NATIONAL BUREAU OF STANDARDS REPORT

4288

EPPB3T OF CHANGES OF ENGINE SPEED ON THE CAPACITY

OF THE 1/2 -TON THERMO KING REFRIGERATING UNIT, MODEL E-IO

;

fey

Minoru Fujil

C. W. Phillips

P, R, Achenbach

to

' Mechanical Engineering Division

Headquarters, Quartermaster Research & Development Command

Natick, Masso

U. S. DEPARTMENT OF COMMERCE

NATIONAL BUREAU OF STANDARDS
U. S. DEPARTMENT OF COMMERCE

Sinclair Weeks, Secretary

NATIONAL BUREAU OF STANDARDS

A. V. Astin, Director

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NATIONAL BUREAU OF STANDARDS REPORT

NBS PROJECT NBS REPORT

1003-20-11832 August 22, 1955 14288

EFFECT OP CHANGES IN ENGINE SPEED ON THE CAPACITY

OP THE 1/2 -TON THERMO KING REFRIGERATING UNIT, MODEL K-10

by

Minoru Fuji!

C. W. Phillips

Po R. Achenbach

Report

t6

' Mechanical Engineering Division

Headquarters, Quartermaster Research & Development Command

Natick, Mass.

U. S. DEPARTMENT OF COMMERCE

NATIONAL BUREAU OF STANDARDS

The publication, 1

unless permission 25,D,C. Suchp

cally prepared If

Approved for public release by the

Director of the National Institute of

Standards and Technology (NIST) on October 9, 2015.

or In part, Is prohibited

Standards, Washington

report has been speclfireport for its own use.

EFFECT OF CHANGES OF ENGINE SPEED ON THE CAPACITY

OF THE 1/2-TON THERMO KING REFRIGERATING UNIT, MODEL K-10

by

Minoru Fuji!, C, W, Phillips, and P, R. Achenbach

Abstract

A study was made of the effect of speed change on

the net refrigerating capacity of a plug-type refrigerating unit. Model K-10, manufactured by the

Uo So Thermo Control Company of Minneapolis, Minn.

The stock unit, of nominal l/2-ton capacity, is

belt-driven by a gasoline engine fixing the speed

ratios between engine and compressor and between

engine and the condenser and evaporator fans at

about I 03 I4 and 1.59, respectively. The results

showed that there was an optimum speed for the

unit ne’ar to the design speed of 2I4 OO rpm. A

maximum net refrigerating capacity was obtained

for engine speeds in the range from 2I4 OO rpm to

2700 rpm for all refrigerator temperatures below

35°F» There was no significant change in capacity

in this speed range. The maximum capacity values

were about I 7OOO Btu/hr for a refrigerator temperature of 35 °Fs 9300 Btu/lir for a refrigerator

temperature of 10°F, and 7^00 Btu/hr for a refrigerator temperature of 2°F in an ambient temperature

of 110°F. It was observed that the net refrigerating capacity increased almost linearly with refrigerator temperature. The gasoline consumption

rate increased with engine speed, and was higher

for any given speed at a refrigerator temperature

of 10 °F than for either higher or lower refrigerator temperatures. During approximately ^00 hours

of operation some adjustment of belt tension and

servicing of the distributor points on the gasoline

engine was required, but there were no component

failures, no breaks in the refrigerant lines, and

no failures in the gasoline engine.

1. INTRODUCTION

At the request of the Office of The Quartermaster General,

tests were made of a l/2-ton gasoline-engine driven refrigerating unit. Model K-10, of the plug type manufactured by the
, '

•. I ^

<

1
» 2 -

U„ So Thermo Control Company of Minneapolis, Minnesota* It

was tested in a 600 cubic foot, portable, walk-in refrigerator of the type usually equipped with this type refrigerating

unit in the field* The refrigerator was employed as a calorimeter for the purpose of these tests*

The purposes of this investigation can be divided into two

main parts as follov/s:

A. Measurement of the net refrigerating capacity of

the unit at design compressor speed for an arabient temperature

of 110°F and at refrigerator temperatures of 10°P and

the lowest temperature attainable without internal load in the

refrigerator

B« Investigation of the effect of changing the compressor speed on net refrigerating capacity of the unit when

the speed ratios between the compressor, and the evaporator

and condenser fans remained fixed at the design value*

The results observed with respect to the above two objectives are reported in Parts A and B of the section on Test

Results, respectively.

