3au of Standards

NBS MONOGRAPH 7

MAR 1 0 I960

Precise Measurement of Heat of Combustion

Witli a Bomb Calorimeter

U.S. DEPARTMENT OF COMMERCE

NATIONAL BUREAU OF STANDARDS
1

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UNITED STATES DEPARTMENT OF COMMERCE • Frederick H. Mueller, Secrelary NATIONAL BUREAU OF STANDARDS • A. V. Astin, Director

Precise Measurement of Heat of Combustion

With a Bomb Calorimeter

R. S. Jessup

National Bureau of Standards Monograph 7

-

Issued February 26, 1960

For sale by the Superintendent of Documents. U.S. Government Printing Office, Washington 25, D.C. - Price 25 cents

Contents

Page

1. Introduction 1

1.1. Definitions of terms 1

a. Heat units . 1 b. Heats of combustion 2

2. General discussion of bomb-calorimetric measure- ments 2

3. Factors affecting accuracy In bomb-calorimetric measurements 3

3.1. Undesirable side reactions 3

a. Incomplete combustion 3

b. Oxidation of crucible and fittings 4

c. Reaction of acids with bomb material. 4

d. Combustible impurities in oxygen 4

3.2. Experimental techniques of individual measurements 4

a. Weight of sample of combustible 4

b. Weight of calorimeter plus water 5

c. Temperature measurements 6

d. Firing energy i 9

e. Materials in bomb 9

f. Acids formed in combustion 9

4. Summarized directions for a bomb-calorimetric ex- periment 10

4.1. Preliminary adjustment of apparatus 10

4.2. Preparation and weighing of sample of benzoic acid 10

4.3. Preparation of bomb 10

4.4. Weighing of calorimeter plus water 10

4.5. Assembly of calorimeter 10

4.6. Adjustment of initial temperature of calo- rimeter 11

4.7. Observation of temperature and ignition

of sample 11

4.8. Analysis of contents of bomb 11

4.2. a. Preparation and weighing of sample of

fuel 11

Page

4. Summarized directions for a bomb-calorimetric ex- periment—Continued

4.3. a. Preparation of bomb 11

4.6. a. Adjustment of initial temperature of calorimeter 11

4.8. a. Analysis of contents of bomb 12

5. Calculation of results 12

5.1. Calibration experiment 12

a. Heat of combustion of benzoic acid 12

b. Correction terms Ci and C2 13

c. Corrected temperature rise 13

d. Example illustrating the calculations E and E, 14

5.2. Calculation of heat of combustion 15

a. Energy equivalent of calorimeter as used 15

b. Corrected temperature rise 15

c. Corrections Ci, Ca, C3 15

d. Calculation of Q, (gross) 15

e. Calculation of Qp (net) 16

6. References 16

7. Appendix 17

7.1. Apparatus 17

a. Bridge 17

b. Thermometer 18

c. Galvanometer 18

d. Bomb 18

e. Calorimeter and jacket 19

f. Balance for weighing samples 20

g. Balance for weighing calorimeter 20

h. Oxygen purifier 20

i. Laboratory table for bridge 21

j. Pressure gage 21

7.2. Glass sample bulbs 21 in

Precise Measurement of Heat of Combustion

With a Bomb Calorimeter

R. S. Jessup

This Monograph gives detailed descriptions of apparatus and methods which are used

at the National Bureau of Standards for precise determinations of heats of combustion

of liquid hydrocarbon fuels. Numerical examples are given of methods of calculating

results of measurements from observed data. The technique of making and fiUing glass bulbs to contain samples of volatile liquid fuels is described. The accuracy of the methods described is about 0.1 percent. This is intermediate

between the accuracy of 0.01 or 0.02 percent attained in certain measurements on pure compounds, and the accuracy of several tenths of one percent obtainable with published

standard procedures for measurements on fuels.

1. Introduction

I Standard methods of moderate precision for

I' bomb-calorimetric measm-ement of heats of combustion of solid and liquid fuels are published b}^

the American Society for Testing Tvlaterials [1,2].^ These methods can be used to measure heat of

combustion with an accm-acy of several tenths

I of one percent. Apparatus and methods for measm-ements of heat of combustion with an

acciu'acy of 0.01 or 0.02 percent have been

described in the literatm*e [3]. The attainment

of such accuracy involves the use of rather

laborious and time-consuming procedures. In some applications of fuels the heat of cembustion

is reciuired with an accuracy of approximately 0.1 percent, which is probably not attainable with the

standard methods referred to [1, 2], but can be

attained with less expenditm-e of time and effort

j than is requii'ed in the most precise meastu'ements.

J This Monograph describes methods and apparaI tus used at the National Bureau of Standards in connection wnth several series of investigations of heats of combustion of aircraft fuels [4, 5, 6, 7]. The restrictions of the treatment to a particular

I set of apparatus, and a particular experimental

procedure involves some loss of generality, but

this is not believed to be serious. Most precise

measurements of heats of combustion of materials

containing essentially only the elements carbon,

hj^drogen, nitrogen, and oxygen have been made in apparatus of similar design, to which the same

procedures are applicable. Descriptions of other

equally satisfactory apparatus for this purpose

will be found in reference [3] and the references

there cited. There will also be found in some

of these references descriptions of apparatus and

procedures suitable for measurements on materials

containing considerable amounts of such elements

as sulfur and halogens. The apparatus and

methods described in this Monograph are not

suitable for measiu-ements on such materials.

Although the apparatus described in this Monograph is suitable for measurement of heat

1 The preparation of this Monograph was supported ui part by the Air Force Ballistic Missiles Division, Air Research and Development Command, U.S. Air Force.

2 Figures in brackets indicate the literature references on page 16.

of combustion with an accuracy of 0.01 or 0.02

percent, the procedm'e in measurements on fuels is such that a considerable saving in time was

achieved at the cost of a somewhat lower precision.

This lower precision resulted from several factors

as follows:

(1) As a rule only two experiments were made

on each fuel, as compared with six or more experiments where the highest possible accuracy is desired; (2) an approximate method was used to obtain the corrected temperature rise of the calorimeter, i.e., the temperature rise corrected for heat

of stirring and heat transfer between calorimeter

and surroundings; (3) because some liquid fuels are quite volatile there may be an appreciable loss of lighter components in handling. In spite of

these effects the results of measurements even on

the most volatile fuels (gasolines) are, in general,

precise to about 0.05 percent. The results contain a nearly constant systematic error of approximately 0.03 percent (for hydrocarbon fuels) due

to neglect of the Washburn correction [18] which

takes account of the difference between heats of

combustion in oxygen under a pressure of about

30 atm and at a pressure of 1 atm.

(If desired, this error can be approximately corrected, in the case of hydrocarbon fuels only, by

reducing the observed heat of combustion by 0.03

percent.)

In other respects the methods described are

applicable in measm'ements of the highest precision, and may serve as a useful guide to persons

interested in such measurements.

1.1. Definitions of Terms

a. Heat Units

The heat units used in connection with measm-e- ments of heats of combustion of fuels are the IT

calorie ^ and the Btu. These units are here

defined as follows

:

3 IT calorie is an abbreviation for International Steam Tables calorie. This calorie was adopted by the International Steam Table Conference

held in London in July 1929 [12], and is in faiiiy general use in engineering

practice both in this country and abroad.

1
1 IT calorie=4.1868 absolute joules

1 Btu= 1055.07 absolute joules.

The definition of the Btu was obtained from that

of the IT calorie by means of the relations

1 pound (avoirdupois) =453.5924 grams

1 IT calorie per gram=1.8 Btu per pound.

b. Heats of Combustion

The quantity directly measured in a bombcalorimetric experiment is generally referred to as the "total (or gross) heat of combustion at constant volume"; this quantity is represented by the

symbol C*^ (gross) in this Monogi'aph. A precise

definition of this term requires a specification of

the initial states of the reactants (oxygen and fuel)

and of the final states of the products of combustion. For most purposes the following definition

is sufficient : The total {or gross) heat of combustion

at constant volume of a liquid or solid juel containing

only the elements carbon, hydrogen, oxygen, nitrogen

and sulfur is the quantity oj heat liberated v^hen unit

weight oj the juel is burned in oxygen in an enclosure

oj constant volume, the products oj combustion being

gaseous CO2, N2, and SO2 and liquid H2O, with the

initial temperature oj juel and oxygen and the iinal temperature oj the products oj combustion at 25° C. Although the total heat of combustion is the

quantity du"ectly measiu'ed in a bomb-calorimetric

experiment, the quantity required in many practical applications is the "net heat of combustion

at constant pressm'e." This quantity is designated by the symbol Qp (net), and may be defmed

as follows: The net heat oj combustion at constant pressure oj a liquid or solid juel containing only the elements carbon, hydrogen, oxygen, nitrogen, and

sulfur is the quantity oj heat liberated when unit

weight oj the juel is burned in oxygen {or air) at a

constant pressure oj one atmosphere, the products oj

combustion being CO2, N2, SO2, and H2O, all in the gaseous state, with the initial temperature ojjuel and

oxygen and the iinal temperature oj products oj

combustion at 25° C. For the most fuels to which these definitions

apply the elements oxygen, nitrogen, and sulfur

will be minor constituents if present at all. The

methods described in this Monograph are not

suitable for accurate measurements on materials

containing more than about 2 percent sulfur.

2. General Discussion of Bomb-Calorimetric Measurements

A bomb calorimeter consists essentially of a

calorimeter vessel containing a measured amount

of water, in which are immersed (1) a thermometer

for measuring the temperature of the water, (2) a

stirring device for maintaining the water at a

uniform, and therefore definitely measui'able

temperature, and (3) a "bomb" of constant volume

in which combustible materials can be burned in oxygen under pressure. In order to control heat

transfer between the calorimeter and its environment ("thermal leakage") the calorimeter vessel

is enclosed by a "jacket" which is separated from

the vessel by an air space about 1 cm thick, and

which for a majority of precise calorimeters, is kept at constant temperatm-e by means of a thermostat.

A schematic diagi'am of a bomb calorimeter and

its jacket is shown in figure 1. The calorimeter

vessel is made in the form shown in order to

facilitate stirring of the calorimetric liquid. The

effectiveness of a screw propeller stirrer is greatly

increased by enclosmg it in a tube which extends

from near the top to near the bottom of the

calorimeter vessel. Putting the tube outside of the main part of the calorimeter vessel, as shown

in figure 1(b), reduces the total volume of the

calorimeter necessary for a bomb of given size. In principle, a measiu-ement of the heat of

combustion of a given material consists in comparing the corrected temperature rise of the

calorimeter in an experiment in which a known

quantity of energy is supplied to it, with that

produced in another experiment by combustion

(a) Figure 1. Schematic diagram of a bomb calorimeter.

(a) Assembled calorimeter with jacket: B, bomb; C, calorimeter vessel;

J, jacket wall; P, resistance thermometer; FL, firing leads; CS, calorimeter

stirrer; JS, jacket stiixer; TV, tube to thermostat valve; H, jacket heater; TB, thermostat bulb; and TH, tubular housing, (b) Top view of calorimeter

vessel OL, opening for firing lead; and OT, opening for thermometer, (c) Firing lead through jacket T, tube; R, rod; and W, washers, (d) Firing lead

to calorimeter vessel CL, clip.

in the bomb of a weighed sample of the given

material. The temperature rise in both experiments should be as nearly as possible over the same

temperatm-e range from a standard initial temperature to a standard final temperature. This

method of using the corrected temperature rise of the calorimeter to compare an unknown with a known quantity of energy eliminates some of the

systematic errors of calorimetric measurements,

in particular those associated with uncertainty

as to the exact location of the boundary of the

2
i calorimeter, and those associated with various

kinds of lag, by virtue of which different parts

of the calorimeter are at different temperatures

when the temperature is changing.

Experiments to determine the corrected temperature rise of the calorimeter when a known

quantity of energy is supphed to it are generally

called calibration experiments, and tlie mean

I result of a series of such experiments expressed as energy supplied per imit of corrected temperature

rise is called the "energy equivalent" of the

calorimeter. The product of the energy equivalent and the connected temperatm-e rise of the

calorimeter in an experiment in which a material

of unknown heat of combustion is burned in the

bomb gives the energy liberated in the combustion reaction plus any side reactions which may

occur. Calibration experiments are conveniently carried

out by burning in the bomb weighed samples of a standard material, such as benzoic acid (NBS

Standard Sample 39), the heat of combustion of

which is accurately known. If the calorimeter is

to be used for measm-ements of heats of combustion of volatile liquids, such as gasolines, it is deshable to supplement the calibration experiments

with benzoic acid by making a series of measure- ments on NBS Standard Sample 217 of 2,2,4-

trimethylpentane, which is a volatile liquid with a

certified value for its heat of combustion. The

difference between the measixt'ed and certified values of heat of combustion will give an overall

check on the accuracy of the measm'ements, and in

particular, on the effectiveness of precautions

taken to eliminate effects of volatility.

Since a measurement of heat of combustion in- volves the comparison of two nearly equal changes

in temperatm'e covering the range from a standard

initial temperatm"e to a standard final temperature,

the temperature scale used in the measm-ement is not important. Thus temperatures can be ex- pressed in terms of the resistance in ohms of a

given platinum resistance thermometer with the

energy equivalent of the calorimeter in calories

per ohm, or in Celsius degrees with the energy

equivalent in calories per degi'ee.

