NI8T

„ PUBLfCATlONS REFERENCE

A11107 DSflD3fl

NBSIR 75-931

The National Measurement

System for Pressure

Peter L.M. Heydemann

Institute for Basic Standards

National Bureau of Standards

Washington, D.C. 20234

September 1976

>. DEPARTMENT OF COMMERCE

10NAL BUREAU OF STANDARDS

NBSIR 75-931

THE NATIONAL MEASUREMENT

SYSTEM FOR PRESSURE

Peter LM. Heydemann

Institute for Basic Standards

National Bureau of Standards

Washington, D.C. 20234

September 1976

U.S. DEPARTMENT OF COMMERCE, Elliot L. Richardson, Secretary

Edward O. Vetter, Under Secretary

Dr. Betsy Ancker-Johnson, Assistant Secretary for Science and Technology

NATIONAL BUREAU OF STANDARDS. Ernest Annbler. Acting Director

DIVISION SUMMARY REPORT

A Study of

THE NATIONAL MEASUREMENT SYSTEM FOR PRESSURE

Peter L.M. Heydemann

Heat Division

EXECUTIVE SUMMARY

The field of this study is the measurement of pressure and vacuum. This field

has developed from humble beginnings in the

laboratory of Evangelista Torricelli in 1644

to one that encompasses practically all manu- facturing processes, the operation of many

types of machines, important scientific en- deavors and our daily life. Industrial

applications cover 17 decades of pressure

and science adds another five. Within its

range are billion-dollar industries using

pressure or vacuum in a significant way to make or control semiconductors, radiators,

beer cans, aircraft, automobile tires, missiles, freeze dried food, power plants and

many other products or manufacturing activities. Savings of great economic significance can be achieved through accurate mea- surements of pressure. The safety of aircraft and of nuclear reactors is ensured

through, among others, pressure measurements.

A significant part of the measurements in

this system are referred to NBS primary

standards mainly through the instrumentation

industry. NBS calibration services now cover about five decades of pressure and an expansion to seven decades is planned for

the near future. The calibration services

are based on a number of primary standards,

whose accuracy and precision are adequate

for most purposes and work continues to update and improve these standards. The

dissemination of accurate measurements into

the field is not always satisfactory. Large

uncertainties added in the calibration

chains are due to inadequate technical and

mathematical procedures, lack of good transfer standards and lack of training of the

personnel involved. The calibration services

are backed up by training of industrial

personnel, preparation of technical manuals

and reports, data evaluation service, laboratory evaluation service and extensive

consultations. All of these back-up ser- vices were prompted by early results from

this investigation.

The field of pressure measurements is currently undergoing rapid changes through

the introduction of new types of measuring

instruments, through an expansion of the

range of pressures used in industry and

through the requirement for lower uncertainties. There is close cooperation between

NBS and the professional and engineering

societies on the development of the necessary measurement standards and test codes.

Increased awareness of possible safety

problems with gages and transducers, and

particularly with high pressure vessels was prompted by the Occupational Safety and

Health Act. Here again NBS is cooperating

with the concerned parties to provide the

necessary engineering standards. All of

these activities tend to lead to an infrastructure that is more strongly focused on the primary standards of pressure at NBS and

the related dissemination services.

A critical need, involving the safety of

people, is felt in the pressure range re- quired for the calibration of aircraft altimeters. No central standards are presently

available to ensure uniform measurements

throughout this country and the rest of the

world in this important range. A new standard is, however, under construction. Also,

a measurement assurance program is under

study to bring this area under control.

Nuclear reactors require numerous pressure

transducers and at least those related to safety should be referenced to central,

national standards to ensure that necessary

regulations can be enforced.

A Delphi technology forecase was undertaken to determine the need for vacuum measurements, which are not provided by NBS. This study is not complete, but is has provided valuable information for planning and

management decision making. Of approximately 125 randomly selected members of the

American Vacuum Society 68% stated that they

require vacuum calibrations in the range lO"^

to 1 torr with an uncertainty of 0.1%, or better, now; 42% now require vacuum measure- ments to better than 1% in the range 10"^ to 10"^ torr. A much larger percentage of ex- perts in various fields of vacuum measurements foresee these needs for the immediate

future. The most demanding requirements for vacuum measurements were found in the manu- facture of semiconductors, in the space industry, and in the vacuum instrumentation

industry for the calibration of standard

leaks and the measurement of pumping speeds.

BB -14

CONTENTS

EXECUTIVE SUMMARY

1 . INTRODUCTION

2. STRUCTURE OF THE MEASUREMENT SYSTEM

2.1 Conceptual System

2.2 Basic Technical Infrastructure

2.2.1 Documentary Specifications System

2.2.1.1 Standardization Institutions

2.2.1.2 Survey of Documentary Standards

2.2.2 Instrumentation System

2.2.2.1 Measurement Tools and Techniques

2.2.2.2 The Instrumentation Industry

2.2.3 Reference Data

2.2.4 Reference Material

2.2.5 Science and People

2.3 Realized Measurement Capabilities

2.4 Dissemination and Enforcement Network

2.4.1 Central Standards Authorities

2.4.2 State and Local Offices of Weights and Measures

2.4.3 Standards and Testing Laboratories and Services

2.4.4 Regulatory Agencies

2.5 Organizational Input-Output Transactions Matrix

2.5.1 Analysis of Suppliers and Users

2.5.2 Highlights re Major Users

3. IMPACT, STATUS AND TRENDS OF MEASUREMENT SYSTEM

3.1 Impact of Measurements

3.1.1 Functional, Technological and Scientific Applications

3.1.2 Economic Impacts -- Costs and Benefits

3.1.3 Social, Human, Man-on-the-Street Impacts

3.2 Status and Trends of the System

4. SURVEY OF NBS SERVICES

4.1 The Past

4.2 The Present -- Scope of NBS Services

4.2.1 Description of NBS Services

4.2.2 Users of NBS Services

4.2.3 Alternate Sources

4.2.4 Funding Sources for NBS Services

4.2.5 Mechanism for Supplying Services

4.3 Impact of NBS Services

4.3.1 Economic Impact of Major User Classes

4.3.2 Technological Impact of Services

4.3.3 Pay-Off from Changes in NBS Services

4.4 Evaluation of NBS Program

4.5 The Future

5. SUMMARY AND CONCLUSIONS

Appendix A. Methodology of the Study

Appendix B. Summary of Background Documents

References
LIST OF FIGURES

Figure 1. Piston gage, schematic

Figure 2. Controned Clearance piston gage, schematic

Figure 3. Infrared Laser Interferometer Manometer, schematic

Figure 4. Calibration Chain

Figure 5. NBS Calibration

Figure 6. Variation of Tire Gage Calibrations

Figure 7. Input/Output by Type

Figure 8. Input/Output by Group

Figure 9. Delphi Questionnaire

Figure 10. Instructions for Delphi Questionnaire

Figure 11. Delphi Evaluation

LIST OF TABLES

Table 1. Conversion Factors for Pressure Units

Table 2. Standards for Altitude and Pressure Measurements on Aircraft

Table 3. Tire Gage Calibration

Table 4. Density of Manometric Fluids

Table 5. Fixed Points on the Pressure Scale

Table 6. Uncertainty of Primary Pressure Standards at NBS

Table 7. Required Accuracy for Vacuum Measurements

Table 8. Commercial Calibration Laboratories

Table 9. Organizational Input-Output Transaction Matrix

Table 10. Estimated Cost of Operating a Small Secondary Pressure Standards Laboratory

Table 11. Committee Memberships

Table 12. Listing of Users of NBS Pressure and Vacuum Calibration Services

Table 13. Annual Production Volume of Some Industries Benefiting from NBS Services

Table 14. Persons Contacted During the National Measurement System Analysis
1. INTRODUCTION

Few types of measurements pervade our daily life as much as the measurement of

pressure. Many items in daily use depend

on it: automobile engines, tires, refrigerators, furnaces and airplanes. Many industrial processes require pressure measurement and control: semiconductors, vacuum

melting of steel, transportation through

pipe lines, powder metallurgy and freeze

drying of foods. And yet the field of pressure measurement is not well structured.

There is neither a thermodynamic nor a practical pressure scale as is the case in the

temperature measurement system; there is no agreement on transfer devices. Pressure mea- surements are made and used in science and

industry over the range from about 10"^° to 10^2 Pa, that is 22 decades. No one method

of pressure measurement covers more than a few decades. The problems at 10^^ pg not

have anything in common with those encountered

at 10"^°Pa. Instead of one or two different

units, as in length or temperature measurements,. there are half a dozen or more. In many areas, as, for example, tn manometry, vacuum and very high pressure measurements, the field is still in its infancy. Few texts deal with the subject. There are few,

if any, professional organizations devoted

entirely to this field. This lack of structure in a field that covers so very many

applications makes a comprehensive investi

gation a task that is beyond the expertise of

a single individual . Nevertheless we shall

give here an overview of the field to convey

the flavor of the problems inherent in the

accurate measurement of pressure and its

dissemination.

It was the purpose of this study to learn

as much as possible about the national mea- surement system of pressure, to identify

deficiencies and shortcomings that could

then be eliminated, and to provide sufficient

information for long range planning.

Personal contact through interviews, visits,

letters and telephone conversations was the

main source of information. Due to the lack

of structure in the pressure measurement

system, committees and other organizations

proved to be of very limited use as sources of information. The field of vacuum measurements was treated differently. No vacuum calibration services are presently provided by

'IBS and consequently there are few personal

contacts. Therefore, a Delphi study was begun to forecast the requirements of the

measurement system below about 10 kPa.

The study proved to be an extremely valuable exercise. It has helped us to pinpoint

shortcomings. It has changed the attitude

of the staff and it has fostered manv close

and continuous contacts with industry. This

has increased the calibration workload to the

advantage of our customers.

2. STRUCTURE OF THE MEASUREMENT SYSTEM

2.1 Conceptual System

History :

Pressure, both hydraulic and pneumatic,

was used extensively in antiquity, but we

have no indication that measurements of

pressure were ever attempted until the early

17th century. At that time Galileo and his

contemporaries struggled with the concept of

the horror vaoui in order to explain observations made in their early gas-thermometers.

Evangel ista Torricelli, disciple of Galileo,

was the first to report an actual measurement of barometric pressure. The details of

his experiments are contained in a letter to

his friend Ricci. The news of the experiment spread rapidly and it was repeated and

refined by many of the famous thermodynamicists of the time: Descartes, Boyle, Pascal, Guericke, Watt and so forth. Until

the introduction of the high pressure steam

engine by Richard Trevi thick about 1800, the

barometer served mainly as a surveyor's

tool for the purpose of leveling. Watt and

a few others used it for their measurements

of the properties of steam. But the advent

of the high pressure steam engine created a "national need." Within the next 50 years

the manometer was perfected, the piston

gage was developed, a pressure gage was invented by Mr. Schinz and then patented

by Mr. Bourdon. In the next one-hundred

years until about the middle of this century

these instruments were refined, the aneroid

barometer was invented and the foundations

for the transducer industry were laid. Today we find ourselves in the midst of a rapid

development of essentially electronic devices

for the measurement of pressure prompted by

the advent of automatic data acquisition and

process control. At the same time, the range

of pressures used in an industrial environment is broadening and the number of measurements is increasing.

Definitions : Over most of the pressure range used in

scientific and industrial applications pressure is homogeneous in space, hydrostatic,

and independent of time. It can therefore

be defined as force per unit area
or as energy density

where F is the Helmholtz free energy with

F = U - TS dF = pdV - SdT.

At very low pressures, that is in the

medium or high vacuum area, it would be more appropriate to use moleculeir concentration for

tne characterization of a space. But it is nevertheless customary to indicate pressures

even though one rarely, if ever, measures it

as such.

At high pressures, or when viscous liquids

or even solids are used to transmit pressure,

one has to be more specific in the definition of hydrostatic pressure. This has re- cently been pointed out by Decker et al . in

their excellent review of high pressure calibrations, in J.Phys.Chem.Ref .Data 1,773(1972).

In order to specify precisely the terminology and describe the non-ideal features

of an arbitrarily stressed system in a particular non-ideal laboratory situation, we introduce the symmetric stress tensor, T,

which is defined at each point in space.

Measurable stress over finite areas can then

be calculated by averaging. We note that

each component of the stress tensor, in

general, depends upon position and time,

Tjj(r,t) where r is the position vector.

The pressure, which is also a function of

position and time, is defined as the negative of the average of the three normal

stress components

P(r,t) = - i (Til + T22 + T33).

The shear stresses are given by the deviatoric stress tensor

T'ij(r,t) = Tij(r,t) - 6ijP(r,t).

We are now in a position to define unambiguously the meaning of hydrostatic pressure,

which is characterized by zero deviatoric

shear stress and isotropic normal stresses.

Mathematically we write: T'jj(r,t) = 0 and

-P(r,t) = Tii(r,t) = T22(r,t) = T33(r,t).

We note that this definition does not re- quire static (i.e., time-independent) conditions although time-dependent changes generally involve shear. The equations define

hydrostaticity at a point. If a region of

space is to be hydrostatic, each point in

the volume must satisfy the hydrostatic

condition, which does not necessarily imply

homogeneous (i.e., constant in space) conditions. According to the above definition of

pressure, nonhomogeneity in a hydrostatic

medium at equilibrium can arise only from

volume (body) type forces such as gravitational, magnetic, or electric forces, which

in practice are often very small compared

to the applied forces. It is important to note that neither homogeneous pressure nor homogeneous stress implies hydrostaticity,

but simply constancy in space.

In laboratory pressure systems the time

dependence in T-jj generally arises from a change of the system from one equilibrium

state to another. In a practical manner we define equilibrium as the state, which a system approaches asymptotically within a laboratory time scale. All systems under pressure will support time-dependent shear

stress components with some characteristic

relaxation time, t, when momentarily disturbed from the equilibrium state. In many

cases, especially with gases and liquids at low pressure, x may be a small fraction of

a second and is usually (but not always)

negligible compared with measuring times;

while in highly viscous liquids and solids

the relaxation times may be of the order of

many hours or even years. In the formalism

discussed one can clearly distinguish between plastic solid and viscous liquid behavior during this stress relaxation. For the liquid case, the deviatoric stress T'-jj

will in time approach a zero value, but for

solids T'ij will approach some non-zero

final deviatoric stress state. The approach to equilibrium will be highly dependent upon the details of the system.

