NBS

lefereoce Pub! >-

...NAT'L INST. OF STAND & TECH rTM0nS

A 11 10 5 ^73^5

/c WW \

a NBS TECHNICAL NOTE 1161

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

Testing to Quantify

the Effects of Handling

of Gas Dielectric

Standard Capacitors

QC

100

.15753

No. 1161

1932
NATIONAL BUREAU OF STANDARDS

The National Bureau of Standards' was established by an act of Congress on 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's technical work is per- formed by the National Measurement Laboratory, the National Engineering Laboratory, and

the Institute for Computer Sciences and Technology.

THE NATIONAL MEASUREMENT LABORATORY provides the national system of

physical and chemical and materials measurement; coordinates the system with measurement

systems of other nations and furnishes essential services leading to accurate and uniform

physical and chemical measurement throughout the Nation's scientific community, industry,

and commerce; conducts materials research leading to improved methods of measurement,

standards, and data on the properties of materials needed by industry, commerce, educational

institutions, and Government; provides advisory and research services to other Government

agencies; develops, produces, and distributes Standard Reference Materials; and provides

calibration services. The Laboratory consists of the following centers:

Absolute Physical Quantities2 — Radiation Research — Chemical Physics —

Analytical Chemistry — Materials Science

THE NATIONAL ENGINEERING LABORATORY provides technology and technical ser- vices to the public and private sectors to address national needs and to solve national problems; conducts research in engineering and applied science in support of these efforts; builds and maintains competence in the necessary disciplines required to carry out this research and technical service; develops engineering data and measurement capabilities;

provides engineering measurement traceability services; develops test methods and proposes

engineering standards and code changes; develops and proposes new engineering practices;

and develops and improves mechanisms to transfer results of its research to the ultimate user. The Laboratory consists of the following centers:

Applied Mathematics — Electronics and Electrical Engineering2 — Manufacturing

Engineering — Building Technology — Fire Research — Chemical Engineering2

THE INSTITUTE FOR COMPUTER SCIENCES AND TECHNOLOGY conducts

research and provides scientific and technical services to aid Federal agencies in the selection, acquisition, application, and use of computer technology to improve effectiveness and

economy in Government operations in accordance with Public Law 89-306 (40 U.S.C. 759), relevant Executive Orders, and other directives; carries out this mission by managing the Federal Information Processing Standards Program, developing Federal ADP standards

guidelines, and managing Federal participation in ADP voluntary standardization activities; provides scientific and technological advisory services and assistance to Federal agencies; and

provides the technical foundation for computer-related policies of the Federal Government.

The Institute consists of the following centers:

Programming Science and Technology — Computer Systems Engineering.

'Headquarters and Laboratories at Gaithersburg, MD, unless otherwise noted; mailing address Washington, DC 20234. 'Some divisions within the center are located at Boulder, CO 80303.
Testing to Quantify the Effects of Handling

of Gas Dielectric Standard Capacitors

WATTOMJU. BOBEA0

OP STAtfDAB&a Charles R. Levy "W 24 198Z

Electrical Measurement and Standards Division

Center for Absolute Physical Quantities

National Measurement Laboratory

National Bureau of Standards

Washington, DC 20234

U.S. DEPARTMENT OF COMMERCE, Malcolm Baldrige, Secretary

NATIONAL BUREAU OF STANDARDS, Ernest Ambler, Director

Issued October 1982
National Bureau of Standards Technical Note 1161

Natl. Bur. Stand. (U.S.), Tech. Note 1161, 44 pages (Oct. 1982)

CODEN: NBTNAE

U.S. GOVERNMENT PRINTING OFFICE

WASHINGTON: 1982

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

Price $4.75

(Add 25 percent for other than U.S. mailing)
CONTENTS

Page

1. Abstract 1

2. Introduction 1

3. Methodology 2

4. Equipment Fabricated for Tests 5

5. Testing Procedure and Results 6

6. Connector Test 8

7. Orientation Test 8

8. Small Angle Test 10

9. Tilt Test 12

10. Knock Soft Test 16

11. Knock Hard Test 17

12. Drop Test 17

13. General Considerations 19

14. Data Reduction 19

15. Unusual Problems 21

16. Conclusion 22

17. Additional Observations 23

18. References 24

19. Appendix 1 27

20. Appendix 2 35

iii
LIST OF FIGURES p aQe

Figure 1. Tilting table for Angle Test 3

Figure 2. Special fixture for Knock Test 4

Figure 3. Reference points - all tests 7 Figure 4. View showing web assembly 9

Figure 5. Orientation test - side A 11

Figure 6. Angle Test - special block for 3 degrees in place .. 13

Figure 7. Tilt Test in progress - side A horizontal 14

Figure 8. Tilt Test in progress - test unit inverted 15

Figure 9. Set-up for simulated Drop Test 18

ACKNOWLEDGMENT

I acknowledge with appreciation Dr. Arthur 0. McCoubrey

for his expertise, time, and effort in the editing of this

paper.

IV
TESTING TO QUANTIFY THE EFFECTS OF HANDLING

OF

GAS DIELECTRIC STANDARD CAPACITORS

by Charles R. Levy

1. Abstract

A test method known as the "Handling and Stability Test" is

currently being used at NBS as part of the requirement for the

five-part-per-million calibration of gas dielectric standard

capacitors. This test is used to achieve qualitative information

on the effects of mechanical shock from shipping and handling on

standard capacitors and to rank them quantitatively with respect

to these effects.

2. Introduction

Nitrogen dielectric capacitors are used as the reference

standards for measurements of capacitance in many metrology

laboratories in the United States, as well as in many other parts

of the world. Since the most common standard used has a capacity

of 1000 picofarads, this report is based on tests of capacitors

of this value, but an equivalent test is performed on 10 and 100

pF capacitors as well.

Recognizing the range of mechanical shocks which these

capacitors may receive in service or in transport, NBS devised

tests to confirm the mechanical integrity of the units before

calibration and to determine the magnitude of any related effects

[1]. Such effects may then be taken into account quantitatively

in the systematic error portion of the uncertainty statement.