3. Factors Affecting Accuracy in Bomb-Calorimetric Measurements

The overall en-or in the result of a bombcalorimetric measui'ement is the algebraic sum of individual errors contributed by various factors. Some of the individual errors may be positive and

others negative, so that to some extent they may

be expected to cancel. However, the extent to which cancellation may take place is imcertain,

and may vary from one experiment to another, so that the percentage contribution of each individual

error should be kept weU below the permissible

overall en-or of the determination. In the present

Monograph the permissible dift'erence between duplicate determinations is taken as 0.05 percent. In

order to attain this precision, the error contributed

by each of the various independent factors should

be kept below 0.01 percent so far as possible.

The factors wliich may affect the accuracy of a bomb-calorimetric determination may be divided

roughly into two classes, (1) side reactions which

may take place in the bomb, the effect of which

cannot be readily evaluated, and (2) the experimental techniques used in making the various

individual measurements. These two classes of factors are discussed in sections 3.1 and 3.2,

respectively.

3.1. Undesirable Side Reactions

These include (a) formation of carbon monoxide,

carbon, or other products of incomplete combustion of the samples; (b) oxidation of the crucible

in which combustion of the fuel sample takes place, or oxidation of other parts of bomb or fittings;

(c) reaction of acids formed m combustion with

the material of the bomb; (d) oxidation of combustible impurities such as hydrogen or hydrocarbon gases in the ox3^gen used.

a. Incomplete Combustion

In tests of volatile liquid fuels which must be

enclosed in glass bulbs to prevent loss by evaporation, incomplete combustion sometimes occurs as a result of breakage of the bulb before ignition of

the sample. In such cases large amounts of

carbon will usually be formed, and the results of such tests should be discarded. Normally a bulb containing a volatile liquid sample does not

break before ignition of the sample, and in such

cases there is generally little or no evidence of

incomplete combustion. It occasionally happens

that globules of glass remaining in the crucible

after combustion are gi'ay or black in color. This

may be due to metal oxides formed in combustion

of the fuse wire, or to inclusion of a small amount

of carbon in the glass while it is molten. Determinations of carbon dioxide in the products of

combustion of pure liquid hydrocarbons which

were enclosed in soft glass bulbs indicate that the

amount of carbon in such glass globules does not

exceed 0.01 percent of the carbon in the sample.

Incomplete combustion of solid materials, such

as benzoic acid, is very likely to occur if these

materials are placed in the crucible in powdered

form. For this reason, such materials should be

compressed into pellets. For samples of nonvolatile liquid fuels which are not enclosed in

glass bulbs, and for pelleted samples of benzoic

acid, incomplete combustion almost never occirrs in apparatus such as that described in the

appendix.

3
b. Oxidation of Crucible and Fittings

Oxidation of the crucible and its support and

other fittings exposed to the direct action of the

flame can be avoided by using platinum crucibles

and fittings (see appendix)

.

c. Reaction of Acids with Bomb Material

Reaction of acids formed in combustion with

the material of the bomb can be made negligible

by the use of suitable corrosion-resistant material

for the bomb. The bomb described in the appendix has been found satisfactory in this respect.

Bombs provided with gold or platinum linings

have also been found to be satisfactory.

d. Combustible Impurities in Oxygen

Combustible impurities in the oxygen used may

introduce very serious errors into the results of

bomb-calorimetric measurements. Oxygen pre- pared by electrolysis of water may contain enough

hydrogen to cause errors of 1 percent or more. The possibility of error from this source is eliminated by passing the oxygen over copper oxide

at 500° C before admitting it to the bomb (see

appendix).

3.2, Experimental Techniques of Individual

Measurements

The most important of the individual measure- ments which go to make up a bomb calorimetric

determination include measurements of (a) the

weight of the sample of combustible; (b) the

weight of the calorimeter plus water; and (c) the

temperature rise of the calorimeter, including

correction for thermal leakage and heat of stirring.

A given percentage error in any one of these

measurements introduces an equal percentage

error into the final result of the experiment. Other

individual measurements which determine rela- tively small correction terms and which can be

easily made with the necessary precision include

measurements of (d) the quantity of energy used

to fire the charge of combustible; (e) the amounts

of oxygen and other materials in the bomb; and

(f) the amounts of nitric and sulfuric acid formed

in the combustion reaction.

a. Weight of Sample of Combustible

The sample of combustible (0.8 to 1.5 g) is weighed with a precision of about 0.02 mg using an undamped semimicro balance with a keyboard

arrangement for adding and removing the smaller

weights (see appendix). Calibrated high grade

(class M) weights are used, and the corrections

to the weights given on the calibration certificate are applied in determining the weight of the

sample. If weighings are made in the ordinary manner with weights and object weighed on

opposite pans of the balance, the balance should

be tested to determine whether the balance arms

are of equal length. If the arms are found to

differ in length by a significant amount a correction

for this difference will be necessary.

The necessity for such a correction can be

eliminated by using the method of weighing by

substitution. This consists in placing on the left- hand pan a fixed weight or "tare," somewhat

greater in weight than the heaviest object to be

weighed, then placing the object to be

weighed on the right-hand pan together with

sufficient weights, including rider adjustment, to balance against the tare. The object being

weighed is then removed and weights and rider

again adjusted to obtain balance. The difference

in the weights requu'ed in the two cases is the

desired weight of the object. In addition to eliminating the effect of any difference in length

of the balance arms, this method also eliminates

any change in sensitivity with load, since the load

is constant. The following description of the

procedure is for this method of weighing by

substitution.

Each half of the balance beam from the center

knife edge to the outer knife edge is graduated in

100 equal divisions, so that if a 1 mg rider were

used a change in position of the rider by one

division on the scale would correspond to a change

in weight of 0.01 mg. However, it may be inconvenient to use such a small rider because of the

likelihood of its being deformed or lost. Instead,

a 10 mg rider may be used so that one division on

the scale corresponds to 0.1 mg in weight, and

hundredths of a milligram obtained from the

deflection of the pointer, as described below.

Weighings are normally made with the weights

to the nearest milligram on the right-hand pan of

the balance, and the 10 mg rider so placed (between 0 and -|-10 divisions on the beam scale)

that the tare Aveight on the left-hand pan is balanced to the nearest 0.1 mg. The equilibrium

position of the pointer with reference to the zero

of the index scale is then determined by observing

the turning points of the pointer in an odd number

of swings (3 or 5). The procedure in using the balance will be

illustrated by an example of the weighing of a sample of benzoic acid. All weighings are made

after time has been allowed for temperature

equilibrium to be established, and for air currents

set up when the balance case was open to die out. To determine when equilibrium has been established, the apparent weight is observed from time

to time until no change has been observed during

a period of about 10 min.

The following observations were made in determining the weight of a pellet of benzoic acid, the

first set of observations were made with the empty

crucible, and the second with the crucible containing the pellet:

4
Crucible empty:

Weights 12. 493 g Turning points of swings:

Rider . 0.0002 -2.6 2.0

Pointer deflection X2, -0.55X0.07X10 —5. — .00004 -2.5

Total weights ' 12.49316 -2.55 2.0

2 X pointer deflection= — 0.55 Crucible plus pellet: Weights 10. 971 Turning points of swings:

Rider 0.0003 -2.3 2.6

Pointer deflection X2, 0.45X0.07X10-3 . 00003 -2. 0 Total weights 10. 97133 -2. 15 2. 6

2 X pointer deflection = +0.45

Observed weight of pellet 1. 52183

Weight correction " + . 00005

Corrected weight of pellet 1. 52188

» The weight correction is the algebraic sum of the certificate corrections to the individual weights used with the empty crucible minus the algebraic sum of the corrections to the weights used with crucible plus pellet.

In the above example the weight corresponding

to the pointer deflection from the zero of the

index scale was obtained by multiplying twice

the average deflection (in divisions on the scale)

by one-half of the sensitivity reciprocal of the

^balance, in this case 0.07=1/2X0.14 mg per scale

division. (The same result would be obtained, of course, if the average deflection were midtiplied by

the sensitivity reciprocal, e.g., for the first weighing of the above example the averasre pointer

deflection = —0.28 and —0.28X0.14= —0.04 mg).

The same procedure is foUowed, for example,

in weighing a sample of a volatile liquid which

must be enclosed in a glass bulb to prevent loss by evaporation, except in this case it is- not necessary to include the crucible in either weighing.

The empty btdb is first weighed against the tare, and then the fifled bulb together with glass re- moved in sealing is weighed against the tare. It shoifld be emphasized that the balancing weight

against the tare with only the weights on the righthand pan should be determined in weighing both

the empty bulb and the filled bulb. The use of a

fixed value for the balancing weight against the

tare with only the weights on the right-hand pan

would introduce errors if the rest point of the bal- ance changes between the times of weighing the

empty and filled bulbs. This is likely to occur if a considerable length of time elapses between the

' weighings with the bulb empty and filled. The use

' of a fixed value for the balancing weight with the

' empty crucible on the right-hand pan is rided out

by the fact that the crucible is subject to changes

in weight in use.

i b. Weight of Calorimeter Plus Water

! The calorimeter plus water (3,700 g total

weight)* is weighed on a magnetically damped

balance having a capacity of 5 kg and a sensitivity

i of 0.5 mg (see appendix). The procedm'e in this weighing is as foUows: The calorimeter is filled with approximately the desired quantity of water,

the temperature of which is adjusted to a value

such that after assembly of the calorimeter its temperature wiU be a few tenths of a degree

* The weight should correspond to an amount of water in the calorimeter such that when the bomb is immersed in the water the calorimeter cover

can be put in place with its lower side in contact with the water.

below the desired initial temperature in the

experiment. The choice of the temperature to which the water is to be brought before weighing

will depend upon a number of factors, including room temperature, the desired initial temperature

in the calorimetric experiment, and the relative

heat capacities of calorimeter vessel, water, and

bomb. No definite rule can be given for this

choice, but the operator will learn by experience

how to select the proper temperature under the

conditions of his particular laboratory and apparatus.

After adjustment of the temperature the calorimeter is placed on one pan of the balance with

the desired weights (see footnote 4) on the other.

The amount of water in the calorimeter is adjusted

so that the total weight of calorimeter plus water

exceeds slightly that of the weights on the opposite

balance pan, and the balance case is closed.

Because of evaporation of water the weight of

calorimeter plus water decreases slowly, and the

pointer on the balance gradually moves toward

the zero of the index scale. When the pointer

reaches the scale zero the balance beam is arrested,

the case is opened, and the calorimeter is removed

from the balance and placed in position in its jacket preparatory to making a calorimetric

experiment.

It is not feasible in weighing the calorimeter to wait for it to come to temperature equilibrium

with the air, partly because this would require a very long time and partly because the desired

initial temperature in the calorimetric experiment

wfll usually differ from room temperature. Any

difference in temperature between calorimeter

and the air in the balance case will cause convection currents which will affect the observed

weight. The effect of this will partially cancel

if the difference between the temperature of calorimeter and the air in the balance case when the

calorimeter is weighed is the same in all experiments, including both calibration experiments and

measurements of heat of combustion.

The change in weight of the water in the

calorimeter due to evaporation after weighing wiU

affect the energy equivalent of the calorimeter,

but the effect of this is small and will cancel if the

527973—60 2

5
procedure in placing the calorimeter in its jacket

and completing the assembly of the system is carried out in the same manner and in the same

length of time in the calibration experiments as

in measurements of heat of combustion.

In addition to the water in the calorimeter

vessel, 1 cm^ of water is placed in the previouslj'-

dried bomb before each experiment to insure that

the space in the bomb is saturated with water

vapor at the beginning of the experiment as well

as at the end, when water formed in combustion

will be present in any case. This water is introduced into the bomb by means of a buret or pipet accurate to about 0.01 cm^, immediately

before introducing the sample of combustible.

c. Temperature Measurements

The temperature rise of the calorimeter in a combustion experiment is approximately 3° C,

and must be measured with an accuracy of 0.0003°

if the error in temperature measurement is to contribute not more than 0.01 percent to the

resiilt of the calorim.etric experiment. Measurements of temperature differences with this precision can be made conveniently with a suitable

25 ohm platinum resistance thermometer, and

a Mueller bridge of the type commonly designated

as G-2.

In precise measurements of temperature with a

resistance thermometer it is customary to eliminate the resistance of the thermometer leads

completely by making two resistance readings,

with the commutator set first in the A'^ position

and then in the R position. In bomb calorimetric

measurements where only a small temperature

difference is to be measured and where time for making the temperature measurements is limited,

it is permissible to omit changing the commutator

setting. However, if this procedure is followed it

is very important that all measurements in all ex- periments be made with the same commutator

setting, since the change in observed resistance per

degree change in temperature may be different for the A'' and R positions of the commutator. The

possibility of error due to change in the commutator setting may be eliminated by permanently

disconnecting one of the current leads (c or t) of

the thermometer from the bridge. The thermometer is then effectively a 3-lead thermometer

and the bridge can be operated with only one

setting of the commutator.