In solid-media systems , stress relaxation will

be very complicated. For systems containing

only fluids in which the viscosity of the

fluid can be assumed constant thoughout the

system, the approach to equilibrium is

characterized by the stress components decreasing with time approximately exponentially. Spatial pressure differences within

the system will obey the expression aP

e~'t/T where the characteristic time, t, is

directly proportional to the viscosity of

the liquid but highly dependent upon the

geometry of the chamber. One of the important consequences associated with this discussion is the fact that time-dependent

shear stresses are generally associated

with pressure changes, and adequate time

(several times x) must be allowed for the

system to approach equilibrium before re- liable measurements can be made.

To measure pressure experimentally one must measure the normal stress over a finite area. If P is homogeneous over that

area, as it very nearly is in a fluid

chamber, the simple force per unit area relationship is valid, and pressures can be

determined to high accuracy. (See the
section on the primary pressure scale.)

Pressures determined by force per unit area

in solid-media systems measure some average

stress over the specified area which may

differ from the true average normal stresses

on the surface as well as from pressures at points inside the bulk of the chamber. In general, such errors will be of the order of

the shear strength of the solid materials.

The errors associated with measurements of

material properties under these non-hydrostatic stress conditions may be more serious

than the associated error in pressure since

the property measurement errors are dependent upon the stress sensitivity of the

parameter being considered.

Pressure Units :

Since pressure is defined as force per

unit area or energy density, the units of

pressure should be rather simple to list.

However, on closer inspection one realizes

that there are many units in which to ex- press force and area and that consequently

there is an even larger number of combinations possible. A further complication is

added by the fact that people using manometers tend to measure pressure by the height

of the fluid column rather than in terms of

the force generated by the weight of the column over its area at the reference level.

This adds a list of bastard units, the use of

which has caused innumerable difficulties

with less-well -trained or thoughtless personnel. These bastard units should be

stamped out. This is, of course, more easily

said than done, since manometry is a field

full of old traditions, having developed

since 1644 or about 200 years earlier than

the piston gage.

The official unit of pressure in the Systeme Internationale (S.I.) is the Newton/

square meter or N/m^. This unit also carries

the name Pascal (Pa) after the well known

French scientist, who sent his brother-in-law

up the mountain with a mercury barometer to measure elevation.

Some of the more frequently encountered

pressure units are listed in Table 1. In this text we shall use the unit (Pa) and we

shall frequently indicate in brackets the same pressure in more familiar units. These

units will vary with the range, torr being

preferred for vacuum measurements, psi for

low and medium pressures, kbar for high

pressures

Table 1. Conversion Factors

to Pascals for various pressure units

To convert from multiply by*

atmosphere(normal=760 torr) 1 .013 250 E+05

atmosphere ( tec hni cal =1 kgf/ cm^ )9.806 650 E+04

ba r 1 .000 000 E+05

cen 1 1 rneter ot rnercury ^ u l y 1 .333 22 E+03

Lcil L- 1 lllc Lcr UI waLcr \'t j 9 .806 38 E+01

Hop 1 V* 1 .000 000 E+04

uy rlc/ Cell L 1 MIc Lcr 1 .000 000 E-01 Tuu u UT wd ter \jy,c r j 2 .988 98 E+03

gram ^ Torce j / cenn mexer 9 .806 650 E+01

iiiLii UT iiicrcury v*^^ 'J 3 .386 389 E+03 mcn OT mercury ^ou 3,.376 85 E+03

inch OT water r) 2,.490 82 E+02

incn Or waxer ^du rj 2,.488 4 E+02

kgf/centimeter^ 9,.806 650 E+04

kgf/meter^ 9,,806 650 E+00

kgf/mi 1 1 imeter^ 9,.806 650 E+06

K 1 p/ \ ncn ^ KS1

y

6,.894 757 E+06

mi 1 1 i bar 1

,

.000 000 E+02

mi 1 11 mexer ot mercury 1

,

.333 224 E+02

newton/meter^ 1

,

.000 000 E+00

poundal per square foot 1 ,.488 164 E+00

pound-force/foot^ 4,.788 026 E+01

pound-force/inch2(psi

)

6,.894 757 E+03

torr(mm Hg 0° ) 1

,

.333 22 E+02

*The notation E+n means multiplication by

ion
Pressure Scale :

In any branch of metrology, the establishment and universal acceptance of a primary

scale upon which all interpolation and extrapolation functions and devices can be based

and to which they can be referred is of fundamental importance. In general, the establishment of such a scale will involve specific procedures, apparatus, and precautions

associated with the measurement in question

and will be as closely related to the fundamental definition of the measured quantity

as apparatus will permit.

The techniques and apparatus associated

with the primary scale should be as simple

as possible and the process should be a direct measurement of the quantity itself.

It is preferable not to involve averaging,

differentiation, integration, or other mathematical manipulations implied by a theoretical treatment. It is not expected that

specific procedures and details will be

permanent but, rather, that they will be

temporarily accepted by the scientific

community until a more direct and reliable

or more accurate technique can be demonstrated. The fundamental nature, the

reliability, and the accuracy will be of

prime importance in contrast to sensitivity,

convenience, and availability. It would be

desirable to have but one technique or apparatus extend over all ranges of the

measured quantity, but such a condition is

generally not possible.

Based on criteria of this type, two basic

measuring systems with a variety of modifications have been proposed and

used rather widely as a basis of a primary

pressure scale: (a) the mercury manometer

(including multiple and differential manometers), and (b) the free-piston or deadweight pressure gage (also called a piston

manometer, pressure balance, or unpackedpiston gage). In a practical sense the use

of the mercury manometer, has been limited to pressures between 0.1 Pa and 200 kpa.

One very elaborate system built by Bett,

Hayes, and Newitt (1954) was designed for

use to 230MPa although no measurements a- bove 70 MPa have been reported by these re- searchers. The free-piston gage is in common use to over 1 GPa (10 kbar) and has

been successfully used to 2.6 GPa by Johnson

and Heydemann (1967) and to 2.5 GPa by

Konyaev (1961) but with serious difficulty.

Since pressure comparisons require uniformity of pressure throughout the system or combination of systems, a primary pressure

scale must be based on a truly hydrostatic

system. Both of the systems proposed above

are so based. At higher pressure (above 5 GPa), this requirement of hydrostaticity

represents a rather severe ultimate limitation on a primary pressure scale as discussed herein. Other scales applicable to higher pressures have been proposed as discussed in other sections of this report,

but they cannot be considered to fit the

above criteria of a primary scale and cannot

be currently considered as such. At pressures above 2.5 GPa, several apparatus of the piston-cylinder type have been

built and pressure values have been reported.

In these systems, various techniques have

been used to approximate the conditions of

the free-piston gage, but to date such

approximations have diverged rather drastically from the criteria outlined above. It appears obvious that the best current approximations to a primary scale above 2.5

GPa are the piston-cylinder systems. However, significant improvement must be made

in this technique before its reliability

will be greater than indirect extrapolation

techniques directly tied to the primary

scale at lower pressures.

In the U.S. the pressure scale is primarily based on direct pressure measurements with controlled clearance piston gages

as described in the next section and, to some extent, mercury filled manometers.

Great Britain, Canada and the USSR base their

pressure scales also upon piston gages but

employ theories and evaluation methods different from those used in the U.S.; Germany

and Italy are presently setting up standards

of pressure using the controlled clearance

piston gage principle.

A large variety of transfer standards and

interpolation gages are in use. Two problems are obvious: There are no practical

fixed points on the pressure scale below

about 700 MPa and none of the many types of

interpolation gages is as adequately

characterized as the platinum resistance

thermometer serving as the interpolation

gage for a large region of the temperature

scale. Both of these facts have led to the

initiation of research and development work

aimed at improving this situation.
2.2 Basic Technical Infrastructure

2.2.1 Documentary Specification System

2.2.1.1 Standardization Institutions

It is not surprising that the very widespread use of pressure had led to a proliferation of organizations writing often

conflicting standards for pressure gages,

transducers and measurement procedures.

There is no formal international agreement

on a pressure scale nor is there any one organization clearly to succeed.

International organizations with a stake

in pressure standards are

Bureau International des Poids et Mesures (BIPM),

Organization Internationale de

Metrologie Legale (OIML),

International Union for Pure and Applied Chemistry (lUPAC),

International Union for Pure and

Applied Physics (lUPAP),

International Association of High

Pressure Researchers (AIRAPT).

Of these organizations only lUPAC has made a serious attempt to prepare an international

pressure scale. OIML has written two manufacturing standards for pressure gages.

Legal and regulatory standardizing organizations within the U.S. are

Federal Aviation Administration (FAA),

Energy Resources Development

Administration (ERDA).

Voluntary standards for pressure measurements and gages are written by a large number

of organizations. The more important ones

are:

American Society of Mechanical

Engineers (several committees) (ASME)

American Society for Testing and

Materials (ASTM)

Society of Automotive Engineers (SAE),

Factory Mutual System (FME)

Compressed Gas Association (CGA),

Instrument Society of America (ISA),

Scientific Apparatus Makers Association

(SAMA)

American National Standards

Institute (ANSI),

American Vacuum Society (AVS),

Air Transport Association (ATA),

American Welding Society (AVS).

The military specifications written by

the Department of Defense for the purpose

of procurement have had great influence on practices in the dissemination of pressure

measurements, quality control and design.

These "Mil specs" occupy a place intermediate

between the voluntary and legal standards.

ANSI through its associated engineering

societies has probably the most significant

influence on the system. Working on voluntary standards for pressure measurements are the B40 Committee for pressure Gages, the

High Pressure Research Committee, the High

Pressure Technology Committee of the Pressure Vessel and Piping Division, the Power

Test Code Comnittee (all ASME), an ISA Committee on Manometry, cormiittees of the

Compressed Gas Association, the American

Can Manufacturers, the SAE, the ATA and many

others. Traditionally these committees were concerned with writing standards, often

dictated by safety considerations, for gages

and components. The pattern is now beginning to change and more attention is being

paid to the problems of disseminating ac- curate measurements, assessing errors and

providing uniform measurements throughout

the system. NBS is actively participating

in this work.

2.2.1.2 Survey of Documentary Standards

The large number of standards that influ- ence the field of pressure and vacuum measurements might be divided into three groups:

a. ) Standards for measuring devi ces . These

may either be performance standards or they may be construction standards. Most

American standards are of the first kind,

while several standards recently proposed

by OIML are construction standards. This

duality poses a problem for export of

American-made devices.

b. ) Standards for procedures. These

standards describe the type of instrumentation and the methods to be used to achieve a certain calibration, as for

example, of an altimeter. Also in this

category are standards or recommendations

on numerical evaluation and error calculation. Not much attention has been paid

to the latter but, through our urging,

several such standards or recommendations

are now being considered.

c. ) Safety standards. A large group of

important standards deals with safety

aspects in various ways. These may be

standards for pressure vessels, pressure

switches, components etc. They often re- quire pressure measurements and calibration only in the design or test phase.

In other cases-as, for example, on nuclear

reactors, aircraft, compressors or tanks-
continuous measurements within certain

close tolerances are required.

It would serve no purpose to list here

all standards applicable to the national

pressure measurement system. However, in

order to illustrate the variety of standards

we are including a table of standards that

must be observed in airtransportation

.

(Table 2).

Subject

Table 2 (cont.)

Organization Identification

Table 2,

Subject

Standards for Altitude and

Pressure Measurements on Aircraft

Organization Identification

Al timeter, Airborne RT

Al timeter, Pressure SAE

Actuated

Al timeter, Pressure SAE

Compensated

Altimeter, Airborne RT Radar

Altitude Digitizer SAE

Equipment

Altitude Commercial SAE

Transport

Pressure Actuated SAE

Rate of Climb Ind.

Pressure Actuated SAE

Rate of Climb Ind.

Crew Oxygen Mask SAE

Assembly

Pressure Fueling SAE

Simul. for Aircraft

Pressure Fueling SAE

Systems

Pressure Instrum., SAE

Fuel ,Oil & Hydraulic

Pressure Refueling SAE

of Aircraft

Pressure Release De- SAE

vices .Aircraft wheels

Pressure Trans, for ISA

Aerospace testing

Pressure Trans, for ISA

Aerospace testing

High Pressure Hose SAE Assem. .Aircraft

Airplane Cabin SAE

Pressurization

Recommended Practice ISA

for Manometer Tables

DO 103

392 CAS

415 AS

DO 123

855 AS

505 AIR

429 AS

394 AAS

452 AAS

816 ARP

775 ARP

408 BAS

74 AIR

707 AS

S37.6

RP 37.3

614 AARP

367 CARP

RP 2.1

Airborne Transponder Equipment

Aircraft Hydraulic

& Pneumatic Systems

RT

SAE

Aircraft Hydraulic SAE

& Pneumatic Systems

Aircraft Hydraulic SAE

& Pneumatic Systems

Aircraft Hydraulic SAE

& Pneumatic Systems

Aircraft Engine SAE

Perform. Altitude

Graphs

Remote Serviced Air SAE

Data Instruments

Maxim. Allowable SAE

Airspeed Instrum.

Pi tot or Pi tot-Static

Pressure Tubes SAE

Mach Meters SAE

DO 112

243 BARP

575 ARP

714 AS

578 ARP

1 AAS

791 AS

418 AAS

390 AS

436 AS

Other important standards, issued by the

ASME in the B40-series, deal with pressure

gages of various types. The ASME pressure

vessel code, although extending only to

20 MPa (3000 psi), is of great importance

for the design, testing and certification of

thousands of pressure vessels and its ex- tension to higher pressure is now before

the appropriate committees as urgent business. Pressure measurements are also

covered in the performance test code for

power plants (PTC 19). A complete list of

applicable standards is found in "An Index

of U.S. Voluntary Engineering Standards,

NBS Special Publication 329," and its

suppl ements

.

2.2.2 Instrumentation System

2.2.2.1 Measurement Tools and Techniques

Physical Reference Standards :

The pressure scale from about 1 kPa to 2.5 GPa is based upon direct pressure

measurements with piston gages of various

designs. Up to about 5 MPa (600 psi) gases

and above 5 MPa liquids are used as pressure

transmitting media. At pressures exceeding

2.5 GPa (25 kbar), piston and cylinder or piston and die assemblies are used to generate and measure pressure. All of these
gages have two problems in common: the

determination of the effective area and the

determination of the change of this area with pressure. Of the various types of

gages, only the controlled clearance piston

gage lends itself to a direct and primary

calibration. All other piston gages need

to be calibrated by comparison. However,

they constitute transfer standards with a precision and stability that are not matched

by many other mechanical devices.

