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NBS TECHNICAL NOTE 1 035

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

s^yy
NATIONAL BUREAU OF STANDARDS

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

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'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.
A Portable Calibration Densimeter

"-'^

AW 15 1981

for Use in Cryogenic Liquids J^J

"^

.tiCTSS

no. b3&^

J.D. Siegwarth

J.F. LaBrecque

K\8S -{ecK,.*u^c6^ nof^

Thermophysical Properties Division

National Engineering Laboratory

National Bureau of Standards

Boulder, Colorado 80303

^if-*" "' -« .* w^_ _ \t. o

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

NATIONAL BUREAU OF STANDARDS, Ernest Ambler, Director

Issued March 1981
NATIONAL BUREAU OF STANDARDS TECHNICAL NOTE 1035

Nat. Bur. Stand. (U.S.), Tech. Note 1035, 20 pages (March 1981)

CODEN: NBTNAE ^1

U.S. GOVERNMENT PRINTING OFFICE

WASHINGTON; 1981 For sale by the Superintendent of Documents, U.S. Government Printing Office, Washington, D.C. 20402

Price $1,50 (Add 25 percent additional for other than U.S. mailing)

,
CONTENTS

Page

1. INTRODUCTION 1

2. THE PORTABLE REFERENCE DENSIMETER 2

3. DENSITY MEASUREMENT AND SYSTEMATIC UNCERTAINTIES 4

4. CALCULATIONS OF THE RANDOM UNCERTAINTY FOR THE PORTABLE

DENSIMETER 8

5. CONCLUSIONS 12

6. REFERENCES 13

m

A PORTABLE CALIBRATION DENSIMETER FOR USE IN CRYOGENIC LIQUIDS

J. D Siegwarth and J. F, LaBrecque

Thermophysical Properties Division

National Engineering Laboratory

National Bureau of Standards

Boulder, Colorado 80303

A portable densimeter designed specifically for calibrating

liquefied natural gas densimeters but suitable for density measurement

of a wide range of liquids, is described. The densimeter has been

compared to the Density Reference System densimeter at the National

Bureau of Standards and found to agree to well within the combined

systematic error. The density results of this instrument and that of

the DRS densimeter are statistically indistinguishable.

Key words: Densimeter; density reference system; liquid methane; LNG.

1. INTRODUCTION

The cryogenic liquids Density Reference System [1] (DRS) at the National

Bureau of Standards in Boulder, Colorado, originated as a device for testing

calibration and performance of commercially built densimeters for liquefied

natural gas (LNG) service. A number of commercial densimeters were tested with

this system and the results reported [2]. All the densimeters tested functioned

adequately. These tests, however, disclosed a need for better calibration

methods for the commercial densimeters.

We implemented a transfer standard program at NBS as a method of providing

a suitable calibration for conmercially built LNG densimeters. In this program,

a manufacturer or user of densimeters selects a commercial densimeter against

I which he will calibrate his densimeters by comparison in a homogeneous fluid.

'We periodically calibrate his selected densimeter in the DRS and this becomes

I the transfer standard.

B We rebuilt the density reference system into a flexible configuration so

! densimeters of various designs could be more easily introduced and more rapidly

calibrated [3]. The new DRS was designed also with the intention of demonstratj ing a type of system that could safely handle combustible gases in an industrial

calibration laboratory since for best results, a densimeter should be calibrated

in a fluid similar to that in which it will be used.

I
After the DRS was rebuilt, the availability of an electronic balance capable

of weighing 200 g to 0.001 g resulted in our rebuilding the DRS densimeter to

incorporate this balance. We found this new balance both compact and sufficiently robust to suggest the possibility of constructing a portable silicon

densimeter. This portable densimeter could then be installed in any commercial

calibration facility to calibrate the reference densimeter of the facility. This

method has the definite advantage that it permits us to test a whole facility

rather than just the calibration densimeter used in the facility. The transporting of this portable densimeter should not add any additional uncertainty to the

instrument since the reference weight makes it self-calibrating.