The results of the handling test were also expected to be useful

to guide the user concerning changes in capacitance to be

expected as a result of transport and normal handling. The use

of standard gas filled capacitors in transport service is rapidly

becoming a more important basis for the intercomparison of

reference standards among calibration laboratories. A similar

approach, based upon the use of a compressed gas capacitor

(specifically designed for high voltage) in transport service,

has also been investigated in connection with the intercomparison

of high voltage capacitance measurements [2].

The test procedure has also been used for the development of

a very stable, transportable capacitance standard. In

particular, the results of these tests were used in the selection

of capacitors and in the design of a suitable transport box for

shipping the capacitors.

If the procedures described within this paper are used, gas

dielectric capacitors not normally shipped to a primary reference

facility may be tested to identify possible handling problems

that may not otherwise be detected. In some metrology

laboratories, step-up or step-down methods are used (3, 4, 6, 7,

8) to compare capacitors having a range of value with a high

1
quality (e.g., fused silica) reference capacitor. In such cases,

the uncertainties assigned to the measurements should include

appropriate allowances for capacitance changes associated with

normal laboratory handling. The handling test procedures are an effective tool for this purpose and, in those laboratories having

the necessary facilities and data analysis skills, there is a significant possibility for reducing the cost of traceability to

primary standards. Such cost savings may result from a reduction

in the number of reference standards in inventory to replace

those taken out of service for calibration at a remote facility,

a reduction of the cost of a larger number of such calibrations,

and a reduction in the risks of shipping fragile devices.

3. Methodol ogy

Tests simulating the acceleration forces during various

types of handling had to be defined. Some guidance in this

regard was obtained from the manufacturers' specifications. On

the basis of these tests, specialized equipment, relatively simple in design, was developed by Jerome Morrow and George Free of

NBS [1]. This equipment and the procedures used to affect these

tests are described in later portions of this document. In

addition, dimensions of the equipment are given in Appendix 2 as

an aid to those who wish to duplicate it.

The tests which have been developed address many of the

common handling problems users may encounter. The test names and

their purposes are:

o CONNECTOR TEST - Determines looseness of either or both

connectors and associated parts. Also used to determine

conductance problems that may be due to poor bonding of the

connector to the case of the capacitor standard, poor solder

joints, or loose inner pins of the connectors.

o ORIENTATION TEST - Used to determine the change in

capacitance due to a ninety-degree rotation of the capacitor

from its upright position; i.e., to a position of lying on

its side. The capacitor is tested six times, once to each

side. Excessive changes may indicate looseness of fasteners

within the capacitor housing.

o ANGLE 3 DEGREES - Most lab table surfaces vary from the

horizontal and this test was planned [1] on the basis of the

estimated worst case in normal calibration laboratory

operations considering a survey within NBS. The value of

three degrees does not reflect a sampling of other

laboratories. The change in measured capacitance allows the

user to relate errors to surface differences of lab tables.

TILT TEST - This test was devised to check for movement of

component parts within the capacitor housing that may not be

detected by measurement of the standard capacitor in its

normal resting position.
•4-> (/) 0)

+->

Qi

C7> C

A3

S-o

4-

<1J m

+ja>c

<u

cn
$ooo

4->

2.

to

CM

0)

D)
o KNOCK SOFT OR HARD TEST - To simulate impacts the standard

capacitor receives while in transit or through rough

handling in the lab.

o DROP TEST - Designed to simulate the capacitor standard

being dropped during transport or excessively rough handling

within the users' lab or shipping facility.

4. Equi pment Fabri cated for Tests

Special equipment was constructed at NBS to accomplish the

tests described in this paper. For the convenience of those

interested in reproducing this equipment, drawings are included

following the text.

For the angle test, a tilting table was built consisting of

a platform supporting a hinged plate that can be tilted in half

degree increments. Three adjustable feet are used to level the

platform. To adjust the angle of the tilting plate, an

adjustable screw rod with a large knob is provided as shown in

figure 1. This knob can vary the angle of the adjustable plate

from zero to three degrees or more as required. However, as an

angle of three degrees was selected as the most useful test angle

and to save time in adjustment of the elevated table, a removable

block with an adjustable screw was fashioned and fixed in the

three degree position.

For the knock tests, a stand was made to support a hammer

with interchangeable heads. Referring to figure 2 , an angle

plate on the side of the stand was marked off in increments of

10, 15, 20, 25, and 45 degrees. A stop is used to prevent the

hammer from going back further than the desired angle. The plate

is movable in the vertical plane to accommodate taller capacitors

without affecting the desired striking angle.

The hammer selected was manufactured by the Nicholson File

Co., Providence, R. I., and is designated as No. 155. The two

detachable heads used are manufactured by J. H. Williams and

Company, United Greenfield Division of TRW, Inc., Buffalo, N. Y.;

they are designated as HSF-15-SS (soft head) and HSF-15-N (hard

plastic head) 1

.

In the case of the drop test, a block of wood having dimensions 3.8 cm (1.5 inches) by 8.9 cm (3.5 inches) was used. By

inserting this block with the smaller dimension under one of the

capacitor sides and then quickly removing it, one drops the

capacitor to that side. This test simulates dropping a capacitor

contained within an appropriate shipping container on one of its

sides or corners.

Certain commercial equipment, instruments, or materials are identified in this

paper in order to adequately specify the experimental procedure. Such identification does not imply recommendation or endorsement by the National Bureau of

Standards, nor does it imply that the materials or equipment identified are necessarily the best available for the purpose.

5
5. Testi ng Procedure and Resul ts

To produce uniform results, a systematic method of testing

was developed. The results collected

fore, consistent significance and may

of impending problems, or to predict

value of the capacitor may change.

of the handling and stability of the

test results can be used to screen capacitors not suitable for

precision measurements and to select capacitors for a bank.

over the years have, therebe examined for indications

the direction in which the

They also provide a history

standard. Furthermore, the

A long set of coaxial leads was connected to the test

capacitor and to the appropriate transformer tap on the NBS-TYPE

2 Bridge (8, 9). A known capacitance standard was connected to

the appropriate tap on the standard side of the bridge as outlined in Table 1.

