.*",T " oo.\

\

\

If) J

NBS TECHNICAL NOTE 884

**fAU of

U.S. DEPARTMENT OF COMMERCE/ National Bureau of Standards

bration of

Low and Medium Di

Magnetic Disk Pi

100

no. "o^
NATIONAL BUREAU OF STANDARDS

The National Bureau of Standards 1 was established by an act of Congress March 3, 1901. The Bureau's overall goal is to strengthen and advance the Nation's science and technology

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

research and provides: (1) a basis for the Nation's physical measurement system, (2) scientific and technological services for industry and government, (3) a technical basis for equity in trade, and (4) technical services to promote public safety. The Bureau consists of the Institute for Basic Standards, the Institute for Materials Research, the Institute for Applied Technology,

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

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

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

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

Sciences 2 — Applied Radiation 2 — Quantum Electronics * — Electromagnetics 3 — Time

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

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

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

Radiation — Physical Chemistry.

THE INSTITUTE FOR APPLIED TECHNOLOGY provides technical services to promote

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

technological standards (including mandatory safety standards), codes and methods of test; and provides technical advice and services to Government agencies upon request. The Institute consists of a Center for Building Technology and the following divisions and offices: Engineering and Product Standards — Weights and Measures — Invention and Innovation — Product Evaluation Technology — Electronic Technology — Technical Analysis — Measurement Engineering — Structures, Materials, and Life Safety 4 — Building

Environment 4 — Technical Evaluation and Application 4 — Fire Technology.

THE INSTITUTE FOR COMPUTER SCIENCES AND TECHNOLOGY conducts research and provides technical services designed to aid Government agencies in improving cost effec- tiveness in the conduct of their programs through the selection, acquisition, and effective utilization of automatic data processing equipment; and serves as the principal focus within

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

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

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

THE OFFICE FOR INFORMATION PROGRAMS promotes optimum dissemination and

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

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

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

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

Publications — Library — Office of International Relations.

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

2 Part of the Center for Radiation Research.

3 Located at Boulder, Colorado 80302.

* Part of the Center for Building Technology.
OF 8TAK5JABD8

tIBRAST

NOV 2 6 1375

Calibration of Unrecorded Low and Medium Density

us

Type Magnetic Disk Pack Surfaces

6,1 Jjt ' CZL

Nicholas P. Goumas

Computer Systems Engineering Division

Institute for Computer Sciences and Technology

National Bureau of Standards

Washington, D.C. 20234

*< ">*

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

James A. Baker. Ill, Under Secretary

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

NATIONAL BUREAU OF STANDARDS, Ernest Ambler, Acting Director

Issued October 1975
National Bureau of Standards Technical Note 884

Nat. Bur. Stand. (U.S.), Tech. Note 884, 23 pages (Oct. 1975)

CODEN: NBTNAE

U.S. GOVERNMENT PRINTING OFFICE

WASHINGTON: 1975

For sale by the Superintendent of Documents, U.S. Government Printing Office, Washington, D.C. 20402 (Order by SD Catalog No. 03.46:884). Price 45 cents. (Add 25 percent additional for other than U.S. mailing).
Table of Contents

Page

1. Introduction 1

2. General Description of the Disk Calibration Services 2

3. Component Parts of the Measurement System 2

3.1 Disks 2 3.2 Heads 3 3.3 The Disk Test Bed 3

a. The Mechanical System 3 b. The Write Channel 4

c. The Read Channel 4

4. The Disk Calibration Procedure 10

4.1 Calibration Procedure for Low Density Surfaces 11 4.2 Calibration Procedure for Medium Density Surfaces 12

a. Amplitude Reference Calibration 12

b. Data Reference Calibration 13

5. Preparation of the Disk Calibration Report 14

List of Figures

Figure No.

1. Block Diagram of the Write Channel 5 2. Write Current Waveform 6

3. Geometry of Head Disk System 7 4. Block Diagram of the Read Channel 8

5. Correction Curve for the Digital Read Out Unit 9

Appendices

A. Low Density Disk Calibration Report Form 16

B. Medium Density Disk Calibration Report Form 17

C. Magnetic Measurements 18

D. Typical Track Width Diagram 19

Calibration of Unrecorded Low and Medium Density Type

Magnetic Disk Pack Surfaces

Nicholas P. Goumas

Computer Systems Engineering Division

Institute for Computer Sciences and Technology

National Bureau of Standards

Washington, D.C. 20234

This publication describes the design requirements and the

operation of the NBS test bed that is used for calibrating unre- corded magnetic disk pack surfaces for low and medium density use. The signal level calibration is made with respect to a reference

level that is derived from the NBS Standard Amplitude and Data Reference Surfaces that are held in repository at NBS. The techniques for calibrating the measurement system with the NBS repository Reference Surfaces and the calibration of candidate reference

disks are described in detail.

Key words: Calibration factor; computer amplitude reference;

computer storage media; disk calibration; disk pack; magnetic

disk; standard reference surface; unrecorded disk surface.

1. Introduction

Magnetic disk packs are used with electronic computers for high speed random access storage of digital information. The magnetic characteristics of disk pack surfaces have

been specified in national and international standards to help achieve unrecorded disk pack

interchange between disk storages and associated information processing systems. NBS and

the American National Standards Institute (ANSI) have agreed to the establishment of Standard Reference Surfaces and a disk calibration service in support of disk standardization.

These Standard Reference Surfaces are held in repository at NBS, and are used for calibrating the test bed which is then used for measuring the relative signal amplitude characteristics of the candidate disk surfaces that are submitted by industry. These calibrated

secondary reference surfaces are then used by industry as their in-house primary standards

for controlling disk characteristics between given tolerances.

The ANSI X3 Committee presented a low density (1100 bits per inch) Standard Amplitude

Reference Surface to be held in repository and to be used for calibrating low density sur- faces by NBS. For calibrating medium density (2200 bits per inch) surfaces, both the Amplitude Standard and a Data Standard Reference Surface are required. Medium density disk packs are used in drives that employ data type heads, but the signal amplitude qualities of these sur- faces can be more completely characterized by calibrating them with both an amplitude and a data test head. A number of medium density candidate amplitude and data reference surfaces were submitted to NBS by several industry manufacturers. Read-back signal amplitude measurements were then made of these candidate surfaces using a group of amplitude and data test heads.

