The comparisons of the Transfer Reference to 2800 (Working Group C) are
summarized under the heading REFERENCE CELL STATISTICS. The
standard deviations are the standard deviations of a single observation from
the redundant measurement design [3]. The emfs of the reference cells in
2800 and the 1.018-V outputs of the Transfer Reference (labelled Z1 to Z10)
are expressed in microvolts reduced by 1.01 volts. The mean values from
the two calibrations are used as the values of the Transfer Reference for
calculating the values of the solid-state standards under test. The emf dif-
ferences between two calibrations are calculated and printed in the Differ-
ence column to aid the operator in detecting abnormal changes.

At the bottom of Fig. 3a a short summary of the DVM calibrations appears.
In this example the DVM was compared to two different resistors before
and after each set of Zener measurements ([3 sets +1]x2) for a total of 8 cali-
brations. The mean DVM error on the 1-V range was found to be (2.0 + 0.1)

ppm.

Fig. 3b, page 2 of the printout, lists the standards under test, their average
measured voltage, and the standard deviation of the measured voltage as
determined from the repeated sets of measurements (3 in this case). The
repeated measurements are performed over a relatively short period of time
(1 to 2 hours) to reduce the variability of the final averaged values. The
complete set of measurements described above is considered to be a single
measurement of each standard consisting of the one averaged value for that
standard.

5. Assignment of the Final Value

Client standards are measured in terms of NBS working cells once a day
(using the above procedure) for at least ten working days. The measured
values are plotted and the data are reviewed by laboratory staff to determine
if the outputs exhibit sufficient stability for a report to be issued. If so, the
mean of the all the daily measurements is used as the final assigned value
for the output of the standard. In some rare cases standards have shown
an initial drift for the first few days after arriving at NBS. If this happens
additional measurements are taken so that at least ten stable
measurements are obtained, and the early unstable readings are not used
in determining the final value. Even if the standard behaves properly, the
exact number of measurements used for any calibration is not fixed; it
depends on when the standard was received, when the client requires that
it be returned, etc., but at least ten measurements are always used and
usually not more than fifteen.

Figure 4 shows the measured values of three outputs of a typical Zener
standard. The outputs are all derived from the same Zener reference
device; the 1.018-V and 1-V outputs are obtained from internal resistance
dividers connected to the 10-V output.


13 nominal temperature of group 1, actual temperature of
group 1, nominal temperature of group 2, actual
temperature of group 2

14 ENDDATA command to indicate end of DESIGN data set
15 comment
16 ZMEAS command to start accepting Zener measure-

ment data for unknown Zeners to be measured

17-19 Zener ID, nominal voltage (volts), scanner channel
number

ENDDATA command to indicate end of ZMEAS data set

comment

REDESIGN command, repeat previous cell
intercomparison specified by the DESIGN data

7CALC command, calculate standard cell measure-
ment designs and final Zener voltages. ZCALC re-
quires that a DESIGN measurement on the transfer
reference be done before and after the ZMEAS mea-
surements.

8 BES

"S" files

"S" files contain the raw standard cell intercomparison data; only one set is
shown here. A file may contain additional sets taken on the same day. The
similarity between lines 4 through 12 of the Runfile with lines 2 through 10
of the "S" file is no coincidence!

Sample data: S860730
07 30 86 13:54:46 Micky

2 1600 4 Zener 10
40 8165.455 0.034

11110000000000

1601 1602 1603 1604 Z1 Z2 23 Z4 Z5 26 Z7 Z8 Z9 Z10
15171911111326282 102 122 14
35373931131346484 104124 14
1341149474541431231038363
1321129272521411211018161

30 30.011 30 30

157.742 199.267 87.808 173.733 173.267 149.658
169.544 170.258 159.317 157.133 154.942 196.600
84.825 170.825 170.467 151.842 171.758 172.492
161.475 159.417 -169.067 -169.608 -83.544 -195.217
-153.542 -160.475 -162.833 -173.717 -173.000 -153.217
-166.683 -167.267 -81.258 -192.875 -151.333 -163.433
-165.633 -176.592 -176.000 -156.083

Bee eee
HEE RE RR ERO eIATAONDE


second "calibration" of the Transfer Reference reduces the error caused by
any drift in the Reference during the measurements. The low-thermal
scanner switch is used to connect the standard cells and solid-state
standards to the measurement system.

10x 250Q

digital voltmeter

10.18V
Zener
reference
- ‘Po— standard
R , under test
2
Ry
of

Fig. 2. Simplified wiring diagram of the measuring system set up to
compare a standard-under-test to the calibrated resistive divider
(Transfer Reference). Not shown is the low-thermal crossbar se-
lector switch

Measurement Theory

With this system the digital voltmeter is used to read only a fraction of the

voltage of the standard-under-test which reduces the contribution of the

DVM uncertainty to the overall measurement uncertainty. When compar-

ing the standard-under-test to the Transfer Reference (with voltage taps at

ree V increments) the maximum reading required of the DVM is 0.509 V.
us,

Vz- 01
yu% a 0: 8 oye
where Upym is the DVM uncertainty expressed as a percentage of reading,
Vz, is the voltage of the solid-state standard, N is the number of resistors,
and U is the final DVM uncertainty as a percentage of Vz. For a worst case
of Vz = 5.6 V, U = 0.09 Upvn, and for Vz =10 V, U = 0.02 Upvm. Therefore,
if the DVM can be calibrated to an accuracy of 1 ppm the worst case error
contribution is 0.09 ppm.


Other potential sources of error include uncompensated thermal emfs in
the crossbar switches and in the leads to the standards and circulating
ground currents within the measuring instrument and between the mea-
suring instrument and the client standard. Several quality control proce-
dures are periodically performed to estimate or eliminate these uncertain-
ties.

Thermal emfs in the leads from the standards to the crossbar switch are
evaluated regularly. The positive and negative leads are shorted together at
the end where they would normally connect to the standard and a redun-
dant measurement design is done between the two sets of shorted leads us-
ing the DVM as the detector. These thermal emf measurements are per-
formed as standards leave the laboratory and leads become free. Experi-
ence has shown that the thermal emfs are usually small and fairly con-
stant (approximately 15 nV or less).

Leakage resistance and leakage current measurements arc made period-
ically between the different parts of the measuring system using an elec-
trometer to ensure that they are at acceptable levels.

A 10-V standard that is not moved is measured every day as part of the
workload and serves as a check standard to monitor the long term stability
of the measurement system. The 10-V output of the standard is plotted ver-
sus time in Fig. 5.

1.2
104

€

é 08 4 ° e °

>

2

€

g

§

5 slope = 0.20 ppm/year

=

oO i =

8 o24 standard deviation = 0.067 ppm
0.0 + + + +

0 ee) 100 180 200
Time in days

Fig. 5. Ten volt output of a check standard as measured by the system.

16


PREFACE

Calibrations and related measurement services of the National
Bureau of Standards provide the means for makers and users of
measuring tools to achieve levels of measurement accuracy that
are necessary to attain quality, productivity and competitive-
ness. These requirements include the highest levels of accuracy
that are possible on the basis of the most modern advances in
science and technology as well as the levels of accuracy that are
necessary in the routine production of goods and services. More
than 300 different calibrations, measurement assurance services
and special tests are available from NBS to support the
activities of public and private organizations. These services
enable users to link their measurements to the reference
standards maintained by NBS and, thereby, to the measurement
systems of other countries throughout the world. NBS Special
Publication 250, NBS Calibration Services Users Guide, describes
the calibrations and related services that are offered, provides
essential information for placing orders for these services and
identifies expert persons to be contacted for technical
assistance.