Tabl e 1 Connection of test unit (unknown) and standard

reference capacitor to NBS-type 2 bridge

Nomi nal

Val ue

1000 pF

100 pF

10 pF

Test Capaci tor

Bridge Transformer

Tap

+ 1 Fx

+ 1 Fx

+ 1 Fx

Standard Capaci tor

Nominal Bridge Transformer

Value Tap

100 pF

100 pF

10 pF

- 1 Fs

- 1 Fs

- 1 Fs

The procedure adopted at NBS provided for testing the

capacitors in a specific order: connector, orientation, angle

three degrees, tilt, knock soft, knock hard, and finally the drop

test (1). The detailed description of each is given later in

this paper.

A uniform method of identifying the sides of the capacitor

was established as specified in figure 3. Throughout this

report, mention will be made to the reference position.

Referring to figure 3, the reference position in the case of all

tests except the knock test is that position of the capacitance

standard resting upright on the lab table with Side A facing the

operator. In the case of the knock tests, Side A faces the

striking force (refer also to figure 2).

The long coaxial leads to the capacitor are used to allow

for complete movement of the capacitor within the testing area as needed. The capacitor in all tests is connected to the bridge

with the capacitance standard selected using Table 1 and in

accordance with NBS procedures outlined in references (3), (4),

and (5). All measurements start with the capacitor in the

reference position and a return to the reference position after

all movements associated with a particular test are accomplished.
a 3QIS o

LUa

a)

LUa

V

SIDE

B

O '/ aav3 d3u do ±Niod

_ ^y\

w < «•

</> z H LU z S oa

LU

CC

UJ </> O < z UJ

UJ 2 (/) LU

_i

H

LLUJ < 0)

HI

a

O

1-

u. -1

-I<1

</> HZ5

Q.

UJOz

LU

CC

LU

Lo- rn

CCCO Zo

LU

CC

if) UJ

a

D o CI o <

LL Z Li.

•*

DC

— o< oo

_l

CC ^ LU LU

a.O >

Q.o

CO
The long leads to the unit under test do not affect the accuracy

of these measurements because only changes from the initial

values are significant.

6. Connector Test

This test was devised to detect several common problems

experienced with capacitors received for calibration at the NBS.

Such problems are not necessarily related to transportation

effects. Many standards arrive with loose connectors, loose

connector inner conductors, or broken or damaged connector parts;

for example, a leaf may be missing from the inner conductor of

the GenRad 874 connectors, those most commonly used.

Repairs made at the user's lab have been found to create

significant problems due, particularly, to improper soldering

techniques. Unacceptably high or unstable conductances are often

the result of cracked or cold solder joints at the inner pins of

the connectors or at points of connection to the capacitor;

likewise such faults may be associated with a loose inner conductor in either or both connectors. Modifications of the unit

often lead to similar problems (6).

The procedure to determine if a connector defect exists is

to apply a gentle force in a rotary fashion to each lead connector. The bridge must be balanced prior to this test. Any

changes in capacitance or conductance are noted. Then, with the

source oscillator voltage to the bridge off (to avoid transients

in the bridge circuits), each connector is disconnected and then

reconnected. Repeat, recording any difference observed. Changes

greater than 0.05 ppm capacitance or 10 micromhos conductance

are cause for concern. If a problem is apparent, check the

points mentioned above to see if improvement can be achieved. In

some cases, the rotation procedure will improve the readings

without doing anything to the connector; this suggests dirt or

oxidation on the connector surfaces or one of the other problems

mentioned earlier. In such cases, the test should be repeated in

an effort to obtain consistent results.

7. Orientation Test

This test is a good indicator of looseness in the web

assembly (refer to figure 4), the case components, and possible

problems within the sealed capacitor assembly.

The most frequent fault detected is a loose web assembly

(refer to figure 4). Web assembly screws loose by one or more

turns have been found to cause changes of several tenths of a part per million. Defects within the sealed capacitor, such as

strains and mechanical displacement of the capacitor plates, may

also be detected ( 6).

The procedure for this test is quite simple. The test

capacitor is first placed with Side A facing the operator (refer

to figure 3) and the capacitance bridge is balanced. This is the
(/)

XI

c

•r-o

<L>

0)

s-o>
reference position. Then the capacitor is moved from the upright

position to a position of rest on Side A (refer to figure 5), the

bridge rebalanced, and the capacitance value recorded. Without

bringing the capacitor to the upright position, it is turned

successively to positions of rest on Sides B, C, and D, the

capacitance value for each position being recorded. After

capacitance values for positions on all sides have been measured,

the capacitor is returned to the upright position facing Side A

(reference position) and a final measurement is made. The last

reading, identified as ref2, is used to check the first (refl)

for drift (ref 1-ref 2=dri f t ). Should the difference between the

reference value and the value associated with any position be

greater than ten ppm, the test should be repeated to determine

whether or not the change is consistent or erratic. Two or more

tests are desirable to reduce the possibility of a reading or

recording error. Always use the last reference point in a series

of measurements. If the capacitance value changes are not

consistent from test to test, it is advisable to check the

capacitor for defects such as loose screws, nuts, etc.,

including, in particular, those associated with the trimmer

capacitor contained inside the outer enclosure and adjacent to

the sealed capacitor assembly.

As indicated earlier, capacitors supplied from two different

sources were evaluated. In the case of the models involved, the

manufacturer's published specifications for maximum capacity

changes under the conditions of the tests ranged from 1 ppm to 10

ppm. Changes greater than manufacturer's published specifications were considered as evidence of a defective capacitor.

