NBS

Publi - cations

A111D5 TflM005

NBSIR 83-2752 (/C)

Advanced Mass Calibration in

State Laboratories

U S DEPARTMENT OF COMMERCE

National Bureau of Standards

Center for Absolute Physical Quantities

Washington, DC 20234

August 1 983

Sponsors Report

Prepared for: U S. DEPARTMENT OF COMMERCE

National Bureau of Standards

Office of Weights and Measures

Washington, DC 20234

SEP 1 3 1983.

NBSIR 83-2752

ADVANCED MASS CALIBRATION IN

STATE LABORATORIES

Randall M. Schoonover

U S DEPARTMENT OF COMMERCE

National Bureau of Standards

Center for Absolute Physical Quantities

Washington, DC 20234

August 1 983

Sponsors Report

Prepared for: U S. DEPARTMENT OF COMMERCE

National Bureau of Standards

Office of Weights and Measures

Washington, DC 20234

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

NATIONAL BUREAU OF STANDARDS, Ernest Ambler, Director

ADVANCED MASS CALIBRATION IN STATE LABORATORIES*

R. M. Schoonover

Center for Absolute Physical Quantities

National Bureau of Standards

Washington, D. C. 20234

ABSTRACT

This report describes the initial phase of a development program to

improve maintenance of mass reference standards in State Weights and

Measures Laboratories. Four one-kilogram objects of special design were

calibrated by five State Laboratories, a highly qualified '’check”

laboratory, and NBS. The results indicate that certain improvements

should be made in critical areas of the recalibration process. Errors

associated with air buoyancy correction, for example, are significant;

for this reason, it is necessary to improve the methods and instrumentation used for the measurement of air pressure, air temperature, and

relative humidity in State Laboratories. Proposed future work is focused

upon the air buoyancy correction, the simplification of calculation

procedures, and the provision of NBS Reference Standards to improve the

values assigned to the State Reference Standards.

The work reported was based upon the collaboration of the NBS

Length and Mass Measurements and Standards Division, the NBS Office of

Weights and Measures, Sandia Laboratories, and the five participating

State Laboratories.

*This report includes privileged information and it is intended for

internal NBS use only.
2

PURPOSE

Recent measurement control programs coordinated by the Office of

Weights and Measures have disclosed some apparent anomalies in mass

measurements in State programs that may be the result of inconsistencies

in the mass standards that are in the possession of the State laboratories.

The measurements reported here were designed to detect problems that

would be encountered if the State laboratories were given the opportunity

to perform measurements necessary to recalibrate State reference standards

for mass and to determine the techniques that should be developed to

perform this task most effectively.

METHOD

A. General

A weighing scheme has been devised using four one-kilogram

weights, some with special features, to examine the following aspects of

the participating State laboratories:

1. The accuracy of the State Kilogram Mass Standards

2. Laboratory techniques and environmental influences on

assigned values

3. Adequacy of the required instrumentation

4. Data reduction difficulties and degree of automation

Cleaning of test weights by State Laboratories was specifically excluded

from the measurement procedures.

In the measurement sequence, Division 523 cleaned the weights by

vapor degreasing and calibrated the circulating weights before and after

calibration by each State. This procedure enables early detection of a

mass change in any weight due to accident or mistreatment.
3

Fives states participated in the measurements

1. California

2 . Colorado

3 . Nevada

4 . New York

5. Utah.

Sandia laboratory also participated at the request of NBS in order to provide Division 523 with additional measurement confirmation in areas for

which the associated instrumentation is critical to success. Unlike

most State laboratories, the Sandia laboratory maintains a high level of

competence in pressure and temperature measurements that are important

in mass calibrations involving significant buoyancy corrections. [1 ]*

B. The measurement format

Each participating laboratory was asked to assign mass values

to the four one -kilogram test weights by comparison with its primary

kilogram standards using a specified format. The four weights, designated

Al, S2, R2, and H2 were paired as follows:

Pair 1 Al S2

Pair 2 R2 H2

Each pair was to be calibrated by performing all possible comparisons

with the two State kilogram standards. This procedure yields a total of

six intercomparisons per pair, shown in table I, which were to be accomplished by the method known as double substitution weighing. Air density

parameters were observed at the beginning and end of the sequence involving

pair 1 and during the third and sixth observations for pair 2. The

sequence of observations was modified for pair 2 to insure that air

density parameters were noted during the most sensitive comparisons
4

(see table I). State metrologists were not given special instructions

concerning the measurement of the air density parameters: temperature,

barometric pressure, or relative humidity.