2* DESCRIPTION OF TEST SPECIMEN

The specimen refrigerating unit was identified as follows®

(NBS Test Specimen 100-53)

U*S, Thermo Control Co*, Minneapolis, Minn*

Model K-10

Serial Number GQ109QS

The unit was of the plug type and used Freon 12 as the refrigerant* The compressor was identified as Thermo-King,

Model 2R, Serial Number 2093* The refrigerating unit was

driven by a gasoline engine manufactured by D, W. Onan &

Sons, Inc., of Minneapolis, Minnesota with the following nameplate data?

Model CK-MS

Specification 190J

Serial Number lOl-IjliliS^S

Maximum Horsepower 10,1 at 3>000 R.P.M.

- 3

“

Three photographs of the specimen refrigerating unit

are attached as Figures 1-3 inclusive. Figure 1 and Figure

2 show the exterior views of the front (condenser section)

and rear (evaporator section) respectively. Figure 3 is a side view showing the relative locations of the engine, compressor, condenser and evaporator section with the top perforated panels removed and side doors opened.

The condenser fan was of the propeller type made of

cast aluminum. It has 12 blades, 6 l/l4 "long, and the diameter was 20 inches. Ambient air was drawn through the

condenser, forced around the engine and compressor and discharged from the top and sides of the condensing unit section.

The evaporator fan was also of the propeller type made

of cast aluminum. It had 12 blades, 5" long, and the diameter was 20 inches. The air was drawn from the top and sides

of the evaporator section and forced through the evaporator.

The air leaving the evaporator was discharged into the refrigerator through a duct of about one third the cross-section area

of the face of the evaporator. The top and side panels of the

discharge duct projected straight out from the edges of the

evaporator whereas the bottom panel projected upward, thereby

reducing the cross-sectional area to approximately one-third

of the evaporator face as shown in Fig. 2 and A defrosting

damper pivoted on a horizontal axis was located in the discharge opening.

The condenser and evaporator fans were mounted on opposite ends of a common shaft, Ihe pulleys of the engine, compressor and fans were connected in that order in a clockwise

direction, as viewed from the condenser end of the unit, by two

Gates Vulco Ropes. These ropes, or belts, connected the pulleys in such a way that the direction of rotation of the fan

pulley was opposite to that of the engine and compressor pulleys. The outside diameters of the engine, compressor, and

fan pulleys were 6 I/I4 ", 8” and 9 l/2", respectively.

The physical dimensions of the refrigerating unit were

as follows

(1) Gross weight, without batteries, lb., 703

(Gross weight Indicated on nameplate 720 lbs)

(2) Overall dimension, inches Width Height Depth

without batteries, including door handles and

lifting hooks hk 33 1/2 57 1/2
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'.i/r
Width He i ght Depth

(3) Condensing unit section

inches 44 33 1/2 24 l/2

( 4 ) Evaporator section, in.

including plug section

but not the discharge

duct 36 3/16 27 lA 20 1/8

{^) Compressor

a) Model - 2 R

b) Serial Number - 2093

c

)

Type - 2 cylinder vertical

d) Bore, inches - 2 i/k

e) Stroke, Inches - 1 3A

f) Speed, rpm.

design - 1875

f) Displacement

,

cu. in. - 13.91

(6) Condensers

aj Width, in. overall 31 3 A

b) Height, in. overall 27 1/2

c ) Depth, in. overall 14 1/2

d) Fin size, in. 27 1/2 X 3

e) Fin spacing 6 per inch

f) Tube size, in. 1/2 O.D.

g) Nuin be r of tubes 66

h) Series or parallel 22 tubes in series

per circuit, 3

circuits in parallei

i) Primary surface area , sq . ft. 20.5

.1 ) Secondary 'Surface area, sq.ft. 218.3

k) Total surface area. sq, ft. 238.8

1) Ratio primary to total surface, % 8 e 6

m) Tube material Copper

n) Fin material Aluminum

Engi]ne

a) Model number CK-MS

b) Specification I9OJ

c

)

Serial number ioi-l4 lti)545

d) Type 4 cycle, 2 cylinder opposed

horl zontal. air-cooled

- 5

-

Engine (Cont'd)

e) Material Aluminum

f) Speed, design, rpm 2400

g) Rated maximum brake

horsepower at

3000 rpm rH 0 «

1

1

h) Lubrication Force feed

i) Fuel system Pump feed to carburetor

j) Ignition system Special 12-volt motorgenerator built into flywheel of engine provided

automatic starting, battery charging and ignition.

k) Cooling system Fan on armature of engine

generator forced air over

engine block and finned

cylinder heads as guided by

engine shrouding.