NBS Special Publication 250 has recently been expanded by the
addition of supplementary publications that provide detailed
technical descriptions of specific NBS calibration services and,
together with the NBS Calibration Services Users Guide, they
constitute a topical series. Each technical supplement on a
particular calibration service includes:

o specifications for the service
© design philosophy and theory
o description of the NBS measurement system
o NBS operational procedures
© measurement uncertainty assessment
error budget

systematic error
random errors

o NBS internal quality control procedures

iii


f6 PRINT TEXT Print the file on the printer select
code from f5.

f7 CAT Print a catalog of specified disk.

f8 END PROGRAM _ Stop the program without saving
the file.

In addition all the normal H/P program editing keys may be used — insert
line, delete line, insert character, delete character, and all the arrow keys.

File Formats

All files are ASCII files with data stored as strings. Each line in the file
comprises one string and may contain an arbitrary number of items. All
files are read using a free-field format; only the order of the data is impor-
tant, not the position on any one line. Each item is separated from its
neighbor by one or more spaces (usually one). For convenience we have
adopted a convention of grouping several items on a line together to make it
easier for the operator to "read" the file. For each file an example of actual
data is given. Line numbers are shown for identification purposes only,
they are not included in the file. The exact number of lines is unimportant;
data may be stored all one line, or one item per line if desired.

Runfiles

Runfiles contain the measurement design data and commands to control
operation of the READZEN program. The exact format will vary depending
on the measurements required but will follow the general pattern of mak-
ing cell measurements between the reference cell group and the Transfer
Reference (DESIGN), measuring the unknown Zeners (ZMEAS), repeating
the cell measurements (REDESIGN), and calculating the final results
(ZCALC).

Note: The ten resistor voltages of the transfer reference are
considered to be equivalent to cells for the purposes of the mea-
surement programs and files. The ten voltages are assigned a
group name of ‘Zener’ and individual IDs of Z1, Z2, Z3,...,Z10.

Sample data: RUNFILEA

2.1138921114E11 Zener_workload

! 4x10 design, 1600 vs transfer Zener
DESIGN

2 1600 4 Zener 10

4010 40 8165.455 0.034

oR whe


0-0-0-O~ 2 6-6-0 + 10.0000450

5007/3

~~ — 1,00000345

5007/2
Bm et 101800012
@
]
0

Days

Fig. 4. The three voltage outputs of a typical solid-state standard sent to
NBS for calibration. The outputs are plotted in ppm from their
mean voltage. The solid line is drawn at the mean value which is
also labelled (in volts) on the right axis.

in int:

The measuring system was designed principally to measure solid-state
standards in the range 5-10 V. Table I lists the sources of uncertainty in
the measuring system extrapolated to the worst case unknown voltage in
the 5-10 V range. The system may be used over the 1 - 5 V range but with
somewhat reduced accuracy when the unknown voltage is not near a car-
dinal value (1.018, 2.036, 3.054,... V).

Explanation of Table Items

By far the most critical component of the system is the digital voltmeter.
The DVM gain is measured during the course of the Zener measurements
and the gain error of the 1-V range is calculated and applied as a correction
to the DVM readings. An allowance is included in the Table for the in-
accuracy of this gain measurement.

The DVM linearity was initially checked on the 10-V range using a
calibrated, manual 7-dial Kelvin-Varley divider, and measurements on
similar DVMs indicate little or no change in the linearity with time. The
linearity error is typically a maximum of 0.7 ppm or less at half-scale input
and no correction to the data is made for it. Provision has been made for the
system to calibrate automatically the linearity of the DVM at 10 points on
the 10-V range by measuring the 10 voltages 1.018, 2.036, ..., 10.18 V
although this is not done routinely.


Appendix A - Report of Calibration

ow <] UNITED STATES DEPARTMENT OF COMMERCE

f Standards

8 ¢| National Bur:
ylund 20833

XY SF | GatthersburgM:

REPORT OF CALIBRATION

Solid-State Voltage Standard
XXXXX Model YYYY DC Reference Standard
Serial No. 1111111

Submitted by
XXXXXXXK
XXXXXXXXXXXX
XXXXXXXXXXXXXXX

This voltage standard was calibrated during the period June 1,
1987 to June 18, 1987, by comparing its output voltages with
those from a calibrated ten step divider driven by a stable ten
volt source. The difference voltages were measured using a cal-
ibrated digital voltmeter. This measuring system was stan-
dardized by saturated standard cells calibrated in terms of the
U.S. Legal Volt (volts-NBS) using standard NBS. procedures. The
solid-state voltage standard was received with the IN CAL light
on and with a thermistor resistance of 3.91 kilohms. The unit
was plugged into the ac power line for all measurements.

The values given in the table below are the mean values of the
results of the individual calibrations summarized in the Ap-
pendix. The uncertainties given are three standard deviation
estimates equal to three times the root-sum-square of the esti-
mated 0.095 ppm one standard deviation uncertainty in the mea-
surement system (primarily systematic or type B), and the one
standard deviation random (type A) uncertainty of the mean of
the measurements on the individual voltage standard output.*

Nominal Output Measured Voltage Uncertainty Uncertainty

(volts-NBS) (ppm) (microvolts)
10 volts 100000450 0.29 2.86
1.018 volts 1.0180001 0.29 0.29

* The nomenclature 'type A' and ‘type B' and the procedure used
to combine them to obtain the total uncertainty has been recom-
mended by the International Bureau of Weights and Measures
(BIPM) Working Group on the Statement of Uncertainties, see
Metrologia, vol. 17, No. 2, p. 73 (1981).

Test No. 521/XXXXXXXXX

19



TABLE I

UNCERTAINTIES IN THE FINAL ASSIGNED VALUE

Source of Uncertainty 1 std. dev. estimate
(ppm)
DVM gain uncertainty 0.020
DVM linearity uncertainty 0.062
DVM leakage/bias currents 0.023
Standard cell leakage currents 0.006
Scanner switch thermal emfs 0.013
Random uncertainty in calibrating
the Transfer Reference 0.007
Random uncertainty in calibrating
the client standard (within-day) 0.013
RSS subtotal 0.072
Uncertainty in value of Working Group C 0.040
Change in Working Group C during the day 0.047

Uncertainty of the value assigned to the
client standard

RSS Total 0.095

The DVM leakage and bias currents and standard cell leakage currents
were measured directly with an electrometer and their effects on the mea-
surement calculated.

The crossbar scanner switch thermal emfs were measured with the DVM
by shorting the input leads and measuring the residual voltages.

The random uncertainties in calibrating the Transfer Reference and the
client standards are estimated from the pooled standard deviations of the
measurement designs. Although these uncertainties tend to be reduced by
averaging ten measurements of the client standard we prefer to include
them for completeness.

14


6 11110000000000

vi 1601 1602 1603 1604 Z1 Z2 Z3 Z4 Z5 Z6 Z7 Z8 Z9 Z10

8 29 30 31 32 19 20 21 22 23 24 25 26 27 28

9 1517191111132628210212214

10 35373931131346484104 12414

11 1341149474541431231038363

12 1321129272521411211018161

13 30 30.011 30 30

14 ENDDATA

15 !

16 ZMEAS

17 5018-1 108

18 5018-2 1.018 7

19 5018-3 1 12

20. ENDDATA

al !

22 REDESIGN

23 ZCALC

Description:

1 time and date in seconds (standard H/P time as re-
turned by TIMEDATE), identifying label (up to 30
characters with no spaces) displayed by READZEN to
identify different Runfiles

2 any line preceded by an exclamation point (!) is ignored
by the program; this is a comment for the operator

3 command to the program to expect DESIGN data for
intercomparing cell groups to follow

4 number of cell groups, group 1 name, number of cells in
group 1, group 2 name, number of cells in group 2

5 a meaningless design number - not used but something
must be there; total number of observations, mean of
reference group, accepted standard deviation (the
mean of the reference group is expressed in micro-
volts reduced by 1.01 volts; i.e., 8165.455 | 1.018165455
volts)

6 restraint - one number for each cell in each group, 1
identifies the cell as included in the mean of the ref-
erence group, 0 the cell is not included

7 cell IDs - group 1 first, group 2 next

8 cell circuit number corresponding to above cell IDs

9-12 measurement design - cells in line 7 are assigned posi-

tion numbers 1 through N (14 here). Pairs of num-
bers indicate difference measurements to be made,
ie., 1 - 5 indicates to measure cell "1" (1601) minus
cell "5" (Z1). The number of pairs of numbers must
agree with the number of observations specified on
line 5.