The values which were derived were submitted to the ANSI X3B7 Committee. The measured values

obtained by industry at their laboratories when using their own disks and heads were also

submitted to X3B7. Using these submissions, X3B7 established the values of standard amplitude

and standard data references for given test conditions as shown in the following table:

Surface Test Head Tunnel/Straddle Track Standard Amplitude/

Density Type Era se Current Location Frequency

megahertz

Data Reference

mi Hi volts Low amplitude on 000 0.625 11.3 Low amplitude on 200 0.625 6.6

Low amplitude on 200 1.25 3.5

Medium amplitude off 000 0.625 11.5

Medium amplitude off 200 0.625 7.0

Medium data on 200 1.25 3.8

Medium data on 200 2.5 1.5
A Medium Density Standard Amplitude Reference and several Data Reference Surfaces were selected by NBS from the candidates. Groups of working reference surfaces for each density

were also selected. These surfaces were chosen on the basis of their read-back signal

amplitudes relative to those of the established standard amplitude references, their signal

envelope uniformity and smoothness, and the absence of dropouts or dropins within the tests

areas. Their resolution values are within the range specified by the ANSI and ISO disk pack

standards. A grouo of amplitude and data reference heads were also selected on the basis of

their signal amplitude, resolution, and resonant frequency characteristics as specified in th< ANSI and ISO standards. These primary and working standard reference surfaces and test heads

are maintained in repository at NBS. Periodically, the signal amplitude characteristics of

these surfaces and heads are measured on the NBS test bed. Any change in the characteristics

of a disk or head would indicate possible damage or serious change to the head or disk, and

would initiate a search for a replacement. To date, no change has been noted in any of the

disks or heads held in repository for the past two years.

2. General Description of the Disk Calibration Services

Candidate disks are calibrated on the basis of the read-back signal amplitude characteristics of their surface test areas relative to those that are obtained from the Standard

Reference Surface. The test areas are located on track 000 (outer diameter) and on track 200

(inner diameter). The test areas are each 225 microseconds in length, starting 50 microseconds from a scribe mark on the surface with the disk rotating at 2400 rpm. The write

current level, frequency, and tunnel or straddle erase conditions to be used for recording

on the test area are specified for each test condition. The NBS test bed and the instrumentation are first calibrated to the Standard Amplitude Reference level using the Standard

Reference Surface end test head on the test bed. The candidate disk is then substituted for

the Standard Reference Surface and is recorded in a similar fashion. The Measured Amplitude

of the candidate disk test area is then derived from observations made of the read-back

signal. The method of deriving the Measured Amplitude is described in section 4.2.b. The

Calibration Factor is then calculated as the ratio of the Standard Amplitude Reference value

to that Measured Amplitude of the candidate disk. Several calibrations are made on each disk

using different heads. Any erroneous readings are detected by this process.

A data base of measurements derived from the primary surfaces and the families of workin<

reference surfaces and heads is being maintained in order to determine the precision of the

measurements. From this data base, the computed standard deviation a for each disk Calibration Factor is recalculated as the number of measurements n increases. The Calibration Factoi

of each candidate disk has an associated Uncertainty Factor of 3o that is the current largest

value of 3a computed from the NBS family of reference surfaces. Assuming a normal distribution, the chance of any single error having an absolute value greater than three times the

computed standard deviation 3a is one in four hundred. The Uncertainty Factor is an index of

precision for the calibration process, equipment, and the phenomenon that is being measured

A candidate disk is reported to have a Calibration Factor and an Uncertainty Factor, i.e., Cn + 3a, for e^ery test condition.

The reported Calibration Factor for any disk is the average value of several independent

calibrations made over a period of time. By calibrating the candidate disk several times,

the probability of a single observational error biasing the resulting Calibration Factor is

reduced. Constant system errors are not eliminated by repetitive calibrations. Uniformly

occurring measurement errors will not affect the validity of the reported Calibration Factor

The designation of plus or minus 3a as the precision of the calibration service results in a conservative estimate of precision from the data base.

3. Component Parts of the Measurement System

3.1. Disks

Disks that are used in the various types of disk packs usually consist of an aluminum

substrate which is coated with a magnetic oxide formulation. The thickness and formulation

of the coating depends upon the application of the disk, i.e., low, medium, or high density

use. The mixture is spin-coated onto the aluminum substrate so as to produce a wedge-shaped

coating whose thickest section is located toward the outer diameter of the disk. This is

2
done in order to assure that the ratio of the read head signal levels which are produced

by the outer disk track and the inner disk track is approximately two to one. The read

system electronics can then easily be designed to operate within this range. The double

frequency encoding/decoding system of recording is employed. In this mode of recording,

a flux reversal takes place every clock time. A flux reversal also occurs between clock

times for every "one" bit. The coating formulation and the head design are also controlled

to produce a head signal at the basic frequency which is approximately twice the output at twice the basic frequency (all "ones" pattern) . There are two types of surfaces used for low

and medium density recording. The low density (1100 bpi) is used in the six-high disk pack.

The medium density (2200 bpi) is used in t^e eleven-high disk pack and the single-disk front

and top loading cartridges. In recent years, some manufacturers are utilizing the medium

density type, rather than the low density type, in the six-high disk pack.

3.2 Heads

The IBM 2316 (2311 type) amplitude test head and the IBM 2314 data test head (or their

equivalents) have been specified in the ANSI and ISO standards as being acceptable type test

heads for the appropriate disk type testing. The 2316 amplitude test head that is used with

the six-high disk pack has a stainless steel pad, is center-pivot loaded, and flies above

the disk surface at a height of approximately 100 microinches. There are two tunnel erase windings on the head pad immediately following the write winding. The tunnel erase gaps are

parallel to, and overlap each end of, the write gap. When written with tunnel erase current

off, the track width of an amplitude test head is 0.201 + 0.0025 mm (0.0079 ± 0.009 inches).