Pressure measurements below 200 kPa (2atm)

down to 10 mPa (10"'+) torr are frequently

based on measurements with manometers. These

are considered primary standards as they can frequently be calibrated in terms of the

basic units of length, mass and time. They

are, in principle, capable of operating with

lesser uncertainties than piston gages.

Many types are commercially available but

few of them are designed to minimize the

largest single source of uncertainty:

temperature. Mercury is commonly used as manometric fluid. Vacuum pump oils are used

at very low pressures. The overlap of the

operating ranges of manometers and piston

gages from 1 to 200 kPa provides a welcome

chance for a comparison between basically

different primary pressure standards.

In the vacuum range, that is below about

10^ Pa (1 torr), a variety of standards are

used, which are based on volume expansion,

pressure division (conductance devices) or

pressure multiplication (compression manometers) .

Piston Gages :

Figure 1 (a) recalls the principle of a simple piston gage. A piston of well known

area Ap is fitted into a cylinder. Pressure

is supplied to the cylinder and the resulting force pA is counteracted by loading the

piston with sufficient weight to balance this

force. In practice, the effective area Aq

of the piston gage is not equal to the

Area An of the piston but is more nearly

equal to the arithmetic mean of the area of

the cylinder A(.y-| and that of the piston Ap

Ao = }

(Ap + Acyi ) and p

=

Both Ap and Acyi are not constants, but are functions of pressure, temperature, and load.

It is possible that the effective area is also a function of the viscosity of the

pressure transmitting medium; it may be a function of speed and direction of rotation

and there may be other influences that we are

not presently aware of. The largest single

effect on Ap is the dilation of the cylinder

due to the internal pressure p. This is

(a) (b)

Figure 1. Piston gage, schematic

taken into consideration by multiplying Aq

with(l+bp), where b is called the pressure

coefficient of the effective area. In order

to reduce the dilation effect, which becomes

so large at high pressures as to make gages

inoperable beyond a certain pressure, the

re-entrant cylinder shown in figure 1(b) was introduced. Here the pressure p actually

tends to reduce the area of the cylinder at higher pressures.

In either case it is extremely difficult

to predict the pressure coefficient on theoretical grounds, and before use of any such

piston gage its effective area and pressure

coefficient must be determined by a calibration . To circumvent the difficulty posed by the

pressure coefficient, the so-called control led -clearance piston gage was introduced by Johnson and Newhall. The principle

is shown in figure 2. Here the cylinder is

surrounded by an annular space which is filled with oil and pressurized (jacket pressure

pj) by a separate pump. The dilating effect of the internal pressure can thus be

compensated and the cylinder can be made to conform to the piston. Now the change in

effective area is determined by the small

change in area of the piston, which is a- menable to mathematical analysis (Johnson),

and by the clearance between piston and

cylinder controlled by the jacket pressure

Pj-
Figure 2.

f

P

Control led -clearance oiston

gage, schematic

The change of effective area of controlled

clearance piston gages due to piston distortion is taken into consideration by multiplying Aq with ( 1 + bp ). The pressure

coefficient b for control led-clearance

piston gages can be determined from the

elastic constants of the piston material.

Note that the pressure coefficient b for

control led-clearance piston gages may be two or more orders of magnitude smaller

than that for simple piston gages.

The effect of the jacket pressure on the

effective area can be described by the term

1 + d (p^ - Pj), where Pz is the jacket

pressure for which, at a particular internal

pressure p, the clearance between piston

and cylinder is reduced to zero, p^ is therefore a function of pressure p or load

M, d is the fractional change of area with

jacket pressure.

With these terms and a few additional

corrections the pressure generated by a control 1 ed-cl earance piston gage is described by

Mg (1 - Pai/p^) + yC

AqLI + (ac + ctp) (T-Tp)](l + (d + em + -

fM^) (p^Q + s^M + q^M^

where Mg(l - Pg-j^/P[v|) "is the force exerted

on the piston by the mass of weights M with

a correction for air buoyancy, yC is the

force generated by the surface tension y of

the pressure fluid through which the piston

emerges. C is the circumference of the

piston at that point of emergence. [1 +

(ac + otp) (T - T)-)] corrects for changes of

temperature; a^- and cip are the thermal

expansivities of the cylinder and piston

respectively. T^ is the temperature for

which Aq is known and T is the temperature

of the gage at the time of measurement. The

other terms have been discussed above and are here expanded to quadratic functions of

M since our experience shows that the

characteristics of piston gages often

require quadratic functions for their description.

Control 1 ed-cl earance piston gages are commercially available both for pneumatic

and hydraulic operation over a wide range

of pressures. A controlled clearance piston

gage capable of generating more than 2500

MPa (370,000 psi) is in operation at NBS.

Manometers :

Manometers measure pressure by balancing

the pressurfeof a column of liquid against

the unknown pressure. The force

per-unit-area generated by a liquid is

p = ghp

where g is the local acceleration due to gravity, h is the height of the column and

p is the density of the liquid. Numerous

devices have been proposed to accurately

locate the position of the meniscus of the

liquid column and to transfer this position

to a scale. The most precise manometers

use capacitive detectors for the location of

the menisus. Interferometric methods appear

most promising for the future and an interferometer manometer is under construction at

NBS. It uses beams of light from a carbon

dioxide laser (10.6 ym wavelength) to locate

the menisci and then measures their displacement in terms of the wavelength of the laser

light. Fig. 3 shows a diagram of the principal components. The phase modulator permits the extraction of directional information from the output signal.

4cm DIA. BEAM

FOLDING MIRROR

BEAM SPLITTER DETECTOR

FOCUSING LENS

Figure 3. Infrared Laser Interferometer

Manometer, schematic
There are a number of major sources of

error. The most conspicuous one is the

density p of the manometric fluid and its

temperature dependence. The density of

mercury is known to about 1 or 2 parts per

million at 20°C but it changes by more than

180 parts per million per degree Celsius.

Although this is a well known fact, it is

frequently not sufficiently taken into ac- count in commercial manometer designs.

Vertical adjustment of the scale is another

source of error. Many difficulties arise

from the low viscosity of mercury, which

makes it difficult to locate its surfaces in

the presence of vibration. Mercury surfaces

are easily contaminated though oxidation of

dissolved contaminants. Its high surface

tension causes a depression of the meniscus

that depends predictably on the size of the

tube and depends unpredictably on the properties of the surface that the mercury is

in contact with.

Dissemination of Accurate Measurements :

There are, in principl e,two ways to transfer the accurate pressure measurement from

NBS standards into the field: by sending a calibrated transfer standard into the field

for in situ calibration of the customer's

standard or by receiving the customer's

standard for calibration at NBS. With few

exceptions the latter way is presently

favored. One of the primary reasons is the

cost of the transfer standard and the time

required to perform a calibration, including

shipping the transfer standard forth and

back. NBS would require many transfer

standards and a large amount of work to keep

them in calibration. Conversely a customer

sending his standard to NBS would lose the

use of his standard only once every three

years for a period of between 4 and 8 weeks.

Until a transfer standard can be developed

that is as stable and precise as a piston

gage, but easier to use and ship, we expect

to do most of the calibration work at NBS by

comparing the customer's instruments

with NBS standards in "cross-float" operations . In a piston gage "cross-float", if

properly executed, the pressure p' generated

by the test gage equals that generated by

the standard gage pS at the reference level

of the test gage. This pressure is referred

to simply as pressure p.

Mg (1 Pair

-T

1 + (an +

) + yC

Tv

is the force exerted on the test gage piston.

«o,o

kT

bi ,0

'2,0

is the effective area of the

test gage,

is the fractional change of

effective area of the test gage

with pressure, and

is the fractional change of

effective area of the test

gage with the square of the

pressure.

Note that for simplicity the temperature

correction of the area has been lumped with

the force pT. All data computations are performed by a digital computer where a comprehensive analysis of the cross-float is obtained. The

available information is tabulated and

plotted in various ways to test for errors

in the compilation of input data and for deficiencies in the gages or the calibration

procedure and to present the results of the

piston gage cross-float. A comprehensive

calibration report is supplied to the

customer.

In principle, instruments other than

piston gages can be calibrated at NBS and

used as transfer standards. This would entail generating a number of well known pressures with one of the primary standards and

recording the output of the transducer con- nected to the standard. The results can be

evaluated by fitting polynomials of various

degrees in either pressure or output reading to the experimental data. This has been

done in the past, but the lack of adequately

characterized transducers presently precludes the wide use of this method. However, we are presently preparing a program,

which will lead to the production of re- liable data on transducers with the expectation that one or two transducers will emerge

which can be used with confidence as transfer standards. This program is described

elsewhere in this report. One must note that,

in the important area of flight control instrument calibration, large numbers of transducers are used in spite of the absence of

generally available data on lorig-term stability etc.

P = (aI (1

^ O'O bi ,oP bl.oP")

where
Very similar principles are followed when

calibrations are performed against the primary standard manometer. In any calibration

the customer receives a detailed report of

the test with all pertinent data, with a statistical analysis and with clear statements of the estimated uncertainty. The

customer, often an instrument manufacturer,

is only the first link in the long calibration chain down to the end point of use.

The Calibration Chain

Fig. 4 illustrates an example of a calibration chain. In the NMS for pressure there

exist hundreds of such chains, which are often connected with each other and usually

have an interface with NBS somewhere near

the top. The calibration chain illustrated

here begins with one of the primary standards

at NBS; in this case the 150 MPa controlled

clearance piston gage. This gage has

uncertainty of about 35 ppm. A gage manu- facturer's plant standard was compared to the primary standard at a number of pressures using the cross-float methods discussed in the section on Dissemination. A total of 26 comparisons were made. The

evaluation renders a set of constants for

the gage manufacturer's plant standard with

uncertainties determined by those of the

primary standard and by the comparison. The

gage manufacturer's Plant Standard is in

turn used to calibrate instruments sold to customers. The customer elevates the gage

NBS

Primary Stanilard

Working

Piston Gage

Plant

Manijfactrjrer

Stanitard

Worliine

Pressure Gage

Plant SU

Working

Pressure Gage

Point of EnrI Use

Example of a Calibration Chain

Figure 4. Calibration Chain

purchased from the gage manufacturer to the

level of plant standard and uses it to calibrate working standards. These working

standards are then transferred to the various

divisions and departments where they may be

used to calibrate pressure gages and transducers used in a production process. We are

now six transfers away from the top of the

calibration chain. This calibration chain

is typical. It is by no means a particularly long one; many examples can be found

where there are more than six links in the

calibration chain.

Not all calibration chains start at NBS. There are a number of laboratories with

primary pressure standards. In a few cases these are controlled-clearance piston gages.

Mostly, however, they are mercurial manometers. Both of these instruments are capable of being calibrated in the field.

Some chains beginning with primary standards

not at NBS nevertheless have links at lower

levels with chains connected with NBS,

affording them a certain degree of measurement assurance.

The instruments used in the transfer of

measurements are almost exclusively piston

gages, at least in the upper part of the

chains. Piston gages of very high quality

are capable of a precision and repeatability

that occasionally approach one part per

million but usually range from 3 to 12 ppm. Even if allowance is made for uncertainties

introduced by various sources of error during a transfer, it is possible to propagate

pressure measurements with errors of less

than 20 ppm per link under carefully con- trolled conditions, with well trained

personnel and high quality instrumentation.

In many situations these conditions cannot

be met. Well trained personnel and/or con- trolled environments may not be available.

Cost is often an important limitation.

Speed may be a consideration. Consequently

one finds that near the lower end of a calibration chain transducers replace piston gages and instead of adding 20 ppm per

link the uncertainty is increased by a factor between three and ten. This tends to drop the accuracy of pressure measurements

very rapidly and thwarts even the boldest

attempts near the top of the chain to improve the situation.

Until now we have been looking down the

calibration chain; let us now change our

point of view and look up the chain from the

point of use. Take tire inflation pressures

as the measurement to be performed as illustrated in table 3. This is an example

of a chain that is uncomfortably long even

for such a crude measurement. An uncertainty of no more than 1 psi should be allowed

for the inflation pressure. The figure
Table 3. Tire gage calibration

Operator i^ect/Instrument Rfouired Acc. Acc. Lost in Transfer

You AND I Tire 1 PSI Factor 3

You A^D I Tire Gage 0.3 PSI Factor 3

Tire Gage Manu. Bench Std. (Bourdon) 0.1 PSI Factor 3

Tire Hage fWiu. ''orking Std.(^urdon) 0.03 PSI Factor 3

Tire (tage "anu. Plant vStd. (Piston Gage) 0.01 PSI 30 PPM

Press. Gage '^anu. ''orking Std, (Piston Gage) ^.007 PSI 20 PPM

Press. Gage Hanu. Plant Std. (Piston Gage) 0.005 PSI 20 PPM

f!BS Primary Std. (CCPG) 0.003 PSI

shows clearly the accuracies required at various levels and the increments in

uncertainty at each link of the chain.

An area with particularly critical needs

for pressure measurements, as we shall demonstrate later, is air transportation. Be- cause of the demand for accurate measurements and because NBS had for years neglected this area, most airlines and aircraft

manufacturers have set up mercury manometers

to serve as in-house primary standards.

From these they disseminate accurate measurements through a variety of transducers to the point of use, namely the altimeter. The

manometers are calibrated in terms of the

basic units of length, mass and time as maintained by NBS and they are, in some

cases, compared with NBS pressure standards

by means of piston gages. In a measurement

of such importance an additional assurance

of the accuracy of measurement could be derived from a comparison between a calibrated

altimeter and a transfer standard calibrated

at NBS. Such measurement assurance program

applied to standards near the end points of

use could eliminate the need for many long

calibration chains and at the same time re- duce the uncertainty and the cost of the

operation. But its main feature is the as- surance at the point of use that a measurement of sufficient accuracy can indeed be

rendered and that it is in agreement with

the rest of the national measurement system.

2.2.2.2 The Instrumentation Industry

The instrumentation industry plays an

important role in the national measurement system, since the majority of pressure

measurements are disseminated through the

instrument makers ' standards laboratories.