8. Smal 1 Angl e Test

This test was originally done for changes from zero to one,

zero to two, and zero to three degrees. However, this series of

tests was time consuming, and a single test for a change from

zero to three degrees was found to be just as useful. The angle

of three degrees was selected as the worst case by measuring a number of laboratory tables and finding that surfaces are usually

within plus or minus two degrees of the horizontal. The change

in capacitance due to small changes in angle of tilt is generally

found to be a linear function of the angle within the zero to

three degree limits. A worst-case acceptable value of 0.2 ppm

per degree of angle change has been established for capacitors in

satisfactory condition [1],

The procedure for this test is not difficult. However, the

placement of the capacitor, adjustment of the elevated table, and

provisions to avoid movement of the laboratory table or the

elevated table are all important for consistent results (refer to

figure 6). Be sure the adjustable table is horizontal (zero

degrees). Then place the capacitor in the center of the tiltable

plate on the adjustable table upright with Side A facing the

operator. The measured capacitance under these conditions becomes the reference value and should be recorded as such. Use the

adjustable knob to increase the angle to three degrees or, alter10
<L)

toI

0)

4-> co

£>

CO

+-> c

<U

•r—s- o

ID

S-en

11
n a t e 1 y , use the special block made for three degrees, located

securely in a suitable slot to avoid error in the elevation of

the table. (Refer to figure 6). Measure the capacitance values

with Sides A, B, C, and D facing the operator. Differences

greater than 0.2 ppm between the reference value and the value

associated with any other position indicates that the test should

be repeated. If the results of a second series of measurements

are similar to those of the first run, the changes probably

reflect a characteristic of the particular capacitor. If the

test is repeated, and the results are significantly different

from those of the first test, subtract the smallest change from

the largest change and record one half the result; this value

should not be greater than 0.2 ppm. If it is larger, check for

looseness in the web assembly.

9. Tilt Test

The tilt test (rotation through angles of 90 degrees, 180

degrees and return) serves several purposes. The geometry of

parallel plate capacitors is such that movement of the plates

will cause changes in capacitance. For example, shifting of the

capacitor plates by only a few thousandths of an inch will cause

changes ranging from a few tenths of a part per million to

several parts per million. If all accessible fasteners holding

the capacitor to the web assembly are tight and a sudden change

in capacitance can be detected upon tilting and it does not

disappear within plus or minus 0.1 ppm when the capacitor is

placed 'back in the upright position on the table, it may be

assumed that a problem exists within the sealed capacitor

assembly.

The testing procedure is as follows. First, place the

capacitor in the reference position on the table with leads

connected as in previous tests. Balance the capacitance bridge

as usual. Then grasp the handle of the capacitor and by

balancing the weight of the capacitor in the other hand (refer to

figure 7), turn (gently) the capacitor to a position with Side A

in the horizontal plane. Note any erratic changes of the

you place the capacitor in the upright posiRecord any change observed. Repeat the test

D. Finally, repeat the test for the inverted

Figure 8). Grasp the capacitor and turn it

observing the detector for changes as you

to the normal orientation and place it back

detector reading as tion on the table,

for Sides B, C, and

position (refer to

upside down again

return the capacitor

on the table.

Changes greater than 0.2 ppm between any two measurements of

the capacitance of the upright capacitor indicate the possibility

of a problem and the test should be repeated at least once. If

the change is greater than 0.2 ppm, and consistent with the

previous test, the test may be considered to reflect a characteristic of the capacitor. If changes during a second test

are significant and do not repeat the results of the first test,

check for looseness in the web assembly and capacitor assembly

mounting parts. In the worst case, you might hear a distinct

12
0Jo

"a. c

(A

03a>

sa> -aro

j-o*4- -^Oo

f0

•I—a

a>o.

to M

CO

a>

+->

a>c

vo

O)

en

13
Figure 7. Tilt test in progress - side A horizontal

14
Figure 8. Tilt test in progress - test unit inverted,

15
"clunk" as you invert the capacitor indicating that the capacitor

assembly within the case will probably be found to be loose.

Visual observation of the detector during this test is

essential. Erratic behavior, indicated by a sudden change in the

detector zero point, may be evidence of problems such as movement

within the capacitor. This should be recorded to determine, in

the future, if the quality of the capacitor degrades with time.

It is also suggested that such a capacitor be calibrated more

frequently as behavior of this nature is generally a sign of

impending deterioration. Before reaching a conclusion that the

capacitor is faulty, be sure to check all fasteners previously

mentioned as well as connection points and cables. In many

cases, a correctable fault can be eliminated.

10. Knock Soft Test

The mechanical shock in the knock soft test is approximately

equal to a mild, wrist -act ion motion of one's hand against the

side of the capacitor. The test is a controlled impact of a soft

rubber-tipped hammer swung, in the vertical plane, from a position of 45 degrees. The hammer strikes the test unit on a side, just below the horizontal plane of the top surface, and

midway between adjacent sides.

This test may reveal several conditions. For example, defects within the sealed capacitor enclosure, or looseness of

fasteners in the web assembly of the capacitor, may be

responsible for shock-induced changes of capacitance.

During transit, the capacitor is generally subjected to

larger mechanical forces than this test will produce. However,

small impulsive forces often create more noticeable changes

within a capacitor than larger, less impulsive forces. This test

is intended to simulate a typical impulsive force that a wellpacked capacitor may receive during handling in transit. There

are no specified manufacturers' limits for such shocks.

For the test, use the stand with the soft tip end of the

hammer head toward the face of Side A of the capacitor. Balance

the bridge and record the reading. This capacitance value is the

reference value. The hammer is then drawn back to the 45 degree

position and released to strike the capacitor (refer to Appendix

2 , Figures 2 and 4). However, it is not allowed to strike a second time following rebound. Again the capacitance value is

recorded. The capacitor is moved to position A Corner (refer to

figure 3) for a hammer strike. The measurement is repeated

through all sides and corners in the order: Side A, Corner A,

Side B, Corner B, Side C, Corner C, Side D, Corner D.

If changes are greater than 0.2 ppm between any two tests in

this series, it is suggested that the series be repeated. If the

relative changes between a first series and a second series is

noted to be approximately the same, subsequent tests are not

necessary.