When written with tunnel erase current on, the track width is 0.140 ± 0.013 mm (0.0055 + 0.0005

inches). The 2314 data test head that is used with the eleven-high disk pack and the singledisk cartridges has a ceramic pad, is center-pivot loaded, and flies above the surface at a height of approximately 75 microinches. There are two straddle erase windings on the head

pad immediately following the write windings. The straddle erase gaps are perpendicular to,

and are located at the ends of, the write gap. When written with straddle erase current on,

the track width of a data test head is 0.175 + 0.005, -0.010 mm (0.0069 + 0.0002, -0.004

inches). When measured at the head cable connector, the resonant frequency of one-half of

the center tapped write/read coil of acceptable amplitude test heads must be 2.0 megahertz,

or greater; while that of acceptable data heads must be 4.2 megahertz, or greater. Methods

of measuring the characteristics of track width, resolution, and resonant frequency are described in Appendix C.

Every disk surface in the pack has an associated head. Heads are flown in pairs diametrically positioned on opposite sides of each disk. Space limitations require that the

heads are displaced on either side of a radial line to the center of rotation. The heads move in and out on a line of access parallel to this radial. Four different head designs

(A-up, A-down, B-up, and B-down) are needed because of these position requirements. The

head write gaps are radial to the center of rotation at only one track location. The write

gaps have an increasingly positive or negative skew angle to the radial on either side of

that track.

The flying height of the head is determined by its aerodynamic design, the surface

velocity, the physical characteristics of the disk surface, the head-load force, and the

temperature and humidity conditions. The amplitude of the head read signal is determined

by the head flying height and the frequency of the recorded signal. The read-back signal

amplitude decreases as the signal half-wavelength approaches the gap length dimension of the

read head. It has been found that the read-back characteristics between the same type test

heads of a given manufacturer may vary as much as 20 percent when writing with tunnel erase current on. One reason for these differences between heads may be that the read/write and

the tunnel/straddle erase winding assemblies are manually installed in the head pad during

the manufacturing process. The differences in the characteristics between heads are some- what less when writing with the tunnel erase current off.

3.3. The Disk Test Bed

a. The Mechanical System

The ANSI and ISO standards for the various types of disk packs specify the required

mechanical and electrical characteristics of the test bed to be used for measuring and

3
calibrating disk surfaces. The NBS test bed used for calibrating low and medium density

surfaces is the Applied Magnetics Corporation Model HTU-210 2AB tester. It has been specifically designed to meet these requirements. The mechanical assembly consists of a precision

spindle, a fixed speed 2400 ± 24 rpm motor, a precision head loading and actuating mechanism,

an electromechanical control panel, and is equipped for mounting industry-compatible IBM 1316

and IBM 2316 type disks and 2316 and 2314 type heads. A permanent magnet assembly is used to provide dc erase for the disk surfaces. The heads can be moved to any desired track position

by means of a track counter unit. Calibrated head load forces can be set to a net loading

force of 3.43 + 0.10 newtons (350 + 10 grams) by means of precision gram gauges. The total

head loading force, including head spring tension, is 3.73 ± 0.10 newtons (380 + 10 grams).

The disk surface is supported on the test bed at both the inner and outer diameters. Test

heads are flown from above the top surface only, with the direction of rotation of the spindle

being selected to maintain proper direction relationship when using "up" or "down" type heads.

Reference to this particular test bed is for informational purposes only and does not imply

any endorsement by the National Bureau of Standards. Any equivalent type test bed that meets

the mechanical and electrical requirements may be used.

b. The Write Channel

The block diagram of the write channel operation of the NBS disk measurement system is shown in figure 1. Front panel control switches give the operator the options of selecting

the proper write current level, write frequency, and tunnel/straddle erase currents for the

various testing conditions. The WRITE 1/REA0 mode is used when calibrating disk surfaces.

In this mode, upon pressing the START switch, a dc erase operation is performed during the

first revolution, writing takes place during the second revolution, and continual reading

occurs on successive revolutions. Appropriate type read/write driver circuit cards are employed for using either amplitude or data test heads. When driving an applicable dummy

resistive load, the balanced current driver is capable of switching the write current between

its two outputs in less than 75 nanoseconds when the 2311 driver card is selected, and less

than 25 nanoseconds when the 2314 driver card is selected. Appendix C describes the applicable

resistive load. The write current waveform may be observed from the output of a current transformer'. This transformer consists of a one turn loop around a lead to one end of the write

head winding. The write current waveforms for both amplitude and data test heads meet the

characteristic requirements as shown in figure 2. These write current waveforms are specified

in national and international disk pack standards. Saturation recording technique is employed

The tunnel /straddle erase current is 40 mA dc. The write current of the 2316 amplitude head

is 35 mA for all tracks. When using the 2314 data head, tracks 000 through 127 are written

with 35 mA of write current, and tracks 128 through 200 are written with 30 mA of write current

Three basic frequencies are used for calibrating low and medium density surfaces. The IF frequency is 0.625 MHz (1.25 megatransitions per second), the 2F frequency is 1.25 MHz (2.5

megatransitions per second), and the 4F frequency is 2.5 MHz (5.0 megatransitions per second).

The physical location of type A and type B heads with respect to the line of access and the

radial to the center of rotation is important in order that the read/write gaps display proper

skew angle. Figure 3 shows the physical location of heads on the test bed. Track 000 (R2)

is located 166.726 mm (6.54 + 0.010 inches) from the center of rotation, and track 200 (R\)

is located 115.062 + 0.25 mm (4.530 + 0.010 inches) from the center of rotation.

c. The Read Channel

The block diagram of the read channel of the NBS test bed is shown in figure 4. When

measured at the head termination connector, the amplitude test heads see a differential input

impedance at the read amplifier of 7500 ohms resistance in parallel with a capacitance of

approximately 45 picofarads. Data test heads see a differential input impedance at the read

amplifier of 3000 ohms resistance in parallel with a capacitance of approximately 20 picofarads. The input read amplifier has a gain of 100, and a bandwidth that is flat within

+ 3% in the frequency range from 300 kilohertz to 4.0 megahertz. The output of the read

amplifier goes to an oscilloscope and to the Calibration Factor Correction Unit. The

Calibration Factor Correction Unit contains five precision potentiometer networks. The

output of each potentiometer is "And gated" with switched dc signals which are selected by

TRACK and FREQUENCY or switch settings. The output of the Calibration Factor Correction

Unit goes to a Digital Read Out Unit.