We have built such a portable densimeter and tested it in the DRS. We

describe this densimeter and these comparison measurements in this report. From

these results, we have generated an accuracy statement for this densimeter.

2. THE PORTABLE REFERENCE DENSIMETER

The Portable Reference Densimeter (PRD) shown in fig. 1 is very similar to

the DRS densimeter described in ref. 3. The PRD is an Archimedes type densimeter

consisting of a silicon single crystal float of accurately known weight that is

weighed while immersed in the liquid whose density is desired. We weigh the

162 g silicon single crystal with a conmercially built electronic balance capable

of weighing 220 g to 1 mg. The sensing unit has been removed from the instrument

case and installed in a 178 nin diameter closed aluminum case designed to

withstand an internal pressure in excess of 1 MPa.

A simplified drawing of this instrument is shown in fig. 1. "A" is the

silicon single crystal shown lifted from the suspension cage "B" and clamped

between supports "C" and "D". The bottom supports "D" are raised when the 15 mm

ID thin walled stainless steel tube "E" is lifted by a pneumatic cylinder

actuating the bell crank mechanism "F". The upper supports are fixed to the

25.4 mm OD by 0.6 m long thin wall stainless steel tube "G". This length was dictated by the distance from the top plate to the sample liquid in the DRS where

it was tested and may be changed. The cage "B" is suspended from the electronic

balance sensing unit "H" by the 1.6 mm diameter tube "I". The balance "H" is

mounted on a cant i levered plate "J" so that should gas tight balance case "K",

capable of holding 10 bar interval pressure, flex when pressurized the leveling

of the balance is unaffected. This permits a lighter construction for the

balance case than used for the DRS densimeter. The tungsten reference weight
K\\\\\\\V\\\\\\\\\\\\\\^^

/

/

/

/

/

^\\\\\\^

S^

£

V

N

g^

A

'i\^\\\ \'\ ^^ A

B

Figure 1. Portable Reference Densimeter

3
'el 1I

"L", used to monitor the balance calibration, is shown placed on its suspension

cage "M". This cage replaces a section of the suspension tube "I". Some swiv

joints in this suspension permits flexing but not rotation. A pneumatic-cylinder

driven bell crank, similar to "F" but not shown in the drawing raises "N" which

lifts the reference weight "L" off the suspension and clamps it against the fixe

top support "0". The "0" ring seal at "P" connects this densimeter into an LNG

sample container. This joint is a gland seal rather than a flange seal so the

instrument can be tilted according to the bull's eye level "Q" to align the

suspension "F" to tube "E". A gas tight seal "R" is provided for the electrical leads "S". The case "K" is clamped together with sufficient bolts through

the flanges to withstand a 10 bar internal pressure.

The apparent mass of the reference weight is approximately the same as that of the silicon float when the latter is suspended in liquid methane. We

compared this apparent mass value, corrected to a true mass value by using the

temperature and pressure of the sample gas to determine the buoyant effect, to

the true mass values of the tungsten as determined by precision weighing. This

procedure provides a continual monitoring of the balance calibration. Tungsten

was chosen for the reference weight because its high density reduces the

magnitude of the buoyancy correction hence any effect of uncertainty in the gas

density in the region of the reference is minimized.

3. DENSITY MEASUREMENT AND SYSTEMATIC UNCERTAINTIES

The density of the liquid in which the float is immersed is given by the

relation [3]:

p = p^d - (M^ - i^jm^) (1)

where Mg (=162.2502 + 0.0005 g, 99% confidence interval) is the true mass of

the silicon single crystal, M^ is the apparent mass value with the silicon on the suspension and immersed in the liquid, Mqq is the weight of the

suspension in the liquid (this reading is zero jf 0.001 g as a general rule

because the balance is zeroed in this configuration) and pg is the density of

the single crystal silicon at the liquid temperature and pressure. The silicon

single crystal density at 110 K from ref . 2 is assumed to be 2.3308 + 0.00007

g/cm^. This includes both the estimated experimental uncertainty in the

density and the thermal expansivity.
The true mass values of the silicon crystals and the reference weights for

the DRS and the PRD are given in table 1 along with an estimate of total

uncertainty for each value. Total uncertainty is defined as t/- the sum of the

systematic error limit plus the 99% confidence limit for the average of the n weighings. Each true mass value is an average of n weighings made on separate

days. The systematic error in each case is the systematic uncertainty specified

for the set of class S weights used.