ZENER MEASUREMENT LOG FOR: 9 SEP 1987 [1.0] WEEK: 8737

RUNFILE: RUNFILEA:,700,1

DESIGN: (2800 - Zener ) Zener = 7970.159 sD = .022

Zener measurement completed -- Set 1

Zener measurement completed -- Set 2

Zener measurement completed -- Set 3

DESIGN: (2800 - Zener ) Zener = 7970.155 sD = .017
REFERENCE CELL STATISTICS

022 017 Standard Deviations (uV)
+008 006 Left-Right Components (uV)
30.0135 30.0135 Reference Cell Box Temps (C)
Reference Value at Difference Value at Mean
Cells Res 06:03:56 (uv) 10:07:02 Values

2801 1 8166.558 <  .003> 8166.561 8166.560
2802 1 8164.409 <-0.000> 8164.409 8164.409
2803 1 8165.602 <  .007> 8165.609 8165.606
2804 1 8166.147 <-0.010> 8166.137 8166.142
Mean 8165.679 < 0.000> 8165.679 8165.679
Transfer Zener
21 7978 .379 < 0.000> 7978 .380 7978.379
22 7976 .446 < -.021> 7976.425 7976 .436
23 7932.627 < -.006> 7932.621 7932 .624
24 7956.967 < ~-.004> 7956.963 7956.965
25 8052.081 < -.011> 8052.071 8052.076
26 7954.107 < -.015> 7954.092 7954.099
27 7956.854 < -.006> 7956.848 7956.851
zB 7966.002 < -.006> 7965.996 7965.999
29 7958.452 <  .019> 7958.471 7958.461
210 7969.671 <  .015> 7969.686 7969 .678
Mean 7969.671 < -.004> 7970.155 7970.157

DVM CALIBRATION SUMMARY
Number of DVM calibrations

= 8
Mean DVM error (ppm) = 2.0
Std. Dev. of mean error = 1

Fig. 3a. Page 1 of a typical daily printout from the measurement pro-
gram READZEN.

10


2. Description of Service
f Standards At for Calibrati

NBS offers a calibration service for solid-state voltage standards that are
complete functioning standards which continuously produce the output
voltages to be measured. We do not accept bare reference devices or
modules that require a stabilized power supply for operation. Also not ac-
cepted are instruments with multiple outputs that must be manually
switched to one set of output terminals. Each output voltage to be measured
must be continuously available at a set of terminals dedicated to only that
output. A potentiometric (non-loading) method is used for the measure-
ment of the standards so there is no requirement that the standard be capa-
ble of delivering current to the measuring system. Standards having out-
put resistances as high as 1 kQ can be accurately measured.

Shipment of Standards

Solid-state standards may be carried to NBS by hand, or shipped to NBS by
common carrier; arrangements can be made with a local delivery service
for pickup and delivery to local airports. Standards may be shipped under
battery power to maintain temperature control and/or to power the refer-
ence device, or they may be shipped with all power turned off. If they are
shipped with the power turned off we have observed that a short (2-3 days)
settling time is occasionally necessary before high-accuracy measurements
can be started.

Calibration Procedures

Most solid-state standards operate with the reference device in a tempera-
ture-controlled oven. The temperature of the oven is recorded when the
standard is first put on test (if the standard contains a temperature mea-
suring device), but unless specially requested, the temperature of the oven
is not measured or recorded during the remainder of the test.

Routine calibrations of solid-state standards typically take 2 to 3 weeks to
complete. The voltages are read daily for a period of ten working days. Af-
ter ten readings the data are reviewed by NBS personnel and if the stability
and day-to-day scatter of the outputs are within acceptable limits, a calibra-
tion report is issued. If the measured voltages fluctuated or drifted unduly
during the measurement period, the report will reflect these circum-
stances. In some cases additional measurements may be taken to confirm
the existence of a problem. If the standard exhibits an initial drift but later
becomes stable (indicating a transportation problem), additional measure-
ments will be taken until at least ten stable readings have been obtained.
Only in rare circumstances would more than twenty measurements be
taken; if the standard has not stabilized by that time it is likely that it is de-


separated by at least one space. All files are read using a free-field format;
only the order of the data is important, not the position on any one line. File
names are generally composed of a unique prefix identifying the type of file
and a suffix based.on the date. The date suffix is either of the form
YYMMDD or YYWW where YY is the year, MM is the month, DD is the
day, and WW is the number of the week in the year. WW starts with 1 for
the first week of the year and automatically changes on Sunday for the next
week. The exact style of the date suffix depends on whether the file contains
one day's data or one week's. The exact format of the data stored in the files
is discussed below.

The creation and use of the files is as follows: Intercomparisons between
the reference standard cells and the Transfer Reference produce 40 voltage
difference readings (4 cells by 10 resistors). These difference readings are
stored in an "S" file, ic, SYYMMDD. All intercomparisons performed in
one day are stored one after the other in the file. READZEN performs a
least-squares calculation on the "S" data and saves the calculated cell val-
ues and "resistor-voltages" from the Transfer Reference in a "D" file, i.e.,
DZEN_YYWW. All standard cell and Transfer Reference voltages for the
entire week are stored in this file. The raw difference readings between the
Zener standards under test and the Transfer Reference are stored in "Z"
files, i.e, ZYYMMDD. After calculation the final Zener values are stored
in "R" files, ie., RAEN_YYWW.

The files are initially created with a specified size and once created cannot
be increased beyond this size. They have generally been made large enough
to handle one or two sets of measurements per day. If the file size is
consistently too small the initial size specification may be increased. "S"
files are created in line 4780 of READZEN and are specified as 68 records of
256 characters/record. "D" files are created in line 11840 and are specified
with a length of 20 records of 256 characters/record. "Z" files are created in
line 8100 with a length of 20 records of 256 characters/record. "R" files are
created in line 12980 with a length of 20 records of 256 characters/record.

Program EDITA

EDITA is a general purpose ASCII text file editor for viewing, printing,
and modifying the input and output files generated by READZEN. Prior to
running READZEN a RUNFILE is created using EDITA which details the
measurements to be made for that day. The specific RUNFILE format is
described in the next section on file formats.

Running the program.

When the program is started it asks for a file storage device. This is the
floppy disk or hard disk on which a previously created file is stored or a new
file is to be stored. Enter a standard H/P disk designator without any
quotes, e.g., :,700,0. The program then prompts for a file name; enter the


Appendix B - Measurement Software

The READZEN program is a generalized measurement program that runs
under the direction of an ASCII text file (RUNFILE) and is used to make all
the measurements required for solid-state standard calibrations. Program
EDITA is a general purpose editor program for creating and modifying
RUNFILEs. The following sections describe how to operate the programs
and the purpose of some of the important program variables. It is assumed
that the reader is familiar with the BASIC language environment (Rocky
Mountain BASIC) used with the Hewlett/Packard model 200/300 desktop
computers.*

Program READZEN

READZEN is the main measurement and calculation program. It has the
capability of doing standard cell intercomparisons, least-squares cell data
reduction, Zener-Transfer Reference comparisons, and Zener data re-
duction. The operation of the program is controlled by an ASCII text file
named RUNFILExxx that is stored on the program disk. The xxx indicates
a1 to 3 character suffix of numbers or letters. The suffix is used to dif-
ferentiate between several Runfiles that may be stored on the same disk but
were created for different measurement procedures, i.e., normal workload
measurements, special tests, etc. At runtime the operator is prompted to
select one.