The Digital Read Out Unit contains a post amplifier, peak detector and buffer, sampleand-hold, scaling network, and a digital voltmeter for visual readout. The analog read head

4
TUNNEL/STRADDLE

ERASE SELECTION

WRITE CURRENT

SELECTION

2316/2314

HEAD SELECTOR

CURRENT

CONTROL

WRITE MODE

SELECTION

1

'

i MASTER

CONTROL

SEQUENCER

START

WRITE FREQUENCY

SELECTION

'

1

1 CLOCK

FREQUENCY OSCILLATOR

S ELEC ro R

/\ s\

READ/WRITE

HEAD

FIGURE 1. BLOCK DIAGRAM OF THE WRITE CHANNEL
2316 AMPLITUDE TEST

HEADS

2314 DATA TEST HEADS

'w

35±1 mA (ALL TRACKS) 35+1 mA (TRACK 000)

30 ±1 mA (TRACK 200)

'o <10%l w <8%lw

Tr 170±30 NANOSECONDS 140+20 NANOSECONDS

T

F 170±30 NANOSECONDS 140+20 NANOSECONDS

TR =T

F ±20 NANOSECONDS +20 NANOSECONDS

Tl =T2 ±2% ±2%

FIGURE 2. WRITE CURRENT WAVEFORM
TYPE HEAD Far Fbr

DATA 12.125mm (0.4774") 10.987 mm (0.4326")

AMPLITUDE 11.999 mm (0.4724") 10.861 mm (0.4276")

RADIAL TO CENTER OF ROTATION

LINE OF ACCESS

R/WGAP

B HEAD

FIGURE 3. GEOMETRY OF HEAD DISK SYSTEM
<zo

LING

/ORK lH

ITAL 5LT TER < £ U uJ

"• z

S2 > i

i—z at

LU LU \ u.

Li.

O 3 rj O o

CD at

Q

h- <LU

at

_i

T

-Jo 1 < LU o

^2

—i

Q.s<

CDO < u LU LU

CL 1-

111

CO

CO

Q

at

_i

LUQo t

MPLE GGER

WORK S at

LU 1/1 1 1 »- z

to

— o O ct x < :

INPUT A INPUT B

OSCILLO- SCOPE t 1

at

LU

VARIABLE

TIME-DELAY NETWORK

1/1 <1 >-

CO

at

{ATION :tor

ECTION NIT

J . Oo

i— <

ALIBI FA< :orr

U

a \ ••* LU _l |—

—'

UL < U <

a.Ou | —

—=

atCO

u « 3 Z "j z

0.

O - O

to

-j .• 1 < •* [ \ < : PUT AD

LIFIER 00) OPE

ERENT.,

OBE 2 O /-\ to

QLU

_ LU Q. •—

u u. at vi |l a. |z

CO

»— < Q «/) u> A 1 1

CM \ '

1^> Id

— t 1 r.

HEAD

DISK, I

_J

cd<

CO

LU

ocCD
t CM

O 00 (d/d aw) inoavaa ivuoia

c^
signal from the input amplifier is first amplified by the post amplifier which has a gain of

10, and is then peak detected. The peak detector has a very fast charge time and a very slow

discharge time. The detector senses the higher peaks of the analog signal, and misses very

low amplitude pulses. The output of the detector is sample-and-held once ewery revolution

of the disk. The sample time is triggered by an index sensor at the spindle, and is adjustable over 85 percent of a disk revolution by means of a variable time delay network. The

sample time is 60 microseconds in duration, at the end of each sample time the instantaneous

level of the sampled signal is held until the next sample time. The sample-and-hold output goes

through a scaling network into a digital voltmeter for visual readout. The system noise of

the digital voltmeter is less than 0.20 mV as displayed. The Digital Read Out Unit response

is not exactly linear with respect to signal amplitude and frequency. The output reading of

the digital voltmeter must therefore be corrected according to the correction curve as shown

in figure 5.

The "Measured Amplitude" of the read signal is the actual peak-to-peak signal amplitude

value averaged over the measured area, when recorded in accordance with given test conditions.

The measured areas are located by scribe marks on the surface of the disk at R\ and at R2. The measured areas begin 50 microseconds after the trailing edge of the scribe mark and

extend 225 microseconds in length, with the disk rotating at 2400 rpm. The scribe mark

produces a discontinuity of the surface magnetic media. The result of the scribe mark is readily observed on the oscilloscope as a pair of positive and negative pulses. The TIME DELAY potentiometer to the sample-and-hold circuit can be adjusted so that the trailing edge

of the sample-and-hold trigger occurs simultaneously with the scribe mark as displayed on the

oscilloscope. Under these conditions, the value displayed by the digital voltmeter is a measure of the average peak-to-peak read head signal produced during the first 60 microseconds of the measured test area. Five additional time delay steps to the trigger pulse

can be inserted by the TEST AREA switch on the Read Out Unit in order to allow read-out

observations of five discrete areas within the 225 microsecond test area. Calculations are

then made of these five observations to determine the average Measured Amplitude of the test

area.

The Measured Amplitude of the candidate disk test area is used in calculating the Calibration Factor. The Standard Reference Surface and test head are first put on the disk test

bed. The appropriate Calibration Factor Correction Unit potentiometer is then preadjusted

to produce a readout on the digital voltmeter, when averaged over the test area, that is

the Standard Amplitude Reference value for that condition of track and frequency. The

candidate disk is then substituted in place of the Standard Reference Surface, and the

Measured Amplitude is then observed. The setting of the Calibration Factor Correction Unit

is left unchanged during this operation. The Calibration Factor for the candidate disk is then calculated as being the ratio of the Standard Amplitude Reference to that of the value

of Measured Amplitude of the candidate disk.

4. The Disk Calibration Procedure

The equipment that is used in the NBS laboratory for calibrating low and medium disk

surfaces consists of an Applied Magnetics Corporation (AMC) Model HTU-210 2AB test bed, AMC

Model HTR-300 B Digital Read Out Unit, AMC Model CFC-210 Disk Calibration Factor Correction

Unit, Renco Model R3-S counter (used for counting track position), and a Tektronix Model 545

oscilloscope with a dual input 1A5 plug-in unit. Switch names are capitalized in the operating procedure description that follows. HEAD SELECT switches are located on both the HTU-210

2AB test bed and the Disk Calibration Factor Correction Unit. The front panel of the test

bed also contains the following switches: TRACK, FREQUENCY, WRITE CURRENT, MODE, and TUNNEL/

STRADDLE ERASE. The Calibration Factor Correction Unit also contains a CALIBRATE/BYPASS

switch and the following potentiometers: TRACK 000 1/2F, TRACK 200 IF, TRACK 000 IF, and

TRACK 200 2F. The CALIBRATE/BYPASS switch is normally left in the calibrate position. The

Read Out Unit contains a five position TEST AREA switch and a TIME DELAY potentiometer.