There are two layers of instrument makers:

the producers of piston gages and the makersof transducers and indicating gages. The

former receive calibrations from NBS and

each of them transfers these to about forty

customers per range. The transducer and

gage manufacturers among these customers in turn transfer the measurements to their customers many of whom are not end users yet.

The multiplication factor of these two layers is extremely large and, at least in quantity, the direct NBS to end user transappears to be insignificant.

Commensurate with the importance of the

makers of standards there is close and

growing contact between this industry and

NBS, and recent activities at NBS have also

started a growing cooperation between the

transducer and gage manufacturers. There

are approximately 150 manufacturers of

gages and transducers with annual sales

ranging from less than $100,000 to several tens of millions of dollars.

With one exception, the piston gage manu- facturers produce devices that are classed

as transfer or secondary standards. One

manufacturer makes primary standard piston

gages. Two manufacturers offer vacuumbacked piston gages intended primarily for

use in altimeter calibrations. Throughout

this part of the instrumentation industry

the price per instrument varies from a few hundred to several ten thousand dollars
as the sophistication of the instruments

increases. With the exception of the maker

of primary standards (controlled clearance

piston gages) all manufacturers maintain

standards calibrated by NBS. Many of the

primary standards are also sent to NBS for

cal ibration

The transducer and gage manufacturers

traditionally have not been direct NBS

customers. This is beginning to change as transducers and gages with greater precision

become available. Several manufacturers

have recently begun to obtain calibrations

for their plant standards directly from NBS.

In the future this cooperation will become

much closer as NBS is beginning a transducer

evaluation program. A similar program for

gages may be instituted later. The variety

of transducers and gages is even wider than

that for piston gages. Among the gages the

conventional Bourdon tube predominates with

accuracies ranging from 5% down to 0.1%.

Three manufacturers produce quartz Bourdon

tubes. At least two manufacturers produce

force balanced Bourdon tube gages with

significantly reduced hysteresis. Among the

transducers strain gage types appear to dominate. Others use LVDT, reluctance or capacitive systems to sense the deflection

of an elastic element. A new class of

transducers with rather high resolution uses oscillating components, the resonance

frequency of which is changed by the application of strain or stress. The prices for

gages range from a few dollars to about

one thousand dollars, those for transducers

with or without read-out from below $100

to about $10,000.

In our opinion the lack of information on the performance of transducers and gages over longer periods of time is the chief

obstacle to their extended use as transfer

standards in the calibration chain. Much

information about certain types has been

accumulated by users, but the information is

proprietary and not publicly available. We have found that manufacturers frequently

misstate performance characteristics. Both

overstatements and understatements are found.

Inequities in the market place arise from

the fact that users often write purchase

specifications for transducers, which they

cannot verify. As a result of NBS actions this

situation is expected to change(see 4. 2. If).

Another group of instrument manufacturers

produces manometers. These range in price

from a few tens of dollars to about $50,000

for the most elaborate instruments. Mercury,

oil and water are frequently used as filling

fluids. The market for the most sophisticated

instruments used in altimeter calibration is dominated by two companies. Altogether there are about 25 manufacturers in this field.

Since manometers are primary

standards, calibratable in situ in terms of

the basic units of mass, length and time,

NBS does generally not provide calibrated

pressure standards for these manufacturers.

However^ the cal ibrationsof the more accurate

ones in the field are frequently compared

with NBS primary standards through suitable

transfer standards.

The situation in the vacuum gage industry

is confused at best. Since there are no U.S

national standards, there are now very few

direct connections between NBS and the industry. Large numbers of ionization gages,

capacitance manometers, thermocouple gages

etc. are produced with no chance to compare

their calibration with a national standard.

The absence of dependable primary standards

in a central laboratory also precludes any

measurements on the reproducibility,

resolution and long term stability of these

gages. Other countries as for example Japan

Great Britain and the Federal Republic of

Germany, are in the position of having

excellent standards laboratories serving

their own, relatively small vacuum industries. This could easily convert the present U.S. positive trade balance for vacuum equipment into a negative one.

2.2.3 Reference Data

Although the use of reference data in the

national measurement system for pressure is

not extensive, there are several sets of

data which are directly or indirectly of

great importance. Among these are densities

of manometric fluids, the standard atmosphere, the steam tables, fixed points on the pressure scale and the equation of

state of sodium chloride.

Density of Manometric Fluids

Mercury is the fluid with the highest

and best known density. It has

traditionally been the choice manometric

fluid although it has many properties that

adversely affect its use and that are not

well understood. The more prominent ones

are its low viscosity, the rapid deterioration of its exposed surfaces, its high sur- face tension, and the variability of its

contact angle with containment materials.

The most precise determination of the density is that of A.H. Cook and N.W.B. Stone

published in Phil, Trans. Royal Soc. London,

Ser. A. 250, 279 (1957). The best determination of the thermal expansivity of

mercury is that of J. A. Beattie et al

published in Proc. Am. Acad. Sci. 7£. 371 (1941). The density and thermal expansivity
of a siloxane manometer fluid (DC 704) were

determined by R.H. Orcutt and published in J, Vac. Sci. Technol. 10. #4, 506 (1973).

Selected data are given in Table 4. Table 4. Density of Manometric Fluids

Substance Temp. Density Thermal Ex (g/cm^)

Mercury o°c 13.59504

Mercury 20°C 13.54584

Water 0°C 0.999840

Water 3.98°C 0.999972

Water 20°C 0.998206

DC 704 23°C 1 .0731

1 .8159

7.2

Standard Atmosphere

The Standard Atmosphere consists of a set of tables of atmospheric properties vs. geopotential and geometric altitude. It covers the range from -5000 to +300,000 m

and forms the basis for all altimeter calibrations and related measurements. It was prepared under the sponsorship of the Geophysics Research Directorate and the U.S.

Weather Bureau. NBS maintains a computer

code for the computation of altitude vs pressure tables in finer increments than

those given in the published document.

Steam Tables

The Steam Tables are an international ly

agreed-upon tabulation of the thermodynamic

properties of steam as a function of pressure and temperature. A large effort is ex- pended in their improvement. Their area of

application is mainly the power industry for

the purpose of designing and operating steam

turbines and engines. The tables are distributed in this country by the ASME.

Fixed Points on the Pressure Scale

For the calibration of high pressure

apparatus in situ fixed points marked by

phase transitions have been determined by

a large number of researchers in the field.

These points cover the range from about 500 MPa (5 kbar) to over 20 GPa (200 kbar). An international conference hela at NBS agreed

on a set of fixed points to be used for high

pressure calibrations and assigned best

values to them. These have been published

by E.G. Lloyd, C.W. Beckett and F.R. Boyd

in Science 164 , 860 (1969) and are in widespread use. Table 5.
Transition

Table 5. Fixed points on the pressure scale

Pressure

Mercury Freezing Point at 0°C 756.9 MPa

Bismuth I to II transition at 25°C 2.555 GPa

Thallium I to II transition at 25°C 3.67 GPa

Barium I to II transition at 25°C 5.5 GPa

Bismuth III to V transition at 25°C 7.7 GPa

Uncertainty

0.2 MPa

0.006 GPa

0.03 GPa

0.2 GPa

0.3 GPa

Two of the published points were determined

at NBS. The mercury melting line is also used

to provide reproducible pressure points for in

situ calibrations. The publication in

Science by Lloyd et al contains an equation

for the pressure and temperature relation

along the melting line. NBS is currently

involved in generating improved data for

this phase line particularly at pressures

below 0.7 GPa.

Equation of State of Sodium Chloride

The equation of state of sodium chloride

is used extensively for pressure measurements at very high pressures (above 2.5 GPa).

It was determined by three or four groups

and the results of these- groups agree within the estimated uncertainties of the measurement. Sodium chloride is mixed in

with the charge or the material under test and its density is determined from x-ray

lattice measurements. The pressure can then

be calculated from the known equation of

state. Other materials proposed for the same purpose are copper and aluminum.

2.2.4 Reference Materials

It appears highly desirable to provide

reference materials for the substances

discussed under 2.2.3 Reference Data. This

is particularly true for manometric fluids

like mercury and siloxane, where the lot to lot variation may exceed the inaccuracy of

the density determination. The creation of

such standards is now under discussion.

2.2.5 Science and People

The measurement of pressure is rarely an end in itself and it is therefore generally

left to the national laboratories to improve existing primary standards and to develop new ones. The instrumentation industry is naturally interested in developing

new, saleable products. The scientific and

research oriented organizations are primarily users in this system. One must not

overlook, however, the large amount of

basic research underlying all present primary and transfer standard developments.

The manometer and the piston gage are both

the fruit of basic research in thermodynamics. Strong impulses for improvements in

the basic standards still come from areas of

scientific research such as equation of

state, thermodynamics, statistical physics,

meteorology, oceanography and others. Strong

contributions are made particularly for

measurements at very high pressures by re- searchers in statistical and solid state

physics.

Professional societies appear to have

only a limited effect on the system. With

the exception of the American Vacuum Society

there is no professional society dedicated

directly and exclusively to the measurement system for pressure, although important contributions are made by several

committees of the American Society of

Mechanical Engineers, the Instrument Society

of America and others. Most of these

contributions are in the form of standards,

procedures or recommendations. Another important, though indirect, contribution is

the organization of professional meetings

strengthening the dissemination of information which is otherwise scattered

throughout the engineering literature. NBS

contributes by organizing meetings,

publishing papers, technical notes and

monographs. Many of these NBS publications

are the only available written material on the subject.

The lack of publications devoted to the

subject of pressure measurement is symptomatic of the lack of formal education and

training in this field. Most, if not all,

persons now working in this part of the

measurement system have no formal training

in the measurement of pressure. Only one member of the NBS Pressure and Vacuum

Section had prior experience with pressure

standards. Engineers and technicians working in industrial pressure standards
laboratories typically acquire all their

specific, job-oriented skills on the job. Two manufacturers of pressure standards

have formal courses for industrial personnel naturally inclined towards use of their

own instrumentation. The American Vacuum

Society arranges short courses in vacuum

measurements, a commendable practice worth

being copied by other societies. NBS has

recently instituted training courses for

pressure measurements in standards laboratories, which appear to be quite well

accepted

2.3 Realized Measurement Capabilities

2.3.1 Pressure Ranges

The range of industrial applications

covers about 17 decades of pressure and vacuum from 10"^ Pa (10"^ torr) to 10^° Pa (100 kbar). Scientific applications exceed

this range by several orders of magnitude

in either direction. Of this total range

NBS presently covers about one third,

namely the range from 2 x lO^Pa (0.2 psi) to

4 X lO^Pa (60,000 D?i)(see fiq. 5).

m

us»M312»-

18-

Calibratlon Service

Acuraci i( fimvi Slaidards

I I I -7 -6

I

' I I' ' I -2 -1 0 I 2 ! 4 S 6

1

Calibratiei Service Ruse

atnosphetic pressme

Figure 5. mS Calibration

An extension down to lO-^ Pa (10"'* torr) and

up to 7 X lO^Pa (100,000 psi) is anticipated for FY 76. NBS will then cover about

60% of the industrially used range. There

will be no service to the research community

working at pressures above about 7 x lO^Pa

(7 kbar) and there will be no calibrations

for the medium and high vacuum ranges unless

a management decision is made to begin a vacuum calibration effort at NBS.

2.3.2 Accuracy and Precision

NBS pressure standards have systematic

uncertainties ranging from 33 to 60 ppm for

standards used in the calibration service

and up to 1000 ppm for primary standards not

in the calibration service(Tabl e 6).

Table 6. Uncertainty of primary pressure

standards at NBS.

Range

[Pa]

Uncertainty

[ppm]

2 ^ 103(0.3 psi) 38

to 105 psT) 33

to

3 x 105 (^50 psi) 34

to

4 X 106 (600 psi) 49

to

1 .4 X 108 (20,000 psi ) 60

to

4 X 108 (60,000 psi) 235 this range

is not in cal ibration

to service

2.5 X 109 (25 kbar) 1000 this range

is not in

cal ibration

to service

5 X 10^ (50 kbar)

The precision of these standards ranges from

about 3 ppm to about 15 ppm within the

calibration service range.

At the other end of the calibration chain

are the required accuracies at the end point

of use. These are generally difficult to assess in detail. A large group of measurements with fairly well defined tolerances

serves altimeter and airspeed indicator

calibrations: measurement assurance for

plant standards requires an accuracy of 20

ppm of the reading. Similar requirements

hold for altimeter setting indicators (ASI).

Maintenance of military equipment calls for

accuracies of the order of 100 ppm at pressures up to 7 X lO^Pa (10,000 psi). Oil well

pressure gages are specified with a resolution of 1 ppm and a comparable precision is

required of the standard at pressures up to 108 Pa (12,000 psi). Gage manufacturers

need standards up to 7 x 108pa (100,000 psi)

with an accuracy of 300 ppm. Transducer

manufacturers, however, require accuracies

of 20 ppm or better for the evaluation and
calibration of their products over a wide

range of pressures. At pressures up to 10^

Pa (10 kbar) or higher, uncertainties of

less than 100 ppm are now required for many

scientific purposes, while 1000 ppm appears

adequate for industrial purposes. For the

range below 10^ Pa (1 torr), information on required accuracy was obtained from the

Delphi study. (Data in table 7 for different applications). There are several areas

of great economic significance, which now require measurements to better than 0.1% and

tighter tolerances in the future. None of

the requirements can be met by NBS.

Table 7. Required accuracy for vacuum measurements (all in percent)

Range

Application (in torr) 1 to lO'^ 10-3 to IQ-^ lO'^ to lO"^ below 10"^

Nuclear Reactor Safety

Systems

...

. ^ V 10 50

Chemical Plant,

Refining etc. r 10 50

Space Simulation Vehicle

Satellite Test

1 1 10 10

Semicond. Manuf.

Substrate S-j , Gg 0.1 0.1 10 50

Semicond. Manuf.

Vac. Deposition 0.1 1 10 50

Metal Preparation

Steel Specialties 1 . 1 50

Electron Tubes,

CRT, Displays 1 1 10

Cal ibr. of Leak

Detectors 0.1 0.1 10 10

Calibr. of Std.

