AIllQE MASEtM

NBS

UBLICATIONS

NAFL INST OF STANDARDS & TECH R.I.C.

, A11 102485264

QC100 .US56 N0.174 1986 C.I NBS-PUB-C 19 THERMOMETER

CALIBRATION

NBS MONOGRAPH 174

Jacquelyn A. Wise

Robert J. Soulen, Jr.

U.S. DEPARTMENT OF COMMERCE

NATIONAL BUREAU OF STANDARDS

"QC

100

.U556

No. 174

1985

C 2
Tm he National Bureau of Standards' was established by an act of Congress on March 3, 1901. The

f Bureau's overall goal is to strengthen and advance the nation's science and technology and facilitate their effective application for public benefit. To this end, the Bureau conducts research and provides: (1) a

basis for the nation's physical measurement system, (2) scientific and technological services for industry and

government, (3) a technical basis for equity in trade, and (4) technical services to promote public safety.

The Bureau's technical work is performed by the National Measurement Laboratory, the National

Engineering Laboratory, the Institute for Computer Sciences and Technology, and the Institute for Materials

Science and Engineering

.

The National Measurement Laboratory

Provides the national system of physical and chemical measurement;

coordinates the system with measurement systems of other nations and

furnishes essentiaJ services leading to accurate and uniform physical and

chemical measurement throughout the Nation's scientific community, in- dustry, and commerce; provides advisory and research services to other Government agencies; conducts physical and chemical research; develops,

produces, and distributes Standard Reference Materials; and provides

calibration services. The Laboratory consists of the following centers:

• Basic Standards^

• Radiation Research

• Chemical Physics

• Analytical Chemistry

The National Engineering Laboratory

Provides technology and technical services to the public and private sectors to address national needs and to solve national problems; conducts research in engineering and applied science in support of these efforts; builds and maintains competence in the necessary disciplines required to carry out this research and technical service; develops engineering data and measurement

capabilities; provides engineering measurement traceability services; develops

test methods and proposes engineering standards and code changes; develops

and proposes new engineering practices; and develops and improves

mechanisms to transfer results of its research to the ultimate user. The

Laboratory consists of the following centers:

Applied Mathematics

Electronics and Electrical Engineering^

Manufacturing Engineering

Building Technology

Fire Research

Chemical Engineering^

The Institute for Computer Sciences and Technology

Conducts research and provides scientific and technical services to aid Federal agencies in the selection, acquisition, application, and use of computer technology to improve effectiveness and economy in Government

operations in accordance with Public Law 89-306 (40 U.S.C. 759), relevant Executive Orders, and other directives; carries out this mission by managing

the Federal Information Processing Standards Program, developing Federal ADP standards guidelines, and managing Federal participation in ADP

voluntary standardization activities; provides scientific and technological advisory services and assistance to Federal agencies; and provides the technical foundation for computer-related policies of the Federal Government. The Institute consists of the following centers:

• Programming Science and

Technology

• Computer Systems

Engineering

The Institute for Materials Science and Engineering

Conducts research and provides measurements, data, standards, reference

materials, quantitative understanding and other technical information fundamental to the processing, structure, properties and performance of materials; addresses the scientific basis for new advanced materials technologies; plans

research around cross-country scientific themes such as nondestructive

evaluation and phase diagram development; oversees Bureau-wide technical programs in nuclear reactor radiation research and nondestructive evaluation; and broadly disseminates generic technical information resulting from

its programs. The Institute consists of the following Divisions:

Ceramics

Fracture and Deformation ^ Polymers

Metallurgy

Reactor Radiation

'Headquarters and Laboratories at Gaithersburg, MD, unless otherwise noted; mailing address

Gaithersburg, MD 20899. ^Some divisions within the center are located at Boulder, CO 80303. 'Located at Boulder, CO, with some elements at Gaithersburg, MD.
Hierniometer Calibration:

A IVIodel for State Calibration Laboratories

RESEAECH nTFOEJflATION CEIITER

/Oo

no.

Jacquelyn A. Wise

Robert J. Soulen, Jr.

Temperature and Pressure Division

National Measurement Laboratory

National Bureau of Standards

Gaithersburg, MD 20899

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

NATIONAL BUREAU OF STANDARDS, Ernest Ambler, Director

Issued January 1986

o

o

4r
Library of Congress Catalog Card Number: 85-600636

National Bureau of Standards Monograph 174

Natl. Bur. Stand. (U.S.), Monogr. 174, 88 pages (Jan. 1986)

CODEN: NBSMA6

US. Government Printing Offi<ce

Washington: 1986

For sale by the Superintendent of Documents, U.S. Government Printing Office, Washington, DC 20402
TABLE OF CONTENTS

Page

1. INTRODUCTION 1

2. LABORATORY AND EQUIPMENT 1

2.1 Laboratory Space and Environment 1

2.2 Temperature Baths 6

2.2.1 Ice bath 6

2.2.2 Variable, constant-temperature baths ... 8

2.3 Laboratory Standards 11

2.3.1 Primary standards 11

2.3.2 Check standards 13

2.4 Personnel 13

3. PERFORMING CUSTOMER'S CALIBRATIONS 14

3.1 Examination and Preparation of Thermometers

for Calibration 14

3.2 Proper Method for Calibrating Specific

Thermometers 15

3.2.1 Total-immersion thermometers 15

3.2.2 Partial-immersion thermometers 15

3.2.3 ASTM thermometers 16

3.2.4 Calorimetric thermometers 16

3.2.5 Beckmann thermometers 17

3.2.6 Clinical standard thermometers 17

3.2.7 Kinematic viscosity thermometers 17

3.2.8 Electronic temperature measuring

systems 18

iii
Page

3.3 Calibration Procedure 18

3.3.1 Number and choice of calibration

points 18

3.3.2 Use of the ice-point bath 18

3.3.3 Steam point (100 °C) as reference

point 19

3.3.4 Role of primary liquid-in-glass

thermometers 20

3.3.5 Routine adjustment to primary

standard thermometer calibration

data 20

3.3.6 Placement of thermometers in

calibration bath 24

3.3.7 Reading sequence and recording data ... 25

3.3.8 Computation of bath temperature from

readings of primary standard

thermometers 27

3.3.9 Determining corrections for

thermometers being calibrated 30

3.3.10 Stem-temperature corrections 31

4. DOCUMENTATION 36

4.1 Report of Calibration 36

4.2 Report of Test 39

5. MAINTAINING LABORATORY STANDARDS AND ESTIMATION OF

LABORATORY UNCERTAINTY 46

5.1 Estimates of Inaccuracy of Calibration of

Liqu id- in-G lass Thermometers 46

5.2 Maintaining the Accuracy of Primary

Laboratory Standards 46

5.3 Using Check Standards for Quality Assurance ... 47

iv
Page

6. CONCLUSION . .

.' 51

7. ACKNOWLEDGMENTS 54

8. REFERENCES 54

9. COMMERCIAL SOURCES 55

9.1 Suppliers of Calibration Baths 55

9.2 Suppliers of Oils for Calibration Baths 55

9.3 Suppliers of Thermometers 56

9.4 Suppliers of Faden Thermometers 56

10. APPENDIX 57

Liquid-In-Glass Thermometry, NBS Monograph 150 57

V

1. INTRODUCTION

This publication is written primarily to assist State

calibration laboratories to establish temperature calibration

capabilities. It can also be used by any industrial organization

or independent testing laboratory that wishes to establish a similar facility.

The range of the calibrations to be conducted extends

roughly from 0 to 200 ^C, while the target accuracy is

±0.1 ^C. Several types of thermometers could be used to maintain

this stated range and accuracy, but liquid- in-glass thermometers

have been chosen because they are dependable, easy to use and

maintain, and economical.

Presented in this publication are detailed descriptions of

the laboratory environment, equipment, type of training and

personnel, calibration procedure, calculations, data analysis,

and documentation needed for the calibration of thermometers.

These descriptions and procedures are based on experience

acquired at the National Bureau of Standards (NBS) for over 50

years.

If a laboratory is established and maintained as described

in this publication, liquid-in-glass thermometers and electronic

temperature measuring systems can be calibrated to an accuracy

somewhat better than 0.1 over a range of 0 to 200 ^C, and they

will be traceable to NBS. A deviation from the procedures or

specifications given may be acceptable, but any consequent errors

should be determined and evaluated for acceptability by the

manager of the laboratory. If laboratories follow these procedures, there will be more uniformity among these laboratories and

this will result in better service to industry and to the public.

A checklist for establishing a calibration laboratory to the

specifications described in this publication is provided in

Section 6 (see Table 10).

2. LABORATORY AND EQUIPMENT

2.1 Laboratory Space and Environment

Liquid-in-glass thermometers submitted for calibration may

be as long as 615 mm. They may be accidentally broken if an

adequate distance is not kept between laboratory furnishings.

Futhermore, any electronic temperature measuring systems

submitted for calibration will require additional room around the

calibration bath to place the digital readout device or other

electronic components. The room chosen for the calibration laboratory should therefore be spacious; dimensions of approximately 7mx7mx3m are adequate. A photograph of one room of the

liquid-in-glass thermometer calibration laboratory at the NBS is

shown in Figure 1 as an example. A floor plan of the entire

facility is shown in Figures 2 and 3.

1
4J

touO

4J

fO

1-1 o

(0c0

•H

+J

fO

i-l XI•HH

<0uu

Q)

<D x: •

en fO .H c c w

•H 0

•H

•H (D iJ 3

CQ

to

s-<

O

•1-4 fO fa 2; 2

Counter and Storage Area Counter Sink

Exhaust Hood

on

Bath

Ice Shaver

Sand Blaster and Thermometer Pick-up

Figure 3. Floor plan of higher-temperature

calibration room and room used for ice

preparation and shipping of thermometers.

The laboratory should be maintained at a constant

temperature and humidity. Many thermometers that will be

submitted for calibration will be intended for use at partial

immersion, in which case it is important that the emergent stem

remain at a constant temperature while being tested. For example, if there is a one-degree change in the laboratory

temperature, an error of approximately 0.01 can result in the

calibration of a partial-immersion thermometer with 50 degrees

emergent from the bath. The humidity should be maintained near

50% to keep any electronic components that may be in the

laboratory from being adversely affected.

Any calibration bath that uses an oil as the medium should

be operated under a hood. The hood should not be connected to

the ventilation system of the building; instead it should be

exhausted directly outside. Any filters in the system should be

periodically checked and changed if necessary. Figure 4 shows

the exhaust hood over the oil baths at NBS.

There must be a place in the laboratory to safely store the

primary liquid-in-glass thermometer standards, as well as those

submitted for calibration. A map case, having corrugated paper

placed on the bottom of each drawer to keep the thermometers from

rolling into each other when the drawer is opened, can be used

for this purpose (see Figure 5). Another method commonly used is

to store the thermometers vertically in a thermometer rack

especially designed for this purpose. This method is shown in

Figure 6.

4
Figure 4. Exhaust hood over oil baths.
Figure 6. Thermometers placed in

thermometer rack.

2.2 Temperature Baths

2.2.1 Ice bath

An ice bath (see Figure 7) is perhaps the most valuable and

essential item in a thermometer calibration laboratory. This is

because all liquid-in-glass thermometers suffer changes in bulb

volume (due to thermal expansion and other factors). The method

for correcting for such changes involves recal ibr at ion in most

cases at the ice point (0 ^c) . (See Section 5.2, NBS Monograph

150 [I]-'-, and a discussion later in this Monograph.)

A typical ice bath consists of a Dewar flask approximately

36 cm deep and 8 cm in diameter. The outside of the flask should

be covered with electrical or waterproof adhesive tape and enclosed in a metal sleeve to protect the operator in the event

that it should break. The flask should be supported by a structure that will not allow it to tip over easily. An appropriate

-•-Figures in brackets indicate references at the end of this

Monograph.

6
Figure 7. Ice bath at NBS

.

clip is placed above the top of the flask to hold the thermometer

perpendicular to the telescope used for reading. A siphon is

placed in the bottom of the flask to remove excess water that

will form due to the melting of the ice during the day. A small

soft mallet, which is used to gently tap the thermometers before

reading, should be available.

Shaved ice (of quality described below) and distilled water

are placed in the Dewar flask and allowed to reach a constant

temperature, a process that takes approximately 15 to 30 minutes.

After this time, any excess water should be siphoned off and

additional ice added. When the ice bath is ready for use, there

should be no floating ice in the flask and no excess water on the

surface. There should be as much ice as can be forced into the

flask and sufficient distilled water to fill the small crevices

between the ice chips. Failure to follow this procedure can lead

to vertical or radial temperature gradients, which can result in

a calibration error as large as 0.01 at the ice point.

Whenever the ice bath is used, care should be taken not to

contaminate the ice or water. Thermometers should be rinsed with

cool distilled water before being placed in the ice and plastic

gloves should be worn by the operator or hands washed often while
taking ice points. It is important that impurities, such as salt

from the hands, not be allowed to come in contact with the ice.

This will cause a depression of the freezing point and result in

an error in subsequent ice-point determinations.

From what has been stated in this section, it is clear that

a calibration laboratory will need an adequate supply of

distilled water and pure ice (see below). A still and storage

tank can be purchased for this purpose. The distilled water that

is produced can be frozen to form blocks of distilled water ice.

The freezer and containers used for this purpose should not be

used for anything else and kept as clean and free from impurities

as possible. The containers should be washed thoroughly with

soap and water and rinsed two or three times with distilled water

before being used initially.

Ice purchased commercially can also be used if it is crystal

clear. Some companies produce large blocks of ice by freezing

the outside first. When ice is made in this manner, the

impurities are forced to the center where the ice will be cloudy

and white. This unclear portion is removed and only the crystalclear portion is used.

An ice shaver, which can be purchased from restaurant supply

companies, is used to shave the ice to the proper consistency.

The ice chips should be approximately 2 to 5 mm in diameter.

These particles are smaller than those produced by crushing or

chipping the ice.

2.2.2 Variable, constant-temperature baths

In addition to the ice bath, which provides a calibration at

a single temperature, it will be necessary for the laboratory to

provide calibrations at several other temperatures from +1 to

+200 oc. This requires the use of temperature baths (see

Section 9.1 for sources) in which the temperature can be varied

and then set to constant values for moderately long periods of

time to permit calibrations. Spanning this temperature range

requires, in general, two baths using different working media

(water and oil). The water bath used at NBS is shown in

Figure 8,

For calibration in the range of approximately +1 to +95 ^C,

water can be used as the bath medium. Distilled water is

preferable, since no chemical deposits will form at the top of

the bath. Water is plentiful, clean, and does not produce

harmful vapors. In the range of approximately +95 to +200 ^C, an

oil should be used as the medium. Manufacturers of the calibration baths will generally recommend the type of oil that should

be used in their baths and list suppliers (see Section 9.2 for

sources). It is important that an exhaust hood be placed over

the oil bath to remove vapors from the laboratory and that the

oil is not used near its flash point. The division of the

temperature range into two regions using two different media is

not always required. In particular situations a laboratory might

8
Figure 8. Variable, constant-temperature

water bath.

find it more convenient to use a single oil bath to perform

calibrations from +1 °C to +200 °C.

A cover should be placed over any openings of the bath to

prevent foreign material from entering the medium that might

cause contamination or obstruct stirring.

Several factors must be taken into account in the decision

for the purchase or construction or such baths. The most

critical ones are: volume, temperature homogeneity, and

temperature stability.

Constant-temperature baths may be purchased from any of

several companies or they can be made at the calibration laboratory. Before the calibration bath is purchased or designed, one

must determine what types of thermometers will be calibrated. If

calibrations will include long, total-immersion liquid-in-glass

thermometers, the depth of the bath must be sufficient to accommodate them. If several thermometers are to be tested at one

time, the diameter at the top of the bath must be large enough to

accept a holder containing several thermometers. The bath should

have a provision for overflow or removal of the expanding fluid

9
that will result as the temperature of the medium is increased.

It should also be easy to add fluid so that the liquid level can

be maintained within 6 mm to 12 mm of the bottom of the

thermometer holder.

A constant-temperature bath should have the heating and

cooling coils isolated from the volume where the thermometers

will be placed (see Figure 5 in NBS Monograph 150 [1]). There

should be an unrestricted path of flow for the bath medium, which

is stirred or pumped at a sufficient rate to maintain a uniform

temperature throughout the medium. There should be adequate

insulation around the bath. The holder for the thermometers

should be designed so as to prevent heat loss.

Every bath should be checked to determine the magnitude of

the horizontal and vertical temperature gradients. One way of

doing this is to regulate the bath sequentially at several

temperatures throughout the range. At each temperature a thermometer is placed in one position and constantly read to

determine if the temperature of the bath is changing. A second

thermometer, either another liquid-in-glass type or preferably a faster responding one, such as an electronic digital readout

using a thermistor or small resistance device as the sensor, is

placed next to the first one. When it is observed that the

thermometer readings are not changing or are increasing at a rate

of no more than 0.01 to 0.05 in 5 minutes, stability can be

considered to have been reached. At this time both thermometers

are read and the readings are recorded. While the stationary

thermometer is monitored, the second thermometer is moved to

other locations and depths in the bath and readings on both

thermometers are recorded. If a liquid-in-glass thermometer is

used to probe the bath, adequate time must be given for it to

stabilize, since the response time may be slower than that of an

electronic thermometer. After adjusting the reading of the

second thermometer for any change that may be noted by the

stationary thermometer, the change in the second thermometer

reading at various locations will indicate the uniformity of the

bath temperature. The accuracy of either thermometer is not

important for this test, only its temperature sensitivity.

A common cause of bath temperature nonun i f or m i ty is

inadequate circulation of the fluid. If this is suspected to be

the case, it is necessary to increase the circulation rate and

recheck the temperature gradients. On the other hand,

circulation rates cannot be increased beyond the point where

overheating and cavitation of the bath medium will occur

(observable by the generation of temperature fluctuations). It

is best to determine optimum rates experimentally.

A second problem can occur when the bath is used at elevated

temperatures. In this situation, heat escapes from the top of

the fluid, causing a vertical gradient in the upper 25-50 mm

region of the fluid. Clearly, for accurate results, no part of

the bulb or contraction chamber of a liquid-in-glass thermometer

or the sensing portion of an electronic thermometer should be in

10
this region. When the bath is first installed, this gradient

should be determined as a function of temperature.

Attached to all calibration baths, including the ice bath,

should be a telescope mounted perpendicular to the axis of the

thermometers that will be placed in the holder. If the telescope

cannot be mounted on the bath, it can be placed on a stand near

the bath, but care must be taken to see that it is always set

perpendicular to the thermometer axis to avoid reading errors due

to parallax. The telescope should be approximately 10 power and

it is desirable to have a cross-hair to aid in reading the

meniscus within the scale divisions.

2.3 Laboratory Standards

2.3.1 Primary standards

The laboratory should be equipped with two sets of "primary"

standards and one set of "check" standards. The primary standards should be calibrated versus the International Practical

Temperature Scale of 1968 (IPTS-68). This temperature scale is

the latest version of a series of international temperature

scales that have been developed by national standards laboratories like the NBS throughout the world. A complete description

of this scale is given in the Metrologia article entitled, "The

International Practical Temperature Scale of 1968 Amended Edition

of 1975" [2]

.

The inaccuracy of IPTS-68 (that is, the best estimate of the

difference between the scale and the correct temperature) in the

region 0 to 200 is estimated to be ±0.04 It is possible

to register temperature in this region with an imprecision much

smaller than the inaccuracy of the temperature scale. For

example, platinum resistance thermometers can be used to

repeatedly register the freezing point of Gallium (near 29 ^C)

with a standard deviation of only 0.0001 (the imprecision) yet

the temperature assigned to the Gallium point on the IPTS-68 will

still remain inaccurate to ±0.04 until a better temperature

scale is developed. When such a temperature scale is developed

(approximately every 10 or 20 years; last done in 1968) and compared to IPTS-68, a new difference table will be constructed. At

that time at each temperature, a different systematic correction

(or offset) must be applied.

By comparison to platinum resistance thermometers, the imprecision of 1 iqu id - in-g la ss thermometers under the best

conditions has been found to be ±0.03 °C (see Section 5.3).

Thus, a State calibration laboratory can maintain a temperature

scale based on the IPTS-68 using liquid-in-glass thermometers and

the procedures given in this Monograph in which the inaccuracy

and imprecision are comparable. Using liquid-in-glass

thermometers, the laboratory will be able to provide calibrations

with errors of about one-third the tolerance permitted in

customers' thermometers.

11
Since total-immersion thermometers (which have virtually no

mercury-containing stem exposed to ambient temperature) are more

accurate than partial-immersion thermometers, the former should

be chosen as standards. Given in Table 1 are ranges, graduation

intervals, total length, and imprecision of thermometers that are

recommended for use as primary standards. Commercial suppliers

of these thermometers are given in Section 9.3. The reproducibility and accuracy of the thermometers chosen for standards

should be better than or equal to that of the thermometers being

calibrated.

TABLE 1

PRIMARY LIQUID- IN-GLASS THERMOMETERS

ASIM^ Thenrcmeters[3]

Asm No.

63C

64C

65C

66C

67C

68C

TheriTDneter Range

-8 to +32 °C

+25 to +55 °C with auxiliary scale at 0*^

+50 to +80 °C with auxiliary scale at 0°C

+75 to +105 °C with auxiliary scale at 0°C

+95 to +155 °C with auxiliary scale at 0 °C

+145 to +205 °C with auxiliary scale at 0 *-t

Graduation

Interval

0.1 °C

0.1 °C

0.1 °C

0.1 °C

0.2 °C

0.2 °C

Inprecision at Calibration It>ints Tbtal Length After NBS Calibration

374-384 irni 0.03

374-384 mm 0.03 ^

374-384 mm

374-384 mn

374-384 mn

374-384 mm

0.03 °C

0.03 °C

0.05 °C

0.1 °C

ANSI^ Thermometers [4]

Thermometer

SftMA'^ No. Range

CT40

CT45

CT50

-1 to +51 °C

-1 to +101 °C

-1 to +201 °C

Graduat icn Interval

0.1 oc

0.1 °C

0.2 "^t

Tbtal Length

460 tim

610 nm

610 mm

Imprecision at Calibration Itoints After NBS Calibration

0.03

0.03 °C

0.05 °C \jp to 100 °C

0.1 °C above 100 °C

^ASM is an acronym for the American Society for Ttesting and Materials. °MSI is an acronym for the American National Standards Institute. ^SAMA is an acronym for the Scientific /^aratus Makers Association.

"For imprecision between the calibration points see Section 5.4 of NBS Monograph 150 [1]

.

These thermometers must be free of foreign material in the

capillary (such as glass chips or oxides of mercury), be numbered

correctly (in proper numerical sequence), and be uniquely

12
identified by a serial number. The scale must be placed on the

thermometer according to NBS specifications (see Section 7.3, NBS

Monograph 150 [1]), which do not permit it to be graduated close

to an enlargement in the capillary. The thermometers must meet

all other requirements given in NBS Monograph 150 [1], such as

being made of the proper glass and giving temperature indications

within the specified tolerances. When ordering liquid-in-glass

thermometers that will be used as standards, it is advisable to

request the company to guarantee in addition that they will pass

the NBS visual inspection. This service may increase the initial

cost of the thermometer, but it will be replaced at no additional

cost in the unlikely event that it is found ineligible for test

by NBS.

To equip the laboratory with adequate primary thermometers,

two liquid-in-glass thermometers from each range should be

purchased from the choices given in Table 1. Then these thermometers should be sent for calibration to the NBS or another

competent laboratory, accompanied by a letter or purchase order

requesting calibration. In the former case, the thermometers

will be calibrated by NBS personnel and returned to the sending

laboratory along with Reports of Calibration, which list the

corrections to be made at each calibration point. By virtue of

their calibration at the NBS, such standard thermometers will

automatically bear the IPTS-68 scale.

2.3.2 Check standards

The third set of liquid-in-glass thermometers, also chosen

from the list given in Table 1, should be purchased for use as

check standards. This set differs from the primary standards in

that it is not sent to NBS for calibration. Rather, the appropriate thermometer from the set is incorporated as one of the

thermometers in the calibration sequence (to be described in

Section 3.3.7). By treating the check standard as one of the

thermometers being calibrated in terms of the primary standards,

it is used to check the calibration procedure, helping to insure

that the calibration was done properly. A check standard may also

be used as temporary backup for a primary standard in the event

that one of a primary thermometers is broken. More than one

check standard in each temperature range may be needed due to

bulb volume expansion (see Section 3.3.5).

2.4 Personnel

The staffing level as well as the experience and education

needed by the calibration staff will depend on several factors

that are specific to the laboratory: budgeting, calibration income, calibration volume, and turnaround time. Thus a general

prescription here would not be especially useful. For the purpose of reference, however, we mention the staffing situation at

the NBS. The Liquid-in-Glass Thermometer Calibration Laboratory

at the NBS is staffed by one individual who has had 3 years of

13
college education and 20 years experience in the laboratory.

Approximately 800 liquid-in-glass thermometers and a small number

of other types of thermometers are calibrated per year in this

laboratory. An individual chosen to be employed in a thermometer

calibration laboratory should be adept in mathematics, a stickler

for details, and always striving for perfection. Depending on

the initiative of the individual, we estimate that it would take

12 months of practice and experience for a high school graduate

to operate a calibration laboratory as described in this

publication. We strongly recommend that an individual chosen for

this responsibility attend a session of the NBS Precision

Thermometry Seminar which is given twice yearly. In the Seminar,

participants are given lectures on all common types of

calibration including liquid-in-glass thermometers, and the

attendees also perform simulated calibrations at the NBS

facility.

To summarize: when the laboratory has been properly

equipped and staffed, and when NBS-cal ibr a t ed primary

thermometers have been incorporated into a careful measurement

program, then the State calibration laboratory can be considered

to be ready to perform calibrations, based on the IPTS-68, for

its customers. We now turn to a discussion of the procedures to

be followed in performing those calibrations.

3. PERFORMING CUSTOMER'S CALIBRATIONS

3.1 Examination and Preparation of Thermometers for Calibration

Every thermometer submitted for calibration must be examined

under a microscope (preferably 15 or 20 power) for defects. One

reason for this visual inspection is to detect a separated mercury column or small droplets of mercury that may be at the top

of the thermometer. This condition can occur because of rough

handling encountered during shipping. All of the separated mercury must be reunited with the main column before the thermometer

can be calibrated.

