NIST Measurement Services:

NEW NIST PUBLICATION

April 20, 1989

High Vacuum Standard

and Its Use

Sharrill Dittmann

U.S. Department of Commerce

National Institute of Standards and Technology

(formerly National Bureau of Standards)
Center for Basic Standards

The Center for Basic Standards develops and maintains the scientific competencies and laboratory facilities nec- essary to preserve and continue to refine the base physical quantities upon which the Nation's measurement

system is constructed. The Center is responsible for the standards of voltage, current, resistance and

impedance, temperature, vacuum, leak rate, pressure, mass, length, time and frequency. The Center collaborates closely with the standardizing laboratories of other countries to ensure that measures used in international

trade are fully compatible. The Center also performs research at the frontiers of physics in order to improve

measurement capability and quantitative understanding of basic physical processes that underlie measurement

science.

Electricity Division

Maintains and improves the national standards of electrical measurements; conducts experiments to realize the

electrical units in terms of the mechanical units; develops stable electrical standards and improved methods for the dissemination of the units of electrical measurements; determines fundamental physical constants related to

electrical units; develops instrumentation for precise electrical measurements directly referenced to the national

standards; implements techniques for their use and puts them into practical applications; assists in applying scientific and technological development to measurement problems, especially in other areas of NIST research.

Temperature and Pressure Division

Maintains and improves the national standards of temperature and pressure; conducts research towards the development of new concepts for these standards; calibrates practical standards for the U.S. scientific and technical community in terms of the primary standards; develops methods and devices to assist user groups in the

assessment and enhancement of the accuracy of their measurements of temperature and pressure; prepares

and promulgates evaluations and descriptions of temperature and pressure measurement processes; coordi- nates these standards and measurement methods nationally and internationally; and determines the accuracy of

the national standards of temperature and pressure with respect to fundamental thermodynamic relations.

Length and Mass Division

Develops and uses the competence necessary to implement a more accurate measurement system for length

and mass; applies research, utilizing the most advanced technology available, to the improvement of our present

standards and devises new standards and new methods for their transfer. Time and Frequency Division

Maintains, develops, and improves the national standards for time (which are coordinated with the United States

Naval Observatory) and frequency, and the time scales based on these standards; carries out research in areas

of importance to the further fundamental improvement of frequency standards and their applications focusing on

microwave and laser devices, atomic and molecular resonances, and the measurement of fundamental physical

phenomena and constants; adapts time and frequency standard devices and concepts to special scientific and

technological demands; develops time and frequency measurement methods in the radio frequency, microwave,

infrared, and visible radiation regions; coordinates the national time and frequency standards, time scales, and

measurement methods nationally and internationally in conjunction with the United States Naval Observatory;

operates time and frequency dissemination services, such as radio stations and broadcasts, for the purpose of

traceability to the national standards of time and frequency.

Quantum Piiysics Division

Engages in research in atomic and molecular physics at the forefront of the field, and performs basic, highly accurate measurements and theoretical analyses that help establish a reliable foundation for scientific and technological measurement and data.

Quantum IVIetrology Division

Engages in forefront research in quantum metrology; contributes to new determinations of the fundamental

physical constants, and the extension and refinement of the electromagnetic scale and devises, where possible,

tests of basic symmetries and invariances.
NIST MEASUREMENT SERVICES:

High Vacuum Standard and Its Use

Sharrill Dittmann

Center for Basic Standards

National Measurement Laboratory

National Institute of Standards and Technology

Gaithersburg, MD 20899

March 1989

U.S. DEPARTMENT OF COMMERCE, Robert A. Mosbacher, Secretary

Ernest Ambler, Acting Under Secretary for Technology

NATIONAL INSTITUTE OF STANDARDS AND TECHNOLOGY

Raymond G. Kammer, Acting Director

NOTE: As of 23 August 1988, the National Bureau of

Standards (NBS) became the National Institute of Standards and Technology (NIST) when President Reagan signed into law the Omnibus Trade and

Competitiveness Act
Library of Congress Catalog Card Number. 88-600605

National Institute of Standards and Technology Special Publication 250-<34

Natl. Inst. Stand. Technol. Spec. Publ. 250-34, 68 pages (March 1989)

CODEN: NSPUE2

U.S. GOVERNMENT PRINTING OFFICE

WASHINGTON: 1989

For sale by the Superintendent of Documents, U.S. Government Printing Office, Washington, DC 20402-9325
PREFACE

Calibrations and related measurement services of the National Institute of

Standards and Technology provide the means for makers and users of measuring

tools to achieve levels of measurement accuracy that are necessary to attain

quality, productivity and competitiveness. These requirements include the

highest levels of accuracy that are possible on the basis of the most modern

advances in science and technology as well as the levels of accuracy that are necessary in the routine production of goods and services. More than 300

different calibrations, measurement assurance services and special tests are available from NIST to support the- activities of public and private

organizations. These services enable users to link their measurements to the

reference standards maintained by NIST and, thereby, to the measurement

systems of other countries throughout the world. NIST Special Publication

250, NIST Calibration Services Users Guide , describes the calibrations and

related services that are offered, provides essential information for placing

orders for these services and identifies expert persons to be contacted for

technical assistance.

NIST Special Publication 250 has recently been expanded by the addition of

supplementary publications that provide detailed technical descriptions of

specific NIST calibration services and, together with the NIST Calibration

Services Users Guide, they constitute a topical series. Each technical

supplement on a particular calibration service includes:

• specifications for the service

• design philosophy and theory

• description of the NIST measurement system

• NIST operational procedures

• measurement uncertainty assessment

error budget

systematic errors random errors

• NIST internal quality control procedures

The new publications will present more technical detail than the information

that can be included in NIST Reports of Calibration. In general they will

also provide more detail than past publications in the scientific and

technical literature; such publications, when they exist, tend to focus upon

a particular element of the topic and other elements may have been published

in different places at different times. The new series will integrate the

description of NIST calibration technologies in a form that is more readily

accessible and more useful to the technical user.

The present publication, SP 250-34, NIST Measurement Services: The National

Institute of Standards and Technology High Vacuum Standard and Its Use, is one of approximately 20 documents in the new series published or in preparation by

the Center for Basic Standards. It describes calibration technology and

procedures utilized in connection with NIST Service Identification Numbers

from 30030C to 30050S listed in the NIST Calibration Services Users Guide

1986-88/Revised (pages 45-46) . Inquiries concerning the contents of these
documents may be directed to the author or to one of the technical contact

persons identified in the Users Guide (SP - 250)

.

Suggestions for improving the effectiveness and usefulness of the new series

would be very much appreciated at NIST. Likewise, suggestions concerning the

need for new calibration services, special tests and measurement assurance

programs are always welcome.

Joe Simmons, Chief

Office of Physical Measurement Services

Katharine Gebbie, Director

Center for Basic Standards

iv
TABLE OF CONTENTS

Page

Preface iii

I. Introduction 1

II. Description of the NISI Vacuum Calibration Service 1

III. Theory, Design Philosophy and Description of the Calibration

System 2

The Chamber 3

The Flowmeter 4

The Orifice 6

Extension to Lower Pressures 8

Calibrations at Higher Pressures 9

Component Selection Criteria 11

IV. Operational Procedures for Calibration Using the NIST High

Vacuum Standard 15

Installation and Preparation of Gages 16

The Day Prior to the Calibration 17

The Morning of the Calibration 18

Measured Flow to Upper Chamber 19

Measured Flow to Lower Chamber 20

Steady Flow Generator 20

Shutting Down 21

Venting the System 21

Data Analysis 21

V. Assessment of Uncertainties 23

Systematic Errors 23

Random Errors 27

Stjmmary of Errors 28

VI. Quality Control 30

Comparisons Against Additional NIST Primary Standards 30

International Comparisons 31

Continuous, Repeated Calibrations of Vacutun Gages 32

Additional Checks 33

VII. Needs and Plans 34

VIII . Acknowledgments 35

IX. References 36

X. Bibliography 37
TABLE OF CONTENTS rCONTINUED)

Appendices

A. Sample Report of Calibration for Ionization Gage

B. Sample Report of Calibration for Molecular Drag Gage

G. Dimensions and Conductance of the Orifice in the NIST

Dynamic Expander

vi
LIST OF TABLES

Page

I. Stunmary of the vacuum calibration service 2

II. Primary standard configurations used for calibrations 15

III. Primary standard nonllnearltles 25

IV. Uncertainties of the NIST high vacuum standard with nitrogen 29

V. Results of first International Intercomparlson 31

VI. Ion gage sensitivities, nitrogen gas 33

LIST OF FIGDRES

1. Schematic of the primary high vacuum standard 39

2. Schematic of the vacuum chamber 40

3. -Axial section of the orifice plate 41

4. Representative measured low-pressure nitrogen sensitivities for

different Ion gages 42

5. Deviations from Initial value for calibrations of five molecular

drag gages 43

6. Ion gage sensitivity in nitrogen for two ultra-high vacuum

ionization gages 44

7. Detail of central duct through orifice plate 45

vii

I INTRODUCTION

Vacuum measurements are an important tool for many segments of the

American economy and for American defense efforts. The accuracy of these

measurements affects a large variety of industries ranging from space simulation to semiconductor processing. The National Institute of Standards and

Technology provides calibrations for a variety of vacuum gages over a broad

range of pressures - from atmospheric pressure (about 10^ pascal) to high

vacuum (about 10"^ pascal).

This document presents an in-depth discussion of the NIST primary high

vacuum standard, used between 10"^ and 10"^ Pa. Included are discussions of

the theory, design, and construction of the standard. In addition, the systematic and random errors in the standard and the methods used to check the

accuracy of the standard are presented.

Also included is a brief discussion of the molecular drag gage and its

performance as a transfer standard between 10"^ and 10"^ Pa.

II. DESCRIPTION OF THE NIST VACUUM CALIBRATION SERVICE

The National Institute of Standards and Technology provides calibrations

of suitable high vacuum gages over the range 10"^ to 10"^ Pa (pascal) (10"^ to

10""^ Torr) using gases compatible with the standard. Gages must be stable and

capable of precision at the level of a few percent to be considered suitable

for calibration. Molecular drag gages (MDGs) are calibrated against the NIST

primary high vacuum standard at 5x10"'' Pa (4x10'^ Torr). Hot cathode

ionization gages (IGs) are calibrated against calibrated molecular drag gages over the range 10"^ to 10"-^ Pa. They are calibrated against the NIST primary

high vacuum' standard over the range 10"^ to 10'^ Pa (10"^ to 10'^ Torr).

These services are summarized below:

1
TABLE I. Summary of the Vacuum Calibration Service

Sensor

Calibration

Range Standard Uncertainty

Pa percent

MDG 5x10-3 Primary high 1.4-1.6

vacuum

IG

IG

10"^ - 10-1

10-6 . iQ-t,

Calibrated MDG

Primary high

vacuum

1.4-2.6

1.4-5.2

If calibration of a gage other than a hot cathode ionization gage or a molecular drag gage is desired, contact the Temperature and Pressure Division

at the address below.

The Report of Calibration for an MDG gives the effective accommodation

coefficient or calibration coefficient of the rotor, and a discussion of the care and use of the gage for best results. The Report of Calibration for an ionization gage gives the correction factor for the gage's indicated pressure

as determined as a function of pressure and the characteristics of the con- troller (grid and filament biases and electrometer performance if applicable)

,

and provides a discussion of the treatment and use of the gage for best

results. See Appendices A and B for sample reports.

For information concerning these services or to schedule a calibration,

please contact Temperature and Pressure Division, Bldg. 220, Room A55,

National Institute of Standards and Technology, Gaithersburg , MD 20899.

III. THEORY. DESIGN PHILOSOPHY AND DESCRIPTION OF THE CALIBRATION SYSTEM

High vacuum calibrations are performed at the National Institute of

Standards and Technolgoy (NIST) with the Primary High Vacuum Standard - an orifice-flow system (see fig. 1) - between 10-^ and 10-^ Pa and against a group of calibrated molecular drag gages (also called spinning rotor gages)

between 10"'' and 10" ^ Pa. The three main components in the primary standard

are the vacuum chamber, the flowmeter and the orifice. Pressures are generated by producing a known flow of gas which passes through an orifice of

known conductance, thereby producing a known pressure drop across the orifice.

This pressure drop is calculated from first principles and constitutes a primary standard. The details of the system are discussed below.

2
THE CHAMBER

The design of the vacuum chamber, illustrated in figure 2, is largely

determined by the need to maintain a low background pressure, to provide room

to mount the gages to be calibrated, and to maintain a geometry where the

orifice conductance calculations will be valid and the necessary degree of

pressure uniformity can be assured. The chamber is in two cylindrical halves,

each 27 cm in diameter and 34 cm long, separated by a central wall with the

orifice in the center. This size chamber permits the incorporation of eight

3.5-cm i.d. mounting ports on the upper half 13.3 cm above the plane of the

orifice. A circular baffle 5 cm below the top of the upper chamber insures

that gas molecules entering through the top of the chamber from the flowmeter

will experience several collisions with the chamber walls before entering a gage or passing through the orifice at the bottom. These collisions are essential to insure the random distribution of molecular velocities, which is

necessary for pressure uniformity and which is assumed in the conductance

calculation

.

As discussed below, the uniform distribution of molecular flux will be

perturbed by the molecules entering the chamber from the inlet line and by

those escaping through the orifice. These perturbations can be kept within

acceptable bounds by using an orifice whose area is a small fraction of the

surface area of the chamber. At the same time, the orifice conductance must

be large enough to maintain a low background partial pressure and ensure a high enough flow that gage pumping and outgassing are minor perturbations. In our system, the compromise used is a nominal 1.1 -cm diameter orifice,

described in detail below, with a nitrogen conductance of about 12x10"'^ m'^s'^

and an area of 0.03% of the chamber surface.

Because of the finite pumping speed available at the lower chamber, some fraction of the molecules passing through the orifice from the upper chamber

will return back through the orifice. This is a second order perturbation

that can be readily accounted for if these returning molecules originate from

a uniform flux distribution. This is achieved by making the lower chamber an almost symmetric duplicate of the upper chamber, with a baffle 12 cm above the

inlet of the nominal 0.5 m^sec"^ turbomolecular pump attached to the bottom of

the chamber. The chamber is not perfectly symmetrical as the baffle is

located 24 cm below the orifice (the top baffle is 29 cm above the orifice)

.

The effective rate of exhaust from the lower chamber has been measured to be

0.3 m^ s~^ for nitrogen. The pump speed is somewhat limited by the baffle in

3
the lower chamber and by the 15-cm i.d, connection between the lower chamber

and the pijmp. The turbomolecular pump was chosen for its stable high pumping

speed, low base pressure, and compatibility with all gases.