1) Governor Pierce Governor Company

GC 1408 9 1

Evaporator

a) Width, in. finned 31 1/8

b) Height, in. finned 22 5/8

c

)

Depth, in. finned 6

d) Fin size, in. 6x22 1/2

e) Fin spacing 5 per* inch

f) Tube size, in. 1/2 O.D.

g) Number of tubes 108

h) Series or parellel l8 tubes in series per circuit, 6 circuits in parallel

i) Primary surface

area, sq. ft. 36,6

j) Secondary surface

k)

area, sq. ft. 245.0

Total surface

area, sq. ft. 281.6

1) Ratio primary to

total surface, % I3o0

m) Tube material Copper

n) Fin ” Aluminum

3, TEST PROCEDURE

The specimen refrigerating unit was installed in a 600

cUo ft. portable walk-in refrigerator for the capacity tests.

The refrigerator was located in a test room where the ambient
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- 6 - temperature could be controlledo Refrigerator temperature was

maintained at the desired level by an electronic rheostat controlling the input to electric heaters which were used for

the internal load, The heat output of these heaters was measured by means of watthour meters in the supply circuits. Temperatures were measured by means of calibrated thermocouples

using an electronic, constant-balance type potentiometer,

.

'1 The engine speeds were varied by adjusting the governor.

The engine, compressor and fans were connected with a pair of

Gates Vulco Ropes throughout the tests. Therefore, the compressor and fan speeds were proportional to the engine speed

as determined by the constant pulley ratio.

After steady state test conditions were reached, they

were maintained for a period of at least five hours while

observations of pressures, temperatures, speeds, voltage,

current and wattage were made at 30 -mlnute intervals,

4, TEST RESULTS

Part A

In preparation for the capacity determinations, a test

was made to determine the heat transmission factor of the

600 cubic foot, portable, walk-in refrigerator used as a calorimeter. During this test the refrigerator temperature

averaged the ambient temperature averaged 129o9°F

for five hours during steady state conditions. The heat

transmission factor was computed to be 65o03 Btu/hr per °P

temperature difference between inside and outside.

The net refrigerating capacity of the Model K-10 Thermo

King unit at design engine and compressor speeds of 2I4 OO RPM

and 1870 RPM respectively werej 7,370 Btu/hr, 9,230 Btu/hr

and 16,620 Btu/hr at refrigerator temperature of 1,8°F,

10o9°F and 35oij°F respectively. The net refrigerating capacity varied approximately in linear relation to the refrigerator

temperature at these engine and compressor speeds. This is

shown graphically by one of the curves in Fig. refrigerating capacity increased 290 Btu/hr per degree rise in

refrigerator temperature, on the average.

Part B

Results of the capacity tests for a range of engine

speed from I 8OO rpm to 3 OOO rpm at an ambient temperature of

- 7

-

110°F and refrigerator temperatures of 10°P, and lowest

attainable without Internal load are summarized in Table 1

and shown graphically in Fig. 5-

The engine speed bore an average ratio of 1.3U to 1

relative to the compressor speed and 1»59 to 1 relative

to the fan speed during these tests. The pulley ratios

(outside diameters) were 1.28 and 1.52 respectively indi°

eating that very little belt slippage was occurring during

most of the tests. There was a tendency toward more belt

slippage at an engine speed of 3 OOO rpm than at lower speeds

as indicated by tests 5 a-^d 15 in Table 1. In particular in

test 15 the observed ratio of engine to compressor speed was

1.51 a.s compared to an average of 1.33 'the remainder of

the tests.

Figure 5 shows that the net refrigerating capacity reached

a maximum value in the range of engine speed between 2 i400 rpm

and 2700 rpm and that it was less for speeds below 2ij00 rpm

and above 2700 rpm* This is characteristic of a unit for which

the fan speeds increase proportionally to the engine speed by

virtue of a fixed pulley ratio. It would be expected that the

total refrigerating capacity of the compressor would increase

proportionally with engine speed if the volumetric efficiency

remained constant. Howeverj the power to drive the evaporator

fan would increase as the cube of the engine speed. Thus at

some engine speed the Increment of Increased compressor capacity caused by an increase in engine speed would be exactly

offset by the Incremental Increase in evaporator fan power.

