDS LABS 3 I 4

T—I?—r~ 113 4 10 GOVERNMENT

REGULATORY AGENCIES

11 DEPARTMENT OF DEFENSE

(EXCL. STDS. LABS

.

3 2|2

4 4

2 _J

12 OTHER

FEDERAL GOVERNMENT

13 CITY, COUNTY,

STATE GOVE RNMENT

14 INDUSTRIAL

TRADE ASSOCIATIONS

15 MINING AND CONSTRUCTION IND. [S1C_ GROUP B_4 C^^_

16 TEXTILE. LUMBER 8 PLASTICS INDUSTRIES

(SIC CODES 23 TO 28)

17 PRIMARY t FABRICATED

METAL PRODUCTS

(SIC CODE 33 8 34) 18 MECHANICAL AND ELECTRICAL MACHINERY

(SIC CODE 35 S 36)

19 TRANSPORTATION

EQUIPMENT

(SIC GROUP 3 7)

WHOLESALE TRADE

(SIC GROUP F)

J2 !3

' 3' 3 2'2 '

' 2 2

4

! 2

2

3

2;3

l2 7 7—

3 3

•7 4 ^I 4

7 ?!

-4-2 - -

'1 ? 2

i 3

3 ! 3 2

3 ; 3

See Difcumenliary S^nddrqizattqn Orga(niz9ti>ns

-+- -H \ .-.5ee D(cumenjary S1anddrcizdti<n Organizations

i ^ , . i — -.See D(cumen^ary Si andarc 1 zdtic n Organl zatijons

t

3 I 3

2r

7 7—

2 : 2

f- - ^—

2

4 3

72 1

2 ' 2

1 1

1 2

2 n 2 2 2 2

21 RESEARCH AND DEVELOPMENT LABS

(SIC CODE 739 1)

12 T 3,3

-2

2 3

2

3 7

3 , 2 7

3 , 2

22 MISCELLANEOUS

INDUSTRY AND C0W1ERCE

GENERAL PUBLIC 1 I .UL

2 ]2

! 2 2

Table 8 - Direct Measurements Transactions

Matrix for Force

18
FORCE STSTEM USER PROFILE BT SELECTED 2 DIGIT SiC CODE

16 r

14

12

10

GROUP

16 CONSTRUCTION

28 CHEMICALS

30 RUBBER & PLASTICS

33 PRIMARY METALS

34 FABRICATED METAL PRODUCTS

35 MACHINERY

36 ELECTRICAL EQUIPMENT

37 TRANSPORTATION EQUIPMENTS

38 MEASURING INSTRUMENTS

41 HIGHWAY TRANSPORTATION

45 AIR TRANSPORTATION

73 BUSINESS SERVICES

96 ECONOMIC PROGRAMS

97 NATIONAL SECURITY

16 33 I.I

TWO DIGIT SIC CODE

Figure 10 - Impact of force measurements in the standard industrial

classification 2 digit codes

stability. The basic relationships among

the parts of the system are well established,

are generally not cnanging or growing, and

in the area of static force measurement are adequate from the users point of view. The

only change in the system which appears here

is the type of measurement being made. Current growth is in the measurement of dynamic

properties of materials. In many cases, although static force measurements are com- pletely adequate, an increasing number of

materials and goods are now being tested and

used under dynamic conditions. The test

methods needed to test these materials, the

standards against which the dynamic forces

can be verified and in many cases, the basic

understanding of the motions of the dynamic

systems themselves are not available. These

inadequacies show up heavily, for example,

in the Documentary Standards and NBS portions

of the matrix.

2.5.2 Highlights of Major Users

The force measurement industry, an out- growth of the need to measure large v/eights

and the properties of materials, has direct

measurable sales of less than $70 million

per year. This includes the direct costs

of transducers ($40 million) and the fees

collected for calibration of standards to

be used for verification of production line

testing machines

.

Several of the major uses of force mea- surements have been discussed in earlier

sections of this report. These include the

impact of the testing machine industry,

government and industrial R and D laboratories and conmerical testing laboratories.

In addition to these direct measuren:ents

of force, other areas of measurement such

as liquid and cryogenic gas flow rate, flow

totalizer calibration, wind tunnel balance,

and the measurement of electric current are derived from force measurement. These areas

will not be discussed here.

2.5.2.1 Primairy '-letals Production

The objective of this section is not to describe the size of the total primary

19
metals industry ($44.5 billion) [7], nor to detail the use of force measurements in the

industry. It is, rather, to give some feel as to the number of measurements made and

to provide a typical example of the traceability of force in an important industry.

Of the total primary metals industry, steel

is the single largest fraction. In 1973,

steel production in this country was about

136 million tons, valued at $22.3 billion.

Each of the almost 100 companies producing

steel perform tests for strength and ductility of the product as part of quality control. In 1973 about 3.8 million such mea- surements were made, just by the primary

steel manufacturers. Several million additional tests are performed each year by

finished product producers such as automobile and appliance manufacturers.

One manufacturing plant of a major aluminum company sends 7 load cells to NBS for re- calibration every 2 years. These are used

to calibrate 100 materials testing machines

for compliance with ASTM E 4 specifications.

These testing machines are used in the cor- porate quality assurance program to insure

accurate measurements of mechanical properties. In addition, the testing machines are used to calibrate other force devices such

as dynamometers used in fatigue machines and

load cells, for use in cranes used for material handling. Load cells used in the rolling

mill to control plate thickness are calibrated directly against the laboratory standard.

Uncertainties not exceeding 0.25 percent are

required for calibrating testing machines;

5 percent can be tolerated in rolling mills.

A staff of 3 people is employed at each of

the seven corporation plants making such

calibrations.

2.5.2.2 Torque

A complete, almost self-contained sub-system of the Force Measurement System which is

concerned with torque measurement has been

identified. This part of the system is only

indirectly served by the remainder of the

measurement system through the transducer

manufacturers and the Office of Weights and

Measures. The greatest percentage of mea- surements are made with torque wrenches.

Most machinery manufacturers specify torque

levels for the fasteners which join parts

togetner. Applications range from spark

plug installation in automobiles to chemical

reactor vessel seals, steam generators, etc. Specified torque levels for these fasteners

span a range of at least a million to one.

Primary torque standards use the application of known forces to a lever arm of a precisely measured length. The forces are traceable to primary standards through the

paths mentioned above. Typical accuracies

for primary torque standards range from 0.25

to 1 percent. Accuracies of torque measurements range up to 10 percent for fasteners

tightened with impact wrench equipment.