The estimated errors for the various measured quantities are shown in

table 2 which is similar to the same table in ref. 3. We have not included the error for temperature gradient and drift of a liquid sample [3] because we are now examining the systematic error of the densimeter only, not the densimeter

plus sample container. As before [3], the total systematic error shown in

table 2 is assumed to be the square root of the sum of the squares of the

individual contributions.

If the densimeter is used in liquids at temperatures well removed from 110 K

an expression for ps(T) is related to the O^C density by

PgCT) = PgCO^Od - 3 e(T) + negligible terms) (2).

where

Pg(0°C) = 2.32940 + 0.00007 g/cm'^

fnd the thermal expansivity

3(T) = - (L(273.2) - L(T))/L(273.2) (3)

(here L is a crystal dimension. Gibbons [4] gives values of 3(T) for T = 40 to

loo K at intervals of 10 K. He reports the probable error for expansivity to be

less than 0.5% at each point. Gibbons does not define probable error, but in one definition [6] it is 0.6475 times the standard deviation. This suggests that his

Estimated standard deviation for expansivity is less than 0.8% for each value.

le also does not mention if inherent systematic error in his experimental

ipparatus was considered. Differences between his values and those of other

experimenters are noted in the paper.

As a possible aid to interpolation in Gibbon's table for the expansivity of

silicon the following fourth degree polynemial was fit to his values of 3(T) for

T = 70 to 300 K:
TABLE 1. Weight Values

*99% Systematic

Avg of n Conf Limits Error from Total % Total

Measure- for Class S Uncer- Uncerments

127.4351g

n

6

Average Weights

+ .00020g

tainty

+ .00042g

tainty

DRS + .00022g + .0003

Sil icon

Crystal 1

DRS 102.9022g 1 + .00055g + .00017g + .00072g + .0007

Reference J

Weight

(brass)

1

PRD 162.2502g 5 + .00025g + .00024g + .00049g + .0003

Silicon

Crystal

PRD 132.2457g 1 + .00055g + .OOOlSg + .00073g + .0006

Reference /

Weight

(tungsten)

*From t /-^/ir where a is estimated to be O.OOOUg based on the pooled standard

deviation of silicon weighings (9 degrees of freedom). The Student-t statistic

for a 99% confidence interval in this case is 3.25.
Table 2. Systematic Errors

Variable, x 3p/3X Max Magnitude

0.012 cm"^

fix 3p/p(%)*

Silicon

crystal mass

(Pg - p)/\ 5 X lO"'* g 0.0015

Apparent masses

silicon

-P3/M3 0.015 cm"^ 0.001 9 0.0038

Suspension P3/M3 0.015 cm"^ 0.001 g 0.0038

Silicon density p/pg 0.17 7 X 10"^ g/cm"^ 0.0030

TOTAL 0.006

*for p = 400 kg/m^
(4)