It IS also important that the thermometer be

immersed to the same depth in all experiments, so that its contribution to the energy equivalent of

the calorimeter will be the same in all experiments,

and so that the temperature distribution along the

leads wUl be the same. The observations of the temperature of the

calorimeter in a bomb-calorimetric experiment

must be made in such a manner as to provide data

from which can be derived a value for the temperature rise corrected for thermal leakage and heat

of stirring. The following procedure is for a

calorimeter with the jacket maintained constant <| at a standard temperature.^ \\ Temperatures of the calorimeter are measured ' \ at definite times during three periods: (1) An ini-

!

tial period of 6 to 10 min during which the tem- j' perature change results solely from thermal

leakage and heat of stirring; (2) a middle period |' of about 12 min, at the beginning of which the ' charge in the bomb is fired, and during which the • temperature change is due partly to thermal

leakage and heat of stirring but mostly to the f

heat liberated by the combustion reaction in the |

'

bomb —this period continues until the rate of ' change of temperature has become constant; and ' (3) a final period of 10 min or more, during which ' the temperature change is again due solely to ' thermal leakage and heat of stirring. It is de- ' sirable to choose the initial temperature of the ! calorimeter so that its final temperature wUl be '

'

slightly below that of the jacket. This reduces

the total thermal leakage during the middle period I

\ and eliminates heat transfer by evaporation of * |' water from the calorimeter and condensation on ' the jacket wall which would occur if the tempera- f ture of the calorimeter exceeded that of the jacket. ' During the initial and final periods the resistance f of the thermometer should be measured with the I* highest possible accuracy, since the overall accu- \ racy of the determination depends directly upon ) the accuracy of these temperature measurements. ) During the middle period, because of the very

rapid rate of temperature rise, it is not possible i to make readings as accurately as during the initial and final periods, but this is not important because ; the readings of the middle period are used only for calculating the relatively small correction for thermal leakage and heat of stirring.

With a G-2 bridge and a thermometer having ' a resistance of approximately 25.5 ohms at 0°C, one step in the last dial of the bridge (0.0001

olim) corresponds to approximately 0.00 1°C in

temperature. Readings of the bridge are made to the nearest 0.00001 ohm (0.0001°) by interpolating between two adjacent settings of the last ' dial of the bridge. This interpolation is accom- ; plished by observing the deflection of the galvanometer as follows: The galvanometer is located i about 2 meters from a gi-ound glass scale graduated

in millimeters, which is so placed as to be conven- . iently observable by the operator of the bridge.

[ In front of the galvanometer mirror is placed a I lens of such focal length that the reflected image \ of the vertical filament of a light source is focused i on the ground-glass scale. ^ The bridge cmTent is adjusted so that with constant thermometer

resistance a change of 0.001 ohm in the bridge

5 The NBS calorimeter is located in an air-conditioned laboratory, the temperature of which is maintained at about 25° C. The jacket tempera- i ture is always maintained constant at 28° C and the calorimetric experiments i cover the range from 25° to 28° C. If room temperature is not controlled the standard temperature of the jaclcet should be taken high enough so that it will be above room temperature at all seasons. A jacket temperature below

that of the room is undesirable as it may result in the condensation of water ; on the calorimeter, the initial temperature of which is 3° C below that of the

jacket.

6 Lamp and scale devices for indicating galvanometer deflection are available commercially, and are as satisfactory as the device described here.

6
setting causes a galvanometer deflection corre- sponding to a 2.5 cm displacement^ of the filament

image on the scale. This is equivalent to a dis- placement of 2.5 mm per step in the last dial of

the bridge, i.e., to 2.5 mm per mUlidegree. This

sensitivity is doubled by reversing the current through the bridge and observing the resulting

change in position of the filament image. The

sensitivity is such that when the bridge current

is reversed a displacement of the filament image

by 0.5 mm corresponds to 0.00001 ohm (0.0001°).

Reading the bridge by observing the deflection of

the galvanometer when the bridge cm-rent is reversed not onl}' increases the sensitivity but

also reduces the effect of changes in the galvanometer zero. The reversal of the bridge current

is conveniently made by means of a double-pole

double-throw toggle switch connected in the

battery circuit. The above discussion of the deflection of the

galvanometer when the bridge current is reversed

tacitly assumes that the resistance of the thermometer remains constant dm-ing this operation.

In an actual calorimetric experiment the resist- ance of the thermometer is usually changing

with time; in the initial period for example the

resistance of the thermometer is increasing at a

rate of approximately 0.0007 ohm/min. (0.007°

C/rain). During this period readings are made

once per minute in such a manner as to give

the resistance of the thermometer to the nearest 0.00001 ohm at intervals of exactly 1 min.

These readings are made as follows: The

switch for reversing the bridge cxirrent is thi'own

in the direction such that as the temperatm'e of the calorimeter rises the image of the filament of

the galvanometer light source moves toward

the obsei-ver's right. The bridge dials are adjusted occasionally so as to keep the filament

image slightly to the left of its zero position.

The time is observed by means of a stop watch,

or preferably hj means of a combination of stop

watch and audible second signals from a standard

clock. The position of the filament image is observed to the nearest 0.5 mm at a time 4 sec before the even minute and the reversing switch

is thrown immediately. The position of the

image is observed again at 4 sec after the even minute* and the switch is then thrown back to

its original position in preparation for later readings. If the second position of the filament

image is exactly the same as the first, then the

galvanometer deflection on the even minute was

zero, and the reading of the bridge is correct with

zero in the next place after the reading of the

' With the particular combination of bridge, fhermometer, galvanometer, and galvanometer to scale distance used, the bridge current of 6.8 ma (thermometer current of 3.4 ma) gives the sensitivity indicated. Much higher bridge current is iindesirable because of the increased heating of the thermometer and the consequent increased variation of thermometer resistance with

slight variations in bridge current. A 6-volt low-discharge storage battery

is a convenient source of bridge current.

8 The time which should elapse between the two observations of the galva- nometer deflection depends upon the free period of the galvanometer. The

reading time of an underdamped galvanometer with a relative dampmg

constant of 0.8 or 0.9 is roughly equal to the free period [19].

last dial. If the second position of the filament

iniage is x mm to the left (or right) of the

original reading, then the reading of the bridge

should be increased (or decreased) by 0.00002a;

ohm to obtain the reading corresponding to the even minute. For example, if the reading

of the bridge is 27.3956 olmis and the upper

and lower arrows in figure 2(a) represent the

initial and final positions, respectively, of the

filament image, then the corrected reading of the bridge at the even minute would be 27.39563

ohms. On the other hand the situation repre- sented in figure 2(b) for the same setting of the

bridge dials would correspond to a corrected

bridge reading of 27.39556 ohms.

(a) (b)

Figure 2. Diagram to illustrate use of galvanometer deflec- tions in reading bridge.

With a little practice an observer can learn to anticipate each bridge reading fairly closely after the first one or two of a series, and to preset the

bridge dials for each subsequent reading so that

the change in galvanometer deflection upon re- versal of the current will not exceed 2.5 mm

(0.00005 ohm or 0.0005° C).

As a result of the preliminary adjustment of the

temperature of the water in the calorimeter before

weighing it as described in section 3.2.b., it is to be expected that after assembling the calorimeter

the temperatiu-e will be a few tenths of a degree

below the desired initial temperature. The temperatm-e should then be raised to about 0.05°

(0.005 ohm) below the temperature at which the

initial period is to start, and allowed to drift up

to this temperature as a result of thermal leakage

and heat of stirring. The observations of the

initial period are then made as described above.

The heating of the calorimeter to 0.05° below

the startmg temperature is most conveniently done

by means of an electric heater which is a permanent part of the calorimeter system. Another

method which is frequently used is to place a heated brass rod {ji in. X 2 m.) on top of the

calorimeter cover. The rod is inserted through a

hole in the jacket cover which is normally closed

by a brass plug. Care is taken to avoid having

the heated rod come in contact with the jacket and

thereby upset the control of its temperature.
The above instructions may be illustrated by an

example. The calorimeter temperature before

weighing was adjusted to a value such that after assembly of the calorimeter the thermometer reading was 20.5 +7.420 ohms.^ By means of the

heated rod referred to above, the temperature was

raised until the reading of the bridge was 20.5 +

7.465 ohms. The rod was then removed, the plug

replaced in the hole in the jacket, and the calorimeter temperature allowed to drift upward until

the bridge reading was 20.5 +7.4701 olmis when

the readings of the initial period were begun.

These are made once per minute as indicated

previously.

Immediately after the observations of the initial period are completed the charge of combustible in the bomb is ignited by means of an electric fuse,

the sensitivity of the galvanometer is reduced by

reducing the bridge current to about one-third of

its original value, and the observations of the

middle period are begun. These must be made so rapidly that there is no time for reversing the

bridge current for each observation. The observations are made by setting the bridge dials

successively at certain predetermined readings and

observing the times at which the galvanometer deflection becomes zero. After about 3 min the

rate of temperature rise has decreased to such an

extent that the bridge current can be increased to

its original value and the precise readings of resist- ance each minute are resumed. These precise

readings are continued until the rate of temperature change has been constant for at least 10 min.

The observations made after the rate of temperature change has become constant constitute those

of the final period.

Table 1. Observations of time and thermotneter resistance during a bomb-calorimetric experiment

Time

minutes

Resistance minus

27.5 ohms

Difference X lOs Time

minutes

Resistance minus

27.5 ohms

Difference X 106

0

123

4

5

6

6. 528

6.621 6.682

6. 754 6.820

6. 894

6. 970

7. 066

7.170

7. 320

7. 399

7. 496

0. 47015

. 47078

. 47142

. 47206

6364

64

(64)

(64) 63

7. 614

7.747

7.922

9

10

1112

1314

1516

17

1819

20

2122

23

24

25

26

27

28

0. 7000

.7100

.7200

. 7.5050

. 75920

. 76270

.76410 .76470

.76499

. 76515

. 76525

. 76534

. 76540

. 76546

. 76553

. 76560

. 76566

870

350

140

6029

16

10

9

6

6

7

620^ (6) (7) 6

7

7

. 47334

. 47397

.4900

.5100 .5300

. 5500

. 5700 .5900

.6100

.6300

.6500

.6700

.6800

.6900

. 76586

. 76592

. 76599

. 76606

» The bridge has a 20..5-ohm coil in addition to the 10-, 20-, 30-ohm coils, etc., in the lOxlO-ohm decade (see appendi.t).

Bridge readings made during the coiu-se of a

typical bomb calorimetric experiment are given in \ table 1, and are shown graphically in figure 3. The values given for resistance represent the observed resistance of the thermometer minus 27.5

olmis. The initial period extends from t=Q to

f = 6 min, the middle period from i=6 to ^=18

min, and the final period from ^=18 to ^=28 min.

The constancy of the differences between consecutive readings in the initial and final periods is an | indication that the system has attained a steady

state. This is a necessary condition for the applicabUit}' of the method given in section 5 for calcu- I lating the corrected temperature rise.

1 I \ \ L_ I I 0 5 10 15 20 25 30

TIME, MINUTES Figure 3. Bridge readings versus time in a bomb calori- metric experiment.

In the particular experiment illustrated in table

1 and figure 3 the time readings during that part

of the middle period from ^=6 to ^=9 min were made by means of an electric timer which prints

the time at which the operator presses a button

when he observes the galvanometer deflection to pass through zero. This instrument is a con- venience and would be an important part of the

equipment in measurements of extremely high

precision. It is not a necessary part of the apparatus for measurements of the precision aimed at

in the procedures described in this Monograph. For

such precision it is also not necessary to obtain

as many readings in the middle period. It is sufficient to make three or four measurements in the range from 55 to 70 percent of the total temperature rise, and to read the time to the nearest second (or the nearest 0.01 min) by means of a

stopwatch.

i
The con-ected temperature rise for the experiment illustrated in table 1 and figure 3 is calculated in section 5.

d. Firing Energy

The quantity of energy used to fire the charge

is the sum of the electric energy used to ignite the

fuse plus the heat of combustion of the amount

of fuse wire burned. The current for igniting the

I fuse is obtained from a small transformer with a

! secondary voltage of about 10. The total firing

energy can be determined in a series of blank ex- periments in which only the fuse wire is burned.

In such experiments the temperature rise of the

calorimeter is very small, and the initial temperature of the calorimeter should be very near that

of the jacket. The rate of change of temperature

in the initial and final periods will then be small,

and the correction for thermal leakage wUl also be small.

When a 2-cm length of fuse wire is used in the manner described in the appendix it has been

found that if the wire is burned completely the

firing energy is constant and equal to 5.2 cal. No

appreciable error wUl result from using this value

in any case where the same type of fuse is used,

i.e., a 2-cm length of Parr fuse wu-e wound into a

helix and attached to platinum leads. The firing

energy in this case is only 0.05 percent of the en- ergy normally produced in a combustion experiment, and any error in the value 5.2 cal will largely

cancel if this same value is used for both calibration experiments and measurements of heats of

combustion, and if the temperature rise is about

the same in the two kinds of experunent.

The most serious error likely to be encountered

in accounting for the firing energy is that which

may result from a short-circuit between the firing

electrode and the bomb, either because of failure

of the insulation around the electrode or because

of moisture getting into the mica insulation inside

of the bomb. This mica normally is wet by the

acid solution formed in combustion and by the

water introduced in washing the interior of the

bomb after each experiment. It should be dried

thoroughly before each experiment, and the insulation should be tested before attaching the fuse

to its platinum leads. An insulation resistance of

less than 10^ ohms usually indicates either moisture in the insulation or some other failure of the

insulating material which needs to be corrected.

A "megger" or volt-ohmeter or other similar in- strument may be used for testing the insulation

resistance.