This would be the point of maximum net refrigerating capacity.

At higher speeds the rapid Increase in evaporator fan power

would cause a decrease in net refrigerating capacity.

Assuming that the volumetric efficiency of the compressor

remained constant for the speed range under consideration and

that the evaporator fan power increased as the cube of the

fan speed, the characteristic curve for the net refrigerating

capacity can be expressed by an equation of the form^

R = AS - BS® where

R is the net refrigerating capacity in Btu/hr

S is the engine speed in rpm

A and B are constants which can be evaluated using

observed values of R and S for two test conditions.

- 8 » Figure 5 shows that the net refrigerating capacity

reached a maximum value of about 17,000 Btu/hr at an engine

speed of about 2600 rpm for a refrigerator temperature of

35°F„ The corresponding maximum net refrigerating capacities

for refrigerator temperatures of 10°F and 2°F were about 9300

Btu/hr and ?i|00 Btu/hr respectively. Figure 5 shows that the

net refrigerating capacity was affected less by speed change

at the lower refrigerator temperatures than for a refrigerator

temperature of 35°F. The refrigerating unit was unable to cool

the refrigerator to 0°F in an ambient temperature of 110°F at

any engine speed because of the large heat leakage of the 600

cubic foot refrigerator.

The net refrigerating capacity observed in Test 1 is believed to be too lowo The low suction pressure and low temperatures observed on the evaporator indicate excessive frost

on the evaporatoro If the volumetric efficiency in Test 1

were as high as for Tests 2 and 3 the net refrigerating effect

would have been about 12000 Btu/hr for Test lo This is a safe

assumption since volimnetric efficiency usually Increases rather

than decreases as the compressor speed is decreasedo The curve

for a refrigerator temperature of 35°F Figure 5 is drawn for

an estimated capacity of 12000 Btu/hr for Test 1 , although the

observed capacity is also plotted for this testo

Figure I4 shows that the net refrigerating capacity of the

unit varied approximately in linear relation to the refrigerator

temperature at any given engine speed in the range from loOO

rpm to 3000 rpmo This figure also shows the capacity to be

approximately equal at engines speeds of 21400 rpm and 2700 rpm

and to be slightly lower at an engine speed of 3 OOO rpm. In

figure I4 the curve for a speed of I 8OO rpm is drawn through

a computed capacity value of about 12000 Btu/hr rather than

the observed value of 7770 Btu/hr corresponding to a refrigerator temperature of 35°F because the data for Test 1 reveal

that the evaporator probably had excessive frost on it when

this test was made.

Table 1 shows that the temperature difference between

inlet and outlet air for the condenser remained reasonably

steady as the engine speed was increased. This indicates

that the total heat rejection of the condenser increased

proportionally with engine speed since the air delivery of a

fan increases proportionally with speed. On the other hand,

the temperature difference between inlet and outlet air for

the evaporator decreased somewhat as the engine speed was
|:

I
~ 9

-

Increased indicating that the heat transfer to the evaporator

increased less than proportionally as the engine speed was Increased.

There was a gradual decrease in suction pressure as the

engine speed was increased for a given value of refrigerator

temperature. This also Indicates that the total heat transfer to the evaporator was increasing as the engine speed was

increased even though the net refrigerating capacity may not

have increased. Table 1 also shows that the compression ratio

increased slowly as the speed was Increased primarily because

the suction pressure decreased.

Figure 6 shows how the gasoline consumption rate was affected by engine speed and refrigerator temperature. At any

given refrigerator temperature the fuel consumption rate In^

creased with the speed although not along a straight line.

Fuel consumption rate could be affected by belt tension,

carburetor adjustment, and perhaps other factors that would

prevent obtaining a smooth curve for this relationship. It

will be noted in Figure 6 that the highest fuel consumption

rate was observed consistently for a refrigerator temperature

of 10°F and the lowest rate was observed with the lowest attainable refrigerator temperature.