Reference to the particular test equipment used is for informational purposes only and does

not imply any endorsement by the National Bureau of Standards. Any equivalent type test equipment may be used.

All electrical power to the equipment should be applied at least one-half hour before

taking any measurements in order to allow the operating temperature to stabilize. The

operating laboratory temperature should be 23° + 3°C (73° ± 5°F), and the relative humidity

should be in the range between 40 percent and 60 percent. The analog signal output of the

read amplifier is connected to one input of the plug-in unit of the oscilloscope, and the

10
sample trigger pulse of the Read Out Unit connected to the other input. This is done in order to accurately position the observation points within the test area. Set the Track

Counter to 000, with the test bed positioned so that the head position dial indicator reads

0. All of the disks and heads should be acclimatized for at least 24 hours under ambient

laboratory temperature and humidity conditions before initiating the calibration procedure.

The test bed, disks, and heads should all be wiped clean before each measurement with a solution of 91 percent alcohol and 9 percent water by volume.

4.1. Calibration Procedure for Low Density Surfaces

(1) Placement of the Standard Reference Surface on the NBS test bed:

(a) Place the Low Density Standard Amplitude Reference Surface and the 2316

amplitude test head on the test bed.

(b) Load the head onto the disk with 350 grams of net load force.

(c) Position the head to track 000.

(2) Switch Settings:

(a) WRITE CURRENT 35 mA,

(b) TUNNEL ERASE on,

(c) WRITE FREQUENCY 0.625 MHz,

(d) MODE write 1/read,

(e) TRACK 000,

(f) HEAD SELECT 2311.

(3) Oscilloscope Adjustments:

(a) TEST AREA switch to 0. (b) Adjust TIME DELAY potentiometer until the trailing edge of the sample trigger

pulse occurs at the same time as the scribe mark. (This can be observed on the oscilloscope.

(4) Calibrating the NBS test bed at 1/2F test conditions:

(a) Press the START button.

(b) Adjust the TRACK 000 1/2F potentiometer so that the average value displayed by

the digital voltmeter is 11.3 mV. (This adjustment should be made while switching the TEST

AREA switch to each one of its five positions.)

(c) Position the head to track 200.

(d) Position the TRACK switch to 200.

(e) Repeat step 3. (f) Press the START button.

(g) Adjust the TRACK 200 1/2F potentiometer in the manner that is described in step (4.b) so that the average value displayed by the digital voltmeter is 6.6 mV.

(5) Calibrating the NBS test bed at IF test conditions:

(a) Position the WRITE FREQUENCY switch to 1.25 MHz.

(b) Press the START button.

(c) Adjust the TRACK 200 2F potentiometer in the manner that is described in step

(4.b) so that the average value displayed by the digital voltmeter is 3.5 mV.

(6) Candidate Disk Calibration at 1/2F test conditions:

(a) Substitute the candidate disk in place of the Low Density Standard Amplitude

Reference Surface.

(b) Load the head onto the disk with 350 grams of net load force.

(c) Position the FREQUENCY switch to 0.625 MHz.

(d) Position the TRACK switch to 000. (From this point on, do not change the settings

of the potentiometers from those obtained during steps 4, 5.)

(e) Position the head to track 000.

(f) Repeat step 3. (g) Press the START button.

11
(7) Calculation of Calibration Factor for Test A: Calculate the average value of the five

readings displayed on the digital voltmeter that are obtained when switching the TEST AREA

switch to each one of its five positions. This calculated average value is the Measured

Amplitude of the candidate disk for low density Test A conditions. The Calibration Factor

(C n

) of the candidate low density disk for Test A is then calculated according to the

following equation:

T t

a.

r

_ Standard Amplitude Reference (11.3 mV)

D Measured Amplitude of candidate disk (mV)

(8) Calculation of Calibration Factor for Test B: Position the head to track 200. Position

the TRACK switch to 200. Repeat step 3. Press the START button. Calculate the average

value of the five readings displayed on the digital voltmeter that are obtained when switching the TEST AREA switch to each one of its five positions. This calculated average value is

the Measured Amplitude of the candidate low density disk for Test B conditions. The Calibration Factor (Cn) of the candidate low density disk for Test B is then calculated according to the following equation:

T

<..,. R

. r

_ Standard Amplitude Reference (6.6 mV) 11 D Measured Amplitude of candidate disk (mV)

(9) Calculation of Calibration Factor for Test C: Position the WRITE FREQUENCY to 1.25 MHz. Press the START button. Calculate the average value of the five readings displayed on the

digital voltmeter that are obtained when switching the TEST AREA switch to each one of its

five positions. This calculated average value is the Measured Amplitude of the candidate low

density disk for Test C conditions. The Calibration Factor (Cn) of the candidate low density

disk for Test C is then calculated according to the following equation:

Tpct r- r Standard Amplitude Reference (3.5 mV)

D Measured Amplitude of candidate disk (mV)

4.2. Calibration Procedure for Medium Density Surfaces

a. Amplitude Reference Calibration

(1) Placement of the Standard Reference Surface on the NBS test bed:

(a) Place the Medium Density Standard Amplitude Reference Surface and the 2316

amplitude test head on the test bed.

(b) Load the head onto the disk with 350 grams of net load force.

(c) Position the head to track 000.

(2) Switch settings:

(a) WRITE CURRENT 35 mA,

(b) TUNNEL ERASE off,

(c) WRITE FREQUENCY 0.625 MHz,

(d) MODE write 1/read,

(e) TRACK 000,

(f) HEAD SELECT 2311.

(3) Oscilloscope adjustment:

(a) Position the TEST AREA switch to 0. (b) Adjust the TIME DELAY potentiometer until the trailing edge of the sample trigger

pulse occurs at the same time as the scribe mark. (This can be observed on the oscilloscope,

(4) Calibrating the NBS test bed at 1/2F test conditions:

(a) Press the START button.