Table 9 - Torque Instrument

Acurex Corp.

Mountain View, Cal . 94042

Ametek, Inc. East Mo line, 111. 61244

BLH Electronics, Inc.

Waltha'^', Mass. 02154

Camis Corp.

Meadowbrook, Pa. 19046 Carlson Co. Oceanside, N.Y. 11572

John Chatillon & Son

Kew Gardens, N.Y. 11415

Daytronic Corp.

bavton, Ohio 45429

W. C. Dillon & Co. , Inc. Van Nuys, Cal . 91407 Dynamic Precision Controls Corn.

S. Windsor, Conn. 06074

Fal k Corp

.

Milwaukee, Wise. 53201 General Thermodynamic Corp.

Wilmington, Mass. 01887

Gentran, Inc. Santa Clara, Cal . 95051

Manufacturers

S. Himnelstein & Co. Elk Grove Village, 111 . 6 Howell Instruments

Fort Worth, Tex. 76107

Kahn Industries, Inc. Wethersf ield. Conn. 06109

Kavlico Electronics, Inc. Chatsworth, Cal . 91311

Leho'-' Associ ates , Inc

.

Trov, Mich. 48084

Link Enqineerim

Detroit, Mich. 48227

McFauden Electronics Co. South Gate, Cal . 90280

McMab, Inc. New York, N.Y. 10012 Meridian Laboratorv, Inc. Middleton, Wise. 53362 • Moxon, Inc. Irvine, Cal . 92664 Owatonna Tool Co. Ovatonna, Minn. 55060

Pov"er Instru . , Inc . Skokie, 111. 60076

Sensotec, Inc. Columbus, Ohio 43212

Snan-on Tools Corp.

Kenosha, Wise. 53140

Sturtevant/Richmont- Dresser

Industries, Inc. Newtown, Conn. 06470

Sunshine Scientific Instru. Philadelphia, Pa. 19115

B. K. Sweeney Mfq. , Co. Denver, Col . 80216

Transducers, Inc. Whittier, Cal . 90606

Tyco Instruments Waltham, Mass. 02154

Utica Tool Co. , Inc

.

Orangeburg, S.C. 29115

Vibrac Corn. Chelmsford, Mass. 01824 Waters Manufacturing, Inc. Wavland, Mass. 01778

West Coast Research Corp.

Los Angeles, Cal. 90025

20
An approximate value for torque instrument

sales and calibration is not presently available. As shown in Table 9, torque measuring

equipment is supplied by about 35 manufacturers.

2.5.2.3 Airfreight Industry

This final study report is introduced to show the dependence of a fairly compact industry on load cell weighing systems.

When an eastern United States camera maker

recently needed to increase stocks of a new

camera on the West Coast rapidly, he flew

70,000 cameras on Saturday for Monday delivery. This is typical of the use of air

freight. This year such use will result in

freight revenues of about $1 billion. More

than 5.5 billion ton-miles of freight, mail

and express packages will be air freighted

on domestic airlines this year. An additional 2 billion pounds of goods worth $12

billion will be shipped in the international

import-export trade. This industry is growing by 20 percent per year.

Load cell weighing systems are used at

many different levels in this industry. Of

primary importance, the aircraft are weighed

and the maximum weight capacity and center

of gravity of the aircraft are determined.

These are important to the pilot for control

purposes and determining the range of the

aircraft. Of less critical importance is

determining the weight of the cargo being

loaded to aid in planning the proper location in the aircraft. The cargo is first

weighed by the air freight shipper, containerized for easier handling, and consigned to an aircraft. Cargo containers are then distributed in the aircraft according to the

aircraft's predetermined plan for load distribution. 23,000 kg (50,000 lb) of containerized cargo, in addition to almost 400 passengers and baggage, can be loaded into a single jumbo jet.

2.5.2.4 General Public

As in most measurements, the ultimate

beneficiary of force measurements is the

general public. Most of the time, the mea- surements have occurred many times during

an item's manufacture and the public is

totally unaware of their existence. In some cases, however, the public makes the final

measurement. The most common example of this occurs at the supermarket produce counter

where apples or tomatoes are purchased by the

pound and at home when the overweight dieter

checks his weight on the bathroom scale.

3. IMPACT, STATUS AND TRENDS OF THE MEASUREMENT SYSTEM

3.1 Impact of Measurements

3.1.1 Economic Impact

In considering the economic impact of force

measurements, it should be noted that the

purpose of such measurements is generally

either to determine the strength of a load

carrying element or to weigh the amount of

material in a commercial transaction. From

these considerations we see that the principal

economic impact could be classified as "equity in trade," with the measurements

either entering directly into the transaction

e.g. weighing operations, or more indirectly

as in the quality control checks on a production process. In either case, the mea- surement is made to assure both parties that

a product is delivered in accordance with a contractual agreement, explicit or implied.

In many cases, these contractual agreements

refer, directly or by implication, to one or more voluntary standards. A comprehensive

study of the economic impact would therefore

include the use of such standards and also

the processes and organizations that are involved in their development. However, here

it will suffice to note that such standards

contribute heavily by providing widely ac- cepted procedures for testing the product

without the buyer and seller having to devise and agree upon test methods, which could

frequently be an expensive and time-consuming

effort. Although the diversity of use of

these voluntary standards makes it difficult

to assign a dollar value to their benefits, we noted above that over 40 organizations

producing voluntary, consensus standards involving some form of force measurement operate

within the United States with support from

industry and general interest groups.

There is a second impact which force is

only beginning to have on the economy. There

has recently been a rapid growth in the num- ber and scope of federal and state mandatory

regulations on the strength and durability

of consumer products. Such regulations are

the result of government ' s deci si on to pro- tect the people from certain specific hazards

for the public good without the direct con- sent of those to be protected. Clearly the

costs for testing required by the regulations

are added to the price of the item, and the

incremental cost is felt directly by the

final consumer. As an example, let us look

at the testing of eyeglass lenses. Federal

regulations require that every eyeglass lens

be proof tested before being used. The lens

is mounted on a fixture and a steel ball

21
weighing 16 g (0.04 lb) is dropped on the lens

center from a height of 127 cm (50 in). The

lens, of course, must not crack. The manu- facturers estimate that this test cost them

$1.00 per lens and this cost is passed directly on to the consumer. Americans buy

about 200 million pairs of glasses and sun- glasses each year [9], so the add on cost for

this testing is about $400 million. Clearly

in such cases, government must weigh the increased economic costs against the expected

social benefits.