At the beginning of the program code of READZEN are comments on the
purpose of the program, array variable dimension statements, and a num-
ber of variables that define the various defaults of the program. These vari-
ables control things like the specification of the program disk and data disk
storage units, the 488 bus addresses of the measuring system instruments,
and other internal variables used to control calculations and data storage.
These variables are described in detail below. Program operations that may
be optionally changed by the user are generally controlled by these vari-
ables. No modification to the program code itself should be necessary.

Running the program

When the program is started an informational screen is shown giving the
name, version number, and purpose of the program, and all the scanner
channels are opened. If the scanner is not connected or turned on a mes-
sage is printed and the program is stopped. Similarly, the printer is
checked and the disk drives are checked for the program and data disks. If

* Specific model names are used for clarification only and their use does not constitute an
endorsement by NBS as the best available equipment. Other instruments may be as
suitable, or more suitable, for the applications described.


Table of Contents

page
List of Figures ..........:ccceseccsseececesseceeeerseeseeeeensteseeceeeseceseesereees vi
List of Tables .........:::::ssesssssceceeeeesesessesessnsssenenseeeeeeeeseseesereeeees vi
ADStract.....eeeececececceeeeeeeeeseseessseenneeeecccescesseeeereseesaseeasaessaeeeenee 1

1. Introduction.. 1

2. Description of Service
Types of Standards Accepted for Calibration.
Shipment of Standards....
Calibration Procedures...

8. Calibration Apparatus and Measurement Approach
System Description
Zener Referenc:
Resistive Divider
Scanner..........
Digital Voltmeter
Working Group C
Measurement Process
Measurement Theory.

4, Operational Procedures
Client Standards Handling .

Daily Procedures...........
Calibration of Working Group C
Calibration of Solid-State Standards
Analysis of the Daily Data

COBDOHND ANNITP AL ww NNNN

5. Assignment of the Final Value
Uncertainty ..............e
Explanation of Table Items .

6. Quality Control Procedures
Measurement System Verification

Client Standard Evaluation... 17
7. Bibliography.............csssssccsssscsesseecesesseseccnsesseseseeseeeessseeeeoeee 18
Appendices

A Report of Calibration
B Measurement Software ...


The new publications will present more technical detail than the
information that can be included in NBS Reports of Calibration.
In general they will also provide more detail than past
publications in the scientific and technical literature; such
publications, when they exist, tend to focus upon a particular
element of the topic and related elements may have been published
in different places at different times. The new series will
integrate the description of NBS calibration technologies ina
form that is more readily accessible and more useful to the
technical user.

The present publication, SP 250-28, NBS Measurement Services:
Solid-State DC Voltage Standard Calibrations, by Bruce F. Field,
is one of approximately 20 documents in the new series published
or in preparation by the Center for Basic Standards. It
describes calibration technology and procedures utilized in
connection with NBS Service Identification Numbers from 53110 to
53150 listed in the NBS Calibration Services Users Guide.
Inquiries concerning the contents of these documents may be
directed to the author or to one of the technical contact persons
identified in the Users Guide (SP-250).

Suggestions for improving the effectiveness and usefulness of the
new series would be very much appreciated at NBS. Likewise,
suggestions concerning the need for new calibration services,
special tests and measurement assurance programs are always
welcome.

Joe Simmons, Acting Chief
Office of Physical Measurement Services

Helmut Hellwig, Acting Director
Center for Basic Standards

iv


ZENER CALIBRATION SUMMARY FOR:

9 Sep 1987

Zener TD Voltage Std. Dev.
(volts) (ppm)
188/1 9.99999654 -145
188/2 1.01813796 220
5950/1 9.99999937 006
6015/1 10.00000245 -002
5027/1 10.00002831 004
5027/2 1.01799749 -011
5027/3 +99999292 0.000
5010N/1 10.00003443 0.000
5010N/2 1.01800020 -008
5010N/3 1.00000492 0.000
5011/1 10.00002566 20321
5011/2 1.01799755 028
5011/3 1.00000773 +036
5018/1 9.99999464 004
5018/2 1.01799740 007
5018/3 «99997592 0.000
BFF1/1 1.01899358 020
49105/1 9.99987041 026
49105/2 1.01798278 .010
49105/3 -99998092 0.000
49105/4 6.99989104 052
49111/1 9.99990381 027
49111/2 1.01798826 017
49111/32 -99998952 042
49111/4 6.99992458 047
5035/1 10.00000727 014
5035/2 1.01799416 013
5035/3 «99999792 0.000

ALL CALCULATIONS COMPLETED AND SRM UPDATED

Fig. 3b. Page 2 of a typical daily printout from the measurement pro-
gram READZEN.

i



NBS MEASUREMENT SERVICES:
SOLID-STATE DC VOLTAGE STANDARD CALIBRATIONS

Bruce F. Field

Abstract: This document describes the procedures used al NBS to calibrate
solid-state dc voltage standards in terms of the U.S. Legal Volt. The process
involves calibrating client standards at NBS approximately 10 times over a
two week period. The operational procedures and apparatus used to com-
pare the client standards to the U.S. Legal Volt are discussed in detail.

L Introduction

DC voltage standards based on solid-state devices are now available with
stabilities approaching 1 ppm/year. Such voltage standards can be used to
maintain and disseminate (transfer) a laboratory unit of voltage with ac-
curacies approaching those obtainable with saturated standard cells.
Presently-available voltage standards’contain at least one (and sometimes
more) Zener diode reference devices which develop a voltage that is scaled
using internal resistive dividers and low-noise amplifiers to produce a
range of output voltages from 1 to 10 V. These standards typically have
their best accuracy at some voltage other than the 1.01-V level of standard
cells. NBS has developed a calibration service based on a specialized mea-
suring system that automatically compares the output of any arbitrary volt-
age source, within the voltage range of one to ten volts, to a 1.018-V ref-
erence standard.

Solid-state voltage standards that are accepted for calibration must be com-
plete instruments, must be powered by the ac line or internal batteries, and
must continuously produce one or more stable voltages. Such standards,
when sent to NBS, are allowed to stabilize, and then are directly compared
to NBS standards which are in turn calibrated in terms of the U.S. Legal
Volt. Typically ten comparisons are made (once each working day) over a
two week period. At the conclusion of the test a report is issued with a
statement of the values of the voltage outputs of the standard and the esti-
mated uncertainties for the values while the standard was at NBS. No ad-
ditional uncertainty is included for transportation effects, long term drifts
of the outputs, or measurement errors in the client's laboratory.

The calibration of solid-state standards is closely tied to, and dependent on,
the calibration of working groups of standard cells. The reader is thus re-
ferred to reference 1, NBS SP250-24, NBS Measurement Services: Standard
Cell Calibrations, for a complete explanation of the apparatus and proce-
dures used fo~ the calibration of standard cells.


i ni Evaluation

Tests for ac effects are done for each new type of standard received for test.
Some models of standards produce a de shift of the output voltage when an
ac source is connected to the output terminals. Experiments have shown
that for some standards the DVM can produce a significant shift [2]. These
experiments also showed that the effect was model dependent, not standard
dependent; all items of a particular model tested either had the problem or
they did not. Any new, unfamiliar model received is tested for ac effects by
applying a small ac signal to the output while observing the dc voltage.

17


NBS Measurement Services:

Solid-State DC Voltage
Standard Calibrations

NBS
Special
Publication
250-28

Bruce F. Field

U.S. Department of Commerce
National Bureau of Standards

|


al connection to a 16 Mbyte hard disk which is common to several measur-
ing systems in the laboratory and which is used for program and data stor-
age. The floppy disk system is used for backup data storage in addition to
the main storage on the hard disk.

Also not shown in Fig. 1 is the group of four saturated standard cells used
as a reference for the system (Working Group C). All solid-state voltage
measurements are expressed as a numerical ratio times the mean emf of
these four cells. These cells in turn are calibrated daily in terms of the U.S.
Legal Volt using standard NBS procedures [1].