The superheat of the refrigerant gas at the bulb of the

thermostatic expansion value as compared to saturated evaporator temperatures near the coil inlet ranged from seven to

llj degrees F for all tests except tests 1 and 2, for which the

superheat was 0.5 and i\ok degrees F, respectively. Neglecting

these two exceptions, the average superheat was 12.8 degrees

F for a 35°F refrigerator temperature, 10.9 degrees F for a

10°F refrigerator temperature, and 9o8 degrees F for the lowest temperature attainable in the refrigerator.

Experience with the maintenance of the Model K-10 unit

during the tests indicated that the distributor points and

condenser were too inaccessible considering the amount of

servicing required. It x-jas also observed that the Pierce

governor was sluggish and did not provide very precise speed

control for the unit with changes in load. Hunting of the

engine speed was also observed during some of the tests.

In the course of testing this unit, it was operated

approximately 500 hours. During this period there were no

component failxores, no breaks in the refrigerant lines, and

no failures in the gasoline engine. Adjustment of belt tension

and servicing of the distributor points was required at intervals .

- 10

5« DISCUSSION AND CONCLUSIONS

The capacity tests of the Thermo King refrigerating unit

showed that the maximum net refrigerating capacity was obtained

for engine speeds in the range from 2lj00 rpm to 2700 rpm for all

refrigerator temperatures below 35°F when the condenser and

evaporator fans and the compressor were belt-driven with fixed

pulley ratioSo There was no significant change in capacity

in this speed range, which included the design engine speed of

2l|.00 rpm« These tests were not planned to establish whether

or not the speed ratio between compressor and fans was the

optimum for this uni to

The warehouse used to test this unit was of such size

that the specimen refrigerating unit was not able to cool it

to 0°Fo At a 0°F refrigerator temperature the net refrigerating

capacity of the unit would be about 650O Btu/hr for an engine

speed of 240O rpm as determined by extrapolation of the appropriate curve in Figure 4»
^ '

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.,:te?.': -,f V: ..

r .,

'll- 4..' ..ii.il.
12 13 14 15

Test

7370 7360 7210 7150

Net

2305 2416 2694 2999

Engi 1753 1564 2050 19^1

Comp: 1502 1569 1700 1909

Evap

Rati 1.31 1.30 1,31 1.51 b)

Com

Rati 1.53 1.54 1.55 1.57

Eva

3.5 4.5 5.5 6.6

Gaso l.S 1.6 2.6 3.7

Aver 110.6 110.4 109.2 110.2

Aver 171 171 173 172

Disc 3.9 3.4 3.1 3.5

Suet 9.9^ 10.26 10.54 10.26

Comp

Suet

7^.2 76.4 75.0 75.2

Temp 112.7 113.7 114.9 114.6

Con

Temp 120.3 120.4 121.9 122.7

Con

Air 7o6 6.7 7^0 5.1

Aer

Temp 5.0 4.1 5.3 6.4

Eva

Temp 0.1 -0.2 1.1 2.5

Eva

Air 4.9 4.3 4.2 3.9

Aer

ipacity in Test 1 indicates that the evaporator needed

compressor indicates belt slippage.

TABLE 1

EFFECT OF ENGINE SPEED AND REFRIGERATOR TEMPERATURE

ON UNIT CAPACITY AT CONSTANT AMBIENT TEMPERATURE

THERMO KING, MODEL K-10 UNIT

Test Number

Net Refrigerating Capacity

Engine Speed

Compressor Speed

Evaporator Fan Speed

Ratio of Engine Speed to

Compressor Speed

Ratio of Engine Speed to

Evaporator Fan Speed

Gasoline Consumption

Average Refrigerator Temp.

Average Ambient Temperature

Discharge Pressure

Suction Pressure

Compression Ratio

Suction Gas Temperature

Temperature of Air Entering

Condenser

Temperature of Air Leaving

Condenser

Air Temperature Difference

Across Condenser

Temperature of Air Entering

Evaporator

Temperature of Air Leaving

Evaporator

Air Temperature Difference

Across Evaporator

1 2 3 4 5 6

BTU/HR 7,770 a) 13,890 16,620 16,790 15,590 8,19(

R.P.M. 1837 216S 2393 2719 3005 1777

H.P.M. 1378 1648 1789 2068 2145 1360

R.P.M. 1178 1446 1491 1715 1765 1121

1.33 1.32 1.34 1.31 1.40 1.31

1.56 1.50 1.60 1.59 1.70 1.59

lb /hr 3.2 3.9 5.4 6.5 6.8 3.3

op 35.1 34.8 35.4 34.0 34.3 10.7

op 109.7 109.8 111.3 109.9 109.3 110.'