3.1.2 Technological Impact

Improved test methods and instrumentation

results in better material utilization,

better made products and improved product

safety. Thus force measurements are a means

to an end. The goals of advancing technologies are to produce better products at lower cost. Making better measurements and

measuring quantities which have never re- quired measurement before are vital to this

effort. This does not result in dramatic

breakthroughs in force measurement, but

rather in a steady advance of the total

testing techniques which results in improved

test results.

3.1.3 Social Impact

The social impact of the Force Measurement

System is felt largely in the area of increased safety. The system promotes safety

through enforcement of engineering standards

and better, more accurate measurements.

These have been discussed before. The social

implications of mandatory safety requirements,

while real enough, are hard to assess in

their present early stage of development.

In the future, an increasing number of com- monly used items may come under the control

of a safety related regulatory body. Some

of these have been cited above: tires, car door beams, bumpers, bicycles, glass doors,

toys, etc.

3.2 Status and Trends of the System

Our assessment of the National Force

Measurement System is that some parts of it are stable and adequately maintained while

other parts scarcely exist. Static force

measurement, the historical area of major

effort, over the range from 0.5 N (0.1 Ibf)

to 5 X 10^ N (10^ Ibf), is fully adequate

with the level of accuracy now provided, and

will remain so for the foreseeable future.

There does not appear to be any requirement

to improve the static force standards in this range by the factor of 2 presently

22

possible with an air density correction when

devices are calibrated.

However, in the areas of dynamic measure- ment of force, measurement of very small

forces, and measurement of torque, the system

is not in good shape.

The measurement system for dynamic force

does not exist. The standards and test methods that are needed to assure the accuracy

of measurements and safety, reliability and

efficiency of load carrying systems are not

available. This lack of standards is a deficiency which gives every appearance of becoming a major concern. The American Society

for Testing and Materials in its recently

published, "Standard Method for Verification

of Fatigue Machines" (ASTM Designation E 467)

states that current methods of measuring dynamic forces make untested assumptions about

the cyclic behavior of materials under dynamic conditions. Virtually every industry

surveyed had some interest in being able to improve the ability to make dynamic measurements. The metals industry needs to be able

to characterize materials better for resistance to crack propagation, forming, and improvement of extrusion and rolling mill procedures. The ability to measure the high

speed effects will become economically important as plant speeds increase in the at- tempt to increase productivity. Motion

weighing is already speed limited. Several

important structural factors including the

dynamic response of the weighing cells are unknown. Railroad operations, track and aircraft weighing operations and batch process

operations are all limited by the available

equipment. Other examples of dynamic force

measurements which have unknown uncertainties

due to being made without knowledge of the

dynamic behavior of the force device include:

(1 ) Impact Forces

(a) Barrier crash tests of

automobiles

(b) Tests of automobile seat belts

(c) Biomechanical failure studies

of human bone under impact

(2) Miscellaneous High Speed Transient

Forces

(a) Dynamic penetration of tires

(b) Rocket and jet engine thrust

measurements

(c) Weapons testing

(d) Skid resistance of highway

pavement

(3) Cyclic Forces

(a) Fatigue testing of all tyoes

(b) Mechanical impedance

measurements
The measurement of very small static and

dynamic forces has also been identified as

an area in which work is needed. This mea- surement directly impacts the biomedical

field where a means is needed to measure the

pulling strength of a single muscle fiber.

It also impacts the acoustic measurement

field which has need of a way to map the output of ultrasonic transducers. If forces of

5 X 10"8 Newtons (10"- Ibf) could be accu- rately measured a method would then be

available to provide an absolute calibration

of the radiated power of the transducer at every point in the transmitted field. This

would improve quality control of the transducers when they are manufactured and of the

measurements made with these systems. The

problem of calibration of ultrasonic transducers is one of current urgency and considerable effort is being spent on development

of an "interim" technique.

All the aspects of information transfer

and preparation for metrication need strengthening. The information transfer sector of the

measurement system is quite cumbersome. As was noted above, this results from the diversity of the groups performing force measurements. The system is now only beginning to establish the lines of information return

from the end user to the source of the first

measurement in the calibration chain. In conjunction with the adoption of the new ASTM

Standard for Calibration of Force Measuring

Devices, E74-74, about 30 government and

corporate metrology laboratories are co- operating- in a round-robin calibration program. This is being done to assess the new standard in field use, to improve conmunication among the parts of the measurement

system and to evaluate the magnitudes of the

random and systematic errors which result

when force standards are transferred to field

measurements. NBS has agreed to provide the

instrumentation packages and perform the

statistical evaluation of the data for the

round-robin group. It is hoped that variations in the data resulting from nonuniformity of test procedure and nonuniform methods

of data presentation can be controlled. A workshop session to discuss problems en- countered in trying to implement the new calibration method E74-74 has been held and

34 government and corporate laboratories

participated. This workshop preceded adoption by NBS of E74-74 as its usual method of

standards calibration.

Any discussion of trends in the measurement system must include the conversion from

U. S. Customary Units of Measurement to the

SI System of Measurement. For the force

measurement system, the problems range from

confusion over which unit will finally be

adopted for customary usage to how to overcome the problem of long term depreciation

of capital equipment.

The SI unit of force is the newton. However, many European countries are using the

ki logram- force (kgf) as the force unit in much the same way as the pound-force unit is used instead of the Poundal in the U. S. Customary System. This has, and will continue to confuse engineers in this country.

The present trend in the U. S. is to use the

newton, despite the references in the literature to European use of kilograms-force.

This confusion is added to the problems of

education of engineers and technicians cited

in the (J. S. Metric Study IlOl. Engineering

education at the university level will tend

to fellow the increased usage of the SI system. If the common practice is to change,

the education must start at the first levels

of formal education. It has been estimated

that 10 to 20 years might be required for

the change.

We have also noted that most force measurement and producing equipment currently ex- isting in the U.S. is graduated in U.S. customary units. This equipment is long

lived and its continued use may delay the

conversion to SI units.

As a result of these two reinforcing trends,

the newton may be one of the last SI units in common usage. Even then, confusion may con- tinue if the problem of newton versus kilogram-force is not resolved.

4. SURVEY OF NBS SERVICES

4.1 The Past

As was discussed in Section 2 above, the

history of force measurement in the United

States is closely linked to the history of

NBS. The development of high-grade instruments for the accurate measurement of large

forces began shortly after the end of World

War I with the invention of the WhittemorePetrenko proving ring [1, 11]. The proving

ring, because of its accuracy, constancy and

convenience, was further developed commercially and is now the most widely used device

for the calibration of testing machines.

About 4400 of these devices are now in service in commercial and government laboratories.