Ultra-high vacuum construction practices are employed throughout the

chamber; the chamber material is type 304 stainless steel, only all-metal

seals are used, and the chamber and all gages are baked at. about 250 °C after

each venting to air. However, the small orifice clearly would restrict the

attainment of a low base pressure in the upper chamber. Therefore, the

orifice is contained in a plate that seals into a gallium-filled groove

located in the central wall as illustrated in figure 2. During pump-down or bakeout the orifice plate can be lifted through a bellows at the top of the

chamber, opening a 12.7-cm hole between the upper and lower chambers. For the

gallium to form a seal when the orifice plate is lowered, it must remain

supercooled for prolonged periods below its 302.95 K melting temperature.

Should the gallium freeze, the edge of the orifice plate would fail to make an integral seal with the central wall. Leaks of 10 cm^ s~ or larger will have a significant effect on our measurements. In 4 years of operation we have never observed any erratic calibration data for our check standards which would

indicate unreproducible sealing or failure of the seal. Nor have we seen any

evidence of gallium vapor in the chamber, which is consistent with gallium's

predicted vapor pressure of 10"^® Pa at room temperature.

This system routinely attains base pressures of about 10"^ Pa, the

residual gas being almost entirely hydrogen. The base pressure in the upper

chamber increases by about a factor of two when the orifice plate is lowered

and sealed in the gallium groove.

THE FLOWMETER

The flowmeter is used to generate a known flow of gas through the orifice

which produces a pressure drop across the orifice. The differential pressure

thus produced plus the pressure below the orifice is measured by the gage

under test. The flow is produced by a P(dV/dT) method in which gas is allowed

to leak out of a reservoir which is maintained at a constant pressure by

changing its volume through the advance of a piston. The time taken for the

piston to advance a known distance is measured and the flow is thus known.

This use of the orifice-flowmeter system offers three distinct advantages over

a static expansion system. First, because this is a steady flow system,

transient behavior associated with adsorbtion and desorbtion become negligible

4
after a sufficiently long time following establishment of a steady gas flow.

Second, stable pressures are readily generated for long periods of time.

Third, the only dimensions which need to be characterized are those of the

orifice and the volume of the piston, and these are not dependent on the

position of valves. A detailed examination of the flowmeter is found in

Ref.l.

Before examining the flowmeter, we briefly review the equations used to

calculate the flow rate of the gas and the pressure drop across the orifice.

The equations used are:

for the throughput Q,

Q = Pf^[(dV)/(dt)](T,i,/Tf J in Pa m^s'i (1)

for the orifice conductance C,

C = C*(T^h/298.15 K)i/2 in m^ s" ^ (2)

for the pressure at the upstream side of the orifice,

P = (Q/C)Rp/(Rp - 1) in Pa (3)

where Q is the throughput of the gas, C* is the conductance of the orifice, (I

in fig. 1) at 298.15 K for the gas in question, P is the pressure on the

upstream side of the orifice, and Rp is the ratio of the pressures in the

upper and lower chambers. The superscript refers to quantities calculated

at 298.15 K, the subscript "fm" refers to quantities measured in the flowmeter

and the subscript "ch" refers to quantities measured in the chamber.

All letter references in the following discussion are those indicated in

fig. 1.

Q is generated as follows. Initially, two interconnected reservoirs (B

and E) are filled to the same pressure from a ballast volume (A) . Gas flows

from the ballast volume into the reference reservoir (B) and through the

working reservoir (E) into the chamber (H) . The rate of flow into the vacuum

chamber is regulated by an all-metal leak valve (G) . To initiate a calibration, the reference reservoir (B) is isolated. At this time, P^^ is measured

in the reference reservoir with a capacitance diaphragm gage (CDG) (C) . P^

is between 0.07 and 133 kPa and is within the range where the CDGs have

adequate accuracy. After the reference reservoir is sealed, the flow from the

ballast volume continues into the chamber through the working reservoir (E)

.

Gas purity is continuously monitored with a residual gas analyzer. No special

precautions are taken once the gas leaves the high-pressure supply cylinder,

but for some gases, it has been found necessary to purchase high purity

samples.

5
The next step is to isolate the working reservoir (E) from the ballast

volume so that flow is out of the working reservoir alone into the chamber.

This flow of gas will tend to reduce the pressure in the working reservoir but

constant pressure is maintained by advancing a piston (F) to displace volume

in the working reservoir. The piston enters the working volume through an elastomer and PTFE sliding seal. A differential CDG (D) reads the pressure

difference between the two reservoirs and the piston is advanced by a stepping

motor in such a manner as to keep the reading of that CDG at zero. For optimum sensitivity and control, the volume of the piston should be a large fraction of the volume of the working reservoir. This has been achieved

without sacrificing a wide range of available flow rates by constructing two

working reservoirs in series. Each reservoir has its own piston and the

piston areas are 0.8 cm^ and 5.1 cm^ . The piston not in use during a measurement is driven into its working reservoir in order to minimize dead

volume in the flowmeter. We shall refer to a single working reservoir and

piston in this document with the understanding that there are actually two.

The piston is allowed to travel 2.54 cm as measured by a micrometer

screw, at which time the measurement is terminated. The time needed for the

piston to complete its travel is monitored and displayed after each turn of

the micrometer screw. The temperatures of the working reservoir, T^^, and the chamber, T^^^, are

rp'^orded before and after the calibration as is the pressure in the reference

reservoir, Pf m

.

THE ORIFICE

C in eqs (2) and (3) is the conductance of the orifice (I in fig..l).

From theory and dimensional measurements, the conductance of the orifice

corresponding to the chamber temperature is calculated for each gas , as indicated in eq (2).

The orifice was designed so that its conductance could be easily

calculated. In addition, it was desirable that its area be a small fraction

of the vacuum chamber area and that the desired pressures in the vacuum

chamber correspond to the accurate range of the flowmeter.

The orifice, is a sharp-edged hole at the center of a 15.2-cm diameter,

1.27-cm thick, stainless steel plate (fig. 3). The outer edge of the plate

includes a lip (K in fig. 3) that fits into the gallium-filled groove in the

wall between the chamber halves. The plate was fabricated by machining the

6
upper side to a depth of 0.61 cm from the center out to a diameter of 8.89 cm,

and a centered 1.12-cm diameter hole was bored through the plate. The

underside of the plate was machined to form a hollow, truncated 80° half- angle

cone, leaving a thickness of 0.74 mm at the edge of the center hole. The

orifice edge region was formed by lapping the hole with a steel ball of

1.587-cm (0.6250 in) diameter to nominally equal depths from above and below.

Upon final lapping with an unworn ball, the two concave spherical surfaces

thus generated met at a sharp circular edge with a measured diameter of 1.1240

cm (0.4425 in ±0.0003 in) and a computed edge angle of 90.1°. Microscopic

examination revealed a clean, sharp, burr- free edge.

Calculation of the orifice conductance is based on the molecular

transmission probability approach initiated by Clausing [2]. It assumes free

molecular flow and cosine- law scattering at surfaces. A full derivation of

the orifice conductance is given in Appendix C. From the work in Appendix C and the dimensions of the orifice, we derive

C* = 0.2464-S cm^s'^ (4)

where S is the mean molecular speed in cm s' ^ . The overall uncertainty in C*

is estimated to be ±0.18%, with the largest contribution being 0.14% due to

the uncertainty in the measured value of the radius of the hole.

Knowing the orifice conductance, C, and the throughput, Q, of gas, we can obtain the pressure difference across the orifice,

Pu - Pl = Q/C, (5)

where Py and Pl designate the pressures in the upper and lower chambers.

Because of the finite pumping speed below the orifice, P^ is not negligible:

Pl is 3.7% of Pu for N2 in our system. It is difficult to measure P^ directly

since this would require a gage calibrated below the range of pressures

generated by the standard. However, if we can measure the ratio, Rp , of the

pressures above, Py , and below, Pl , the orifice,

Rp = Pu/Pl. (6)

then from eqs (5) and (6) we have

Pu = QRp/C(Rp-l). , (7)

The pressure ratio Rp is measured by a molecular drag gage that, as shown in

figure 2, can be connected through 1 1/2 - in '(3.8 cm) bakeable valves to one of the gage ports in the upper chamber or to a similar port in the lower

chamber. Since the MDG operates at the temperature of the gas and is

reproducible at the 1% or better level above 10"^ Pa, it can be used to

determine Rp quite precisely in the high vacuum range. The value of Rp/(Rp-l)

is quite Insensitive to errors in Rp . A more significant problem is that as

7
the pressure is reduced, the random error of the MDG, typically 10"^ Pa or greater, precludes its use for a direct determination of Rp . Rp will be a function only of the orifice conductance and the pump speed, and in the

molecular flow range, the only variable can be the turbomolecular pump speed.

As detailed below, we have indirectly found the pump speed, and therefore Rp

,

to be constant to within a few percent down to a lower chamber pressure of

3x10-8 Pa

EXTENSION TO LOWER PRESSURES

The lowest usable pressure generated with an orifice-flow standard is

determined by the lower limit of acceptable performance of the flowmeter and

the base pressure of the vacuum chamber. In the case of the NIST system, this

limit is determined by the behavior of the flowmeter.

The base pressure is normally 10" ^ Pa and, were it possible, the useful

range of the standard could be extended down to 10"^ Pa with the residual gas

pressure constituting no more than 1% of the calibration pressure. This has

effectively been done by injecting the gas flow into the lower half of the

vacuum chamber, which permits using a higher gas flow to generate a given

pressure. As a result, the usable range of calibration pressures has been

extended two decades lower at the cost of slightly increased uncertainties in

the generated pressure. The pressure gradients in the lower chamber preclude

the direct calculation of the resultant pressure in the upper chamber.

However, we can experimentally determine the ratio of the flows, Rp , into the

two chambers required to generate the same pressure in the upper chamber. A

pressure is established by a measured flow, Qy , from the flowmeter into the

upper chamber. Using the valves identified as K in figure 1, the flow, is then

redirected from the upper chamber to the lower chamber; the flow is then

increased to a value Ql until the same pressure as before is established in

the upper chamber. This measured flow is used to derive Rp = Ql/Qu • Equation

7 is then modified to give

Pu = QlRp/[RfC(Rp-1)]. (8)

Molecular drag and ion gages in the upper chamber are used to determine Rp

.

There is a random error in the determination of Rp of the order of 0.1%. More

importantly, there is an additional systematic uncertainty due to the two

flowmeter measurements required. This effectively doubles the contribution of

the flowmeter error in eq (8) . In the NIST standard, Rp differs from Rp by

8
less than 1% although this may be fortuitous since flux and pressure gradients

of several percent are known to exist in the lower chamber.

The flow ratio cannot be directly measured for pressures lower than those

that can be reliably generated by a measured flow into the upper chamber,

which is 10"^ Pa for a 0.8% random uncertainty. However, since the system is

clearly in the molecular flow regime at these lower pressures, Rp will be

constant with decreasing pressure if the turbomolecular pump speed does not

change with pressure. As described below we have indirect evidence that the

pump speed does not change significantly down to 3x10'® Pa. On one occasion

we found that the turbomolecular pump speed had decreased over a period of

weeks as the bearings deteriorated, causing a noticeable decrease in the pump

rotor speed and in Rp

.

CALIBRATIONS AT HIGHER PRESSURES

Because of the increasing deviations from conditions of molecular flow at

pressures above 10"^ Pa, calibrations in this range are performed against

molecular drag gages which have been calibrated against the primary standard.

Molecular drag gages are used because of their linearity, reproducibility, and

precision in this pressure range. In brief, the molecular drag gage consists

of a magnetically levitated rotor inside the vacuum chamber which is

accelerated to about 400 Hz. It is then allowed to rotate freely and the

pressure is indicated by the deceleration of the rotor as gas molecules strike

its surface (the molecular drag). There is an additional slowing due to eddy

currents induced in the rotor by the magnets in the monitoring and suspension

circuitry which is referred to as the residual drag.

Pressure is computed from the deceleration rate according to the

following formula:

P = (K/a)(-w/w - R.D.) (9)

where w is the rotational frequency of the ball, w/w is the relative

deceleration, R. D. is the residual drag, and K is a constant which is a function of the ball density and diameter, the gas in use, and the

temperature. The effective accommodation coefficient, a, is determined when

the MDG is calibrated. The orifice conductance and MDG are both sensitive to

the molecular weight of the calibration gas. A thorough discussion of the

theory of MDGs may be found in Refs. 3 and 4. A discussion of techniques for

using the MDG may be found in Refs. 5 and 6.

9
MDG performance at pressures above 10"^ Pa is limited by nonlinearities

arising from interactions between the gas surrounding the ball and the walls

of the steel thimble which contains the ball in the vacuum chamber. The

resulting changes may be empirically treated but NIST currently has no facility for evaluating this effect.

Comsa, et al. [7], have investigated the linearity of the MDG at

pressures as high as 2.0 Pa using the primary vacuum standard (a static

expansion system) at the Physikalisch-Technische Bundesanstalt (PTB) . Their

results show nonlinearities in the MDGs of 0.3% at 10"^ Pa. The resulting

errors in pressure are much smaller than those generated by the nonlinearities

in the orifice conductance at these pressures, hence the use of the MDGs as transfer standards. As a general practice, we do not calibrate vacuum gages

above 10"^ Pa using either the primary high vacuum standard or MDGs. The

ultrasonic interferometer manometer is used at these higher pressures.

Of greater concern is the increasing random noise in the residual drag of

the MDG as the pressure is decreased toward 10" Pa. We have investigated the errors in this range by calibrating IGs with both the orifice-flow system and

with MDGs at pressures near 10" Pa. Differences are less than 2% for IG

measurements

.

When using the MDGs as transfer standards, the flowmeter is simply

treated as a gas source used to establish a steady flow of gas into the

chamber. It is not important to know the flow rate, only that it is stable.

The pressure in the chamber is measured by the MDGs, and the orifice serves to ensure a uniform pressure in the calibration chamber. The upper to lower

chamber pressure ratio is computationally unimportant although a change would

imply some degradation of the calibration system. The stability of the flow

over the coUrse of the measurement and a low base pressure are the most

important qualities of the system in this configuration.

With a stable flow, the pressure is measured by the MDGs and readings are taken on the customer's ion gage. Because the MDG is a differential gage, the

customer's gage reading must be corrected to reflect the fact that it reads

total pressure while the MDG reads only the pressure above the base vacuum. This is accomplished by subtracting the base vacuum reading of the ion gage

from the reading at the calibration pressure before the correction factor is

computed.

10
COMPONENT SELECTION CRITERIA

Each part of the calibration system must meet certain performance

criteria to insure that the system is operating under optimum conditions . The

chamber and orifice criteria are discussed above. The three remaining parts

of the system are the molecular drag gages, the flowmeter, and the vacuum

pumps. The criteria that have determined the design and selection of

components are discussed below.