16
11. Knock Hard Test

This test is similar to the knock soft test. The

differences are that the hammer swing is limited to fifteen

degrees and the hard tipped head strikes the capacitor. The

object of this test is to simulate shocks received by the test

capacitor when, in being lifted from a work bench, it accidentally strikes another object. Mechanical shock in transit may,

under some conditions, be similar to this type of shock. This

test is particularly effective for detecting problems associated

with the trimmer capacitor.

Changes larger than 0.2 ppm should be recorded, particularly

when two sets of readings do not repeat. Changes greater than

0.5 ppm are cause for concern and the capacitor should be

carefully checked for correctable defects. If there are no loose

components, the excesssive capacitance change may be a result of

stress changes within the sealed capacitor enclosure.

12. Drop Test

The drop test was devised to simulate a shock which may

occur in service, for example, due to a drop from the hand to a table surface. It may also simulate, to some extent, the dropping of the standard in its shipping container during handling in

transit. Short-duration forces in the vertical direction may

cause various movements of plates or support rods. Intermittent

electrical shorts between plates may sometimes be found using

this method.

Testing the unknown capacitor requires a block of wood

having measurements of 3.8 cm (1.5 inches) x 8.9 cm (3.5 inches)

and from 10 to 15 cm (4-6 inches) long (refer to figure 9).

Balance the capacitance bridge with the capacitor normally

resting on the table, being sure that all four feet of the

capacitor are in contact with table surface. Then place the

block of wood under Side A (smallest dimension vertical) and

carefully, but swiftly, pull out the block. After the block of

wood has been removed and the capacitor has struck the table,

record any changes in capacitance value. The test is repeated

raising each side of the capacitor sequentially (Side A, B, C,

and D). Changes greater than 0.2 ppm between any two capacitance

values, or between the reference value and any other value should

be recorded. Changes greater than 1 ppm indicate the possibility

of a problem capacitor.

17
+J

<D

a.o

i- -ao

Q.3I

CD

S-3

18
13. General Consi derat i ons

All of the tests described in this document should be

performed in a systematic manner. No test should be discontinued

until all specified sides, corners, etc. have been included. The

specified order of tests, connectors, orientation, small angle,

tilt, knock soft, knock hard, and drop test should be followed to

eliminate possible confusion in comparing results with those of

earlier or later tests. This order was determined to give the

most reproducible results. There is one caution that should be

mentioned. If any fasteners are suspected of being loose, either

tighten all fasteners before the handling test is undertaken, or

wait until all tests are completed. Otherwise, it will not be

possible to know if the repairs caused or cured the problem(s).

It is not necessary to complete a series of tests in one

day; it is only important that the tests be done in the proper

sequence. This is the only way to obtain a repeatable characterization of the standard.

Capacitors determined to be faulty should be retested before

any repairs are attempted in order to eliminate the possibility

of other factors such as improper hook-up, faulty electrical

leads, bridge problems, or faulty connections within the test

system being used.

The use of two reference standards, having known characteristics, to which the unit under test is compared, one during the

first half of the test series and the other during the second

half, may help reveal problems external to the test capacitor.

Such problems might include intermittent faults in a reference

capacitor, improper hook-up, bridge malfunction, faulty leads, or faulty connectors.

14. Data Reducti on The data for any of the test series described in the

previous sections of this document may be represented by a string

as f ol 1 ows :

ref, rdg-^ rdg

2 rdg

n where r e f = i n i t i a 1 reference value which appears only at the

beginning of a string for all positions to be tested, and

rdg.j =readi ng for a particular side or corner.

Successive differences or changes would then be as follows:

19
difj = r d g ^ - ref^

d i f 2

= rdg

2

- rdg^

d i f 3 = r d g 3

- rdg2

dif n = rdg^rdg^^

where dif = difference, rdg^ = reading at position i, and ref = reference.

The range between any two values is used for two reasons.

In the first place, most laboratories only need to know the

maximum change expected. In the second place, if changes were

nearly equal but opposite in sign, the average would

underestimate possible changes. Changes noted for each side or

corner in a string usually will not be randomly distributed,

particularly when significant changes are experienced.

For the tests listed, the following data should be obtained:

ORIENTATION: Minimum of six data points required - refl, Side A,

Side B, Side C, Side D, ref2

ANGLE 3 DEGREES: Minimum of six data points required - refi,

Side A, Side B, Side C, Side D, ref

2

TILT: Minimum of six data points required - ref, side A, side B,

Side C, Side D, upside down, rightside up

.KNOCK SOFT OR HARD TEST: Minimum of nine data points required - ref, Side A, Corner A, Side B, Corner B, Side C, Corner C, Side

D, Corner D

In the orientation and angle tests, drift can be determined

from the difference between the first reference and last

reference in a given string. If the test is completed with

minimal time between measurements (less than a half hour), drift

in excess of 0.2 ppm should be recorded. Most excessive drift

will be observed during the first few minutes of testing. In

some cases, drift may continue for several minutes or as long as

a half hour. Such drift may be caused by changes in temperature

of the capacitor under test or changes in temperature of the

standard used as the reference. While the latter may be placed

in a lag box to reduce the effects of rapid temperature

fluctuations, it may not be practical to do so for the capacitor

20
being tested. Therefore, these tests should each be carried out

rapidly to minimize temperature-created anomalies in the data

(which one would have a difficult time sorting out from the

effects of defects) and take place in a relatively wellcontrolled room.

Except for the results of the orientation test, the magnitude of changes caused by handling tests should be included in

the uncertainty of calibration results as a systematic error.

This is particularly important if any of the changes are greater

than 0.5 ppm. In the case of capacitance standards which are not

kept continuously within the metrology laboratory or are otherwise exposed to unusual handling conditions, experience at NBS

suggests that the calibration uncertainty should be increased by

the addition of half the largest change induced by the handling

tests described in this document [1].

The stresses described in this document are believed to be

small compared to those that are encountered in normal shipping

and handling; therefore, unusual effects due to shipping should

be determined by using the results of control measurements made

in the user laboratory before and after shipping in addition to

the results of these tests at NBS.