After the development of a proving ring

design suitable for the calibration of testing machine, it became appropriate for the

NBS to provide means for applying the

precisely known forces to these rings to

23
detennine their performance characteristics.

A deadweight machine was accordingly designed

and placed into service in 1928. The Bureau

has continued to provide deadweight calibration services and has extended the deadweight

accuracy and capability to cover the range

of 50 to 5 X 10^ newtons (10 to 10^ Ibf)

with uncertainties of less than 20 ppm

(fig. 11).

In the early 1940' s the idea of measuring

forces by measuring the elastic deformation

of a steel column under load with the newly

developed resistance strain gages occurred

to several experimenters at about the same time [12, 131. Soon, force measurement devices capable of measuring forces up to 13

million newtons (3 million pounds) became

available. Although basic load cell design

has changed little since the 1940's, improved strain gages, gaging techniques and re- finements in mechanical design have provided

improved transducers. In 1971, an NBS

analytical study of load cell design provided load cells with improved response for

secondary standards for force calibration

[14]. A set of 4.5 MN (1 million Ibf) and

13.3 MN (3 million Ibf) standards was fabricated and is now in service at NBS.

Although the growth of the system seemed

fairly orderly with the design and installation of the first 493,750 N (111,000 Ibf)

deadweight machine at NBS, forecasts of ex- pected needs made by NBS, other Government

Agencies and a load cell manufacturer in the

mid 1950's predicted an urgent need for much

larger capacity, high precision, for mea- suring devices. It was expected that these

would be a primary requirement in the development of an ICBM missile system. Accordingly

new deadweight machines to be used to calibrate these devices were designed and installed at the new NBS site in Gaithersburg,

Maryland. Additional state-of-the-art and

economic studies were made in 1967 and 1971.

Both of these studies were conducted for

internal use only and the results were never published. These studies indicated that,

because of technical developments in other

fields during the planning and construction

of the new deadweight facilities at

004

Q03

PERCENT

OF LOAD

001

UNCERTAINTY OF STATIC FORCE CALIBRATION

002 „

4 6

LOAD. MILLIONS OF Ibf

Figure 11 - Uncertainty of static force calibrations

24
Gaithersburg, the need was not nearly as large as had been predicted. The demand for

force calibrations by NBS rose until it

reached a peak of about 1000 devices per

year in the early 1960 's. It has since

fallen to a current level of about 400 devices per year (fig. 12). This decline began at the time when deadweight machines

were placed into service at DoD and NASA

laboratories and by several transducer

manufacturers

.

4.2 The Present

4.2.1 Description of NBS Services for Force

NBS, because of its diverse program,

affects, in some way, almost every sector

of the measurement system. The NBS program,

in addition to providing primary force calibrations to any client upon request includes

the development of test methods for measurement of the physical properties of structures, metals, building materials, fabrics,

glass, rubber, and many other materials.

Tests performed on these materials attempt

to establish their physical behavior under

various environmental conditions. Materials

are tested in hostile environments which include high vacuum and intense nuclear radiation. Test temperatures range from the near absolute zero of liquid helium to several

thousand degrees. Much of this testing is related to the national problems of energy

conservation and consumer safety. Only the

research areas which relate to the force

calibration and dissemination services of

the National Bureau of Standards are within

the scope of this study.

The services which NBS provides to the

system are of three basic types. These are

1) maintenance and dissemination of force

standards, 2) participation in voluntary

standards activities, and 3) development of

measurement methods and instruments.

Another way in which NBS interacts with

the measurement system is through its development of innovative instrumentation.

As was noted above, in 1971 a new set of

transfer standards was designed for use at

NBS to take advantage of the new force capabilities available with the installation

1000 r

900

800

700

600

400

1939 1945 1950 1955 1960 TIME- CALENOEI YEIIS

1965 1970 1975

Figure 12 - Histogram of force calibrations at NBS

25
of the 54 MN (12 million Ibf) testing machine. This new design was accomplished

through the use of a finite element analysis

of the response of the load cell body to the

applied forces. The new design resulted in

a transfer standard with improved and more

uniform response characteristics. Currently,

a new transducer for the measurement of dynamic loads is under development at NBS [15].

In this development, a fluorocarbon plastic

film is mechanically loaded to straighten

and polarize the polymer chains. When load

is applied normal to the surface of the

polarized film, a piezoelectric effect occurs

and the output voltage is proportional to the

dynamic load applied. This instrument is

currently in use in several medical and defense related applications such as measuring

the forces introduced into anthropomorphic

dummies by seatbelt restraint systems used

in automobiles and aircraft.

4.2.2 Users of NBS Services - Input - Output Analysis

The Engineering Mechanics Section, Mechanics Division, is responsible for maintaining

the national standards of force, for providing a calibration service by which transfer

force standards can be compared to the

national standards, and for providing leadership in government and industry in the advancement of the science of force measurement. An analysis of the input-output flow

chart (fig. 13) shows the influences which

affect the directions of research and calibrations, and the directions in which the

current output of the Section are directed.

4.2.2.1 Input

Inputs into the system are largely of

three types. The single most important input is the use of the base units of length,

mass and time which establish the force

standards. Very careful mass measurements

together with the best available value for

g were used to produce a force standard with

uncertainties of less than 0.002 percent.

The two secondary inputs are in the form

of direct requests for calibration services

and sponsorship of R and D in the areas of

interest which coincide with the NBS mission.

All of the types of organizations shown con- tribute to these inputs to some degree.

However, most of the R and D support is funded by Congress or by other government

agencies. Interaction with industry groups

and standards organizations primarily takes

place through calibration services.

4.2.2.2 Output

The output of the Engineering Mechanics

Section is divided into three major parts:

R&D publications. Standards and Test Me- thods, and Calibration Services.

R and D publications are the result of

research supported by direct appropriation

or supported by other government agencies.

Research constitutes a major percentage of

the total Section effort. Publications take

the form of papers in archival journals.

Technical Notes, and Technical Reports to the sponsoring government agencies.

The writing of standards and Test Methods

results from the voluntary participation of

75 percent of the professional staff in or- ganizations such as ASTM, ASME, etc. Staff

members often perform a major percentage of

the development of such documents.

The staff is occasionally requested by

Government sponsors to write test methods or procurement specifications for items where

research has shown that a new test method

would be beneficial to the government.

Very often, the contact of the Section

with the measurement system is through the

calibration services. Devices are submitted

for calibration by academic, industrial and

government organizations. The direct calibration of force devices constitutes about

15 percent of the section's effort. About

400 force devices per year are calibrated

using the deadweight force standards

maintained in the section.