Rough handling during shipping can also cause gas bubbles

to form in the bulb, a defect that also can be detected during

the visual examination. These gas bubbles will expand at a

greater rate than the mercury that should be in these spaces;

therefore, they must be removed. To reunite separated mercury

and to remove gas bubbles in the bulb, see Section 8.4 of NBS

Monograph 150 [1]

The third reason for examining each thermometer under a microscope is to look for foreign material in the capillary and

defects in the thermometer. The foreign materials most often

found are glass chips or oxides of mercury. These objects can

hold gas or mercury and cause the reading of the thermometer to

vary when the thermometer is repeatedly inserted into a regulated

constant-temperature bath. The capillary of the thermometer must

be clean and free of any foreign material, and it must also

14
appear to have a uniform diameter. If any variation in the

diameter of the capillary can be seen under the microscope, the

thermometer is not acceptable for calibration. The distance

between scale graduations must be uniform, the scale must be

placed on the thermometer according to NBS specifications (see

Section 7.3 in NBS Monograph 150 [1]), and the thermometer must

be numbered correctly and be free of cracks in the glass. If any

of the above conditions are not met in a thermometer that has

been submitted for calibration, it should be ineligible for test

and should be replaced by the customer.

3.2 Proper Method for Calibrating Specific Thermometers

Thermometers usually are calibrated according to their

types. A particular thermometer should be calibrated differently

only if the customer specifically requests it. In this section

is presented a discussion on the calibration of several types of

thermometers.

3.2.1 Total-immersion thermometers

Total-immersion thermometers are calibrated with the bulb

and all but approximately 6 mm to 12 mm of the mercury column

immersed in the calibration bath. The upper 6 mm to 12 mm of the

mercury column extends out of the bath surface to permit reading.

The meniscus (the top of the mercury column) should never be

immersed in the bath fluid, since distillation of the mercury can

occur at high temperatures and cause mercury droplets to condense

at the top of the thermometer.

After remaining at room temperature (23 ^C) for 3 days,

these thermometers are calibrated starting at the lowest temperature requested and proceeding in sequence to the highest point.

At each calibration point the thermometer must be pushed deeper

into the calibration bath to maintain the proper immersion.

If the thermometer has a contraction chamber (see Section 3

of NBS Monograph 150 [1]), it must be immersed well below the

surface of the bath medium. The contraction chamber contains a great deal of mercury and this mercury must be at the same temperature as the mercury in the bulb for the thermometer to give a correct reading. If there is a temperature gradient at the

surface of the bath, as may be the case in high temperature

calibration baths, the contraction chamber must be placed below

this gradient, even if this means changing the calibration point.

If the thermometer being calibrated is too long and cannot

be immersed at the proper depth, a stem-temperature correction

will have to be made as described in Section 5.3 of NBS

Monograph 150 [1] and in Section 3.3.10 of this Monograph.

3.2.2 Partial-immersion thermometers

Partial-immersion thermometers must have either an immersion

line or the proper depth of immersion engraved or printed on the

15
back (most thermometers have both). These thermometers are

immersed only to the immersion line or specified depth. The

portion of the stem that contains mercury above the immersion

line is known as the "emergent stem". It tends to reach ambient

temperature or the temperature prevailing above the calibration

bath. If the emergent stem is to have a specified stem

temperature, as is the case with all ASTM partial-immersion

thermometers, a stem temperature correction must be made as

described in Section 3.3.10,

Like total-immersion thermometers, calibration points are

taken in the same sequence; the thermometer must remain at room

temperature (23 ^C) for 3 days before calibrating; and a contraction chamber must be immersed properly.

3.2.3 ASTM thermometers

Over 120 different thermometer specifications are listed in

ASTM Standard E-1, Standard Specification for ASTM Thermometers [3]. The thermometers were designed to be used in

specific equipment and test methods and are identified by having

the acronym "ASTM" and a number written on the back. Specification E-1 lists the calibration points for all ASTM thermometers, stem temperatures for ASTM partial-immersion thermometers,

and any special procedure that must be followed. An ASTM thermometer should be tested in accordance with the procedure outlined in E-1 and E-77, Standard Method for Verification and

Calibration of Liquid-in-Glass Thermometers [5].

3.2.4 Ca lor ime trie thermometers

In some cases it is only necessary to accurately measure a temperature difference rather than the absolute temperature.

Calorimetric thermometers were designed for this purpose. They

usually are 600 mm long, cover a short range of temperature

(typically 6 to 16 degrees), and are graduated in intervals of

0.01 to 0.05 degree. The thermometers are usually calibrated

every 50 to 100 scale divisions (in intervals of 0.5 or

1 degree). Because of the very small capillary diameter, they

must be tapped lightly at each reading to eliminate a possible

error due to a sticking mercury column, and they must be heated

to the maximum calibration point before testing. Initially

heating to the top point causes the bulb to expand to the maximum

volume and eliminates the error that will occur because of the

gradual expansion of the bulb as the thermometer is heated. In

order to register accurate temperature differences, the bulb must

be expanded before calibration and before use. The thermometer

should be calibrated within 2 to 3 hours after it has been

heated.

These thermometers are capable of measuring temperature

differences with uncertainties of 0.01 ^C, which will be beyond

the capabilities of a State calibration laboratory following the

procedures outlined in this publication. If a customer requests

16
this accuracy, the thermometer must be sent to another

calibration laboratory, such as NBS.

3.2.5 Beckmann thermometers

These thermometers are instruments in which a narrow

temperature interval (typically 6 oc) may be adjusted to fall

within 0 to 100 oc. The graduations are 0.01 oc and the

thermometer is of the enc losed-scale type, which consists of a capillary tube attached to a scale and enclosed in a glass tube.

Since the narrow interval is adjustable, these thermometers are

to be used only to register temperature differences.

The interval may be moved within the range 0 to 100 oc by

adding or removing mercury to or from the bulb. Consider the

following example for adjusting a Beckmann thermometer to

indicate temperature differences from 20 to 26 o^. Before

calibration, the amount of mercury in the bulb is adjusted to

place the meniscus within ±5 scale divisions of the "zero"

indication when the thermometer is immersed in a medium having a temperature of 20 oc. This is referred to as a "setting of

20 oc". The thermometer is then calibrated in the range of 20 to

26 °C. For other settings, the temperature difference must be

multiplied by the appropriate setting factor as shown in

Section 6.4 of NBS Monograph 150 [1].

The Beckmann thermometer is a calorimetric thermometer and

must be heated to the highest test point, tapped gently before

reading, and calibrated every 0.5 ^C. They are capable of

measuring temperature differences with uncertainties of 0.005

and, like the calorimetric thermometers, will have to be sent to

another calibration laboratory, such as NBS, if this accuracy is

desired.

3.2.6 Clinical standard thermometers

Clinical standard thermometers are used by the manufacturers

of fever thermometers, which are used for measuring body temperature. In the United States they are marked in degrees Fahrenheit

with a range of approximately +90 to +112 of. They are provided

with an auxiliary scale at 32 ^F and graduated in intervals of

0.1 Of. The calibration points are usually 32, 98, 102, and

106 °F and they are heated to 108 ^F before calibrating to

initially expand the bulbs. Since they are heated to 108 °F

before calibrating, the ice points are taken after test. They

are generally tested at total immersion and at partial immersion,

immersed to approximately the 92 °F mark.

3.2.7 Kinematic viscosity thermometers

When measuring the viscosity of liquids, it is imperative to

accurately measure one specific temperature. Kinematic viscosity

thermometers are designed for this purpose and have a short 3 to

5 degree range with an auxiliary scale for an ice point. These

17
thermometers are calibrated at two points on the main scale. Ice

points are measured afterwards.

3.2.8 Electronic temperature measuring systems

A thermometry calibration laboratory may occasionally be

requested to calibrate an electronic temperature measuring

system. It usually consists of a digital readout connected to a probe, which can be a thermocouple, thermistor, or resistor (most

commonly an industrial platinum resistance thermometer). It must

first be determined that the probe can physically be placed in

the calibration bath with enough lead wire to enable the readout

unit to be located near the bath. The probe must be long enough

so that the sensing element is below the region of large temperature gradient that may occur at the top of the bath. If the

probe cannot be placed directly in the fluid, it must be put in a tight fitting, protective tube to provide good thermal conductivity. The electronic system is calibrated in the same

manner and readings taken in the same sequence as for liquid-inglass thermometers.

3.3 Calibration Procedure

3.3.1 Number and choice of calibration points

A thermometer submitted for test should be calibrated at

several points over at least 80% of the main scale.

The thermometer should be calibrated at the temperatures

specified by the customer or at intervals of approximately every

100 scale divisions. Calibration points taken at no more than

every 100 scale divisions will give acceptable interpolated corrections for temperatures between the calibration points. (See

Section 5.4 of NBS Monograph 150 [1].)

The thermometer should be calibrated at the reference point,

usually 0 or 100 ^C, if there is one on the thermometer. The

reference point is used to recalibrate the thermometer as

described in Section 5.2 of NBS Monograph 150 [1] and in Sections

3.3.5 and 3.3.8 of this Monograph.

3.3.2 Use of ice-point bath

Make a fresh ice bath on the day that a calibration is to be

performed (discarding any previous day's bath). As ice melts

throughout the day, siphon off the water and add new ice. The

bath is ready for use when no ice is floating and no excess water

is on the surface of the ice.

For proper calibration at 0 ^C, each liquid-in-glass

thermometer should be inserted to a specified depth (the

immersion depth) into the ice-point bath. Total-immersion

thermometers should be inserted into the ice-point bath until the

0 °C mark on the thermometer scale is within one scale division

of the top of the bath. For partial-immersion thermometers, the

18
depth is indicated either by an additional line scribed on the

glass (immersion line) or by a number given on the back of the

thermome ter

.

In order to prepare the bath to accept the thermometer,

loosen an area of ice at the center of the bath with an object

such as a clean glass rod to a depth approximately equal to the

immersion depth. Clean the thermometer to be calibrated at the

ice point by rinsing it with cool distilled water. (Never rinse

with warm water, since this may expand the bulb.) Then gently

place the thermometer through the holder and into the region of

loosened ice. If the thermometer touches a firm surface before

it is immersed to the immersion line or 0 oc mark, then remove

the thermometer and loosen the ice further down into the bath to

permit the thermometer to be immersed to the proper depth. If

the immersion line or 0 °C mark on the thermometer passes below

the surface of the bath before resting on a firm foundation,

repack the ice and loosen the ice to the correct depth.

After the thermometer is properly immersed and perpendicular

to the observer's telescope, firmly pack the ice around the

thermometer. It is important to keep the ice clean while using

the ice bath. The operator's hands must frequently be washed

(with soap and water) or covered with plastic gloves. When the

thermometer is resting on a solid section of ice in the bath and

cannot be immersed further, when it is perpendicular to the

telescope, when ice is firmly packed to the immersion line or one

scale division below the 0 °C mark, and when any excess water is

siphoned from the ice bath, the thermometer is ready to be read

as shown in Figure 9.

The thermometer should remain in the ice bath for approximately one to two minutes. (Thermometers using an organic fluid

instead of mercury as the liquid may require approximately 15

minutes because the organic fluid tends to cling to the wall of

the capillary.) When stability is reached (the meniscus stops

moving), gently tap the thermometer to free the mercury meniscus

(which can sometimes stick) and record the ice-point reading.

3.3.3 Steam point (100 ^C) as reference point

Although the ice point at 0 °C is most often the reference

point, the steam point at 100 °C is sometimes used. A steampoint bath or hypsometer, as described in Section 5.1b of NBS

Monograph 150 [1], is the fixed-point apparatus sometimes used at

NBS to realize this temperature. Since the temperature is

dependent on the atmospheric pressure, a barometer accurate to

0.1 mm of mercury must also be available.

It is not necessary to purchase a steam bath or barometer if

the calibration laboratory has standard thermometers and an oil

bath capable of being controlled at 100 °C. A thermometer having

a reference point at 100 can be calibrated at this temperature

in the oil bath. The change in bulb volume can be determined by

subsequent recalibra tion at 100 ^C.

19
Figure 9. Thermometer properly placed in ice

bath.

3.3.4 Role of primary liquid-in-glass thermometers

Precision liqu id-in-glass thermometers that have been calibrated by NBS or another calibration laboratory are used to

realize the current International Practical Temperature

Scale [2] . When liquid-in-glass thermometers are used as primary

standards, two of them should be used at each calibration point.

Each thermometer is corrected to obtain the accurate temperature

of the bath. The corrected readings of the two primary standards

must agree within a specified tolerance before the calibration

point is acceptable. NBS has found that 0.2 of a scale division

is an achievable tolerance. Using two primary standards usually

will reveal an error due to an incorrect reading of one of the

primary standards. Averaging the carefully taken readings of two

primary standard thermometer temperatures will give a more

reliable value of the temperature of the calibration bath medium,

especially at temperatures that differ from those where the

primary standard thermometers were calibrated.

3.3.5 Routine adjustment to primary standard thermometer

calibration data

There are three conditions that will prevent each primary

1 iqu id- in-g la ss thermometer standard used by the State

20
calibration laboratory personnel from indicating temperatures

correctly. The first is permanent irregularities in the

thermometer, such as nonuniform bore and inaccurate scale

markings. This correction need only be obtained once when the

thermometer is sent to a national standards laboratory such as

NBS. Here the readings of the thermometer are compared with the

temperatures determined from a precision standard platinum

resistance thermometer that has been calibrated on the current

IPTS. On the basis of these measurements, a Report of

Calibration is issued, which gives a table of corrections for

these permanent errors at the calibration points resident on this

thermometer.

The other two conditions, caused by long-term or short-term

changes in the bulb glass, must be corrected by personnel in the

State calibration laboratory on a routine basis according to a procedure to be described later in this section. Briefly, glass,

as a supercooled liquid, will slowly flow, and thus on a longterm basis (over the life of the thermometer) the volume of the

bulb will change, causing the readings of the thermometer to

change. This is easily detected and can be corrected by

determining the correction at the ice point and shifting

uniformly the corrections given on the Report of Calibration (see

Section 5.2, NBS Monograph 150 [1]).

There is also a short-term (three days or less) change in

the bulb glass. To understand this, we appreciate the well known

fact that when a liquid-in-glass thermometer is heated to higher

temperatures, the glass as well as the mercury expands. This

expansion of the bulb, which has a greater effect on the

thermometer readings than changes in the stem glass, can

correspond to a change in temperature reading as great as

0.01 oc for every 10 degrees that it is heated. As the

thermometer is cooled (e.g., to the ice point), the mercury will

return to its original volume, but the bulb has been found to

take approximately 3 days to return to its original volume.

During this three-day period the thermometer will indicate that

the temperature is lower than it actually is, because the

expanded glass bulb volume causes the mercury column to fall

lower in the stem. In order to obtain a correct temperature

value, one must compensate for this temporary, short-term change

in the bulb volume known as an "ice-point depression".

To illustrate this procedure, we provide the following

example. Consider a primary thermometer that, for simplicity, we

assume has no error in the graduations, i.e., all corrections on

the Report of Calibration, which were obtained after the

thermometer had been at room temperature (23 OC) for three days

and calibrated from the lowest calibration point to the highest,

are zero. (See column 2 of Table 2.) We conduct the following

measurements: First, a measurement at 0 ^C, taken after the

thermometer had been at room temperature for not less then three

days, shows no correction. The thermometer then is heated to a given temperature, (e.g., 50 ^C) and then returned within a few

minutes to the ice bath. The volume of the thermometer bulb has

21
been increased by virtue of the heating to 50 so the mercury

will not return to the original height in the glass tube and the

reading at 0 will be lower (e.g., -0.05 OC) . We now cycle the

thermometer to 100 and note the new reading at the ice point

(e.g., -0.10 OC) and then cycle to 150 and back to 0 for a fourth ice-point reading (e.g., -0.15 ^C) . The data thus obtained

for this thermometer may be represented by Column 3 of Table 2.

TABLE 2

EXAMPLE OF ICE- POINT DEPRESSION

Calibration

Point

Or ig inal

Correction

from

Report of

Calibrat ion

Ice-Point

Reading^

Ice -Point

Correct ion^

(OC) (OC) (OC) (OC)

- 0.00 0.00 0.00 0.00

50.00 0.00 -0.05 +0 .05

100.00 0.00 -0.10 +0.10

150.00 0.00 -0.15 +0.15

^Determined after heating thermometer to calibration

point and then returning to ice bath.

Now suppose the thermometer that has been heated to 150 ^C

is immediately returned to 50 *^C. The reading at 50 ^C will be

influenced by the thermometer's having been heated to a higher

temperature. That is, the reading of the thermometer will depend

on its previous thermal history. The ice-point correction taken

after heating the thermometer to 150 ^C was +0.15 *^C and, in this

case, it will remain that value after the thermometer has been

returned to 50 ^C. The ice-point correction taken after the

thermometer had been heated to only 50 ^C was +0.05; therefore,

when the thermometer is used at 50 ^C after being heated to

150 oc, it will indicate that the temperature of the bath is

49.90 oc. The volume of the bulb had increased by an amount

equal to 0.1 [+0.15-(+0.05) ] , thus causing the thermometer to

read lower at 50 OC by 0.1 ^C.

Having understood the long-term and short-term changes in

the bulb glass, we are now ready to describe the procedure to be

followed routinely by the State calibration laboratory.

The first step is to let the thermometer rest at room

temperature (23 ^C) for 3 days to enable the bulb to contract

22
back to its equilibrium volume. After this period of time an ice

point is taken. If the ice point is the same as the one on the

Report of Calibration, then it can be assumed that the corrections given on the report are current and valid. Since long-term

changes can occur in the volume of the bulb with time and use, it

would not be unusual for the ice point to have a different value.

If the ice point has changed, all of the corrections should be

changed by the same amount and in the same direction (see

Section 5.2 in NBS Monograph 150 [1]). This technique gives the

operator a current set of corrections and is a r ecal ibrat ion

procedure followed by NBS for many years. If the primary standard thermometers are to be used only once or twice a week, with

a 3-day waiting period between their use, and they will be used

only from some lower point to some higher point, the corrections

determined after changing the corrections for the change in ice

point can be used. If the primary standard thermometers will be

used several times within a week, as most standards are, then one

must determine and compensate for the ice-point depression caused

by the short-term, temporary change in the bulb volume.

The data in Table 2 provides the basis for establishing

"adjusted scale corrections" in order to compensate for the icepoint depression and long-term changes that will occur in the

volume of the bulb. This is done using the following equation.

Ci = C2 - C3 (1)

Ci = Adjusted scale correction.

C2 = Correction from table supplied in the NBS Report

of Calibration or current set of corrections

obtained from change in ice-point reading.

C3 = Ice-point correction determined by the State

laboratory after heating the thermometer to the

calibration point and returning to the ice bath.

The value C3 can be found after the thermometer is returned

from NBS by taking the ice point after heating the thermometer to

each of the calibration points and cycling back to the ice point

as was done in the example given above (Table 2, Column 3).

Adjusted scale corrections for the primary standard thermometer

used in the example are given in Table 3, Column 4.

Altering the original corrections in this manner enables the

user to know the influence of the ice-point depression on the

thermometer reading as the thermometer is heated. When the

adjusted scale correction is applied to the thermometer reading,

the resulting temperature value will be correct only if the icepoint reading at that time is 0.00. Since the ice point will not

always read exactly 0 ^C after use, either because of the

temporary ice-point depression or long-term changes in the bulb

volume, it is necessary to take an ice point periodically (see

23
TABLE 3

EXAMPLE OF ADJUSTED SCALE CORRECTIONS

Report of

Calibration Calibration Ice-Point Adjusted Scale

Point Correct ion Correct ion^ Correct ion

(C2) (C3) (Ci)

(OC) (°C) (°C) (°C)

0 .00 0 .00 0 .00 0 .00

50 .00 0 .00 +0.05 -0.05

100.00 0 .00 +0.10 -0.10

150 .00 0.00 +0.15 -0.15

^De termined after heating thermometer to calibration

point and then returning to ice bath.

Section 3.3.7) while the thermometer is in use and to record the

ice-point reading. This procedure will be discussed further in

Section 3.3.8.

Given in Table 4 are adjusted scale corrections computed for

two hypothetical primary standard thermometers, 1 and 2, used in

the sample calibration that will be discussed in Section 3.3.8.

3.3.6 Placement of thermometers in calibration bath

The thermometers to be calibrated should be placed in the

calibration bath in a row with one primary standard liquid-inglass thermometer at one end and the other primary standard at

the other end. This placement is shown in Figure 10. Totalimmersion thermometers should be immersed with the meniscus of

the mercury column approximately 6 mm to 12 mm above the surface

of the bath medium and partial-immersion thermometers should be

immersed to the immersion line or to the specified depth. Any

additional thermometers (see Section 3.3.7) used to measure stem

temperatures should be placed on the corresponding test

thermometers. One method of doing this is by joining two

electrical pee-wee clamps together and clamping the additional

thermometer to one clamp. The second clamp is attached to the

thermometer

.

Ideally, for liquid-in-glass thermometer calibrations or

when 1 iqu id- in-g lass thermometers are used as standards, the

temperature of the calibration bath should be rising slowly (no

faster than one scale division of the primary standard

thermometer in three minutes). It is also acceptable to control

the bath temperature (by observing a more sensitive thermometer)

within ±0.005 ^C. The temperature of the bath should not be decreasing during calibration because the mercury column at the

24

i
TABLE 4

ADJUSTED SCALE CORRECTIONS FOR HYPOTHETICAL

PRIMARY STANDARD THERMOMETERS 1 AND 2

Primary Standard 1 Report of Ice-Point Correction Adjusted

Calibration Determined after Heating Scale

Calibration Correction to Calibration Point Correction

Point {C2) (C3) (Ci)

(°C) (°C) (°C) (°C)

0.00 +0.08 +0.08 0.00

20.00 + .11 + .10 + .01

40.00 + .02 + .10 - .08

60 .00 + .07 + .11 - .04

80.00 + .10 + .12 - .02

100.00 + .06 + .12 - .06

Primary Standard 2 Report of Ice-Point Correction Adjusted

Calibration Determined after Heating Scale

Calibration Correction to Calibration Point Correction

Point (C2) (C3) (Ci)

(°C) (°C) (°C) (°C)

0.00 +0.01 +0.01 0.00

20.00 + .17 + .01 + .16

40.00 + .04 + .02 + .02

60.00 + .11 + .03 + .08

80.00 + .12 + .04 + .08

100.00 + .06 + .04 + .02

meniscus can cling to the wall of the capillary of the thermometer and remain in an erroneously high position even though the

temperature of the bath may have decreased. There is less chance

for this problem to occur if the temperature of the bath is

r ising.

3.3.7 Reading sequence and recording data

After the thermometers have been placed properly in the

calibration bath and it is regulated at the lowest point requested, the thermometers are read in the following sequence:

Primary Standard 1, Thermometers Tl-TX, Primary Standard 2,

Primary Standard 2, Thermometers TX-Tl, and Primary Standard 1.

One of the thermometers Tl-TX should be the check standard.

Any additional auxiliary thermometers used for stem

temperature determinations should be read after this sequence.

An auxiliary thermometer can be any thermometer that is accurate

25
Figure 10. Correct placement of

thermometers in calibration bath.

to within 1 or 2 degrees. It is used to measure the average

temperature of the emergent stem, both for total -immer s ion

thermometers used incorrectly at partial immersion, and for

partial-immersion thermometers. NBS uses for this purpose a

thermometer approximately 15 cm long with a range of 0 to 100

and graduated in intervals of 1 ^C.

If two calibration operators are available, one should

record the observed thermometer readings while the other reads

the thermometers. The two operators should switch positions and

take a second set of readings. It is preferable to have two

observers, since this procedure will minimize reading errors.

However, this is not absolutely necessary; at the NBS, all calibrations for 12 years have been done by one individual.

At specified intervals (typically, every 200 scale

divisions), the two primary thermometers should be removed from

the constant temperature bath and reinserted into the ice bath.

The ice-point readings are recorded. This procedure is used to

determine the "ice-point correction" (see Section 3.3.5 and

Section 3.3.8) for the two primary standard thermometers. Then

they are returned to the constant-temperature bath and further

readings are taken at higher temperatures.

26
Table 5(a) gives an example in which three unknown

thermometers (Tl, T2 and T3) and a check standard (T4) were

calibrated versus two primary standard thermometers (Primary

Standard 1 and Primary Standard 2). All the thermometers in this

example were total-immersion thermometers with graduations of

0.2 oc and having a range -2 oc to +102 oc. They were calibrated

at the nominal temperatures 0 oc, 20 oc, 40 oc, 60 oc, 80 oc, and

100 oc. These particular points were chosen because they

correspond to intervals of 100 scale divisions, a common practice

used in calibration of 1 iqu id- in-g lass thermometers. The

calibration proceeded from 0 oc to 100 oc. The primary

thermometers were removed from the bath after the readings were

taken at 20 oc, 60 oc, and 100 oc and remeasured in the ice bath.

This allowed ice-point corrections to their readings to be made

at those temperatures (interpolation was used for the 40 oc and

80Oc points)

.

After a set of data is obtained at a calibration point, it

should be inspected to see that it looks reasonable. The

calibrator should check for reading errors and to insure that the

temperature during the calibration remained constant to i0.005 °C

or was rising at a rate of no more than 1 scale division per

3 minutes. As mentioned earlier, it is not acceptable for the

temperature of the bath to decrease while data is being recorded.

3.3.8 Computation of bath temperature from readings of primary

standard thermometers

Once the data have been recorded in the format shown in

Table 5(a), and after inspection indicates that there are no

suspicious data points, computation of the corrections is done as

shown in Table 5(b).

The first step is to convert the readings of the two primary

thermometers to the correct temperature. This is done by using

the following equation

T = Tr + Ci + C4 (2)

T = True temperature.

Tr = Average of two readings for the primary standard

thermometer

.

Ci = Adjusted scale correction.

C4 = Ice-point correction taken after thermometer has

been used.

Having established the set of adjusted scale corrections the

only term that is necessary to determine the correct temperature

is the ice-point correction determined by taking the ice point

after the thermometer has been used (C4).

As seen in Tables 5(a) and 5(b), if the thermometer is to be

calibrated at several points over a narrow temperature range, it

27
is not necessary to take an ice point after every calibration

point, but one should be taken at intervals of every 200 scale

divisions. The ice-point correction between these points can be

determined by interpolation. Adjusted scale corrections at temperatures between the calibration points can also be determined

by interpolation.

In the far left and right columns of Table 5(b) are shown

the calculations used to obtain T for Primary Standards 1 and 2.