12
(b) Adjust the TRACK 000 1/2F potentiometer so that the average value displayed by

the digital voltmeter is 11.5 mV. (This adjustment should be made while switching the

TEST AREA switch to each one of its five positions.)

(c) Position the head to track 200.

(d) Position the TRACK switch to 200.

(e) Repeat step 3. (f) Press the START button.

(g) Adjust the TRACK 200 1/2F potentiometer in the manner that is described in

step (4.b) so that the average value displayed by the digital voltmeter is 7.0 mV.

(5) Candidate Disk Calibration at 1/2F test conditions:

(a) Substitute the candidate disk in place of the Medium Density Standard Amplitude

Reference Surface.

(b) Load the head onto the disk with 350 grams of net load force.

(c) Position the TRACK switch to 000. (From this point on, do not change the

settings of the potentiometers from those obtained during steps 4 and 5.)

(d) Position the head to track 000.

(e) Repeat step 3. (f) Press the START button.

(6) Calculation of Calibration Factor for Test A: Calculate the average value of the

five readings displayed on the digital voltmeter that are obtained when switching the TEST

AREA switch to each one of its five positions. This calculated average value is the

Measured Amplitude of the candidate disk for medium density Test A conditions. The Calibration Factor (Cnn) of .the candidate medium density disk for Test A is then calculated

according to the following equation:

T

. . r Standard Amplitude Reference (11.5 mV)

JAD Measured Amplitude of candidate disk (mV)

(7) Calculation of Calibration Factor for Test B: Position the head to track 200. Position

the TRACK switch to 200. Repeat step 3. Press the START button. Calculate the average

value of the five readings displayed on the digital voltmeter that are obtained when switching the TEST AREA switch to each one of its five positions. This calculated average value 1s

the Measured Amplitude of the candidate disk for medium density Test B conditions. The

Calibration Factor {C/\q) of the candidate medium density disk for Test B is then calculated

according to the following equation:

T t R r Standard Amplitude Reference (7.0 mV) est a. lad - Measured Amplitude of candidate disk (mV)

b. Data Reference Calibration

(1) Placement of Standard Reference Surface on the NBS test bed:

(a) Place the Medium Density Standard Data Reference Surface and the 2314 data test

head on the test bed.

(b) Load the head onto the disk with 350 grams of net load force.

(c) Position the head to track 200.

(2) Switch settings:

(a) WRITE CURRENT 30 mA,

(b) STRADDLE ERASE on,

(c) WRITE FREQUENCY 1.25 MHz,

(d) MODE write 1/read,

(e) TRACK 200,

(f) HEAD SELECT 2314

13
(3) Oscilloscope adjustment:

(a) Position the TEST AREA switch to 0. (b) Adjust the TIME DELAY potentiometer until the trailing edge of the sample trigger

pulse occurs at the same time as the scribe mark. (This can be observed on the oscilloscope.

(4) Calibrating the NBS test bed at 1.25 megahertz test conditions:

(a) Press the START button.

(b) Adjust the TRACK 200 1/2F potentiometer so that the average value displayed by

the digital voltmeter is 3.8 mV. (This adjustment should be made while switching the TEST

AREA switch to each one of its five positions.)

(5) Calibrating the NBS test bed at 2.-5 megahertz test conditions:

(a) Position the WRITE FREQUENCY switch to 2.5 MHz.

(b) Press the START button.

(c) Adjust the TRACK 200 IF potentiometer in the manner that is described in step

(4.b) so that the average value displayed by the digital voltmeter is 1.5 mV.

(6) Candidate disk calibration at IF test conditions:

(a) Substitute the candidate disk in place of the Medium Density Standard Data Reference Surface.

(b) Load the head onto the disk with 350 grams of net load force.

(c) Position the head to track 200.

(d) Position the FREQUENCY switch to 1.25 MHz. (From this point on, do not change

the settings of the potentiometers from those obtained during steps 4 and 5.)

(e) Repeat step 3. (f) Press the START button.

(7) Calculation of Calibration Factor for Test C: Calculate the average value of the five

readings displayed on the digital voltmeter that are obtained when switching the TEST AREA

switch to each one of its five positions. This calculated average value is the Measured

Amplitude of the candidate disk for medium density Test C conditions. The Calibration

Factor (Cpn) of the candidate medium density disk for Test C is then calculated according to the following equation:

T

- t r

. r

. Standard Data Reference (3.8 mV) esz L ^_^ ' L DD Measured Amplitude of candidate disk (mV)

(8) Calculation of Calibration Factor for Test D: Position the WRITE FREQUENCY switch to 2.5 MHz. Press START button. Calculate the average value of the five readings displayed

on the digital voltmeter that are obtained when switching the TEST AREA switch to each one

of its five positions. This 'calculated average value is the Measured Amplitude of the

candidate disk for medium density Test D conditions. The Calibration Factor (C^q) of the

candidate medium density disk for Test D is then calculated according to the following

equation:

T

. n . r _ Standard Data Reference (1.5 mV) est ^_^

DD Measured Amplitude of candidate disk (mV)

5. Preparation of the Disk Calibration Report

Each disk calibration is repeated nine times for each one of the test conditions.

The calibration process is repeated three times with each one of three appropriate type

amplitude or data test heads. Any observational or operational errors may be detected and

eliminated by means of this process. The calculated average value of the Disk Calibration

Factor that is obtained from these repetitive tests is then reported on the Disk Calibration

Report form. The forms that are used for Low and Medium Density Disk Calibration Reports

14
are shown in Appendix A and in Appendix B. The value of the Uncertainty Factor given for

each test condition is three times the value of the computed standard deviation. The

computed standard deviation is derived from a data base of measurements that is being

continually maintained and updated. These measurements are made using a group of primary

and working reference surfaces and test heads for low and medium density use. A PDP-11/45

computer has been programmed for calculating and updating the current values of Disk

Calibration Factors and Uncertainty Factors that are obtained from this data base.

The assistance of Mr. Sidney B. Geller as technical consultant, of Mr. Michael D. Hogan

in maintaining the data base of measurements of reference surfaces and test heads and the

calculations for computing Disk Calibration Factors and Uncertainty Factors, and of

Mr. William B. Truitt in developing and implementing the computer program that is used for

calculating the Disk Calibration Factors and Uncertainty Factors that are derived from this

data base is gratefully acknowledged. The assistance of Mrs. Candy Leatherman in the

preparation of the manuscript is also gratefully acknowledged. All of the above mentioned

personnel are members of the Computer Systems Engineering Division.