Zener Reference

The Zener reference is a commercial Zener standard that has been modi-
fied to produce 10.1817 V rather than the 10 V normally supplied. With 16
AWG wire connecting the standard and the resistive divider the voltage at
the divider is very close to 10.181 V, approximately ten times the voltage of a
saturated standard cell at 30 °C. Interactions between the Zener reference
and the other parts of the measuring system (notably the DVM) are
negligible even when the standard is plugged into the ac power lines. The
Zener reference is operated on ac power for all the measurements.

Resistive Divider

The resistive divider driven by the Zener reference consists of 10 nominally
equal, stable 250-Q wire-wound resistors. The 250 © choice is a compro-
mise between reducing the resistance to minimize the effects of leakage re-
sistances and increasing the load current that must be supplied by the Zen-
er reference. The resistors are selected for matched temperature coeffi-
cients; all the coefficients are within +1 ppm/°C of one another. ‘The resis-
tors are mounted in a thick-walled aluminum box for temperature lagging.
During a measurement sequence the individual resistors typically change
value less than 0.03 ppm with respect to the total divider resistance.

Scanner

The scanner used to select the voltage inputs is a modified crossbar switch
identical to the VIP scanner described in reference 1. It has 80 2-wire in-
put channels and uses latching relays to reduce the thermal emfs within
the switches. The scanner is wired so that any two of the 80 channels may
be selected by the computer and connected together in series opposition with
the difference voltage presented to the DVM. By selecting the two channels
in reverse order the voltage difference with opposite polarity is presented to
the DVM. The unresolved thermal emfs of the scanner have been period-
ically measured and are less than 0.015 pV per switch. Twenty of the
channels are used for the resistive divider, four channels are used for the
reference standard cells, and two channels are reserved for crossbar ther-
mal tests, leaving 54 channels available for unknown standards.


fective. Only the stable readings are used for the final values in the calibra-
tion report.

8. Calibration Apparatus and Measurement Approach

System Description

The system used at NBS to calibrate solid-state voltage standards is spe-
cially designed; it employs a potentiometric method to measure the stan-
dard which allows voltage measurements to be made without loading the
standard. (This system is also described in reference 2). The system is ba-
sically a scaling device for comparing arbitrary voltage standards to 1.018-V
standard cells. It will measure any dc voltage in the range from 0 to 10 V,
and has input channels for up to 54 unknown standards.

The system is also capable of making high-accuracy standard cell compar-
isons. Standard cells may be connected to the input channels of the scan-
ner in place of (or in addition to) solid-state standards and these cells may
be intercompared with one another.

40-step divider
in a temperature
lagged enclosure

test inputs
0 - 10 Volts

10.18 V 6-1/2 digit
Zener digital
reference voltmeter
\amensanassnecectonsiomsel
dual floppy desktop dot matrix
disk drive computer printer

Fig. 1. Block diagram of the de voltage measuring system.

Figure 1 is a simplified block diagram of the measurement system. It in-
cludes a modified 10.18-V Zener reference, a 10-step resistive divider con-
sisting of 10 nominally equal 250-Q resistors, a low-thermal-emf crossbar
scanner, a 6-1/2 digit digital voltmeter (DVM), and a desktop computer.
The scanner, voltmeter, disk drive, and printer are connected to the com-
puter with an IEEE-488 bus system. Not shown in the figure is an addition-


Zener_volt$ is the partial filename for result files.
Calculated standard cell results preface
Zener_volt$ with a "D" and Zener reference
results preface it with an "R". For both, the
2 digit year and a 2 digit week number
within the year are appended to the file-
name. Zener_volt$ should be 4 characters
or less, otherwise the filename will be too
long.
(Zener_volt$="ZEN")

Heopy controls the printing of measurement data,
intermediate results, and final results to
the printer. If Hceopy=0 (normal default)
one line is printed on the printer each time
a measurement set (cells or Zeners) is com-
pleted. The printer output becomes a log of
the measurements; the actual mea-
surements are not printed. The results of
the cell calculations and the final Zener re-
sults are also printed. If Hcopy = 1 then log
printing is suppressed but cell and Zener
results are printed. If Hcopy = 2 all the
measurement data (cell-pair differences
and Zener differences) are printed to the
printer as well as the results. If Hcopy =—1
nothing is printed to the printer. (Hcopy =
0)

Srm controls whether data is saved on an SRM
in addition to the floppy disks. It should be
set to either Yes or No. If Yes, check to
make sure that Dir$ is correctly set to the
subdirectory desired. (Srm=Yes)

Saveflag controls saving of data on floppy disk and
the SRM. For Saveflag set equal to 1, all
measurement data files and result files are
saved on disk. To save space on disk Save-
flag may be set equal to 0 so only the cell and
Zener result files are saved. If Saveflag =—1
only the final Zener result files are saved.
(Saveflag = 1)

File use

READZEN automatically creates as needed a number of data files. These
are all ASCII files with data stored as strings. Each line in the file is stored
as one string and may contain an arbitrary number of items with the items


all is in order, the Runfile names on the program disk are displayed along
with an ID number, a 20 character identifying line, and the creation date of
the file. The operator must enter the ID number of one of the Runfiles.
Once this has been done the measurements proceed automatically with no
further operator control. At the conclusion of the measurements the re-
sults are printed on the printer and saved on disk.

Program. options

Variables to control the data storage and bus addresses of the measuring
instruments are contained in lines 1270 to 1641 of READZEN. A description
of some useful variables follows with their normal values shown in paren-
theses.

Observer$ is an identifying name for the system.
(Observer$="Micky")

Zsets is a counter for the number of times the
Zener references are compared to the
transfer reference. Under special cir-
cumstances more or fewer measurements
may be desired. (Zsets=3)

Dvmcorr is a "gain" value that is used to multiply all
DVM readings taken on the 0.1 V scale. If
the 0.1 V scale is known to have a signifi-
cant gain error this variable can be set to
compensate for it; e.g., if the gain error is
-100 ppm, the DVM reading is lower than
the actual value, set Dvmcorr = 1.00010.
Generally, the DVM is physically adjusted
if the gain is significantly in error.
(Dvmcorr=1)

Dk$ is the data disk specifier. (Dk$=":,700,0")

Progdk$ is the program disk specifier.
(Progdk$=":,700,1")

Srm$ is the suffix for the SRM.
(Srm$=":REMOTE")

Dir$ is a prefix directory name for the SRM.
This name will preface the data file name,
and Srm$ will be appended to the end to
form a complete filename.
(Dir$="ZENDATA/")


List of Figures

Figure page
1. Block diagram of the dc voltage measuring system............... 3
2. Simplified wiring diagram of the measuring system set

8a.

3b.

Table I.

up to compare a standard-under-test to the calibrated
resistive divider (Transfer Reference) .............ccccsseecseeeeeeenee 6

Page 1 of a typical daily printout from the measurement

program READZEN.. . 10
Page 2 of a typical daily printout from the measurement
program READZEN.............ccecceeeccecesceeeeeseeseseeceeeuneeseeseness ll
The three voltage outputs of a typical solid-state standard
sent to NBS for calibration .............ccccsseseeesseeeesseeeeeeneeeeees 13
Ten volt output of a check standard as measured by
the SYStOM ........eceecceeeeeseesseecceeseeeeeeeeeeceeeeeeeeecaenenseereeeeeeees 16

List of Tables

page

Uncertainties in the Final Assigned Value................+6 14

vi


Two sets of DVM readings are taken for every voltage measurement (both
for standard cells and solid-state standards), one with the input voltage
source connected normally and a second with the voltage source polarity
reversed by the crossbar scanner. Subtracting the second reading from the
first and dividing by two yields a measurement free from any error due to a
non-zero DVM offset, thus no allowance has been included in the Table for
DVM offset errors.

Ac effects observed in an earlier system were found to be negligible in this
system [2]. The ac effects on the Zener reference of the Transfer Reference
were estimated by monitoring changes in its voltage using a passive mea-
surement circuit referenced to a standard cell while ac was introduced into
the system by connecting the DVM to the Transfer Reference taps.