THE ARTIFACTS

The NBS mass standards used by Division 523 (Bl, D2) for the NBS

measurements were manufactured by the Troenmer Co. in the late 1960's.

These weights are made of highly polished stainless steel with height

nearly equal to diameter. Each weight is constructed as one-piece

with an integral lifting knob machined into the object. Each State onekilogram standard is also made from one piece of stainless steel with

height nearly equal to diameter. However, lifting is provided by a

pinched waist, i.e., "hour glass shape". The State standards were

manufactured by several firms from an alloy of nominal density 8.0 g/cm

furnished by NBS.

The package sent to each State contained four one-kilogram weights.

The test weight designated A1 is identical to the NBS standards described

above (Bl, D2) . Weights S2, R2, and H2 were specially manufactured by

Division 523 from stainless steel and were designed with highly unusual

features to detect various weighing errors. S2 is a weight made from

three concentric tubes supported by a tee-shaped base for simultaneous

lifting of the individual pieces. This weight has an extremely large

surface area. Weight R2 is a thick-walled tubular weight with a surface

area which approximates that of H2. H2, although made of stainless

steel, is a hermetically sealed hollow weight of average density 2.9 g/crn^

and, consequently, it has a much larger volume than R2. Table II lists

the important artifact characteristics.
5

DATA REDUCTION

The instructions supplied to each State contained the information

required to perform a complete data reduction and assignment of the

mass values to the four one-kilogram weights. These instructions

included

1. Least-squares matrix coefficients

2. Standard deviation calculation for the least -squares fit

3. Volumes and thermal expansion coefficients of the weights

4. Fractional weight calibration report for the weight set

supplied in the package

5. Air density formula.

Each State was also asked to return the raw data to NBS with

sufficient information to permit the use of the NBS 1182 computer and

the mass code[2] for data reduction.

RESULTS

Each laboratory completed the measurements and reduced the data

according to the instructions provided by NBS. The results varied in

detail for many reasons including round-off errors attributed to hand

calculators and the omission of thermal expansion corrections. Two

laboratories made mathematical errors in the data reduction and were

given the opportunity to correct them. The results of the State calculation, although not used, are given in table IV. These data indicate

that most states had problems calculating the mass of H2 as compared

to the NBS calculated values.

Table III presents the NBS reduced values assigned to the artifacts, the series standard deviations, and corresponding F ratios, check
6

standard agreement, and T values. For the moment, consider the check

standard values. Each series was solved based on the value of a single

kilogram standard and not the sum of the two standards as is the usual

method. The value assigned to the second kilogram standard, referred to

here as the "check standard," is compared to its "accepted value" and

any difference is given in the appropriate columns of table III. Based

on the statistical "T" test, all values except one are within the measurement error limits. Of course, if the Standard and check standard change

1 mass an equal amount, the difference would remain constant and incorrect

mass values would evade the check.

All States performed the weighings on balances of the same make

and model; Sandia and NBS have more modem models that in principle and

performance, however, do not differ significantly from those belonging

to the State laboratories. Standard deviations (Series S.D. in table III)

for these instruments are given for each weight pairing. Experience

with balances of this type indicates that a 60 yg standard deviation is

quite normal and better than the manufacturer's specifications. The

calibration data are in agreement with this experience and indicate that

all balances functioned normally during these measurements.