15. Unusual Probl ems

Gas dielectric capacitors have two sources of capacitance

change which are not repairable by the user; in the first place,

all gas dielectric capacitors manufactured in the United States

use a trimmer capacitor to bring the value of the capacitor close

to its nominal value. Many users adjust this capacitor to meet

specific needs. Two types of trimmers are in use, one having an adjustable core and the other having interleaved plates similar

to a radio tuning capacitor. The plate type is rarely found to

be a problem unless it becomes loose. Since this type is

generally exposed within the capacitor housing, a loose part,

such as a nut or screw, falling between the blades of the trimmer

capacitor may cause an electrical short or mechanical damage to

the plates. Such faults may cause capacitance changes of greater

than 100 ppm. Changes in placement of the leads to the trimmer

may also affect the capacitance by several ppm in either

direction without any change in the trimmer adjustment.

The coaxial adjustable-core type of capacitor trimmer has

been known to short, or worse, fall apart due to extreme abuse in

transit. If there is a possibility of an electrical short, the

trimmer must be replaced because excessive current will be drawn

through the measuring system. Malfunction of the trimmer may

cause a capacitance change of several hundred ppm.

Removal of either type of trimmer will result in capacitance

changes of up to 300 ppm (determined for the capacitors selected

for the NBS portable transport box). If such departures from

21
nominal value are acceptable, it is suggested that the trimmer

capacitor be removed to eliminate a possible cause of

i nstabi 1 i ty

A second, less frequent, problem involves hermetic seal

leakage. The hermetic seals of capacitors manufactured in the

United States are generally trouble free unless undue pressure is

applied on the electrical feed-through connection points or they

are overheated. Cracked seals are not common; however, if this

is suspected, temperature cycling will reveal the fault. Cooling

to freezing followed by warming to approximately forty degrees

Celsius and returning to lab temperature should be sufficient to

verify seal leakage. A capacitor that is cycled will not retrace

to the previous value within the tolerance stated by the manufacturer unless the seals are intact. There may be problems within

the sealed capacitor other than those associated with the seals.

However, any capacitor that cannot be cycled without significant

changes should not be considered satisfactory as a working

standard .

16. Cone! us ion

A review of the graphs for the various test procedures will

disclose the utility of handling tests described in this

document. The overall results of a number of such tests

conducted at NBS are summarized in table 2, following:

Table 2. Summation of tests

TEST ACCEPTABLE LIMITS* (PPM)

Orientation 10

Angle 3 Degrees 0.2

Tilt 0.2

Knock Soft 0.2

Knock Hard 0.2

Drop 0.2

% FAILED

23

48

5

14

13

14

*Acceptable limits excluding orientation test for which one half

the largest change in any test should be added to the uncertainty

of cal i brati on.

The tables and graphs (Appendix 1) in this document are

derived from the results of tests carried out on 200 capacitors

during the period 1972-78. The method of evaluation involved

collection of data from all capacitors received for testing the

first time in order to establish a basis unbiased by the selection criteria and to reflect a good cross-section of all

capacitors received. Data are selected randomly from all

22
capacitors received for test in order to avoid disclosure of

differences between the products of different manufacturers.

Data from succeeding years were also selected randomly for

capacitors not used in previous data evaluations in order to

obtain an overall balance.

Data from the first year of testing were compared with data

from succeeding years with very convincing results. In more than

80% of the cases compared, the results of the capacitors repeated

to within 0.2 ppm of those from the previous year for each

particular test. Most capacitors with changes greater than 0.2

ppm had been subjected to rough treatment. For fewer than 10%

there was no discernible reason for the changes noted.

Referring to the summary of tests, it is apparent that the

orientation and three degree angle tests were failed more often

than the other tests. If a capacitor shows poor results on the

tilt test, the problem can, in principle, be overcome by using a level table. In the case of the orientation problem, there is

not much that can be done other than use the capacitor in the

upright position. It should be recognized that excessive changes

during these tests suggest a lack of mechanical integrity within

the standard capacitor and reliabilty during service is

open to question.

It will be noted that there is no graph in Appendix 1 for

the connector test. Generally,

small and when a problem with a generally no remaining concern.

17. Additional Observations

the changes during this test are

connector is corrected, there is

The care with which a gas dielectric capacitor is packed for

shipping can, to a large extent, determine its useful life.

Manufacturers in the United States ship standard capacitors in

protective reusable containers, which should also be placed in an outer box containing at least 10 cm (4 inches) of shock-absorbing

material on all sides for added protection.

Exposure of the capacitor to abnormal temperatures, such as those found in the trunk of a car on a hot day, should be

avoided. The time for the capacitor to stabilize after being

exposed to excessively low or high temperatures depends on the

design of the capacitor. A similar consideration applies

following a long period of storage with the capacitor resting on

a side; a period of recovery in the normal upright position is

necessary before its use or before calibration at NBS.

Checking all screws, nuts, etc., for looseness before the

capacitor is shipped for calibration will reduce the possibility

of damage due to the movement of parts during transit. Loose

connectors should be properly positioned and tightened before

tests are started.

23
Handling the capacitor in the laboratory or field is as

important as care in shipping if long term stability is

essential. Capacitors receiving rough treatment, such as impacts

against other equipment or furniture, may be damaged in terms of

loosened parts or more serious mechanical changes.

It is important to document values and abnormal changes

found in the handling tests. Both can guide the user in avoiding

problems and removing from service a faulty capacitor; many hours

of needless measurements can, thereby, be saved.

All maintenance and repairs should be performed after

completing the series of tests described in this document unless

a definite problem is detected. In this way, problems discovered

during tests may be evaluated as correctable or otherwise

identified as an intrinsic characteristic of the unit under test.

The material contained within this paper is intended to

serve as a guide; it is not suggested that the problems discussed

are the only problems the user will experience. However, the

handling tests described have been found to disclose the most

frequent problems encountered at NBS. All known causes of

capacitance change should be documented fully and the capacitor

should be monitored systematically if it is to be used at the

five ppm level of uncertainty.