The RSS subtotal represents the ratio error in comparing an arbitrary volt-
age source to a group of 1.018-V standard cells. For establishing an uncer-
tainty for the client standards with respect to the U.S. Legal Volt, the
uncertainty of Working Group C must be added. The uncertainty of the
value of Working Group C computed from earlier measurements is
developed in reference 1 and will not be repeated here. An additional
uncertainty is included, however, for temperature instabilities and other
related emf variations in the present standard cell enclosure used for
Working Group C that cause the cell emfs to change during the day.

As far as is known, all systematic errors listed in Table I are independent
of each other. Therefore, they are combined in root-sum-square (RSS) fash-
ion along with the random errors. The final reported uncertainty for a
client standard (Ureportea) is three times the root-sum-square of the above
measurement uncertainty and the standard deviation of the mean of the ten
daily measurements (sy), i.e.,

Ureported = 3 rR | (0.0957 +s”) | .

6. Quality Control Procedures
Measurement System Verification

As discussed in Section 5 all major sources of error in the measuring sys-
tem are evaluated during the measurements and corrections are applied to
the data as necessary. The printout from READZEN is reviewed daily by
laboratory personnel to verify that all the measurements are in statistical
control.

15


right component, measurement system ID, time,
number of cell groups, average DVM offset

2 group 1 name, number of cells in group 1, nominal
temperature, actual temperature, temperature cor-
rection

3 group 1 cell IDs

4 group 2 name, number of cells in group 2, nominal
temperature, actual temperature, temperature cor-
rection

5 group 2 cell IDs

6-9 restraint flag, cell ID, cell emf in iV reduced by 1.01 V.
The restraint flag=1 if the cell is included in the
reference group mean, =0 if not. The restraint, ID,
and emf triplet is repeated for each cell measured.

"2" files

"Z" files contain the raw Zener measurements taken between the unknown
Zener references and the transfer Zener; only one set is shown here. A file

may contain additional sets taken on the same day.
Sample data: Z860730

860730 15:2847 3 2 3
DVM 1 1.0181223E+6
DVM 2 1.0181238E+6
5018-1 10 -179729.8
5018-2 1 21.9

5018-3 1 -17988.95
DVM 1 1.01812225E+6
DVM 2 1.01812395E+6
5018-1 10 -179730.15
5018-2 1 22.05
5018-3 1 -17988.8
DVM 1 1.0181223E+6
DVM 2 1.0181236E+6
5018-1 10 -179729.9
5018-2 1 21.7

5018-3 1 -17988.95
DVM 1 1.01812225E+6
DVM 2 1.0181237E+6

Be et pe
SRRRES RE BOwmIMAMAwODH

Description:

1 YYMMDD, time, number of repeated sets of Zener mea-
surements, number of DVM calibrations per Zener
measurement, number of Zeners mcasured


2-3 string "DVM" to indicate a direct DVM reading of one of
the resistors of the resistive divider, the number of
the resistor measured (1-10), the reading in pV

4-6 Zener ID, tap number of resistive divider to which Zener
was compared (0-10), measured difference in pV

7-18 repeated measurements, stored the same as lines 2 - 6

"R" files

“R" files contain the final calculated results of the Zener data; only one set
is shown here. A file may contain additional sets of data taken within the
same week.

Sample data: RZEN_8632

860730 12:53:36 3 3

4 8131.1000 1601 1602 1603 1604
5018-1 10.0000047045 0.003333331 76584
5018-2 1.01799810104 0.142727802049
5018-3 0.999989513271 0

oho be

Description:

YYMMDD, time, number of Zeners calibrated, number
of repeated measurements (Zsets)
number of reference cells, mean emf of reference cells,
reference cell IDs
-5 Zener ID, calculated voltage (volts), standard deviation of
the mean (ppm) calculated from the repeated mea-
surement sets

wo wo

System. war iremen:

The measurement software will run satisfactorily on all H/P Basic systems
3.0 and higher. The following BIN files are required for proper operation.

CLOCK CS80
KBD MAT
ERR HPIB

For use with an SRM the following additional BIN files are needed.
SRM DCOMM


Appendix A (cont'd) - Report of Calibration

Solid-State Voltage Standard
XXXXX Model YYYY DC Reference Standard
Serial No. 1111111

These uncertainty figures contain no allowance for the effects
of transport of the standard back to the user. Any valid uncer-
tainty statement applying to the above values when the standard
is used olher than in this laboratory must contain an additional
uncertainty component. Also not included in the above uncer-
tainties is an allowance for long term drift of the output volt-
ages of this standard. This must be determined from historical
data on a case by case basis.

For the Director
National Measurement Laboratory

Norman B. Belecki, Physicist
Electricity Division
Center for Basic Standards

Test No.: 521/XXXXXXXXX
Reference: XXXXXXXXXXXX
Date: September 8, 1987



Library of Congress Catalog Card Number: 87-619904

National Bureau of Standards Special Publication 250-28
Natl. Bur. Stand. (U.S.), Spec. Publ. 250-28, 39 pages (Jan. 1988)
CODEN: XNBSAV

U.S. GOVERNMENT PRINTING OFFICE
WASHINGTON: 1988

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



The linearity of the DVM is checked as necessary, but the gain of the one
volt range of the DVM is calibrated every time measurements are made by
measuring the 1.018 V developed across two of the resistors in the Transfer
Reference; typically resistors Ry and Rg are used (see Fig. 2.). This is done
to evaluate the effect of measuring voltages at ground and above ground.
Gain calibrations from several days can be combined to detect a systematic
bias in the measurements with respect to the particular resistor used.
Such measurement errors can be caused by excessive DVM leakage or bias
currents and will also affect the accuracy of the comparisons between the
Transfer Reference and the standard cell reference group.

Once the Transfer Reference has been calibrated in terms of the reference
standard cells the linearity of the DVM on the 10 V range can be checked by
directly measuring the ten voltages, 1.018 to 10.18 V, developed across the
resistors. Although not a thorough calibration of the DVM linearity, this
test can be used to detect significant changes in the linearity errors.

The DVM is used on the 0.1-V range to improve the resolution of compar-
isons of 1.018-V solid-state standards to the Transfer Reference, and of the
comparisons of the Transfer Reference to the reference standard cells. The
gain error of the 0.1-V range is not calibrated daily by the system but, the
largest expected measurement difference for these comparisons is only
200 pV. Thus for an overall measurement error of 0.005 ppm (of 1.018 V)
the gain need only be known to 250 ppm. The gain of the 0.1-V range is
periodically checked using external standards to ensure that it is within
this limit.

The final value for each solid-state standard under test is computed by
correcting the measured difference between the standard and the Transfer
Reference for the gain of the DVM, and adding this to the value of the
appropriate tap of the Transfer Reference. The calibrated values of the
Transfer Reference are determined using a least-squares analysis of the
redundant measurements that compared the 10 outputs of the Reference to
the reference standard cell group. The mean of the “before” and "after"
calibrations is used.

4, Operational Procedures
Client Standards Handling

Standards shipped to NBS under power are hand carried from the Building
301 Receiving Room as soon as Electricity Division personnel are notified.
The temperature is checked by qualified laboratory staff (if the standard has
a temperature measuring device): and the standard is brought to the Volt
Facility Laboratory. All solid-state standards to be calibrated are connected
to the measuring system scanner at the time of their arrival and are left
undisturbed until the test is completed. When the standard is received it is


Measurements are started by setting the DVM to the 0.1-V range and
shorting the DVM input to discharge the input capacitance. Then the ten
1.018-V outputs of the Transfer Reference are compared to the Working
Group C cell emfs using the redundant measurement design discussed in
section 3. Forty measurements are required and the data is saved on disk;
this process takes about 1/2 hour. Measurements of the unknown solid-
state standards are preceded by gain calibrations of the DVM 1-V range.
DVM gain calibrations consist of directly measuring (in both forward and
reverse polarity) two of the Transfer Reference voltages (1.018 V) using the
1-V range of the DVM as discussed above.