For example, at the nominal temperature 20 ^C, Primary

Thermometer 1 had the following parameters: T = 19.76;

Ci = +0.01; C4 = +0.20, and thus T was calculated to be 19.97 ^C.

By a similar technique. Primary Thermometer 2 also indicated that

the temperature of the calibration bath medium was 19.97 ^C.

After the temperature of the bath has been determined for

each primary standard and the calculated temperatures are found

TABLE 5 (a)

SAMPLE CALIBRATION SHEET

Readings (one observer)

Nominal

Temp (°C) Pr imaryStandard 1 Tl T2 T3 T4

Pr imary

Standard 2

-0.10 -0.10 -0 . 03 +0.01

20 19 .75

19 .76 19.9219.93

19 .94

19 .95 19.9 3 19 .93 19.9920.00

19.7 2 19.73

0

40

-0.20

39.71

39 .72 39.90

39.92 39.90

39.91

39.88

39.88 39.8339 .85

-0 .09 39.70

39.71

60 59.70

59.71

59.86

59.86 59.9359 .94 59.8959 .90 59.8059 .81 59.6859.69

-0.22 -0. 10

80 79.79

79 .80 80.03

80 .04 79.9679 .96 79.9079.91

80.02

80 .03 79.81

79.81

100 99.71

99.71

99.88 99.8899.86 99.88

99.86

99 .87 99.9199 .92 99.7399 .74

-0.23 -0.11

2 8
TABLE 5(b)

SAMPLE CALIBRATION SHEET

CtonputatiDns (one observer)

Primary Primary

Standard 1 Tl T2 T3 T4 Standard 2

-0.10 -0.10 -0.03 +0.01

+ .10 + .10 + .03 - .01^

19.76^ 19.75 19.92 19.94 19.93 19.99 19.72 19.72^

+.01^^ 19.76 19.93 19.95 19.93 20.00 19.73 +.16° +.20^ +.09^

^ 19.92 19.94 19.93 20.00^ —

< ^19.97

+.05 +.03 +.04 -.03

-0.20 : -0.09

5"

.07 -.06 -.04 .00

-.04 +.04 +.10 -.02

g 19.97®

39.72 39.71 39.90 39.90 39.88 39.83 39.70 39.70 -.08, 39.72 39.92 39.91 39.88 39.85 39.71 +.02^ +.21^ +.10*^ 39.91 39.90 39.88 39.84 39-85 < 39.84 ^> 3^.82

59.70 59.70 59.86 59.93 59.89 59.80 59.68 59.68 -.04 59.71 59.86 59.94 59.90 59.81 59.69 +.08 +.22 +.10

59.86 59.94 59.90 59.80

59.88 < 59.87 > ^9.86

-0.22 -0.10 +.01 -.07 -.03 +.07

79.80 79.79 80.03 79.96 79.90 80.02 79.81 79.81 -.02 79.80 80.04 79.96 79.91 80.03 79.81 +.08 +.22 +.10

80.04 79.96 79.90 80.02 80-00 < 80.00 ^> 79.99

99.71 99.71 99.88 99.86 99.86 99.91 99.73 99.74 -.06 99.71 99.88 99.88 99.87 99.92 99.74 +.02

+.23 +.11

99.88 99.87 99.86 99.92

99-88 < ^99.88 ^> 99.87

-0.23 -0.11

.00 +.01 +.02 -.04

^Average of two readings for the primary standard thermometer (whai rounding off

.the ev^ nunber was chosen)

.

Mjusted scale correction corputed for primary standard at indicated tenperature.

^Ice-point correction - tak&n after calibratiOTi point.

Interpolated ice-point ccarrection. Tteiqperature of calibration bath medium as determined by the corrected primary

^tandard.

Average of two readings for the thermDmeters being calibrated.

^Iteicperature of calibraticm batli medium as determined by average of two cor-

, rected primary standard thermometers. Corrections given on Report of Calibration for thermometers under test.

29
to agree within 0.2 of a scale division (in our example,

0.2 X 0.2 = .04 °C) , or within other limits established by the

operator's laboratory, the two values are averaged to obtain the

temperature of the calibration bath medium. In our example this

average is 19.97 oc.

3.3.9 Determining corrections for thermometers being calibrated

Having calculated the correct temperature of the two primary

thermometers as described in Section 3.3.8, we may now obtain the

corrections for thermometers Tl, T2, T3, and T4.

The two readings of each thermometer being calibrated and

the check standard are averaged and compared to the temperature

of the bath medium. If the thermometer under test reads higher

than the bath temperature, the correction is negative; if the

thermometer under test reads lower than the bath temperature, the

correction is positive.

Using the data taken for thermometers in Table 5(b) and the

computed bath temperature of T = 19.97 °C, we calculate that

their respective corrections are +0.05, +0.03, +0.04 and -0.03

at 20 °C. This procedure is followed for the other temperatures,

thereby generating the data for the Reports of Calibration for

these three thermometers. (The Report of Calibration for thermometer Serial Number Tl is given as Figures 17 and 18.)

Ice points should be taken on the thermometers that were

calibrated not less than 72 hours after test. The second ice

point should agree with the initial ice point within the expected

accuracy of the thermometer. Thermometers with a range of 0 to

100 OC and graduated in 0.2 oc have an expected accuracy of

0.05 Oc at the calibration points (see Section 6 of NBS Monograph

150 [1]). The ice points should agree within this limit. If the

two ice points are not in agreement, indicating that the bulb has

not recovered from expanding after heating (due probably to

improper annealing), then the thermometer has to be completely

recalibrated.

The data should be analyzed to see if the results are reasonable. A very useful technique for analysis is to plot the

calibration data. An example for a total-immersion thermometer

graduated in 0.2 oc divisions is shown in Figure 11. The corrections at 80 oc and 100 oc are suspect for two reasons: 1) the

magnitude of the difference between adjacent corrections is close

to one scale division, and 2) the sign of the correction changes.

A deviation curve shown in Figure 11 can occur for liquid-inglass-thermometers. If these data were taken by two observers,

the probability for reading error is small and recalibration in

this case is not necessary. If, on the other hand, the data were

obtained by one observer, we recommend recalibration at 80 oc and

100 oc.

The plot may indicate another effect. If the trend of the

curve is downward as the temperature is increased, the

thermometer probably has gas in the bulb. The gas should be

removed (see Section 8.4 of NBS Monograph 150 [1]) and the

thermometer retested.

30
.20

.10 - co

o

o

w

oo

.10

00 -JI

Uncertainty

1 Scale

Division

20 40 60

Temperature °C

80 100

Figure 11. Thermometer correction versus temperature,

3.3.10 Stem-temperature corrections

If the thermometer being calibrated is a total-immersion

thermometer used at partial immersion or an ASTM partialimmersion thermometer requiring a correction that reflects a specified stem temperature, it will be necessary to correct for

the variation in the stem temperature.

Use the following formula to determine the stem-temperature

correction.

Stem-temperature correction = kn(tobs ~ ^sp) (3)

k = differential expansion coefficient of mercury (or

other thermometric liquid) in the particular kind

of glass of which the thermometer is made,

n = number of scale degrees equivalent to the length

of emergent stem (including the evaluated area

above the immersion line) ,

tobs ~ observed mean temperature of the emergent stem

(auxiliary thermometer reading),

tsp ~ specified mean temperature of the emergent stem

(for which reported scale correction applies)

.

All stem-temperature corrections require that four values be

determined. The first value is the differential expansion

31
coefficient of mercury (or other ther mome tr ic liquid) in the

particular kind of glass from which the thermometer is made (k).

These values have been established and used for many years and

are given in NBS Monograph 150 [1], Table 4, and discussed in

Section 5.3 of that publication.

The second value needed is the number of scale degrees

equivalent to the length of emergent stem (n). This is

essentially the number of degrees that is in the incorrect

temperature environment. If it is a total-immersion thermometer

with a portion of the stem above the bath, then the value n is

the number of degrees from the degree mark at the surface of the

bath medium to the meniscus (the top of the mercury column). If

the thermometer is a partial-immersion thermometer the value n

is the number of degrees from the immersion line to the meniscus.

Since partial-immersion thermometers are not graduated to the

immersion line, there is a portion of the stem containing no

scale divisions. It must be determined how many degrees could be

inscribed in this portion of stem and that number included in the

value n. One way of doing this is to measure the distance from

the immersion line to the lowest scale graduation, then move the

ruler on the thermometer scale and determine how many degrees

this distance represents.

The third value needed is the observed average temperature

of the emergent stem (tobs)* This value is found by actually

measuring the temperature of the environment where the emergent

stem is located. The preferred way of doing this involves the

use of faden thermometers as described in Section 5.3a of NBS

Monograph 150 [1]. Because the availability of faden thermometers is limited (see Section 9.4 of this Monograph), the

emergent stem temperature can also be measured by using one or

more auxiliary thermometers. Whether one, two, or even three,

auxiliary thermometers are needed depends on the length of

mercury column emergent from the bath. Generally an auxiliary

thermometer should be attached for every 120-mm to 150-mm length

of emergent mercury column. If one auxiliary thermometer is

used, then the center of its bulb should be placed at the

midpoint of the emergent mercury column. If two auxiliary

thermometers are used, then the center of their bulbs should be

placed at 1/3 and 2/3 the height of the emergent mercury column.

An average of the two auxiliary thermometer readings is used as

the temperature of the emergent stem.

The fourth value is the specified average temperature of the

emergent stem (tgp). This value is the temperature at which the

emergent stem should be. If the thermometer is a total-immersion

thermometer having a portion of the stem out of the medium being

measured, the specified temperature of that portion of the stem

should be the temperature of the medium. If it is an ASTM partial-immersion thermometer, the specified temperatures should be

the values given in ASTM Specification E-1 [3].

The stem-temperature correction is added to or subtracted

from the initial correction. Examples are given in Tables 6(a)

32
and 6(b) and a discussion is given in Section 5.3 of NBS

Monograph 150 [1].

In Tables 6(a) and 6(b), Thermometer T6 is an ASTM 33C

thermometer and at 40 the correction should reflect a stem

temperature of 30 as specified in ASTM E-1 [3]. The length of

emergent mercury column is equal to 314 mm and the centers of the

auxiliary thermometer bulbs are placed adjacent to the stem

105 mm and 209 mm from the immersion line. This procedure is

shown in Figure 12.

TABLE 6(a)

Nominal

Temp (°C)

EXAMPLE OF STEM-TEMPERATURE CORRECTIONS

Readings (two observers)

Pr imary

Standard 3

T6 ASTM 3 3C

50 mm Immers ion

T7 ASTM 3 4C

50 mm

Immers ion

Pr imary

Standard 4 Auxi Aux40 39.909 39.96 — 39.882 31 26 39.923 39.97 39.883 31 26

39.919 39.96 ~ 39.901

39.933 39.96 39.902

0 -.138 -.151

-.139 -.150

45 44.770 — 44.85 44.751 33 44.784 44.86 44.750 33

44.784 — 44.86 44.751

44.793 44.87 44.752

0 -.139 -.154

-.139 -.153

The indication on auxiliary 1 thermometer gives the average

temperature of the lower portion of the emergent stem of T6 and

that on auxiliary 2 thermometer the average temperature of the

upper portion of emergent stem of T6. The average temperature of

the emergent stem is the average temperature of the two auxiliary

thermometers. Since in the example there are two observers, each

auxiliary thermometer should be read twice. An average of the

two values for each auxiliary thermometer is used.

33
TABLE 6 (b)

EXAMPLE OF STEM- TEMPERATURE CORRECTIONS

Computations (two observers)

T6 T7 ASTM 3 3C ASTM 34C

Pr imary 50 mm 50 mm Pr ima ry Aux^ Aux. Standard 3 Immersion Immers ion Standard 4

39 .916^

-.032°

+.138°

40 .022*^

39.926^

-.032°

+ .138^^

40 .032'

44.777

-.042

+ .139 44.874

39 .909 39.923

3 9.919

39 .933

, 138

.139

44.770

44 .784

39 .96 39.97

39 .96*

+ .06-

39.96

39.96

39 ,96*

+ .08^

+ .07" -.02^

+ .05-

-40 .020'

-40 .036'

-44 .872-

44 .85 44 .86

44 .86

39.882

39.883

39 .901

39 .902

-. 151

-.150

44 .751

44 .750

a 39 .882

-.013"^

+ .150*^

40 .019

39 .902f

-.013'^ +.150°

40 .039"^

44 .750 -.035

+ .154

44 .869

31

31 2626

33

33

+ .01

44.788

-.042

+ .139

44 .885

44.784

44.793

-. 139

-.139

-44 .878-

44 .86 44 .87

44 .86

+ .02

+ .02 -.02

.00

44 .751

44 .752

-. 154

-.153

44 .752 -.035

+ . 154

44 .871

Average of two readings for the primary standard thermometer (when rounding off, the even number

j^was chosen) . ^Adjusted scale correction computed for primary standard at indicated temperature.

^Ice-point correction - taken after calibration point.

^Temperature of calibration bath medium as determined by the corrected primary standard.

^Average of two readings for the thermometer being calibrated.

Temperature of calibration bath medium as determined by average of two corrected primary standard

thermome ter s. ^Correction for thermometer under test excluding possible stem-temperature correction.

^Average correction of two observers.

.Stem-temperature correction (see text for computation).

-"Algebraic addition of average correction of two observers and stem-temperature correction. This

value is given on the Report of Calibration.

34
/N

I

314 mm

A

78 Degrees

209 mm i

20 Degrees

120 mm

105 mm i

60 mm

15 Degrees 15 Degrees

N/ Si/

i i

Figure 12. Measuring stem

ten^erature on an ASTM 33C

thermometer using two

auxiliary thermometers.

Figure 13. Measuring stem

temperature on an ASTM 34C

thermometer using one

auxiliary thermometer.

35
The average observed stem temperature (tobs) 28.5

[(31+26)/2], the specified stem temperature (tgp) is 30 °C, the

coefficient of expansion of mercury in glass for Celsius

thermometers is 0.00016 and the number of degrees emergent from

the bath, including the portion from the immersion line to the

first graduation (n), is 93 degrees.

Stem-temperature correction - 0 .00016 n (tobs~*-sp^

= 0.00016 (93) (28.5-30)

= -0.02 OC

Notice that the values tgp and tpj-,5 are reversed in the above

formula, as compared to the one given in NBS Monograph 150 [1] , since we are applying this correction to a correction and not to

the thermometer reading itself.

As a second example, thermometer T7 is an ASTM 34C

thermometer. Since the length of the emergent mercury column is

only 120 mm, placing the center of the bulb of one auxiliary

thermometer adjacent to thermometer T7, 60 mm from the immersion

line, is sufficient to measure the average temperature of the

emergent stem. This example is shown in Figure 13.

In this example, the average observed stem temperature

(tobs) is 33 °C, the specified stem temperature is 37 °C, the coefficient of expansion of mercury in glass is 0.00016 , and the

number of degrees from the immersion line to the mercury meniscus

is 35 degrees.

Stem-temperature correction = 0.00016 n (tobs~tsp)

= 0.00016 (35) (33-37)

= -0.02 OC

4 . DCXniMENTATION

4.1 Report of Calibration

After corrections are determined (see Section 3.3.9 of this

Monograph), a Report of Calibration is generated for each

thermometer. The Report should include the following:

1) Company, agency, or establishment that submitted the

thermometer,

2) Manufacturer's trade name and serial number of the

thermome ter

,

36
3) Range and graduation interval,

4) Calibration temperatures and corrections,

5) Temperature scale used (IPTS-68)

,

6) Instructions for use of corrections,

7) Estimated uncertainties,

8) Other notes concerning how the thermometer was

calibrated and how it can be recalibrated.

The following list contains examples of notes given on NBS

Reports of Calibration.

ALL TOTAL- IMMERSION THERMOMETERS.

The tabulated corrections apply for the condition of total

immersion of the bulb and liquid column. If the thermometer

is used at partial immersion, apply an emergent stem correction as explained in the accompanying stem correction sheet.

(See Figures 14 and 15.)

ALL PARTIAL- IMMERSION THERMOMETERS WITH NO ST EM -TEMPERATURE

CORRECTIONS.

The thermometer was tested in a large, closed-top,

electrically heated, liquid bath at an immersion of . The temperature of the room was about 2 3 °C (7 3 °F). If the

thermometer is used under conditions which would cause the

average temperature of the emergent liquid column to differ

markedly from that prevailing in the test, appreciable

differences in the indications of the thermometer would

result.

PARTIAL- IMMERSION THERMOMETERS WITH THE SAME STEM TEMPERATURE AT

EACH CALIBRATION POINT.

These corrections are applicable for immersion and

for an emergent stem temperature of . PARTIAL- IMMERSION THERMOMETERS WITH DIFFERENT STEM TEMPERATURES

AT EACH CALIBRATION POINT.

These corrections are applicable for immersion and

for temperatures of the emergent mercury column as given

below : Reading of thermometer

Temperature of emergent mercury . RECALIBRATION NOTE USING ICE POINT.

The tabulated corrections apply provided the ice-point

reading is . If the ice-point reading is found to

be higher (or lower) than stated, all other readings will be

higher (or lower) by the same amount.

37 it)
RECALIBRATION NOTE USING ICE POINT FOR TOTAL- IMMERSION

THERMOMETERS GRADUATED UNDER 15 0 °C AND IN GRADUATION INTERVALS

OF 0.2 OC OR LESS.

The tabulated corrections apply provided the ice-point

reading taken after exposure for not fewer than 3 days to a temperature of about 23 °C (73 °F) is . If the icepoint reading is found to be higher (or lower) than stated,

all other readings will be higher (or lower) by the same

amount. If the thermometer is used at a given temperature

shortly after being heated to a higher temperature, an error

of 0.01 ° or less, for each 10° difference between the two

temperatures, may be introduced. The tabulated corrections

apply if the thermometer is used in its upright position; if

used in a horizontal position, the indications may be a few

hundredths of a degree higher.

STEAM POINT USED AS REFERENCE POINT.

The tabulated corrections apply provided the reading when

the thermometer is immersed in steam at 100 °C (212 *^F) is

. If the reading is found to be higher (or lower)

than stated, all other readings will be higher (or lower) by

the same amount. The temperature of steam is 100 *-*C (212 Of) only if the pressure is 760 mm (29.9 21 inches). If

the pressure differs from 760 mm (29.921 inches), allowance

must be made for this. If the pressure is higher (or lower)

than 760 mm (29.921 inches) the temperature will be higher

(or lower) than 100 ^C (212 °F) by approximately 0.037 °C

per mm difference (1.68 °F per inch difference).

CALORIMETRIC THERMOMETERS HEATED TO TOP POINT BEFORE TESTING.

The thermometer, before testing, was heated to the

temperature of the highest test point. The application of

the tabular corrections to the readings of the thermometer

will give true temperature differences provided the

thermometer is used in its upright position and is heated

previously (within an hour before using) to the highest

temperature to be measured.

SETTING FOR BECK MANN THERMOMETERS.

The tabulated corrections apply for a "setting" of 20 °C.

Setting factors for use with other settings are given on the

accompanying sheet. (See Figure 16.)

ICE POINT TAKEN AFTER TEST, SUCH AS CLINICAL AND KINEMATIC

VISCOSITY THERMOMETERS.

The tabulated corrections apply for the condition of

immersion indicated provided the ice-point reading, taken

after heating to for not fewer than 3 minutes, is

. If the ice-point reading, which should be taken

within 5 minutes after removal of the thermometer from the

heated bath, is found to be higher (or lower) than stated

I 38
all other readings will be higher (or lower) by the same

amount.

If there is any other information concerning the thermometer

that the laboratory supervisor thinks is important for the

user to know, a note should be written and included on the

Report. If the thermometer was calibrated against NBS

standards, it might be advisable to state that the thermometer is traceable to NBS and give the serial numbers of

the standards and the NBS test number that the standards

were tested under. (Figures 17 and 18 show a sample Report

of Calibration that would be issued by NBS for Thermometer

Tl.

)

4.2 Report of Test

A Report of Test is issued for an electronic temperature

measuring system and should also include the name of the company

that submitted the system, identification of the system (serial

number and model), temperature scale used, calibration temperatures, and output of the sensor, which may be in volts, ohms,

hertz, etc. All conditions of the test, such as the immersion

depth of the probe and settings on the electronic readout device,

should also be included in the Report. (Figure 19 is a sample

Report of Test issued by NBS.)

39
CORRECTION FOR EMERGENT STEM

This stem correction sheet is designed to explain

the process of computing the corrections for emergent

stem, but has not been specifically adapted to the

individual thermometer with which it is issued.

If a total-immersion thermometer is actually used

with a part of the liquid column in the capillary

emergent from the bath, a stem correction must be

calculated and applied as explained below, making

use of the formulas given.

The general formula used in computing the correction for emergent stem is: Stem correction=A;x«(*i —t), where fc=the differential expansion coefficient of mercury (or other liquid in the thermometer such as alcohol, toluene, pentane,

etc.) in the particular kind of glass of which the thermometer is made; (for

numerical values see below)

Tirrnumber of degrees emergent from the bath;

fi=temperature of the bath; t=mean temperature of the emergent stem. Example 1.—Suppose the observed reading was

84.76° C and the thermometer was immersed to the

20° mark on the scale, so that 65° of the column projected into the air, and the mean temperature of the emergent column was found to be 38° C, then

Stem correction=0.00016x65 (85°—38°) =+0.49° C. The true temperature is therefore the observed

reading, 84.76° C, + tabular correction as interpolated

from the report, -(- emergent stem correction (+0.49° C).

Example 2.—Suppose the observed reading was

780° F and the thermometer was immersed to the 200° mark on the scale, so that 580° of the column

projected into the air, and the mean temperature of the emergent column was found to be 170° F, then

Stem correction=0.00009x580(780°—170°)=+32° F

as a first approximation.

Since the result shows that the bath temperature was approximately 780° F+32° F, a second approximation should be made, using h=S12° F instead of ii=780° F. This givesStem correction=0.00009x580(812°—170°)=+34° F. The true temperature is therefore the observed

reading, 780° F, + tabular correction as interpolated

from the report, + emergent stem correction (+34° F).

It will be noted that if the average temperature of the stem is below that of the bulb, the sign of the cor- rection will be +, while if the temperature of the stem is above that of the bulb, the sign of the correction will be — .

EXPLANATORY NOTES ON THE EMERGENT STEM CORRECTION

Some thermometers are pointed and graduated by the maker to read correct, or approximately correct, temperatures when the bulb and the entire liquid index In the stem are exposed to the temperature to be measured, while other thermometers are so pointed and graduated that they will read correct, or approximately correct, temperatures when the bulb and only a short length of the stem of the thermometer are immersed in the bath, the temperature of which is to be measured. Thermometers of the former class are known as "total-immersion thermometers," and

those of the latter class as "partial-immersion thermometers."

Total-immersion thermometers are tested under the condition

of total immersion and the corrections resulting from such a test will serve to reduce the observed readings of the thermometer to true temperatures only if the thermometer is used as a total- immersion thermometer. If such a thermometer is actually used

as a partial-immersion thermometer, i.e., with a part of the mercury column emergent into the space above the bath, and

with the emergent stem therefore either colder (or warmer) than

the bulb, the thermometer will obviously read lower (or higher) than it would under the condition of total Immersion. Hence, if a total-immersion thermometer is so used, a so-called stem

correction must be applied to the observed reading in addition

to the correction taken from the accompanying table of corrections. This stem correction is very large if the number of degrees

emergent and the difference of temperature between the bath and the space above it are large. It may amount to more than

20° C (36° F) for measurements made with a mercury

thermometer at 400° C (750° F).

The coefficient k is different for different kinds of glass and, even for the same kind of glass, it differs for different temperature intervals, i. e., different values of (f, — t). Values for k

for two widely used thermometric glasses, for use in calculating stem corrections are tabulated as follows

VALUES OF k FOR MERCURY-IN-GLASS THERMOMETERS

For Celsius thermometers For Fahrenheit thermometers Mean

temp.

t, + t 2

k

for "normal"

glass

k

for "borosillcate" glass

Mean

temp.

t, + t 2

k

for "normal"

glass

A: for "borosilicate" glass 0° 100

150 200

250

300

350

400 450

0.000158 158 158

159

161

164

0.000164 164

165

167

170

174 178

183 188

0° 200

300

400

500

600

700

800

0.000088

8888

8990

92

0.000091

919293

9597

.000100 103

If the kind of glass of which the thermometer is made is known, the value of k to be used in computing the stem correction mav be taken from the above table. If the kind of glass is not known, use fc =0.00010 for Celsius or 0.00009 for Fahrenheit

thermometers. High-grade thermometers are now generally made

of "normal" or "borosilicate" glasses. If a thermometer is grad- uated only to about 450° C (850° F), it may be made of either

of the above glasses; if it is graduated to 500° C (932° F) and

is actually usable at that temperature, it is made of one of the

borosilicate glasses or a similar glass. The expansions of liquids such as alcohol, toluene, etc., vary

quite rapidly with the temperature, so that k varies considerably

for different temperature intervals. An approximate stem cor- rection for such thermometers may be calculated by taking k in the above equation = 0.001 for Celsius thermometers or 0.0006

for Fahrenheit thermometers. The value of f, the mean temperature of the emergent stem, is the most difficult of the terms in the above formula to estimate.

It may be quite accurately measured by the use of special capil- lary thermometers. This is. however, very rarely done except in the testing laboratory, and then only when the stem correction must be determined with considerable precision (to 10 percent or

better). In general, the value of t may be determined to a suffi- cient approximation by judgment or preferably by suspending an

auxiliary thermometer close beside the emergent stem, with the bulb of the aux lary thermometer somewhat nearer to the top

of the bath than to the liquid meniscus.

Figure 14. Stem temperature explanation sheet attached to

reports for total -immersion thermometers.

40
DIFFERENTIAL CORRECTION FOR EMERGENT STEM

This sheet is designed to explain a convenient process

of computing tlie differential correction for emergent stem but has not been specifically adapted to the individual thermometer with which it is issued.

If a calorinietric or differential or a metastatic (Beckmann) thermometer > is actually used with a part of its stem emergent from the bath, a differential stem correction must be calculated and applied as explained below, making use of the formulas there

given.

Example 1. —Suppose the point. 7°. to which the thermometer was immersed was 16° C : its initial reading, T",, was 24.127° : its final reading, T"-, was

27.876° : the mean temperature. t°. of the emergent

stem Avas 26° ; then

Differential stem correction=0.00016(28 — 24) (24° +

28° -16° -26°) = +0.006°.