'1 Combustion of the fuse wire may not be complete In all blank experi- ments to determine the firing energy, although it is practically always complete when a sample of fuel is burned in the bomb. Blank experiments in which combustion of the fuse wire is incomplete should be rejected, or else a correction for the unbumed wire should be applied.

e. Materials in Bomb

A calibration of the calorimeter yields a value

for the energy equivalent E of the system as ac- tually used, consisting of calorimeter vessel, stirrer,

water, thermometer, bomb, and contents of the

bomb at the beginning of the experiment. The

calorimeter vessel, stirrer, mass of water, thermometer, and bomb will ordinarily be unchanged

from one experiment to another, and the crucible

and amount of water placed in the bomb will also be constant. However, the mass of the platinum

leads to the fuse, the mass of oxygen, and the mass and kind of combustible material may change

from one experiment to another. The heat ca- pacity of the largest amoimt of platinum wire in

the bomb amounts to only 0.001 percent of the

total energy equivalent of the system, so that

variations in the amount of platinum wire are entirely negligible. The mass of oxygen in the

bomb is inversely proportional to absolute temperature and directly proportional to absolute

pressure. The temperature is measured to about

0.5° C at the time the oxygen is admitted to the

bomb, by means of a mercurial thermometer with

its bulb near the bomb. The pressure of the

oxygen (about 30 atm=450 psi) is measured to the

nearest 0.1 atm (1.5 psi) by means of a calibrated

Bourdon gage.

The values of the energy equivalent E obtained

in calibration experiments are reduced to the

energy equivalent E^ of a standard calorimeter

system consisting of the calorimeter vessel with

cover, the standard mass of water, stirrer, thermometer, and bomb containing 1 ml of water,

the crucible, and oxygen under an absolute

pressure of 30.0 atm at 28.0° C but no combustible material. In a measurement of heat of

combustion the energy equivalent Eg of the

standard calorimeter system is corrected for any

excess of the oxygen pressure at 28° C over 30

atm and for the heat capacity of the charge of

combustible.

The reduction of the observed E to the corre- sponding value of Es, and vice versa, is discussed

in section 5 on calculation of results.

f. Acids Formed in Combustion

Nitric acid is normally formed m the combustion reaction from nitrogen either in the fuel sample itself or present as an impurity in the

oxygen. If sulfur is present in the fuel, oxides of

sulfur will be formed. These are entu-ely con- verted to sulfuric acid if nitric acid is also formed.

For this reason, if the fuel is known or suspected

to contain a significant amount of sulfur, the air

initially present in the bomb should not be flushed

out with oxygen at the time oxygen is admitted

to the bomb. The sulfur will then be all con- verted to aqueous sulfuric acid, and therefore to a determinate final state. The total acid formed in combustion is deter-
mined by washing the inner surface of the bomb

with distilled water and titrating the washings

against 0.1 sodiima hydroxide, using methyl

orange as the indicator.

If both nitric and sulfuric acids are formed the

total acid is determined by titration as before,

and the sulfur is determined by analyzing the

4. Summarized Directions for a

Summarized directions will now be given for carrying out a bomb-calorimetric experiment with

benzoic acid to determine the energy equivalent

of the calorimeter. Following these directions

there will be a brief discussion of the differences

in procedure in an experiment to determine the

heat of combustion of a fuel.

4.1. Preliminary Adjustment of Apparatus

Observe the temperature of the oxygen purifier

and if necessary bring its temperature to 500° C.

(Caution: The temperature and oxygen pressure

should never be allowed to exceed values consistent with safety with respect to the possibility of

explosive failure of the purifier. The manufacturer's recommendations regarding safety precautions should be adhered to rigidly.) Add water, if necessary, to fill the calorimeter jacket bath, bring

its temperature to the desired standard value and

put the thermostat into operation (see sec. 3.2.C.).

4.2. Preparation and Weighing of Sample of

Benzoic Acid

Weigh out approximately an amount of benzoic

acid somewhat in excess of 1.52 g and compress

it into a pellet. Weigh the pellet on a balance

sensitive to about 0.01 g and adjust the weight, if necessary, to 1.52 ±0.01 g (see footnote 11) by

scraping the pellet. Carefully remove any powdered benzoic acid from the pellet by brushing.

Weigh the crucible on the semimicro balance,

first empty and then after placing the pellet of

benzoic acid into it, following the procedure

described in section 3. 2. a,

4.3. Preparation of Bomb

Test insulation resistance. If less than lO*' ohms, dry mica or replace cone insulator.

Attach fuse wire to platimmr leads as described

in the appendix.^- Measure 1.00 cm^ of distilled water into the bomb by means of a buret or pipet.

Attach the crucible to its support in such a manner

that the fuse is nearly touching the pellet of benzoic

acid and assemble the bomb. Connect the bomb

11 The standard mass of benzoic acid should be such as to give a temperature rise of approximately 3° C. The mass used should be within 0.01 g of the standard mass.

12 It may be convenient to do this while waiting for the attainment of temperature equilibrium in the balance case in coimection with weighing the sample of combustible.

bomb washings after titration, using the procedure ^ described in ASTM Designation D129-58 [14]. The correction for nitric acid is 14.0 kcal/mole of

acid formed. If sulfur is present in the fuel the

total acid is corrected for as if it were all nitric , ( acid, and an additional correction of 1,400 cal/g ji of sulfur is applied. : i

) 1 Bomb-Calorimetric Experiment i

to the oxygen cylinder through the purifier and'

admit oxygen to a pressure of approximately 30

atm. The oxygen should be admitted slowly so as j not to cool the copper oxide in the oxygen purifier

below the temperature at which it will oxidize

combustible impurities in the oxygen. Close the

valve in the oxygen line and open the needle valve on the bomb, allowing the oxygen to escape until

the pressure is reduced to atmospheric. This

procedure removes about 97 percent of the nitrogen

in the air initially in the bomb. Refill the bomb to the pressure of approximately 30 atm, read and

record the temperature and pressure, and disconnect the bomb from the oxygen tank. Attach the

binding post (fig. 6(b)) with the lead wire to the

terminal nut on the bomb.

4.4. Weighing of Calorimeter Plus Water

Fill the calorimeter with water, adjust temperature of water, and weigh calorimeter plus water asj

described in section 3.2.b.

j

4.5. Assembly of Calorimeter

Immediately after the calorimeter is weighed'

carry it to the jacket and place it in its proper

|

position in the jacket space. Place the bomb in

the calorimeter,'* put the calorimeter cover in! place and press it down until its lower surface is in

contact with the water. These operations should

be performed rapidly and as nearly as possible in

the same manner and in the same length of time in

all experiments. Complete the firing circuit by

connecting the lead to the bomb electrode to the

binding post provided for this purpose (fig. l.(c))

and connect the other lead (already attached to the other binding post) to the calorimeter vessel

by means of the clip shown in figure l.(d) (see;

appendix). Close the jacket cover and connect;

the calorimeter stu-rer to its driving shaft. Measure the jacket temperature to 0.001° C (0.0001

ohm) by means of the bridge and resistance thermometer, then remove the thermometer from the

jacket bath, dry it, and put it in place in the

calorimeter.

15 The bomb should be lowered into the calorimeter without touching the water with the fingers. This can be done by using some sort of hook on which

the bomb can be hung and which can be removed after the bomb is in place. A hook made of a piece of brass or nichi'ome l od about He in. in diameter and

bent into the form shown in fig. 6(f) has been found satisfactory for this purpose with the Parr bomb. The hooked ends of the rod are inserted into holes on opposite sides of the screw cap of the bomb, and are easily removed after the bomb is in place. The amount of water which adheres to the hook when

it is withdrawn from the calorimeter is negligible.

10
4.6. Adjustment of Initial Temperature of

Calorimeter

Adjust the calorimeter temperatm-e to 0.05°

(0.005 ohm) below the starting temperature of the

initial period, and allow it to di'ift up to the start- ing temperature as described in section 3.2.c. The starting temperature (in caUbration experiments) always has the same value, which is so chosen that the final temperature of the calorimeter after combustion \vill be at, or preferably a few hundredths of a degree below, the temperature

of the jacket.

4.7. Observation of Temperature and

Ignition of Sample

After the temperatm-e of the calorimeter has

been allowed to drift up to the starting temperature, make and record the readings of time and

temperature of the initial period each minute as described in section 3.2.c. Immediately after the

last reading of the initial period fire the charge by

closing the circuit through the fuse in the bomb.

Observe the ammeter in this cu'cuit when the

charge is fii'ed. The reading of the ammeter

should rise abruptly to about 5 amp and then drop

almost instantly to zero. Failure of the current

to drop immediateh^ indicates a short cu'cuit. and

the charge will probably not be ignited, or if it is ignited the result of the experiment will be in error by an unknown amount because of the excess

electrical energy. If the ignition takes place

normally, make and record the observations of the

middle and final periods as described in section

3.2.C. After the end of the final period remove the

thermometer from the caloruiieter, insert it in the

jacket bath and measiu-e the temperature of this

bath to 0.001°. If the temperature control equipment has been operating properly the jacket temperature should remain constant to 0.005° or better

during an experiment.

4.8. Analysis of Contents of Bomb

Remove the bomb from the calorimeter, dry it, open the needle valve and allow the gas to escape

at a rate such as to reduce the pressure to atmospheric in not less than 1 min. Then open the

bomb and examine the interior for unburned

carbon, which would indicate incomplete combustion. If more than a slight trace of unbmned

carbon is found the experiment should be rejected.

If no appreciable amount of unburned carbon is found wash all interior sm'faces of the bomb mth

distilled water and collect the washings quantitatively in a beaker. The quantity of the washings should preferably not exceed 150 ml. Titrate the washings against a 0.1 solution of

NaOH using methyl orange as the indicator.

Calculate the correction for nitric acid as described in section 5 on calculation of results.

The procedm'e in a measm-ement of heat of

combustion follows the same steps outlined in

paragraphs 4.1 to 4.8, with certain differences

in individual steps as indicated below.

4. 2. a. Preparation and Weighing of Sample of Fuel

If the material whose heat of combustion is to be

determined is a sufficiently nonvolatile liquid,

weigh the empty crucible by the method of

substitution as described previously, decrease

the weights on the pan with the crucible by

the desired weight of sample,^' and add the

hquid drop-wise from a pipet imtil the weights

plus sample plus crucible balance the tare weight on

the opposite pan. Ordinarily this procedm-e wiU

add more than the desired weight of sample, and

the excess may be removed, for example, by dipping the end of a fine glass capillary into the

liquid. Great care should be taken during the

introduction of the sample into the crucible and

maldng the final adjustment of the weight to avoid getting any of the sample on the outside

of the crucible. The final weighing of crucible

plus sample is carried out as described in section

3.2.a. If the material whose heat of combustion is to be determined is a volatile liquid, the enclosure

of the sample in a glass bulb (as described in the

appendix) and the determination of its weight

(as described in sec. 3. 2. a.) will ordinarily have

been done prior to the beginning of the calorimetric measurements. It is then only necessary to place the bulb in the crucible before starting the

procedure described in section 4.3. and 4. 3. a.

4.,?. a. Preparation of Bomb

The procedure here is the same as that described

in section 4.3 except that if the sample contains

sulfur the ah initially in the bomb is not flushed

out before filling the bomb -with oxygen. If the

sample does not contain sulfm* the air should be

flushed out as described in section 4.3 by filling

the bomb with oxygen to 30 atm, allowing the

gas to escape until the pressm'e is reduced to atmospheric, and then refilling the bomb to 30

atm. Wlaere the sample is enclosed in a glass

bulb, premature breakage of the bulb when the

pressure :s fu'st raised to 30 atm can sometimes

be detected by the odor of the fuel in the escaping

gas when the pressm^e is released.

4. 6. a. Adjustment of Initial Temperature of Calorimeter

In the case of a nonvolatile liquid where the

weight of sample is such as to give a temperatm-e

rise equal to that in a calibration experiment

the adjustment of the initial calorimeter temperature is carried out in the same manner as described

in section 4.6.

n Tlie weight of sample should he such as to give approximately the same

temperature rise of the calorimeter as in the cahbration experiments.

11
In the case of volatile samples it is difficult to make the glass sample-bulbs sufficiently uniform

in capacity to give the same temperature rise of

the calorimeter in all experiments. Select the

initial temperature for each experiment so that

the mean of the initial and final temperatures of

the calorimeter will be the same as in the calibration experiments.

4»8.a. Analysis of Contents of Bomb

If the sample contains no sulfur follow procedm"e described in section 4.8. If the sample contains sulfur titrate the bomb washings to determine total acid is described in section 4.8. After

this titration analyze the bomb washings for sulfur

using the procedure described in ASTM Designation D129-58 [14].

5. Calculation of Results

5,1. Calibration Experiment

Calculate a value for the energy equivalent E

fi'om the data of each calibration experiment,

using the following formula:

At

where (?c=heat of combustion of benzoic acid in

IT calories per gram imder the conditions of the experiment,

weight in air, in grams, of sample of

benzoic acid bm-ned.

Ci =correction for ffi'ing energy, in calories.

C2=coiTection for nitric acid, in calories. Ai= corrected temperature rise of calori- meter, in degrees Celsius (or ohms).