NBS assigned seven values to each weight before shipment and upon

return from each laboratory. Prior to each calibration, all weights

were vapor degreased and allowed several days to equilibrate. At least

24 hours before calibration the weights and standards were stored in an

annex to the balance case to allow for further temperature equilibration.

In table III the mean values for each of the weights are given with the

corresponding standard deviations. The standard deviation values are

typical of the methods and instrumentation used in these measurements.
7

It is useful to look at the mass values assigned by each participating laboratory by examining the differences between these values and

the NBS mean value, as shown in table V. The reader should keep in mind

that weights Al, S2, and R2 have nearly the same volume as the standards

(see table I) and therefore most of the buoyant force is self-cancelling.

Weight Al closely resembles the standards in every way whereas S2 has a

much larger surface area and it should be relatively more sensitive to

surface related problems.

Using the range of values for Al as a normal reference, it may be

noted that the ranges of values for S2 and R2 are significantly larger;

that is most likely caused by the thermal sensitivity of the weight

geometry. The range of values assigned to weight H2 is much larger than

the ranges for the other three weights. Moreover , the mean values

assigned to H2 by the States show a large offset from the corresponding

NBS value whereas the mean values of the other weights agree well within

the uncertainty limits discussed in the next section. Although we could

expect thermal problems to cause weight H2 to behave similarly to S2 and

R2, additional errors in the buoyancy correction due to the lower

average density of H2 are dominant here. With the techniques used by

NBS and Sandia in this work the agreement between NBS and Sandia is

typical of the agreement we should expect for a weight such as H2[3].

In summary, it is evident that a somewhat smaller range in the

assigned values for weights Al, S2, and R2 could be economically achieved

by the States with improved thermal control. Improvements in the range

of assigned values for weights such as H2 will require significant

improvements in the capability of States to measure air temperature,

pressure, and relative humidity. These and other improvements are

discussed in the last section of this report.
8

UNCERTAINTY ESTIMATES

Before the beginning of this program, the NBS standards B1 and D2

used in this work were suspected to have lost mass. A recent comparison

with the platinum prototype kilograms does indicate a mass loss of 76

and 40 micrograms, respectively. It is also known that the two NBS

working kilograms originally used in the calibration of all the State

one-kilogram standards have changed. However, there are two partially

compensating effects: mass is increasing from the lack of cleaning and,

at the same time, there is a mass loss from abrasion that occurs with

use. Therefore at this time it is premature to place realistic uncertainties on the accuracy of the measurements reported here. However it

is possible to estimate a limit for the offset between the State kilograms

and the NBS standards used in this work.

Recent recalibration of the NBS working kilograms that were initially

used to calibrate the State kilograms (not B1 and D2) indicates a mass

loss of 100 micrograms from the accepted value. In the early 1970’s

other measurements indicated that even larger departures from the accepted

values could have been possible when the working standards were relatively

clean. Therefore, there might be an offset between the State assigned

weight value for weight A1 and the value assigned by NBS. At most, due

to this cause, the State values should not be in disagreement with

values assigned to A1 by NBS by more than 125 micrograms. Normally the

uncertainty of weights such as Al, as assigned from a "four ones series"

(see table I) where the restraint is a pair of standard kilograms is,

in general

Uncertainty = 3(0.6124 x S.D.) + S.E.
9

where S.E. is the systematic error for the standards and S.D. is the

standard deviation of the least squares adjustment. For an ordinary

one-kilogram NBS calibration this amounts to about 100 yg but it can

vary with the balance used to make the measurement. For most of the

participating laboratories an estimate based on the NBS calibration

certificate for a State standard would be

3(0.6124 x 60) + 100 = 210 yg .

FUTURE DIRECTION

There are two areas that must be improved to advance this project.

The first priority is to reduce the work load on the staff of Division

523 and the State metrologists and the second priority is to recalibrate

the State one-kilogram standards with improved techniques. The first

priority will be met by reducing the number of measurements made at NBS

and by shipping a portable microcomputer with data reduction software

as a part of the weight package so that each participant can reduce the

data using validated methods in his own laboratory.