The result, 0.006°, should be added to the difference between T'z and T°i, found after applying to each

the corrections tabulated on the Report of Calibration.

ICxAMPLE 2. —Suppose a Beckmann thermometer was

immersed to the 0° mark on its scale; its initial reading,

r°,. was 2.058°; its final reading,'r°2, was 5.127°; the mean temperature, *°, of the emergent stem was 24° C ; and the setting, S°, was 25° C; thenDifferential stem correction =0.00016 (5-2) (25° +

2° + 5° -24°) = +0.004°.

The result +0.004°, should be added to the difference l>etween T°: and !r°i, found after applying to each the

corrections tabulated on the Report of Callbratioffi and

multiplying the difference by the setting factor (1.0014

for a setting of 25° )

.

EXPLANATORY NOTES ON THE EMERGENT STEM CORRECTION

Calorimetric and differt'iitial and metastatic (Beckmann)

thermometers are pointed and Kraduated by the makers to read correct or approximately correct temperature intervals when the bnlb and the entire mercury index in the stem are exposed to the temperature to be measured, and such thermometers are standardized for the condition of "total immersion," and the corrections Riven in reports apply for the con- dition of total Immersion.

In practicall.v all cases, however, such thermometers are actually used with only the bulb and a portion of the stem immersed, the remainder of the stem projecting into the air above the bath. In such cases the accuracy attained can be increased by

applying' the necessary corrections for the emergent stem. The emergent stem correction corresponding to any reading may be computed by means of the formula

Stem correction =fcx "( 7'°

f°) ; where fc=:the differential expansion coefficient of mercury in glass ; = 0.00016 for Celsius thermometers ; = 0.00009 for Fahrenheit tlieruKmieters : n=number of degrees emergent from the bath : r° = temperature of the bath:

J° = niean temperature of the emergent stem.

However, in differential measurements the parts of the

scale on which readings are made are. in general, emergent from the bath and it is usually permissible to assume that f

will be constant during the measurement. In this case, in- stead of calculating the stem corrections for the initial and

final readings, and applying them separatel.v. the differential stem eorrection can be more conveniently computed by using

the appropriate formula of the two given below.

Dijfferentinl emerfjent stem correction for ordinary thermomFterK.- -The differential stem correction may he computed from the following formula :

Differential stem correction = fcXrf(7"'i + 7'°:— 7° —f°) ; J-=0.00016 for Celsius thermometers : = 0.00009 for I'ahrenhelt thermometers:

7'°i = the Initial reading;

7'°- = thp final reading:

(7 = 7'°~T°i :

1° = scale reading to which the thermometer was Immersed ; t°=mean temperature of the emergent stem.

It should be noted that this differential stem correction, as

well as the difference d" , may be either positive or negative, and the correction must be applied with due regard to these

signs. Differential emergent stem coiTection for metastatic (licckmnnn) thermometers.—The differential emergent stem

correction may be computed from the following formula, pro- vided the thermometer Is immersed to its 0° mark : Differential stem correction =fcXrf(S° + 7'°i+7"'2— 1°) ; where S° = the "setting" of the thermometer, and fc, d. T°i. T°2 and t° have the meanings given in the preceding para- graph.

It should be noted that this differential stem correction,

as well as the difference d. may be either positive or negative, and the correction must be applied with due regard to these signs. A Beckmann thermometer of the ordinary type should not

be used with any part of the lower portion of the stem ex- posed, as this part ma.v contain from 5 to 10 times as much

mercury per centimeter as the graduated portion, and if ex- posed introduces a large and uncertain error.

In case it Is necessary to use such a thermometer with some

of the lower portion of the stem emergent from the bath, the necessary correction may be computed from the above

formula, provided S° in the formula is replaced by S°+m°,

where TO is the number of degrees the temperature of the thermometer must be lowered to bring the meniscus from

the zero position on its scale to the point of immersion.

If the thermometer Is immersed to some point other than Its 0° mark, as would ordinarily be the case with thermometers

having the 0° graduation at the top of the scale, the differ- ential stem correction may also be computed from the above

formula, provided 8° in the formula is replaced by iS°+m°, where m Is the number of degrees the temperature of the thermometer must be lowered to bring the meniscus from the

zero position on its scale to the point of immersion. Provided

the points at which readings are made are above the point

to which the thermometer Is immersed, the preceding state- ment is applicable whether the point to which the thermometer is immersed is on the scale or below it.

Figure 15. Stem temperature explanation sheet attached to

reports for calorimetric thermometers calibrated at total

immersion.

41
This sheet gives a table of setting factors and an example to explain the processes

employed in calculations. The example, however, has not been specifically adapted to the

individual thermometer with which this sheet is issued.

The thermometer is said to have a given setting, for instance 20°C, if its scale reading is 0° when the temperature of the bulb is 20°C.

The following table, calculated for thermometers of Jena 16'" glass, gives a series of

factors corresponding to different settings by which each observed temperature difference

must be multiplied after applying the corrections given on the preceding page. This factor is made unity for a setting of 20°C.

For any scale length up to 15°

Setting Factor Setting Factor

0°C 0.9935 55°C 1 .0092

5 0.9951 60 1.0102

10 0.9967 65 1.0112

15 0.9984 70 1.0121

20 1 .0000 75 1.0129

25 1.0014 80 1.0137

30 1.0029 85 1.0146

35 1 .0042 90 1.0154

40 1.0056 95 1.0162

45 1.0070 100 1.0170

50 1.0082

As an illustration, suppose the following observations were made:

Setting =25 C. Lower reading = 2.058"

Stem temperature = 24° Upper reading = 5.1 27*' Lower Upper

Observed reading = 2.058 5.127

Correction from certificate = +.005 —.008

Corrected upper reading = 5.119

CorrectedTower reading = 2.063

Difference = 3.056

Setting factor 1.0014 corr. +.004

Emergent stem correction* = +.004

Corrected difference = 3.064

See accompanying stem correction slieet.

Figure 16. Table of setting factors attached to reports for

Beckmann thermometers.

42
U.S. DEPARTMENT OF COMMERCE

NflTIONflL BUREAU OF STftNDftRDS

NflTIONflL MEASUREMENT LABORfiTORY

gaithersburg; md. aeasg

REPORT OF CALIBRATION

LIQUID-IN-GLASS THERMOMETER.

TESTED FOR: NATIONAL BUREAU OF STANDARDS

DIVISION 5££

MARKED: SURETY Tl

RANGE: -£ TO DEGREES C IN 0. £ DEGREE

THERMOMETER CORRECTION

READING (IPTS-68)**

-. lei C . IIZI C

£0. m . 05

40. 00 -. 07

60.00 .01

80. 00 -. 04

100.00 .00

**ALL TEMPERATURES IN THIS REPORT ARE BASED ON THE INTERNATIONAL

PRACTICAL TEMPERATURE SCALE OF 1968, IPTS-6a. THIS TEMPERATURE SCALE

WAS ADOPTED BY THE INTERNATIONAL COMMITTEE OF WEIGHTS AND MEASURES AT

ITS MEETING IN OCTOBER, 1968, AND IS DESCRIBED IN "THE INTERNATIONAL

PRACTICAL TEMPERATURE SCALE OF 1968 AMENDED EDITION OF 1975,"

METROLOGIA 1£, NO. 1, 7-17 (1976).

ESTIMATED UNCERTAINTIES IN THE ABOVE CORRECTIONS DO NOT EXCEED 0.05

DEGREE UP TO 102 DEGREES C . FOR A DISCUSSION OF ACCURACIES ATTAINABLE WITH SUCH THERMOMETERS SEE

NATIONAL BUREAU OF STANDARDS MONOGRAPH 150, LI QUID- IN-GLASS

THERMOMETRY.

IF NO SIGN IS GIVEN ON THE CORRECTION, THE TRUE TEMPERATURE IS HIGHER

THAN THE INDICATED TEMPERATURE; IF THE SIGN GIVEN IS NEGATIVE, THE TRUE

TEMPERATURE IS LOWER THAN THE INDICATED TEMPERATURE. TO USE THE CORRECTIONS PROPERLY, REFERENCE SHOULD BE MADE TO THE NOTES GIVEN BELOW.

THE TABULATED CORRECTIONS APPLY FOR THE CONDITION OF TOTAL IMMERSION OF

THE BULB AND LIQUID COLUMN. IF THE THERMOMETER IS USED AT PARTIAL IMMERSION, APPLY AN EMERGENT STEM CORRECTION AS EXPLAINED IN THE ACCOMPANYING

STEM CORRECTION SHEET.

TEST NUMBER 999999

COMPLETED l-££-85

Figure 17. Facsimile of page 1 of an NBS Report of Calibration

for thermometer, Serial Number Tl.
PfiBE £

THE TflBULfiTED CORRECTIONS APPLY PROVIDED THE ICE-POINT READING, TOKEN

OFTER EXPOSURE FOR NOT FEWER THAN 3 DAYS TO A TEMPERATURE OF ABOUT 23

DEGREES C (73 DEGREES F), IS -0.10 C . IF THE ICE-POINT READING IS FOUND

TO BE HIGHER <0R LOWER) THAN STATED, ALL OTHER READINGS WILL BE HIGHER

(OR LOWER) BY THE SAME AMOUNT. IF THE THERMOMETER IS USED AT A GIVEN

TEMPERATURE SHORTLY AFTER BEING HEATED TO A HIGHER TEMPERATURE, AN ERROR

OF 0.01 DEGREE OR LESS, FOR EACH 10-DEGREE DIFFERENCE BETWEEN THE TWO

TEMPERATURES, MAY BE INTRODUCED. THE TABULATED CORRECTIONS APPLY IF THE

THERMOMETER IS USED IN ITS UPRIGHT POSITION; IF USED IN A HORIZONTAL

POSITION, THE INDICATIONS MAY BE A FEW HUNDREDTHS OF A DEGREE HIGHER.

FOR THE DIRECTOR,

NATIONAL MEASUREMENT LABORATORY

TEST NUMBER 999999

COMPLETED l-c:£-85 ROBERT J. SOULEN, JR.

CHIEF, TEMPERATURE AND PRESSURE DIVISION

CENTER FOR BASIC STANDARDS

Figure 18. Facsimile of page 2 of an NBS Report of Calibration

for thermometer, Serial Number Tl.

44
FORM NBS—259 IREV. I2-7«l U.S. Department of Commerce

NATIONAL BUREAU OF STANDARDS

GAITHERSBURG, MD 20899

REPORT OF TEST

Surety Model 1234 RTD Digital Thermometer S/N 4567

Resistance Thermometer S/N 999

Submitted by

National Bureau of Standards

Gaithersburg, Maryland

Resistance thermometer S/N 999, which was attached to a Surety Model 1234 RTD Digital Thermometer S/N 4567, was calibrated by intercomparison with a standard platinum resistance thermometer in stirred liquid comparison baths at

four temperatures. The unit was allowed to warm up 20 hours before

measurements were made. An ice point was taken before and after test by

immersing the probe in an ice bath. The probe was immersed to a depth of

18 cm for all measurements and the ambient temperature of the laboratory was

23 °C. The results are given below.

Bath Temperature

0.00 °C

50.00

100.00

125.00

150.00

0.00

Reading of

Resistance Thermometer

+0.40 °C

50.49

100.48

125.40

150.23

+0.40

The uncertainty of the bath temperatures given above is estimated not to

exceed 0.01 °C to 100 °C and 0.02 °C from 100 to 150 °C.

All temperatures in this report are based on the International Practical

Temperature Scale of 1968, IPTS-68. This temperature scale was adopted by the

International Committee of Weights and Measures at its meeting in October

1968, and is described in "The International Practical Temperature Scale of

1968, Amended Edition of 1975", Metrologia 12, No. 1, 7-17 (1976).

For the Director

National Measurement Laboratory

Robert J. Soulen, Jr., Chief

Temperature and Pressure Division

Center for Basic Standards

Test No. 999999

Date : May 31, 1985

Figure 19. Facsimile of an NBS Report of Test for an electronic

temperature measuring system.

45
5. MAINTAINING LABORATORY STANDARDS AND ESTIMATION OF

LABORATORY UNCERTAINTY

5.1 Estimates of Inaccuracy of Calibration of Liquid-inGlass Thermometers

The inaccuracy of a temperature measurement refers to a consistent offset or systematic difference between readings and

the proper temperature. Several factors affect the accuracy of

liquid-in-glass thermometer readings; the ones described in this

Monograph include

o Inaccuracy in IPTS-68. See Section 2.3.1.

o Stem corrections for partial-immersion thermometers.

See Section 3.3.10.

o Improper preparation of ice bath. See Section 2.2.1.

o Temperature gradients in temperature baths. See

Section 2.2.2.

o Bubbles or gas in mercury. See Section 3.1.

o Expansion of bulb glass. See Section 3.3.5.

The first factor is beyond the control of the laboratory;

the others will have to be carefully studied for the particular

laboratory, and their influence on accuracy will have to be

assessed.

To indicate how the values for inaccuracy were determined at

NBS, we note the following. Ice baths prepared over a number of

years were compared with a platinum resistance thermometer that

has a measurement inaccuracy of 0.001 OC. It was found that the

ice bath temperatures varied by ±0.005 OC, so we assign an

inaccuracy of this value to this factor. As for temperature

gradients in the constant temperature baths, three platinum

resistance thermometers read simultaneously were used to

demonstrate that, for baths used from 0 oc to 100 oc, the maximum

variation radially or horizontally was found to be 0.005 OC. For

baths used from 100 °C to 200 ^C the gradients were found to be

0.01 OC at 100 OC gradually increasing to a value of 0.025 oc at

200 *-*C. Concerning errors introduced by poor thermometer

quality, thermometers used as primary standards at NBS did not

have bubbles or gas, so these systematic errors were negligible.

Finally, when 1 iqu id- in-g la ss thermometers were used as

standards, expansion of the bulb was corrected for by the

techniques discussed in this Monograph (Sections 3.3.5 and 3.3.8)

and thus introduced no significant residual inaccuracy. We

summarize these results in Table 7.

5.2 Maintaining the Accuracy of Primary Laboratory

Standards

The experience at the NBS has been that no factors other

than those summarized in Section 5.1 have a significant effect on

the accuracy of primary liquid-in-glass thermometers. Indeed,

46
TABLE 7

MAJOR SYSTEMATIC ERRORS (INACCURACY) IN THE NBS

LIQUID- IN-GLASS THERMOMETER CALIBRATION SERVICE

Effect Inaccuracy

Range 1 Range 2

0 - 100 oc 100 - 200 oc

Ice bath 0.005 Oc 0.005 Oc

Bath gradients 0.0 05 Oc 0.01 - 0.025 Oc

TOTAL 0.010 Oc 0.015 - 0.030 Oc

many of them refer to laboratory techniques (ice bath, gradients

in constant temperature bath) that, if properly established at

the outset, will not introduce inaccuracies at a later time.

Of the remaining effects, the single largest source of

calibration offset is the bulb volume change of the thermometers.

If adjusted scale corrections are calculated for each primary

standard thermometer, this effect will be accounted for each time

the primary thermometers are used. By using two thermometers,

the likelihood that the corrections are incorrectly applied will

be small.

Use of these aforementioned techniques will preserve the

original calibration accuracy of the primary thermometers

provided by NBS. Thus the thermometers need not be returned to

NBS for recalibration. In the event that the IPTS-68 is revised,

NBS will provide the data so that the adjusted scale corrections

may be changed accordingly.

5.3 Using Check Standards for Quality Assurance

In the previous section we reviewed the role of the two

primary thermometers used in each calibration and the accuracy of

the temperatures defined by them. In this section we describe

the role of the check standard in evaluating the quality (that

is, the imprecision) of the calibration process. Remember that

the check standard is included in each calibration run. It is

easy therefore to obtain a large number of data points on the

check standard in a short period of time. We recommend that the

laboratory construct a control chart (to be defined below) from

these data. With a sufficient number of points, the data will

47
have statistical significance that, when analyzed, can indicate

clearly the performance of the laboratory.

As an example, we discuss some data obtained at NBS for

check standard Serial Number 48425. This thermometer is a total

immersion type, with a range of -2 to +102 ^C, and with scale

divisions of 0.2 ^c. Using the measurement techniques described

in this Monograph, the measurement resolution was found to be

0.01 OC (the cross hair of the telescope is placed at the center

of the scale division thus dividing the scale division into two

equal parts; the meniscus was read to 1/10 of these divisions).

This thermometer was incorporated as a check standard in several

calibrations conducted from 1976 to 1984. Measurements were

repeated at many temperatures over the full range of the thermometer. In these experiments the bath temperature was not defined

by two primary liquid-in-glass thermometers, but by a calibrated

standard platinum resistance thermometer. The imprecision of

this resistance thermometer is only 0.001 °C, so that no significant source of uncertainty was introduced in the temperature

scale. The thermometer was allowed to rest at room temperature

(23 °C) for 3 days before being calibrated, allowing the volume

of the bulb to recover from a previous thermal cycling. In

Figures 20 and 21 we have plotted control charts for this thermometer. Such charts consist of the correction for the thermometer

(before and after the ice point shift was accounted for) plotted

versus time. We chose to show the data at four temperatures.

Such plots immediately reveal the consistency of the calibration

process and can indicate calculation errors or degradation of the

process.

In addition to plotting the data in a control chart format

shown in these figures, we recommend that the same data be

submitted to elementary statistical analysis. At each

temperature the average correction and the standard deviation may

be calculated with the pressing of a few keys on almost any

scientific hand-held calculator. In this way, the quality of the

calibration process may be quantified. When this was done for

the data for check standard Serial Number 48425, it was found

that the standard deviation at all temperatures was 0,01 oc.

This is perhaps understandable, given the fact that the measurement resolution is also 0.01 oc. At any rate, as more data are

accumulated, these quantifiers may be used to watch for any

changes.

For this liquid-in-glass thermometer, the imprecision (i.e.,

standard deviation) was found to be 0.01 *^C. Since the imprecision of the primary thermometer was so small (0.001 °C) in this

case, no additional uncertainty was introduced by the primary

thermometer. This will not be the case in the State calibration

laboratory, where the primary thermometers will be liquid-inglass thermometers of performance comparable with check standard

Serial Number 48425. Thus, if two primary thermometers with

performance similar to thermometer Serial Number 48425 are used

to measure the bath temperature, and the measurements are assumed

to be independent, then the standard deviation of the temperature

48
0.20

o 0.10

u

0)oo

*0>

0)E

o

E

0) .c

0.00

0.10

-0.20

76 77 78 79 80

Year

81 82 83 84

Figure 20. Control chart for thermometer, Serial Number 48425,

at four temperatures (* at 0 ©C, + at 40 ^C, X at 80 ^c, and

# at 100 °C) . The readings were not corrected for ice-point

shifts.

u

0)ou

w

0)Eo

E

0)

0.20

P 0.10

Figure 21. Control chart for thermometer, Serial Number 48425,

at four temperatures (* at 0 oc, + at 40 oc, X at 80 OC, and

# at 100 oc) . The readings were corrected for ice-point

shifts.

49
will be ^

X (0.01 OC) = 0.014 oc. If we now measure a check

standard (again with a performance comparable with the other two)

repeatedly versus these two primary thermometers, and construct a control chart, the standard deviation will be the square root of

the sum of the squares of the standard deviation of the check

standard and of the thermometers used to define temperature.

Thus we would expect the standard deviation for a State calibration laboratory to be /3 x (.01) = 0.017 ^C. That is an increase

by a factor of^is introduced into the standard deviation

because two liquid -in-glass thermometers are used as the primary

standards instead of a platinum resistance thermometer.

Check standards were studied at NBS versus a standard

platinum resistance thermometer for four ranges in order to

determine the standard deviation. We present these results in

Table 8. Remember: The standard deviation expected at a State

calibration laboratory using two other primary thermometers

should be roughly a factor of larger than that obtained by

NBS. Note too that the standard deviation increases at higher

temperatures, and that this is most likely due to the decreasing

resolution of the liquid-in-glass thermometer and larger timevarying gradients in the bath.

TABLE 8

STANDARD DEVIATION (IMPRECISION) OF TEMPERATURE

OBTAINED WITH LIQUID- IN-GLASS THERMOMETERS COMPARED

WITH A STANDARD PLATINUM RESISTANCE THERMOMETER AT NBS

Temperature Thermometer

Range Gr aduations Resolution Std. Dev.

(OC) (OC) i°C) (°C)

0 - 50 0.1 0.005 0.006

50 - 100 0.1 0.005 0.008

0 - 100 0.2 0.01 0.009

100 - 200 0.2 0.01 0.028

With this table and the discussion above, a State calibration laboratory can estimate its best achievable imprecision.

This table should be compared with the value calculated from the

actual data taken at the laboratory. The extent to which the two

agree or disagree and the course of action to be taken depend on

the goal for precision set by the laboratory.

To construct a final total laboratory uncertainty, the

laboratory should assess the systematic errors described in

Section 5.1 and add them to the estimated imprecision. As an

example, we present in Table 9 the total uncertainty estimate for

50
the NBS liquid-in-glass calibration service. (We define the

uncertainty as the sum of the inaccuracy and three times the

imprecision)

.

TABLE 9

UNCERTAINTY ESTIMATE FOR NBS CALIBRATION SERVICE FOR

LIQUID-IN-GLASS THERM0^4ETERS

(Inaccuracy)

Temperature ( Imprec ision) Systemat ic Total

Range 3 Std. Dev. Error Uncertainty

(OC) (°C) (OC) (OC)

0-50 0 .018 0.010 0.028

50 - 100 0.024 0.010 0.034

0 - 100 0.027 0.010 0.037

100 - 200 0.084 0.015 - 0.030 0.099 - 0.114

This table merits several comments. The uncertainty of the

IPTS-68 is not included; the systematic error originates chiefly

from estimates of the operator's ability to reproduce the ice

point (0.005 ^C) , temperature gradients in the baths (0.005 to

0.025), plus the thermometer resolution. Note too, that we have

tripled the standard deviation to insure that 99.7% of the

measurements will be included. Assessment of systematic errors

is a difficult task, and each laboratory will have to determine

systematic effects according to the prescription given in this

Monograph. This table is intended to provide an indication of

the performance at NBS; practice (and thus these quantities) may

differ at other laboratories.

6. CONCLDSION

In this monograph we have indicated the procedures by which

a laboratory may be equipped and staffed so that temperature

calibrations in the range 0 to 200 ^C may be performed accurately

within ±0.1 ^C or less. The temperature scale thus established

is shown to be traceable to the IPTS-68. Detailed procedures are

given for using liquid -in -glass thermometers as primary standards

and for deriving calibration corrections for a variety of

laboratory thermometers. A checklist for establishing a calibration laboratory to the specifications described is

provided in Table 10.

51
TABLE 10

CHECKLIST FOR ESTABLISHING A

STATE CALIBRATION LABORATORY

A. Range of Calibration: 0 to +200 ^C

B. Accuracy: ±0.1 ^C; traceable to NBS

C. Types of devices that can be calibrated:

1. Liquid-in-glass thermometers

a) total-immersion

b) partial-immersion

c) ASTM

d) calo rime trie

e) Beckmann

f) clinical standards

g) kinematic viscosity

2. Electronic temperature measuring systems

D. Laboratory Specifications:

1. Dimensions: 7m X 7m X 3m

2. Other requirements

a) Constant temperature: 23 ^C ± 2 degrees

b) Constant humidity: approximately 50%

E. Equipment:

1. Thermometer rack or map case to store thermometers

2. Microscope (15- or 2 0 -power)

3. Ice bath

a) tape-wrapped Dewar flask

b) thermometer clip

c) telescope (10-power with crosshair)

d) s iphon

e) soft mallet

f) shaved ice

g) freezer or chest for ice storage

h) distilled water storage tank

i) water distillation apparatus or distilled

water supply

j) plastic gloves

k) ice shaver

52
Equipment: (continued)

4. Constant temperature baths (one oil bath may be

chosen to span the entire calibration range of

0 Oc to +200 OC)

a) Water bath (range: +1 to +95 ^C)

1) Sufficient to accommodate total-immersion

liquid-in-glass thermometers

2) Overflow provision

3) Heating/cooling coils isolated from

thermome ter s

4) Unrestricted flow for bath medium

5) Insulated

6) Thermometer holder designed to minimize heat

loss

7) Capable of maintaining bath at a constant

temperature 10.005 °C or a rise rate

of no more than 0.1 ^C in 3 minutes

8) Telescope (10-power with crosshair)

b) Oil bath (range: +95 ^C to +200 °C)

1) Same specifications as water bath

2) Oil

3) Hood vented to outside

F. Standards:

1. "Primary"

a) Two sets of total-immersion liquid-in-glass

thermometers (See Table 1) guaranteed to pass

NBS visual inspection

b) Calibrated by NBS or other traceable laboratory

2. "Check"

One or more sets of total-immersion liquid-in-glass

thermometers

G. Personnel Requirements:

1. At least a high-school graduate

2. Detail oriented

3. Adept in mathematics

4. Should attend NBS Precision Thermometry Seminar

5. Twelve months of practice and experience

6. Capable of calibrating up to 800 thermometers per year

53
7 . ACKNOWLEDGMENTS

We would like to thank the following individuals for

assisting us in this effort.

Dr. Carroll S. Brickenkamp

Office of Weights and Measures, NBS

Mrs. M. Carroll Croarkin

Statistical Engineering Division, NBS

Mr. L. F. Eason

North Carolina Department of Agriculture

Consumer Standards Division

Mrs. Sylvia C. Ramboz

Temperature and Pressure Division, NBS

Dr. James F. Schooley

Temperature and Pressure Division, NBS

Mr. Charles R. Stockman

Maryland Department of Agriculture

Weights and Measures Section

Mrs. Ruth N. Varner

Statistical Engineering Division, NBS

Mr. Sidney Ween

Ever Ready Thermometer Co., Inc.

8 . REFERENCES

1. Jacquelyn A. Wise, Liquid-in-Glass Thermometry. Nat.

Bur. Stand. (U.S.) Monogr. 150; January 1976. 30 pages.

2. "The International Practical Temperature Scale of 1968,

Amended Edition of 1975", Metrologia 12, No. 1, 7-17

(1976)

3. ASTM Designation E-1, Standard Specification for ASTM

Thermometers, Annual Book of ASTM Standards, Vol. 14.01.

4. ANSI Z236.1, Specifications for General Purpose

Laboratory Glass Thermometers.

5. ASTM Designation E-77, Standard Method for Verification

and Calibration of Liquid-in-Glass Thermometers, Annual

Book of ASTM Standards, Vol. 14.01.