15
Appendix A

LOW DENSITY DISK CALIBRATION REPORT FORM

NBS Magnetic Media Group

Report of Disk Calibration

Test No.: Date of Test: Low Density (1100 bpi) Disk:

Submitted by:

TEST TRACK; FREQUENCY SAR

A

B

C

000; 0.625 MHz

200; 0.625 MHz

200; 1.25 MHz

11.3 mV

6.6 mV 3.5 mV

CALIBRATION FACTOR (C

D^- UNCERTAINTY FACTOR

+ 07

+ 06 + 09

NOTES: 1. 2316 Amplitude Type Test Head used with Tunnel Erase Current on.

2. Uncertainty Factor is defined as three times the standard deviation; i.e., 3a.

3. Calibrated Surface is side of disk inscribed with identifying number and scribes.

4. Calibration Factor defined as:

c

_ Standard Amplitude Reference (SAR)

D

" Candidate Measured Signal Amplitude

PERFORMED BY:

Data Acquisition and Storage Section

Institute for Computer Sciences and Technology

National Bureau of Standards

16
Appendix B MEDIUM DENSITY DISK CALIBRATION REPORT FORM

NBS Magnetic Media Group

Report of Disk Calibration

Test No.: Date of Test: Medium Density (2200 bpi) Disk:

Submitted by:

(Using 2316-Amplitude Type Test Head Without Tunnel Erase Current)

TEST TRACK FREQUENCY SAR CALIBRATION FACTOR (C Ap

)

000

200

0.625 MHz 0.625 MHz

11.5 mV

7.0 mV

II. (Using 2314-Data Type Test Head With Straddle Erase Current On)

TEST TRACK FREQUENCY CALIBRATION FACTOR (C

p p

)

c 200 1 25 MHz 3 8 mV

D 200 2 5 MHz 1 5 mV

UNCERTAINTY FACTOR

+ .04 + .07

UNCERTAINTY FACTOR

+ .10 + .14

NOTES: 1. Uncertainty Factor is defined as three times the standard deviation; i.e.,

3a.

2. Calibrated Surface is side of disk inscribed with identifying number and

scribes.

3. Calibration Factor defined as:

_ _ Amplitude or Data Reference (SAR or SDR' AD DD Candidate Measured Signal Amplitude

PERFORMED BY:

Data Acquisition and Storage Section

Institute for Computer Sciences and Technology

National Bureau of Standards

17
Appendix C MAGNETIC MEASUREMENTS

Track Width

In order to measure the effective track width of a test head, a seven track wide band with

track location 200 in the center of the band is erased. Next, when using an amplitude test

head, a 0.625 MHz pattern on track 200 is recorded with tunnel erase. When using a data test head, record with straddle erase a 1.25 MHz pattern is recorded on track 200 with

straddle erase. Then move the head radially to the disk in increments of 0.0025 mm (0.001

inch) to the left and to the right of track 200 until the read-back signal becomes zero. Determine the read-back signal amplitude at each incremental move, and plot amplitude (Y

axis) versus displacement (X axis). The fringing at both ends of the curve is to be ignored.

See Appendix D for a typical plot of track width.

Test Head Resolution

To measure the test head resolution, dc erase and subsequently record a IF frequency pattern

with an amplitude test head (2F frequency pattern when using a data test head) in the test

area of the Standard Reference Surface, and determine the read-back signal amplitude. Repeat

the same procedure with a 2F frequency pattern in the same area (4F frequency pattern when using

a data test head). Determine the read back signal amplitude. The ratio of the 2F amplitude

over the IF amplitude (4F amplitude over the 2F amplitude when using a data test head) multiplied by 100 represents the resolution of the test head in terms of percentage values.

Test Head Resonant Frequency

To measure resonant frequency, the use of a Vector Impedance Meter, Hewlett-Packard Model

4815A or equivalent is suggested. The frequency tuning control is adjusted until the phase

angle reads zero. At this point, the resonant frequency of the test head is indicated by the

meter.

Write Circuit Dummy Load

The suggested dummy load is two 10 ohm resistors replacing the half coils of the write head.

18
APPENDIX D TYPICAL TRACK WIDTH DIAGRAM

EFFECTIVE EFFECTIVE100%

t—i—i—i—i—r—i—i—i—i—i—

r

DISPLACEMENT (0.025 mm INCREMENTS)

19
U.S. DEPT. OF COMM. BIBLIOGRAPHIC DATA

SHEET

1. PUBLICATION OR REPORT NO. NBS TN-884

2. Gov't Accession

No.

3. Recipient's Accession No.

4. TITLE AND SUBTITLE

Calibration of Unrecorded Low and Medium Density Type Magneti

Disk Pack Surfaces

5. Publication Date October 1975

6. Performing Organization Code

7. AUTHOR(S)

Nicholas P. Goumas

8. Performing Organ. Report No.

9. PERFORMING ORGANIZATION NAME AND ADDRESS

NATIONAL BUREAU OF STANDARDS

DEPARTMENT OF COMMERCE

WASHINGTON, D.C. 20234

10. Project/Task/Work Unit No.

11. Contract/Grant No.

12. Sponsoring Organization Name and Complete Address (Street, City, State, ZIP) 13. Type of Report & Period Covered Interim

14. Sponsoring Agency Code

15. SUPPLEMENTARY NOTES

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

bibliography or literature survey, mention it here.) This publication describes the design requirements and the operation of the NBS

test bed that is used for calibrating unrecorded magnetic disk pack surfaces for

low and medium density use. The signal level calibration is made with respect to

a reference level that is derived from the NBS Standard Amplitude and Data

Reference Surfaces that are held in repository at NBS. The techniques for

calibrating the measurement system with the NBS repository Reference Surfaces

and the calibration of candidate reference disks are described in detail.

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

Calibration Factor; computer amplitude reference; computer storage media; disk

calibration; disk pack; magnetic disk; Standard Reference Surface; unrecorded disk

surface.