Molecular Drag Gages

Molecular drag gages are used as transfer standards to perform

calibrations between 10"'' and 10"^ Pa. Important characteristics of a transfer standard are long and short-term stability, ruggedness, and

predictability. The molecular drag gage combines all of these

characteristics. While there may be small effects on ball performance due to

the vacuum thimble containing the ball and due to the controller, the major

predictor of performance is the ball itself. The ball must contain some magnetic material and must be slightly aspherical in order that the spin and

magnetic axes not align when the ball is rotating. It is the horizontal

component of the magnetic field which produces the signal for the controller.

However, if this asphericity is too large, the residual drag will have a large

frequency dependence. This can be taken into account mathematically but may

reduce accuracy at the lower pressures. In practice, commercially available

steel ball bearings of diameter 4.5 - 4.76 mm may be used as rotors. These

may be either shiny or etched and made of either normal steel or 400 series

stainless steel.

The MDG performance is limited at the lowest pressures by the stability

of the residual drag and its measurement (the offset correction) [8], Factors

affecting it are slow temperature changes, vibration, signal strength and

timing circuitry errors. The factor having the largest impact on system

design is the necessity to minimize vibrations. Temperature changes affect

MDG performance as the dimensions of the ball change. The fact that the ball

is contained in a vacuum helps to reduce the rate of temperature change.

The residual drag is equivalent to an apparent pressure reading of

10"^ Pa or more and its stability is important. Every effort is made to keep

random errors no larger than 10"^ Pa. At this level of stability, the MDG is

capable of performance at the 1% level at pressures as low as 10"* Pa.

11
The performance of these gages is discussed in the section on errors. With careful use, the calibration of the gage will change no more than 2% over

a 2-year period.

The Flowmeter

, All letter references in this section are to figure 1.

The Gases

Because the orifice conductance, the MDG constant K, and the IG

sensitivity are all dependent on the molecular mass of the calibration gas,

gas purity is important. The gas composition in the chamber is routinely

monitored with a residual gas analyzer to check for impurities. The

calibration gases are used as purchased with no further purification and the

purity levels have been more than adequate.

Capacitance Diaphragm Gages

The pressure in the reference reservoir is measured with capacitance

diaphragm gages (C) . Two, with different ranges, are used. Accuracy and

long- term stability are the most important criteria. One of the CDGs is a 1.3 kPa full-scale differential unit which uses the vacuum created by a small

turbomolecular pump as its reference pressure. The other is a 133 kPa

absolute gage. The CDGs are calibrated against the NIST ultrasonic

interferometer manometer which has an accuracy of 15 ppm ± 1.3x10"^ Pa.

A third CDG (D) is used to measure the pressure difference between the

reference and working reservoirs. Short-term stability (20-30 minutes) is the

most important criterion. This unit is only used to measure small pressure

differences and is not calibrated.

The Pistons

The volume of the piston which moves into the working reservoir appears

in eq (1) as dV. Therefore, dimensional stability and characterization are very important. We are using "wire gages" as pistons. These are readily

available, well characterized, and carefully constructed cylinders of tungsten

carbide. One of the pistons has a nominal diameter of 1 inch (2.540 cm +

40 ppm, -0 ppm) and the other has a nominal diameter of 1 cm

(1.0000 cm ± 51 ppm). They will be referred to as the inch and centimeter

12
pistons, respectively. The pistons are coupled to stepping motors through

bellows and twenty- turns -per- inch micrometer screws. There is about

one-quarter turn backlash in the piston drive but this has no effect as the

piston is always driven in the same direction during measurements

.

The piston seals contain elastomers. In order to reduce gas leakage across these seals, the fill pressure is brought to both sides of each seal.

Tests have indicated that the leakage across the seals under normal operating

conditions contributes an error of no more than 0.01%.

The Piston Advance System

A stepping motor turns a micrometer screw which drives the piston into

the working reservoir and the rate at which the motor turns is controlled by

the reading of the differential CDG between the working and reference

reservoirs. The length the piston travels in a given number of turns enters

directly into the throughput and pressure calculations as part of the

displaced volume. Therefore, the threads of the screw must be linear and

accurate. Checks of the micrometer screws have indicated that they are linear

to +0.001 inch.

As the piston is driven into the working reservoir, the elapsed time is

measured with a resolution of 0.1 second and is displayed after each turn of

the piston.

Reservoirs

Thermal stability of the reference volume (B) is important so that its

pressure does not change during a flow measurement (20-30 minutes) . The

working reservoirs (E) must be small in relation to the piston and both

dimensionally and thermally stable. The nominal volumes of the working

reservoirs are 39 cm^ and 32 cm^ for the 1-in and 1-cm pistons respectively

(the nominal volumes displaced by the 1-in and 1-cm pistons are 13 and 2 cm'').

They are thermally insulated and the temperature is monitored. They are fabricated from stainless steel.

Thermometers

One thermometer on the flowmeter measures the temperature of the working

reservoirs. Accuracy is important so a calibrated quartz thermometer is used.

A liquid- in-glass thermometer with a resolution of 0.05 °C is used to monitor

the chamber temperature

.

13
The flowmeter is thermally insulated but not temperature-controlled. It

is insulated with polystyrene to protect it from short-term temperature

fluctuations and insure internal uniformity. A second quartz thermometer is

located inside the polystyrene at a different location from the first. The

difference between the two temperatures is usually tens of millidegrees . The

actual temperatures of the flowmeter and chamber are not important when the

flowmeter is used simply as a pressure generator but they must be known when

used in the primary standard configuration. Changes in the temperature difference between the reference and working reservoirs will cause spurious

differential pressure changes.

Variable Leak Valve

The gas from the working reservoir flows into the chamber through a commercially available, all-metal, variable leak valve (G) . This valve

enables us to have a large fill pressure, Pf ^ , and still introduce only a very

small amount of gas into the chamber to achieve a given pressure. Large fill

pressures can be more easily measured because of the characteristics of the

CDGs and their calibrations, and they minimize the effects of outgassing.

This valve must be dimensionally stable for stable flow and it is set the day

before a calibration to stabilize mechanical drift which may change the

conductance of this type of valve by as much as 10% overnight.

Vacuum Pumps

A small (0.05 m'' s" ^ ) turbomolecular pump backed with a mechanical pump is

used to evacuate the flowmeter reservoirs and to provide a reference vacuum

for the 1 . 3 -kPa CDG on the reference reservoir. A second mechanical pump

serves as a roughing pump for the ballast volume and flowmeter reservoirs. A

larger turbomolecular pump (J) exhausts the chamber. Its speed must be

constant over long periods of time in order to make meaningful measurements.

The pump speed must also be high enough, to ensure a large pressure ratio

across the orifice and a very low base vacuum (two orders of magnitude below

the lowest calibration pressure for both MDGs &nd IGs if possible) . The pump

in use has a nominal pumping speed of 0.5 m^s"-"- for N2 gas. The

turbom.olecular pump is backed by a molecular-sieve trapped mechanical pump.

An additional requirement is that the pump be free from mechanical vibration

in order that the stability of the MDG readings not be compromised. We have

14
found turbomolecular pumps to be satisfactory if the MDGs are mounted with

care

.

IV. OPERATIONAL PROCEDURES FOR CALIBRATION USING

THE NIST HIGH VACUUM STANDARD

This section describes in some detail the operation of the orifice-flow

standard. Users are referred to Ref. 6 for MDG operating procedures.

There are two flowmeters of similar design at NIST and either may be used

with the orifice-flow standard. It is also possible to use either flowmeter

with the primary leak standard. However, there is only one vacuum calibration

chamber with an orifice of the design described above. The chamber has eight

ports of which one is part of a by-pass manifold between chamber halves and

one is occupied by a residual gas analyzer. This leaves six ports for gages

under test and transfer standards. Care must be taken that ion gages sharing

ports do not interact by having ions generated in one gage collected at the

other. This is generally accomplished by avoiding line of sight connections

between ionization gages. Within these limitations, the number of gages that

may be calibrated at one time is limited only by gage geometry and operator

preference

.

As discussed earlier, the IG calibrations fall naturally into three

groups

:

Table II. Primary Standard Configuration Used for Calibrations

Pressure

Pa

Flow in Which

Chamber

Piston

Diameter

10"^ to 10-5

IQ-^ to IQ-''

>io-^

Lower

Upper

Upper

One Centimeter

One Centimeter

One Inch or MDGs

MDGs are calibrated with the flow into the upper chamber using the 1-in

diameter piston. These calibrations are procedurally like the IG calibrations

between 10" ^ and 10"* Pa. The calibration is performed at only one pressure

(5x10" Pa) which represents a compromise between the upper limits of the

orifice conductance linearity and the lower limit of acceptable MDG performance. The MDGs remain linear between 10"'' and 10"^ Pa but the random scatter

in the residual drag becomes a problem near 10"'' Pa if performance at the 1%

level is desired.

15
INSTALLATION AND PREPARATION OF GAGES

Bake -Out

Each venting of the system must be followed by a complete baking of the

chamber and inlet lines. The chamber is designed to be baked at temperatures

up to 250 °C. The gas pison flowmeter may not be baked because of the use of

elastomer seals for the pistons but as much of the gas line to the chamber as possible should be baked. At present, only the chamber and the inlet line

between the variable leak valve and the chamber are baked. The flowmeter

upstream from the variable leak valve is maintained at high enough pressures

to ensure viscous flow so outgassing and backstreaming during the bake are not

so critical.

The MDG balls are cleaned in alcohol and acetone and dried in a stream of

pure nitrogen gas. The thimbles should be degreased if they have been exposed

to contaminants

.

Once the gages have been installed on the chamber with metal -gaske ted

seals, high temperature leads are attached between all IGs and ion gage

controllers. The IGs are operated during the bake-out. Suspension heads must

be removed from the MDGs . The turbomolecular pump and its mechanical roughing

pump are started and the system is checked for leaks.- Once it has been

ascertained that the system is leak- tight, the chamber is evacuated with the

roughing and turbomolecular pumps and the ion gages are turned on. The IGs

should be set for the biases and emission currents at which they will be

calibrated. The oven is carefully lowered over the entire chamber and the

oven controller, safety switch and thermocouple connected. A flow of argon

gas generating about 10"^ Pa (10"^ Torr) as monitored by an IG in the chamber

is established through the inlet line into the chamber. Because of the small

conductance of the inlet line, this will maintain viscous flow in the inlet

line and across the variable leak valve to prevent backstreaming of

contaminants.

The oven controller may now be turned on to increase the temperature

slowly. It should take at least a day to reach 250 °C. Slow heating is

needed because the temperature must remain uniform so that the upper and lower

chamber flanges do not differentially expand with respect to the center plate

between them. The temperature may be monitored with the thermocouple. When

the chamber is at maximum temperature, the inlet line is heated by a bank of

low-voltage transformers passing equal currents of approximately 60 A from the

center of the inlet line through each half. This configuration is used

16
because the inlet line is grounded at the flowmeter and at the chamber. The

inlet line should sizzle when water is dropped on it. In addition, the heater

on the turbomolecular pump should be started.

The oven should remain at temperature for at least 8 hours. Then, the

controller may be set to decrease the temperature slowly. The inlet line

should be cooled slowly and the turbomolecular pump heater switched off. The

goal is keep the temperatures of all components as uniform as possible so that

impurities do not condense in cold spots.

After the external oven walls are cool, the argon flow is stopped and the

oven may be removed. When the MDG thimbles have cooled to room temperature,

the suspension heads may be attached, and the balls levitated and accelerated.

The pick-up signals of the MDGs should be checked and the signal scatters

monitored. If needed, the suspension head may be removed after the ball is

braked to a stop and the ball remagnetized to improve the pick-up signal

strength. This completes the baking procedure.

Degassing

The IGs are normally degassed by operating them during the baking of the

chamber. This has served very well. On some occasions, it may be desirable

to further degas a specific IG after baking by wrapping it in glass wool

building insulation and operating it overnight. Electron bombardment or resistance heating of the grid is not used.

THE DAY PRIOR TO THE CALIBRATION

Preparations for calibration against the NIST Primary High Vacuum

Standard begin the day before the actual calibration. Several procedures must

be completed in preparation - the MDGs must be spun up, the IG voltages and

currents should be checked, the flowmeter pistons must be positioned, and the

leak valve must be set for the proper flow rate.

The MDG balls must be spun up if they are to be used. This is done the

day before in order that they may dissipate the heat produced by the acceleration process. If the ball has been at rest, one must wait several hours (5-7)

before the effect of the cooling is less than the equivalent of 1x10"^ Pa, so

it is essential that this be done the day before calibrations.

If IGs are to be used, the filament and grid biases of those gages on NIST-built controllers should be adjusted as should the emission currents.

This is straightforward. The emission currents of customers' gages should be

17
checked and adjusted if necessary and the zeros of the electrometers should be

set by mechanical adjustment on a high-range scale and electrical adjustment

at low- range scale readings after removing the collector lead.

For pressures at the low end of the range (<10~* Pa), the 1-cm diameter

piston will be used. The 1-in diameter piston covers the higher end (>10"^

Pa) (see table II). The piston not in use should be left at the end of its

travel to make the volume of the working reservoir as small as possible. The

piston to be used should be placed at the top of its travel and then advanced

slightly to eliminate backlash.

The leak valve must be set the night before a calibration to stabilize

mechanical drift. The flowmeter performs most reliably when the piston takes

about 1000 seconds to travel 1-in. All leak valve adjustments should be made

in the direction of increasing conductance as the valve has substantial

backlash.

In general, the system should be left overnight with the flowmeter and

chamber evacuated and the orifice plate up. If very low pressures are to be

generated (near or even below 1x10"^ Pa), it may be advisable to leave the

orifice plate down overnight for maximum stability in the base vacuum.

THE MORNING OF THE CALIBRATION

The first order of business is to check the base readings of the IGs to

be sure that the system is at an appropriate base pressure. If so, the

orifice plate should be gently lowered if it is up and the time noted. Then,

the IG voltages and currents should be rechecked. The chamber temperature

should be noted and the MDGs set up for the day. This will involve entering

the date, run number, gas and ball parameters (molecular weight, viscosity,

diameter, density and temperature of 298.15 K for a second generation gage), an effective accommodation coefficient (1 for any second generation gage to be

calibrated), timing interval of 30 in the second generation units, etc., as appropriate.

After the gage readings have stabilized (about 45 min after lowering the

orifice plate) , the base readings of the IGs and MDGs should be checked for

stability. The MDG readings should be stable to better than ±1x10'^ Pa.