Leaks 0.1 0.1 1 10

Calibr. of Flight

Control Instr. 0.1 10 50

Measurement of Pumping

Speed 0.1 0.1 10 50

Calibr. of Vac. Trans. Stds. 0.1 0.1 10 10
Many of the accuracy requirements listed

above that fall within the range where NBS

presently offers calibration services can be

met by available primary standards. In some

cases, however, the dissemination poses a severe problem. There are three reasons:

the first one is the often inadequate training of the personnel associated with these

measurements, the second is our lack of

knowledge on the performance of transfer

standards, and the third is the length of the

calibration chains. Actions taken to help

improve the situation are described in

chapter 4 of this report.

It is interesting to note that when one purchases a tire gage one has no assurance

of its accuracy nor any means to have it

checked. Neither the notoriously inaccurate

air columns in gas stations nor the tire

gages in the hands of gas station attendants

are checked once they have left the

manufacturers plant. Fig. 6 shows some re- sults of tire gage calibrations done at NBS.

Variation of Tire Gage Calibrations

I i

20-

a>EJ 10-

0- r

Deviation from nominal value of 28 psi

Figure 6. Variation of Tire Gaqe

Cal ibr^ition

Plotted is the number of gages reading a certain pressure for a constant set pressure.

Such data were taken over a period of several

months on tire gages owned by NBS staff. We

will comment elsewhere in this report on the

economic significance and desirability of

this particular measurement (3.1.3).

2.4 Dissemination and Enforcement Network

2.4.1 Central Standards Authorities

Internationally only the organization for

Legal Metrology (OIML) appears to affect the

pressure measurement system. OIML has re- cently proposed a number of standards of

pressure gages. These have not been ratified as yet by the U.S. delegation, nor is

their ratification likely. Nevertheless

these standards may form a trade barrier for

U.S. exporters in those countries that have

adopted these standards.

Within the United States the Department of

Defense (DOD), through military specifications and standards, exerts great influence

that is not restricted to its own suppliers.

Energy Research and Development Administration

(ERDA) and the Federal Aviation Administration (FAA) influence the system through

specifications, recommendations and standards

pertaining to their particular domain. The

FAA lacks the measurement capability for

rigorous enforcement of important standards.

2.4.2 State and Local Offices of Weights

and Measures.

These offices are generally not equipped

to provide pressure calibrations. They do,

however, frequently calibrate weights for

piston gages. Offices have been created in

most states to certify the design of pressure vessels to be installed in the state. These generally follow the ASME Pressure

Vessel Code.

2.4.3 Standards and Testing Laboratories,

and Services.

A large number of commercial testing

laboratories provide some limited pressure

calibration service (see table 8). In a few cases the standards have been calibrated

at NBS and are maintained by personnel

trained at NBS. In the majority of the cases these testing laboratories maintain

a master gage with a calibration from the

manufacturer for the testing and calibration of less accurate gages. An extensive

listing of such testing laboratories is

annually compiled by the National Conference

of Standards Laboratories. Their use of

the term "primary standard" in the listing

may be misleading. These testing laboratories are an important part of the measurement system as an economical source for

calibrations. For many users they are virtually the only available source. Large industrial concerns frequently make

their standards laboratories available to the public or at least to their subcontractors. Some nOD, ERDA and NASA laboratories

do the same for their subcontractors. All of

them perform a significant amount of

calibration work for their own hierarchy.

Standards are provided by NBS where possible.

In the vacuum range, McLeod gages and dynamic

expanders are most frequently used with no opportunity to provide uniform measurements throughout the system.
Table 8. COMMERCIAL PRESSURE CALIBRATION

LABORATORIES

P, "primary" standard in the terms of NCSL,

denotes an accurate transfer standard as for

example a piston gage or a good Bourdon gage

Commercial Laboratories

All is Chalmers MFG P American Geophysical P Avco P Ball Brothers Research

Beckman Instruments

Bendix P Boeing P Burroughs

Collins Radio P Colorado Engineering Experiment P Dayton T. Brown P

E. G. and G. Electrical Testing Labs

E - Systems

Foxboro

Gage Lab

Gauge Repair Service P General Dynamics P General Electric P Harri s Intertype P Hercules

Honeywel

Instruments East

Leeds & Northrup

CI ifton of Litton

Lockheed P Martin Marietta P

- McDonnel Douglas P

3 M NAR, Rocketdyne P National Astro Labs P Raytheon

Rockwell International P Ruska Instruments P Singer p SSCO Standards Lab P Teledyne

TRW Systems P Varian

Vol umetrics P Westinghouse

2.2.4 Regulatory Agencies

The Nuclear Regulatory Commission (NRC)

and Federal Aviation Administration (FAA)

both influence important sectors

of the measurement system. ERDA has excellent calibration laboratories

in several of its plants that are capable of

enforcing and verifying most or all specifications. The FAA lacks these capabilities

at least for its enforcement netv/ork. Standards laboratories exist in the FAA Experimental Facilities. It is left to the airtransport industry to comply with the re- gulations and technical standards orders of

the FAA (and the trade associations) and to provide their own standards and calibration

hierarchy. FAA inspectors visit the certified laboratories and rework facilities to oversee the work, but they have no means to conduct a measurement assurance program. We

at NBS feel that this puts a large and

partially avoidable burden on the industry.

Each of the airlines and aircraft manufacturers presently must maintain a primary

standards laboratory. One central laboratory coupled to the users with a measurement assurance program could achieve the same end at a lesser total expense and provide for uniformity at the same time. The

FAA has so far been unresponsive.

2.5 Direct Measurements

Transactions Matrix

Table 9 contains schematic and summary information about transactions between users

and suppliers of measurements, information

and essential hardware. The community has

been divided into 25 categories, which,

with few exceptions are self-explanatory.

These categories are

1 Knowledge Community: scientific

organizations, professional societies

technical publishing organizations,

trade associations,

2 International Metrological Organiza-

^, tions.

Electric P 3 Documentary Standards Organizations,

- 4 Instrumentation Industry.

5 NBS

6 Other U.S. National Standards Authorities

7 State and Local Weights and Measures

Offices.

Instrumentation Industry.

8 Commercial Standards and Testing Laboratories.
Table 9: Direct measurements transactions matrix

DIIECT

U

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1 e OTHER U.S. WTIOHAL

STMOAKDS

ALTV^BT-TES

0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0

0 0 0 0 0 0 0 0

7 s-i— s 0 0 0

1

1 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 3 0 0 0 0

2

3 2

2 2 1

1 0 0

2 2

1 2 1 a 1 1 1 2 0

2 2 0 2 2

,

3

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1 0

3

3 0 0 0

2

1

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2 0 1 0 0 0 0 0 0 0 3

3

2

3 3

3 3 3 0

10 DEFSS-^S- "

DEFEISE

(excl . Stas- -abs)

1 2 0 22

2 0 0 0 ;

4 1 1 4 1 0 0 0 0 0 3 3 0

-2 2

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11 CIYILIM FEDERAL awEKxexT

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0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0

13 lausniiiu. THACE

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1 2

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1 0 0 0 0 0 0

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0 1 2

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(SIC Major Go 3S)

2

1 0 0 0 0 0 0 0 0 1 0 1 1

1 ? ? X X 1 'a Eucmic IK

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(SIC Major ^ 36)

2

0 1 2

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2 0

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0 0 0

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2 2 2

2

2

1 0 0 1 2 3 0 1 0

2 0

2

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25

' F-EWML PUBLIC 0 1

1 2

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1 0 0

2 2 0 1 1 1 0 0 0 0 0 3 D 2 2

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1

Key to matrix entries:

none

small

large

negative

stabl

slowly growing

growing

fast growing

explosive

not essential matter of convenience,

economically important; strongly desirable,

no satisfactory alternate source; legally

requiretd, very essential; sole source; matter of life and death.

no foreseeable improvements needed or

desi rabl

under control, but could be improved

marginal, significantly inadeauate

seri-ooisly deficient

out ef control, thoroughly unsatisfactory

[^tJ magnitude of transaction: 0

Ij^j rate of change of transactions:

[1 adeouacy of transaction

r

J importance of transaction 1 for the recipient 23
9 Regulatory Agencies

10 Department of Defense (DOD)

11 Civilian Federal Government Agencies.

12 State and Local Government Agencies.

13 Industrial Trade Associations.

14 Extractive Industries.

15 Construction.

16 Food, Textile etc.

17 Chemical and Oil Industry.

18 Metal Products.

19 Machinery except Electrical.

20 Electric and Electronic Equipment.

21 Transportation Equipment.

22 Transportation, Uti 1 ities.

23 Trade.

24 Health Services.

25 General Public.

Each category is entered twice, as supplier and as user. Up to five entries

are made at each intersection. Their meaning is explained at the bottom of the matrix. These entries are not free from

ambiguity: a transaction may be important

for the user but not for the supplier or vice

versa. Two large transactions in different

parts of the matrix may have widely different economic consequences. There are 625

intersections with a total of more than 1000

entries. The discussion will therefore be

limited to major features and major users. The core of the measurement system is

found in the 5x5 element submatrix in the

upper left hand corner. A measure of the

significance of this part is the mean of the

"importance" entries in this submatrix of

2.7 compared to 0.5 for the rest of the

matrix. Many of the transactions taking

place here are information exchange. Only

the dissemination of measurements from NBS

to the manufacturers of standards, gages

and transducers and between the three latter

appear. There is ample indication of a large amount of feed-back in this part of

the system. The second important part is

the 5 x 20 element matrix in the upper right

hand corner. This part shows the transaction

from the core of the system to the users. The mean magnitude entry here is ^A (mean

of the entire matrix 0.6) and mean importance entry is 1.4 (mean of the entire matrix

0.6). The predominant transactions are in

the form of measurements. The 20 x 5 element matrix in the lower left hand corner

gives the reverse interaction, namely, from

the users to the core of the system. Nat- urally, these transactions are all in the

form of information. In magnitude (0.8 vs. 0.6) and in importance (0.8 vs. 0.6) they

rank above the mean. The rest of the total

matrix, a 20 x 20 element submatrix in

the lower right corner, describes interactions in the lower echelons of the measurement system. Here the significant interactions are primarily within each category and. they appear therefore along t-lie diagonal.

NBS appears as a supplier of measurements

to most of the users and at the same time it

receives information from virtually all parts

of the system. In several cases the information feedback is, however, insufficient.

2.5.2 Highlights re Major Users

The major users of measurements in the

system are easily identified as those having

entries 3 or 4 for magnitude. Among these

are the manufacturers of standards, gages,

transducers, the commercial testing laboratories, the industries in categories 17 to 22, and the government agencies. Exchanges

go, for example, from NBS to Standards Manufacturers to NOAA to the Transport Industry,

here primarily the airlines. Another chain

goes from NBS to the Standards Manufacturers

to the Equipment Manufacturers and thence in

the form of essential auxiliary equipment

to any of a variety of users. Most industries are important suppliers of their own services through extensive internal calibration hierarchies.

There are a number of motivations for

using pressure measurements, some of which

with their major constituents are:

Safety: Aircraft Manufacturers

Transportation

DOD ERDA

FAA General Public

Calibration (product requires calibration to operate)

Standards Manufacturers

Gage Manufacturers

Transducer Manufacturers

Equipment Manufacturers
Operational: (process oressures must be

measured)

Aircraft Manufacturers

Transportation Industry

Chemical and Oil Industry

Semiconductor Industry

NASA

NOAA

General Public

Performance: (accurately known pressures

required for evaluation

of performance)

Aircraft Manufacturers

Equipment Manufacturers

Gage Manufacturers

Transducer Manufacturers

Dissemination:

Standards Manufacturers

Commercial Testing

Laboratories

NOAA

Research: ERDA NASA

Industrial R + D Universities

It is now interesting to see whether

these needs, for example those related to safety, are fully met. The pAA (Col. 9) is re- ponsible for tne safety of the nation's airlanes. Rapidly increasing traffic requires

improved control of altitude, vertical and

lateral separation, and air speed of planes

in the major traffic lanes. New instrumentation must be used even though there may not

be sufficient performance data available. In some cases the problem of calibrating the

equipment being installed has not been solved.

This, if not resolved, would constitute

"inadequate transitions". An improvement is

also needed in the aircraft manufacturing and

transportation industries to tie all critical

pressure measurements to national standards.

This improvement would likely be in

the form of a measurement assurance system

provided either by the FAA or the NBS, in

the form of better characterized transfer

standards, and in a few cases of more accurate measurements.

The services provided by NBS to the semiconductor industry are thoroughly unsatisfactory and, in fact, there just are no services. Measurements provided by the

general public to the general public are found to be inadequate. This pertains to the fact that an automobile owner can not

have his tire pressure measured reliably

although the measurement of tire pressures

is of significant economic impact(3.1 .3)

The End User :

The end uses to which pressure and vacuum measurements are put are as varied as the

types of gages, switches, transducers that

are applied. Little can be gained from

enumerating all of them. It may be more revealing to have a somewhat closer look at

an example for an end use that is more demanding than the tire gage calibration. The

example deals with altimeter calibrations.

We are using data obtained by the International Civil Aviation Organization and published in the "Manual for Barometry,"

DOC-Weather bureau.

Altimeters are used principally for the

following purposes which require the measure- ment or control of the altitude of aircraft

with respect to sea level or some other ref- erence surface:

(1) Landing at an airport;

(2) Vertical separation between aircraft

flying in different directions or at different speeds on an airway or elsewhere;

(3) Vertical clearance of terrain under

instrument flying conditions;

(4) Miscellaneous aircraft operations

that require knowledge of pressure

altitude and other parameters; for

example, the setting of engine controls for power or thrust on the

basis of pressure altitude and air

temperature; establishment of opti- mum cruising speed by means of density altitude which depends upon the

two factors last mentioned; air

navigation conducted to take account

of pressure pattern flying, that is,

by cruising on minimum time routes,

determined, with the aid of the

pressure altitude, air temperature

and other factors.

Pressure altimeters contain an internal

mechanism which is essentially an aneroid

barometer calibrated in accordance with the

pressure - altitude relationship of the

standard atmosphere. The pointers of the

typical altimeter are actuated by changes of

pressure exerted on the aneroid element.

Consequently the pointers show deflections

with respect to the base of the internal

mechanism as the ambient pressure varies.

This mechanism includes suitable cams, gears, levers, and other devices designed

for a special purpose. The primary function

is to permit the calibration on a uniform
scale of pressure altitude when the instrument is set on the standard altimeter

setting of 29.921 inches of mercury (101.31

kPa). That is, with this setting, the

pointer on a properly calibrated, perfect

altimeter should indicate altitude corresponding to the ambient pressure, in accord

with the standard atmosphere relationship.