54
9. COMMERCIAL SOURCES

9.1 Suppliers of Calibration Baths*

1) Hart Scientific

P. 0. Box 9 34

Provo, UT 84601

2) Willard L. Pearce Associates

Manufacturers Representative

Box 200

Allison Park, PA 15101

3) Rosemount Engineering Company

4900 West 78th Street

Minneapolis, MN 55435

4) Techne Incorporated

3700 Brunswick Pike

Princeton, NJ 08540

9.2 Suppliers of Oils for Calibration Baths*

1) Dow Corning

Chemical Products Division

Midland, MI 48640

2) E. I. Dupont DeNemours and Co.

Freon Products Division

Wilmington, DE 19899

3) Exxon Company, U.S.A.

P. 0. Box 1288

Baltimore, MD 21203

4) 3M Company

Commercial Chemicals Division

223-6S-04 3M Center

St. Paul, MN 55144-1000

55
9.3 Suppliers of Thermometers*

1) Brooklyn Thermometer Company, Inc.

90 Verdi Street

Farmingdale, NY 11735

2) Ever Ready Thermometer Company, Inc.

401 Park Avenue South

New York, NY 10016

3) H-B Instrument Co.

American and Bristol Sts.

Philadelphia, PA 19140

4) Walter H. Kessler Company, Inc.

160 Hicks Street

Westbury, Long Island, NY 11590

5) Miller and Weber, Inc.

1637 George Street

Ridgewood, Queens, NY 11385

6) Princo Instruments, Inc.

1020 Industrial Highway

Southampton, PA 18966

9.4 Supplier of Faden Thermometers*

Karl Schneider and Son

Am Bildacken 14

Postfach 13

Werthaim Main I West Germany

*This list is not all-inclusive. It is intended to be only

representative of commercial products. Certain commercial

equipment, instruments, or materials are identified in this

paper in order to adequately specify the experimental

procedures. Such identification does not imply recommendation

or endorsement by the National Bureau of Standards, nor does it

imply that the materials or equipment identified are

necessarily the best available for the purpose.

56
10. APPENDIX

APPENDIX A-NBS Mongraph 150

I

Contents

Page

1. Introduction _ 1

2. Thermometer Calibration Services at the NBS 1

2.1. Kinds of thermometers accepted for calibration 1

2.2. Preliminary examination 2

2.3. Reports of Calibration 2

2.4. General instructions to applicants for tests 2

a. Initial arrangements 2

b. Shipping instructions 5

3. Definitions 5

4. Temperature Scales and Standards 6

5. Calibration 8

5.1. Equipment 8

a. Ice Bath 8

b. Steam Bath 8

c. Comparison Liquid Baths 9

5.2. Determination of scale corrections 11

5.3. Corrections for emergent stem 13

a. Measurement of emergent-stem temperature 13

b. Formula for total-immersion thermometers 14

c. Formula for partial-immersion thermometers 15

d. Formula for calorimetric thermometers 16

e. Formula for Beckmann thermometers 16

5.4. Number and choice of test points 16

6. Common Thermometers and Factors Affecting Their Use 17

6.1. Total-immersion thermometers 17

6.2. Partial-immersion thermometers 17

6.3. Low-temperature thermometers 19

6.4. Beckmann thermometers — 19

6.5. Calorimetric thermometers 20

7. Thermometer Design 21

7.1. Materials of construction 21

7.2. Scale design and workmanship 21

7.3. Scale dimensions 22

7.4. Reference point 22

7.5. Marking of partial-immersion thermometers 22

8. Special Notes 23

8.1. Glass changes 23

a. Temporary changes 23

b. Permanent changes 24

8.2. Pressure effects 24

8.3. Lag 24

8.4. Separated columns 25

9. References 26

iii

Liquid-In-Glass Thermometry

Jacquelyn A. Wise

This Monograph, which supersedes NBS Monograph 90, contains information of general interest

to manufacturers and users of liquid-in-glass thermometers. Instructions explaining how to submit

a thermometer to the National Bureau of Standards for calibration are provided, and the techniques £md equipment, such as stirred liquid comparison baths, used in the calibration procedures are

described. A discussion of important principles of acceptable thermometer design and factors affect- ing their use is included. Listed are tables of tolerances reflecting good manufacturing practices and reasonably attainable accuracies expected with liquid-in-glass thermometers. The calculation of corrections for the temperature of the emergent stem is given in detail for various types of thermometers and conditions of use.

Key words: Calibration; emergent stem; liquid-in-glass thermometer; reference point; stirred liquid comparison bath; temperature scale.

1. Introduction

It is the responsibility of the 'National Bureau of Standards ( NBS) to accurately reproduce the Inter- national Practical Temperature Scale of 1968 [1]^ as

a basis for the uniform measurement of temperature

throughout the scientific and industrial community of

the United States. Because of this responsibility, NBS

accepts for calibration with reference to this scale

selected types of temperature-measuring instruments

[2] for use as reference or working standards where

precise temp:^- p.ture measurements are required. Certain liquid-in-glass thermometers are among the instruments accepted.

This Monograph is intended for those who may

wish to submit thermometers for calibration or who

desire information on the use of liquid-in-glass

thermometers for precise temperature measurements.

Practices employed at the NBS in the calibration of these thermometers are discussed. Also included is a

brief discussion of the International Practical Temperature Scale of 1968, a description of the calibration equipment, and the accuracy capability of liquid-inglass thermometers. Details of good thermometer design

and factors affecting their use are presented in a copious manner.

2. Thermometer Calibration Services

at the NBS

The liquid-in-glass thermometer is one of many

precision instruments calibrated by the NBS. A complete description of calibration services offered by

the NBS appears in NBS Special Publication 250 [2]

,

which can be obtained by writing to the Office of Technical Information and Publications, National

Bureau of Standards, Washington. D. C. 20234.

Supplements to NBS Special Publication 250 are issued

periodically indicating any changes in these services.

2.1. Kinds of Thermometers Accepted for

Calibration

Not all thermometers classified as liquid-in-glass are acceptable for calibration by the NBS. In general

they must be of an acceptable design and workman-

'Figures in brackets indicate the literature references at the end of this Monograph.

ship, and capable of being placed in the existing cali- bration facilities. The type known as laboratory or chemical thermometers, consisting of a solid stem

with graduation lines and numbers permanently

etched or placed on the stem, are regularly accepted.

Others approved for calibration are Beckmann, cal- orimetric. and enclosed scale (Einschluss) laboratory

thermometers. Industrial or mechanical type ther- mometers with special mountings can be accepted for

test if they can be inserted in the comparison baths, or if the thermometer can be easily detached from the mounting.

Any thermometer, such as a household thermometer,

consisting of a pa])er, plastic or metal mounting con- taining the scale graduations and attached to the unmarked glass thermometer by metal clips, is not acceptable for calibration. Maximum self-registering

mercury-in-glass clinical thermometers are no longer

tested at the NBS.
It is advisable to contact the liquid-in-glass ther- mometry laboratory at NBS if there is any doubt

concerning the acceptability of a thermometer for

calibration. Every effort will be made to assist and

satisfy the needs of the scientific and industrial community with problems involving temperature measurement.

2.2. Preliminary Examination

Every thermometer submitted for test must be

uniquely identified by a serial number and pass a preliminary examination before final acceptance. These

thermometers are viewed under a microscope having a magnification of 15 or 20X to assure that the mercury

and capillary are clean. Any foreign matter found in the capillary or bulb that, in the judgment of the laboratory personnel, might tend to make the ther- mometer readings erratic, is reason for rejection.

Glass chips or oxides of mercury in the capillary or bulb are considered foreign matter that can cause

the thermometer to indicate different readings at different periods of time. [For example, if a small

chip of glass is located in the capillary at the 30 °C

indication, each time the mercury column advances

past this point, it could cause the reading of the ther- mometer to vary for temperatures above 30 °C, because

of different quantities of gas that may be trapped

around the chip. The movement of the chip to different locations in the capillary, or the trapping of mercury

around the chip when the mercury column recedes,

would also cause erroneous readings.]

Other reasons a thermometer may be ineligible for

test are given below:

(a) Defective design or workmanship.

(b) Part of graduated scale not usable.

(c) Errors in scale graduation or numbering.

(d) Unsuitable bulb glass or inadequate

annealing.

(e) Inadequate gas filling. (f ) Cracks in the glass.

A complete list of all possible causes for rejection

is not feasible. The prime consideration in the pre- liminary examination is that the thermometer be

capable of precise, reproducible readings.

2.3. Reports of Calibration

In most instances a Report of Calibration will be

issued by NBS for every liquid-in-glass thermometer

submitted for test that is found JFree from serious

defects as determined by the preliminary examination.

In order to receive a Report of Calibration, the ther- mometer must be calibrated at two or more calibration

points. If only one point is requested, a Report of

Test will be issued. The Report of Calibration contains the corrections

determined for each point requested and an estimate

of the uncertainties associated with the corrections. Also stated on the document are the agency or firm

requesting the test, the trade mark and serial number

on the thermometer, the NBS test number and completion date, and explanatory notes defining the conditions under which the results of the test are

applicable. When necessary, accompanying the Report

of Calibration is a sheet explaining how to calculate the corrections for emergent stem. If the thermometer

is of the metastatic (Beckmann) type, the report will be accompanied by a second sheet containing a table

of setting factors (See Sec. 6.4) enabling the user to apply the calibration results for settings other than

the 20 °C setting for which the corrections are re- ported. A facsimile of a Report of Calibration is shown in figures 1 and 2. Any departure from the conditions under which

the corrections were obtained, as stated on the Report

of Calibration, may significantly change the values of the corrections. Conditions of immersion are particularly important (Sec. 6). It should be emphasized

that the estimates of error assigned to the scale cor- rections do not assure the user of this accuracy in a temperature measurement.

2.4. General Instructions to Applicants

for Tests

Testing will be conducted in accordance with the

policies of NBS as described in NBS Special Publication 250. The cost of calibration will depend on

the number of calibration points requested, the temperature range, and the quantity of thermometers

submitted. For tests not sf)ecifically outlined in the above publication, NBS should be consulted. If th? required measurements appear feasible, and, in the opinion of NBS, sufficiently important to justify the work, such tests will be undertaken for a special fee determined by the nature of the work and time in- volved. In all requests for test the following procedures

and information are pertinent,

a. Initial Arrangements

A letter or purchase order requesting the test must

be sent to NBS either with the thermometer or separately. No work can be undertaken unless both the thermometer and letter or purchase order have been

received. Information in the request for test should

include the quantity and serial numbers of the ther- mometers sent, the calibration points required, (See

Sec. 5.4) and the suggested method for return ship- ment. If it is desired that the thermometer be insured

upon return, a statement to this effect must appear on the request, and a value assigned; otherwise, the thermometer will be returned uninsured.

NBS will assign a test number to each calibration

request and acknowledge the receipt of the ther- mometer. On the acknowledgment will be stated the estimated coippletion date and estimated cost of

calibration. Any foreseeable difficulty that may be

encountered involving the calibration will be mentioned on the acknowledgment, as well as the name

of someone in the testing laboratory who can be

contacted if questions arise.
U.S. DEPARTMENT OF COMMERCE

NATIONAL BUREAU OF STANDARDS

INSTITUTE FOR BASIC STANDARDS

WASHINGTON, O.C. 20234

REPORT OF CALIBRATION

L IQUIO-IN-GLASS THERMOMETER

TESTED FOR: NATIONAL BUREAU OF STANDARDS

DIVISION 221, SECTION II

MARKED: SURETY 198692

RANGE: -2 TO +102 DEGREES C IN 0.2 DEGREE

THERMOMETER CORRECTION

READING (IPTS-68)**

-.06 C .06 C

20.00 .13

40.00 .05

60.00 .04

80.00 .06

100.00 .04

**ALL TEMPERATURES IN THIS PFPORT ARE BASED ON THE INTERNATIONAL

PRACTICAL TEMPERATURE SCALE OF 1968, IPTS-68. THIS TEMPERATURE SCALE

WAS ADOPTED BY THE INTERNATIONAL COMMITTEE OF WEIGHTS AND MEASURES AT

ITS MEETING IN OCTOBER, 1968, AND IS DESCRIBED IN "THE INTERNATIONAL

PRACTICAL TEMPERATURE SCALE OF 1968, METROLOGIA, VOL. 5, NO. 2, 35

(APRIL 1969).

ESTIMATED UNCERTAINTIES IN THE ABOVE CORRECTIONS 00 NOT EXCEED 0.05

DEGREE UP TO 102 DEGREES C . FOR A DISCUSSION OF ACCUFACIES ATTAINABLE WITH SUCH THERMOMETERS SEE

NATIONAL BUREAU OF STANDARDS MONOGRAPH 150, L I QUI D-I N-GLASS

THERMOMETRY .

IF NO SIGN IS GIVEN ON THE CORRECTION, THE TRUE TEMPERATURE IS HIGHER

THAN THE INDICATED TEMPERATURE; IF THE SIGN GIVEN IS NEGATIVE, THE TRUE

TEMPERATURE IS LOWER THAN THE INDICATED TEMPERATURE. TO USE THE CORRECTIONS PROPERLY, REFERENCE SHOULD BE MADE TO THE NOTES GIVEN BELOW.

THE TABULATED CORRECTIONS APPLY FOR THE CONDITION OF TOTAL IMMERSION OF

THE BULB AND LIQUID COLUMN. IF THE THERMOMETER IS USED AT PARTIAL IMMERSION, APPLY AN EMERGENT STEM CORRECTION AS EXPLAINED IN THE ACCOMPANYING

STEM CORRECTION SHEET.

TEST NUMBER 311-11-74

COMPLETED 5-9-74

Figure 1. Facsimile of page 1 of a Report of Calibration.

3
PAGE 2

THE TABULATED COPRECTIUNS APPLY PROVIDED THE ICE-POINT HEADING, TAKEN

AFTER EXPOSURE FOR NOT LESS THAN 3 DAYS TO A TEMPERATURE OF ABOUT 23

DEGREES C (73 DEGREES F) , IS -0.06 C . IF THE ICE-POINT READING IS FOUND

TO BE HIGHER (OR LOWER) THAN STATED, ALL OTHER READINGS WILL 8E HIGHER

(OR LOWFR) BY THE SAME AMOUNT. IF THE THERMOMETER IS USED AT A GIVEN

TEMPERATURE SHORTLY AFTER BEING HEATED TO A HIGHER TEMPERATURE, AN ERROR

OF 0.01 , DEGREE OR LESS, FOR EACH lO-OEGREE DIFFERENCE BETWEEN THE TWO

TEMPERATURES, MAY BF INTRODUCED. THE TABULATED CORRECTIONS APPLY IF THE

THERMOMETER IS USED IN ITS UPRIGHT POSITION; IF USED IN A HORIZONTAL

POSITION, THE INDICATIONS MAY BE A FEW HUNDREDTHS OF A DEGREE HIGHER.

FOR THE DIRECTOR,

INSTITUTE FOR BA/SIC STANDARDS

TEST NUMBER 311-11-74

COMPLETED 5-9-74

CHIEF, TEMPERATUP

HEAT DIVISION

SECTION

Figure 2. Facsimile of page 2 of a Report of Calibration.

4
b. Shipping Instructions To assure safe arrival of the thermometers, they

should be packed firmly in their individual cases. A rigid material, such as wooden slats or corrugated

paper, can be wrapped around^ the thermometer case

as an additional aid against breakage. The package

should contain enough packing material to absorb any

shock that it may receive. Included with each shipment should be a packing list stating the number of thermometers shipped, the name of the company submitting them, and, if applicable, the company's

purchase order number.

Unless otherwise specified, the thermometers will be returned via the best method in the judgment of NBS. Shipping charges, both to and from the NBS,

will be paid by the applicant. All possible care will be taken in handling thermometers at NBS, but the

risk of damage either in shipment or testing must be

assumed by the agency, firm or individual submitting

them.

3. Definitions

The principle features of a solid-stem liquid-in-glass

thermometer are shown in figure 3. Not all of the

features shown would necessarily be incorporated in any one thermometer.

CONTRACTION CHAMBER

EXPANSION CHAMBER

AUXILIARY_

SCALE IMMERSION

LINE J MAIN

SCALE

Figure 3. Principle features of a solid-stem liquid-in-glass thermometer.

Bulb: The reservoir for the thermometer liquid.

The bulb of a thermometer will contain a volume

equivalent to a specific number of degrees of the scale depending upon the coefficients of expansion of the thermometric liquid and bulb glass. For mercury in a bulb made of "normal" glass, the bulb volume is equivalent to approximately 6,222 times the volume

of a 1 degree length of the capillary on the scale for

Celsius thermometers, or 11,200 times the volume of a 1 degree length of the capillary on the scale for Fahrenheit thermometers. For organic thermometric

liquids with higher coefficients of expansion than mercury, the bulb volumes are correspondingly less. Stem: The glass capillary tube through which the thermometric liquid advances or retreats with changes

in temperature.

Main Scale: The scale graduated in degrees or

multiples or submultiples of degrees; in many in- stances the main scale constitutes the only scale. Auxiliary Scale: A short scale including a reference temperature such as the ice point, to provide a means

for checking the therm.ometer for a change in cali- bration with time (see Sec. 7.4). This scale is added

when a suitable reference temperature is not included

in the range of the main scale. Expansion Chamber: An enlargement at the top

end of the capillary bore having a volume equivalent

to not less than the volume of a 20 mm length of un- changed capillary. Smaller chambers are not regarded

as expansion chambers. The expansion chamber is

provided to prevent the buildup of excessive pressures

in gas-filled thermometers as the liquid filling ad- vances toward the top of the scale (see Sec. 7.1).

Contraction Chamber: An enlargement of the capillary bore which serves to reduce a long length of

capillary or to prevent contraction of the entire liquid

column into the bulb. This chamber is introduced

below the main scale or between the main scale and

an auxiliary scale. Reference Point: A reference temperature, such as the ice point or steam point, used periodically to check the thermometer for changes in bulb volume.

(See Sec. 5.2). The reference point can be included

in the main scale or on an auxiliary scale. Total-Immersion Thermometer : A thermometer de- signed to indicate temperatures correctly when the bulb and the entire liquid column is exposed to the temperature being measured. (See the definition for complete-immersion thermometer.)

Partial-Immersion Thermometer: A thermometer

designed to indicate temperatures correctly when the bulb and a specified portion of the stem is exposed

to the temperature being measured. The remaining

portion of the stem, referred to as the emergent stem,

will be at the ambient temperature, usually different from the temperature being measured. Such thermometers are generally marked with an immersion line to indicate the proper depth of immersion.

Complete-Immersion Thermometer: A thermometer

designed to indicate temperatures correctly when the whole thermometer, including the expansion chamber,

is subjected to the temperature being measured. In gasfilled thermometers the reading will be different for complete, as compared to total immersion, as a result of the effect of temperature on the gas pressure in the thermometer (see Sec. 8.2). Although the differ- ence in readings under the two conditions is particularly significant at high temperatures, it is also

significant at moderate temperatures if the bulb and

expansion chamber are both relatively large.

Calibration Points: The temperatures on the ther- mometer scale (i.e., 0°C) where calibrations are performed and corrections reported.

5
Accuracy: The accuracy of a measurement refers

to the difference between the measured value and the true value of the quantity being measured. In this Monograph the accuracy of a given thermometer refers to its ability to indicate temperatures correctly on the International Practical Temperature Scale of 1968 [1]

within the uncertainty stated (See tables 5-12) provided all corrections are applied and the thermometer is used in the same manner as when it was calibrated. The accuracy attainable is principally limited by the

characteristics of the thermometer itself (See Sec. 6).

Instability of the thermometer glass, capillary forces

at the surface of the thermometric liquid, nonuniformity of capillary bore, and inaccuracies in scale graduation are among the important factors. With

partial-immersion thermometers, uncertainties in cor- rections for the emergent stem may greatly limit the accuracy. Observer errors are also involved, but with

care these can usually be made relatively small. Precision: The precision of measurement refers to the degree of agreement amongst repeated measure- ments at a given time of the same quantity.

4. Temperature Scales and Standards

The scale to which measurements of temperature

should ultimately be referred is the Thermodynamic

Kelvin Temperature Scale (TKTS). Values of tem- perature expressed on the TKTS are designated by

the symbol T. The unit of temperature is the Kelvin,

symbol K, which is a base unit of the Systeme Inter- nationale (SI), and is defined as "the fraction 1/273.16 of the thermodynamic temperature of the

triple point of water" [3]

Because of the difficulties that are encountered in the practical realization of the TKTS, it has been

necessary to define and utilize practical temperature

scales. The International Temperature Scale and de- fining text was first adopted in 1927 and later revised

in 1948. The Eleventh General Conference of Weights

and Measures in 1960 changed the name of the scale

to the International Practical Temperature Scale (IPTS) of 1948 and adopted a revised text of the

scale [4] , although numerically the scale was not changed. In 1968, in accordance with the power given

to it by Resolution 8 of the Thirteenth General Conference of Weights and Measures, the International

Committee of Weights and Measures adopted the Inter- national Practical Temperature Scale of 1968 [1],

henceforth referred to as IPTS-68. The IPTS-68 re- placed the International Practical Temperature Scale

of 1948 and may be expressed either as a Kelvin scale or as a Celsius scale. The present Celsius scale has its zero 0.01 K below

the triple point of water (essentially the ice point).

The value of a temperature expressed on the Thermodynamic Celsius Temperature Scale is designated by

the symbol t and is related to the value on the TKTS by

i = r - 273.15 K. The unit of the Celsius Scale is the degree Celsius,

symbol °C, which by definition is equal in magnitude

to the Kelvin.

Values of temperature expressed on the International Practical Kelvin Scale are designated by the symbol T^a, and values of temperatures expressed on

the International Practical Celsius Temperature Scale

are designated by the symbol ios- The relationship

between Tug and ies is

tas — Tes —273.15 K.

The units of Tea and t^a, like the thermodynamic

scales, are the Kelvin, symbol K, and degree Celsius,

symbol °C. Due to the use of more sophisticated equipment and

the ability to measure with more accuracy the defining

fixed points, the new temperature scale more closely agrees with thermodynamic temperatures. This scale, IPTS-68, is intended to provide scientific and industrial laboratories throughout the world with a com- mon basis for stating temperatures. Calibrations of thermometers at NBS, therefore, are made with ref- erence to values of temperature on the IPTS-68.

Shown in Table 1 is the approximate difference in degrees Celsius between the values of temperature

given by the IPTS-68 and the IPTS-48 [1] . The differ- ences are listed in two separate sections for convenience

and clarity. It can be seen, by studying these values,

that the correction is important if quality control in the laboratory is to be maintained to the nearest 0.01 °C. If, on the other hand, accuracies no better than the nearest 0.1 °C are desired, the correction

becomes less significant in the liquid-in-glass ther- mometer region (—200 to +600 °C) of temperature measurements. Although values of temperatures on the IPTS-68

are expressed in degrees Celsius, thermometers that are graduated on the Fahrenheit Scale can be cali- brated with reference to the IPTS-68 by using the conversion formula:

temperature value in °F = 9/5

(temperature value in °C) + 32

The National Bureau of Standards has been per- forming calibrations with reference to the IPTS-68

since July, 1969. On all Reports of Calibration and

Reports of Test issued after this date, it has been

clearly indicated that the new scale was used. In the range of temperatures normally covered by

liquid-in-glass thermometry, the IPTS-68 is defined

by four fixed points: the equilibrium between the

liquid and vapor phases of oxygen (normal boiling

point of oxygen) at —182.962 °C; the equilibrium

between solid, liquid, and vapor phases of water

(triple point of water) at +0.01 °C; the equilibrium

between the liquid and vapor phases of water (nor6
Table 1. — Approximate difference ( tss — tis), in degrees Celsius,

between the values of temperature given by the IPTS—68 and IPTS-^8

la. For the range — 180°CtoO°C.

Q -10 -20 -30 -40 -50 -60 —70 —o\j on inn

-100 0.022 0.013 0.003 -0.006 -0.013 -0.013 -0.005 0.007 0.012 - 0 0.000 0.006 0.012 0.018 0.024 0.029 0.032 0.034 0.033 0.029 0.022

lb. For the range 0 °C to 1070 °C.

tts °C 0 10 20 30 40 50 60 70 80 90 100

0 0.000 -0.004 -0.007 -0.009 -0.010 -0.010 -0.010 -0.008 -0.006 -0.003 0.000 100 0.000 0.004 0.007 0.012 0.016 0,020 0.025 0.029 0.034 0.038 0.043 200 0.043 0.047 0.051 0.054 0.058 0.061 0.064 0.067 0.069 0.071 0.073 300 0.073 0.074 0.075 0.076 0.077 0.077 0.077 0.077 0.077 0.076 0.076 400 0.076 0.075 0.075 0.075 0.074 0.074 0.074 0.075 0.076 0.077 0.079 500 0.079 0.082 0.085 0.089 0.094 0.100 0.108 0.116 0.126 0.137 0.150 600 0.150 0.165 0.182 0.200 0.23 0.25 0.28 0.31 0.34 0.36 0.39 700 0.39 0.42 0.45 0.47 0.50 0.53 0.56 0.58 0.61 0.64 0.67 800 0.67 0.70 0.72 0.75 0.78 0.81 0.84 0.87 0.89 0.92 0.95 900 0.95 0.98 1.01 1.04 1.07 1.10 1.12 1.15 1.18 1.21 1.24 1000 1.24 1.27 1.30 1.33 1.36 1.39 1.42 1.44

Examples showing the use of these tables are given below to help clarify any difficulty that may be encountered.

I Standard thermometer reading ~36.56 °C

Correction from NBS Report of Calibration + .06 True temperature IPTS-45 "36.50 °C

Correction from the above table at ~36.50 °C + -02 True temperature IPTS-68 -36.48 °C

II Standard thermometer reading 98.32 °F

Correction from NBS Report of Calibration ~ .02 True temperature IPTS-48 98.30 °F

Converted correction from the above tables* ~ .02 True temperature IPTS-68 98.28 °F

*The converted correction is obtained by:

A. Convert the Fahrenheit temperature to Celsius 98.30 °F = 36.83 °C.

B. Find the correction from the table at 36.83 °C. It is —0.01 °C.

C. Convert this correction to Fahrenheit by multiplying bv 9/5. -0.01 °C X 9/5 « -0.02 °F.

mal boiling point of water ) at 100 °C: and the equilibrium between solid and liquid phases of zinc (freezing point of zinc) at 419.58 ^C. Temperatures

in the range —182.96 to 630.7 are defined in terms of a standard platinum resistance thermometer

that has been calibrated at the four fixed points, with

interpolations and extrapolations between and beyond

these points accomplished by employing specified

equations [1].