Offsets should be read for the MDGs and the base readings for the IGs should

be taken. These offsets should be entered into the MDG controllers and the

resulting MDG readings should be observed to be sure that they vary about zero

18
Pa. Then, gas may be introduced into the appropriate half of the chamber from

the flowmeter.

Depending on the calibrations to be done, the flowmeter may be used to

generate a known flow of gas or it may be used simply to establish a steady

flow. The second case is a subset of the first and will be discussed later.

MEASURED FLOW TO UPPER CHAMBER

Order of Pressure Generation

Pressures to be measured should be established sequentially from the

lowest to the highest for each gas. This ensures that the elastomer seals and

any IGs on the system will not absorb gas at higher pressures and desorb at

subsequent lower pressures. This may not always be possible from day to day

but in no case should a higher pressure be run before a lower one within a day. The only exception is to evacuate the chamber in order to check the

residual drag of an MDG.

Establishing a Pressure

The gas flow is directed into the upper chamber for the calibration of

all MDGs and the calibration of IGs at pressure above 10"^ Pa. Gas is introduced from the compressed gas cylinder into the ballast volume and reservoirs

while the test or monitoring gages are observed. When the desired calibration

pressure has been reached, the inflow is halted. The ballast volume now

serves as the source of gas. The fill pressure may be adjusted to achieve the

calibration pressure. At this point, the gas should be allowed to flow for

about 45 minutes while the elastomer seals in the working reservoir absorb

gas.

During this time, the operator should check that the necessary parameters

have been entered into the MDG controllers and check the IG operating

parameters

.

Flowmeter operation is initiated by closing the valve which isolates the

reference reser~voir. The valve should be closed slowly because there is the

possibility that gas will be driven into the reference reservoir by the valve,

raising the pressure. Once the valve is closed, the fill pressure is read

with the appropriate CDG and recorded. At the same time, the chamber and

working reservoir temperatures are read and recorded.

Next, the valve is closed which removes the ballast volume from the

system. The differential CDG is monitored while this valve is closed and the

19
final reading should be slightly positive. Once the valve is closed, the

pressure in the working reservoir will start to fall because gas is flowing

out of the reservoir into the chamber. Any time before the differential

pressure reading becomes negative, piston travel should be enabled. The

piston only travels when the reading is negative and overshoots slightly on each step. There is provision to choose between proportional and on-off

control of the piston advance.

The elapsed time for each turn of the piston is monitored and displayed.

This display is locked for one-half of each turn to enable the operator to

note both it and the reservoir temperature as read by the quartz thermometer.

The mid-point of all gage readings should occur half way through the piston

travel (turn number 10). The IGs are read once before turn 10 and again in reverse order after turn 10 in order to average the effects of any pressure

drift. The MDG readings should be started at times appropriate to the timing

algorithm of the controller so that the mid-point of the readings occurs at

the tenth turn.

After 1-in of piston travel (20 turns), piston travel is stopped and the

working reservoir is reconnected to the ballast volume and the fill pressure

is read as are the chamber and reservoir temperatures. Then the piston is

driven out, the reference reservoir valve is opened, -and the system is either

pumped out or adjusted for the next series of measurements. If a new pressure

is established, the operator must wait another 45 minutes for gas sorbtion in

the elastomer seals to equilibrate.

MEASURED FLOW TO LOWER CHAMBER

To calibrate ion gages between 10"^ and 10'^ Pa, the gas flow is directed

into the lower chamber. The operation of the flowmeter is as outlined above.

The 1-cm piston is normally used in this pressure region. Data are taken as above, the only difference being in the data analysis. The ratio of flow in

the lower chamber to that in the upper chamber, Rp , for a given pressure must

be determined and eq (8) used instead of eq (7)

.

STEADY FLOW GENERATOR

For the calibration of IGs above 1x10"^ Pa, the flowmeter is used only to

generate a steady flow. In this case, the piston is not used. Gas is

admitted into the ballast volume until the desired calibration pressure is

reached. Gas from the ballast volume is allowed to leak into the upper

20
chamber and the gages are monitored until the readings are steady. At this

time the gages are read: MDG readings are started, the IGs are read once, and

then the IGs are read in reverse order. The analysis of the gage readings is

described below with the exception that a group of MDGs is used to measure the

chamber pressure. MDGs which are used as transfer standards should have the

appropriate calibration parameters entered before the offset data are taken.

SHUTTING DOWN

After the last run of the day, the flowmeter should be evacuated with the

mechanical pump and the small turbomolecular pump. The MDG readings at base

vacuum should be checked with the orifice in place to evaluate errors caused

by possible drifts in the residual drag. Then, the orifice plate should be

raised.

VENTING THE SYSTEM

Several precautions should be taken before the system is vented to remove

customers' gages or for any other reason. All IG and residual gas anaylzers

must be turned off. The MDG drive must be disabled. The suspension heads may

be left on or not until they must be removed for baking. If they are to be

removed, the balls must first be stopped using the controllers (and an oscilloscope or spectrum anaylzer to monitor their rotational frequencies if

necessary) and lowered into their thimbles. The residual gas analyzer must be

isolated behind a closed valve. The turbomolecular pump is stopped and its

mechanical pump vented.

The system pressure is increased to slightly above one atmosphere with

argon or dry. nitrogen. The system is then vented.

DATA ANALYSIS

Three tjrpes of data may be gathered in the course of a calibration. The

first are the data pertaining to the flowmeter which will be used to calculate

the pressure in the upper chamber. The second are the MDG readings and the

third are the IG readings. The way in which the system is being used and the

instruments being calibrated will determine which types of data are recorded.

Concerning the first (the flowmeter) , the operator should have the

following numbers available:

The fill pressure before and after the run, Pf „

.

The chamber temperature before and after the run, T^j^ .

21
The reference reservoir temperature before and after the run, . When the flow is into the upper chamber, the formula used to calculate the

pressure is

Pu = (Q/C)(Rp)/(Rp-l) (3)

where Q is the throughput into the upper chamber given by

Q = Pf,(dV/dt)(T,h/T,„) (1)

and C is the conductance of the orifice given by

C = C*(T^j^/298.15 K)i/2 (2)

Pfg, is the average fill pressure which is measured in terms of the output of

the CDG (i.e., in volts). The NIST calibration coefficents of the CDG are used to convert the average of the before and after readings into Torr and

this is converted to pascals using the factor 1 Torr = 133.322 Pa. dV is the

volume of the piston which was driven into the working reservoir. The values

are 1.9949 cm'' for the 1-cm piston and 12.870 cm^ for the 1-in piston. dt is

the total time elapsed for the piston travel (the first reading minus the

twenty-first reading) . C is the temperature-corrected value of the conductance of the orifice and C* is the value of the conductance at 298.15 K

(Appendix C) . Rp is determined experimentally and checked periodically. Its

value depends on the gas in use and the performance of the turbomolecular

pump

.

When the flowmeter is used with the flow into the lower chamber, Q* is

calculated as above and then divided by Rp , the lower to upper chamber flow

ratio. This gives the effective throughput that would be required to generate

the calibration pressure with the flow into the upper chamber. The final form

of the equation for the throughput becomes

Q = Pf„(dV/dt)(T,j,/T,i,)/Rp. (10)

For ion gages read with commercial or customer controllers, the generated

pressure should be divided by the -IG reading minus the IG reading at base

vacuum to obtain a calibration factor:

Gal. Factor = Pressure drop/(IG reading - IG reading at base vacuum) (11)

Attention should be paid that all readings are in the same units of pressure

(usually Torr)

.

Ion gage readings taken on NIST-built controllers require more work to

reduce them to gage sensitivies. The data acquired at each pressure consist

of the collector current and the voltage drop across a known resistor due to

the emission current. Comparable data are also taken at base vacuum. The

following equations are used to calculate gage sensitivity, S:

le = V,/R, (12)

22
S = (I, - I,„)/(I,P)

(13)

(14)

where R,, is a resistor specific to the controller and the emission current

range, is the voltage drop across , 1^ is the emission current, is the

measured collector current, E is a range factor for the electrometer and P is

the generated pressure. The subscript "o" refers to readings at base vacuum. MDG readings are the average of 10 readings taken about the tenth turn of

the piston. The calibration coefficient is determined by dividing the

generated pressure by the MDG reading. The effective accommodation

coefficient is determined by dividing the MDG reading by the generated

pressure

.

In addition to the errors already discussed in Appendix C regarding the

conductance calculation and the measurement of the pressure and flow ratios

,

Rp and Rp , several other possible sources of error must be taken into account.

A high degree of pressure uniformity from port to port in the upper

chamber has been demonstrated by establishing a nitrogen flow to generate a pressure of about 10""' Pa in the upper chamber. The pressure was measured by

four ion gages which were well distributed about the upper chamber. After

recording the gage readings, the orifice plate was raised and the flow was increased by a factor of about 15 to establish the same pressure. The

relative gage readings with the orifice plate raised agreed to within 0.1%

with the reading obtained with the orifice plate lowered. Since nonuniformity

due to axial asymmetry of the flow would be proportional to the flow rate

required to generate a specified pressure, the results indicated any such

nonuniformity to be less than 0.01% irr normal operation with the orifice plate

lowered.

The upper chamber pressure will have a vertical perturbation due to the

net flow of molecules between the gas inlet and the orifice. This will have

two contributions, one due to the molecules exiting through the orifice and

not being scattered back, and the second due to the molecules entering from

the inlet. The first contribution is reasonably straightforward to calculate

V. ASSESSMENT OF UNCERTAINTIES

SYSTEMATIC ERRORS

Pressure Gradients

23
since the orifice is an ideal sink. This contribution will decrease the

pressure at the gage ports from that given in eq (7) by a factor of 0.99956.

The second contribution is more difficult to calculate because the

scattering of the inlet flow produces a diffuse source and it will take

several collisions before the effects of the initial nonuniformity are negligible. However, this contribution is somewhat smaller than the first, but

of the opposite sign. The sum of these two contributions results in a condition in which the pressure at the gage ports is slightly less than at the

orifice so that the net flow of gas is from a region of lower pressure to one of higher pressure, an entirely possible result in the molecular flow regime.

Mounting of the Orifice Plate

Another small error is the perturbation of the calculated conductance

caused by the mounting of the orifice plate. The orifice plate is mounted in

the wall between the two chambers with a short duct, 12.7 cm in diameter and

1.27 cm long, between it and the lower chamber. The probability of

transmission to the lower chamber of molecules exiting the orifice is

estimated to be 0.9997 ± 0.0001. This effectively increases the pressure in

the upper chamber by the factor of 1.0003, offsetting the effect at the gage

port of the small vertical pressure gradient previously discussed.

We estimate that 0.003% is a generous upper bound on the uncertainties

caused by the combined upper chamber pressure gradients and the mounting

configuration of the orifice plate.

High Pressure Nonlinearities

The calculation of the orifice conductance assumes an absence of

molecule-molecule collisions. The number of such collisions increases with

pressure, with a corresponding increase in the error of the conductance

calculation. We have chosen at this time not to attempt to correct for this

effect but rather to restrict the upper pressure limit of the standard to keep

this error within reasonable bounds. The error due to non-molecular flow

conditions amounts to 0.1% at about 8.5x10'^ Pa for and at about 2.5x10'^

Pa for He, and increases linearly with pressure.

The orifice- flow standard was operated above the molecular flow range to assess the total nonlinearity of the system at "high" pressures. MDGs were used as the standards in this case as their nonlinearity is known and is

smaller than the nonlinearity in the orifice conductance (Appendix C) . The

24
results are based on the readings of 12 MDGs taken over a period of k days and are summarized in table III below. Total deviations are expressed as the mean difference between the pressure indicated by the MDGs and pressure generated

by the high vacuum standard ± three times the standard deviation of the MDG

readings

.

Table III. Primary Standard Nonlinearities

Nitrogen Total

Pressure, Deviation MDG Nonlinearity

Pa percent percent

1x10-2 0.07±0.2 0

7x10-2 0.7 ±0.3 0.2 (11 MDGs)

1x10-1. 1.7 ±0.6 0.3

Temperature

The average temperature of the gas molecules is determined by the average

temperature of the chamber walls, which is known with an uncertainty of 0.3 K

in the absence of heat sources, notably ion gages. The normal temperature is

296 to 298 K. Ion gages are heat sources and, depending on the number of

gages and whether they are tubulated or immersed in the chamber, this

uncertainty in the chamber temperature could be as much as an order of

magnitude larger in extreme cases. This will contribute an uncertainty to the

pressure varying from 0.05 to 0.5%. This does not include uncertainties in

the thermal transpiration corrections for ion gages being calibrated.

Flow Rate Errors

The uncertainty of the flowmeter [1] is dominated by the uncertainty of

the capacitance diaphragm gages (CDGs) used to measure pressures. The

uncertainty of the CDGs is based on observed calibration shifts of the CDGs

used in the flowmeter and our experience with the long-term stability of a larger group of CDGs [9]. The estimated total systematic uncertainties of the

flowlneter are listed in table IV.

Changes in Pump Speed

As discussed earlier, the pressure and flow ratios can be reliably

measured only at higher pressures and their use in eqs [7] and [8] at lower

25
pressures assumes that pump speed does not change with decreasing pressure.

Any change in the pump speed will cause errors in the calculated pressure,

with corresponding errors in the sensitivity of any gage calibrated. In

particular, a decrease in pump speed, with decreasing pressure, will cause an apparent increase in gage sensitivity. This will be particularly true for

measurements made with gas flow into the lower chamber since the pressures

calculated from eq (8) depend directly on Rp , which depends directly on the

pump speed. A constant measured gage sensitivity implies that either the gage

sensitivity and pump speed are both constant, or that both change with

pressure in a manner such that the changes exactly cancel. Neither case can

be proven, but it is unlikely that different gages will have the same nonlinearities , particularly if the gages are of different designs, so, as more gages are found to have constant measured sensitivities, the second

possibility becomes increasingly unlikely.

The low pressure ion gage sensitivities presented in figure 4 for

different gages calibrated over a year's time can be used to establish an upper bound on possible pump speed changes. The results include random

uncertainties in both the gages and the standard. The data in figure 4b) and

4c) were for gages calibrated with flow into the lower changer and their

sensitivities show a maximum change (in all cases a decrease) of 2% in going

from 10"* to 10"^ Pa. In a second experiment, data were taken on four

different type of ion gages at pressures from 3x10"^ to 10"* Pa using flow

into both the upper and lower chambers. The difference in the sensitivities

determined by the two methods over a period of up to 5 days was between -1 and

+1%.

Another method of assessing pump speed changes is to look for trends in

ion gage calibration data. Sixteen ion gages were calibrated by directing the

flowmeter into the lower chamber at pressures between 3x10"® and 4x10"^ Pa and

the resulting data were fit to an equation of the form S = A+B(logP), where S

is the gage sensitivity and P is the generated pressure [10]. The slopes (B)

of these 16 equations varied from -2.5 to 3.9% per decade. If the individual

coefficient are weighted by their inverse variances, the average value of B

for the 16 gages is less than 1% per decade.