The value of E will be in calories per unit of the

temperatm-e scale used, e.g., in calories per deg C,

or calories per ohm.

a. Heat of Combustion of Benzoic Acid

The certffied value for the heat of combustion

of the present standard sample (39h) of benzoic

acid under the standard conditions of the bomb

process is 6318.3 IT cal/g weight in air.^*^ The

standard conditions are: A. The combustion reaction is referred to 25° C.

B. The sample is bmned in a bomb of constant

volume in pure oxygen at an initial pressure of

30 atm at 25° C.

C. The munber of grams of sample burned is equal to three times the internal volume of the

bomb in liters. D. The number of grams of water placed in. the

bomb before combustion is equal to tlu-ee times

the internal volume of the bomb in liters. If the conditions in the experiment differ from

the standard conditions A, B, C, and D above,

the value of Qc for the heat of combustion of

benzoic acid under the actual conditions of the

experiment will be obtained from the following

approximate equation, which was derived in

accordance with the procedure recommended by

Washburn [18]: "The value for the heat of combustion of benzoic acid is given here in IT

calories per gram weight in air against brass weights, the densities of air, brass, and benzoic acid being taken as 0.00116, 8.4, and 1.320 g/cm', respecively.

Q,-6318.3 ^l + 10-«

1^20 (^P-30^4-42 (^-^-3^

+30(=^-3)-45(<-25)]}

where P=initial absolute pressure of oxygen, in

atmospheres, at the temperature t, weight of sample, in grams,

Wu,= weight in grams, of water placed in

bomb before combustion,

y=internal volume of bomb in liters,

i= temperature to which the combustion

reaction is referred, in degrees C.

The temperatm*e to which the reaction is referred

is here taken as the final temperature of the

calorimeter. The value of the energy equivalent

E obtained from eq (1) is then that of the initial calorimetric system at the mean temperature of

the calorimeter. By the initial calorimetric sys- tem is meant the system before combustion of the

benzoic acid, i.e., it includes the benzoic acid,

water, and oxygen in the bomb before combustion

takes place.

For example let the following data be given:

Volume of bomb,

V

Weight of benzoic

acid, Ms

Weight of water

in bomb, m„ = Initial oxygen

pressure

= 0.380 liter

a.522g; m,/F=4.01g/liter

a.OOg; 7n„/F=2.63g/liter

= 30.8 atm gage at 25.5° C

= 31.8 atm abs at 298.7° K

Final temperature

of calorimeter, ^=28.00° C=301.2° K

Oxygen pressure

at 28.00° C, P=31.8X

301.2

298.7 =32.1 atm absolute

Substituting in eq (2) we obtain = 6318.3{ l-M0-«[20(32. 1-30) +42(4.01-3)-]-

30(2.63-3)-45(28.00-25.00)]} = 6317.9 cal/g in air.

12
b. Correction Terms Ci and C2 The correction Ci for firing energy is preferably

determined in a series of blank experiments as described in section 3.2.d. The use of the value

Ci =5.2 cal will not lead to a significant error if the fuse is a 2-cm length of Parr fuse wire wound

into a helix and attached to platinum leads. In some cases a greater length (5 cm or more) of

fuse wire is attached to electrodes which are located a short distance away from the crucible. In such

cases the fuse may not all biu"n, and the firing

energy will be different in different experiments

depending upon the length of whe bm-ned. If this procedm'e is followed, the length of wire

burned ia each experiment must be determined

and the correction Ci may be taken as 2.6 Xi

calories, where L is the length of wire biu-ned,

in centimeters.

The con-ection C2 for nitric acid is 14.0 kcal/mole

of nitric acid formed, or 14.0 cal/cm^ of sodium

hych-oxide solution of normality N required to neutralize the acid. Thus if 2.4 cm^ of 0.0987

sodium hydroxide is requued to neutralize the

nitric acid formed, C2 =2.4X14.0X0.0987 =3.3 cal.

c. Corrected Temperature Rise

The calculation of the corrected change in re- sistance of the thermometer in a bomb calorimetric experiment will be illustrated by carrying out

the calculations for the experiment in which the

data given in table 1 were obtained^ As stated

in section 3.2.c. the initial, middle, and final periods in this experiment extend from t=0 to if=6,

t=6 to i=18, and t=l8 to f=28 min, respectively.

The observed change in resistance of the thermometer is taken as the difference between the

bridge readings at ^=6 and t=18 min, namely,

0.76540-0.47397 =0.29143 ohm. This difference

is to be corrected for thermal leakage and heat

of stirring. Correct methods for calculating this

correction will be found in reference [3] and in a number of the references there cited. For the

accuracy aimed at in this manual it is sufficient to use the following procedure which has been

found empirically to give correct results within

about 0.01 percent of the total resistance change.

We first calculate the average rates of change of

resistance ri and ro in the initial and final periods,

respectively. These are seen to be

0.47.397-0.4701.5 0.00.382 ^ , ri= = r =0.000637 ohm mm

6 b

0.76606-0.76540 0.00066 ^ aaaa^^ u r2= =—Jo—=0.000066 ohm mm.

Next we calculate from the data of the middle

period the time tm at which 63 percent of the observed resistance change has taken place. The re- sistance at tm is given by that at the final reading

of the initial period (at f=6), 0.47397, plus 63 per- cent of the observed resistance change. In other

words

5^=0.47397+0.63X0.29143=0.65757 ohm.

Keferring to table 1 it is seen that this resistance

is reached at a time intermediate between 7.17

and 7.32 min, and by interpolation we obtain

tm=7.227 min. The corrected initial and final re- sistances, Ri (corr.) and Rf (corr.), are then obtained by applying corrections to the observed

resistance at t=6 and ^=18 min as follows:^'

Si (corr.)=i26 (obs)+ri(^^-6) =0.47397+ 0.000637 X 1.227 =0.474752

ohm,

Rf (corr.)=fii8 (obs)-ro(18-^„) =0.76540-0.000066X10.773 =0.764689

ohm.

The change in resistance corrected for thermal

leakage and heat of sturing is then

Rf (corr.)-i?i (corr.) =0.764689

-0.474752 =0.289937 ohm.

Practically the same result would have been obtained if only three readings had been taken during

the period of very rapid temperature rise, say at i?=0.6300, 0.6600, and 0.6900 ohm.

This result must be corrected for errors in the

bridge coils by applying a correction equal to the

algebraic sum of the certificate corrections to the

cods used in obtaining Ris (obs) less the algebraic

sum of the corrections to the cods used in obtaining

Re (obs.). It is evident that it is not necessary in

this calculation of the change in resistance to in- clude corrections to cods which enter into both

i?6 (obs) and Ris (obs). In this case such coils in- clude one of 20.5 ohms and one of 7 ohms. The

bridge correction in the experiment under consideration was +0.000029 ohm, making the cor- rected resistance change Ai?= 0.289966 ohm.

Resistance of the thermometer can be converted

to temperatm-e with the aid of the formula or

tables accompanying the NBS certificate for the

thermometer. It is somewhat more convenient,

however, to compute a table of factors At/AR

which when multiplied by the corrected change in

resistance AR will give the desu-ed change in temperature At. Such a table may be calculated as foUows. First the temperatures corresponding to certain given resistances, say i?=27.5000, 27.6000,

. . . - 28.5000. . . , are calculated, using the data in the certificate for the thermometer. These temperatm'es are then substituted into the formula ^ 1 AR p r. , 5 '^5t -] (sy

^"^""L ioo~iooooJ

18 Resistances are read to lln the fifth decimal. The calculations are carried one place farther to avoid errors from roimding off. " Strictly speaking, eq (3) gives the derivative di/dR. This is exactly equal

to Ai/Afl for a finite interval AR, provided it is calculated for the mean temperature of the interval. If, as is customary, the value of dt/dR corresponds

to the mean resistance of a finite interval, then dt/dR is not exactly equal to

Af/Afl, but the error is entirely negligible.

13
where Ro, ce, and 5 ai-e constants given in the cali- bration certificate for the particular thermometer.

The values of R and corresponding values of

At/AR calculated from the above formula are tabulated. The value of At/AR to use in calculating

the corrected temperature rise in a given experiment is that corresponding to the mean of the

coi'rected initial and final values of R for the

experiment. This value of At/AR is obtained from

the table, by interpolation if necessary. For ex- ample, in the example given above the value of

At/AR used should be that corresponding to the

mean of 27.5 +j??i (corr.) and 27. 5+^/ (corr.) or

27.5+(0.47475 +0.76469)/2 =27.5 +0.6197 =28.1197 ohms.'«

It is found that At/AR increases by about 0.03

percent for an increase in temperature of 1 deg C

(0.1 ohm).

It is not necessary, however, to convert re- sistance to temperature. If desired, the re- sistance change can be taken as the measm-e of

the temperature change, with the energy equivalent E expressed in caloreis per ohm. The

corrected resistance change AR (ohms) in a meas- urement of heat of combustion would then be

multiplied by E (calories per ohm) to give the

energy (calories) produced in the combustion

reaction. One disadvantage of this procedure is that E in calories per ohm changes more for a given change in the mean temperature of the

experiment than does E in calories per degree.

For a typical bomb calorimeter it has been

calculated that the change in the value of E for an increase of one degi'ee (0.1 ohm) in the mean

temperature of the experunent is about -1-0.03 percent if E is expressed in calories per ohm,

but is less than 0.01 percent if E is expressed in

calories per degree. Hence if temperatures are expressed in ohms, then for a precision of 0.01

percent, the mean temperature of the calorimeter

should be kept ^vithin about 0.2 degree (0.02

ohm) of some fixed value, or else the value of

Es in each calibration experiment should be cor- rected to some standard temperature, and the

mean value of Es for this standard temperature

should be corrected to the actual mean temperature of each measurement of heat of combustion.

The greater change with temperature of Eg ex- pressed in calories per ohm is due to the fact that

the resistance of the thermometer is not a linear,

function of temperature, i.e., the fact that At/AR

is not constant. A convenient check on the behavior of the

calorimeter and the correctness of the temperature

measurements may be obtained by calculating

the "cooling constant," k, of the calorimeter.

This is defined by

k--

•« strictly speaking, the corrections to all the coils used should be applied

in calculating the mean value of the resistance. In many cases, however,

these corrections are too small to be significant in this calculation and may

be neglected.

R2— R\

where and are the average rates of change

of resistance in the initial and final periods,

respectively, and R^ and R^ are the observed

resistances at the mid-points of these periods.

Thus, using the data of table 1 at the beginning

of this section (5.1c):

k=0.000637-0.000066

0.76573 - 0.47206

=0.001944.

This quantity is a constant of the calorimeter

and should remain constant to better than one

percent in different experiments.

d. Example Illustrating the Calculations of E and E,

The calculation of E may be illustrated by an

example, using the following data taken from

preceding sections as indicated.

Weight benzoic acid, Ws=l.52188 g (sec. 3.2 a)

Heat of combustion of benzoic acid, (?c=6317.9

cal/g (sec. 5.1. a)

Firmg energy (2 cm fuse wu-e), Ci=5.2 cal (sec. 5.1.b)

Correction for nitric acid, C2=3.3 cal (sec. 5.1.b)

Corrected resistance change, Ai?= 0.289966 ohm

(sec. 5.1.C)

Corrected temperatm-e change, Ai=2.87462 deg

C (Ai/AS=9.91364)

Oxygen pressure at 28.0°, P=32.1 atm abs

(sec. 5.1. a)

Substituting the above values in eq (1), the value

of E is found to be „ 1.521 88X6317.9+5.2-^3.3 E= ^^.^^^

^ ^ „ — =33188.7 cal/ohm

0.289966

1.52188X6317.9+5.24-3.3=3347.78 cal/deg.

2.87462

To reduce the above values of E to the cor- responding values of E^, the energy equivalent of

the standard calorimeter system, it is only neces- sary to subtract the heat capacity of the sample

of benzoic acid, and of the amount of oxygen in excess of that requu*ed to fill the bomb to 30 atm

absolute at 28° C, as follows:

cal/° C cal/ohm

Observed value oi E-.- - 3, 347. 78

0. 16 0.44

3,347.18

33, 188.

7

1.6

4.4

33,182.7

Heat capacity of excess oxygen= (32.1-30.0) X0.077-- Heat capacity of benzoic acid= 1.622X0.29= Value of a

The factor 0.077 used in calculating the correction for excess oxygen is the increase in heat

capacity (in calories per degree) per atmosphere

14
increase in pressure in the bomb used, which has an internal volume of 0.380 liter. For a bomb

of volume V the corresponding factor is 0.077

F/0.380. The factor 0.29 used in correcting for the benzoic acid is the specific heat of benzoic

acid in calories per gram degree Celsius. Both

factors are greater by a factor of 10 if £" is in

calories per ohm (for a thermometer having a

resistance of 25.5 ohms at 0° C).

5.2. Calculation of Heat of Combustion

Calculate a value of total (or gross) heat of combustion at constant voliune from the data

of each measurement, using the following formula.

Q, (gross)

EXAt—Ci —C2—C3

(4)

where

:

Qf (gross) =total heat of combustion at constant

volume at the final temperatme of

the experiment, in IT calories per

gram.

£=energy equivalent of the calorimeter as used, in IT cal/deg (or IT cal/ohm)

Ai= corrected temperature rise of calorimeter in degrees (or ohms^

m,=mass of sample, in grams

Ci =fii'ing energy, in calories

C2= correction for energy of formation of

nitric acid, in calories

C3= correction for energy of formation of sulfuric acid, in calories.

a. Energy Equivalent of Calorimeter as Used

The calorimeter as used differs from the standard calorimeter because it contains the sample of

fuel and because, in general, the absolute pressure

of the oxygen at 28° differs from 30.0 atm.