B(T) = (-0.11620 - 7.72448 x lO'^T - 5.40814 x 10"S^

+ 6.2707 X lO'^T^ - 9.83286 x lO'^^T^) x 10"^

The residual standard deviation from this fit is 9.6 x 10' . The target

absolute residual of the polynomial values from those of Gibbons is 2.5 x 10'

which occurs for T = 240 K. This maybe an aberrant point, the next largest in

absolute value is -1.7 x 10' for T = 90 K. A plot of these residuals show a non-random pattern, so another method of interpolation may improve the fit. The

estimated standard deviations for the values of 6(T) predicted by the above

polynomial range from 7.8 x 10' at 70 K and 300 K to 3.4 x 10'^ at 150 K. If

we take three times the largest of these, 2.3 x 10' , as our estimated random

uncertainty for b(T) then the systematic error eq (4) introduces into the density

determination is approximately 1 3p/33lj_QOpX(2.3 x 10' ), where

|3p/36L^Qop is, differentiating eq (2), 3 x 2.3294 = 7.0 g/cm . Thus, if

eq (4) is used fo the e(T) calculation, another 1.6 x 10' g/cm (.007%)

would be added in quadrature to the total in table 2. Even if this error were a factor of 10 larger, it would contribute little to the total systematic error.

4. CALCULATIONS OF THE RANDOM UNCERTAINTY FOR THE PORTABLE DENSIMETER

The PRD has been tested in the Density Reference System. The accuracy and

precision of the DRS densimeter and the portable densimeter are essentially

independent of liquid composition. Their accuracy and precision depend primarily

upon the accuracy and precision of the balance and how well the weight and

density of the silicon crystals are known. The ability to make an accurate

comparison will, of course, depend upon conditions within the sample holder. The

liquid needs to be relatively quiet, well mixed and of a fairly uniform

temperature in the volume in which the measurements are being made.

The liquids chosen for comparing the two densimeters were saturated liquid

methane and a saturated LNG like mixture. The DRS densimeter has been tested in

those liquids [3]. The comparison of the two densimeters is based on data taken

from two fillings of the liquid sample holder, for a total of 35 density values.

We first filled the DRS sample holder with liquid methane, took a series of

measurements in the range of 110 to 125 K, then added nitrogen (about 1% to 2%)

and propane (about 4% to 6%) to simulate LNG and took another series of

j measurements in the same temperature range.

8

I
The series of measurements for a liquid sample in the DRS were made by

either heating or cooling the saturated liquid to a target temperature, waiting

for the liquid to come to equilibrium, and taking the desired schedule of

readings. The sample was stirred and the readings were repeated giving a duplicate set of readings for each temperature. It is reasonable to assume that

the random error of the duplicate density values described above would correlate

more closely than values obtained at another temperature.

Because of the large correlation between duplicate readings, only one of

each of the duplicates can be usefully employed in the analysis. We have

arbitrarily chosen to use the first of each duplicate in our analysis. The

second of each would provide essentially the same information. Using both

duplicates would complicate the analyses without providing any increase in

information.

The model used to analyze the data is:

Ad . . = D-r • • ~ Pr. • • - u + a- "•" e.- (5)

where py- • is the jth density determined by the portable densimeter on the ith

set of runs, pr, . • is the corresponding density value determined by the DRS

densimeter, y represents any systematic difference between the two methods, a^

represents a shift in the mean for measurements on the ith set of runs, and e .

. represents any random contribution to the jth measurement on the ith set of runs. A set of runs are those measurements made with a particular mixture in the liquid

isample holder.

Figure 2 is a plot in chronological order of the Ap-.. There are four

isets of runs shown in this figure, two for methane and two for the LNG like

mixture. The symbol "M" is for a methane value and "L" is for LNG. In this plot

can be seen the extent of any shifts, a., in the mean of the Ap.. from one set

;of runs to the next. We know from our study of the DRS, ref . 3, that such shifts

ioccur and appear to be random. In the DRS study, only methane data was used to

'Study the variability of the a. because the comparison was between the DSR

^densimeter and the Haynes-Hiza [5] temperature relationship for saturated liquid

methane. Because the Haynes-Hiza relationship depends on pure methane, the

(O.may have partly been the function of contaminants in the methane. The

99% upper bound on the standard deviation of the Ap . . , taken from ref. 3, is

.036 kg/m-^. This is the bound we take to represent the variability from one

set of runs to the next for each of the two Archimedes densimeters that we are
>> E

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table

Referen

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in

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ference M

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C C 1—

OJ >i fO O) o +-> > z •1- _l .l- OJ to •4-> o

^ E <U O

4-> OJ J= 0) a:

Q H- Q.