Any assessment of the pump speed will, of necessity, involve random

errors due to the gages involved. Based on these data and the discussion of

ra:ndom errors below, we have set the error due to pump speed variation at 2.5%

per decade below 10"^ Pa.

26
RANDOM ERRORS

Upper bounds on the random errors can be set by evaluating the variation

of repeated gage calibrations. These numbers will, of course, include the

random errors of the gages as well as those of the standard.

A measure of the short-term repeatability at higher pressures can be

obtained from four repeated calibrations of six different MDGs with at

4.9x10"^ Pa. In this case, the calj.brations were made over a period of 8 hours. The pooled standard deviation of the effective accommodation

coefficients of the six gages was 0.00026 (the nominal effective accommodation

coefficient is 1.0) with no significant difference in the random errors associated with any gage.

A second experiment was performed to check longer-term performance of the

system. Twelve MDGs were mounted on the system and were calibrated in

nitrogen at pressures ranging from 10"'' to 10"^ Pa. Each MDG was calibrated

two to six times at each pressure over a 5-day period. The pooled standard

deviations of the effective accommodation coefficients were 0.0011 at 5x10"^

Pa dnd 0.0017 at 1x10'* Pa.

Similar results were obtained when repeated calibrations of 10 MDGs were

done with argon at pressures ranging from 5x10"* Pa to 3x10"^ Pa. In this

case, three to five different calibration series were performed at each

pressure, each series taking place on a different day and involving a complete

calibration cycle, starting with the vacuum chamber and flowmeter at base

vacuum and the orifice plate raised above the gallium- filled groove. The data

from two of the MDGs were eliminated because of clearly excessive random

errors in the gages. The pooled standard deviations of the remaining eight

gages were 0.0010 at 3x10" ^ Pa, 0.0008 at 7x10' ^ Pa, 0.0012 at 3x10" ^ p^, and

0.0041 at 5x10"* Pa.

At the lowest pressures the above results are probably dominated by the

short-term random errors of the MDGs, and MDG data at yet lower pressures will

not be useful in assessing random error's of the standard. Therefore, we must

rely on ion gage data at lower pressures

.

As a measure of the random errors at low pressure, short-term

repeatability data were obtained with three calibrations for a tubulated,

tungsten- filament Bayard-Alpert gage made over a period of several hours. In

this case separate flowmeter measurements were made for each calibration. At

2x10'* Pa the calibrations differed by a maximum of 0.1%, and at 2x10"^ Pa by

0.02%.

27
A more realistic assessment of the random errors can be obtained from

calibrations repeated on different days. For flow into the upper chamber we expect that errors at the lowest pressures will be dominated by random

flowmeter errors. The data in figure 4a) are typical of that for ion gages

calibrated in this manner, indicating random variations equivalent to ±5x10"®

Pa. This is consistent with the random errors shown for a different gage,

again calibrated with the flow into the upper chamber, shown in figure 4d)

.

Where calibrations of the same gages have been performed with flow into the

lower chamber, and repeated flow into the upper chamber, the results agree to

within 1-2%. Examples of such data are. shown in figures 4b) and 4c). These

data indicate an upper bound on the random errors of 2% down to pressures

where the flowmeter random errors will again dominate. For flow into the

lower chamber we expect that these errors will exceed 2% at 1x10"^ Pa.

An additional estimate of the random errors at low pressures is obtained

by examining the results of calibrating sixteen ion gages over the pressure

range 3x10"® to 4x10" Pa by flowing gas into the lower chamber [10].

Repeated calibrations for most of the gages differed by less than ±2.5% and

linear fits of these calibration data had standard deviation of the residuals

between 0.26 and 3.5% with an average standard deviation of 1.95%.

Based on the data above, we have, adopted ±2.5% as the upper bound on random errors of the standard with flow into the lower chamber to 10"^ Pa.

SUMMARY OF ERRORS

The component errors previously discussed are summarized in table IV for

nitrogen at four different pressures . The uncertainties at the highest

pressure designated "molecular scattering" are due to the high pressure

nonlinearities in the conductance of the orifice. The effects due to pressure

gradients in the upper chamber and orifice mounting have been left out as negligible. Errors at the lowest pressure have been assessed on the basis of

flow into the lower chamber. The flow ratio contribution includes random

errors and a systematic contribution for the additional flowmeter measurement

needed. The uncertainty due to changes in the assumed pumping speed is based

on the observed bounds of systematic changes in measured gage sensitivities.

This uncertainty is not applicable at higher pressures because with flow into

the upper chamber the effects of changing pump speed are greatly reduced. The

uncertainty for temperature errors has been somewhat arbitrarily doubled from

the minimum expected value.

28
The tabulated random errors are upper bounds based on repeated MDG

calibrations with nitrogen and argon. These included the random errors of the

standard, flowmeter and gages. The value at 10"'' Pa is based on three times

the short-term deviations observed in ion gage calibrations at this pressure.

We expect that longer term random changes, due to orifice plate sealing and

flowmeter seals leakage, are probably no different from those included in the

higher pressure MDG data.

Random errors are not tabulated at 10"^ Pa because they are included in

the determination of the pump speed uncertainty.

Assessment of errors in some cases is hampered by the limited data

available. Therefore, we must regard these results as tentative at this time.

As we gain more data and experience with the operation of this system, we expect the uncertainties to change. In particular, we expect to reduce the

uncertainties due to the flowmeter by maintaining tighter calibration control

over the CDGs

.

TABLE IV. Uncertainties of the NIST High Vacuum Standard with Nitrogen

Pressure. Pa

10-1 10- 2 10- 10-^

Systematic Contributions 0 18 01 8 0 18 0 18

Orifice Conductance (%) 0 18 0 18 0 18 0 18

Molecular Scattering (%) 1 2 0 12

Flow Rate (%) 0 82 0 82 0 82 2 00

Pressure Ratio (%) 0 04 0 04 0 04 0 04

Flow Ratio (%) 0 90

Assumed Pump Speed (%) 2 5 Temperature (%) 0 1 0 1 0 1 0 1

Total Systematic (%) 2 34 1 26 1 14 5 72

Random Errors (3 S.D., %) 0 21 0 30 0 .30

Total (%) 2 6 1 6 1 .4 5 7

We have chosen to linearly sum the systematic erroijs and three times the

random errors as a worst-case estimate. If the individual systematic errors

were combined using the square root of the sum of the squares and three times

the random errors were linearly added, the results would be 1.68% at 10'

^ Pa,

1.15% at 10-2 IA5% at lO;* Pa, and 3.33% at 10"^ Pa.

II

29
VI. OOALITY CONTROL

The determination of systematic and random errors in a primary standard

is but the first step in characterizing its performance. By its very nature,

a primary standard is difficult to monitor over long periods of time and

various laboratories have developed several methods of doing this. Some of

these are the maintenance of additional primary standards, interlaboratory

comparisons of primary standards of the same or different designs, and the

maintenance of a bank of check standards. We have adopted all three of these

methods

.

COMPARISONS AGAINST ADDITIONAL NIST PRIMARY STANDARDS

The original primary high vacuum standard in use before 1984 [11] was suitable only for the direct calibration of early molecular drag gages. It was based on the integrating principle of the MDG rotor and the integrating

capabilities of the MDG controller, and was operated by exhausting a known

volu-^e of gas into the chamber ("gas burst" method). The MDGs respond not to

pressure per se but to molecular flux and the deceleration experienced by the

gages was due to the total number of molecules striking the ball surface. The

calibration was a two step procedure and gave reproducible calibration factors over time.

Upon coiopletion of the orifice-flow standard, it was compared with the

gas burst method described above using 5 MDGs as transfer standards. The

typical difference between the two methods was 0.1% and the worst difference

was 0.5%. This case occurred using the small (1 cm) piston in the flowmeter.

We have subsequently modified the piston-drive circuit to allow for both

proportional and on-off control. A major advantage of the flowmeter is that

it may be used to calibrate IGs as well as MDGs since the pressure generated

in the chamber is calculated directly and can be kept constant.

A second floimeter has been constructed which operates on the same principle as the first [1]. The primary differences between them lie in the

design of the piston seals and improved fill pressure measurement capability

in the second flowmeter. The second flowmeter does not use elastomer seals

but hydraulleally translates the piston motion to a bellows which is part of

the working reservoir. This second flowmeter is completely bakeable. The

original flowmeter has only two CDGs for measuring the fill pressure - 1.3 and

133 kPa (10 and 1000 Torr) full-scale. The second flowmeter has three CDGs -

0.13, 1.3 and 133 kPa (1, 10, and 1000 Torr) - and a force -balanced quartz

30
Bourdon gage. Some initial work has been done to compare the two flowmeters

using both standard leaks and MDGs as transfer standards. The leak work has

been done at low flow rates while the MDG work has of necessity been done at

much higher flow rates. Eleven MDGs were calibrated on both flowmeters

between 10''' and 10"^ Pa over a 10-day period. The differences in the

calibrations on the two flowmeters ranged from 0 to 0.5%. For eight of the

gages, the differences were within three standard deviations of zero. The

leak data showed differences of <1% at flow rates above 10"^^ mol s'

INTERNATIONAL COMPARISONS

Two calibrated MDG balls were sent to the Physikalisch-Technische

Bundesanstalt (PTB) and the National Physical Laboratory in England (NFL) for

comparison against the static expansion standards at those facilities. The success of these comparisons was limited by instabilities in the balls. The

first ball gave very promising results - the results from all three

laboratories were within 1% of each other and the first and last calibrations,

done at NIST, differed by less than 1%. The second ball was very unstable.

The results are given in table V below. The instability was traced to changes

in the residual drag, not to ball surface changes. Subsequent calibrations at

NIST have indicated that neither ball was particularly stable.

TABLE V. Results of First International Intercomparison

Laboratory Difference from

NIST, % Ball 1 Ball 2 NIST

NPL -0.3 +1

PTB +1.6 +2

NIST -0.2 +4

NIST (17 months later) +1

A second intercomparison was undertaken when PTB sent four balls as part

of an international round robin under the auspices of the International Bureau

of Weights and Measures (BIPM) [12]. The effective accommodation coefficients

of the balls were measured according to the circulated protocol at pressures

be.tween 10' and 10'^ Pa in nitrogen, argon, and hydrogen. Several NIST and

customer balls were calibrated at the same time. Unfortunately, the four

balls were damaged in transit and the shifts between calibrations performed at

PTB before and after the measurements at NIST were as large as 8% in hydrogen.

31
A second set of four balls was sent to NIST in 1986 in a continuation of

the round robin. Preliminary results indicate that the mean of the four NIST

values for argon was within 0.5% of the pilot laboratory's value. The results

for nine laboratories (representing five different types of primary standard)

were within ±2% of the pilot laboratory at argon pressures between 10'^ and

10"^ Pa with the exception of two laboratories whose values were within ±3%

[13].

Several additional informal intercomparisons are underway at standards

labortories. Rotors calibrated at NIST are now in use at Istituto di

Metrologia "G. Colonnetti", Turin, Italy and Korea Standards Research

Institute, Seoul, Korea and two calibrated MDG rotors have been sent to the

National Physics Laboratory, New Dehli, India.

Intercomparisons have been carried out with other primary standards

laboratories at very low flow rates using calibrated leaks as transfer

artifacts [14]. Early results showed differences increasing to 10% at flow

rates of 10"^^ mol s" ^ which correspond to a nitrogen pressure of about 10"^

Pa in the upper chamber. It is expected that improvements since these data

were taken should decrease the differences.

CONTINUOUS, REPEATED CALIBRATIONS OF VACUUM GAGES

Repeat MDG Calibrations

Many NIST-owned MDG balls have had multiple calibrations. Systematic

calibrations were begun in 1983 using the gas burst method and continued with

the orifice-flow standard. One rotor in particular has been calibrated

repeatedly between February, 1983 and April, 1987. A total of 106

calibrations in nitrogen have been performed. The range in the effective

accommodation coefficient is from 0.99 to 1.02. These results are indicated

by the dashed line in figure 5. Results of repeated calibrations for four

additional balls are also shown in figure 5. The data shown have been

clarified by plotting deviations from the initial calibration expressed as a percent of that initial value and by averaging points taken within a few days

of each other. There is some evidence of an increase in the effective

accorfimodation coefficient which is expected. Large abrupt changes in the

calibrations of any of the rotors have always been traced to problems with the

primary high vacuum standard (such as the presence of foreign gases in the

chamber, the use of unbaked rotors or failure of the turbomolecular pump) and

not with the rotor surface.

32
Ion Gage Comparisons

Ion gages are not as stable as MDGs and we do not expect to gain as much

information about system performance from our ion gage data. However, because

of the broad pressure range covered by these gages, we can use them to look

for systematic errors in the high vacuum standard performance depending on whether the flow is directed into the upper or lower chamber and depending on which piston is used in measuring the flow. To date, we have not seen any

systematic differences at the 2% level.

Figure 6 shows data for two ion gages. Two extensive sets of

calibrations were performed on each gage between 10"^ and 10"^ Pa. The two

sets were performed 3 months apart. The gage in figure 6a) is designed for use between 10'^° and 10"^ Pa and showed no shift in sensitivity as a function

of calibration method. Four different configurations of the flowmeter were used as well as a bank of calibrated MDGs. The gage in figure 6b) is designed

for use between 10"^ and 10"^ Pa and showed a definite shift in sensitivity

between the two sets of calibrations but exhibited excellent short-term

stability. Again, we see no systematic differences between results using the

various calibration methods. Table VI gives means and standard deviations for

these two gages at 10"* Pa for two frequently used calibration methods. It

should be noted that each method of determining the pressure has a range of

pressures over which it functions best. For example, we would expect more scatter in data taken at or below 10''' Pa with an MDG than at higher

pressures

.

TABLE VI. Ion Gage Sensitivities, Nitrogen Gas 10 ^ Pa"

Gage 6a Gage 6b

Method Mean Standard Mean Standard Date

Deviation, % Deviation, %

Flow to upper chamber, 6.63 1.4 5.12 0.2 9/85

1 cm piston 6.63 0.6 4.09 0.8 1/86

Molecular drag gages 6.79 0.8 5.17 0^.4 9/85

6.64 2.3 4.25 2.4 1/86

ADDITIONAL CHECKS

Eight MDGs have been returned by customers for recalibration at this

time. The changes in the calibration factors varied from -2.3% in 2 year's

time to +2.7% in 1.7 year's time. All of these are within our expected change

33
of 1-2% in 2 year's time. These pa^rticular customers are knowledgeable users of the gage and we expect that the rotors have been treated carefully between

calibrations.