The solid-state standard voltages are measured by comparing each one in
turn to the tap on the Transfer Reference that is closest in voltage to the
standard's voltage and measuring the difference with the DVM set on the
1-V range (in both forward and reverse polarity). Once all the unknown
standards have been measured the measurement sequence repeats starting
with the DVM calibrations. This measurement sequence (including the
DVM calibrations) is repeated three times and then a final (fourth) DVM
calibration is done. At the conclusion of these measurements, the Transfer
Reference is recalibrated against Working Group C. To summarize, the
measurement sequence is — calibrate the Transfer Reference using Group
C, calibrate the DVM, measure the unknown standards, calibrate the
DVM, measure the unknown standards, calibrate the DVM, measure the
unknown standards, calibrate the DVM, and finally calibrate the Transfer
Reference a second time.

Analysis of the Daily Data

At the conclusion of all the measurements for the day, the READZEN pro-
gram automatically calculates the emfs of the standards-under-test and
produces a two page printout summarizing the calibration of the Transfer
Reference and listing the emfs of the solid-state standards. The emfs of the
standards (as well as intermediate results) are also saved on two disk files,
one on a floppy disk, and the other on the hard disk. These data are com-
bined with other daily data sets for assigning the final emf values to the

standard.

Figures 3a and 3b are copies of the two pages of a typical daily printout pro-
duced by READZEN. At the top of page 1, below the date and the name of
the RUNFILE that was used, is a summary of the measurements made,
and their starting time. In the example shown the Transfer Reference
(denoted Transfer Zener in Fig. 3a) was compared to Working Group C
(2800) at 8:03 and 10:07 am. The unknown solid-state standards were com-
pared to the Transfer Reference three times: 8:35, 9:05, and 9:35 am. This
part of the log is printed as the measurements are being taken so the opera-
tor can track the operation of the program.


NBS MEASUREMENT SERVICES:
Solid-State DC Voltage Standard Calibrations

Bruce F. Field

Center for Basic Standards
National Measurement Laboratory
National Bureau of Standards
Galthersburg, MD 20899

January 1988

U.S. DEPARTMENT OF COMMERCE, C. William Verity, Secretary
NATIONAL BUREAU OF STANDARDS, Ernest Ambler, Director


Description:

month, day, year, time, observer (system) name
number of cell groups, group 1 name, number of cells in
group 1, group 2 name, number of cells in group 2
total number of observations, mean of reference group,
accepted standard deviation
restraint - one number for each cell in cach group; 1
identifies the cell as included in the mean of the ref-
erence group, 0 the cell is not included
5 cell IDs - group 1 first, group 2 next
6-9 measurement design - cells in line 5 are assigned posi-
tion numbers 1 through N (14 here) pairs of numbers
indicate difference measurements to be made, i.e.,
1 - 5 indicates to measure cell "1" (1601) minus cell
"5" (Z1). The number of pairs of numbers must
agree with the number specified on line 3.
10 nominal temperature of group 1, actual temperature of
group 1, nominal temperature of group 2, actual
temperature of group 2
11-17 actual measured cell differences in microvolts

- wow Nye

"D" files

“D" files contain the results of the standard cell intercomparison data; only
one set is shown here. A file may contain additional sets of data taken
within the same week.

Sample data: DZEN_8630

860730 14 40 26 0.08247 0.0133 NBS-Z 12:15:11 2 -0.596

1600 4 30 30.011 -0.6226

1601 1602 1603 1604

Zener 10 30 30 0

Zi Z2 Z3 Z4 Z5 Z6 Z7 Z8 Z9 Z10

1 1601 8103.3617 1 1602 8096.9666 1 1603 8100.4951 1
1604 8099.1765

0 Z1 7945.575 0 Z2 7947.2625 0 Z3 7904.0645 0 24
7927.444

0 Z5 8015.7105 0 Z6 7926.848 0 Z7 7929.7768 0 Z8
7937.7688

0 29 7930.194 0 Z10 7939.9563

oO oo x QnkwWNe

Description:

1 YYMMDD, total number of cells, number of observa-
tions, degrees of freedom, standard deviation, left-


10.

7. Bibliography

Field, B. F., "NBS Measurement Services: Standard Cell Calibra-
tions," Natl. Bur. Stand. (U.S.), Special Publ. 250-24, 52 pages (1987).

Field, B. F., "A Sub-PPM automated 1-10 Volt DC Measuring Sys-
tem," IEEE Trans. Instrum. Meas., Vol. IM-34, No. 2, pp. 327-330
(1985).

Eicke, W. G. and Cameron, J. M., "Designs for Intercomparing
Small Groups of Saturated Standard Cells," Natl. Bur. Stand. (U.S.),
Tech. Note 430, 19 pages (1967).

Eicke, W. G., "Comments on Zener diodes as voltage standards,"
Comité International des Poids et Mesures, Comité Consultatif
d‘Electricité, 10th Session, p. 27 (1963).

Eicke, W. G., "Making precision voltage measurement. on Zener
diodes," JEEE Trans., Paper CP 63-416 (1963).

Eicke, W. G., "The Operating Characteristics of Zener Reference
Diodes and their Measurement," ISA Transactions, Vol. 3, No. 2, pp.
93-99 (1964).

Huntley, L., "The Fluke direct voltage maintenance program," Proc.
1984 Meas. Sci. Conf., pp. 147-151 (1984).

Koep, K. J., "The use of a solid state dc voltage transport standard to
transfer the dc volt in a regional standard cell MAP program," Proc.
1984 Meas. Sci. Conf., pp. 142-146 (1984).

Murray, W. and Dencher, P., "High-accuracy voltage transfer using
a modified commercial solid-state reference," J. Phys. E.: Sci. In-
strum., Vol. 17, No. 5, pp. 354-356 (1984).

Spreadbury, P. J. and Everhart, T. E., "Ultra-stable portable voltage
sources,” JHE Conf. Publ., Vol. 174, pp. 117-120.

18


Center for Basic Standards

The Center for Basic Standards develops and maintains the scientific competences and laboratory facilities nec-
essary to preserve and continue to refine the base physical quantities upon which the Nation's measurement
system is constructed. The Center is responsible for the standards of voltage, current, resistance and
impedance, temperature, vacuum, leak rate, pressure, mass, length, time and frequency. The Center collabo-
rates closely with the standardizing laboratories of other countries to ensure that measures used in international
trade are fully compatible. The Center also performs research at the frontiers of physics in order to improve
measurement capability and quantitative understanding of basic physical processes that underlie measurement
science.

Electricity Division

Maintains and improves the national standards of electrical measurement; conducts experiments to realize the
electrical units in terms of the mechanical units; develops stable electrical standards and improved methods for
the dissemination of the units of electrical measurement; determines fundamental physical constants related to
electrical units; develops instrumentation for precise electrical measurements directly referenced to the national
standards; implements techniques for their use and puts them into practical applications; assists in applying sci-
entific and technological development to measurement problems, especially in other areas of NBS research.

Temperature and Pressure Division

Maintains and improves the national standards of temperature and pressure; conducts research towards the de-
velopment of new concepts for these standards; calibrates practical standards for the U.S. scientific and techni-
cal community in terms of the primary standards; develops methods and devices to assist user groups in the
assessment and enhancement of the accuracy of their measurements of temperature and pressure; prepares
and promulgates evaluations and descriptions of temperature and pressure measurement processes; coordi-
nates these standards and measurement methods nationally and internationally; and determines the accuracy of
the national standards of temperature and pressure with respect to fundamental thermodynamic relations.

Length and Mass Division

Develops and uses the competence necessary to implement a more accurate measurement system for length
and mass; applies research, utilizing the most advanced technology available, to the improvement of our present
standards and devises new standards and new methods for their transfer.