When the highest accuracy is required in a calibration, the thermometer indications are compared di- rectly with temperatures obtained using a standard

platinum resistance thermometer. At NBS all calibrations are performed in this manner, with the exception

of the ice point I See Sec. 5.2 I . If a lower accuracy is required, one of several mercury -in-glass thermometers

with ranges and graduated in the intervals li.?ted below may be used as standards for total-irmnersion comparisons.

Range

Smallest graduationAuxiliary

scale

°C

0 to 50 °C

0.1 0.2

0.1 0.2

0.5

"C

0 to 100 50 to 100 atO atO

atO

100 to 200

200 to 300

Partial-immersion thermometers, known as "like standards," can be maintained for the calibration of thermometers manufactured to similar specifications.

7
These standards are calibrated for stem-temperature

conditions expected to prevail during the use of these thermometers or for specified stem temperatures. This

use of like standards eliminates the need for many of the precautions necessary when dissimilar thermometers are compared and permits the direct comparison

of the indications of similar thermometers as long as

the bulbs are at the same temperature and the stem

temperatures are essentially the same for all of the thermometers under comparison.

All of these liquid-in-glass standards should be

calibrated with reference to the IPTS-68 through

comparisons with a standard platinum resistance thermometer.

5. Calibration

In most instances, the determination of the liquidin-glass thermometer scale corrections is accomplished

by comparing its scale indications with a known temperature from a platinum resistance thermometer or mercury-in-glass thermometer standards. This comparison is achieved by placing the standard and the thermometers under test in a series of stirred liquid

comparison baths (See Sec. 5.1c). A discussion of

the equipment needed, the procedure followed, additional corrections that may be necessary, and the choice of calibration points is given in the following

sections.

5,1. Equipment

a. Ice Bath

An ice bath can be easily assembled and consists of a container, a siphon tube, ice and distilled water. A Dewar flask, approximately 36 cm deep and 8 cm

in diameter, can serve as a container for the ice. A vessel of this type is preferable, since the melting

of the ice is retarded by the insulating properties of the Dewar flask. A siphon is placed in the flask to enable excess water to be removed as the ice melts. The clear or transparent portion of commercially

purchased ice. or ice made from distilled water, can

be used. The ice is shaved or crushed into small chips

measuring 2 to 5 mm. The flask is one-third filled with distilled water and the shaved ice is added.

This mixture is compressed to form a tightly packed

slush and any excess water is siphoned off. Before

the bath is used, adequate time ( 15 to 30 min.) should

be given for the mixture to reach a constant temperature throughout.

Ideally there should be as much ice in the flask as

possible, with the small spaces between the chips filled with distilled water. It will be necessary periodically

to add ice and to remove the excess water while the bath is being u^ed to maintain this ideal consistency.

If care is taken to prevent contamination of the ice and water, the ice point can be realized to better than

0.01 '^C by this means.

b. Steam Hath

A schematic drawing of a steam bath is shown in figure 1. Steam produced from the boiler circulates within a double-walled steam jacket permitting free

circulation of steam around the thermometers sus- pended within this space. A provision is made both

for relieving any excess pressure in the space sur- rounding the thermometers and for determining the excess pressure by means of a small differential manometer.

THERMOMETER

SPACE IN —

\

STEAM JACKET

THERMOMETER

AIRJACKET

CONDENSER

BOILER

TO HEATING COIL

Fk.i hk 4. Srhcmalic draiiing of iteaiii balh.

When the steam bath is used as a fixed-point apjjaratus, a barometer is a necessary accessory, since

the temperature of the steam depends upon the pre- vailing atmospheric pressure. The usual adjustments

are made to the barometer reading, including any

necessary corrections for the value of local acceleration Of gravity, for the difference in height between

o
c. Comparison Liquid Baths

At NBS two types of stirred liquid baths are used

for comparison calibrations in the range —110 to 540 °C (-166 to 1004 °F). Each is equipped with a stirring unit to provide a uniform temperature

throughout the medium and a controlled current to the heating coils for proper temperature regulation.

A 5 to 7 cm thickness of insulation surrounding the baths and an insulated cover is provided to help mini- mize heat loss. Fitted into the top cover is a holder

containing the thermometers. This holder can be ro- tated, enabling each thermometer to appear in the

field 01 view of a vertically adjustable telescope at- tached to each bath.

Table 2. — (Thermometric) condensation temperature of steam [5]

[Asterisk (*) indicates change in integer]

Pressure in mm mercury (standard)

p 0 1 2 3 4 5 6 7 8 9

Temperature in degrees of International Practical Temperature Scale of 1968

500 88.674 0.726 0.778 0.830 0.882 0.935 0.987 *0.039 * 0.090 * 0.141 510 89.193 .244 .295 .347 .398 .449 .499 .550 .601 .652 520 89.702 .753 .803 .853 .904 .954 * .004 * .054 * .104 .154 530 90.203 .253 .303 .352 .402 .451 .500 .550 .599 .648 540 90.697 .746 .795 .843 .892 .941 .989 * .038 * .086 * .135

550 91.183 .231 .279 .327 .375 .423 .471 .518 .566 .614 560 91.661 .709 .756 .803 .851 .898 .945 .992 * .039 * .086 570 92.133 .179 .226 .274 .320 .367 .413 .460 .506 .552 580 92.598 .644 .690 .736 .782 .828 .874 .920 .965 * .011 590 93.056 .102 .147 .193 .238 .283 .328 .373 .418 .463

600 93.5079 .5527 .5975 .6422 .6868 .7315 .7760 .8204 .8645 .9092 610 93.9534 .9976 * .0419 * .0860 * .1300 * .1740 * .2179 * .2617 * .3055 * .3493 620 94.39.30 .4366 .4802 .5237 .5671 .6105 .6538 .6972 .7404 .7835 630 94.8266 .8696 .9126 .9555 .9985 * .0413 * .0841 * .1268 * .1694 * .2120 640 95.2546 .2972 .3396 .3819 .4242 .4665 .5087 .5508 .5929 .6351

650 95.6771 .7190 .7609 .8027 .8445 .8862 .9280 .9696 * .0112 * .0526 660 96.0941 .1355 .1769 .2182 .2595 .3007 .3419 .3830 .4240 .4650 670 96.5060 .5469 .5878 .6286 .6693 .7100 .7506 .7912 .8318 .8724 680 %.9128 .9532 .9936 * .0339 * .0741 * .1143 * .1546 * .1947 * .2347 * .2747 690 97.3147 .3546 .3945 .4343 .4741 .5138 .5535 .5931 .6327 .6722

700 97.7117 .7511 .7906 .8300 .8693 .9085 .9477 .9869 * .0260 * .0651 710 98.1042 .1431 .1821 .2210 .2598 .2986 .3373 .3761 .4148 .4534 720 98.4920 ..5305 .5690 .6074 .6458 .6842 .7225 .7608 .7990 .8372 730 98.8753 .9134 .9515 .9896 * .0275 * .0654 * .1033 * .1411 * .1789 * .2107 740 99.2544 .2922 .3298 .3673 .4049 .4424 .4798 .5172 .5546 .5920

750 99.6293 .6666 .7038 .7409 .7780 .8151 .8522 .8893 .9262 .9631 760 100.0000 .0368 .0736 .1104 .1472 .1839 .2205 .2571 .2937 .3302 770 100..3667 .4031 .4395 .4760 .5123 .5486 .5845 .6210 .6572 .6934 780 100.7295 .7656 .8016 .8376 .8736 .9095 .9453 .9811 * .0169 * .0528 790 101.0885 .1242 .1598 .1954 .2310 .2665 .3020 .3376 .3730 .4084

the steam bath and the barometer, and for any excess pressure above atmospheric in the steam jacket. After

the corrected pressure reading is obtained, the temperature of the steam can be derived from the values given in Table 2. With a barometer accurate to 0.1 mm Hg. this procedure is capable of an accuracy of 0.002 to 0.003 (0.004 to 0.005 °F). The Fortin

type barometer will usually suffice for all but the most

exacting measurements. The steam bath can also be used as a comparison

bath, with the temperature of the steam being deter- mined at the time of test by means of a previously

standardized thermometer. This method, which does

not require the use of a barometer, may be preferable,

particularly when a platinum resistance thermometer

can be used as the standard.

9
The type of bath shawn in figure 5 is suitable when

the medium is liquid at room temperature. It consists of two cylindrical wells of different diameters with

connecting passages at the top and bottom. The heating

coil and stirrer are located in the smaller well, leaving

the larger well unobstructed for the insertion of the thermometers. The fluid is forced past the heating

coils, through the bottom opening into the larger well, around the thermometers, and back into the smaller

well through the connecting passage at the top.

THERMOMETER

HOLDER

INSULATION

HEATING —

COILS

AIR SPACE

SPACE BETWEEN

TUBES

BEARING

SUPPORT

Figure 5. Stirred liquid baih.

The bath illustrated in figure 6 is designed for use with a medium that is solid at room temperature. A

bath of this type is used at NBS for high temperature

calibrations. Two coaxial cylinders are arranged in such a way as to permit the medium to circulate be- tween the walls of the two cylinders and through the inner cylinder by means of openings at the top and

bottom. The stirring propeller is situated near the bottom of the inner cylinder, leaving the majority of

the space for the reentrant tubes into which the ther- mometers are inserted. The heater coils are wound

on the outside of the outer cylinder. Both types of

STIR DRIVE

INSULATION

TESTING AREA

AIR SPACE

HEATING COIL

Figure 6. Stirred high temperature bath using liquid having

freezing point above room temperature.

baths are designed to shield the thermometers from

direct radiation from the heating units. For calibrations in the range 1 to 99 °C (34 to 210 "F) water is used as the bath liquid. One grade

of petroleum oil is used between TOO and 200 °C

(212 to 392 ^F) and a second grade between 200 and

300 (392 and 572 °F). The oils must not have

flash points below the highest test temperatures, nor

be too viscous to prevent adequate stirring at the lower test temperatures.

Calibrations from - 1 to -110 (30 to -166 °F)

are made in a cryostat similar to the one described

by Scott and Brickwedde [6]. The cryostat, shown in figure 7. consists of an inner Dewar flask, D, which

contains the bath liquid. This flask is surrounded by

liquid nitrogen contained in the outer Dewar flask, C. The rate of heat transfer between the bath liquid and

the liquid nitrogen is retarded by evacuating the space

10
J

Figure 7. Vertical section of cryostal.

between the walls of the inner Dewar flask through

the side tube. M, which is connected to a vacuum

system. Vigorous stirring of the bath liquid is maintained by the propeller, I, which circulates the liquid

around the walls of the stirrer tube, P, similar to the flow of the medium in the high temperature bath. The temperature of the bath is thermostatically controlled by heater coils, J. wound on the outside of the

stirrer tube. The thermometers are immersed inside the stirrer tube, thus shielding them from the heater

coils. The bath liquid is a five-component mixture containing by weight 14.5 percent of chloroform, 25.3 percent of methylene chloride. 33.4 percent of ethyl

bromide. 10.4 percent of transdichloroelhylene. and

16.4 percent of trichloroethylene. This mixture freezes

at approximately -150 °C (-238 °F), but readily

absorbs moisture and becomes cloudy at somewhat

higher temperatures. For this reason calibrations are not performed in this bath below — 110°C

(-166 °F).

A comparison measurement can be made at approximately — 196 °C (boiling point of nitrogen) and — 183 °C (boiling point of oxygen). A silvered Dewar

flask with a narrow transparent vertical strip is used

as a container for the liquid nitrogen or oxygen. The

liquid is agitated by bubbling nitrogen or oxygen gas

in the corresponding liquid through a glass tube with an outlet near the bottom of the flask.

5.2. Determination of Scale Corrections

Thermometers submitted to NBS for test are generally calibrated by starting at the lowest test point

requested and advancing to the higher test points. In most cases the ice point is the lowest point. Ice is packed firmly around the thermometer which has been

placed in an ice bath and immersed to the proper

depth as determined by the type of thermometer. The

thermometer is gently tapped before reading to prevent

the sticking of a falling meniscus. Care is taken not

to tap too vigorously, as this may cause the mercury

to rebound to an erroneously high reading.

The thermometer scale corrections quoted in the Report of Calibration apply as long as the ice-point

reading remains the same as observed during the NBS

calibration. Subsequent changes in the ice-point read- ing of the thermometer will result from small changes

in the glass of the thermometer bulb which affects its volume. The volume of the capillary also changes, but

the volume of mercury contained in the stem is so

small, in comparison to the amount of mercury in the bulb, that changes in the stem volume can usually be

ignored. As a result, changes in the ice-point reading

of the thermometer, taken after an exposure of not less than 3 days at a temperature of approximately 23 °C

(73 °F), will be reflected by similar changes in readings at each point along the scale. Therefore, when

the correction at the ice point is found to be higher

(or lower) than that observed at the time of calibration, the other reported scale corrections will be higher

11
for lower) by the same amount. An example is given

below:

Original Corrections from NBS Report

Thermometer

Reading Correction + 0.011 °C -0.011 °C

10.000 - .015 20.000 - .020 30.000 + .008 40.000 - .033 50.000 .000

A later ice-point reading may be +0.019 °C. This means that the ice-point correction will be —0.019 °C

and a new set of corrections should be made by subtracting 0.008 from all of the original corrections. The new table would look as follows:

Thermometer

Reading Correction + 0.019 °C -0.019 °C

10.000 - .023 20.000 - .028 30.000 .000 40.000 - .041 50.000 - .008

Once a thermometer has been calibrated, an ice point

check is all that is necessary to obtain a current cali- bration, provided the thermometer has not been

abused.

Ice-point readings on the thermometers being cali- brated are not taken after each test point on the

scale. For thermometers graduated below 300 °C or 600 °F, ice points are taken before the comparison

calibrations and 3 days (with the thermometer at room

temperature) after the last test point to assure that the bulb has recovered within acceptable limits. With

thermometers graduated above 300 °C or 600 °F, an

ice point is taken and it is immediately tested at the highest test point on the scale. After a rest period of 3 days at room temperature a second ice point is taken. The agreement between the first and second

ice point must be within the stated accuracy limit of the thermometer, or it is deemed unsuitable for cali- bration and no further testing is done.

The ice point is the only test temperature taken in the manner described above. Comparison measure- ments are made with the temperature of the bath

slowly increasing at a steady rate, which does not exceed one scale division in 3 to 10 minutes [7]. All thermometers are read using a 10 power telescope,

which is attached to each calibration bath and per- pendicular to the thermometers. The telescope is a necessity if errors due to parallax are to be avoided.

When a liquid-in-glass thermometer is intended to be used as a standard, an ice point should be taken

after heating it at each test temperature. The adjusted

scale correction is determined by adding the ice-point

reading to the current correction at the corresponding

test temperature. The example shown above is modified

in this way.

Thermometer

Reading Current

Correction

Ice-Point Reading After Heating to Test Temperature

Adjusted

Scale Correction + 0.019 "C -0.019 °C +.019 0.000 °C

10.000 - .023 +.019 - .004 20.000 - .028 +.017 - .011 30.000 .000 +.010 + .010 40.000 - .041 .000 - .041 50.000 - .008 -.012 - .020

When this thermometer is used as a standard, the ice point should again be taken after each calibration

point. To obtain the actual temperature of the bath, the ice-point correction is added to the appropriate

adjusted scale correction and thermometer reading.

If the calibration temperature is 25 °C, the thermometer reads 24.983 °C, and the ice-point reading taken

after the calibration is +0.014 °C, the actual temperature is computed as follows:

Thermometer reading 24.983 °C

Interpolated Adjusted Scale Correction — .001

Ice-point correction — .014

Actual Temperature of Bath Medium 24.968 °C

This method eliminates the need to wait 3 days for

the bulb to recover and avoids the error due to the

ice-point depression (See Sec. 8.1a) that appears when a thermometer is heated. The standard can be

used at any time and at any temperature with no con- cern about the change in bulb volume, provided the

ice point is taken after it is used.

It is advisable to use two liquid-in-glass standards

when calibrating, in order to detect reading errors and maintain a cross check of the standards. The

calibration procedure can best be described by follow- ing a hypothetical calibration of four thermometers,

T, through T4. Table 3 shows hypothetical observations taken in obtaining the corrections applicable to the thermometers at 20 °C. For simplification, all of the entries in the table reflect perfect thermometer performance with

a temperature rise of 0.001 °c between each observation

and no observer error.

12
Table 3. — Comparison of test thermometers

with liquid-in-glass standards

Ice-point readings of test thermometers

Observer A .. Observer B .. Mean ice points

Ice-point corrections

SI Tl T2 T3 T4

+0.02 + .02 -0.02 - .02 +0.02 + .02 0.00

.00 + .02 - .02 + .02 .00 - .02 + .02 - .02 .00

S2

Thermometer comparisons

Observer A reading

left to right 19.87 19.98 19.96 20.02 20.03 19.89 Observer A reading

right to left 19.88 19.99 19.97 20.03 20.04 19.89 Observer B reading

left to right 19.88 19.99 19.97 20.04 20.05 19.90 Observer B reading

right to left 19.89 20.00 19.98 20.04 20.05 19.90 Means 19.88 19.99 19.97 20.03 20.04 19.89.-.

Ice Points of Standards

Observer A

Observer B

Mean ice points

Ice-point corrections Adjusted scale corrections

-0.01 - .01 - .01 + .01 + .12

-0.08 - .08 - .08 + .08 + .04

Calculations of corrections

Correction to standards Mean temperature, each standard Mean temperature

of all readings

Corrections to test thermometers

+0.13

20.01

+0.02

20.01

+0.04 -0.02 -0.03

+0.12

20.01.

The first observations are the ice points of the thermometers under test. The test thermometers are then mounted in the comparison bath between the two

standards. It is preferable to have two observers (A and B ) . Observer A reads in the order left to right as the thermometers appear in the table, and immediately repeats the observations in the order right to

left, while observer B records the data. Observer B

promptly reads in the same manner while observer A

records. The bath temperature is increasing linearly

with time and the observations are spaced uniformly

in time. For this reason the mean of the observations

with any one thermometer will correspond to the mean temperature value of the comparison bath me- dium durins; the observations of all of the thermometers. Immediately after the comparison observations,

ice points of the two standards are observed and

recorded. With the ice point data and the adjusted

scale corrections for the standards, the temperatures

indicated by the standards are calculated, and an

overall mean temperature for the observations is obtained. This mean temperature is compared with the mean of the observations for each thermometer to

obtain a correction to the scale of the thermometers.

The thermometer comparisons are repeated in the same manner at the next higher test point until the calibration is completed.

When a platinum resistance thermometer is used

as a standard, the sequence of observations is the same, except that the one resistance thermometer is read in place of the separate observations of two liquidin-glass standards. For more information on platinum

resistance thermometers refer to NBS Monograph 126,

Platinum Resistance Thermometry, available from the Superintendent of Documents, U. S. Government

Printing Office, Washington, D. C. 20402.

5.3. Corrections for Emergent Stem

When a thermometer is calibrated and used under

conditions of total immersion, no difficulty is encountered when the reported scale corrections are to be

applied. The temperature of the thermometer bulb

and the portion of the stem containing the mercury

is definitely defined as the temperature of the bath

medium. The corrections apply as given on the report, when this thermometer is used at total immersion.

Occasionally it becomes necessary to use a total- immersion thermometer with a portion of the stem emergent from the bath medium. The temperature of

the environment above the bath, or apparatus con- taining the thermometer, may differ markedly from

the temperature of the thermometer bulb. It is also possible to have pronounced temperature gradients

along the length of exposed mercury column. A cor- rection can be calculated to account for the difference

in temperature between the bulb and the emergent

stem. A reliable estimate of the mean temperature of the emergent stem is required and should actually

be measured. The determination of the stem temperature should be repeated each time the thermometer is used in this way, or the accuracy of the correction

will depend upon the constancy of the environmental

temperature over a period of time. Significant variations in the temperature of the emergent stem may

occur due to air circulation and variations in ambient

temperature, even though the location of the ther- mometer does not change.

The same situation occurs in the case of partialimmersion thermometers. The reported scale corrections apply only for the indicated depth of immersion

and a particular stem temperature. If the thermometer

is used under conditions other than specified, the reported scale corrections are no longer applicable,

and a stem temperature correction is required.

The following paragraphs describe methods for determining stem temperatures and calculating corrections. It will be seen how important the stem

temperature correction is in relation to a desired accuracy.

a. Measurement of Emergent-Stem Temperature

There are two methods available for measuring the approximate mean temperature of the emergent stem. The first method consists of placing one or more small

13
auxiliary thermometers near the emergent stem as shown in figure 8(c). For a more accurate measure- ment faden or thread thermometers [8, 9] can he

used. These thermometers have long bulbs measuring

variously 5 to 20 cm, with wall thicknesses and bore

sizes nearly the same as the stem of an ordinary

thermometer. The bulb length is selected to approximate that of the emergent stem whose temperature is to be measured. The stem of the faden thermometer

has a finer capillary than the bulb and is usually

graduated in intervals of 2, 5, or 10 degrees Celsius. Stem temperature measurements taken at NBS are based upon the use of faden thermometers whenever

possible.

A convenient method for measuring the emergent- stem temperature of a total-immersion thermometer

that is used at partial immersion involves the use of

a faden thermometer. The top of the faden thermometer bulb is placed on a horizontal plane with the top

of the mercury column of the thermometer whose stem

temperature is being measured. The faden thermometer chosen must have a bulb which is long enough to cover the vertical area of unknown temperature

gradient necessary to be measured. This sometimes

involves placing part of the faden thermometer bulb

in the bath medium, since the top portion of the medium can be at a temperature different from the temperature of the total-immersion thermometer bulb. (This is especially true for measurements above

150 °C). The reading of the faden thermometer will indicate the mean temperature value of the area sur- rounding the bulb, which is also the mean temperature

value of the adjacent portion of the total-immersion

thermometer stem. A faden thermometer used in this manner is illustrated in figure 8(a).

If the stem temperature of a partial-immersion

thermometer is to be measured, a similar approach

is followed. In this case it is necessary to measure the mean temperature from the immersion line to the top

of the mercury column of the partial-immersion ther- mometer. One or more faden thermometers with

appropriate bulb lengths are chosen to accomplish this measurement. This procedure is shown in figure 8(b).

FADEN THERMOMETER

'BULB

FADEN

- THERMOMETER

BULB

. LIQUID COLUMN

OF THERMOMETER

IMMERSION LEVEL

(b) paRTiiL IMMERSION

7777 '/////

(c) PARTIAL IMMERSION

(O) TOTAL IMMERSION

Figure 8. Schemes for measurement of emergenl-slem

temperature.

In calculating the emergent stem correction, it is convenient to express the length of the thermometer

stem adjacent to the faden bulb in terms of degrees on the thermometer scale. If a faden thermometer,

having a bulb which is 10 cm long, is used for a stem

temperature measurement, then the number of degrees

corresponding to the 10 cm length must be found by

measuring a portion of the thermometer scale. This

measurement should be made over the portion of the graduated scale which was adjacent to the faden

thermometer bulb. This is particularly important with

high-temperature thermometers, where the length of

a degree is generally not the same throughout the

entire length of the scale. In some instances the adjacent portion of the thermometer stem is not graduated.

This is especially true with partial-immersion ther- mometers in the area above the immersion line. This

ungraduated length between the immersion line and

the first graduation must be evaluated in terms of

scale degrees and included as part of the distance covered.

b. Formula for Total-Immersion Thermometers

When a thermometer which has been graduated and

calibrated for use at total immersion is actually used

at partial immersion, the correction for the emergent

stem may be calculated by the general formula,

stem correction = kn (t-i - t)

where

k = differential expansion coefficient of mercury

(or other thermometric liquid) in the particular kind of glass which the thermometer is made (see table 4),

n = number of thermometer scale degrees adjacent

to the faden thermometer bulb,

t = average temperature of n degrees of the ther- mometer stem (faden thermometer reading),

ti = temperature of the thermometer bulb.

The coefficient k varies for different kinds of

glass, or for different temperature intervals, i.e., differ- ent values of (t, — t). For purposes of computing the emergent-stem correction, the value of k may be

considered as depending on the average of <i and t. Values of k as the function of ('j + t) /2 for two widely used thermometric glasses are given in table 4. If the kind of glass is not known, it is acceptable to use k = 0.00016 for mercury thermometers graduated in degrees Celsius and k = 0.00009 for mercury ther- mometers graduated in degrees Fahrenheit.

The expansions of liquids such as alcohol, toluene,

etc., vary quite rapidly with the temperature causing

k to vary considerably for different temperature in- tervals. An approximate stem correction for such

thermometers may be calculated by setting the value

of k in the above equation as equal to 0.001 for Celsius thermometers or 0.0006 for Fahrenheit thermometers.

Calculation of the stem correction may be illustrated by the following example: A total-immersion ther- mometer indicates a reading of 90 °C in a bath when

14
Table 4. — Values of k for mercury-in-glass thermometers

Mean temperature k for "normal" glass k for "borosilicate" glass

ti +t

2

For Celsius thermometers

0° 0.000158 0.000164 100 .000158 .000164 150 .000158 .000165 200 .000159 .000167 250 .000161 .000170 300 .000164 .000174 350 .000178 400 .000183 450 .000188

For Fahrenheit thermometers

0° 0.000088 0.000091 200 .000088 .000091 300 .000088 .000092 400 .000089 .000093 500 .000090 .000095 600 .000092 .000097 700 .000100 800 .000103

immersed to the 80 °C graduation mark on the scale. A 10 cm faden thermometer placed alongside the thermometer is adjacent to the scale between the 60

and 90 °C graduations and indicates a stem temperature of 80 " C. For this case n = 90-60 or 30 degrees.

The stem correction is:

0.00016 X 30 (90-80) = +0.048 °C.

This correction is added to the corrected thermometer

reading to obtain the actual temperature of the bath

medium. Note that when the temperature of the emergent stem is lower than the bath temperature, the sign

of the correction is +, since the thermometer would

indicate a higher temperature reading if immersed

properly.

If a faden thermometer was not available in the above example, the emergent-stem temperature could

be estimated by suspending a small auxiliary ther- mometer above the bath adjacent to the thermometer.