The mechanism must permit this relationship

to hold despite the fact that uniform steps

of increasing altitude correspond to gradually decreasing, nonuniform steps of pressure.

In addition to the altimeter proper we have

on board the pitot-static probe and the lines

connecting it with the altimeter. Modern altimeters are usually provided with an electrical readout and digitizer, and are con- nected to a transponder for transmission of

data to the flight controller on the ground.

Two or three independent altimeter systems

are found on most commercial aircraft.

Of the four major purposes for which

altimeters are required, vertical separation

and vertical clearance of terrain seem to be

the most critical applications. During

take-off and landing most commercial aircraft

are controlled by ground-based radar beacons

and on-board radio altimeters (up to 1500

feet (454 m) , 2900 ft (884 m) on 747) and the

miscellaneous operations do not require

comparable accuracy.

An error computation and reasonable

estimates for the vertical separation of

aircraft are difficult, if not impossible,

to obtain. Several experts in the field,

as for example Dr. DuFeu (personal communication) of Smith Aviation in Cheltenham,

England, believe that the uncertainties in

the vertical separation of aircraft often

approach the separation presently used, i.e.

1000 ft (300 m) at altitudes up to 29,000 ft

(8800 m) and 2000 ft (600 m) above that.

Some representative data may be taken from a report of the ICAO Panel on Vertical Separation of Aircraft

"Consider the case of two aircraft assigned

different cruising altitudes or flight levels

where normally one will pass over the other

with an indicated vertical separation of

1000 ft (300 m) ; for example one aircraft

at an indicated altitude of 5,000 ft (1500 m)

,

the other perhaps opposite bound at an indicated altitude of 6,000 ft (1800 m). The

ICAO Panel mentioned above found that a loss

of vertical separation of 546 ft (166 m) is

likely to be equaled or exceeded 3 times

out of 1000 between aircraft passing on headings which normally require a 1000 ft

(300 m) vertical separation provided that a constant, standard setting of 29.92 inches of

mercury (101.31 kPa) is employed for the

setting of the altimeters of both aircraft.

The actual vertical separation of these

I planes would then be 454 feet (138 m). A further substantial reduction in the vertical separation may result if the two air- ' craft come from different airports and have

markedly different altimeter settings (QNH).

Thus, if the higher of the two aircraft em- ploys a QNH value which is, say, 0.38 inches

of mercury (1.29 kPa) greater than that em- ployed by the lower craft, the vertical

separation will be reduced by another 350 ft

(170 m)."

"The question arises whether one should assume the most probable values of errors

for the purpose of providing safe conduct of

an airplane, or whether it would be more

appropriate to use extreme values. Since

the purpose of most aviation operation is

the accomplishment of individual missions,

it appears best to assume a definite probability of the occurrence of extreme errors.

A careful distinction must be made between

mean tolerances pertaining to errors as derived from statistical probabilities based

on the behavior of large numbers of altimeters, aircraft etc. and the actual largest possible errors that could affect adversely the operation of the particular

aircraft during a given flight. Since the crew and the passengers are more immediately

concerned with the latter than the former,

they will naturally consider it essential to give forethought to the contingencies and

deem it discreet to allow for the chance that

the errors may combine sometimes with the

most adverse sign to a maximum degree."

Without going into more details we shall

mention that errors of similar magnitude as those discussed for the instrument must also

be expected for the static pressure system.

Static system errors can be and are compensated automatically on many aircraft and

consequently only the standard deviation of

the errors needs to be considered for a total uncertainty determination.

3. IMPACT, STATUS AND TRENDS OF THE MEASUREMENT SYSTEM.

3.1 Impact of the Measurements

3.1.1 Functional, Technological and

Scientific Applications.

Pressure is an important parameter in

most manufacturing processes and must therefore be measured for control or optimization.

Without such pressure measurements certain

processes could not be used at all (leaktesting of radiators, can tops), others could

not be optimized (power plants, refining,

jet engines). Pressure is used to determine flow in pipelines (also for the detection of leaks). In the semiconductor
industry partial pressures of the residual

gases present during an evaporation process

strongly influence the yield of the process.

Safety is an important reason for pressure measurements. Examples are measurements at numerous points in nuclear reactors,

aircraft altimeter, cabin oxygen gages and

automobile tires.

Scientific applications are primarily in

the equation of state of fluids and solids,

gasthermometry, oceanography and meteorology.

Military applications include the operation of aircraft, missiles and submarines to name only a few more critical areas.

3.1.2 Economic Impacts -- Costs and Benefits

Estimates for the cost of measurements

come from three sources (a) our own estimate

of cost for equipping and operating an industrial pressure standards laboratory

excluding airlines and makers of aircraft;

(b) information provided by calibration

laboratories; (c) annual production volume

of pressure and vacuum gages.

(a) Table 10 attempts to give an estimate of the cost of operating a small

pressure standards laboratory sufficient for

a large industrial plant or a small to medium size maker of pressure gages and

transducers. The cost per laboratory is

$43,000 annually. With a total of about 250

makers of pressure gages and transducers this

adds up to $10 million. Naturally the cost

of operating a calibration laboratory for an airline, aircraft manufacturer or maker of

pressure standards is a multiple of the

amount shown in Table 10.

Table 10. Estimated Cost of Operating

A Small Secondary Pressure Standards Laboratory

Capital Investments

Pressure Standards 16,000

Transfer Standards 6,000

Auxiliary pressure equipment 3,000

General purpose equipment, 5,000

furniture 30,000

Amortized capital cost/year 3,000

Annual Operating Expenses

Salary for one person 20,000

Overhead 16,000

Capital cost 3,000

Operating funds, computer etc. 4,000

Total operating expenses $43,000

(b) During the Delphi study on vacuum calibrations, thirteen vacuum standards

laboratories reported an estimated annual

operating cost of $134,000 per laboratory

(average)

(c) The annual production volume for

pressure and vacuum gages, SIC codes 38 211 46/47, and 38 213 31 was $25.4 mil 1 ion(l 963)

This is only a small part of the total production of pressure measuring devices (see

also 4.3.1) many of which are not separately

1 isted.

These are examples for the cost of pressure measurements in the first and second

echelons below NBS. The cost of pressure

measuring devices in lower layers is much

larger as shown in 4.3.1, and so are the

operating costs. The manager of a nuclear

(BWR) plant estimates that his plant spends

$25,000 annually just to investigate and

report setpoint drift, mostly in pressure

switches. There are about 100 such switches

in each plant with a total acquisition cost

of $60,000.

One of the prime benefits derived from

pressure measurements is safety. If accurate altimeter calibrations derived from

NBS standards can prevent one crash or collision of a passenger airplane, the

attendant savings can pay for the Pressure

and Vacuum Section for 30 years or more.

3.1.3 Social, Human, Person-on-the-Street

Impacts

All of us who travel by plane, car, railroad or ship are indirectly affected

by pressure measurements required to operate

or control these vehicles. Oil and gas

furnaces and airconditioners are controlled

with pressure switches, so are pressure cookers.A striking example for the savings that

can accrue when each car owner keeps his

tires properly inflated (to within 7 kPa or

1 psi ) goes as follows:

One out of every four cars has seriously

underinfl ated tires. Proper tire inflation

can improve qas mileage by 1 mile/gallon.

Average gas mileage in the U.S. is 13.7

miles/gallon. Annual gasoline consumption

in 1973 was 6.6 million barrels/day. Cost

of gasoline is $0.51/gallon. 100 million

cars are on the roads. If all car owners

kept their tires properly inflated we could accrue a potential annual savings of $940

millions or about $40 for each car with

underinfl ated tires. At the same time the

life of the tires on these cars can be increased leading to another savings of

about $7 per car. Both for the individual

and for the national economy these are significant benefits. The necessary expense
for the individual is about $2.50 for a tire

gage, while the national measurement system

should provide for the calibration of these

gages. Neither airtowers nor hand held tire

gages made available to the automobile driver

at most gas stations are checked against adequate standards unless they malfunction in a most conspicuous way. Airtowers were shown

to be highly unreliable in an NBS report on this subject (NBS-TN 512). Hand-held tire

gages are more reliable (Fig. 6) as long as they are protected from damage and contamination. The adequacy of calibration of these

gages is obviously only part of the problem,

but one that must be solved to reap economic

benefits. Note that a more detailed calculation could include the benefits derived

from the safer operation of a car with

properly inflated tires.

Another pressure measurement that is very

widely used by the generaly public is that of

barometric pressure. Each radio and TV station disseminates the barometric pressure

several times each day and numerous newspapers report it daily. It is used by many to satisfy their curiosity about tomorrow's

weather, but it has economic impact for all who have to rely on weather forecasts such

as gardeners, farmers, transportation,

travellers, utilities etc.

3.2 Status and Trends of the System

The current status of the pressure mea- surement system within the range for which

NBS provides calibrations is generally quite

good. Primary standards available now agree

with others in overlapping ranges and with

standard in other national laboratories.

They provide adequate accuracy for most

applications and no major improvements are either needed now or anticipated for the near future.

At very high pressures, where no calibrations are now provided, a slow expansion

is foreseen. This should be facilitated by

the development of new transfer standards

for very high pressures. New technologies

in powder metallurgy, metal forming, coal

gasification and synthesis of materials will

require more and better measurements. Plans

have been made to meet these requirements.

Scientific requirements in equation of state,

geophysics, explosive development, and re- lated fields will be not be met.

In the range of atmospheric pressures and

below into the low vacuum range, standards

are either inadequate or nonexistent. Development of new standards is, however, rapidly

progressing and this gap will be closed.

There are no national standards and no current plans for them for the medium and

high vacuum ranges. This part of the measurement system quite clearly is out of control. There is nothing to provide uniformity

of measurement throughout the system and it

appears that a large industry depending

critically on vacuum measurements is without

measurement support. This situation has in

fact existed for at least the last 20 years

with little effort on the part of the Bureau

to substantially improve the situation. The

industry seems to have resigned itself to the

situation and tries to make the best of it. But most experts agree that it is severely

handicapped. Difficulties arise in the transfer of technology, in providing uniform measurements, in specifying equipment and verifying such specifications and in measuring

pumping speed. The lack of uniform measurements is especially felt in the sale and

purchase of vacuum equipment, where it leads

to inequities in the market place.

There are no facilities for dynamic calibrations nor for calibrations under extreme

conditions of vibration, temperature, radiation or corrosion. Requests for dynamic

calibrations come primarily from developers

of steam turbines and engines, space vehicles

and from researchers in the field of explosives. Some current research might eventually help to provide such services.

We feel that the part of the measurement

system that deals with critical applications

in air transportation and nuclear reactors

is not completely under control. To ensure

both accurate and uniform measurements of

pressure measurement assurance programs (MAP)

should be set up. Such measurement assurance

programs could also benefit other areas of the

measurement system where high accuracy is

required at the point of use.

A further improvement of the accuracy at

the point of use could be achieved by better

error analysis in the lower parts of the

calibration chains. Some work in this direction is now done in cooperation with NBS

in two ASME committees. This will also re- quire continued efforts towards better training of personnel in the lower echelon standards laboratories.

One problem that has not been systematically addressed by anybody is the performance

of transfer standards, particularly transducers, used extensively in the lower echelons

of the system. Unless reliable data on the

long term performance characteristics of

transducers and gages are available it will not be possible to estimate errors propagated

in the system nor will it be possible to use these devices in MAPs. With the rapid expansion of automatic

process control and with increasing desire

to reduce personnel cost the use of transducers will undoubtedly expand quite rapidly.

The characterization and an economic calibration service will therefore gain in
importance in the future.

Metrication does not seem to present a serious problem in the upper parts of the

system. NBS has for several years written

all calibration reports in the S.I. system

and we have rarely had any inquiries about

_

conversions. The question of optimal metric

ranges for gages and transducers is now

under study in committees. Several manufacturers already offer metric scales and

calibrations.

4. SURVEY OF NBS SERVICES

4.1 The Past

A Pressure and Vacuum Section was formed

at NBS in 1960. In 1965 it was divided

into a Pressure Section and a Vacuum Section,

which were then reunited in 1971. Finally

the vacuum part of the section was abolished

in 1973. Since its inception the Pressure

Section has supported the highest echelons

of the calibration hierarchy and has, to some extent, neglected the bulk of the system. It has also in the past neglected the

systematic development of standards particularly in the manometric ranges. Consequently

there has been a great need in recent years

to bring the Section's standards up to the

state of the art. The neglect of the mano- metric range has caused the airtransport

industry to set up Its own standards at great expense and without the benefit of

being firmly integrated into the national

measurement system. The Vacuum Section

began to develop standards in 1965 but never delivered any significant calibration ser- vice except to a few top echelon laboratories

over a limited range.

Among the major accomplishments of NBS in

the past is the development of the controlled

clearance piston gage principle and the

refinements in the art of using these gages.

Controlled clearance piston gages are now

used as primary standards at NBS and in

several other National Standards Laboratories.

4.2 The Present - Scope of NBS Services

4.2.1 Description of MBS Services

(a) Calibration

The NBS Pressure Section provides calibrations over the range from 2 x 10^ to

4 x 108 Pa (0.3 to to 60,000 psi),(Fig. 6).

These calibrations are performed by comparing

transfer standards maintained by NBS. Characteristic data obtained from these

comparisons are compiled in a detailed re- port for the customer. The report also

contains ample information on the conduct

of the test as well as estimates of systematic uncertainty and random errors. There is no restriction on the types of

instruments that NBS will accept for calibration. The time required for most instruments is eight weeks and the cost is approximately $500 per range. The calibration of

controlled clearance piston gages takes three

to four months and the cost averages about

$4000.

(b) Data Evaluation

Laboratories wishing to use NBS programs

and codes for the evaluation of their own data can do so by submitting these data on NBS work sheets.' The data will be evaluated

and returned within about 4 work days. The

. cost is about $100 per standard set. This

provides the user with the best possible

evaluation of his data, including the statistical analysis.

(c) Measurement Assurance

NBS is prepared to set up measurement

assurance programs for a wide range of

pressures. This would entail shipping a calibrated transfer standard from NBS to the customer's point of use. It would there

be calibrated against the customers standard

and returned to NBS. The customer will then

receive a report with an evaluation of his

measurement. Frequency, turn-around time

and cost will be determined individually

for each application.