The energy equivalent E of the calorimeter as used

is obtained by adding to the energy equivalent

Es of the standard calorimeter the heat capacity

of the charge of combustible and the heat capacity

of the oxygen in excess of that required to fill the

bomb to 30.0 atm absolute at 28° C. The calculation is illustrated as follows, taking the sample

of combustible as 0.92231 g of gasoline, and the

oxygen pressure at 28.0° as 29.0 atm absolute:

cal/°C cal/ohm

Energy equivalent of standard calorimeter, E,-.- =

Heat capacity of excess oxygen, (29.0-30.0) X0.077- =

Heat capacity of sample, 0.922X0.51 =

Energy equivalent of calorimeter as used, E=

3, 347. 18 -.08

+.47

33, 182. 7 -0.8 +4.7

3, 347. 57 33, 186. 6

The factor 0.51 used in calculating the heat

capacity of the sample is the specific heat of

gasoline at 26.5°C in cal/g°C. The factor

0.077 used in calculating the heat capacity of the

excess oxygen is defined in section S.l.d. Both

factors are greater by a factor of 10 if E^ is in

calories per ohm (for a thermometer having a

resistance of 25.5 ohms at 0°C). The heat

capacity of the glass bulb (less than 0.1 g) used

to enclose the sample of gasoline may be neglected.

b. Corrected Temperature Rise

Directions for calculating the corrected temperature rise are given in section 5. I.e. The value

to be used in the example to be given later may

be taken as 0.309617 ohm or 3.06943°C.

c. Corrections Ci, C2, C3 The correction Ci for firing energy with a 2-cm

length of fuse wire may be taken as 5.2 cal. The

corrections Co and C3 are calculated as follows:

The correction C2 is calculated as if all the acid

found by titration were nitric acid, as described in section 5.1.b. Thus if it required 14.2 ml of

0.0987 sodium hydroxide to neutralize the total acid the correction C2 would be given by C2=

14.2X14.0X0.0987 = 19.6 cal. If the test for sulfm* showed 0.0042 g sulfur present in the bomb

washings the correction for sulfuric acid, as given

in section 3.2.f., would be C3=1400X0.0042 = 5.9

cal.

d. Calculation of Qv (gross)

Substituting the values of E in calories per

degree (or calories per ohm), (or AR), Ci, C2, C3, and given in sections 5.2. a., b., and c. into formula (4) yields

:

Q, (gross)

3347.57X3.06943-5.2- 19.6-5.9

0.92231

or

0^ (gross)

=

33186.6X0.309617-5.2 —19.6-5.9'

0.92231

Both of these expressions yield the value 11107.4

cal/g weight in air for the total (or gross) heat of

combustion of the liquid gasoline at constant

volume. The corresponding value in terms of

engineering units is

(gross) = 11107.4X1.8 = 19993 Btu/lb.

The method of calculation is such that the value

obtained for Q„ (gi'oss) is referred to the final temperature of the calorimeter, in this case

28.1° C. The observed value may be reduced tO; the standard temperatm-e of 25° by means of the,

following formula

:

n Taken from NBS Miscellaneous Publication M97 [15] for a gasoline' having an API gravity of 70. Values of heat capacity of petroleum products

given in this publication for a temperature of 80° F range from 0.42 to 0.52 for API gravities ranging from 10 to 80. 15
Q„ (gross, 25° 0= ^ (gross, t° C)+.4(f-25) (5) Where the value of the I'actor A is given iii the

following table.

A ''v \S_TOSS)

Btv/lb Btujlb" C Bill/lb Btvllb" C

18, 500 0. 68 19, 600 1.14

18, 600 .72 19, 700 1. 18

18, 700 .76 19, 800 1.22

18, 800 .80 19. 900 1.26

18. 900 .85 20, 000 1.30

19, 000 .89 20, 100 1.35

19, 100 .93 20, 200 1.39

19, 200 .97 20, 300 1.43

19, 300 1.01 20, 400 1.47

19, 400 1.05 20, 500 1.51

19, 500 1.09 20, 600 1.55

Substituting the value of (gross, 28.1° C) =

19993 Btu/lb and the corresponding value A=1.S0

from the above table in eq (5) we obtain

Q, (gross, 25° C) = 19993 + 1.30X3.1 =

19997 Btu/lb.

MTbe values of A were calculated using the specific heats of liquid petroleum

fuels, of gaseous o.xygcn and carbon dioxide, and of liquid water. The masses

of oxygen consumed and of carbon dioxide and water formed in combustion of unit mass of fuel were obtained from an empirical relation between Q, (gross) and hydrogen content. See reference [5], In general, the change in Q, gross

(Btii/lb) with temperature for any fuel may be calculated from the relation do, (gross)/dt=K, (reactants) —A', (products). Here K, (reactants) denotes the heat capacity at constant volume of 1 lb

of fuel plus the oxygen consumed in its combustion, and K, (products) denotes the heat capacity of the products of combustion of one pound of fuel, including gaseous carbon dioxide and liquid water. In calculating K, the heat capacities of solids and liquids may be taken as heat capacities at con- stant pressure.

e. Calculation of Op (i^et)

If the hydrogen content of the fuel is known the

value of the net heat of combustion, Op (net, 25°), can be calculated from that for 0^ (gross) by

means of the following formula [4]

:

Qp (net, 25° C) = Q, (gross, 25° C)- (6)

91.23 (%H)

Thus if (gross, 25° C) = 19997 Btu/lb and

(%H) = 13.9, we obtain

Op (net, 25° C) = 19997-91.23X13.9 =

18729 Btu/lb.

If the hydrogen content of the fuel is not known,

an approximate value for Op (net, 25° C) can be

calciilated from the following empirical relation

:

Qp (net, 25° C) (7) =4310 +0.7195 Ov (gross, 25° C).

Substituting the value Q, (gross, 25° C) = 19997

Btu/lb in this equation we obtain

Qp (net, 25° C)=4310+0.7195X19997 = 18698 Btu/lb (approximate)

Equation (6) is preferable to eq (7) for calculating

net from gross heat of combustion, since the latter equation is purely empirical. A fairly accurate

value of (%H) is required in eq (6), since an error

of 0.1 in this (quantity introduces an error of about

0.05 percent in Qp (net).

6. References

[1] ASTM Method D 240-57T. ASTM Standards 7, 143, (1958).

[2] ASTM Method D 271-58, ASTM Standards, 8, 999 (1958).

[3] F. D. Rossini, Experimental thermochemistry (Inter- science Publishers, New York, N.Y., 1956), and the

references there cited.

[4] R. S. Jessup and C. S. Cragoe, Nat. Advisory Comm.

Aeronaut. Tech. Note No. 996 (1945) (copies not

available)

.

[5] S. Rothberg and R. S. Jessup, Ind. Eng. Chem. 43, 981 (1951).

[6] R. S. Jessup and J. A. Cogliano, ASTM BuU. No. 201

(Oct. 1954).

[71 G. T. Armstrong, R. S. Jessup, and T. W. Mears,

Chem. Eng. Data Ser. 3, 20 (1958).

[8] H. C. Dickinson, Bui. BS 11, 189 (1914) S230.

[9] N. S. Osborne, H. F. Stimson, and T. S. Sligh, Bui. BS 20, 119 (1925) S503.

[10] C. H. Mevers, J. Am. Chem. Soc. 45, 2135 (1923).

[11] T. W. Richards and F. Barry, J. Am. Chera. Soc. 37, 993 (1915).

[12] H. N. Davis, Mech. Eng. 51, 791 (1929).

[13] See for example:

(a) N. Irving Sax, Handbook of dangerous materials (Reinhold Publishing Corp., New York,

N.Y., 1951).

(b) Manufacturing Chemists Association, General

Safety Committee, Guide for safety in the chemical laboratory (D. Van Nostrand and

Co., New York, N.Y., 1954).

(c) National Safety Council, Accident prevention

manual for industrial operations (Chicago,

111., 1951).

(d) National Board of Fire Underwriters, National

electrical code (Chicago, 111., 1940).

[14] ASTM Method D 129-58, ASTM Standards 8, 81 (1955).

[15] C. S. Cragoe, Thermal Properties of Petroleum Products, NBS Misc. Publ. M97 (this paper is out of print but may be found in many public hbraries).

[16] E. J. Prosen and F. D. Rossini, J. Research NBS 37,

289 (1941) RP1420.

[17] J. Coops, D. Mulder, J. W. Dienske, and J. Smittenberg, Rec. trav. chim. 66, 153 (1947).

[18] E. W. Washburn, J. Research NBS 10, 525 (1933)

RP546; see also chapters 5 and 6 of reference [3]. [19] F. K. Harris, Electrical measurements, p. 57 (John

Wiley & Sons, Inc., New York, N.Y., 1951).

[20] E. J. Prosen, W. H. Johnson, and Florence Y. Pergiel,

J. Research NBS 62, 43 (1959) RP2927.

16
7. Appendix

7.1. Apparatus

The major items of apparatus used in bombcalorimetric measurements on fuels at the National

Bureau of Standards are listed in table 2. On the

basis of prices hsted in manufacturers' catalogs at the present time (1959), the estimated total cost

of replacement of the equipment listed is in the

neighborhood of $15,000.

Table 2. Major items cf equipment used in measurement

of heats of combustion of liquid fuels at the National Bureau of Standards

Mueller bridge. Eesistance thermometer. Galvanometer.

Calorimeter, jacket, and thermostat. Combustion bomb.

Platinum crucibles and fittings. Balance.

Set of class M weights for above. Heavy duty balance

Set of weights for above. Oxygen purifier. Temperature controller for o.xygen

purifier. Laboratory table for bridge. Laboratory bench.

Pellet press. Reducing valve. Julius suspension.

The approximate arrangement of the calorimeter

and accessory apparatus is shown in figure 4. The

reading station E provides support for the lamp L

and ground glass scale S, as well as for some electrical equipment for measm-ements not covered in

this Monograph. It also contains a convenient

"desk" for use in recording the data of bomb calo- rimetric experiments. The arrangement of the

equipment for purifying oxygen and admitting it to the bomb is shown schematically in figure 5. The arrangements shown in figures 4 and 5 are not necessarily the best under all circumstances.

For example, the bench BE (fig. 4) is considerably

larger than necessary for the calorimeter alone, as part of this table is used for other purposes.

The essential features of the various items of apparatus are descrilied below. Items not obtainable commercially, and modifications of commercial items are described in considerable detail.

FEET

Figure 5. Arrangement of apparatus for adynitting oxygen

to bomb.

B, bomb; D, dryer; G, pressure gage; N, needle valve; 0, o.xygen cylinder; P, oxygen pm'ifier; and K, reducing valve.

a. Bridge

The bridge used is a special Mueller bridge

designed especially for bomb-calorimetric measurements. It is substantially equivalent to the commercially available bridge designated as 0-2. In

addition to the usual 10-, 20-, 30-ohm coils of

the lOXlO-ohm decade this bridge has a 20.5-ohm

coil. Such a coil is useful in bomb-calorimetric

measurements, where the change m resistance of

the thermometer in an experiment is of the order

of 0.3 ohm, since it makes it possible to avoid a change in setting of the dial of the lOXl-ohm

decade which would otherwise be necessary in

certain circumstances. Thus if the initial and

final resistances of the thermometer are 27.8721

BR BR

I I I I I I I I I I I 0 1 2 3 4 5 FEET

Figure 4. Arrangement of calorimetric apparatus. BE, bench; C, calorimeter; BR, bridge: G, galvanometer; L, light source; M, stirring motor; R, reading station; and S, ground glass scale.

17
and 28.1806, the corresponding dial settings with

the 20.5-ohm coil would be 20.5 +7.3721 and

20.5 +7.6806 and the change in dial setting would

involve only the 10X0.1 -ohm and lower decades.

If no 20.5-ohm coil were available, it would be

necessary to change the dial setting of the 10X1-

ohm decade from 7 to 8 at a time when the temperature of the calorimeter was rising very rapidly.

This would greatly increase the difficulty of following the temperature of the calorimeter during the

period of rapid temperature rise, and might also involve a larger correction to the observed change

in resistance of the thermometer.

b. Thermometer

The platinum resistance thermometer has a

resistance of approximately 25.5 ohms at 0° C.

It is of the strain-free type with coU windings very

close to the wall of the enclosing glass tube. It was purchased under a specified requirement that

the temperature of the coil with a cm'rent of 1 ma

flowing tht'ough it should not exceed that of the

bath in which the thermometer is immersed by more than 0.0003° C. Calorimetric resistance

thermometers enclosed in metal sheaths are available commercially. These are less subject to breakage than glass-enclosed thermometers, but

are subject to damage from bending the metal

sheath. Such damage might not be obvious on

casual examination, but could be detected by

periodic determination of the ice point of the

thermometer.

MM M 'PPiSiP c. Galvanometer^ I^^T^S^v!?