18. AVAILABILITY |xZ Unlimited

~2 For Official Distribution. Do Not Release to NTIS

|X ! Order From Sup. of Doc, U.S. Government Printine Office Washington, D.C. 20402, SD Cat. No. C13. 46:8%4

_j Order From National Technical Information Service (NTIS)

Springfield, Virginia 22151

19. SECURITY CLASS

(THIS REPORT)

UNCLASSIFIED

20. SECURITY CLASS

(THIS PAGE)

UNCLASSIFIED

21. NO. OF PAGES

23

22. Price

\s$ cents

•JSCOMM-DC 29042-P74 * U.S. GOVERNMENT PRINTING OFFICE: 1975-210-959/9
NBS TECHNICAL PUBLICATIONS

PERIODICALS JOURNAL OF RESEARCH reports National Bureau

of Standards research and development in physics,

mathematics, and chemistry. It is published in two sections, available separately:

• Physics and Chemistry (Section A)

Papers of interest primarily to scientists working in these fields. This section covers a broad range of physical and chemical research, with major emphasis on

standards of physical measurement, fundamental constants, and properties of matter. Issued six times a

year. Annual subscription: Domestic, $17.00; Foreign,

$21.25.

• Mathematical Sciences (Section B)

Studies and compilations designed mainly for the mathematician and theoretical physicist. Topics in mathematical statistics, theory of experiment design, numerical analysis, theoretical physics and chemistry, logical design and programming of computers and computer

systems. Short numerical tables. Issued quarterly. Annual subscription: Domestic, $9.00; Foreign, $11.25. DIMENSIONS/NBS (formerly Technical News Bulletin) —This monthly magazine is published to inform

scientists, engineers, businessmen, industry, teachers,

students, and consumers of the latest advances in science and technology, with primary emphasis on the work at NBS. The magazine highlights and reviews such

issues as energy research, fire protection, building tech- nology, metric conversion, pollution abatement, health and safety, and consumer product performance. In addition, it reports the results of Bureau programs in measurement standards and techniques, properties of matter and materials, engineering standards and services, instrumentation, and automatic data processing. Annual subscription: Domestic, $9.45; Foreign, $11.85.

NONPERIODICALS Monographs—Major contributions to the technical liter- ature on various subjects related to the Bureau's scientific and technical activities. Handbooks—Recommended codes of engineering and

industrial practice (including safety codes) developed

in cooperation With interested industries, professional

organizations, and regulatory bodies. Special Publications—Include proceedings of confer- ences sponsored by NBS, NBS annual reports, and other

special publications appropriate to this grouping such

as wall charts, pocket cards, and bibliographies.

Applied Mathematics Series—Mathematical tables, manuals, and studies of special interest to physicists, engineers, chemists, biologists, mathematicians, computer programmers, and others engaged in scientific and technical work.

National Standard Reference Data Series—Provides

quantitative data on the physical and chemical properties of materials, compiled from the world's literature and critically evaluated. Developed under a world-wide

program coordinated by NBS. Program under authority

of National Standard Data Act (Public Law 90-396). NOTE: At present the principal publication outlet for

these data is the Journal of Physical and Chemical

Reference Data (JPCRD) published quarterly for NBS

by the American Chemical Society (ACS) and the American Institute of Physics (AIP). Subscriptions, reprints, and supplements available from ACS, 1155 Sixteenth

St. N. W., Wash. D. C. 20056. Building Science Series—Disseminates technical infor- mation developed at the Bureau on building materials, components, systems, and whole structures. The series presents research results, test methods, and performance criteria related to the structural and environmental functions and the durability and safety characteristics of building elements and systems.

Technical Notes—Studies or reports which are complete

in themselves but restrictive in their treatment of a

subject. Analogous to monographs but not so comprehensive in scope or definitive in treatment of the subject area. Often serve as a vehicle for final reports of work performed at NBS under the sponsorship of other government agencies.

Voluntary Product Standards—Developed under pro- cedures published by the Department of Commerce in Part 10, Title 15, of the Code of Federal Regulations. The purpose of the standards is to establish nationally

recognized requirements for products, and to provide

all concerned interests with a basis for common under- standing of the characteristics of the products. NBS

administers this program as a supplement to the activities of the private sector standardizing organizations. Federal Information Processing Standards Publications (FIPS PUBS)—Publications in this series collectively constitute the Federal Information Processing Standards Register. Register serves as the official source of information in the Federal Government regarding stand- ards issued by NBS pursuant to the Federal Property and Administrative Services Act of 1949 as amended,

Public Law 89-306 (79 Stat. 1127), and as implemented

by Executive Order 11717 (38 FR 12315, dated Mav 11, 1973) and Part 6 of Title 15 CFR (Code of Federal

Regulations). Consumer Information Series—Practical information, based on NBS research and experience, covering areas

of interest to the consumer. Easily understandable language and illustrations provide useful background

knowledge for shopping in today's technological

marketplace.

NBS Interagency Reports (NBSIR)—A special series of interim or final reports on work performed by NBS for outside sponsors (both government and non-government). In general, initial distribution is handled by the sponsor; public distribution is by the National Technical Information Service (Springfield, Va. 22161) in paper

copy or microfiche form.

Order NBS publications (except NBSIR's and Biblio- graphic Subscription Services) from: Superintendent of Documents, Government Printing Office, Washington,

D.C. 20402. BIBLIOGRAPHIC SUBSCRIPTION SERVICES

The following current-awareness and literature-survey

bibliographies are issued periodically by the Bureau:

Cryogenic Data Center Current Awareness Service A literature survey issued biweekly. Annual subscription: Domestic, $20.00; foreign, $25.00. Liquefied Natural Gas. A literature survey issued quarterly. Annual subscription: $20.00. Superconducting Devices and Materials. A literature

survey issued quarterly. Annual subscription: $20.00. Send subscription orders and remittances for the pre- ceding bibliographic services to National Technical Information Service, Springfield, Va. 22161.

Electromagnetic Metrology Current Awareness Service Issued monthly. Annual subscription: $100.00 (Special rates for multi-subscriptions). Send subscription order and remittance to Electromagnetics Division, National Bureau of Standards, Boulder, Colo. 80302.
U.S. DEPARTMENT OF COMMERCE

National Bureau of Standards

Washington, DC. 20234

OFFICIAL BUSINESS

Penalty for Private Use. $300

POSTAGE AND FEES PAID

U.S. DEPARTMENT OF COMMERCE COM-215

SPECIAL FOURTH-CLASS RATE BOOK

dUJT/O/y

^e-igi16