The numbers on the branches of the Output

Section of figure 13 show the portion of

the Section's effort in force measurement

that is expended for the direct benefit of

the organizations represented by the branch.

This may not correlate well with the total

benefit from such an activity. For example,

the one percent effort that goes toward

participation in voluntary standards organizations may have widely distributed benefits of

significant magnitude to the users of these

standards. Users of the standards are re- presented on other branches of this chart

and the benefit to them is indicated there.

The percentage of direct benefit as shown

on figure 13 should be compared with the

percentage of calibration services provided

as shown in figure 14. It is interesting

to note the similarity between the two charts.

Even though calibration constitutes only

about 15 percent of the Section's output, it

is distributed in about the same manner as

26
INPUT

GOVERNMENT AGENCIES REQUESTS fOR SERVICES. SPONSORED R&D. REPORTS &

PUBLICATIONS. RtOUESTS

FOR INFORMATION

NBSMASS

LENGTH> ACCELERATION

TIME

VOLUNTARY STANDARDS AND

RFC PRACTICES

ANSI

ISO ASTM-E 74

PROFESSIONAL ORGANIZATIONS ASTM ASCE ASME SESA

ISA FTDG OTHER COMMITTEE ACTIVITIES INFORMAL CONTACTS

INDUSTRY REOUESTS FOR SERVICES. PUBLICATIONS

COMPULSORY STANDARDS

ICC GOVERNMENT CONTRACTS

MIL SPECS

FOREIGN CONTACTS

ISO REOUESTS FOR SERVICES

ENGINEERING MECHANICS

R&D PUBLICATIONS

3MMUNITY

TECHNICAL SOCIETY INFLUENCE AND PUBLICATIONS

STANDARDS & TEST METHODS

RESEARCH COMMUNITY

UNIVERSITIES INDUSTRIAL R&O

LABS

NBS

CCRL 401 04 FLUIDS 213 06

CBT 460 00 1%

10%

VOLUNTARY STANDARDS

E4 ASTM E8E467

MIL 1312

FOREIGN GOVERNMENT 1%

1%

PROFESSIONAL ORGANIZATIONS ASTM ASME

ASCE

STATE GOVERNMENT 5%

1%

INDUSTRIAL ORGANIZATIONS

QUALITY CONTROL

BATCH WEIGHTING 25%

CALIBRATION SERVICES

10% OTHER GOVERNMENT

NASA

DOT

INTERIOR

JUSTICE

25%

COMMERCIAL ORGANIZATIONS

LARGE SCALE WEIGHTING SYSTEMS

TESriNG LABS

TESTING MACHINE MANUFACTURERS

AIRLINES RAILROAD 20% lEFENSi

USA USN

USAF

DEFENSE CONTRACTORS

1%

OUTPUT

REGULATORY AGENCIES

j ICC FAA CONSUMER PRODUCT SAFETY

Figure 13 - Input-output flow chart shov^ing distribution of

section effort

27
INPUT

GOVtRNMENT AGENCIES REQUESTS FOR SERVICES. SPONSORED R&D. REPORTS &

PUBLICATIONS. REQUESTS

FOR INFORMATION

NBSMASS

LENGTH> ACCELERATION

TIME

VOLUNTARY STANDARDS AND

REC. PRACTICES

ANSI

ISO

PROFESSIONAL ORGANIZATIONS ASTM ASCE ASME SESA

ISA FTDG OTHER COMMITTEE ACTIVITIES INFORMAL CONTACTS

ENGINEERING MECHANICS

R & 0 PUBLICATIONS STANDARDS & TEST METHODS

RESEARCH COMMUNITY

UNIVERSITIES INDUSTRIAL R&O

LA8S

NBS

CCRL 401 04

FLUIDS 213 06 CBT 460 00 3%

VOLUNTARY STANDARDS

E4 ASTM E8E467

MIL 1312

FOREIGN GOVERNMENT 5%

.5%

PROFESSIONAL ORGANIZATIONS ASTM

ASME

ASCE

STATE GOVERNMENT 5%

3%

INDUSTRIAL ORGANIZATIONS

QUALITY CONTROL BATCH WEIGHTING 33%

3%

37%

20%

21%

I NP^ISTRY requests for services, publicaVions

C OMPULSORY STANDARDS

ICC GOVERNMENT CONTRACTS

MIL SPECS

FOREIGN CONTACTS

ISO REQUESTS FOR SERVICES

RESEARCH COMMUNITY

TECHNICAL SOCIETY INFLUENCE AND PUBLICATIONS

CALIBRATION SERVICES

1

OTHER GOVERNMENT

NASA

DOT

INTERIOR JUSTICE

COMMERCIAL ORGANIZATIONS

LARGE SCALE WEIGHTING SYSTEMS

TESTING LABS

TESTING MACHINE MANUFACTURERS

AIRLINES RAILROAD

OUTPUT

Figure 14 - Input-output flov^ chart for force calibration services

28

DEFENSE USA USN

USAF

DEFENSE CONTRACTORS

REGULATORY AGENCIES

ICC FAA CONSUMER PRODUCT SAFETY
the total section benefit output is distributed.

4.2.2.3 Recalibration Rate Profile

A study of the recalibration histories of

a statistical sample of all transducers which

have been calibrated against the NBS deadweight force standards was made. The file of

calibration records was sampled at random for

each of the two principal types of transfer

standard. The sampling procedure was as

fol lows

:

1) The calibration record for a single

device was selected at random from

the file. (The file for proving

rings contains about 3500 entrees.

The file for load cells contains

about 2300 entrees.)

2) If the interval since the last calibration exceeded 5 years, the information was recorded but was not

included in the computation of the

recalibration interval.

3) If the interval since the last

calibration was less than 5 years,

the last two calibration intervals

were averaged and recorded.

4) No device was included which had

not been calibrated three times.

The study revealed that approximately 30

percent of proving rings and 40 percent of

load cells do not have a recent NBS calibration, i.e., have not been calibrated within

the last 5 years. Of the devices on which

the calibration is current, figure 15, over

half of the proving rings and almost 80 percent of the load cells have been calibrated

within the interval reconmended by ASTM

E 74-74 (2 years). Approximately half of the

current NBS clientele submit devices annually

for calibration. Many of these clients submit multiple sets of rings or cells on a rotating basis such that the interval on each

ring reflects the results presented above.