The galvanometer has a sensitivity of about

0.2 juv/mm at 1 m, an external critical damping

resistance of 40 olnns and a period of about 5 sec. The damping resistance used is so chosen that the

galvanometer is slightly underdamped. The galvanometer is protected from air currents and

abrupt changes in temperature by an enclosure

made of cork pipe covering (for 6 in. pipe) about

10 in. in outside diameter. The galvanometer is supported by a Julms suspension, which is very

effective in reducing the effect of \'ibrations.

d. Bomb

The combustion bomb is made of a corrosionresistant alloy (lUium). It is provided with a check valve for admitting oxygen and also with a needle valve for releasing the gases contained in it. It is sealed by a self-sealing neoprene gasket.

The original bomb has been modified by replacing

the three supporting feet furnished with it by new

ones of monel metal about 12 mm in height. This

permits freer circulation of water under the bomb.

The base-metal electrode inside of the bomb,

and the crucible-support rod were cut off leaving

lengths of about 3 cm from the point at which they

enter the lock nuts. To the shortened electrode

21 Parr Instrument Company Catalog No. 1101.

was hard soldered a }{& in. platiniun rod about 3^ cm in length. To the shortened crucible-support 1^ rod was hard soldered a 5-cm length of % in. o.d,-

platinum tube having a 0.01 in. waU. The ar- rangement of electrode, crucible support, crucible,

electric fuse and 0.3 mm platinum leads thereto

is shown in figure 6(a) (valves not shown). An

5

enlarged view of the platinum crucible is shown

'

at (c) figure 6. The crucible is put in place by'

sliding the split tube i%2 in. o.d.) shown at the

right of figure 6(c) over the end of the % in. li platinum crucible-support tube referred to above | until the fuse is nearly in contact with the sample

of combustible. The split tube may be deformed

j slightly with the fingers to make it fit the ji in. »l tube tightly enough to be held in place by friction. 'J The length of the crucible-support, and the posi- 5 tion of the crucible on this support should be such ' that the crucible cannot come in contact with the

water in the bottom of the bomb when the bomb

is assembled. If the crucible should come in j contact with the water, combustion will be in- 3 complete.

(f) Figure 6. Bomb and accessories.

(a) Bomb, showing arrangement of crucible, crucible support, electrode,

etc. (Valves not sho\vii, see fig. 1); (b) Binding post and lead wire; (c) Enlarged view of crucible; (d) Platinum lead flattened at end; (e) Fuse wire

attached to platinum leads; and (f) Hook for handling bomb. A supply of platinum lead wire is wound around

the electrode and crucible support rods as shown,

and a sufficient amount of this to reach down to the

crucible is unwound for each experiment. The

electric fuse is a 2-cin length of Parr fuse wire

(No. 34 AWG chromel C) which is formed into a

small helix by winding it around a rod about 0.04

in. in diameter. The method of attaching the

fuse to the platinum leads is illustrated at (d) and

(e), figure 6. The ends of the lead wires are first flattened by hammering as shown at (d). The

flattened ends are then bent and the fuse inserted

as shown at (e), after which the turned-up ends

18
I of the lead wires are hammered down with a small

j hammer or squeezed together with pliers to clamp

the ends of the fuse wires.

The terminal nut attached to the electrode outside of the bomb was replaced by one of the form

shown at (a), figm'e 6. The binding post shown

at (b), figure 6. has a split tube at its lower end

which can be slipped over the upper part of the

terminal nut, where it fits tightly so as to make

good electrical contact. The lead wire shown, at

(b), figure 6, connects this binding post to one on

the calorimeter jacket which is connected in the

firing circuit. This wire is soldered to spade-type

terminals at each end, the larger of which normally

is permanently attached to the binding post shown

at (b). This wire should be so insulated as to avoid any electrical contact with the calorimeter

vessel. With alternating current the contact of the lead wire and bomb electrode with the water

in the calorimeter does not result in any appreciable electrical leakage, provided actual metalHc

contact with the calorimeter vessel is avoided.

e. Calorimeter and Jacket

The calorimeter now used for fuel testing is of

the type described by Dickinson [8]. The calorimeter vessel is of the form shown at (a) and (b),

figure 1 . The calorimeter cover is in the form of a flat circular disk with vertical wall about 1 cm

in height at the edge. The holes OT and OL (fig. 1(b)) are surrounded by metal collars about 1 cm

high. The cover fits tightly enough inside of the

calorimeter vessel so that it is held in place by

friction. Its lower surface is in contact with the

water of the calorimeter. The calorimeter vessel

is supported inside of the jacket by pins of stainless steel, in the manner described by Dickinson

The Dickinson type calorimeter was constructed

in the XBS shops. There is a commercially available "submaiine" calorimeter, based on a design

developed at the Xational Bureau of Standards

[20], which has been found to be substantially

equivalent in performance to the Dickinson calorimeter. The submarine calorimeter is supplied

I without a thermostat for the jacket bath, so that

I a thermostat for controlling the bath temperature

to a few thousandths of a degree must be purchased

or constructed.

The thermostat ^ used with the particular Dickinson type calorimeter used for fuel testing is similar to one previously described by Osborne,

Stimson, and Sligh [9]. It consists of a "bulb"

filled with toluene and connected to a glass capillary U-tube (1.5 mm i.d.) containing mercury.

Expansion of the toluene as a result of rising tem-

' 22 Catalog Xo. 6.3090, Precision Scientific Co., Chicago, Hi. This calorimeter embodies an improved method of supporting the calorimeter vessel inside

of the jacket [20).

23 Temperature-control units for which a control accuracy of 0.001° C. is claimed are available commercially. S'umerous other temperature-control

devices are described in the literature. See for example the papers c'ted in reference [3]. For any temperature-control unit it is important that the rela- tive locations of heater, stirrer, and temperature-sensing element should be

as shown in figure 1, that is, the bath liquid should flow from the heater, through the stirring propeller, and over the temperature-sensitive element by

the shortest path possible.

perature causes the mercury to make contact with

a needle closing an electrical circuit through an

electronic relay which shuts off the current in

the electric heater in the jacket bath. The "bulb"

TB (fig. 1) consists of about 3 m of in. copper

tubing with 0.01 in. wall thickness wound into a

helix and so located in the same tubular housing

as the jacket bath stirrer that the water flows over

the b\ilb immediately after leaving the stuTer. The glass U-tube and attached valve for admitting

or removing toluene are shown in figure 7. The

valve has tubular openings at the top and bottom

for attaching the reservou- B and the capillary

U-tube as described below.

FiGTJEE 7. Valve and glass parts of thermostat.

V, valve; TB, tube to thermostat bulb; B, toluene reservoir; F, capiUary

U-tube; E, enlargement in capillary; H, platiuum wii-e; L, leads to relay; C, Pyrex tube; D, brass cap; S, screw; and X, steel needle. The Pyi'ex reservoii' B contains a supply of

toluene which is separated from that in the "bulb"

TB (fig. 1 ) by the valve V when it is closed. This

valve is opened when the calorimeter is shut down

thus permitting toluene to flow from the reservoir

into the bulb as the bath cools. Failure to open

the valve Avhen the bath cools usuaUy results in

ah' being di^aAvn into the system thi'ough the

capillary F (fig. 7). The Pyrex bulb C (fig. 7) also contains toluene which protects the mercury

surface in the capfllaiy from oxidation. The en- largement E in the capfllary is larger in volume

than the mercury in the capillary so that no mercury will be di'awn into the metal parts of the

system if the valve V is inadvertently left closed

when the calorimeter is shut down.

2' American Instrument Company Catalog Xo. 4-5301.

19
Electrical connection between the Valve V and

the mercury in the capillary U-tube is provided by

a platinum wire H, long enough to reach from

the top of the capillary to near the bottom of the U

.

The upper end of the wire is flattened and bent at a right angle to the rest of the wire so as to insure

electrical contact with the valve.

The glass reservoir B and the capillary glass Utube are soldered into the tubular openings in the

brass valve Fwith pure tin following the procedure

described by Meyers [10]. To fill the thermostat with toluene the tube

C (with cap D removed) is connected to a large

bulb G above C (at least 250 cm^, not shown in

fig. 7) by means of a short piece of Tygon tubing.

Somewhat more than enough toluene to fill the

thermostat is placed in the bulb G, and some is also placed in the reservoir B. Air can be removed

from the thermostat system bv heating the

"bulb" (TB, fig. 1) to about 100° Cand repeatedly

evacuating the bulb G above the toluene and

then admitting air. To test for air in the system,

air is admitted above the toluene in G, the valve

V is opened, the toluene meniscus is brought

into the capillary at F, and the valve is closed.

If the system is free of air, moderate an- pressure

applied by mouth above the toluene will not

cause a movement of the meniscus in the capillary of more than 0.2 or 0.3 mm. The removal

of air from the system should be done before

introducing mercury into the capillary. If air

is inadvertently admitted to the system after mercury has been introduced into the capillary,

the mercury should be removed before attempting

to remove the air. In prejDaring for a calorimetric experiment the

jacket is fiLrst heated to the deshed temperature,

the heating current is reduced to slightly more

than enough to maintain the temperature constant, and the valve V is closed. Final adjustment of the temperature can be made by moving

the needle A' up or down by rotating the cap

D i-elative to the screw S (fig. 7). The heating

current should be adjusted so that the on and

oft" periods are about equal m length.

The method of bringing the firing leads through

the jacket is illustrated in figure l(c).^^ Two

metal rods, one of which is shown at B, are ce- mented with wax into tubes T wliich pass through

the jacket, so that the rods are insulated from the

jacket. Each rod is drilled and tapped at each

end for a 4-40 brass screw, which is used to clamp

the terminal of the lead wire between the two

washers, W, thus forming a binding post at each

end of the rod. The terminal at the left end of the

lead wire shown in figure 1(d) is attached per- manently to one of the inner binding posts and the

clip at the other end of the wire (d) is slipped over the edge of the calorimeter vessel when it is

This method is not applicable to some types of calorimeters, for example, submarme calorimeters. The important featm-e of the method is that the

leads are in good thermal contact with the jacket, but are electrically insulated from it.

assembled preparatory to making an experiment.

The other lead wire, from the binding post on the

bomb (see fig. 6(b)), is attached to the other inner

binding post after the bomb has been put in the

calorimeter and the calorimeter cover has been

put in place. The two outer binding posts are connected through an ammeter and a switch to a small transformer having a secondai-y voltage

of 8 or 10. The switch should De of the momentary

contact push-button type normally open except

when held closed by the operator.

f . Balance for Weighing Samples

The balance used for weighing samples of combustibles is an undamped semimicro balance

(capacity 100 g) having a sensitivity reciprocal

of 0.14 mg per division on the index scale. The

balance is provided with a keyboard arrangement

for adding and removing weights from 1 to 100 mg.

Temperature gradients parallel to the balance

beam are reduced by covering the walls of the

balance case inside and out with aluminum foil, E and by locating the balance where temperature ' gradients in the room are small. It has been

found, however, that best results are obtained

with the balance when there is a vertical temperature gradient such that the top of the balance

case is warmer by 0.03 to 0.1° C than the bottom.

|| The weights used with the balance are highgrade one-piece (class M) weights and were cali- brated in the Mass and Scale Section of the i' National Bureau of Standards.

For most purposes a good analytical balance

would be satisfactory if provided with high grade

calibrated weights, and properly used.

g. Balance for Weighing Calorimeter

This is a magnetically damped heavy duty

balance having a capacity of 5 kg and a sensitivity

of about 0.5 mg. The tantalum-plated weights

used with this balance are of good quality but

are not calibrated. Calibration was considered

unnecessar}' since the purpose of weighing the

calorimeter is not to determine its absolute weight,

but to reproduce the same weight of calorimeter

plus water for each experiment. For this purpose

it is important that the weights remain constant

in value, and that the same weights be used at all times, but the actual masses of the weights used

need not be known accurately.

h. Oxygen Purifier Combustible impurities in the oxygen are re- moved by passing it through a cylinder containing

cupric oxide maintained at a temperature of

500° C (932° F). The purifier consists essentially of a heavy-walled alloy-steel cylinder having

inside dimensions of approximately 1 in. diameter

by 10 in. length, and capable of withstanding

a pressure of at least 2500 psi at 500° C. The

cylinder is provided with an electric heater. It

American Instrument Company 1942 Catalog No. 406-31.

20
. has inlet and outlet at opposite ends, and a well

-j for inserting a thermocouple for measnrement of

ji temperature. The cyhnder is filled with wire-form

I cupric oxide. The temperature and ox^^gen pres- sure in the pm-ifier should not be allowed to rise

i above values consistent with safety. The manufacturers recommendations regarding safety pre- cautions should be adhered to rigidly. The

j strength of materials decreases rapidly with in- creasing temperature.

I The temperature of the purifier can be conj trolled by means of a "Fail safe" controller. Such

\ controllers are available commercially.^'' The

! pressure is easily controlled by a reducing valve

attached to the oxygen cylinder.

i. Laboratory Table for Bridge

This table should be large enough to accomf modate the bridge comfortably, and of a height

i which will afford convenience in the manipulation

of the bridge dials. A table 22 in. square by

18 in. high is convenient for use with a G-2 bridge

i as it allows space for batteries, etc

j. Pressure Gage

j The Bourdon pressure gage used has a 6 in. dial

' and is graduated from 0 to 800 psi in steps of 10

psi, so that it can be read to about 1 or 2 psi.

The gage was calibrated in the Mechanical Instru- ments Section of the NBS and was set so as to read correctly at 450 psi.