The bulb of the auxiliary thermometer would be

placed at the center of the emergent stem or at the 85 °C graduation. The reading of the auxiliary ther- mometer will indicate the approximate mean temperature of the 10 degrees (80° to 90 °C) emergent from

the bath. For this correction the value for n would

be 10. If the auxiliary thermometer indicates a reading of 60 "C, the stem correction would be:

0.00016 X 10 (90-60) = +0.048 °C.

This method is not usually as reliable as the method

involving the use of a faden thermometer [9].

c. Formula for Partial-Immersion Thermometers

The scale corrections for partial-immersion ther- mometers calibrated at NBS are applicable when the thermometer is immersed to the immersion mark and,

unless otherwise requested, for the unspecified stem

temperatures which prevailed over the comparison

baths at the time of calibration. Frequently it is necessary to report scale corrections which are applicable when specified mean temperatures of the emergent stem are requested. In such cases the emergent stem temperatures are measured during calibration and the observations are corrected as necessary

to account for any differences found between the

specified stem temperatures and the stem temperatures

observed during test. The magnitude of the stem cor- rection will be proportional to the difference between

the specified and observed stem temperatures, and

may be calculated for Celsius mercurial thermometers

by using the following formula:

stem correction ~ 0.00016 n (Up — tots),

where

tsp = specified mean temperature of the emergent

stem (for which reported scale corrections

apply),

tobs = observed mean temperature of the emergent

stem (faden thermometer reading), n = number of scale degrees equivalent to the length of emergent stem (including the eval- uated area above the immersion line).

15
This formula may also be used to correct the indications of a partial-immersion thermometer, when, dur- ing use, the actual stem temperatures differ from the

specified ones for which the scale corrections apply.

d. Formula for Calorimetric Thermometers

The stem correction is often important when ther- mometers are used for differential temperature measurements, as in calorimetry (Sec. 6.5). The correction

may be computed from the following formula, which

involves the difference between the initial and final readings, provided the mean temperature of the stem

remains constant. Stem correction = kd {h +iF — / — t)

where

k = factor for relative expansion of glass and

mercury,

ti = initial reading,

tF — final reading,

/ = scale reading to which the thermometer is immersed,

t = mean temperature of the emergent stem,

J = — h.

This correction must be applied (added if positive, subtracted if negative) to the difference of the read- ings to give the actual temperature difference. Example: The thermometer was immersed to the 20° mark: the initial reading, ii, was 25 °C; the final reading, U, was 30 and the stem temperature was

20 °C. The correction is:

0.00016 x 5 (25 +30—20—20) = +0.012 °C.

Since the difference between U and Zf was 5°, the

actual difference between the initial and final temperature readings was: — + correction = 5.012 °C.

e. Formula for Beckmann Thermometers

The formula used to compute the stem temperature

correction for Beckmann thermometers (Sec. 6.4) is similar to the one used for calorimetric thermometers.

The formula given below is applicable provided the thermometer is immersed near the zero degree indi- cation on the scale and the temperature of the stem remains constant. stem correction = kd (5 + + ?f — ;), where

k — factor for relative expansion of glass and

mercury,

t\ ~ initial reading,

tr = final reading,

t = mean temperature of the emergent stem,

d — If — (1 S = setting of the thermometer (Sec. 6.4).

A Beckmann thermometer should not be used with

any part of the lower portion of the stem exposed to ambient temperature. Since this part may contain

5 to 10 times more mercury per centimeter than the graduated portion, a large and uncertain error will be introduced if this section is not in the bath medium.

If it is unavoidable, and such a thermometer must

be used in this way, the necessary correction may be

computed from the above formula, provided S in the formula is replaced by S + m, where m is the number

of degrees the temperature of the thermometer must

be lowered to bring the meniscus from the zero mark

on the scale to the point of immersion.

If the thermometer is immersed to some point other than the zero mark, as would ordinarily be the case with thermometers having the zero graduation at the top of the scale, the differential stem correction may

be calculated from the above formula if S is replaced

by 5 + m. The formula is applicable whether the point of immersion is on the scale or below it, provided

the points at which readings are made are above the point to which the thermometer is immersed.

5.4. Number and Choice of Test Points

A thermometer is usually calibrated at points spaced

uniformly over the entire range of the main scale. The number of calibration points chosen depends on

the range of scale, graduation interval, and accuracy

desired. The interval between the calibration points

should not be unnecessarily small, nor should it be

so large as to destroy confidence in interpolated cor- rections for temperature values between the calibration

points. For thermometers not graduated above approximately 200 °C, it is generally accepted that the inter- val between test points should not exceed 100 scale

divisions, if the corrected temperature values between

the calibration points are to have an expected accu- racy of approximately one-half of one scale division.

If accuracies of one or two-tenths of a scale division are desired, it will be necessary to reduce the calibration interval to every 40 or 50 scale divisions. If a thermometer is graduated above 200 °C, a 40 to 50

scale division calibration interval is required to pro- duce corrected temperature values with expected ac- curacies of approximately one-half of a scale division,

and a 20 to 25 scale division calibration interval is necessary for expected accuracies to be approximately one or two-tenths of a scale division. The above results were derived from analysis of calibration data taken on more than 50 thermometers

purchased from 1930 through 1956 for use as labora- tory standards. The data indicated that there was considerable variation between individual thermometers

and that scale corrections obtained over a given inter- val for a particular thermometer were not sufficient to predict whether or not more calibration points were

required. The above studies were made with only a few of the many types of thermometers submitted to NBS for calibration, and may not necessarily be

16
applicable to other types. Experience with a particular

type of thermometer seems to be the most reliable guide in the choice of its calibration points.

If a thermometer is submitted to NBS for calibration and the calibration points are not specified on

the purchase order, it will be tested at a reference

point and at intervals of approximately every 100

scale divisions. A calibration should never consist of fewer than two points on the main scale, and should

always be tested at a reference point, whether on the main scale or on an auxiliary scale.

6. Common Thermometers and Factors Affecting Their Use

A listing of tolerances and expected accuracies for common types of liquid-in-glass thermometers, which

are accepted for calibration, are given in tables 5 through 12. The scale tolerances shown are chosen to be indicative of good manufacturing practice. When

a thermometer is manufactured, small errors in pointing (marks placed on a blank thermometer at various

temperatures to be used as guides for the placement

of the graduation lines) and graduating are inevitable. These graduation marks are also subject to variations due to the inherent properties of the glass. The toler- ances must be sufficiently restrictive to insure a satis- factory high-grade thermometer, and at the same time

not cause undue manufacturing difficulties. In addition to the scale tolerance limit, the error in any temperature interval should not exceed 5 percent

of the nominal value of the interval. The purpose of

this requirement is to eliminate thermometers having

large corrections of alternating signs, which lead to uncertainties in the interpolation of scale corrections

between the calibration points.

The word "accuracy" used in these tables refers to the best values attainable in the use of thermometers

when all corrections are applied. The accuracy bounds

may seem broad in some instances, but the definite limitations of liquid-in-glass thermometry become ap- parent when all factors are considered. For example,

if the scale is expanded by reducing the diameter of the capillary, a practical limit is reached beyond

which capillary forces, in combination with the elasticity of the thermometer bulb, will prevent a smooth

advance or retreat of the mercury column. The move- ment of the mercury meniscus may be erratic and

occur in steps appreciably large in comparison to the graduation interval. This is particularly true when

the temperature of the medium is decreasing. Less

rigid bulbs (relatively large diameters and/or thin

walls), as well as capillaries of small diameters, may

cause large "meniscus jumps." Excessively elliptical or flattened bores are not recommended for the same

reason. Therefore, increasing the length of a degree on the scale, for practical bulb sizes, improves ther- mometric performance only to a certain point. Beyond

this point precision of reading may readily be mistaken

for accuracy in temperature measurement. A study of

the effects of bulb and capillary dimensions on ther- mometer performance, made by Hall and Leaver [10],

provides valuable guidelines for design purposes.

Other factors such as ice-point changes, unless ex- actly accounted for, and differences in external pres- sure may also account for inaccuracies much greater

than the imprecision with which a scale having 0.1 or

0.2 degree graduations may be read.

6.1. Total-Immersion Thermometers

Thermometers pointed and graduated by the manufacturer to indicate correct temperatures when the bulb and the portion of the stem containing the ther- mometric liquid are subjected to the temperature being

measured are known as total-immersion thermometers.

While these thermometers are designed for immersion

of all the mercury, it is not desirable to immerse the portion of the stem above the meniscus. The heating

of this portion to high temperatures could cause ex- cessive gas pressures resulting in erroneous readings

and possibly permanent damage to the bulb. In practice, a short length of the mercury column

often must be left emergent from the bath or medium

being measured so that the meniscus is visible. If the temperature difference between the bath medium and

its surroundings is large, an appreciable temperature

gradient may exist around the thermometer stem near

the surface of the bath. This condition becomes more

serious when a total-immersion thermometer is inten- tionally used at partial immersion. If either situation

exists, an emer,s;ent stem correction, as explained in section 5.3b, will be necessary. The correction may be

as large as 20 Celsius degrees (36 Fahrenheit degrees)

if the length of the emergent liquid column and the

difference in temperature between the bath and the space above it are large. Tolerances and accuracies

expected of total-immersion thermometers are given

in tables 5 and 6.

6.2. Partial-Immersion Thermometers

In many instances the use of a total-immersion

liquid-in-glass thermometer for temperature measure- ments is inconvenient or impossible. For this reason

partial-immersion thermometers are designed with

scales graduated to indicate correct temperatures when

the thermometers are immersed to specified depths. Unless otherwise stated, each Report of Calibration issued by NBS gives corrections which are applicable for temperatures prevailing above the comparison baths. No stem temperature correction is necessary when

17
Table 5. — Tolerances for Celsius total-immersion mercury

thermometers

Temperature range in degrees Graduation

interval in degrees

Tolerance

in degrees

Accuracy

in degrees

Thermometer graduated under 150 °C

°C

0 up to 150

0 up to 150

0 up to 100

1.0 or 0.5

.2

.1

0.5

.4

.3

0.1 to 0.2

.02 to .05

.01 to .03

Thermometers graduated under 300 °C

0 up to 100 Above 100 up to 300 .... 0 up to 100 Above 100 up to 200 ....

I 1.0 or 0.5

1

f 0.5 (

) 1.0

( 0.4

1

-5

).l to 0.2

.2 to .3 .02 to .05

.05 to .1

Thermometers graduated above 300 °C

0 up to 300 Above 300 up to 500 .... 0 up to 300 Above 300 up to 500 ....

1 2.0

1

1.0 or 0.5

{ 2.0

( 4.0

( 2.0

I 4.0

0.2 to 0.5

.5 to 1.0 .Ito .5

.2 to .5

Table 6. — Tolerances for Fahrenheit total-immersion mercury thermometers

Temperature range in degrees Graduation

interval in degrees

Tolerance

in degrees

Accuracy

in degrees

Thermometers graduated under 300 °F

32 up to 300

32 up to 300

32 up to 212

2.0

1.0 or 0.5

.2 or .1

1.0

1.0 0.5

0.2 to 0.5 .Ito .2 .02 to .05

Thermometers graduated under 600 °F

32 up to 212 Above 212 up to 600 .... 1 2orl I 1.0

1 2.0

0.2 to 0.5

.5

Thermomete rs graduated a bove 600 °F

32 up to 600 ] c ( 4.0 0.5 to 1.0 Above 600 up to 950 .... { ^ | 7.0 1.0 to 2.0 32 up to 600 ( oil 3.0 0.2 to 1.0 Above 600 up to 950 .... j ^ or i

^ g ^ ^ ^ q

emergent stem, t gether with the inherent difficulty of estimating or measuring the emergent-stem temperature with sufficient accuracy, can contribute

markedly to the uncertainty of a given thermometer

indication. For these reasons tolerances and accuracies expected of partial-immersion thermometers,

as given in tables 7 and 8, are not as stringent as those for total-immersion thermometers.

Table 7. — Tolerances for Celsius partial-immersion mercury thermometers

Temperature range in degrees Graduation

interval in degrees ^

Toler- ance in degreesAccuracy ^ in degrees

Thermometers not graduated above 150 °C

0 up to 100

0 up to 150

1.0 or 0.5

1.0 or 0.5

1.0

1.0

0.1 to 0.3

0.1 to 0.5

Thermometers not graduated above 300 °C

0 up to 100 Above 100 up to 300 ....

1.0

1.0

1.0

1.5

0.1 to 0.3

.5 to 1.0

Thermometers graduated above 300 °C

0 up to 300 Above 300 up to 500 .-. 1

2.0 or 1.0

f 2.5

I 5.0

0.5 to 1.0 1.0 to 2.0

Table 8. — Tolerances for Fahrenheit partial-immersion mercury thermometers

Temperature range in degrees Graduation

interval in degrees »

Toler- ance in degreesAccuracy l> in degrees

Thermometers not graduated above 300 °F

32 up to 212 2.0 or 1.0 2.0 0.2 to 0.5 32 up to 300 2.0 or 1.0 2.0 0.2 to 1.0

Thermometers not graduated above 600 °F

32 up to 212 2.0 or 1.0 2.0 0.2 to 0.5 Above 212 up to 600 .... 2.0 or 1.0 3.0 1.0 to 2.0

Thermometers graduated above 600 °F

these thermometers are used under conditions similar

to those existing during calibration. However, if conditions differ, especially the temperature of the emergent stem, the necessary emergent stem correction

must be calculated as shown in section 5.3c. An un- steady or irreproducible environment surrounding the

32 up to 600 Above 600 up to 950

5.0 or 2.0

5.0 10.01.0 to 2.0 2.0 to 3.0

" Partial-immersion themiometers are sometimes graduated in smaller intervals than shown in these tables, but this in no way

improves the performance of the thermometers, and the listed tolerances and accuracies still apply.

^ The accuracies shown are attainable only if emergent stem

temperatures are closely known and accounted for.

18
6.3. Low-Temperature Thermometers

The use of mercury-in-glass thermometers for low

temperature measurements is limited bv the freezing

point of mercury, which is —38.9 °C' (— 38.0°F).

This limit may be extended to lower temperatures by

forming an eutectic alloy consisting of mercury and 8.5 percent by weight of thallium. The freezing point of

this alloy is approximately —59 °C ( — 74 °F). However, small differences in the quantity of thallium

present have the effect of markedly raising the freez- ing point of the alloy. It has also been found that some thermometers containing this liquid have be- haved erratically in the range of —59 to —57 °C. Consequently, thermometers using a mercury-thallium

alloy as the liquid should not be used below —56 °C

(—69 °F).

Temperature measurements from —56 °C ( —69

°F) to approximately —200 °C (—328 °F) can be

made by using thermometers containing organic li- quids. Alcohol, toluene, pentane, or other organic

liquids, alone or in mixtures, have been used as fluids for low temperature thermometers. All of these fluids have limitations, and thermometers containing organic

liquids are not considered to be as reliable as ther- mometers containing mercury or mercury-thallium

alloy.

Organic liquids have the disadvantage of wetting a surface and leaving a film on the wall of the capillary

when the liquid column recedes. This may lead to significant errors in the temperature indications if sufficient precautions are not taken. The thickness of the film on the capillary wall will depend, among other

things, on the viscosity of the liquid, the interfacial action between the liquid and glass, and the rate at which the thermometer is cooled. Whenever possible the rate of cooling should be slow with the bulb cooled

first, enabling the viscosity of the organic fluid in the capillary to be kept as low as possible until the

final temperature is reached. This minimizes the amount of liquid left on the capillary wall. Sufficient time should always be allowed to assure complete

drainage. Under adverse conditions this may take an

hour or more.

In addition to good drainage characteristics, a satis- factory low-temperature fluid should not contain water,

dirt, or other foreign material which will separate

from the liquid. Because low-temperature thermometers are frequently designed for use above room

temperature, the vapor pressure of the liquid must

be low to prevent distillation at these higher temperatures. Any dye, added to the fluid for improved

visibility, should be chosen for good color fastness with respect to light exposure or chemical action with

the organic liquid.

Tolerances and accuracies applicable to low-temperature thermometers are given in tables 9 and 10.

Table 9. — Tolerances for low-temperature total-immersion thermometers

Temperature

range in degrees

Type of thermometerGraduation

interval in degrees

Toler- ance in degrees

Accuracy

in degrees

Celsius thermometers

-35 to 0 -35 to 0 -56 to 0 -56 to 0 -200 to 0

Mercury

do Mercurythallium. do Organic

liquid.

1 or 0.5

.2

.5

.2 1.0

0.5

.4

.5

.4 2.0

0.1 to 0.2

.02 to .05

.1 to .2

.02 to .05

.2 to .5

Fahrenheit thermometers

-35 to 32 -35 to 32 -69 to 32 -69 to 32 -328 to 32

Mercury

do Mercurythallium. do Organic

liquid.

1 or 0.5

.2 lor .5

.2 2 or 1.0

1.0 0.5

1.0

0.5

3.0

0.1 to 0.2

.05 .Ito .2

.05

.3 to .5

Table 10. — Tolerances for low-temperature partial-immersion thermometers

Temperature

range in degrees

Type of ( thermometerGraduation

mterval in degrees

Toler- ance in degrees

Accuracy

in degrees

Celsius thermometers

-35 to 0 -56 to 0 -90 to 0

Mercury

Mercurythallium. Organic

liquid.

1.0 or 0.5

1.0 or .5

1.0

0.5

.5

3.0

0.2 to 0.3

.2 to .3

.4 to 1.0

Fahrenheit thermometers

-35 to 32 -69 to 32 - 130 to 32

Mercury

Mercurythallium. Organic

liquid.

1.0 or 0.5

1.0 or .5

2 orl

1

1

5

0.3 to 0.5

.3 to .5

.8 to 2.0

6.4. Beckmann Thermometers

A metastatic, or Beckmann thermometer, usually of

the enclosed-scale type, is constructed in a manner

that permits mercury to be removed from, or added

to, the bulb, making it possible to use the same ther- mometer for differential temperature measurements in various temperature ranges [11]. The scale usually

consists of 5 or 6 Celsius degrees, although some micro

types have a scale containing only 3 Celsius degrees.

19
The "setting" of a Beckmann thermometer refers

to the temperature of the bulb when the reading on

the scale is 0°. When the setting is changed, enabling

the thermometer to be used at a higher or lower tem- perature range, the quantity of mercury affected by a temperature change is not the same. Therefore, two

equal changes in temperature at different settings cause different indications on the scale, and a "setting factor" must always be used to convert reading differ- ences into actual temperature differences whenever

the thermometer is used at any setting other than the one at which the scale was calibrated. These setting factors combine corrections for the different quantities of mercury during equal temperature changes,

and the difference between the mercury-in-glass scale and the International Practical Temperature Scale of 1968. Table 11 lists setting factors calculated for thermometers of Jena 16"' - glass or, its American

equivalent, Corning normal. The scale calibrations for Beckmann thermometers as reported by NBS are

applicable for a setting of 20 °C. Consequently, the

Table 11. — Setting factors for Beckmann thermometers

Setting Factor Setting Factor

0 °C 0.9935 55 °C 1.0092 5 0.9951 60 1.0102 10 0.9967 65 1.0112 15 0.9984 70 1.0121 20 1.0000 75 1.0129 25 1.0014 80 1.0137 30 1.0029 85 1.0146 35 1.0042 90 1.0154 40 1.0056 95 1.0162 45 1.0070 100 1.0170 50 1.0082

As an illustration, suppose the following observations were made:

Setting — 25 °C. Lower reading = 2.058° Stem temperature = 24 °C. Upper reading = 5.127° Lower Upper

Observed reading = 2.058 5.127 Correction from certificate — +0.005 "0.008

Corrected upper reading 5.119 Corrected lower reading 2.063 Difference = 3.056 Difference multiplied by setting factor (1.0014) = 3.060 Emergent stem correction (see accompanying

stem correction sheet) — + .004 Corrected difference — 3.064

^Certain commercial products and instruments are identified in this paper

in order to specify adequately the experimental procedure. In no case does such identification imply recommendation or endorsement by the National Bureau of Standards, nor does it imply that the products or equipment

identified are necessarily the best available for the purpose.

setting factor is 1.0000 at this temperature. For a set- ting other than 20 °C, the observed temperature differ- ence must be multiplied by the appropriate setting factor as shown in the example given below the table. For most Beckmann thermometers, the large bulb

is joined to a fine capillary, which is backed by a

milk-glass scale, by a capillary of much larger diameter. This large capillary is a source of some uncertainty when the thermometer is used at partial

immersion and this portion is not entirely immersed. For appropriate emergent-stem corrections refer to section 5.3e. Tolerance requirements for Beckmann thermometers

are given in table 12. Under the heading "Accuracy

of interval in degrees" is given the estimated accuracy

attainable in the measurement of any interval within

the limits of the scale. No tolerances for scale error

are given, although it is desirable that it be no larger

than 0.02 °C over a 1.0 °C interval.

Table 12. — Tolerances for Beckmann and calorimetric thermometers

Gradua- Allowable change Accuracy of Type of tion in- in correction in interval in thermometer terval in degrees degrees

degrees

Beckmann 0.01 °C 0.01 over 0.5° 0.002 to 0.005 ..- interval for setting of 20 °C Bomb calorimetric .01 °C 0.02 over 1.5° .005 to .01 interval Do .02 °C 0.02 over 1.5° .005 to .01 interval Do .05 °F 0.04 over 2.5° .01 to .02 interval Gas calorimetric ... .1 -F 0.15 over a 5° .02 to .05 interval

6.5. Calorimetric Thermometers

Calorimetric thermometers include a specialized

group of solid-stem mercury-in-glass thermometers

which are used for accurate differential measurements.

Since the accuracy of these thermometers at any one

temperature is of less iinportance than the accuracy

of the temperature intervals, no reference point is required.

Table 12 gives the scale error required of some

typical calorimetric thermometers. Like the Beckmann

thermometer, no tolerances for scale error are given,

but it is desirable that the scale corrections be no larger

than approximately five graduation intervals.

20
7. Thermometer Design

Reproducibility and accuracy of readings are influ- enced markedly by the design, choice of materials,

and actual construction of the thermometer. The cali- bration laboratory personnel exercise judgment in deciding whether individual thermometers are likely

to maintain a calibration; however, general recom- mendations for acceptable thermometer design and

construction are given in the following sections.

7.1. Materials of Construction

While the cleanliness of the thermometer bulb, bore,

and liquid filling have a pronounced effect upon the performance of a thermometer, equally important is the proper choice of the glass from which the ther- mometer is manufactured. The thermometer, and

especially the bulb, must be made of glass suitable for use in the temperature range indicated on the stem. Some types of glasses commonly used in the manufacture of thermometer bulbs and reasonable upper

temperature limits are estimated by Thompson [12]

in table 13. Thompson's estimates are based upon the work of Liberatore and Whitcome [13]. The results show that significant changes in bulb volume may

occur if the bulb is heated for long periods of time

at temperatures higher than 130 °C (234 °F) below

the strain point of the bulb glass. The strain point of a

glass is defined as the temperature at which the glass

has a viscosity of 10"^ poises [14]. Thermometers

may be used intermittently, to approximately 70 °C

(126 °F) of the strain point. It should be noted that the use of a glass with a high strain point, such as

borosilicate glass, results in better thermometer per- formance and stability even in thermometers used at temperatures much lower than the exposure limits given in table 13.

Table 13. — Temperature exposure limits for various thermometer glasses ^

Strain Exposure limits

point Continuous Intermittent

"C "C °F "C

Corning normal 7560 500 370 700 ''430* 1-805* Kimble R 6 490 360 680 420 790 Jena 16 III 495 365 690 425 795 Coming borosilicate 8800 529 400 750 460 860 Jena borosilicate 2954 548 420 790 480 900 Corning 1720 668 540 1005 600 1110 Jena Supremax 2955 665 535 995 595 1100

a From reference [12].

b *405 °C or 760 T if Corning Standard Thermometer 0041

glass is used for the stem.

The thermometer must also be adequately annealed

to enable the bulb volume to remain reasonably stable

and causing only small changes in the thermometer

readings with continued use. This is especially important for a thermometer graduated above 300 °C or 600 "F. The quality of the thermometer glass and the adequacy of the annealing process may be judged by

the stability of the reference-point readings. A method

for testing bulb stability is described in The American

Society for Testing and Materials Method E77.

All high-temperature thermometers should be filled with a dry inert gas such as nitrogen under sufficient pressure to prevent separation of the mercury at any

temperature indicated on the scale. Total-immersion

thermometers graduated above 150 °C or 300 °F must

be gas filled to minimize the distillation of mercury

from the top of the mercury column. An expansion

chamber at the top of the capillary or a specified

length of unchanged capillary above the highest graduation is essential for thermometers containing a gas. The gas must have an adequate area for compression

when the mercury is advanced toward the top of the

scale; otherwise, excessive pressure in the capillary

will cause the thermometer to burst. For thermometers graduated below 150 °C or 300 °F a gas filling is optional, but is strongly rec- ommended. The mercury column of a vacuous ther- mometer will tend to separate easily if the thermometer

is inverted or subjected to a sudden shock.

7.2. Scale Design and Workmanship

Solid-stem thermometers must have the graduation

marks placed directly on the stem and opposite the enamel back. If the thermometer is of the enclosedscale type, the graduated scale must be securely fastened to prevent relative displacement between the

scale and the capillary, or a datum line should be

conveniently located to indicate at any time whether

the scale is in its original position. The graduation

marks should be clear cut, straight, of uniform width,

and in a plane perpendicular to the axis of the thermometer.

The scale should be graduated in 1.0, 0.5, 0.2, or 0.1 degree intervals, or in decimal multiples. The divisions should be numbered in such a manner that the identification of any graduation is not unnecessarily

difficult. Thermometers with scales graduated in 0.25 degree intervals, or submultiples, are sometimes diffi- cult to read and the discontinuance of their production

is desirable. If the thermometers are graduated in 0.1 or 0.2 degree intervals (or decimal multiples), every

fifth mark should be longer than the intermediate ones and a number should appear at every tenth mark.

Thermometers graduated in 0.5 degree intervals (or

decimal multiples) require three lengths of graduation

marks. These consist of alternating short and inter- mediate marks, with every tenth line distinctly longer

than the others. A number should appear at every

10th or 20th mark.

21
The scale must not be extended to temperatures

beyond which the particular thermometer glass is suited. A thermometer made of borosilicate glass, for example, should never be graduated to 500 °C

(932 °F). It would be ruined in a short time if used

at that temperature.