Psig 173 189 191 183 187 181

Psig 7.2 a) 15.2 15.8 13.3 13.0 8,1

8.57 6.81 6.74 7.06 7.28 8.58

op S2.2 81.3 82.4 81.0 81.4 81.0

op lU.O 113.9 113.4 111.0 113.3 116.

op 121.9 124.6 123.4 121.2 123.6 125.:

op 7.9 10.7 10.0 10,2 10,3 8.3

op 37.0 37.2 37.2 35.3 35.7 12.5

op 30.4 28.8 28.7 27.6 28.7 6,6

op 6.6 8,4 8.5 7.7 7.0 5.9

7 8 9 10 11 12 13 14 15

8800 9230 9270 9250 6990 7370 7360 7210 7150

2086 2351 2708 2991 1788 2305 2416 2694 2999

1580 1724 2068 2261 1356 1753 1864 2050 1981

1315 1391 1708 1815 1154 1502 1569 1700 1909

1.32 1.36 1.31 1.32 1.32 1.31 1.30 1.31 1.51 b)

1.59 1.69 1.59 1.65 1.55 1.53 1.54 1.58 1.57

4.7 6.4 6.8 7.5 3.0 3.5 4.5 5,5 6.6

10.5 11,0 10.7 10.8 6.0 1.8 1,6 2.6 3.7

110.7 111,5 110.8 110.5 109.1 110,6 110.4 109.2 110.2

177 184 176 174 174 171 171 173 172

7.1 6.9 5.5 4.6 6.3 3.9 3.4 3.1 3.5

8,79 9.20 9.44 9.78 8.99 9.98 10.26 10.54 10.26

79.1 80.0 81.9 80.2 79.1 78.2 76,4 78.0 78.2

116.

A

116.9 114.4 113.2 114.0 112.7 113.7 114.9 114.6

121.1 124.9 124.0 123.4 121.4 120.3 120.4 121.9 122,7

7.9 8.0 9.6 10.2 7.4 7.6 6.7 7.0 8.1

13.2 13.4 12.8 13.1 9.0 5.0 4.1 5.3 6.4

7.3 8.1 8.0 8.4 3.4 0,1 -0.2 1.1 2.5

5.9 5,3 4.8 4.7 5.6 4.9 4.3 4.2 3.9

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THE NATIONAL BUREAU OF STANDARDS

Functions and Activities

The functions of the National Bureau of Standards are set forth in the Act of Congress, March

3, 1901, as amended hy Congress in Public Law 619, 1950. These include the development and

maintenance of the national standards of measurement and the provision of means and methods

for making measurements consistent with these standards; the determination of physical constants and properties of materials; the development of methods and instruments for testing materials,

devices, and structures; advisory services to Government Agencies on scientific and technical

problems; invention and development of devices to serve special needs of the Government; and the

development of standard practices, codes, and specifications. The work includes basic and applied

research, development, engineering, instrumentation, testing, evaluation, calibration services, and

various consultation and information services. A major portion of the Bureau’s work is performed

for other Government Agencies, particularly the Department of Defense and the Atomic Energ)

Commission. The scope of activities is suggested by the listing of divisions and sections on the

inside of the front cover.

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The results of the Bureau’s work take the form of either actual equipment and devices or published papers and reports. Reports are issued to the sponsoring agency of a particular project

or program. Published papers appear either in the Bureau’s own series of publications or in the

journals of professional and scientific societies. The Bureau itself publishes three monthly periodicals, available from the Government Printing Office: The Journal of Research, which presents

complete papers reporting technical investigations; the Technical News Bulletin, which presents

summary and preliminary reports on work in progress; and Basic Radio Propagation Predictions,

which provides data for determining the best frequencies to use for radio communications throughout

the world. There are also five series of nonperiodical publications: The Applied Mathematics

Series, Circulars, Handbooks, Building Materials and Structures Reports, and Miscellaneous

Publications.

Information on the Bureau’s publications can be found in NBS Circular 460, Publications of

the National Bureau of Standards ($1.25) and its Supplement ($0.75), available from the Superintendent of Documents, Government Printing Office. Inquiries regarding the Bureau’s reports and

publications should be addressed to the Office of Scientific Publications, National Bureau of

Standards, Washington 25, D. C.
NBS