7.2. Glass Sample Bulbs

For measurements of heat of combustion of

kerosenes, gasolines, or other volatile liciuids

' it is necessary, in order to permit accurate weighing of the sample and to insure that it is all in the

liquid state until it is ignited in the bomb, to enclose the sample in a thin-walled glass bulb,

so constructed and filled that it will not break

I under the pressure of the oxj'gen. The type of

1 bulb 2* used in testing of volatile fuels at the

! National Bureau of Standards, and the method

I of making such bulbs are illustrated in figure 8.

I Attempts to obtain bulbs of this type from manu-

] facturers of glass laboratory apparatus have been

I unsuccessful to date, and such bulbs are therefore

made in our own laboratory. A brief description

of the method of making the bulbs will be given

I below. It should be recognized, however, that

' it is virtually impossible to describe the method

I in such detail as to enable an inexperienced

" worker to make bulbs successfully on his first

j attempt. The best that can be done is to outline

I the method, and to depend upon the individual

j worker to develop the necessary techniques by

; trial. It is to be expected that in the early

i attempts failures will far outweigh successes, and

that a reasonable degree of skill can be acquired

, only by long and patient pi'actice.

2' For example American Instrument Company Catalog No. 49-9570. Arthur S. Lapine and Company Catalog No. 357-70.

2i Bulbs of this type were first described by Richards and Barry [U].

The starting material for making bulbs is soft glass tubing 4 to 6 mm in outside diameter.^^

Approximately 2 cm of such a tube is heated in a moderately hot air-gas flame of an ordinary

laboratory blast burner. When the glass becomes

soft it is removed from the flame and drawn down

to a fine capillary having a minimum outside

diameter of about 1 mm. This procedure is then

repeated in such a manner as to leave a short

section of the same diameter as that of the original

tube, as shown at figure 8(a) . The two capillaries are then broken at about their midpoints, leaving a piece such as that shown at figure 8(b). The

remaining glass-working operations are carried

out with an air-gas micro blast burner capable of

producing relatively small flames. Using such a micro burner the piece illustrated at (b), figure 8,

is rotated and heated by a small hot flame at the

point indicated by the arrows, and is then drawn

down to 1 mm outside diameter. This procedure

is repeated on the other side of the enlargement in the tube, the final product being as illustrated at

(c) , figure 8 . This piece is then rotated and heated

by a small sharp flame at the center of the enlargement as indicated by the arrow at (c), and the

two halves of the piece are pulled apart as shown

at (d), figure 8, where the enlarged end of the

piece at the left has been reduced in size by removal

of surplus glass. This removal of surplus glass is accomplished by heating the end of the enlargement until it softens, and then pulling off small

filaments of glass until an enlargement of the

proper size remains. After such removal of

surplus glass from both of the pieces shown at

(d) a spherical bulb, not greater than 14 mm in

diameter, is blown on each piece (fig. 8(e)). The

spherical bulb is then flattened on one side by

holding the bulb above a small soft flame (no

primary air) about }i in. in diameter by % in. high,

and gradually lowering it until the glass on the

under side of the bulb softens and flattens. This

procedure is then repeated so as to flatten the

opposite side of the bulb. The final form of the

bulb is shown at figure 8(f). The operations of

bringing the bulb down on to the flame and

removing it after flattening is complete should be

done gradually, since too rapid heating or cooling

of the bulb may cause it to break.

The most difficult part of the procedm-e just

described is that of blowing a bulb of the proper

size. Success in this operation depends in part

upon having the proper amount of glass on the

end of the tube (fig. 8 (d)) and in part upon the

type of flame used, as well as upon the procedure

followed in blowing the bulb. A flame which

has been found to be satisfactory is one about

ji in. in diameter by % in. in height with just

enough primary au- so that there is a smaU yellow

tip in the flame. The end of the tube (fig. 8(d)) on which the bulb is to be blown is heated in the

29 Pyrex glass has been found unsatisfactory for this purpose, as unburned

carbon is usually found on such glass after a combustion. This may be

related to the fact that the Pyrex glass does not melt completely when the sample is burned, as does soft glass, and therefore partially retains its original form, thus inhibiting access of o.xygen to the fuel. 21
Figure 8. Steps in making sample bulbs.

flame until sufficiently hot, and is then removed

from the flame and the bulb blown immediately.

In order to prevent sagging of the end of the

tube when heated, the tube is held at an angle

of about 30° to the vertical and is rotated about

its axis continuously. Usually the first few at- tempts to blow a bulb will be unsuccessful because

the initial size of the bulb is so small that a rela- tively high pressure will be required to overcome

surface tension and the viscous resistance to flow of the glass. If the pressure is not high

enough the bulb will expand so slowly at first that the glass will harden before the desired size

is reached. If the pressure is high enough to expand the bulb at a sufficiently high initial rate,

the bulb may suddenly expand to too large a

size before the glass hardens. The ideal procedure

would be to blow very hard at first and then less so as the bulb becomes larger. This is very difficult to do because the whole operation must be carried

out in a very short time before the glass hardens.

It is therefore usually more successful to repeat

several times the procedure of blowing the bulb

to a diameter of 4 or 5 mm and then shrinking

it by heating, before attempting to blow the

bulb to the required size. This procedure also serves to provide a somewhat more uniform dis- tribution of glass in the final bulb.

Too large an amount of glass in the bulb and

too hot a flame are both conducive to too large

size of the bulb. Too smaU an amount of glass

and too low a flame temperature have the opposite

effect. Success in blowing a suitable bulb of the

proper size depends upon the proper combination

of flame temperature, amount of glass and technique of blowing.

The spherical bulb should be not more than 14

mm in diameter, and the completed bulb should

not contain more than about 1 .3 cm^ nor less than

0.7 cm^ of liquid. The flat sides of the bulb should

be thin enough to deflect visibly, but not so thin

as to break, under moderate air pressure applied

by mouth. The weight of the glass in the bulb

after sealing off the stem near the bulb should be

between 0.05 and 0.10 g. The outside diameter

of the capillary adjacent to the bulb should be i between 1.0 and 1.3 mm. The inside diameter,!

should be at least 0.5 mm and preferably a little 1 larger in order to permit easy insertion of a No. 27 | hypodermic needle (0.4 mm diam.) in filling the^ bulb. The appearance of the fiat sides of a com-i|

pleted bulb will give some indication as to whether i it will be satisfactory. Usually the flat sides I present a scalloped appearance, although if the ; amount of glass m the bulb is greater than normal i they may be nearly plane. If the scallops are very

'

fine, say 1 mm or less apart, the bulb is probably

too thm and fragile to be satisfactory. ; Samples of liquid fuel can be conveniently intro- ? duced into the previously weighed glass bulbs by 1 means of a hypodermic syringe fitted with a No. 27 i needle. The liquid should be introduced into the i bulb slowly in order to permit escape of the air I initially present without undue increase in pres-

| sure. The bulb should be filled completely so that

the liquid extends up into the enlargement in the

stem as indicated at figure 8(f). Usually a small

bubble will be trapped in the bulb at the point

where it joins on to the stem, but such a bubble tj can usually be removed without too much diffi- 1 culty with the aid of the hypodermic needle or a j piece of wire. In filling the bulbs the usual safety

precautions for the handling of volatile organic | solvents should be taken, particularly if the fuel 1 contains tetraethyl lead [13]. i In filling bulbs with very volatile liquid fuels i such as gasolines, great care should be taken to

j avoid loss of material by evaporation, as this i causes fractionation of the material with a conse- I quent change in heat of combustion. The container should be kept tightly closed at all times

except when withdrawing a sample with the hypodermic syi'inge. The sample can be withdrawn more quicldy if the needle is detached from the

syringe. In replacing the needle after withdrawal

of the sample care should be taken to avoid contact of gasoline with the hands, particularly if it contains tetraethyl lead.

.A-fter the bulb is filled, the capillary is sealed off so as to leave a stem about 3 or 4 mm long attached

to the bulb. In order to do this the bulb must be

cooled so as to draw the liquid out of the capillary,

and the bulb must be protected from the fiame during the sealing operation. To protect the bulb

from the flame a small piece of asbestos paper with

a hole in the center and with a cut between the

hole and the edge (fig. 8(g)) is slipped over the

capiflary via the cut so that the capillary goes

through the hole, and the piece of asbestos paper is in contact with the bulb. The bulb is then cooled

by placing it on a mass of shaved ice in a beaker,

or on a copper plate in contact with ice, and when

the meniscus in the capillary disappears below the

asbestos paper, the capfllary is sealed off with a

smafl hot air-gas flame and removed from the

bulb. The bulb should then be removed promptly

from the ice to avoid possible breakage due to

22
excessive contraction of the liquid. The filled ;bulb is conveniently handled by the short projecting capillary by means of forceps of the type ordi-

;narily used for handling balance weights. Particles of asbestos which may adhei'e to the bulb

should be removed by means of a camel's hair

brush. Any liquid remaining in the capillary

detached from the bvdb in sealing it should be re- moved before weighing the filled bulb together

with this detached capdlary. If the liquid is sufficiently volatile it can be removed by flaming

the capillary several times with the micro-burner

flame. The flame should be applied first to the

closed end of the capillary, and moved gradually

to the open end in such a manner as to avoid sof- tening the glass. If the liquid is of low volatility,

it should be washed out with a volatile solvent,

such as benzene or petroleum ether, introduced

into the capillary by means of a hypodermic syr- inge before flaming as described above.

When fii-st sealed, the bulb may contain a small

bubble of air. This will dissolve in the liquid after a time leaving the bulb completely filled with

liquid. Bulbs prepared in the manner described

should withstand changes in room temperature of

10° C or more, and pressures of at least 30 atm

(450 psi) without breaking.

Glass sample bulbs of different types have been

described by a number of workers [16, 17].

Washington, April 14, 1959.

U.S. GOVERNMENT PRINTINS OFFICE: 1360 23

THE NATIONAL BUREAU OF STANDARDS

The scope of activities of the National Bureau of Standards at its major laboratories, in Washington,

D.C., and Boulder, Colo., is suggested in the following listing of the divisions and sections engaged

in technical work. In general, each section carries out specialized research, development, and engineering in the field indicated by its title. A brief description of the activities, and of the resultant

publications, appears on the inside front cover. WASHINGTON, D.C.

Electricity and Electronics. Resistance and Reactance. Electron Devices. Electrical Instru- ments. Magnetic Measurements. Dielectrics. Engineering Electronics. Electronic Instru- mentation. Electrochemistry.

Optics and Metrology. Photometry and Colorimetry. Optical Instruments. Photographic Technology'. Length. Engineering Metrology.

Heat. Temperature Physics. Thermodynamics. Cryogenic Physics. Rheology. Molecular

Kinetics. Free Radicals Research.

Atomic and Radiation Physics. Spectroscopy. Radiometry. Mass Spectrometry. Solid State

Physics. Electron Physics. Atomic Physics. Neutron Physics. Radiation Theory. Radioactivity. X-ray. High Energy Radiation. Nucleonic Instrumentation. Radiological Equipment.

Chemistry. Organic Coatings. Surface Chemistry. Organic Chemistry. Analytical Chemistry.

Inorganic Chemistry. Electrod^osition. Molecular Structure and Properties of Gases. Physical Chemistry. Thermochemistry. Spectrochemistry. Pure Substances.

Mechanics. Sotmd. Mechanical Instruments. Fluid Mechanics. Engineering Mechanics. Mass

and Scale. Capacity, Density, and Fluid Meters. Combustion Controls.

Organic and Fibrous Materials. Rubber. Textiles. Paper. Leather. Testing and Specifications. Polymer Structure. - Plastics. Dental Research.

MetaUurgy. Thermal Metallurgy. Chemical Metallurgy. Mechanical Metallin-gy. Corrosion.

Metal Physics.

Mineral Products. Engineering Ceramics. Glass. Refractories. Enameled Metals. Constitution and Microstructure.

Building Technology. Structural Engineering. Fire Protection. Air Conditioning. Heating

and Refrigeration. Floor, Roof, and Wall Coverings. Codes and Safety Standards. Heat

Transfer. Concreting Materials.

Applied Mathematics. Numerical Analysis. Computation. Statistical Engineering. Mathematical Physics.

Data Processing Systems. SEAC Engineering Group. Components and Techniques. Digital

Circuitry. Digital Systems. Analog Systems. Applications Engineering.

• Office of Basic Instnunentation. • Oflfice of Weights and Measures.

BOULDER, COLORADO

Cryogenic Engineering. Cryogenic Equipment. Cryogenic Processes. Properties of Materials.

Gas Liquefaction.

Radio Propagation Physics. Upper Atmosphere Research. Ionosphere Research. Regular

Prediction Services. Sun-Earth Relationships. VHF Research. Radio Warning Services.

Airglow and Aurora. Radio Astronomy and Arctic Propagation.

Radio Propagation Engineering. Data Reduction Instrumentation. Radio Noise. Tropospheric Measurements. Trophospheric Analysis. Propagation-Terrain Effects. Radio-Meteorology. Lower Atmosphere Physics.

Radio Standards. High-Frequency Electrical Standards. Radio Broadcast Service. Radio and

Microwave Materials. Electronic Calibration Center. Microwave Physics. Microwave

Circuit Standards.

Radio Communication and Systems. Low Frequency and Very Low Frequency Research.

High Frequency and Very High Frequency Research. Modulation Systems. Anteima Research.

Navigation Systems. Systems Analysis Field Operations.