4,2.3 Alternate Sources

The alternate sources for the primary calibration of static force standards have been

discussed earlier in this report (table 1). No alternative source exists for the 4.5

million newton (1 million Ibf) deadweight

standard at NBS. With one exception, force

standards available elsewhere are limited

to about 450,000 N (100,000 Ibf) and only a few of these exist. Force standards below

100,000 N (20,000 Ibf) are more generally

available.

As was noted above, the availability of

deadweight force standards increased greatly

in the early 1960's with the growth in military and space rocket research. These ma- chines were installed where a quick-turnaround of force calibration was necessary

for completion of a mission. It is important

to note that almost no new facilities have

been built since that time. The cost of such

an installation today might well preclude any

such new development in the foreseeable future.

As a result, the existing facilities constitute a natural monopoly.

As noted in section 2.2, most force standards used at the working level are calibrated using a back-to-back technique against a set of standards which have been previously

calibrated with a primary standard. NBS has

encouraged this type of calibration chain.

It provides accuracies at a level needed by

40

PROVING RING

0 - IS 31-60 INTERVAL mos

61-120

r~7i 0-18 19-30 31-60 61-120 >120

INTERVAL mos

Figure 15 - Recalibration interval for

force transducers at NBS

29
the users at a price which is reasonable,

and keeps the workload on the primary standards at a manageable level.

In the field of dynamic force measurement,

no standards exist at NBS or at any alternate

source. The impact of this will be discussed

in a later section.

4.2.4 Funding Sources for NBS Services

As was discussed earlier in this section,

about 15 percent of NBS direct force measure- ment output is force calibrations for customers. These are paid for on a reimbursable

fee basis. The remaining budget is supported

by direct appropriation (about 35 percent)

and other agency contract (about 50 percent).

The only constraints associated with the

funding are those imposed by restrictions in the NBS organic act and the functional statements of NBS and IBS.

4.2.5 Mechanism for Supplying Services

These mechanisms have all been discussed

earlier in section 4.2.2.

4.3 Impact of NBS Services

4.3.1. Economic Impact ,

The users of force measurements in the

measurement system have been identified in sections 2.4 and 2.5 of this report. Although, as can be seen from figure 1, NBS

is not unique as a source of accurate primary force standards, support for all segments of the force measurement system is ultimately traced to NBS, much of it through

the NBS force standards and some more directly through the basic units of mass, length

and time. Therefore, it is important to note the general economic impact of force

measurements as discussed above in section

3 and to realize that the greatest impact of

NBS is not directly on the economics, but on the technology of the measurement system

through the development of better measurement methods and tools.

4.3.2 Technological Impact

The technological impact of NBS services

results from all the measurement services

provided other than the direct calibration

services (fig. 13). The historical role NBS

has played and continues to have primarily

through the development of force measuring

instrumentation, voluntary participation in standards writing committees, and cooperation with government laboratories in the

promotion of better measurement techniques

has been mentioned above. In recent years,

this latter area of participation has ex- panded to include influence in the preparation of mandatory standards by several

federal regulatory agencies. Examples of

recent work where the NBS effort has been

to encourage or develop new test techniques

include: 1) conducting tests in support of

new voluntary standards for the testing of

ypsum wall board for home construction;

2) developing standardized tests for pultruded plastic products; (3) developing

testing techniques in support of new mandatory standards for architectural glass,

bicycles, and lawnmowers; and (4) developing

testing techniques in support of new government specifications such as those for

threaded fasteners, airport runway marker

lights, and automobile ignition locks.

Typically our greatest impact has been on the technologies which require new tests or measurement techniques where experience has

shown that strength or force measurements

are now needed.

4.3.3 Pay-Off from Changes in NBS Services

As was noted briefly above, NBS has re- cently changed its calibration procedure to conform to ASTM Method E74. As a result,

the same procedures are now used at NBS for

the calibration of all forms of force measurement devices. This is a radical departure

from the methods used historically where

each type of device was calibrated differently, often using procedures defined

by the customer. Many of these procedures were not representative of the manner in which the device was used in service and,

as 5 result, were capable of masking errors which were inherent in the instrument design. The new procedures search out many

of these systematic errors and are able to detect deterioration in performance which

may occur as a result of normal use. In this way, at almost no increase in cost to the user, marked improvements in measurement accuracies were achieved at the user

level. The benefits of these are directly

traceable to the economic impact of force

measurements on the economy. NBS, through

its recognized leadership role in the field,

is encouraging the other measurement laboratories to adopt the new procedure with

the hope of establishing a nationwide uniform procedure.

The adverse consequences of the adoption

of these procedures will be felt by those

organizations who use secondary standards

in the field to calibrate testing machines,

loading frames, etc. These companies and

labs will almost certainly be forced to use

30
more devices to verify the total range of the

testing machines being calibrated. Certainly

there will be a small increase in the cost

of the service from these companies, but

this is outweighed by the benefits of the

improved measurement accuracy.

NBS's role in establishing, changing, and

improving calibration standards and measurement procedures is derived from the role of

NBS and its statutory responsibility. It would be difficult to find another organization without a vested interest in force

measurement to which this role could be

entrusted

.

4.4 Evaluation of NBS Program

NBS has, and disseminates to anyone who

needs them, the most complete set of deadweight force standards in the world. These

standards are closely coupled to a wellequipped laboratory staffed by diversified

people interested in development of test

methods for mechanical properties of materials

and structures. The role of NBS is widely

accepted by the technological and commercial

community as the authority in force measurement. Through this role NBS provides the

necessary stability to the static force mea- surement system. As a result, the needs of

this historically important area appear to

be adequately served and will probably re- main so in the future.

As noted several times already, the weaknesses in the measurement system, and

therefore, in the NBS program are in the areas of standards and procedures for the

measurement of dynamic force and very small

forces, in preparation for metrication, and

improvement of conriunication between the

various segments of the force measurement

system. The plans in these areas will be

discussed next, but it is important to note

here that NBS, because of its historic

leadership in the measurement field, must

take the lead again in these areas.

4.5 The Future

The anticipated needs of the measurement

system have been discussed at length in earlier sections of this report, and reasons

for NBS involvement have been presented.

These needs include: maintenance of existing

static force standards along historic lines,

establishment of dynamic force standards,

basic research into techniques for measurement of very small forces, and improved

communication and uniformity throughout the

measurement system.

Because of its size and importance, the

program for establishment of dynamic force

standards has been subdivided into four parts

and a standards and test method program has

been outlined in each of these areas. The

program has been divided as follows:

- closed frame testing such as in measuring the fatigue properties of

material s

.

- motion weighing such as on railroad

track scales.

- forces in flexible couplings such as cables

.