7.3. Scale Dimensions

Although optimum line width depends in some

measure upon the intended use of a particular ther- mometer, coarse graduation marks do not represent

good design. If the thermometer indications are to be

observed precisely (for example to 0.1 of a division),

the width of the graduation marks should not exceed

0.2 of the interval between the center lines of the graduations. In instances where the thermometer must

be read quickly or in poor light, and less precision is expected, somewhat wider lines may be acceptable.

The graduation marks must not be too closely

spaced. The closest permissible spacing depends upon

the fineness and clearness of the marks. In no case should the distance between center lines of adjacent

graduation marks on a solid-stem thermometer be less than 0.4 mm. The minimum permissible interval be- tween graduation marks for an enclosed-scale ther- mometer is 0.3 mm if the lines are ruled on a milkglass scale; otherwise, the minimum is also 0.4 mm.

The minimum in no case represents good design, and

well-designed thermometers will have graduation inter- vals considerably larger than the specified minimum.

In order for a thermometer to be usable over the

entire range of the scale, the graduation marks must

not be placed too close to any enlargement in the

capillary. Appreciable errors may result if any of the following conditions exist. (1) There is insufficient immersion of the mercury in the main bulb or a capillary enlargement; (2) the graduation marks have

been placed over parts of the capillary that have been

changed by manufacturing operations; or (3) the graduations are so close to the top of the thermometer

that excessive gas pressure results when the mercury

is raised to this level. The distances given below are considered as minimum limits commensurate with

good thermometer design

(a) A 13 mm length of unchanged capillary

between the bulb and the lowest graduation, if the graduation is not above 100 °C (212 °F); a 30 mm

length if the graduation is above 100 °C (212 °F).

fb) A 5 mm length of unchanged capillary be- tween an enlargement and the graduation next below,

except at the top of the thermometer.

( c ) A 10 mm length of unchanged capillary be- tween an enlargement, other than the bulb, and the

graduation next above, if the graduation is not above

100 °C (212 ° F) ; a 30 mm length if the graduation

is above 100 °C (212 °F).

(d) A 10 mm length of unchanged capillary above

the highest graduation, if there is an expansion chamber (see definitions, Sec. 3); a 30 mm length if there

is no expansion chamber.

7.4. Reference Point

Thermometers graduated above 150 °C or 300 °F,

or precision thermometers with an expected accuracy

of better than 0.1 °C or 0.2 °F when calibrated for actual temperature measurement, must have a refer- ence point. The effects of changes in the bulb volume

on the thermometer indications may be followed

throughout the life of the thermometer by periodic

testing at the reference point. From the reference point

observations, the thermometer corrections can be kept

current as explained in section 5.2. If a suitable ref- erence point, such as the ice or steam point, is not

included in the range of the main scale, a short auxiliary scale containing a fixed point should be provided.

To prevent the thermometer from being unduly long,

a contraction chamber may be introduced between the auxiliary scale and the main scale. Any auxiliary scale must have graduations identical to those on the main

scale, both dim.ensionally and in terms of temperature,

and they must extend for a short interval both above

and below the reference point. Similarly, when the main scale ends near a reference point, the graduations

must be continued for a short interval beyond the reference temperature.

Reference points are not needed on thermometers

intended for differential measurements (such as calori- metric thermometers), nor on thermometers graduated

below 150 °C or 300 °F, if they are not to be cali- brated to an accuracy of better than 0.1 '°C or 0.2 °F.

7.5. Marking of Partial-Immersion

Thermometers

Partial-immersion thermometers must be plainly

marked "partial immersion" or the equivalent (for

example, "76 mm immersion"), or a conspicuous line permanently placed on the stem to indicate the proper

depth of immersion. This line must not be less than

13 mm above the top of the bulb. Special partialimmersion thermometers, made for use in a specific manner or instrument (for example, viscometers and

flash-point testers where the thermometer is held in a ferrule or other mounting arrangement suited to the instrument), need not be marked.

22
8. Special Notes

The following brief notes on the characteristic be- havior of mercury-in-glass thermometers are added to aid the user in understanding the behavior of these thermometers, and to better utilize the information

contained in the Reports of Calibration.

8.1. Glass Changes

The changes which occur in the glass of a ther- mometer bulb after first heating to a high temperature

within the acceptable exposure limits of the glass and

subsequent cooling to ambient temperatures, are an

involved function of time and temperature. They will depend upon the thermal history of the glass (both

during manufacture and previous use) . the time of exposure to the high temperature, and the rate of cooling. Evidence from many investigations [13, 15,

16] seems to indicate that when a glass is held indefinitely at some fixed temperature, density (and volume)

changes proceed toward a preferred density corres- ponding to a quasi equilibrium condition characteristic of the particular kind of glass and the temperature.

Since these changes involve molecular rearrangements,

they proceed more rapidly at high temperatures where

the viscosity of the glass is lower, and the molecular

mobility consequently higher. For this reason, a close approach to quasi equilibrium may be reached in the order of hours at annealing temperatures, while infinite time may be required at much lower temperatures.

If a glass that has been heated to a high temperature is allowed to cool rapidly, it will be seen that equilibrium is not reached at the lower temperatures

during cooling, and an equilibrium density more

nearly corresponding to the high temperature is "frozen" into the glass. This characteristic behavior

of glass has a lasting effect on the performance of

liquid-in-glass thermometers. For the entire lifetime of the thermometer it may retain a "memory" of the thermal history at the higher temperatures.

The techniques of good manufacture are designed to produce in the thermometer glass a state which will result in maximum stability for the range of temperature indicated on the scale. To achieve perfect stability for all conditions of use is not possible in thermometer

manufacture; therefore, changes in the ice-point read- ings are observed periodically. The changes observed

at the ice point are reflected at all points on the scale by the same magnitude and sign, since they are the

result of changes in the bulb volume (see Sec. 5.2)

(changes in the stem have very little effect). Because

of this behavior of glass, the changes in the bulb

volume can be either temporary or permanent.

a. Temporary Changes

Upon heating to a high temperature, the bulb of a thermometer will expand from its initial state. After

a short period of time, an equilibrium condition corresponding to that particular high temperature will appear to be reached. If the thermometer is cooled

slowly through critical temperature regions, the glass

will nearly return to its initial state, and the ice- point reading will show no change. If, on the other hand, the thermometer is cooled rapidly (such as cooling naturally in still air), the bulb will retain a portion of its expanded condition, and the ice-point

reading will be lower than the reading taken before

heating. This phenomenon is known as "zero, or ice- point depression." Thermometers which have been

heated to high temperatures recover from this ice- point depression in an unpredictable way, and fre- quently there will be no significant recovery after a period of one year at room temperature. Since the

ice-point depression has a reproducible value, ice- point readings may be used reliably to show changes

in the volume of the thermometer bulb with time and

use, provided the thermometer is allowed to cool in

still air and the ice point is taken within a reasonable

period of time, not to exceed one hour, after being

heated. Thermometers used below approximately 100 °C will usually exhibit a relatively rapid recovery from the

ice-point depression, and the original bulb volume will recover within the equivalent of 0.01 or 0.02 degrees

C in approximately 3 days. This phenomenon has

an important bearing on the precision attainable with

mercury thermometers, and must be taken into consideration, especially in the range of 0 to 100 °C.

If a thermometer is used to measure a given temperature, it will indicate an erroneously low value if it has, within a short period of time, previously been

exposed to a higher temperature. With the better grades of thermometric glasses the error resulting from

this hysteresis will not exceed (in the interval of 0 to 100 °C) 0.01 of a degree for each 10-degree

difference between the temperature being measured

and the higher temperature to which the thermometer

has recently been exposed. With the best glasses the error may only be a few thousandths of a degree for each 10-degree difference. The errors due to this hysteresis become somewhat erratic at temperatures

above 100 °C. For these reasons it is customary, in precision thermometry, to apply a scale correction

based upon an ice-point reading taken immediately

after the temperature measurement (see Sec. 5.2).

Where the range of temperature is small and the time between observations is short (as in the use of calorimetric thermometers), it is more satisfactory,

each time the thermometer is used, to first heat to the highest temperature to be measured, so that all of the depression has taken place before the observations

are begun. The condition to be observed is that the time required for observations is so short that no

appreciable recovery shall have taken place during

this time. As this condition is fairly well satisfied in calorimetric work, and as it is the only one for which

23
consistent measurements of the same temperature

interval can be made repeatedly, calorimetric ther- mometers should be used in this way.

b. Permanent Changes

A "secular change" in thermometer glasses, which

may progress with time, results in a non-recoverable

decrease in the bulb volume as indicated by an in- crease in the ice-point reading. At room temperature

there may be a gradual change which will continue

for years, but at high temperatures the changes will be markedly accelerated. With better grades of ther- mometer glasses the change will not exceed 0.1 °C

over a period of many years, provided the thermometer has not been heated to temperatures above approximately 150 °C. Initially, at high temperatures, the secular change usually progresses more rapidly, but

with continued heating and time it tends toward a lower rate of change. The rate of secular change will depend upon the kind of glass used in the thermometer

bulb and the particular heat treatment given the thermometer in manufacture. Thermometers manufactured according to good practices will evidence

only small secular changes. However, thermometers

made of glass unsuitable for the temperature range

indicated on the scale, or improperly annealed, may

show changes as large as 12 °C or 21 °F after heating

for approximately 200 hours at high temperatures

[17] . Permanent changes in the bulb volume have

also been observed when thermometers have been

repeatedly cycled at low temperatures (between —30

and +25 °C) [18].

When using thermometers for high temperature

measurements, one must use care to avoid overheating. After only a few minutes of heating the thermometer

at a temperature higher than its intended range, the increased gas pressure above the liquid column may

cause a permanent distortion of the bulb resulting in lower thermometer indications.

8.2. Pressure Effects

Since glass exhibits elastic properties, the volume

of a thermometer bulb will change when either the

internal or external pressure changes. Therefore, at a given temperature, the reading of a thermometer in a

horizontal position will differ from the reading in a

vertical position. Thermometer readings will also vary

with altitude. Changes of approximately 0.1 °C

(0.2 °F) per atmosphere have been observed for many thermometers with bulb diameters between 5 and 7 mm. This value can be used with some confi- dence for estimating the probable effect of an external pressure change. The effect of change of internal pressure is approximately 10 percent greater.

Formulas for both external and internal pressure

coefficients have been derived by Guillaume [19].

He found the relation.

I3. = k

where and R\ are external and internal radii of

the bulb, and A: is a constant containing elastic properties of the glass and a conversion factor for express- ing the volume change in terms of change of thermometer reading in degrees. In the above formula,

the external pressure coefficient ^ is defined as the change in scale reading in degrees resulting from a change of 1 mm Hg in external pressure. For Celsius

thermometers, Guillaume found a value of 5.2 X 10~^

degrees C/mm Hg for k, but Hall and Leaver [10],

by experiment, found a value approximately 25 percent

lower for their thermometers. In cases where an ac- curate correction is necessary, the value, /?e, should

be determined experimentally. A simple apparatus for

this determination is shown in figure 9.

Figure 9. Apparatus for measurement of the external pressure

coefficient.

The internal pressure coefficient, /Sj, is more diffi- cult to determine accurately, but may be calculated from the value, /3e, by means of the relation,

/?, = iSe + 1.5 X 10-6

for thermometers in Celsius degrees, or

j8i = /3e + 2.7 X 10-6

for thermometers graduated in Fahrenheit degrees.

8.3. Lag

Practically all theoretical treatments concerning

thermometer lag are based on the assumption that Newton's law of cooling is applicable (that the rate of change in the reading of the thermometer is proportional to the difference between the thermometer

temperature and the bath temperature). Consequently,

when a thermometer is immersed in any medium, it does not indicate the temperature immediately, but

approaches it asymptotically. A detailed discussion

of this subject has been given by Harper [20].

If the temperature of the medium is varying uni- formly, the thermometer always indicates what the temperature was at some previous time. The thermom24
eter readings are said to "lag" behind the temperature

of the medium by an amount which may or may not be negligible, depending upon the rapidity of the temperature variation and the physical characteristics

of the thermometer. In this case, the lag. A, may be

defined as the interval in seconds between the time

when the bath reaches a given temperature and the time when the thermometer indicates that temperature.

This lag is dependent upon the dimensions and material

of the thermometer bulb, the medium surrounding

the thermometer and the stirring rate of the medium.

If a thermometer is suspended in still air, the lag

may be as much as 50 times that of the same ther- mometer when it is immersed in a well-stirred water

bath. Since the value of the lag for mercurial thermometers is not large I from 2 to 10 seconds in a well-stirred water bath), it is not generally necessary to correct

for it. For example, if two thermometers, one having

a lag of 3 seconds and another of 8 seconds, are read

simultaneously in a bath with the temperature rising

at a rate of 0.001 degree in 5 seconds, the former will read 0.001 degree higher than the latter, due to the

lag. In the intercomparison of thermometers, the rate of temperature rise can usually be kept small, making

the lag correction negligible.

A second interpretation of thermometer lag involves

immersing a thermometer in a bath where the temperature of the medium remains constant. A certain time must elapse before the thermometer reading

agrees with the temperature of the medium to 0.1 of a degree, and still longer for agreement to be

within 0.01 of a degree. In this case the lag. A, is the time required for the original difference in temperature between the thermometer and bath medium to be

reduced to 1/e (that is 1/2.7) of itself. In a length

of time 4 A the difference will be approximately 1.5 percent of the original difference, and in a length of time 7 A approximately 0.1 percent. Determinations

of A for solid-stem laboratory thermometers, representative of American manufacture, have yielded values

of approximately 2 to 3 seconds in a well-stirred water

bath. Figure 10 shows the approach of thermometer

readings to the water bath temperature for three

selected thermometers having different values of A.

If the thermometer having A = 2.2 seconds is initially at 25 "C. and is immersed in a constant temperature

bath at 75 °C, the thermometer reading will be within

0.05 "C (0.1 percent of 50 °C) of the bath temperature in 15 seconds, and within 0.01 °C in 19 seconds. The curve showing A = 3.1 was obtained for an American Society for Testing and Materials (ASTM)

specification 56C calorimetric thermometer with an

outside bulb diameter of 7.9 mm and a bulb length

of 44 mm. The value of A = 2.2 was found for an ASTM 7C thermometer having bulb dimensions of 5.4 by 12 mm. The third curve, where A = 1.7, was ob- tained for a bulb with dimensions of 5.4 by 34 mm.

It is probable that most solid-stem thermometers of American manufacture will have values of A lying

within the range of the three curves shown. It will be noted that, according to Harper [20], the value

1 1 1

1 r

TIME, seconds

Figure 10. The approach to temperature in a stirred water bath

for three thermometers uith typical lag constants.

of A for a given thermometer in a well stirred oil bath

will be approximately twice its value in a water bath. For a thermometer which is used to measure temperature changes (as in calorimetry )

, it has been

shown by White [21] that the lag enters into the observations in such a way as to be eliminated from

the results in applying the usual radiation corrections. Therefore the lag need not be considered, providing

the initial and final readings are made when the temperature is varying uniformly. This is not strictly true, however, in the case of some Beckmann ther- mometers, which have no true value of A, as is ex- plained in the paper referred to above.

8,4. Separated Columns

Many inquiries are received concerning separated

mercury columns which occur especially during ship- ment. Since no means of avoiding such occurrences has yet been found, some directions for joining the mercury may be helpful and are described below.

(a) The bulb of the thermometer may be cooled

in a solution of common salt, ice, and water (or other cooling agent) to bring the mercury down slowly into the bulb. If the salt solution does not provide sufficient cooling, carbon dioxide snow (dry ice) may be used. Since the temperature of dry ice is approximately

25
—78 °C ( —108 °F), and mercury freezes at approximately —40 °C (—40 °F), the mercury will solidify.

Cool only the bulb and never the stem or mercury

column. Moderate tapping of the bulb on a rubber

stopper or similar soft spongy object, or the application of centrifugal force, by swinging the thermometer

in a short arc (i.e. use of centrifugal force), usually

serves to unite the mercury in the bulb. Care must be

taken to warm the top of the bulb first, so that pres- sures in the bulb due to expanding mercury may be

relieved. (b) If there is a contraction chamber above the bulb or an expansion chamber at the top of the ther- mometer, the mercury can sometimes be united by

warming the bulb until the column reaches the sep- arated portions in either enlargement. Great care is necessary to avoid filling the expansion chamber completely with mercury, which might produce pressures

large enough to burst the bulb. (The expansion chamber should never be more than 2/3 full.) Joining the mercury is more readily accomplished if the quantity

in either cavity has first been shattered into droplets

by tapping the thermometer laterally against the hand.

This procedure should not be used if it requires

the thermometer to be heated above 260 °C (500 °F)

and the bulb should never be heated in an open flame.

(c) As a last resort, especially for thermometers

having no expansion chambers, small separated portions of the column can sometimes be dispersed by

warming into droplets tiny enough to leave space for the gas to by-pass. The thermometer is heated, and

the droplets are collected by the rising mercury

column.

The procedure for thermometers containing organic

liquids is similar. Separated liquid in the stem can be

vaporized and permitted to drain down the capillary.

Another method consists of gently tapping the stem above the separation against the palm of the hand,

forcing the organic liquid to break away from the wall of the capillary and flow down the bore to join the main column.

Minute gas bubbles, which are sometimes found

along the surface of the mercury in the thermometer

bulb, may be collected by "washing" the bulb with

a large gas bubble. Bring all of the mercury into the bulb as outlined in section (a). Hold the thermometer

in a horizontal position and gently tap it against the hand to form a large gas bubble. Force the bubble to travel around the walls of the bulb by rotating the thermometer and tapping it against the palm of the hand. When the entire surface has been "washed"

rotate the bubble to the top of the bulb and reunite

the mercury as described above.

All of these manipulations require patience, and

experience is helpful, but they will yield results if care is used. A convenient method of ascertaining

that all of the liquid has been joined is a check of the

ice point or some other reference point on the scale.

9. References

[1] "The International Practical Temperature Scale of 1968," Metrologia, Vol. 5, No. 2, 35 (April, 1969).

[2] Calibration and Test Services of The National Bureau of Standards, NBS Spec. Publ. 250 (1970 Edition) , Part 5. (Copies available from NBS on request).

[3] Comptes Rendus des Seances de la Treizieme Conference

Generale des' Poids et Mesures (1967-1968), Resolutions 3 and 4, p. 104. [4] Stimson, H. F., The International Practical Temperature

Scale of 1948, J. Res. Nat. Bur. Stand. (U.S.), 65A

(Phys. and Chem.), No. 3, 139-145 (May-June 1961).

[5] Based on the data of Osborne, N. S., and Meyers, C. H., A formula and tables for the pressure of saturated water vapor in the range 0 to 374 °C, J. Res. NBS 13.

1 (1934) RP691.

[6] Scott, R. B., and Brickwedde, F. G., A precision cryostat with automatic temperature regulation, BS J. Res. 6. 401 (1931) RP284.

[7] American Society for Testing and Materials Designation

E77-72, Calibration at Temperatures Other Than Fixed

Points, Section 11.2. [8] Buckingham, E., The correction for emergent stem of the mercurial thermometer, Bui. BS 8. 239" (1912) S170.

[9] Pemberton, L. H., Further consideration of emergent col- umn correction in mercury thermometry, J. Sci. Instr. 41, 234 '(1964). [10] Hall, J. A., and Leaver, V. M., The design of mercury

thernK^meters for calorimetry, J. Sci. Instr. 36. 183

(195P).

[11] Ween, Sidney, "The Beckmann Differential Thermometer:

Its Principles of Construction and Application," Materials Research and Standards, MTRSA, Vol. 12, No. 8, August 1972, p. 31. [12] Thompson, R. D., Recent developments in liquid-in-glass thermometry. Temperature, Its Measurement and Control in Science and Industry 3. Part 1 (Reinhold Publishing Corp., New York, 1962) p. 201. [13] Liberatore, L. C, and Whitcomb, H. J., Density changes

in thermometer slasses, J. Am. Ceram. Soc. 35, 67

(1952).

[14] American Society for Testing and Materials Designation

C162-56, Standard Definitions of Terms Relating to Glass and Glass Products.

[15] Lillie, H. R., and Shaver, W. W., Method of tempering

glass, U.S. Patent No. 2,148,630 (Feb. 28, 1939).

[16] Liberatore, L. C, Method of stabilizing the molecular arrangement of glass thermometers, U.S. Patent No.

2,610,445 (Sept. 16, 1952).

[17] Ruh, E. L., and Conklin, G. E., Thermal stability in ASTM thermometers, ASTM Bui. No. 2,33. 35 (Oct. 1958).

[18] Martin, W. I., and Grossman, S. S., Calibration drift with thermometers repeatedly cooled to ~30 °C, ASTM

Bui. No. 231. 62 (July 1958).

[19] Guillaume, C. E., Traite Practique de la Thermometrie,

Gauthier-Villars et Fils, Paris (1889) p. 99. [20] Harper 3d. D. R., Thermometrie lag, Bui. BS 8, 659

(1912) S185. [21] White, W. P., Lag effects and other errors in calorimetry,

Phys. Rev. 31, 562 (1910).

26

•CrV.S. GOVERNMENT PRINTING OFFICE; 198 6-1+91 070 20331+
NBS-n4A (REV. 2-8CI U.S. DEPT. OF COMM. 1. PUBLICATION OR 2. Performing Organ. Report No. 3. Publication Date BIBLIOGRAPHIC DATA

REPORT NO.

SHEET (See instructions) NBS/MONO-174 January 1906

4. TITLE AND SUBTITLE

Thermometer Calibration: A Model for State Calibration Laboratories

5. AUTHOR(S)

J. A. Wise and R. J. Soulen, Jr,

6. PERFORMING ORGANIZATION (If joint or other than NBS, see in struct/on sj

NATIONAL BUREAU OF STANDARDS

DEPARTMENT OF COMMERCE

GAITHERSBURG, MD 20899

7. Contract/Grant No.

8. Type of Report & Period Covered

Final

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

Same as #6.

10, SUPPLEMENTARY NOTES

Library of Congress Catalog Card Number 85-609636

I I Document describes a computer program; SF-185, FlPS Software Summary, is attached.

11. ABSTRACT (A 200-word or less factual summary of most significant information. If document includes a significant

bi bliography or literature survey, mention it here) This document describes the means by which a state calibration laboratory

can establish a calibration service based on liquid-in-glass thermometers.

Discussed are: ice-point baths, controlled-temperature baths, thermometer

inspection, calibration techniques, and control chart procedures.

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

Calibration; IPTS-68; liquid-in-glass thermometers; temperature

13. AVAILABILITY

I >j Unl imited

I I For Official Distribution. Do Not Release to NTIS

f~53 Order From Superintendent of Documents, U.S. Government Printing Office, Washington, D.C.

20402.

[ I Order From National Technical Information Service (NTIS), Springfield, VA. 22161 fr Sir

14. NO. OF

PRINTED PAGES

88

15. Price

II

Technical Publications

Periodical

I Journal of Research—The Journal of Research of the National Bureau of Standards reports NBS research

{ and development in those disciplines of the physical and engineering sciences in which the Bureau is active. These include physics, chemistry, engineering, mathematics, and computer sciences. Papers cover a broad

range of subjeas, with major emphasis on measurement methodology and the basic technology underlying

standardization. Also included from time to time are survey articles on topics closely related to the Bureau's

technical and scientific programs. Issued six times a year.

Nonperiodicals

Monographs—Major contributions to the technical literature on various subjects related to the Bureau's scientific and technical activities. Handbooks—Recommended codes of engineering and industrial practice (including safety codes) developed in cooperation with interested industries, professional organizations, and regulatory bodies.

Special Publications—Include proceedings of conferences sponsored by NBS, NBS annual reports, and other

special publications appropriate to this grouping such as wall charts, pocket cards, and bibliograpiiies.

Applied Mathematics Series—Mathematical tables, manuals, and studies of special interest to physicists,

engineers, chemists, biologists, mathematicians, computer programmers, and others engaged in scientific and

technical work.

National Standard Reference Data Series—Provides quantitative data on the physical and chemical properties

: of materials, compiled from the world's literature and critically evaluated. Developed under a worldwide pro- gram coordinated by NBS under the authority of the National Standard Data Act (Public Law 90-396).

NOTE: The Journal of Physical and Chemical Reference Data (JPCRD) is published quarterly for NBS by

the American Chemical Society (ACS) and the American Institute of Physics (AIP). Subscriptions, reprints,

and supplements are available from ACS, 1155 Sixteenth St., NW, Washington, DC 20056.

Building Science Series—Disseminates technical information developed at the Bureau on building materials,

components, systems, and whole structures. The series presents research results, test methods, and perfor- mance criteria related to the structural and environmental functions and the durability and safety

characteristics of building elements and systems.

Technical Notes—Studies or reports which are complete in themselves but restrictive in their treatment of a

subject. Analogous to monographs but not so comprehensive in scope or definitive in treatment of the subject

area. Often serve as a vehicle for final reports of work performed at NBS under the sponsorship of other

• I government agencies.

Voluntary Product Standards—Developed under procedures published by the Department of Commerce in Part 10, Title 15, of the Code of Federal Regulations. The standards establish nationally recognized re- quirements for products, and provide all concerned interests with a basis for common understanding of the

characteristics of the products. NBS administers this program as a supplement to the activities of the private

' sector standardizing organizations.

; Consumer Information Series—Practical information, based on NBS research and experience, covering areas

f of interest to the consumer. Easily understandable language and illustrations provide useful background

knowledge for shopping in today's technological marketplace.

Order the above NBS publications from: Superintendent of Documents, Government Printing Office,

Washington. DC 20402. Order the following NBS publications—FIPS and NBSIR 's—from the National Technical Information Service, Springfield, VA 22161.

Federal Information Processing Standards Publications (FIPS PUB)—Publications in this series collectively

I constitute the Federal Information Processing Standards Register. The Register serves as the official source of

information in the Federal Government regarding standards issued by NBS pursuant to the Federal Property

, and Administrative Services Act of 1949 as amended. Public Law 89-306 (79 Stat. 1127), and as implemented

by Executive Order 1 1717 (38 FR 12315, dated May 11, 1973) and Part 6 of Tide 15 CFR (Code of Federal

Regulations).

NBS Interagency Reports (NBSIR)—A sp)ecial series of interim or final reports on work {performed by NBS

for outside sponsors (both government and non-government). In general, initial distribution is handled by the sponsor; public distribution is by the National Technical Information Service, Springfield, VA 22161, in paper

icopy or microfiche form.

NBS
U.S. Department of Commerce

National Bureau of Standards

Gaithersburg, MD 20899

Official Business

Penalty for Private Use $300