I

OJ OJ a>

0) -E . S- o S 4-> E

o

Differences

bet

Densimeter

and

Reference

Syste

liquid

methane,

o CVJ

UJ/6>| 'V-Vsc/v

CT

10
3

comparing; so that the 99% upper bound on the Ap. . could be -^/T x .036 kg/ m

= .051 kg/m^. For the four sets of runs considered in this study, the standard

3 deviation of the o. for the means of the Ap . . is estimated to be 0.007 kg/m . So we do not see any conflict in assuming, for the time being, that the new portable densimeter will behave in a manner like the DRS densimeter with respect

to the variability in the a.. Based on the Ap . . shown in fig. 2, the e.. has an estimated standard

deviation of 0.0099 kg/m-^. An upper 99% bound on the standard deviation is

0.014 kg/m^. If we were to divide this equally between the two densimeters,

then the 99% upper bound for the standard deviation for variability within sets

of runs would be 0.01 kg/m^ for each densimeter. This value is less than

expected based on the results presented in ref. 3, and suggests that our method

of computing the density using the averaged sample temperature may have

contributed a significant amount of the variability in our study of the DRS.

Each balance is calibrated by weighing its reference weight in air and

adjusting the balance to read the correct apparent mass value in air, not the

true mass value. Each reference weight has approximately the same apparent mass value as the corresponding silicon crystal when the latter is submerged in liquid

methane.

As a test of the densimeter balance calibrations each reference weight is

jmeasured and recorded. At each sample temperature setting, the measured apparent

(mass value is corrected for gas buoyancy and compared to the true mass value.

iThis correction is readily made for methane or methane-nitrogen samples but not

!;for methane-propane-nitrogen samples because propane suppresses the methane vapor

^pressure by an uncertain amount and because the gas mixture is slow to reach

equilibrium. The method of calculating the buoyancy correction is detailed in

ref. 3. The reference weight of the PRO is located in ambient temperature gas.

This arrangement and the tungsten weight reduce the buoyancy correction

:thus reducing associated uncertainties. The true mass value of the reference

^weight for the DRS densimeter is 102.9022 g with a total uncertainty from table 1 of + 0.72 mg. Comparison of the reference weight readings for methane and LNG,

showed some differences which are attributed to the buoyancy corrections made for

the two liquids. A 99% confidence interval for the average of the reference

•weight true mass values measured in the methane atmosphere is 102.9024 g + .0004 g and that for the average measured in the LNG vapor is 102.9015 + .0004 g.

11
While a statistical difference is present between these two values, the

difference has no practical significance and both are consistent with the

accepted true mass value of the reference weight.

The measured true mass value of the reference weight for the portable

densimeter is 132.2457 g with a total uncertainty from table 1 of + 0.73 mg.

Comparison of the average true mass value of the reference weight measured in

both methane and LNG atmospheres shows no significant difference between the two

values. A 99% confidence interval for the combined average in methane and LNG

atmospheres is 132.2455 + .0003 g, which is consistent with the accepted true mass value for the reference weight.

The analyses of the reference weight true mass values taken during the

comparison of the two densimeters show the balance. of both densimeters to be both

stable and accurate.

5. CONCLUSIONS

From our experience with the improved DRS densimeter, we determined that a portable silicon densimeter of essentially the same design and similar accuracy

is feasible. We have built such a densimeter and compared its performance to the

DRS densimeter by installing it in the DRS. Its performance exceeds the

performance of the DRS mainly because of the larger silicon crystal and the

relocation and higher density of the reference weight.

The total uncertainty of a single measurement we estimate to be the systematic uncertainty added to three times the estimated within run and the between

run variability added in quadrature:

3(.036^ + .014^)^/^ = 0.116 kg/m"^ (6)

At a density of 422.63 kg/m-^ (methane normal boiling point) the 3<t uncertainty

is .027% and, adding the systematic uncertainty of 0.006%, the total uncertainty

of one measurement is 0.033%.