- open testing such as impact in automobile bumper testing.

Recently, the program for studying motion

weighing was presented to a meeting of the

National Scale Men's Association. The

economic benefits of such a set of standards

and test methods were immediately appreciated by the group and their cooperation was enlisted for the program. Similar economic

benefits are expected from other phases of

the program. In fatigue testing, for

example, lower costs of testing, improved

measurements and better material utilization

are among the anticipated benefits.

Since the formal assessment of the trends

in the National Force Measurement System

began, the potential benefits of a switch

in calibration procedures was appreciated

and the change to E74 was made. In addition

a workshop has been held and a round-robin

calibration of transducers has been initiated

to promote the use of E74. Current response

to the workshop and round-robin has been

favorable and some thought is being given to the development of a training workshop for

calibration technicians. Several laboratories

have indicated that this would be a useful

education tool. Even at this early stage

of the round-robin calibration program, significant differences in indicated load v/ere

observed from at least two participants when

the calibration data were compared with data

taken at NBS both before and after the devices were sent to their laboratories. It

is expected that several such discrepancies

in measurement will be observed during this

set of round-robin tests. In each case, efforts to determine the reason for them will

be made. In the future, the program may be

expanded in depth, to include the secondary

and tertiary levels of calibration.

31
5. SUMMARY AND CONCLUSIONS

The static force measurement system is

stable in size. The accuracies presently

available appear to be adequate for the foreseeable future. There does not appear to be

any requirement to improve the accuracy of

static force standards by the factor of two

possible by correcting for the actual air

dens i ty

.

The measurement system is only beginning

to include the positive feedback of information from the users of the measurement to the suppliers. A round-robin calibration of

force standards is underway among about 30

government and corporate metrology laboratories. This will encourage the communication

needed in the system and will allow an evaluation of the magnitude of the random and

systematic errors which exist in transferring

force standards to field measurements. This

study will continue into 1977.

Through its basically contractual nature,

the force measurement system impacts a major

fraction of the technological, economic and

social sectors of our society. An analysis,

via the SIC codes of the users of calibration services, shows the greatest impact in

the manufacturing and transportation industries. Secondary level impact can be shown

in textiles and chemicals. Examples of this

impact have been cited.

The measurement system for dynamic force

does not have the standards and methods that are needed to assure the accuracy of measurements and the safety, reliability, and efficiency of load carrying systems. Uncertainties introduced by the rapid application

of forces have large economic and safety

ramifications where they affect such diverse

things as eyeglass lenses, seat belt

restraint systems, crane overload indicators

and in-transit weighing systems. A few of

these problems are under study but more

research on the overall effects is needed.

Torque has been identified as an element of

the force measurement system which is probably inadequately served. Traceability of

the measurements back to the basic standards

of force and length is tenuous at best. In some cases the path is not presently visible.

The measurement of very small static and

dynamic forces has also been identified as an area in which research is needed. The

technologies which require this measurement

capability are growing rapidly. The greatest

future impact might be felt in biomedical

research and in nondestructive testing of

manufactured parts.

The National Bureau of Standards, through

its calibration of force standards, has a significant impact on a substantial fraction

of the force measurement system. Through

the combination of its calibration and re- search programs, NBS's impact is widespread and deep. Almost every material used

in the manufacturing, construction, and

transportation sectors of the economy is influenced by research programs or standardization activities at NBS.

This study has served to verify many of the

current concepts of the condition of the

measurement system. Several strengths and

weaknesses have been pointed out. The re- search program at NBS will continue to be

influenced by the effects of the study. It is expected that the Measurement System

Study will be a continuing activity, contributed to be the NBS staff as it is continuously aware of the relationship of NBS to the National Force Measurement System.

32
APPENDIX A. METHODOLOGY OF THE STUDY

The study of a complex structure such as the National Force Measurement System revolves around defining the major structural

elements, detecting the paths of interaction

among them and filling out the detailed

information about interaction rates and

magnitude of impact.

For the force study, the major elements of

the structure were generally well known

through our long history in the field of

physical measurements. The first step was

to sort the NBS calibration service and

research contact clientele into these major

groups. A sample of each group in the

structure was contacted by telephone, a visit to their location or personal discussion at NBS. The sampling was continued

in each group until the interaction of that

structural element with other structural

elements could be defined, and a picture

of its measurement needs and impacts on society could be solidified.

At the same time, non-client members of

each group were identified from several sources ranging from the literature file to formal introduction by other contacts. The

basic interaction impacts and needs of these

contacts with the system were compared with

those of NBS clients, and the differences,

if any, were investigated for cause and

effect. A list of many of the contacts

made during this study is given in Table Al

.

Economic data were gathered from open

literature or public information sources

such as the Census Bureau and several trade

associations

.

33
Table Al - Listing of Organizations Contacted I

Airlines & Aircraft Companies

Allegheny Airlines Inc. Boeing Corp.

Butler Aviation

Dulles International Air Freight Corp.

McDonnell - Douglas Aircraft Corp.

Pan American Airlines - Air Cargo

Load Master

Operation Engineering

Weight & Ledger Control

Instrumentation

TWA

Calibration Services

American Calibration and Testing Inc. Labquip

RW Hunt Co. Service Physical Testers

5 Star Calibration Service

Twin Cities Testing and Engineering Lab

United Calibration Corp.

Government

Census Bureau

Consumer Product Safety Commission

Dept. of Agriculture - Forest Products Lab

Dept. of Labor - Bureau of Labor Statistics

Interstate Commerce Cormiission

Maryland State Highway Dept., Soils Lab

National Bureau of Standards - Cement and Concrete Reference Lab

Office of Weights and Measures

U.S. Army - Corps of Engineers

U.S. Army - Metrology & Calibration Center

U.S. Army - Picatiny Arsenal

USAF - Aerospace Guidance & Metrology Center

USAF - Ogden AFB

USN - Eastern Standards Lab

USN - Mare Island Naval Ship Yard

USN - Metrology Engineering Center

USN - Western Standards Lab

Industrial Users

ABL, Cumberland, Maryland

ASEA Electric Inc. Brunswick Corp. '

.

Burndy Corp.

Ford Motor Company

General Dynamics (Pomona)

Goodyear Tire and Rubber Company

Hamilton Std. Div., United Aircraft Corp.

Martin Marietta Corp., Denver, Orlando

Rocketdyne Corp.

Sandia Laboratories

Schwinn Bicycle Co. Tektronics Corp.