18. References

[1] Free, G. and Morrow, J., Transportable 1000 pF capacitance

standard. Nat. Bur. Stand. (U.S.) Tech. Note 1162; 1982.

[2] Anderson, W. E., Davis, R.S., Petersens, 0., and Moore, W.

J. M., An International Comparison of High Voltage Capacitor

Calibrations, IEEE Trans. Power Appar. Syst. P A S - 9 7 ( 4

)

, pp.

1217-1223 (1978).

[3] Thompson, A.M., The precise measurement of small

capacitances. IRE Trans. Instrum. 1-7(3-4) : 245-255; 1958

Dec.

[4] Cutkosky, R.D., Four-terminal pair network as precision

admittance and impedance standards. IEEE Trns. Commun.

Electron. 70:19-22, 1964 Jan.

[5] Homan, D. N., Application of coaxial chokes to a-c bridge

circuits. J. Res. Nat. Bur. Stand. 72C (2) : 1 61- 165 ; 1968

Apri 1 -June.

[6] A highly stable reference standard capacitor. GR

Experimenter. 37(8); 1963 August.

24
[7] Thoma, P., Absolute calorimetric determination of dielectric

loss factors at = 10s" 1 and application to measurement of

loss factors of standard capacitors at room temperature.

IEEE Trans. Instrum. Meas. IM-29(4); 1980 Dec.

[8] McGregor, M. C, Hersh, J. F., Cutkosky, R.D., Harris, F.

K., and Kotter, F. R., New apparatus at NBS for absolute

capacitance measurements. IRE Trans. Instrum. 1-7:253-261;

1958 Dec.

25

Appendi x 1

Histograms of the Effects of Handling

and Stability Tests on Standard Capacitors

The following histograms are based upon the results of

performing each of the Handling and Stability Tests on each

capacitor of the test group. The maximum capacitance change

value caused by a given test performed a number of times on each

capacitor was used to determine the numbers of capacitors for

which such change values fell within the ranges indicated in the

hi stograms.

27
w

•

1:

n

CM

Ixl z

o

z

•-< o

(9

(9 00

T

53

(SJ

k.o

** • U C0

CO •

fth

°0

"IS

Is

a

c •

«

*•

2

o • 6*

CO • a ° 5

CO «- iS

a

. CO *- o

• E

II

M *" -':

si

a co < >

OU. 3Z-«I-C/) h- LJ (/) h- UJ Q

28
ifi UJ

UJ

Ul

<zO

UJa

-io *o

I-5« n

CL

Q.

UJOz<Iu

UJuz<<< uaa • o » o < E3e«co: • • • e £ § :

0) c o «

OU. DZ^hW HUJWKUD

»-* 6 « o

•o

a I I a • < >

29
©

I-o

O)

UJ

*• o

too a

(0

coa

(0

tt

ro

u

** -

Q.

0. o

(0

CD [ " w ** - UJoz E3C o < o I TM CO

(VI UJ

COco 3

(0o Uz u

(0

k. < (P t- «*- *

1-4

— <

(P •

0)c

«- < OE

CO

(0 £•o 1 o

(0 X

<0 c

(0 «»o

COa

COu 1 <u E3 1 ca W3 1 1 1 1 ' 1 "T~ 1 i E

ea ca ea ea ea ea iZ

X

CO ea ca si ea ea ea i- E

ea 00

a *• CM

H

«m

•*" o 5e to o d> M Ob. DZ^HW 1- U|fOQiC=. (UIKO a 3 a "co < >

30
c•

LLi

U.o

10

*:(Joz 11

Ml

n

CL

0.

UJOz uO

<*>

CD

k.o

COa

(0u

CDw©E3c

to©o

(0ooeno

(0CD

— < ^ I (0 3 —(M u

UJ co

*•

COo u u

CO •

k»A < * [ - u<

a. • •eCO 1 " < oE

(0 o 1 — *~

CDu ?

0)o

(0 i• e

COo

COa

CO 1 o 1 ts •3

0) E

r~ 3mm I 1 i i 1 1 1 E 1 1 i 1 MM •TM

a S S Q S S IL

CO • • * • • • % E a ts cS 8 s s ^

ts 00 CO ^* (M

X <»» in o oe CD

a>

CO3 H CO Oli. DZ~t-tf> HU(/)hUJQ aa< >

31
ee

IdQo

Oloz<Iuuu<

Q.<

OIL DZ-«l-</> »- UJ (/) »- UJ

Q o aoH (0w V O k. <* CD o X

(0ft J£ u oOe

«M * o •

ho•

**

A >» E A 3e • • (0

(0 3 c IB © O o

10 ••* • • £ e» +* e

«*- IB £O E

«o •

h. o

0) e o ID ** **

(0

O

X

a

ID 10 O • E

%m 3 3

0) E

u.

XaE

«*•

X o o (ft c• •3 aa ID < >

32
e®

10

ui

Q.O

CLQ o *• s s s s s a s

00 CD s

(M s M OIL ZDZ^H- (/) h- UJIOI- WO

a

(0 l- a a au i_

n «•-

a

Q.

©

Q. (0 £ ^^ w «M

Id • >£ o E z 3 o < C © I ^^ (0 u (0 3 c (Q u o O u +* 0) z o fa. < (0 ©

»- fa. *

I-* u

®

© < £ 0)

Q_

** c <u o

re £U E ©

(0 o o c

re **

(0 o X

rea

re I <0 o

a> E

k. 3 3 E

0) •TM *

IL re » E ^

«-

X o

e^X» •» e © • 3 aa<

re>

33

Appendi x 2

Drawings of Test Apparatus

The following drawings represent the apparatus used at the

National Bureau of Standards for the tests described in this

publication. Dimensions are given in metric units and customary

units used in the United States.

35
Appendix 2, Figure 1.

KNOCK STAND ANGLE PLATE PATTERN

ZERO DEGREE

POINT USING

CENTER POINT OF

HOLE AT TOP OF

PATTERN

SCALE I I I I I I I I I I I

1 2 3 4 5 cm.