The comparison of the DRS and PRD densimeters has also shown the systematic

uncertaintly of 0.022% for the DRS is over estimated. The agreement between the

measured density values suggests that this sample temperature gradient used for

the systematic error estimate in ref. 3 is too large. The estimated gradient is

the dominant contribution to the systematic error estimate for the DRS [3]. The

gradients across the sample in the region of the silicon crystals, whose centers

are separated by 15 mm, must be much smaller than estimated in ref. 3.

12
6. REFERENCES

[1] Siegwarth, J. D., Younglove, B. A., and LaBrecque, J. F., Cryogenic Fluids

Density Reference System: Provisional Accuracy Statement, Nat. Bur. Stand.

(U.S.), Technical Note 698, (Nov 1977).

[2] Siegwarth, J. D., Younglove, B. A., and LaBrecque, J. F., An Evaluation of

Commercial Densimeters for Use in LNG, Nat. Bur. Stand. (U.S.), Technical

Note 697, (Oct 1977).

[3] Siegwarth, J. D., and LaBrecque, J. F., Cryogenic Fluids Density Reference

System: 1980, Nat. Bur. Stand. (U.S.), Technical Note, to be published.

[4] Gibbons, D. E., Thermal Expansion of Some Crystals with the Diamond

Structure, Phys. Rev 122, 136 (1958).

[5] Haynes, W. M., Hiza, M. J., Measurements of Orthobaric Liquid Densities of

Methane, Ethane, Propane Iso Butane and Normal Butane, J. Chem.

Thermodynamics 9^, 179, (1977).

[6] Kendall, M. G., and Buckland, W. R., A Dictionary of Statistical Terms, 3rd

ed., Hafner Publishing Co., New York, New York, 1971.

13
NBS-n4A (REV. 2-BC) U.S. DEPT. OF COMM.

BIBLIOGRAPHIC DATA

SHEET (See instructions)

1. PUBLICATION OR

REPORT NO.

NBS TN-1035

2. Performing Organ. Report No 3. Publication Date

March 1981

4. TITLE AND SUBTITLE

A Portable Calibration Densimeter for Use in Cryogenic Liquids

5. AUTHOR(S)

J. D. Siegwarth, J. F. LaBrecque

6. PERFORMING ORGANIZATION ('/fjo/nt or other thon NBS, see instructions)

NATIONAL BUREAU OF STANDARDS

DEPARTMENT OF COMMERCE

WASHINGTON, D.C. 20234

7. Contract/Grant No.

8. Type of Report & Period Cover^

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

Gas Research Institute

10 West 35th Street

Chicago, Illinois 60616

(Grant 5014-361-0131)

10. SUPPLEMENTARY NOTES

\2Z\ Document describes a computer program; SF-185, FlPS 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 portable densimeter designed specifically for calibrating liquefied natural gas

densimeters but suitable for density measurement of a wide range of liquids, is

described. The densimeter has been compared to the Density Reference System

densimeter at the National Bureau of Standards and found to agree to well within the

combined systematic error. The density results of this instrument and that of the

DRS densimeter are statistically indistinguishable.

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

Densimeter; density reference system; liquid methane-, LNG.

13. AVAILABILITY

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5n Order From Superintendent of Documents, U.S. Government Printing Office, Washington, D.C

20402.

Order From National Technical Information Service (NTIS), Springfield, VA. 22161

14. NO. OF

PRINTED PAGE

20

15. Price

$1.50
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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

'X)-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, I 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 a.s a supplement to the activities of the private sector standardizing

organizations. Consumer Information Series—Practical information, based on N BS 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 technological marketplace. Order the above NBS puhlicalions from: Superintendent of Documents, Government Printing Office-, Washington, DC 20402. Order the following NBS publications—Fl PS and NBSIR.i—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 II, 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.
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