VII. NEEDS AND PLANS

The major limitations of this system are 1) the presence of elastomer

seals in the gas piston flowmeter, 2) the accuracy with which the pressure in

the reference reservoir can be measured when a low fill pressure is used, and

3) the limitation of an achievable base vacuum to 10"^ Pa.

In an effort to address the first limitation, we have constructed the

second flowmeter which is used primarily in the primary leak standard. The

differences between the two are discussed above. Future flowmeters will

undoubtedly be based on the hydraulically-coupled flowmeter design in order to

eliminate outgassing from the elastomer seals and to permit baking the

flowmeter

.

The second limitation is a major source of certainty in the flow rate

measurements. We have implemented a system to permit in situ calibrations of

the flowmeter CDGs with the ultrasonic interferometer manometer [15,16].

A new program is underway to develop an ultra-high vacuum standard which

will help us overcome the third limitation. A major part of the work is

directed toward lowering the achievable base vacuum. This has involved high

temperature baking over extended periods (e.g., 500 °C for 200 hours) to

reduce the room temperature hydrogen outgassing rate from the stainless steel

walls of the vacuum chamber. In addition, experimentation with various types

of ultra-high vacuum pumps (e.g., turbomolecular , titanium sublimation, ion,

and cryogenic pumps) is in progress.

The increasing availability of ion gages designed for use under ultrahigh vacuum conditions suggests that the performance of the standard at

pressures below 1x10"^ Pa could be investigated using these gages as transfer

standards. Expected limitations in this pressure region include the stability

of the base vacuum and the linearity of the gage sensitivities and the gage

residual currents.

, The primary high vacuum standard is used for both experiments and

calibrations, and the increasing number of calibrations means that it is

becoming increasingly difficult to perform necessary experiments. One means

of increasing the productivity of the system is to automate as many of the

operations as possible. Certain operations lend themselves very readily to

34
automation, among those being the recording of the piston drive information,

the recording of CDG and temperature information, and the recording of MDG

transfer standard readings. Those IGs which are tested on NIST-designed

controllers may also be read automatically. However, we do not foresee at

this time the automatic reading of customer gages owing to the large variety

of instruments that are submitted for calibration.

It is hoped that automatic data acquisition will not only lead to

increased productivity but will allow us to more easily document system

behavior as data are collected. Both the orifice- flow standard and MDGs are relatively new and our understanding of their operation and capabilities will

be expanding as new results are available. Having those results in a form

amenable to ready analysis will promote optimum performance.

A second means of improving productivity is to install a larger chamber

which would allow the calibration and testing of more gages at one time.

There are fundamental limitations to this system which are not amenable

to treatment with equipment or technique. At the highest pressures, we have

overcome the inherent limitations of the orifice by using MDGs as transfer

standards. However, they become increasingly nonlinear and we have no way of

calibrating them because NIST currently has no vacuum standard to cover the

pressure range 10"^ to 0.5 Pa. The orifice- flow standard and ultrasonic

interferometer manometer cover the low and high pressure ends of this range

but there is no provision for direct comparison. This is in part because of

the mercury vapor in the manometer. One appropriate step would be to put

several calibrated MDG balls on the manometer with some protection against

direct mercury exposure. However, the problem of sensitivity to the molecular

weight of the test gas remains for the MDGs.

We anticipate that the working range of the NIST primary high vacuum

standard will remain 10"^ to 10"^ Pa for the foreseeable future. Our task at

this time is to improve performance within this range with technique and

equipment until our ultra-high vacuum program is well established and until a new primary standard is designed and bililt which will bridge the gap between

the ultrasonic interferometer manometer and the f lowmeter-orifice standard.

VIII. ACKNOWLEDGMENTS

We gratefully acknowledge the major contributions of Drs . K.E. McCulloh

and C.R. Tilford in the conception and design of this standard and their work

in the early phases of characterizing the standard. We also thank Mr. D.F.

35
Martin for his efforts in the manufacture and assembly of the many components

involved.

IX. REFERENCES

[I] K. E. McCulloh, C. R. Tilford, C. D. Ehrlich, and F. G. Long, "Low-Range

Flowmeters for Use with Vacuum and Leak Standards," J. Vac. Sci. Technol.

A 5, 376 (1987)

.

[2] P. Clausing, Ann. Physik 12, 961 (1932); English translation appears in

"The Flow of Highly Rarefied Gases through Tubes of Arbitrary Length," J.

Vac. Sci. Technol. 8, 636 (1971).

[3] J. K. Fremerey, "The Spinning Rotor Gauge," J. Vac. Sci. Technol. A 3,

1715 (1985).

[4] J. K. Fremerey, "Spinning Rotor Vacuum Gauges," Vacuum 32, 685 (1982).

[5] M. Hirata, H. Isogai, M. Ono , "Effect of Measuring Conditions on the

Accuracy of a Spinning Rotor Gauge," J. Vac. Sci. Technol. A 4, 1724

(1986) . .

[6] S. D. Wood and C. R. Tilford, "Vacuum Calibrations Using the Molecular

Drag Gage," material accompanying a course given at the National

Institute of Standards and Technology, Gaithersburg , Maryland.

[7] G. Comsa, J. K. Fremerey, B. Lindenau, G. Messer, and P. J. Rohl,

"Calibration of a Spinning Rotor Gas Friction Gauge against a Fundamental

Vacuum Pressure Standard," J. Vac. Sci. Technol. 17, 642 (1980).

[8] K. E. McCulloh, S. D. Wood, and C. R. Tilford, "Zero Stability of

Spinning Rotor Vacuum Gages," J. Vac. Sci. Technol. A 3, 1738 (1985).

[9] R. W. Hyland, C. R. Tilford, "Zero Stability and Calibration Results for

a Group of Capacitance Diaphragm Gages," J. Vac. Sci. Technol. A 3, 1731

(1985)

.

[10] C. R. Tilford, S. Dittmann, K. E. McCulloh, "The NIST Primary High Vacuum

Standard," J. Vac. Sci. Technol. A 6, 2853 (1988).

[II] K. E. McCulloh, "Calibration of Molecular Drag Vacuum Gages," J. Vac.

Sci. Technol. A 1, 168 (1983).

[12] G. Messer, P. Rohl, G. Grosse, W. Jitschin, "High Vacuum Measured by the

' Spinning Rotor Gauge: Status Report of the Bureau International des Poids

et Measures Intercomparison, " J. Vac. Sci. Technol. A 5, 2440 (1987).

[13] W. Jitschin, PTB, Private communication.

[14] C. D. Ehrlich, NIST, Private communication.

36
[15] P. L. M. Heydemann, C. R. Tilford, and R. W. Hyland, "Uitrasonic

Manometers for Low and Medium Vacua under Development at the National

Bureau of Standards," J. Vac. Sci. and Technol. 14, 597 (1977).

[16] R. W. Hyland, "NIST Measurement Services: Calibration at the National

Institute of Standards and Technology over the Low Vacuum Range from 1 to

1.5x10^ Pascal," to be published as NIST Special Publication SP 250-35.

[17] K. F. Poulter, "The Calibration of Vacuum Gauges," J. Phys . E 10, 112

(1977).

[18] R. P. Iczkowski, J. L. Margrave, and S. M. Robinson, "Effusion of Gases

through Conical Orifices," J. Phys. Chem. 67, 229 (1963).

[19] D. R. Willis, "Mass Flow through a Circular Orifice and a Two -Dimensional

Slit at High Knudsen Numbers," J. Fluid Mech. 21, 21 (1965).

X. BIBLIOGRAPHY

In addition to the references cited above, the interested reader may wish

to consult the following.

Intercomparisons :

"Intercomparisons of Vacuum Standards of Countries within the European

Community in the Range 8x10"^ to 8x10"^ Pa," K. F. Poulter, A.

Calcatelli, P. S. Choumoff, B. lapteff, G. Messer, G. Grosse, J. Vac.

Sci. Technol. 17, 679 (1980).

"Intercomparison of the National Low Pressure Standards in the LNE and PTB,"

G. Messer and L. Rubet, Proceedings of the Eighth International Vacuum

Congress, Cannes, France, 1980 J. P. Langeron and L. Maurice Eds., V. II,

259 (1980).

Standards :

"The Measurement of Gas Throughput in Range lO''' to 10'^° Pa m^s'^," G. N.

Peggs, Vacuum 26, 321 (1976).

"Summary Abstract: Long-Term Stability of Low-Pressure. Standards in the

German Calibration Service Deutscher Kalibrierdienst , " G. Grosse and G.

Messer, J. Vac. Sci. Technol. A 5, 2463 (1987).

37
Vacuum Gage Calibration :

"Calibration of Spinning Rotor Gas Friction Gauge Against a Fundamental Vacuum

Pressure Standard," G. Comsa, J. K. Fremerey, and B. Lindenau (KFA) and

G. Messer and P. Rohl (PTB) , J. Vac. Sci. Technol. 17, 642 (1980).

"The Calibration of Vacuum Gauges," W. Steckelmacher , Vacuum 37, 651 (1987).

Vacuum Gages :

"Molecular Drag on Freely Rotating Balls and Vanes," G. Comsa, J. K. Fremerey

and B. E. Lindenau, Proceedings of the Eighth International Vacuum

Congress, Cannes, France, 1980 J. P. Langeron and L. Maurice eds, V. II,

218 (1980).

"Long Term Stability of Various Reference Gauges over a Three Years Period,"

G. Messer, Proceedings of the Eighth International Vacuum Congress,

Cannes, France, 1980 J. P. Langeron and L. Maurice eds, V. II, 191

(1980)

.

"Long Term Behaviour of Ionization Gauges," K. F. Poulter and C. M. Sutton,

Vacuxim 31, 147 (1981)

.

"Spinning Rotor Vacuum Gauges," J. K. Fremerey, Vacuum 32, 685 (1982).

"Reliability of High Vacuum Measurements," C. R. Tilford, J. Vac. Sci.

Technol. A 1, 152 (1983)

.

"Long-term Stability of Two Types of Hot Cathode Ionization Gauges," S. D. Wood and C. R. Tilford, J. Vac. Sci. Technol. A 3, 542 (1985).

"Sensitivity of Hot Cathode Ionization Gages," C. R. Tilford, J. Vac. Sci.

Technol. A 3, 546 (1985)

.

"An Electrostatically-Suspended Spinning-Rotor Vacuum Gauge: A Feasibility

Study," J. D. Nuttall and D. C. Witt, Vacuum 37, 347 (1987).

"Total Pressure Gauges," R. K. Fitch, Vacuum 37, 637 (1987).

"The Use of Hot Filament Ionization Gauges," P. Nash, Vacuum 37, 643 (1987).

38
Gas

Inlet

Line

Gas

Supply

Cylinder

n

Ballast

Volume

A

Working

Reservoir

Variable

Leak

H

1 to

Chamber

u

Reference

Volume

B

Figure 1. Schematic of the Primary High Vacuum Standard Including the

Chamber, the Flowmeter and the Orifice. Letters Indicate

Components which are Discussed in the Text.

39
Valve

Gas

Bakeable

Orifice Plate Lifting

Mechanism

Baffle

Orifice

Plate

Baffle

Bakeable

Valve

)

Spinning

Rotor

Gage

^

Bakeable

Valve

To

Turbopump

Figure 2. Schematic of the Vacuum Chamber Showing the Lifting

^ Mechanism and Sealing Method of the Orifice Plate. Also

Shown is the Arrangement of Valves and Gages Permitting the

Measurement of the Upper to Lower Chamber Pressure and Flow

Ratios

.

40
Figure 3. Axial Section of the Orifice Plate, with Dimensions in Inches.

Details of the Central Duct are Shown in figure 7. This Plate can be Raised to Provide a Large Conductance Between the Upper

and Lower Chambers of the Dynamic Expander During Evacuation.

For Generating Calculable Pressures by Flow Through the Orifice,

the Knife Edge K is Lowered into a Groove Gilled with Gallium to

Provide a Gas-tight Seal.

41
Figure 4. Representative Measured Low-Pressure Nitrogen Sensitivities for

Different Ion Gages. Sensitivities for Each Gage are Relative to

its Value at 10'* Pa. The +'s Indicate Flow into the Lower

Chamber, the O's are into the Upper Chamber. The Data in a) are for a Nude Gage with an Electrode Structure Commonly Found in

Tubulated BA Gages. The ± 20 - 30% Variation at the Lowest

Pressure is Typical of Results for Other Gages Calibrated at the

Same Time.

42
Figure 5. Deviations from the Initial Value of the Effective Accommodation

Coefficient of Five Molecular Drag Gage Rotors as Determined by

Calibrations Against the NIST Primary High Vacuum Standard. All

Data were Taken Using Nitrogen Gas. Values have been Smoothed

for Clarity by Averaging Points Taken within a Few Days of Each

Other.

43
0.10

(0 a

I 0.05 - c

(0

-5 -4 -3 Log Pressure, Pa

0.06

(0 Q. 0.05

0) I 0.04

XX XX

5 X >, X X * t A?

0.03

-8 -6 -5 -4 -3 Log Pressure, Pa

a) and b) Ion Gage Sensitivity in Nitrogen for Two Different

Ultra High Vacuum Ionization Gages' as Determined by

Calibrations Against the NIST Primary High Vacuum Standard and

Against Calibrated Molecular Drag Gages. ^

- Flow into Upper

Chamber, 1-cm Piston; + - Upper Chamber, 1-in Piston; X - Lower

Chamber, 1-cm Piston; - Lower chamber, 1-in Piston;

• - Molecular Drag Gages. Gage 6a is Designed for Use Between

10"^° and 10' 2 Pa and Gage 6b is Designed for Use Between 10"^

and 10"^ Pa.

44
I

i (0, 0, -a)

I

I

z

Figure 7. Detail of the Central Duct Through the Orifice Plate. The

Surfaces Marked A and B are Spherical Segments Lapped to

Radius of Curvature R, and Their Centers of Curvature are Marked with Coordinates (0,0, -a) and (0,0, a). Respectively.

Throat and Entrance Apertures have Radii Tq and r^^ , Respectively. Planes of the Throat, Entrance Aperture, and Exit

Aperture are z = 0, z = -b, and z = b. Approximate Dimensions

in Inches are R = 0.312, =0.221, r^ =0.235, a = 0.221,

and b = 0.015. The Direction of Gas Flow is Downward.