Westinghouse Corp., Research Lab, Steam Div.

iring National Force Measurement System Study

Metals Industry

Alcoa Research Laboratories

Armco Steel Co. - Research Labs

Metrology Lab

Bethlehem Steel Co. Kaiser Aluminum Co. Lukins Steel Co. Reynolds Metals Company

U. S. Steel Research Laboratories

Testing Labs

ATI, Advance Technology, Inc. Central Testing Labs

Franklin Institute Research Laboratories

Lawrence Calibration Lab

National Standards Testing Lab

Pittsburg Testing Labs

Time National Laboratories

U. S. Steel Research Labs

U. S. Testing Co.

Testing Machine Manufacturers

Ametec

GCA Precision Scientific Co. Gilmore Industries

Instron Corp.

MTS Systems Corporation

Satec Systems

Testing Machine, Inc. Thwing Albert Instrument Co. Tinius Olsen Testing Machine Co.

W. C. Dillon Co.

Torsion Industry

Camis Corporation

S. Himmel stein and Company

Utica Tool Co. , Inc.

Trade Associations

American Council of Independent Labs

National Concrete Masonry Association

National Association of Home Builders

National Scale Manufacturers Association

National Scale Men's Association

Transducer MFR

AH Emery Co. BLH Electronics

Daytronics, Inc. Interface, Inc. Lebow Associates, Inc. Martin Decker Co. Morehouse Instrument Co. Revere Corp. of America

Soiltest, Inc. Toroid Corp.

Transducers, Inc.

Universities

Catholic University of America

Virginia Polytechnic and State University

34
REFERENCES

1. Petrenko, S. N., Elastic Ring for Verification of Brinell Hardness Testing

Machines, Trans. Am. Soc. Steel Treating,

IX, 420 (1926).

2. Standard Methods of Calibration of

Force-Measuring Instruments for Verifying the Load Indication of Testing

Machines, ASTM Designation E74-74,

American Society for Testing and Materials, Philadelphia, Pa.

3. Tate, D. R. , Absolute Value of g at the

National Bureau of Standards, J. Res.,

NBS 70C2-225, p. 149 (April-June, 1966).

4. Selected Instruments and Related Products, Current Industrial Reports Series:

MA38B - (71) - 1, U. S. Department of

Commerce (1973).

5. 1967 Census of Business, Volume 5,

Selected Services Area Statistics, Part

1 U.S. Summary, U. S. Department of

Commerce (1971).

6. 1967 Census of Business, Selected

Services, Miscellaneous Subjects, U. S. Department of Commerce (1971).

7. U. S. Industrial Outlook 1974, U. S. Department of Commerce (Oct. 1973)

8. Statement #100, Financial and Operating

Statistics, Class 1 Railroads, Interstate Commerce Commission (1972).

9. USA, EUROPE, AND EYE SAFETY, The

Optician, p. 30, (July 23, 1971).

10. U. S. Metric Study Interim Report, The

Manufacturing Industry, National Bureau

of Standards Special Publication 345-4

(July, 1974).

11. Whittemore, H. L. and Petrenko, S. N. Proving Ring, U. S. Patent 1,648,375

(1926).

12. Tate, D. R., Application of Resistance

Wire Strain Gages to High Capacity Load

Calibrating Devices, NBS Circular No. 528, p. 121 (1951).

13. Thurston, A. L. and Cushman, R. W.,

Precision Determination of Weights by

Means of Bonded Strain Gages, Proc. Soc. of Experimental Stress Anal., Vol III,

No. 1, p. 62 (1945).

14. Mitchell, R. A., Woolley, R. M., and

Fisher, C. R., Formulation and Experimental Verification of an Axisymmetric

Finite-Element Structural Analysis,

J. Res., Vol 75C, Nos . 3 and 4 (1971).

15. Edelman, S., Piezoelectric Polymer

Stress Gages, presented at tne symposium on Advancements in Instrumentation for Civil Engineering

Applications, Air Force Weapons Laboratory, Kirtland AFB, New Mexico,

(May 1973).

35
NBS-IMA (REV. 7-73)

U.S. DEPT. OF COMM. BIBLIOGRAPHIC DATA

SHEET

1. I'UIU.K A VION OR REPORT NO. NBSIR 75-9292. Gov't Accession

No.

3. Recipient's Accession No.

4. TITl.K AND SUBTITl.i:

A STUDY OF THE NATIONAL FORCE MEASUREMENT SYSTEM

5. F^ublication Date June 1975

6. Performing Organization Code

7 AIITHOR(S>

Donald E. Marlowe

8. Performing Organ. Report No.

9. PERFORMING ORGANIZATION NAME AND ADDRESS

NATIONAL BUREAU OF STANDARDS

DEPARTMENT OF COMMERCE

WASHINGTON, D.C. 20234

10. Project/Taslc/Work Unit No. 213010A

11. Contract/Grant No.

12. Sponsoring Organization Name and Complete Address (Street, City, State, ZIP)

Same as above

13. Type of Report & Period Covered

14. Sponsoring Agency C^ode

15. SUPPLEMENTARY NO] ES

16. Af^STRACT (A 200-word or less (actual summary ot most significant information, tf document includes a siffnificant bibliography or literature survey, mention it here.) A Study of the National Force Measurement System has been conducted. The overall

structure of the system has been defined, and the size of several of its component

elements has been measured. The interactions of many of these components within the

system have been assessed. The position which NBS has and the role it plays in the

system are better understood as a result of this study.

The best assessment of the condition of the National Force Measurement System is

that areas such as static force measurement are fully adequate with the levels of

accuracy now provided while other areas such as dynamic force measurements and information transfer, are very inadequate to serve even our present needs.

17. KEY WORDS (six to twelve entries; alphabetical order, capitalize only the first letter of the first key word unless a proper name; separated by semicolons

)

Dynamic force; economics; force; national measurement system; metrication; standards

18. AVAIEAHILITY Unlimited

For Official Distribution. Do Not Release to N TIS

[^3 Order From Sup. of Doc, U.S. Government Printing Office Washington, D.C. 20402, SD Cat. No. C13

Order From National Technical Information Service (NTI.S) Springfield, Virginia 22151

19. SECURITY CLASS

(THIS REPORT)

UNCL ASSIFIED

20. SE( URITY CLASS

(THIS PAGE)

UNf:LASSIFIED

21. NO. OF PAGES

40

22. Price

$4.00

USCOMM-DC 29042. P74


APRIL2013

I

A study of the National Force Measurement System

Marlowe, Donald E. QC100.U56 no.75-929 1975

NIST Research Library

[26] studyofnationalf7592marl

nbsir75-929

Jul 16, 2015