Other improvements that will be made in future measurements include

a prescribed procedure for thermal conditioning of the weights, the

addition of calibrated air density instrumentation in the weight package

and the addition of two one-kilogram standard masses that will enable

the States to recalibrate their one-kilogram standards.

ACKNOWLEDGMENTS

The author is indebted to A. Tholen and Henry Opperman of Division

120 for their support and interest in this project.

Table I

The sequence of observations known as a "four ones series" [4] for both

pairs of artifacts calibrated by the State participants in terms of

their standards.

OBSERVATION OBSERVED KILOGRAMS PAIR 1

ORDER DIFFERENCE STATE 1 STATE 2 A2 S2

1* 1 + -

2 2 + •*

3 3 + -

4 4 + -

5 5 +

, B

6* 6 + -

KILOGRAMS PAIR 2

STATE 1 STATE 2 R2 H2

1 1 + -

2 2 + -

4 3 + -

3* 4 + -

5 5 + -

6* 6 +

*Air density parameters recorded.
Table II

Artifact Characteristics

KILOGRAM

DESIGNATION

VOLUME (cm

3

) @ 20°C

ASSUMED THERMAL

COEFFICIENT

CUBIC (ppm)

APPROXIMATE ~ SURFACE AREA (cnT)

B1 127.3852

D2 127.6250

STATE 1 125*

STATE 2 125*

A1 127.384

S2 126.545

R2 126.392

H2 337.666

45 145

45 145

45 145

45 145

45 145

45 660

45 270

45 270

^Assumed based on nominal mass and a density of 8.0 g/an at 20 °C.

Table IV

State participants data reduction.

Kilograms

A1

1000 g

New York .009302

California .009303

Nevada .009107

Colorado .009076

Utah .009244

Assigned Mass Values

S2 R2 H2

999 g 999 g 1000 g

.988628 .994656 .005920

.988787 .994744 .004497

.988485 .994433 .004384

.988551 .994543 .003347

.988679 .994780 .004115
Table V

NBS values minus participant values

LABORATORIES IN

MEASUREMENT

ORDER A1

New York -163 yg

California -169

Nevada 62

Sandia 30

Colorado 64

Utah -107

X -47

RANGE 233

(as calculated by Division 523)

KILOGRAM DESIGNATION

S2 R2 H2

-72 yg -152 yg -2100 yg

-236 -239 -664

77 32 -763

-186 67 -263

-49 -77 -762

-136 285 64

-100 -14 -748

313 524 2164
REFERENCES:

1.

F. E. J. Jones, Res. Nat. Bur. Stand. (U.S.) 83^ 419 (1978).

2.

R. N. Varner and R. C. Raybold, "National Bureau of Standards Mass

Calibration Computer Software," NBS Technical Note 1127 (July 1980).

3. R. M. Schoonover, R. S. Davis, R. G. Driver and V. E. Bower,

J. Res. NBS 85_, 27 (Jan-Feb 1980).

4. J. M. Cameron, et al., "Designs for the Calibration of Standards

of Mass," NBS Technical Note 952 (June 1979).
NBS-114A (REV. 2 .BO

)

U.3. DEPT. OF COMM. 2. Performlnf Organ. Report NoJ i. Publication Date BIBLIOGRAPHIC DATA

SHEET (See instructions)

1. PUBLICATION OR

REPORT NO.

4. TITLE AND SUBTITLE

Advanced Mass Calibration in State Laboratories

5. AUTHOR(S)

R. M. Schoonover

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)

10. SUPPLEMENTARY NOTES

[2U 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)

This report describes the calibration of four one -kilogram objects byfive State laboratories. The project is of mutual interest to the Length

and Mass Measurements and Standards Division (523) and to the Office of Weights

and Measures (120). The work is ongoing at the present time, however, the

initial phase is completed and reported here. Also discussed is the proposed

direction of future work.

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

Air density; combinational weighing; mass; uncertainty; weighing.

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