45

XI . APPENDICES

APPENDIX A

SAMPLE REPORT OF CALIBRATION FOR

IONIZATION GAGE

Ion Gage Tube and Controller

Submitted by

CUSTOMER NAME

CITY, STATE ZIP

TPD Calibration Number: HV XX

NIST Identification of Gage: XXX

Supplier of Gage: SUPPLIER

Gage Model: AB123

Manufacturer of Controller: MANUFACTURER

Controller Model: 123

Controller Serial Number: 456

Gas: Nitrogen

Date received: MONTH DAY, YEAR Test completed: MONTH DAY, YEAR

Procedures

This calibration is for a thoria-coated filament ion gage and the

associated controller. The gage was calibrated with the electrode axis

vertical and the collector pin up. The ambient temperature was 24 °C with

free but not forced air circulation about the gage tube.

The calibration data presented in this report were obtained with the

controller and gage tube operated as a unit. Operating with an emission

current of 5.4 mA as determined by the controller, the dc filament bias was

30.9 V and the grid bias was 132.3 V.

The calibration was performed on a turbomolecular- pumped, all metal

chamber designed to minimize pressure gradients and maintain gas purity. The

generated pressures, which are pressure increases above base vacuum, were measured with the primary vaccum standard from 10'^ to 10"^ Pa (10"^ to 10'^

Torr) and with calibrated molecular drag gages from 10"'' to 10"^ Pa (10"^ to

10" Torr). Estimated uncertainties in the generated pressures using the NIST

primary vacuum standard were 1% above 1x10"^ Pa increasing to ± 5% at 1x10'^

A-1
Pa. Using calibrated molecular drag gages, the estimated uncertainty in the

generated pressure between 10"'' and 10" •'• Pa is 1.6% ± 2x10"^ Pa,

The gage tube was mounted on the calibration chamber and evacuated and

baked overnight at 250 "C. It was degassed by being operated during the

bakeout

.

Results

The tube designated XXX was calibrated with the controller designated in

this report from MONTH DAY to DAY, YEAR. The tube was operated with a measured emission current of 5.4 mA as established by the controller. All

gage readings were obtained from the front panel display. The data presented

below were obtained after overnight pumpdown to pressures below 5x10'® Pa.

The generated pressures are pressure increases above base vacuum and within

each day were generated starting with the lowest and proceeding sequentially

to the highest each day. The corrected controller readings were obtained by

subtracting the controller reading at base conditions just prior to a calibration series from all subsequent readings. The controller readings at

base conditions are due to the system base pressure and the gage residual

currents. The calibration factor is the ratio of the generated pressure in

Torr to the corrected controller reading.

Generated Pressure Controller Reading Corrected Calibration

Pascal Torr Torr Reading, Torr Facto

(1 Torr=133.32 Pa)

9.19x10'^ 6 89x10"

3

1 2x10-8 0.8x10"

8

0.56

3.77x10"^ 2 83x10-8 5 2x10"

8

1.4x10"

8

0.54

1.41x10-5 1 06x10"^ 2 0x10'^ 2.0x10'^ 0.54

4.28x10-5 3 21x10'^ 5 8x10"^ 5. 8x10"

7

0.56

1.34x10-* 1 00x10"^ 1 8x10"

8

1.8x10"

8

0.56

3.79x10-* 2 85x10'^ 5 3x10-8 5. 3x10"

8

0.54

3.80x10"* 2 85x10"

6

5 3x10-8 5.3x10-8 0.54

4.13x10-* 3 10x10-8 5 7x10-8 5. 7x10"

8

0.54

1.28x10-3 9 59x10-8 1 7x10-5 1.7x10-5 0.56

1.33x10"

3

9 99x10"

8

1 8x10-5 1.8x10-5 0.56

4. 20x10"

3

3 15x10"

5

5 9x10-5 5.9x10-5 0.53

5. 28x10"

3

3 96x10"

5

7 .5x10-5 7. '5x1

0-5 0.53

1.22x10-2 9 16x10-5 1 .7x10-* 1.7x10-* 0.54

4.10x10-2 3 07x10"* 6 .6x10'* 6.6x10"* 0.46

6.39xl0"2 4 79x10"* 10 .0x10-* 10.0x10"* 0.48

A-2
The operating parameters of the electrometer were independently checked

and are noted below, for your information. The controller required

approximately 24 hours after being turned on to reach a stable indication.

Current at Collector

Input. Amps

Indicated Pressure,

Torr, N2

8.337x10"!°

2.634x10-9

2.601x10-9

2.074x10-8

2.074x10-8

2.043x10-^

2.043x10-^

2.216x10-6

2.216x10-6

3.916x10-5

1.0x10-8

10.0x10-8

1.0x10"^

10.0x10-^

1.0x10-6

10.0x10-6

1.0x10-5

10.0x10-5

1.0x10-^

10. 0x10"

Changing emission current or bias voltages can cause changes in the gage

tube sensitivity and therefore the calibration factor.

In order to obtain optimum stability from this tube, we recommend that it

not be operated with significant pressures of air or oxygen, that long periods

of outgassing be avoided, that after outgassing it be allowed to equilibrate

for eight or more hours, and that after exposure to atmospheric air or operation with other gases it be baked out at a high temperature or wrapped in

fiberglass insulation and operated for 8 hours or more in a high vacuum. An

overnight equilibration period should be allowed after bakeout. Operation

with 10-^ Pa N2 for 1 or 2 hours followed by baking may help to stabilize

subsequent N2 readings. When a sequence of pressures is to measured, the gage

will perform most reliably when an increasing sequence is used. Preferably,

the gage should be stabilized by operation for several hours at a pressure at

least a factor of 10 below the lowest pressure to be measured.

A3

APPENDIX B

SAMPLE REPORT OF CALIBRATION FOR

MOLECULAR DRAG GAGE

Spinning Rotor Vacuum Gage

Submitted by

CUSTOMER NAME

CITY, STATE ZIP

TPD Reference Number: HV X

NIST Identification of Flange: 654321

Manufacturer's Identification of Flange: 123456

Manufacturer: MANUFACTURER

Model Number: ABC XYZ

Serial Number of Controller: 123456

Serial Number of Suspension Head: 12345

Diameter of Ball: 4.5 mm

Gas: Nitrogen

Previous test record: HV XX

Date Received: MONTH DAY, YEAR Test Completed: MONTH DAY, YEAR

Procedure

This unit was tested on an all-metal vacuum chamber designed to maintain

a low residual gas background and minimize pressure gradients. The chamber was pumped with a turbomolecular pump to a base pressure of about 10"^ Pa. After

being mounted on the chamber, the thimble and ball were baked overnight in

argon at 250 °C at a pressure of 10"* Pa and then cooled slowly. After the

thimble had cooled, the suspension head was installed and the ball was spun

up.

During the calibration, known pressures of about 5x10'^ Pa were generated

in the chamber. Prior to the calibration, the residual drag of the ball was measured at a base pressure of about 10'° Pa and was subtracted from

subsequent readings. This correction must be redetermined at the time of use, as noted below.

Before calibration, the signal strength and timing uncertainty of the

unit were examined. Errors of the timing circuitry were evaluated by applying

a stable 200 uV p-p, 414 Hz signal to the input of the electronics unit. The

signal scatter (SSC) with this synthetic signal was X.Y usee. Visual

inspection of the ball indicated that the surface was smooth and shiny.

B- '
At the time of test, the pickup signal from the ball, the residual drag

and the frequency dependence of the residual drag were all monitored. All of

these factors may change during use and must be redetermined at the time of

use. The ball was remagnetized until its signal strength exceeded 100 mV.

The frequency dependence of the residual drag was checked and the residual

drag was found to change from XxlO''^ Pa at 415 Hz to AxlO"^ at 405 Hz.

Corrections for frequency dependence were made in the calibration data.

Results

Any number of calibrations were performed against the primary vacuum

standard between MONTH DAY and MONTH DAY, YEAR. The parameters used were 28.0134 for the molecular weight of nitrogen gas, 175.7 upoise (at 298.15 K)

for the viscosity of nitrogen gas, 4.5 mm for the diameter of the ball, and

7.78 g cm"^ for the density of the ball. From these calibrations, the ball

was found to have an average effective accommodation coefficient of

1.00

in nitrogen gas for pressures less than 10"^ Pa. The residual standard

deviation of the measured effective accommodation coefficients was 0.X%. The

estimated total uncertainty of the NIST primary standard used for this

calibration is 1.5%. Nonlinearities in the gage, not included in the above

effective accommodation coefficient will increase with pressure and amount to

about 0.1% at 0.05 Pa and 1% at about 0.5 Pa. Corrections are made for this

nonlinearity by the controller using the viscosity data entered by the user. The uncertainty in this correction had not been quantified.

Please note that the parameters for this ball and gas constitute an integral part of the calibration. For the proper use of this calibration, the

operating temperature and the appropriate viscosity must be entered and the

other parameters must remain unchanged.

Discussion

This ball was calibrated as part of a system consisting of the ball, the

thimble, the suspension head and the controller. We expect that the effective

accommodation coefficient will depend primarily on the properties of the ball

and the ball's surface. However, we cannot predict the effect on the

effective accommodation coefficient of changing the other components of the

system.

B-2
The effective accommodation coefficient given for the ball applies only

for nitrogen.

Before the gage is used, the chamber temperature, in kelvins, must be

entered from the front panel of the controller as a parameter. Temperature

corrections must be made for the viscosity, and the residual drag must be

determined for the temperature of use. Care must be exercised in using a spinning rotor gage to achieve optimum

performance. For use at the lowest pressure, the ball and thimble should be

baked with the vacuum system. Mechanical or chemical damage to the surface of

the ball should be avoided and the ball should always be stopped before it is

lowered in the thimble. External magnetic fields (time dependent fields in

particular) will have an unpredictable effect on the behavior of the unit. To

obtain the optimum rotational timing signal, it may be necessary to

periodically remagnetize the ball, particularly after it has been baked.

Since the magnitude of the residual drag can change, it must be

redetermined each time the ball is suspended, and preferably before each

series of low pressure measurements. The residual drag should be determined

by monitoring the indicated pressure at "zero" pressure. The most accurate

values will not be obtained until 5 to 7 hours after spinning up the ball from

rest. This allows for dissipation of the ball's induced heat. The frequency

dependence of the residual drag can change as well when the ball is

resuspended, and therefore should be checked.

The error of a measurement made with this gage, assuming pure nitrogen

and appropriate parameters, will include the 1.5% uncertainty of the NIST

standard, random reading errors of the gage, and any shift in the effective

accommodation coefficient as a function of time because of changing ball

surface conditions. Random reading errors will depend on the strength of the

rotational timing signal, the measurement time and local temperature and

vibrational instabilities. They should be evaluated by the user by observing

repeated measurements at a stable pressure. This is most easily accomplished

by making repeated observations of the indicated zero or residual drag at low

(<10"^ Pa) pressure. Operating the ball in a different thimble from the one in

which it was calibrated may change the calibration by up to 1%. The

mechanisms contributing to this change are not well understood. Experience to

date indicates that if the ball is not mechanically scuffed or exposed to

aggressive gases, changes in the effective accommodation coefficient should bt,

less than 2% over 1 or 2 year's time.

B-3

APPENDIX C

DIMENSIONS AND CONDUCTANCE OF THE ORIFICE

IN THE NIST DYNAMIC EXPANDER*

Introduction

The orifice in the NIST dynamic expansion apparatus was designed and

fabricated in 1980 to satisfy three conditions;

1. Sufficient thickness to ensure dimensional stability,

2. A molecular flow conductance differing by less than 1% from that of an infinitely thin orifice having the same diameter, and

3. A geometric form to facilitate accurate calculation of this

conductance by the integral equation method introduced by Clausing [2].

An orifice with a sharp circular edge in the plane of minimum cross section

best satisfies these conditions. Spherical geometry was chosen for the

surfaces meeting at this edge in order to facilitate the conductance

calculations. In addition, these surfaces could be generated by lapping.

A generalization of Clausing' s theory is required because of the

departure from cylindrical geometry. Although the present formulation

emphasizes probabilities for retrograde escape from the orifice duct, while

Clausing' s is based on forward escape probabilities, the two are equivalent.

Either formulation can be derived from the other or directly from first

principles.

Structure and Dimensional Details

An axial section of the orifice plate as designed and fabricated of

stainless steel is shown in figure 3, and details of the central duct appear

in figure 7. The two surfaces A and B in figure 7 are spherical segments

generated by lapping to a radius of curvature R = 0.3125 in. The diameter of

the throat where these surfaces meet at a sharp edge was measured at NIST by

R. Veal and W.

*A Technical Report submitted by The Catholic University of America to the

Temperature and Pressure Division of the National Institute of Standards and

Technology in Compliance with the Terms of Grant NB82NAHA3005 , K E. McCulloh,

Principal Investigator.

C-1
Gallagher, who reported a value ItCq = 0.4425 in ±0.00003 in, where rQ is the

radius. Each of the spherical segments terminates at an axial distance

b = 0.0150 in ±0.0005 in from the plane of the throat. Transition to the full

thickness of the orifice plate is made above by a cylindrical step 0.240 in

high at a diameter of 3.50 in, and below by a truncated cone 0.230- in high

with an apex half-angle of 80°.

Microscopic examination gave no evidence for burrs or significant

irregularities at the throat edge. The edge angle computed from the above

dimensions is 90.1°, in close agreement with the nominal design value of 90°,

which was chosen specifically for the purpose of minimizing burr formation.

Calculation of the Molecular Flow Conductance

Rectangular coordinate axes Oxyz are chosen with the origin in the center

of the throat and the axis Oz in the axis of rotational symmetry. The axial

coordinate z increases downward, in the direction of flow. Symbols are listed

in Appendix C-I. The plane of the throat is assumed to be a plane of symmetry

for the central duct. Any error from this assumption is absorbed within the

error bounds assigned to dimension b. The cross sections in the planes Z = -b

and Z = b, where the surfaces A and B terminate, will be referred to as entrance and exit apertures, respectively. Centers of curvature for surfaces

A and B are at the points (0,0, -a) and (0,0, a), respectively, where

a - K - ro . Following Foulter [17], the molecular conductance L can be adequately

approximated in the form

L = nr^^m^KzC/U, (Cl)

where r^ is the entrance aperture radius. The molecular transmission

probability W of the central duct is to be calculated by neglecting the

cylindrical step above the plane z = -b and the conical transition below the

plane z = b. This neglect requires the introduction of two correction

factors: for the effect of the step and for that of the cone. The

symbol c represents the mean molecular speed in a Maxwellian distribution.

No adequate basis for estimating has been found in the literature.

Indeed, the draft international standard ISO/DIS 3570/1 for dynamic expanders

requires no correction for a step having the dimensions given above. Although

a complete analysis of this subject is beyond the scope of this report, simple

estimates based on a generalization of Clausing' s theory [2] provides the

bounds

C-2
0.99995 < < 0.99997.

A carefully designed conical transition in place of an abrupt step would

require a correction factor much closer to unity. Unfortunately, design and

fabrication preceded recognition of this point.