(d) Laboratory Evaluation

In urgent and critical cases NBS will

send a team to a standards laboratory to evaluate standards and procedures and to calibrate them against an NBS transfer

standard in situ. This has the advantage

that several plant standards can be compared

and that qualified NBS personnel are at hand

to operate the transfer standard. The cost

of this service is quite high and it should

therefore be used only under urgent conditions.

(e) Training

Calibration services are supplemented

with training courses for industrial standards laboratory personnel. These courses

'take two days at a cost of $105. The number

of participants is limited so that there is time for everybody to get hands-on experience. Two morning sessions are devoted to theory of piston gages, design, data collection, analysis and interpretation of results,

calibration of transducers and a short re- view of current NBS activities in this field.

The afternoons are spent in the laboratory.

Such courses also can be and have been

held outside NBS.
(f) Transducer Evaluation

A transducer evaluation service is now available from NBS. Under this program manufacturers or users can submit pressure transducers to NBS for an extended performance

test. Sensitivity, resolution*, dependence

(3n environmental and operational parameters,

drift, hysteresis and long term stability

during thousands of pressure cycles are determined. The user will be provided with

an extensive report containing all the data

generated during the test and their analyses.

The submitter is urged to make these data

available to others upon request. The purpose is to generate reliable data for transducers to facilitate their introduction into more critical areas of the measurement system

and to provide equity in the market place.

The cost will be about $1500 for the first

and $400 for any other transducer of the same type and range. The test will last for

6 to 8 months.

(g) Research and Development

The National Bureau of Standards will

undertake specific research and development

programs within its mission for government

agencies and other appropriate sponsors.

These may be the development of new principles of measurements, generation of specific data, writing of reports etc. for which

NBS has particular expertise.

(h) Publications

NBS provides information on topics re- lated to the national measurement system in

the form of talks, seminars, reports and

publications. New technical reports are now

in preparation on measurements with piston

gages and manometers and on numerical analysis of data and error computation.

Such publications present also* the results

of development and research at NBS as, for

•example, data on fixed points, phase lines

and equations of state.

(i) Consultations

Consultations are an important part of

the services rendered by NBS. They are generally free of charge. Consultations can

be made by visit to NBS or in the quarters

of another organization, by letter and by

telephone. They cover all matters in which

NBS can provide assistance and may range on one day from the operation of an Italian

cappuccino machine to high-frequency dynamic

pressure calibrations. In these consultations information flows in both directions.

Cooperation of NBS experts in professional

societies could be listed under consultations

as well. Members of the staff are actively

working in a number of organizations, a few

of which are listed in table 11.

Table 11. Committee Memberships

(Pressure Measurements)

ASME Committee on Pressure Gages

ASME Research Committee on Press. Technology.

ASME Power Test Code Committee

ASME Joint Committee for High Press. Technol

ISA Committee on Manometry

The general position of NBS in the InputOutput Transactions Matrix of Table VIII was already discussed in section 2.5. Some additional information is shown in figs. 7 and 8. Fig. 7 identifies schematically the types of

input received and outputs delivered by NBS,

while fig. 8 shows the sources of the input

and the recipients of the output.

Input amp Oum/r by Type

(measured by W invested)

Requests for Laboratory Evaluation -

Performance Tests -

Calibration - Data EvaluationTraining - Information -

Talks and Discussions at Conferences -

Scientific and Technical PapersVisitors AND VisitsContracts -

Creative ThoughtsPressure AND Vacuum Section

— tNSTRl>Em"ATIOM

20% — Calibrations

2Cf^ — CONSULTAT ION ^ 6X — PERFOR^ftNCE TeSTS 5Z— Talks mo Papers

iJX — Laboratory Evaluation

57 — Training 2% — Data Evaluation

Figure 7. Input/Output by Type
Imput and nurpuT by Group 4.2.4 Funding Sources for NBS Services

Individuals Pressure Gage ttANUFAcTu?ERS Pressise Transducer Ianufaourers

Air and Space Industry CtCMiCAL Industry

Industry, oticr

Dec GoVERWtNT, OTHER THAN DOD

lillVERSITIES, feEARCH LABS. Professional Societies

Standards Bodies Foreign Governhjits

PRESSIRE m VAQJIM SEaWI

3°:-

'—277.-

iNDUSTRY 12% Pressure Gage Man. Pressure Transducer H A Air and Space lie. 3? Chemical Industry 27 Other Industry

GOVERWBfT

2SZ Dept. of Defense

2% Other Goverwcnt

-ITSrtltJivEBsiTiES, Research Labs. ~ 5S—

f°W3F, '30C1ETIES, Stanpards B, ] I'Ol ^IjUALS

The development and maintenance of NBS

facilities are conducted with NBS funds with

considerable other agency funds for special

development programs.

The cost of performing calibration ser- vices is charged to the customer.

Figure 8. Input/Output by Group

4.2.2 Users of NBS Services

Table 12 contains a list of users of

NBS calibrations services, which confirms

our earlier statements about the wide-spread

application of pressure in almost all

industries. However, one must keep in mind

that as a rule only the upper echelons of

the calibration hierarchy send standards to NBS for calibration. The manufacturers of

pressure gages listed under 1.) in Table 12 are in this upper echelon and carry a much

reater weight in the system than their numer in the table indicates.

4.2.3 Alternative Sources

For pressures above atmospheric very few

primary standards outside NBS are accessible

to the public. Most of them are placed in

laboratories of the Department of Defense,

which do not provide calibrations for the

public. In the manometric range a large

number of primary standards can be fou d, some of which are available for dissemination of calibrations. Several of the labo a- tories listed in table 8 provide this service.

Ng other source can provide calibrations of the same or lower uncertainty

than NBS. In principle, other laboratories

could set up primary pressure standards and

provide the service, but the calibration and

further development of these standards re- quires technical back-up services, for which

NBS has unique expertise and facilities. It

is therefore not likely that commercially

operated, primary laboratories would be able

to provide adequate services at economic

cost.
Table 12. Listing of users of NBS pressure and vacuum calibration services

1. ) Manufacturers of pressure and vacuum gages

Chandler Engineering Co. Ametek - Mansfield & Green

Pneumercator Co. Seegers Instrument Corp.

Texas Instruments, Inc.

Gil more Industries, Inc. Ruska Instrument Corp.

Exactel Instrument Co. Ideal - Aerosmith

H.J. Green Insts. , Inc. Amtkor

2 . ) DOD

Naval Ship Research & Development

Army Map Service

Dept. of the Navy

Naval Ordnance System

David Taylor Model Basic (Navy)

Dayton Air Force Station

U.S. Naval Air Station

Naval Air Station

Benicia Arsenal (Dept. of Army)

Eastern Standards Lab

Frankford Arsenal (Dept. of Army)

Redstone Arsenal (Dept. of Army)

Naval Ordnance Laboratory

U.S. Army Procurement Office

NASA

Pratt & Whitney

Tooele Army Depot

Newark Air Force Station

Naval Air Station

Harry Diamond Lab

Picatinny Arsenal

3.) NASA

NASA - Langley

NASA - Lewis

NASA - Houston

NASA - Goddard

NASA - MSC

NASA - Cape Kennedy

NASA - Ellington

NASA - White Sands

4. ) Government Agencies (except DOD, NASA)

F.A.A. Sandia Corp.

Tennessee Valley Authority

5. ) Chemical Industry

Union Carbide Corp.

01 in Mathieson Chemical Corp.

Monsanto Research Corp.

Thiokol Chemical Corp.

Du'Pont de Nemours & Co. Manard Molasses Co. Thiokol Chemical Corp.

Tulsa, OK

Solon, OH

New York, N.Y. Barrington, IL Houston, TX Cleveland, OH

Houston, TX Mountain View, CA

Cheyenne, WY

Westbury, N.Y. Brooklyn, N.Y.

Maryland

Maryl and

New London, CT

Pomona, CA

Washington, D.C. Dayton, OH San Diego, CA

Alameda, CA

Benicia, CA

Washington, D.C. Philadephia, PA

Al abama

Maryl and

Fort Monmouth, NJ Greenbelt, MD West Palm Beach, FL Tooele, UT

Newark, OH

Johnsville, PA Washington, D.C.

Dover, N.J.

Langley, VA

Cleveland, OH

Houston, TX Greenbelt, MD. Huntsville, AL Cocoa Beach, FL Ellington, AFB, TX

Las Cruces, NM

Atlantic City, N.J. Albuquerque, NM Chattanooga, TN

East Alton, IL Miamisburg, OH

Elkton, MD Gibbstown, NJ Gretna, LA Brigham City, UT
Du Pont de Nemours & Co. Eldorado Terminals Corp.

Refinery Supply Co. Pacific Molasses Co. Hercules Powder Co., Inc. Union Carbide

Sucrest Corp.

National Molasses Co. Util ities

Detroit Edison Co. Ohio Fuel Gas Co. United Fuel Gas Co. Air and Space Industry

Hughes Aircraft

McDonnell - Douglas Astronautics Co. North American Rockwell Corp.

MCDonnel Aircraft Corp.

Aerojet - General Corp.

Rocketdyne - North America Avia.

Martin Marietta Corp.

Douglas Aircraft Co., Inc. General Dynamics Corp.

Goodyear Aircraft, Corp.

Rocketdyne - North Amer. Avia.

Hughes Aircraft, Co. Republic Aviation Corp.

Boeing Company

Lockheed Aircraft Corp.

Pratt & Whitney Aircraft

Avro, Inc. General Dynamics / Astronautics

Lockheed Aircraft Corp.

Lockheed Aircraft Corp.

North American Aviation

United Air Lines

North American Aviation

Martin - Marietta Corp.

North American Rockwell

Industry (except Chemical, Utilities,

Air and Space)

Varian Assoc. Westinghouse Electric

Raytheon Co. Chicago Bridge & Iron Co. Intercontinental Dynamics Corp.

General Electric

ILC Industries Inc. E-A Industrial Corp,

Industrial Instruments, Inc. Firestone Tire & Rubber Co. Bendix Corporation

Wright Instruments, Inc. Leeds and Northrup Co. AVCO Research & Development

(Allison Div.) General Motors

General Electric Co. Blout Brothers Construction Co. General Electric Co. Cincinnati Milling Mach., Co. Bendix Corporation

Aiken, SC Bayonne, NJ Tulsa, OK

Houston, TX Radford, VA Oak Ridge, TN New York, N.Y. Baltimore, Md.

Detroit, MI Columbus, OH

Charleston, W. VA.

Anaheim, CA

St. Louis, MO

Sacramento , CA

Canoga Park, CA

Denver, CO Santa Monica, CA

Fort Worth, TX Akron, OH Neoshe, MO

Culver City, CA

Farmingdale, N.Y. Seattle, WA Marietta, Georgia

West Palm Beach, FL Arnold Air Force Station, TN San Diego, CA

Sunnyvale, CA

New York, N.Y. Downey, CA

San Francisco, CA

Tulsa, OK

Baltimore, MD Downey, CA

Baltimore, MD Andover, MA

New Castle, DE Englewood, N.J,

Bay St. Louis, MS

Dover, DE Chamblee, GA

Cleveland, OH

Pottstown, PA New York, N.Y. Vestal , N.Y. Philadelphia, PA Wilmington, MA Indianapolis, IN King of Prussia, PA Montgomery, AL Cincinnati , OH

Cincinnati , OH

Baltimore, MD
Research Manufacturing Co. Muirhead Instruments Inc. Bendix Corp.

Astra Corp.

Brown Boveri Corp.

Bendix Corp.

General Electric

Marquardt Corp.

Mel par

General Electric Co. Metal Bellows Corp.

Hildebrant Engineering Co. Curtiss-Wright

Carrier Corp.

Litton Systems, Inc. IBM Hercules, Inc. Rosemont Eng. Co.

9. ) Universities and Research Labs

Carnegie-Mellon University

Argonne National Laboratory

Brookhaven National Lab.

Battel le Memorial Institute

University of Michigan

University of California

University of Oklahoma

Lawrence Radiation Lab.

10. ) Foreign Organizations

Physik - Institut der Univ. Zurich

Australian Government

Phoenix, AZ Mountainside, N.J. Teterboro, N.J. Hatboro, PA New York, N.Y. Kansas City, MO. Ballston Spa. , N.Y. Ogden, UT

Falls Church, VA

Schenectady, N.Y. Sharon, MA Houston, TX Woodridge, N.J. Syracuse, N.J. Woodland Hills, CA

San Jose, CA

Cumberland, MD

Eden Prairie, MN

Pittsburgh, PA Argonne, IL Long Island, N.Y. Columbus, OH Ann Arbor, MI Los Alamos, N.M. Norman, OK

Berkeley, CA

Zurich, Switzerland

Austral ia
4.3 Impact of NBS Services

4.3.1 Economic Impact of Major User Classes

The manufacturers of piston gages benefit

most immediately from pressure measurements

disseminated from NBS standards. They, in

turn, provide the transducer and gage manufacturers with pressure calibrations. From

this second level a large variety of important industries is affected as outlined

in previous sections of this report. Table

13 lists the annual production of a number

of products, which benefit from NBS services.

The large variety of these products, which

require pressure for their operation, for' their manufacture or for safety reasons,

indicates the impact of pressure measurements on economy and society.

4.3.3 Pay-off from Changes in NBS Services

Most recent changes in NBS services for

the national measurement system for pressure have been gradual improvements in

quality (accuracy, detailed information,

training) or range. These changes were aimed

at satisfying requests from the manufacturers

of gages and transducers or other users of

our services.

A rather important change was the

elimination of the vacuum calibration

service. This service had never developed

a high use rate, but extensive consultations

were given to industry on topics related to vacuum measurements. Discontinuing this

service left the vacuum industry without any

type of national standard or central

authority.

4.4 Evaluation of NBS Program

For the pressure range 2 x 10^ Pa (.2psi)

to 4 X 10^ Pa (60,000 psi) NBS has adequate

primary and working standards and a good

dissemination program to the next echelons

in the calibration hierarchy. This is backed

by training, data evaluation, laboratory

evaluation, by writing manuals and technical

notes on state of the art measurements and

by extensive consultations. There is close

cooperation between NBS and the industries

concerned. Adequate plans are in effect for

the expansion of the range and the improvement of the standards.

The adequacy of present services is

measured by comparing the services delivered

with the requests received. These requests

come directly as purchase orders for calibrations, as inquiries or they result from our frequent personal contacts in meetings and

training courses.