Time and Frequency Division

Maintains, develops and improves the national standards for the unit of time, the unit of frequency, and time
scales based on these standards; carries out research in areas of importance to the further fundamental im-
provement of frequency standards and their applications focusing on microwave and laser devices, atomic and
molecular resonances, and the measurement of fundamental physical phenomena and constants; adapts time
and frequency standard devices and concepts to special scientific and technological demands; develops time
and frequency measurement methods in the radiofrequency, microwave, infrared, and visible radiation regions;
coordinates the NBS time scales with international time scales and the time scales maintained by the United
States Naval Observatory; collaborates with national and international organizations in the development of im-
proved methods for the measurement of time related quantities; operates time and frequency dissemination ser-
toast such as radio stations and broadcasts, for the purpose of traceability to the national standards of time and
ency.

Quantum Physics Division

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

Quantum Metrology Division

Engages in forefront research in quantum metrology; contributes to new determinations of the fundamental
physical constants, and the extension and refinement of the electromagnetic scale and devises, where possible,
tests of basic symmetries and invariances.



name of an existing file or a name for the new file to be created. If the
program cannot find the file on the specified disk, it will ask if a new one
should be created. Enter Y or N as appropriate.

Most operations are controlled by the function keys at the top of the key-
board. They are:

fl PREVSCREEN Scroll up to display the previous
screen of data.

f2 NEXT SCREEN Scroll down to display the next
screen of data.

f8 FIND CHRS Find a string of characters within
the text.

f4 2NDKEYLEVEL Change the key functions to the
alternate (2nd) set.

f APPEND ON (OFF) With append on additional lines
may be added to the file, with ap-
pend off the original length is pre-
served.

f6 GOTOLINE Go to the specified line number.

f7 MOVE/DEL COPY Move, delete, or copy a block of
lines.

f8 END PROGRAM _ Stop the program without saving
the file.

2nd Key Level

fl STORE TEXT Save the file in the previously
specified name.

f2 MERGE FILES Append another file to the end of
the existing one in memory.

f8 NEWSTORAGE Specify a new disk designator

f4 1ISTKEYLEVEL _ Return to the original key
functions

f NEW PRINTER Specify the printer select code
(usually 701). Note: this must be
done before printing the first time.


Appendix A (cont'd) - Report of Calibration

Solid-State Voltage Standard
XXXXX Model YYYY DC Reference Standard
Serial No. 1111111

APPENDIX
(values in volts-NBS)
Date Time 10 volts
06/01/87 9 10.0000448
06/02/87 8  10.0000452
06/03/87 2  10.0000451
06/04/87 6 10.0000454
06/05/87 7 ~—-10.0000447
06/08/87 7 10.0000450
06/09/87 2  10.0000450

06/10/87 09:34 10.0000450
06/11/87 09:33 16.0000451
06/12/87 11:20 10.0000448
06/15/87 11:03 10.0000453
06/16/87 11:03 10.0000449
06/17/87 09:39 10.0000448
06/18/87 09:38 10.0000450

Mean (volts-NBS) 10.0000450

Standard Deviation

of the Mean (ppm) 006
Number of readings 14
Test No.: 521/XXXXXXXXX

Reference: XXXXXXXXXXXX
Date: September 8, 1987

1.018 volts

1.0180001
1.0180002
1.0180001
1.0180001
1.0180001
1.0180001
1.0180001
1.0180001
1.0180002
1.0180001
1.0180001
1.0180001
1.018000
1.0180001

1.0180001

007

14



Digital Voltmeter

A 6-1/2 digit DVM with 0.1 pV resolution on the 0.1-V range is used as the
system voltmeter. The voltmeter is programmed and read by the computer.
All voltages are measured by the DVM in both the forward and reverse di-
rection to cancel offset voltages in the DVM. The 0.1-V range is used when
measuring microvolt level voltages associated with standard cell differ-
ences, and an integration time of 9 s for each polarity yields a random mea-
surement uncertainty of 0.03 pV (1s) for a single measurement. For all
measurements of solid-state standards, except at the 1.018-V level, the volt-
meter is calibrated and used on the 1-V range.

Working Group C

Working Group C consists of four saturated standard cells housed in a
commercial temperature-regulated enclosure. The enclosure contains an
internal thermistor and Wheatstone bridge for measurement of the cell
temperatures with a resolution of 0.001 °C. The enclosure is powered from
a regulated 12 V dc power supply to minimize any effects of a changing ac
line voltage.

Measurement Process

The general measurement procedure is to 1) self-calibrate the system using
Working Group C, 2) compare the unknown solid-state standards to the
calibrated system, and 3) self-calibrate the system again to make sure no
significant drifts have occurred. The exact procedure is controlled by a
measurement procedure file (RUNFILE) which is executed by the com-
puter. The measurement procedure file is created and modified daily by the
operator as necessary.

Figure 2 shows some critical parts of the measuring system in more detail.
The modified 10.18-V Zener reference is permanently connected to the
resistive divider; this combination (the Transfer Reference) is considered to
be a stable voltage source with ten series-connected 1.018 V outputs. The
‘Transfer Reference is calibrated by comparing the voltage drop across each
resistor to each cell in Working Group C, measuring the microvolt-level
difference voltages with the digital voltmeter. All four Working Group C
cells are compared to the ten Transfer Reference voltages for a total of 40
measurements which results in a redundant set of measurements. [1] By
referencing each output tap to the bottom tap, the Transfer Reference can be
used as a source of ten calibrated voltages from 0 V to 10.18 V in steps of
1.018 V. Then (as shown in Fig. 2) the output voltage developed across N of
the divider resistors is compared to the voltage of the unknown standard-
under-test using the DVM. N is chosen to minimize the magnitude of the
DVM reading. All the solid-state standards to be measured are compared
in turn to the divider tap appropriate for that standard. Then the Transfer
Reference voltages are compared a second time to the standard cell voltages
after all the standards-under-test measurements have been completed. The


assigned a unique ID number, usually a subset of the serial number, and
this is entered into a log book along with the date and the condition of the
standard. Measurements are begun the day after the arrival of the unit.
After the test is completed, the client is notified and the standard is
repackcd in its original shipping matcrial and returned to the client.

Daily Procedures
Calibration of Working Group C

Working Group C is used as the reference for all solid-state measurements
in the Volt Facility Laboratory. Working Group C is compared daily to two
groups of primary cells (Primary Groups A and B) using the automated.
standard cell comparison system VTP [1]. These measurements are made
early in the morning before measurements of solid-state standards are be-
gun and the data is immediately reduced to determine the values of the cell
emfs of Working Group C. These values are used for the solid-state
standard measurements done that day.

Calibration of Solid-State Standards

The operator must set up a measurement procedure file (RUNFILE) that
has a list of all the standards to be calibrated. Program EDITA, a general
purpose text editor, is usually used to modify the previous day's file. The
exact format of the file is described in Appendix B, but in general the file
contains the measurement sequence for calibrating the Transfer Refer-
ence, a list of identification numbers of the standards-under-test, their
scanner circuit numbers, and the approximate voltage of each of the stan-
dards. The operator manually measures the temperature of Working
Group C before the measurements are begun and enters this into the file
along with the cell emf values (from the comparisons with the primary
groups). When the file is complete the measurement program (READZEN)
is started and measurements proceed automatically with the fully corrected
values of the solid-state standards printed out and saved on disk at the con-
clusion of the measurements.

READZEN first checks the measurement system apparatus to make sure
everything is functionally working. This includes sending a clear-all-
channels command to the scanner and checking to see if it responds, send-
ing an initialization command to the DVM and checking it's response,
checking that the manual front/rear input switch on the DVM is set
properly, checking to see if the hard disk/file server is connected and turned
on (if it is to be used), checking that the printer is turned on and loaded with
paper, and finally, checking that a properly initialized data disk and a pro-
gram disk containing a RUNFILE are in their respective disk drives. Mul-
tiple RUNFILEs may be included on the program disk for different sets of
measurements; the operator selects the desired one at the time the program
READZEN is run.