From the computed results and uncertainty estimates published by

Iczkowski et al. [18] fcr conical orifices

0.9999 < K2 < 1.0000.

On the basis of these inequalities a nominal value of 0.9999 for the

product Kj^K2 is employed in the subsequent computations.

The transmission probability W remains to be calculated. It is assumed

that molecules from a solid angle lix of a Maxwellian distribution enter the

central duct through the entrance aperture and that the wall, consisting of

surfaces A and B, scatters impacting molecules according to the cosine law.

Clausing' s treatment of this law [2] provides a basis for calculating several

probabilities required for the conductance calculation. These probabilities,

which depend only upon axial coordinates because of rotational symmetry, are defined as follows:

h(z) : Points of initial impact on the wall by molecules entering the duct

through the entrance aperture are distributed with respect to z with

probability density h(z)

.

g(u,z): If the axial coordinate has the value u at a scattering point P on

the wall, points of next impact n the wall by molecules scattered

from P are distributed with respect to z with probability density

g(u,z).

Pj, (z) : Molecules scattered from the wall at (x,y,z) make direct

line-of-sight escape back through the entrance aperture with

probability Pq(z).

p(z): Molecules scattered from the wall at (x,y,z) made eventual escape

(direct plus indirect) back through the entrance aperture with

probability p(z), which equals p^Cz) plus the probability of escape

after again striking the wall one or more times

.

w(z): Molecules scattered from the wall at (x,y,) make eventual forward

escape through the exit aperture with probability w(z) . As the

walls are assumed not to capture molecules, p(z) + w(z) = 1.

If W is the molecular transmission probability of the orifice, the

complementary probability 1-W is given by the integral

C-3
- W = J

h(z)p(z)dz, (C2)

-b

where p(z), translating its verbal definition into mathematical form, is the

solution of the inhomogeneous Fredholm integral equation of the second kind

p(z) = Po(z) + [ g(z,u)p(u)du. (C3)

J-b

These two equations provide the basis for the conductance calculation.

The probabilities and probability densities appearing in these equations

depend only on geometric parameters, not on any particular distribution of

molecular flux. Hence they must hold at equilibrium, where conditions for

detailed balance lead to the reciprocal relations

Trr^^hCz) = 27rR p„ (z) (C4)

and

g(z,u) = g(u,z) . (C5)

The factor 27rR occurs in eq (C4) and the kernel g(z,u) is S3rmmetric because

the differential 27rRdz is an element of wall area. Also, because the plane

z = 0 is a plane of symmetry for the central duct, w(z) = p(-z). Since

p(z) + w(z) = 1,

p(z) + p(-z) = 1 (C6)

No further use of the forward escape probability w(z) is required.

From eqs (C2) and (C4)

.

r ^ r^^d-w) = 2R p„(z)p(z)dz. (C7)

This form is useful because expressions for Po(z) can be conveniently

formulated.

From Clausing' s auxiliary theorems, [2]

2Rp(z) = R + a - b; -b < z < 0,

and equating two expressions for the probability that a molecule scattered

from the wall will next pass through the throat,

r ° p„(z) + g(z,u)du = (R-a)/(2R); 0 < z < b,

J -b

Further, if z and u are both positive or both negative,

2Rg(z,u) = 1.

C-4
Because of eq (C6) it suffices, to expand the integral eq (C3) only in the

range of positive z. After some manipulation the result becomes

pb

2Rp(z) = R-a+A-2R g(-z ,u)p(u)du; 0 < z< b

,

where

j.b

p(u)du.

Integrating eq (C8)

where

(2R-b)A = b(R-a) - B, (C9)

P 0 p b

B = 2R dz g(z,u)p(u)du.

J-b 0

Substituting for p^Cz) in eq (C7) and introducing an effective transmission

probability K based on throat area Trr^^ , where

r^^K = r^^w, (CIO)

r„2(l-K) = b(R-a) - (2a- ? )A-B (Cll)

Eliminating the double integral B from eqs (C9) and (Cll) yields the

unexpectedly simple result

K = l-2A/(R+a) , (C12)

while eq (Cl) becomes

L = nr^^YK^K2c/^. (C13)

Equation (C13) is preferable to eq (Cl) for assigning uncertainties because

the uncertainty of 0.14% in the area factor ^rr^^ is dominant, while that in

the factor K is only about 0.03%, and that in the product Kj^K2 is estimated to

be about 0.01%. The uncertainty in K is due almost entirely to the assigned

imprecision in the dimension b. Hence with K = 0.99167 from Appendix B and

^12 ~ 0.9999, the final result becomes

L = 0.9916-7rr„2c/4_

with an uncertainty of 0.04% for the formula and an additional 0.14% when the

measured value is assigned to the throat diameter.

The molecular flow conductance is proportional to the square root of the

absolute temperature through the factor c. Values for several gases at 298.25

K are tabulated below, with an estimated uncertainty of 0.18%.

C-5
Gas Conductance CL/s)

H2 43 . 52

He 30.89

N2 11.675

Ar 9.777

The effect of deviations from the cosine law of scattering is beyond the

scope of this report. A simple model, based on the assumption that a small

fraction of molecules undergoes specular reflection, suggests that the small

difference 1-K is roughly proportional to the fraction of molecules undergoing

cosine law scattering.

The correction for finite Knudsen number in the transition region near

the limit of free molecular flow will require an empirical approach. The

theoretical treatment by Willis applies to an ideal infinitely thin orifice

with zero pressure on the downstream side [19]

Conclusion

The orifice in the NIST dynamic expander satisfies the three conditions

stated in the Introduction. The simple form of eq (C12) and the ability to

impose narrow bounds on the transmission probability are consequences of the

spherical geometry and the symmetry of the central duct with respect to the

place of the throat. Application of more precise dimensional metrology would

decrease the uncertainty in the calculated conductance.

C-6
APPENDIX C-I

List of Symbols

R: radius of curvature of surfaces A and B (fig. 7).

r^, : radius of the throat

,

a: distance from the center of the throat to the center of curvature of

surface A or surface B. a^ = - r^,^

b: distance from the center of the entrance aperture to the center of the

throat; also from the center of the throat to the center of the exit

aperture

.

r^ : radius of the entrance (exit) aperture, r^^ = R^ - (a-b)^

L: molecular flow conductance of the orifice,

c: mean molecular speed, c^ = 8kT/(7rm)

W, K, h(z), g(z,u), Po(z), p(z), w(z): probabilities defined in text.

C-I-1

APPENDIX C-II

The Integral A and the Transmission Probability K

The Integral A and the transmission probability K have been narrowly

bounded without solving integral eq (C8) . First the integrals

f ^

I(z) = 2R g(z,u)du; -b < x < 0

J 0

and

J = I(z)dz

J-b

were evaluated by numerical quadrature. Although a suitable analytical

expression for g(z,u) has not been found, argximents based on the spherical

geometry and Clausing' s first auxiliary theorem [2] identify the product

27rRI(z) with a particular area on the (mathematical) sphere of which surface A

(fig. 7) is a segment. This is the area traversed by molecules scattered from

a point (x,y,z) on surface A and next striking surface B.

The quadratures were performed in BASIC on a minicomputer by Simpson's

rule. Several hours of running time on the otherwise idle computer were

required. The grid was sufficiently fine to compute I(z) at 101 points with

art uncertainty less than 0.001 b. The integral J was computed with an

uncertainty less than 0.001 b^

.

Absolute bounds based on generous overestimates of uncertainty yield the

inequalities

Ii = 0.63-b < I(z) < 0.79-b =12; -b < z < 0,

and

Ji = 0.69-b2 < J < 0.71-b2 = J2.

Then if p^ , pg , A^ , and Ag are absolute bounds such that

Pi < p(z) < P2; 0 < z < b,

and

Ai < A < A2 .

one obtains from eqs (C8) and (C9) the system

2Rpi = R-a + Aj - I2P2

,

2Rp2 = R-a + A2 - IiPi

,

(2R-b)Ai = b(R-a) - J2P2.

(2R-b)A2 = b(R-a) - J^p^

.

C-II-1
Assigning nominal values to the orifice dimensions and solving this system for

the bounds,

0Ti476T<~i(zyT'O48l27^^^^^^ 0 < z < b

From eq (C12) and these bounds on A,

' 0.991672 < t < (T. 991677.

Hence K = 0.99167 to five decimal places, with an uncertainty of 0.03% due to

dimensional imprecision.

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NBS-n4A (REV. 2-80) U.S. DEPT. OF COMM.. BIBLIOGRAPHIC DATA

SHEET (See instructions)

1. PUBLICATION OR

REPORT NO.

NIST/SP-250/34

2. Performing Orgjui. Report No, 3. Publication Date March 1989

4. TITLE AND SUBTITLE

NIST Measurement Services: High Vacuum Standard and Its Use

5. AUTHOR(S)

Sharrlll Dlttmann

6. PERFORMING ORGANIZATION (If joint or other than NBS. see instructions) NATIONAL INSITTUTE OF STANDARDS AND TECHNOLOGY

(formerly NATIONAL BUREAU OF STANDARDS)

U.S. DEPARTMerfT OF COMMERCE

QATTHERSBURQ. MD 20699

7. Contract/Grant No.

8. Type of Report & Period Covered

Interim

9. SPONSORING ORGANIZATION NAME AND COMPLETE ADDRESS (Street. City, State. ZIP)

Same as Item #6

10. SUPPLEMENTARY NOTES

library of Congress Catalog Card Number; 88-600605

I I Document describes a computer program; SF-185, FIPS Software Summary, is attached.

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

bibHography or literature survey, mention it here)

This document presents an in-depth discussion of the National Institute

of Standards and Technology primary high vacuum standard, used between 10" and

10' Pa. Included are discussions of the theory, design, and construction of

the standard. In addition, the systematic and random errors in the standard

and the methods used to check the accuracy of the standard are presented. Also

included is a brief discussiog of the_^olecular drag gage and its use as a transfer standard between 10" and 10" Pa.

12. KEY WORP,S (Six to twelve entries; alphabetical order: capitalize only proper names: and separate key words by semicolons]

dynamic expander; flowmeter; high vacuum standard; ionization gage calibration;

molecular drag gage; orifice conductance; orifice-f lowmeter system; primary high vacuum standard; spinning rotor gage; vacuum gage calibration

13. AVAILABILITY

Unlimited

I I For Official Distribution. Do Not Release to NTIS

[ Order From Superintendent of Documents, U.S. Government Printing Office, Washington, D.C.

20402.

Order From National Technical Information Service (NTIS), Springfield, VA. 22161

14. NO. OF

PRINTED PAGES

68

15. Price

USCOMM-DC 6043-P80

Jl



THE SP 250 SERIES ON NIST MEASUREMENT SERVICES*

SP 250-1 Spectra) Radiance Calibrations SN003-003-O2792-8 $3.50

SP 250-2 Far Uttravlolet Detector Standards PB87227609

SP 250-3 Radiometric Standards in the Vacuum

Uitraviolet PB87227625

SP 250-4 Filcke Dosimetry In IHigh-Energy Electron Beams

SN003-003-02816-9 $2.75

SP 250-5 Alpha-Patllde Caiitjratlons SN003-003-02823-1 $2.00

SP 250-6 Regular Spectral Transmittance PB88108550

SP 250-7 Radiance Temperature Calftxatlons SN003-003-02827-4 $2^5

SP 250-8 Spectral Reflectance PB88109905

SP 250-9 Csrilbratlon of Beta-PaiUde-Emlttlng

Ophthalmic Applicators

SN003-003-02817-7 $2.00

SP 250-10 Radioactivity Calit)ratlons with the "4ir" Gamma Ionization Chamber and Other

Radioactivity Callttratton Capabilities

SN003-003-02824-0 $2.25

SP 250-1 1 Dosimetry for High Dose Applications

PB88201587

SP 250-12 Neutron Personnel Doslmetiy

SN003-003-02811-8 $2.50

SP 250-13 Activation Foil irradiation with Californium

Fission Sources SN003-003-02866-5 $2.25

SP 250-14 Activation Foil Irradiation by Reactor Cavity

Fission Sources SN003-003-02861^ $3.25

i SP 250-15 Photometric Calibrations

I PB881 53747

SP 250-16 Calibration of X-Ray and Gamma-Ray

Measuing Instruments SN003-003-02862-2 $7.00

I SP 250-17 The NBS Photodetector Spectral Response

' Calibration Transfer Program

SN003-003-02857-6 $3.25

SP 250-18 Neutron Source Strength Calibrations SN003-003-02863-1 $3.25

SP 250-19 Calibration of Gamma-Ray-Emitting

Brachytherapy Sources SN003-003-02923-8

SP 250-20 Spectral iiradlance Calibrations SN003-003-0282&-1 $5.50

SP 250-21 Calibration of Beta-Particle Radiation instrumentation PB88201579

SP 250-22 Platinum Resistance Thermometer

Calibrations PB88138367

SP 250-23 UqiM-ln-Glass Thermometer

Calibration Sen/Ice

SN003-003-02891-6 $6.00

SP 250-24 Standtfd Ceil Calibrations PB88123690

SP 250-25 Calibration Sen/ice for inductive Voltage Dividers

SP 250-26 NBS Phase Angle Calibration Sen/lces

SN003-003-O2871-1 $5.00

SP 250-27 AC-DC Difference Calibrations

SP 250-28 Solid-state DC Voltage Standard Calibrations SN003-003-02842-8 $2.00

SP 250-29 Traceable Frequency Calibrations SN003-003-02844-4 $2.25

SP 250-30 GOES Satellite Time Code Dissemination:

Description and Operation

SN003-003-02845-2 $2.75

SP 250-31 Mass Calibrations SN003-003-02919-0

SP 250-32 A Calibration Sen/lce for 30 MHz

Attenuation and Ptiase Shift SN003-003-02875-4 $13.00

SP 250-33 A Calibration Senrtce for Voltage Transformers and High-Voltage Capacitors

SN003-003-02880-1 $1.25

SP 250-34 High Vacuun Standard and Its Use

* Those entries containing a stock number (003-003

) and price can be puchased from the Superintendent of Documents, U.S. Government Printing Office, Washington, DC 20402. GPO will accept checks, money orders, VISA, and Masterchaige. For more infomatk)n, or to place an order, call (202) 783-3238. Be sure to use the stock number In all orders.

Entries containing PB numbers can be purchased from the National Technk;al information Sen^, Springfield, VA 22161. NTIS will accept American ^ress In addition to the payment methods listed for GPO. For more Infoimatkm call (703) 487-4650; to place an order

call (800) 336-4700. Be sure to use the PB nunber on all orders.

Titles without stock numbers are In preparation.
U.S. Department of Commerce

National Institute of Standards and Technology

(formerly National Bureau of Standards)

Galthersburg. MD 20899

Official Business

Penalty for Private Use $300