Table 13. Annual production volume

of some industries benefiting from NBS services

SIC-code Industry Ann. Prod Year

in M$

- Piston gages 2 1974

- Pressure trans. 26 1973

38 211--47 Pres. and vac. 4 1963

indicators

38 211 46 Pres . ratio indi

2 1963

38 214 98 Mech. pres. gages 53 1963

for motor veh.

38 213 31 Industr. pro. 18 1963

instru. : pres. and

vac. trans.

38 111 08 Altimeters (pres.) 24 1972

10 1963

38 111 10 Air Speed indi

16 1972

(pressure) 3 1963

38 111 13 Rate of climb 10 1972

indicators

38 111 32 Barometers 3 1963

38 111 64 Pres. regul . equip . 19 1972

for combus. , air

pres. , cabin pres. > demand oxy. for aero.

appl ication

38 220 16 Pres. sensitive 22 1963

controls for air

cond., refrig, and

heating

34941 Auto, regul. and 498 1972

control valves for

sophis. instru.

loop

349435 Safety valves 30 1972

3561 Pumps and compres. 2,439 1969

1 ,249 1963

3585 Refrig. mach 4,355 1969

3511 Steam eng. and 1 ,502 1969

turbines 616 1963

3721 Aircraft 12,446 1969

3011 Ti res 4,717 1969

2034 Dried, dehy.

,

481 1969

freeze dried food 335 1963

2813 Industrial gases 649 1969

340 1963

3674 Semiconductors 2,018 1973

1 ,493 1969

623 1963

3671 Electron tubes 1 ,147 1973

Data from Bureau of Census and McGraw-Hill

Market Forecast.
If NBS were to discontinue its pressure

calibration services, then the industry

would find itself in the position of either

setting up their own primary standards laboratories or of gradually moving away from

uniform measurements throughout the country

with chaotic consequences for the gage and

transducer industries and all of their

customers. The maintenance of safety would

become impossible. Setting up good primary

pressure standards is not a feasible

commercial venture as it requires the

sophisticated metrology back-up that only a national laboratory can provide and it re- quires systematic research and development

of new or improved methods. '

4.5 The Future

While present services are adequate from

2 X 10^ to 4 X 10^ Pa except for the manometric range from lO'* to 10^ Pa, there are

no such services above and below this range.

Presently there is very definite and urgent

need for calibration services at lower

pressures and a need for services at higher

pressures is anticipated for the future.

Measurements in the range from 2 x 10^

Pa down to about 10"^ Pa are now urgently

needed for the calibration of flight control

instrumentation, for safety systems on nuclear

reactors, and for a large variety of industrial applications. These needs were

identified and plans to provide the service were made. It is anticipated that a primary standard covering the upper part of

this range will be in operation late in FY 76 and that the rest of the range will be

covered by a new standard late in FY 77

Measurements in the range above 4 x 10^ Pa will be needed in the future by the emerging

new technology for high pressure forming and

extrusion. Again, plans have been made to provide these services in the near future.

These and the plans for expansion of services

into the vacuum range have been discussed with

the Division Advisory Panel and have the

strong support of most members of the panel

Training of personnel in industrial

standards laboratories is a continuing need.

Courses offered by NBS will be extended to include manometry and several manuals will be

issued shortly.

Major new needs arise from the increased

application of pressure transducers, which are rarely well characterized. It is therefore

difficult to choose the right types and to estimate the propagation of errors. Great

inequities exist in the market place. NBS

has therefore developed a transducer evaluation program, which will be offered as

a service beginning in FY 76. At the same time a low-cost calibration service for

transducers and gages will be offered.

A study of the needs for vacuum measurements is still under way and a decision on resumption of vacuum work will be

made later.

5. Summary and Conclusions

The state of the national measurement

system for pressure in the range where NBS

presently provides calibrations and related

services is satisfactory. Needs

for expansion of the range have been identified and most of these needs will be satisfied in the near future.

Rapid changes in the needs of the system

are generated by the increased use of

transducers to transfer measurements, and

plans are being implemented at NBS to provide all necessary services.

No preparations have been made to establish primary vacuum standards at NBS. A Delphi study on vacuum calibration needs is

under way. A management decision must be

made, when the study is completed.
APPENDIX A, METHODOLOGY OF THE STUDY

The study was conducted in five parts:

(1) Investigation of two calibration chains

to the point of end use,

(2) Solicitation of written comments on needs for measurements at very high

pressures

,

(3) Delphi study for vacuum measurements,

(4) Survey of existing literature, particularly the applicable standards,

(5) Personal interviews.

Table 14 contains a list of persons

contacted during this study. It does not

include the participants in the Delphi study,

who will remain anonymous. We are indebted

to all of them, listed and unlisted, for

their often considerable effort to help us

in this assessment.

In the investigation of the calibration

chains we attempted to gain insight into

the details of the dissemination: the

instruments used, the personnel and their

training, numerical methods and error estimation etc. This part of the investigation

caused us to begin training programs and to devote time to write a manual on numerical

evaluation methods. It also showed the

need for measurement assurance and for a reduction in the length of the calibration

chains

Comments on the need for high pressure

calibrations were invited from a number of

researchers and workers in this field.

The names of those contacted are contained

in table 14. The letters provided information about the pressure range, required

accuracy and applications.

The Delphi study on vacuum calibrations

is still under way. It was begun with a letter to a small number of experts asking

for rather detailed answers to a large number of questions ranging from the presently

used source of calibrations to problems

foreseen in the future. This was followed

by the dissemination of the questionnaire

shown in Figs. 9 and 10. More than one hundred of the questionnaires have been

received and evaluated. An example of part

of this evaluation is shown in Fig. 11. It shows under the same key numbers as the

original form the lower and upper quartiles,

the median and the experts estimates as to when a certain measurement is required for

the two application areas. We are presently

working on improving the consensus. The

study will be published separately.

The literature was searched for information

pertinent to this study. Little important

information beyond production numbers and

general statements was found. We also

examined existing standards to find whether

they contained adequate information on methods, instrumentation and numerical

evaluation to achieve their stated purpose.

This is generally not the case, and we are encouraging and helping to make the

necessary improvements.

Visits, telephone conversations, letters,

conferences, and training courses were used

to obtain additional, more detailed or con- firming information. This process is

continuing and the flow of information

(both ways) is probably the most valuable

effect of this study.

N 5 S S ^ - - f = » s !—

- 5 i

!

3 3 a S i

1 !

s

i S

1

!

5 9

!

!

5 i = 3

Figure 11. Delphi Evaluation
Table 14. Persons contacted during

Andeen C, Case Western Reserve Univ.

Andrews W. , Bureau of Mines

Aust R.B., Eastman Kodak

Austin C. , IBM Babb, S.E., University of Oklahoma

Balkosky G., Detection Technology

Ballingall J.M., Varian Inc. Bartz T. , Lockheed

Benson J., Haystings-Raydist

Blevinn G. Brunswick, Corp.

Bone A., RAF. Great Britain

Brecht M., United Airlines

Brown G.V., NASA Lewis Research Center

Bundy P.P. , G.E. Burris L. , Argonne Nat. Lab.

Carbaja B., Texas Instruments

Carrozzo T. , Detection Technology

Cerbs D. , Detection Technology

Charton J., General Foods

Christoe C.W., Feltman Research Lab.

Clapham P., N.P.L. Great Britain

Cohrs G. , Boeing

Costantino M.S., U.S. Military Academy

Council T. , FAA Craig 0., USN, Air Systems Command

Damrel J.B., Texas Instruments

Daniels W.B., University of Delaware

Drickamer H.G., University of Illinois

Dufeu A.N., Smiths Aviation

Duvall G.E., Washington State University

Edwards H.B., NASA Langley

Fabbroni J., Kollsman Instruments

Farrish B., United Airlines

Favell G. , Wallace and Tiernan

Feierabend R. , Gilmore Industries

Frack R.B. , FAA Frisbie D. , Varian Inc. Fritz I.J. , Sandia Lab.

Friz G., Wallace and Tiernan

Fruit J. , Mensor Corp.

Fulton W.C. , NASA Houston

Gray H.L. , NASA Langley

Hablania M.H. , Varian

Hale D. , Wallace and Tiernan

Hall H.T., Brigham Young University

Hardison H.V., DuPont Corp.

Harra D. , Varian Inc. Harris H., EXXON

Hill E., Varian

Hill W.W., Westinghouse

Huntress A., DOW Corning

Irwin J .W. , Boeing

Jamieson J.C., University of Chicago

Jayaraman A., Bell Telephone Lab.

Johnson A. , Dresser Industries

Jones D. , Boeing

Joslyn D.E., Viatran Corp.

Kafalas J. A. , M.I.T.

Kahn E.H., Kollsman Instrument Corp.

Kohatsu I., U.S. Army Materials Research Center

Kopp M., Validyne Corp.

Krause E.J., DOC, Office of International Trade

Kremerskotter F. , United Airlines

Kullman G. , USN, Western Standards Lab.

Lafferty J.M. , G.E. Lange W.J., Westinghouse

Lauer A. , Varian Inc

Layman W. , United Airlines .

the National Measurement System Analysis

Lazarus D. , University Illinois

Lee A. , Boeing

Lewis G.K., E.I. de Pon de Nemours

Liebenberg D.H., Los Alamos Scientific Lab.

Littmann A.J., Amthor Testing Instrument Co. Lombardo J., High Vac. Equipment Corp.

Low G.M. , NASA HDQRT

Lunde A. , Boeing

Lush W., Ideal Aerosmith

Matychuck D. , Rosemont

Meyer H., United Airlines

Meyers D.E., Leybold-Heraeus

Miks C, Ruska Instruments

Musa R.S., Westinghouse

Nerken A. , VEECO

New R. , USAF Newark

Olson L., United Airlines

Osterstrom G. , Sargent-Welch

Overacker W., Temescal Corp.

Patton P. , Leybold

Patterson D. , Boeing

Ralston K. , Philadelphia Electric Company

Redhead P. A. , N.R.C. Canada

Reed W. , Republic Steel

Reesby C. , Texas

Riley J., NASA Kennedy Space Center

Romana A.R., Kollsman Instrument Corp.

Rudolph J.F. , FAA Rush D. , Varian Inc. Russo F. , Brunswick, Corp.

Samara G.A., Sandia Lab.

Sanford T.W. , FAA Scheve G., United Airlines

Schirber, J.E., Sandia Lab.

Schrager J . , FAA Schultz R., USN, Pomona

Smerke J.J., Philadelphia Electric Company

Smith C.S., University of North Carolina

Sobolewski R. , Republic Steel

Southern R.L., Allegheny Ludlum

Spengl er C. ,• Boeing

Spetzler H., Sandia Lab.

Steffens B., EXXON

Strauss T., Wallace and Tiernan

Street W.B., U.S. Military Academy

Struisa F. , G.E. Stubblefield R.L., NASA HDQTR

Taylor W., Sandia, AEC

Tongas D. , Boeing

Twigg W.P., Westinghouse

Vedam K. , Penns. State University

Vissers D., Argonne Nat. Lab.

Waterman H., FAA Wheeler W., Varian Inc. Whitlock M., United Airlines

Wolfe R. , MKS Instruments

Wolny R., Oak Ridge Nat. Lab.

Woolrich R. , Ideal Aerosmith

Zielesch E., Ford Motor Company

U.S. DEPARTMENT OF COMMERCE

National Bureau of Standards

Washington. D C. 20234

Dear Sir:

In May of 1973 I asked a small number of experts in the field of vacuum for information on vacuum calibration practices and needs. The reason for this request for information was explained in a letter, a copy of

which is included with today's letter. The response to our request exceeded our expectations both in the

number of people responding as well as in the completeness of the information provided. We are very much indebted to all of you who contributed. We have studied the information and have designed from it

the enclosed Delphi questionnaire. This is the first of two or possibly three communications which we

shall use to establish consensus about the time at which certain vacuum measurements become critical.

In any given application of vacuum in an industrial process there will come a time when an accuracy of.

say, 1% in a certain range, for example 10'^ to lO"* torr, must be reached to obtain optimum results. If

the accuracy is not reached the measurement of vacuum becomes a technological barrier for the given

process. We want to develop a best estimate for the dates at which certain accuracies in certain pressure

ranges must be attained for a number of important processes or applications.

Our Delphi questionnaire lists several important applications of vacuum. For each process there is a

choice of four vacuum ranges, and within each range there is a choice of three accuracies to be attained. We would like you to enter the year in which you think a given accuracy and range wi II be required for the

processes listed. Let me use the manufacture of widgets (row 16) as an example. Widgets, according to

Webster's, are unnamed articles considered for purposes of hypothetical example as the typical product of

a company. In the Delphi questionnaire 1 identified myself as an expert on widgets by checking col. 3. I

then selected the range 10"^ to lO''' torr as the most important range for widget production, ana I know that

as early as 1968 an accuracy of about 1% in this range would have been desirable for economic production.

That is not to say that widgets were not made, but a better and more uniform product could have resulted. So I entered "1 968" in col. 12. Of course, all of us who are f ami liar with widget production, know that

improvements have been made but that no later than 1975 an accuracy around .1% in the same pressure range

must be met to ensure economical production. Therefore, that year was entered in col. 11. Similarly I

entered my estimates for the time when accurate measurements at much lower pressures will be needed. I

also checked col. 18 to indicate that reactive gases are used in the process. Please attempt to fill in all columns as appropriate.

We would now ask you to consider the real industrial processes listed and to enter your best estimates.

After the questionnaires are returned we will determine the interquartile ranges and the median year for

each application. The second Delphi round will include this information, thus allowing the respondents

to revise their estimates in the light of the panel's earlier response.

We may have left out processes that you believe are important. Please use rows 13 to 15 to enter those. We may then ask others to give their best estimates for these additional applications.

Please identify yourself on the form so that I can contact you again about specific questions. The information provided by you will be kept confidential. Only myself. Dr. J. Seeds, Scientific Assistant to the

Director, and Dr. H. Oser, Chief, Mathematical Analysis Section, will have access to the questionnaires.

Only statistical data will be published. All participants in this Delphi study will receive a copy of our

final report.

If you have any further questions, please, do not hesitate to call me on (301 ) 921 -2121

Ypucs sincerefly.

Peter L.M. HeydemaW Chief

Pressure and Vacuum Section

gure 10. Instructions for Delphi Questionnaire