I I I I I 2 inches

36
Appendix 2, Figure 2.

Hammer for use with Knock Stand (see following figure)

HARD RUBBER HEAD

SOFT RUBBER HEAD

HAMMER-HEADS

TOTAL Wt. OF HAMMER

grams lbs.

41.03 .09

46.70 .10

441.14 .97

528.87 1.16

1 3/8"

(3.44 cm)

4

12 1/2"

(31.25 cm)

111/4"

(28.13 cm)

(10.63 cm)

37
CO LU

_l

(fl E

<D < O "

O) S» I »

il <o1- \-

CM * o X o z

"5c

(D S 1 a LU a * < CO

38
to -1 s O

UJ CO o Qz<

to „

</> </) < 1-

cc 3 m Z

to

cc

UJZ

1-W)<

CO

O)

iZ

C\J oc

0)aa<

UJ s< 2O * O u z

UJ

UJUi UJ < -1 m (') r a < oo5 < ; >

rr

oo _i CD in a.

CO

HIIoz

COZ

39
CM

T3C

0)aa<

C/> K

</) IDH ^^, LU

_loz<

LU<u

cco o

u.

H

LU

_l oz m T < o Q Ul

HI *

h- (/) <>

LU

UJ

40
O)

iloc

<Daa<

iHII- ^ o<o 

_lCD< OzIo Q UJ

UJ *

I- </> >

LU

-I

111

41

NBS-114A (REV. 2-8C) U.S. DEPT. OF COMM. BIBLIOGRAPHIC DATA

SHEET (See instructions)

1. PUBLICATION OR

REPORT NO.

NBS TN 1161

2. Performing Organ. Report No.| 3. Publication Date October 1982

4. TITLE AND SUBTITLE

Testing to Quantify the Effects of Handling of Gas Dielectric Standard Capacitors

5. AUTHOR(S)

Charles R. Levy

6. PERFORMING ORGANIZATION (If joint or other than NBS. see instructions)

NATIONAL BUREAU OF STANDARDS

DEPARTMENT OF COMMERCE

WASHINGTON, D.C. 20234

7. Contract/Grant No.

8. Type of Report & Period Covered

9. SPONSORING ORGANIZATION NAME AND COMPLETE ADDRESS (Street. City. State, ZIP)

same as #6

10. SUPPLEMENTARY NOTES

[] 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

bibliography or literature survey, mention it here) A test method known as the "Handling and Stability Test" is currently being used at

NBS as part of the requirement for the five-part-per-million calibration of gas-dielectric standard capacitors. This test is used to achieve qualitative information on the

effects of mechanical shock from shipping and handling on standard capacitors and to

rank them quantitatively with respect to these effects.

12. KEY WORDS (Six to twelve entries; alphabetical order; capitalize only proper names; and separate key words by semicolons)

calibration; measurement assurance; reference standards; standard capacitors;

standard qualification.

13. AVAILABILITY

^Unlimited

| | For Official Distribution. Do Not Release to NTIS

C>?[ Order From Superintendent of Documents, U.S. Government Printing Office, Washington, D.C. 20402. Q Order From National Technical Information Service (NTIS), Springfield, VA. 22161

14. NO. OF

PRINTED PAGES

44

15. Price

$4.75

$U. S. GOVERNMENT PRINTING OFFICE: 1 9 8 2 - 3 8 " 9 9 7 / 2 3 9 USCOMM-DC 6043-P80

NBS TECHNICAL PUBLICATIONS

PERIODICALS

JOURNAL OF RESEARCH—The Journal of Research of the National Bureau of Standards reports NBS research and develop- ment in those disciplines of the physical and engineering sciences in which the Bureau is active. These include physics, chemistry,

engineering, mathematics, and computer sciences. Papers cover a broad range of subjects, with major emphasis on measurement

methodology and the basic technology underlying standardization. Also included from time to time are survey articles on topics

closely related to the Bureau's technical and scientific programs. As a special service to subscribers each issue contains complete

citations to all recent Bureau publications in both NBS and non- NBS media. Issued six times a year. Annual subscription: domestic

$18; foreign $22.50. Single copy, $4.25 domestic; $5.35 foreign.

NONPERIODICALS

Monographs—Major contributions to the technical literature 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 in- terested industries, professional organizations, and regulatory

bodies. Special Publications—include proceedings of conferences 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 worldwide program coordinated by NBS under

the authority of the National Standard Data Act (Public Law

90-396).

NOTE: The principal publication outlet for the foregoing 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, 1 155 Sixteenth St., NW, Washington, DC 20056.

Building Science Series—Disseminates technical information 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 procedures

published by the Department of Commerce in Part 10, Title 15, of

the Code of Federal Regulations. The standards establish nationally recognized requirements for products, and provide all concerned interests with a basis for common understanding of the

characteristics of the products. NBS administers this program as a supplement to the activities of the private sector standardizing

organizations. Consumer Information Series—Practical information, based on NBS research and experience, covering areas of interest to the con- sumer. Easily understandable language and illustrations provide

useful background knowledge for shopping in today's tech- nological marketplace. Order the above NBS publications from: Superintendent of Documents, Government Printing Office, Washington, DC 20402. Order the following NBS publications—FIPS and NBSlR's—from

the National Technical Information Services, Springfield, VA 22161

.

Federal Information Processing Standards Publications (FIPS

PUB)—Publications in this series collectively constitute the Federal Information Processing Standards Register. The Register

serves as the official source of information in the Federal Govern- ment regarding standards 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 May 11, 1973) and Part 6

of Title 15 CFR (Code of Federal Regulations).

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 dis- tribution is handled by the sponsor; public distribution is by the National Technical Information Services, Springfield, VA 22161,

in paper copy or microfiche form.
U.S. Department of Commerce

National Bureau of Standards

Washington, D.C. 20234

Official Business

Penalty for Private Use $300

POSTAGE AND FEES PAID

U.S. DEPARTMENT OF COMMERCE

COM-215

THIRD CLASS BULK RATE