for Reference Standards

and Field Standard

Weights and Measures

6. Specifications and Tolerances for Thermometers

 NIST
Preface

The effort to develop a 105-series handbook for field standard temperature measuring devices was undertaken by Joe

Rothleder (CA) in 1990 in response to the need for specifications, tolerances, and procedures for such devices used in

weights and measures enforcement activities. This publication is the result of the draft prepared by Joe Rothleder.

Special thanks regarding this handbook is also given to Karl Herken, metrologist with the State of Kansas, for his

assistance with review of reference materials, evaluation of comments submitted during peer review, and for typing and

editing the document in WordPerfect format. Thanks are also given to numerous metrologists (of both State and

industry laboratories) for their technical review of several drafts.
Table of Contents

Preface . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . iii

Introduction ................................................................................ 1

1 Scope . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1

1.1 Thermometer Types ........................................................... 1

1.2 Classification ................................................................. 2

1.2.1 Type ................................................................ 2

1.2.2 Specific Class Applications .............................................. 2

1.3 Retroactivity ................................................................. 2

1.4 Safety . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2

1.4.1 MSDS of Measured Products ............................................. 2

1.4.2 Specific Concerns ...................................................... 2

2 Reference Documents . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3

2.1 NIST . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3

2.2 ASTM . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3

3 Terminology . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3

4 Specifications ....................................................................... 3

4.1 General . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3

4.2 Temperature Range ............................................................ 4

4.3 Materials and Workmanship .................................................... 4

4.4 Physical and Mechanical ........................................................ 4

4.5 Graduations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4

4.5.1 Readability ........................................................... 4

4.5.2 Type II and Type III .................................................... 4

4.6 Immersion ................................................................... 4

4.7 Applications ................................................................. 4

5 Tolerance . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4

5.1 Tolerance . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4

5.2 Testing Failures . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4

6 Verification Requirements .............................................................. 4

6.1 Legal Requirements ............................................................ 4

6.2 Traceability .................................................................. 5

6.3 Calibration Verification ........................................................ 5

6.4 Initial and Periodic Verification .................................................. 5

6.5 Reverification Type I .......................................................... 5

6.6 Reverification Type II and Type III ............................................... 5

6.7 Failing Tolerance .............................................................. 5

7 Test Methods ........................................................................ 5

7.1 Documented Test Procedures .................................................... 5

7.2 Tolerance / Calibration ......................................................... 5

7.3 Temperature Span ............................................................. 5
iv

8 Uncertainties ........................................................................ 6

8.1 Legal Applications ............................................................ 6

8.2 Sources of Variation ........................................................... 6

8.2.1 Accuracy Limit ........................................................ 6

8.2.2 Repeatability .......................................................... 6

Notes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7

Table 1. Typical suggested ranges and associated tolerances ...........................................8
NIST Handbook 105-6 October 1997 1

SPECIFICATIONS AND TOLERANCES 

FOR REFERENCE STANDARDS AND FIELD STANDARD

WEIGHTS AND MEASURES

6. Specifications and Tolerances for Thermometers

These specifications and tolerances are recommended as minimum requirements for temperature measuring devices,

hereafter referred to as thermometers, used by State and local weights and measures programs for thermometer testing

associated with device and quantity control enforcement activities.

Key words: field standards, liquid-in-glass thermometer, temperature specifications, temperature standards,

thermometers.

 

Introduction

The thermometers and associated tolerances specified herein are intended for weights and measures applications where

temperature is a factor in the procedures used during testing for compliance to National Institute of Standards and

Technology (NIST) Handbook 44 and Handbook 133. 1 2

This handbook is intended to provide the weights and measures official with a useful summary of the extensive body

of information on the manufacturing, use and calibration of thermometers. It is intended to be used as a guide when

purchasing and using thermometers determined to be suitable for weights and measures applications in terms of meeting

the specifications and tolerances contained herein. The specific temperature range, type and physical characteristics of

each thermometer are to be selected by the user consistent with application requirements and to meet specifications. 

Tolerance testing or calibration of thermometers included in this handbook are based on the following:

Temperatures are relative to "The International Temperature Scale of 1990 (ITS-90)". Guidelines for realizing these 3

temperatures are available in NIST Technical Note 1265. 4

Testing at, or correcting data to, the applicable immersion depth of the thermometer under test.

Testing conducted while operating the thermometer in accordance with the manufacturer’s instructions.

Additional information may be obtained by reviewing the applicable documents referenced in this handbook.

1 Scope

1.1 Thermometer Types

This publication covers various types of thermometers within the temperature range of -30 EC to 50 EC [-20 EF to 120

EF]. This is the extreme overall temperature testing range for weights and measures enforcement activities, as

incorporated in Table 1. The actual temperature range of purchased thermometers may differ due to local requirements

and type of thermometer desired.

1.2 Classification

1.2.1 Type

Thermometers are divided into types based on design.

1.2.1.1 Type I

Liquid-in-glass thermometers that have a 0 EC [32 EF] reference point. These thermometers tend to be stable with time

and use. Once tolerance tested, the thermometer can be checked at the reference point to verify that it remains in

tolerance.
NIST Handbook 105-6 October 1997 2

1.2.1.2 Type II

Electronic thermometers using analog or digital readout. These thermometers have active electrical components that

can change with time, requiring periodic retesting of tolerance and possibly requiring adjustment, repair or replacement.

1.2.1.3 Type III

Thermometers of any design other than Type I and II, but capable of meeting the specifications.

1.2.2 Specific Class Applications

1.2.2.1 Class 1

Thermometers typically used as accessory thermometers for the testing of the moisture in grain (e.g., ASTM No. 63F).

See Handbook 44 and Table 1 for additional details. 

1.2.2.2 Class 2

Thermometers that are used in general weights and measures programs, including device testing and quantity control

functions.

1.3 Retroactivity

These specifications are not intended to make obsolete those field standards in use prior to publication of these

specifications. All new thermometers must meet these requirements prior to verification for legal use. The described

specifications apply to all thermometers placed in service after the publication of this standard; tolerances apply to all

thermometers in service. A thermometer in service prior to the publication of this standard that has maintained

tolerances between verification tests shall continue to be acceptable.

1.4 Safety

This standard may involve hazardous materials, operations, and equipment. This standard does not purport to address

all safety problems associated with its use. It is the responsibility of the user of this standard to establish appropriate

safety and health practices and determine the applicability of regulatory limitations prior to use.

1.4.1 MSDS of Measured Products

Commercial liquid-measuring devices, tested with these field standards, are typically used to measure quantities of

petroleum products. Petroleum products are known hazardous materials and hazardous wastes. The user is encouraged

to obtain Material Safety Data Sheets (MSDS) from the manufacturer of any product encountered. Federal and local

safety and disposal regulations concerning hazardous materials encountered should be reviewed by the user. 

1.4.2 Specific Concerns

The use of liquid-in-glass thermometers can bring the user and others into contact with broken glass and spilled mercury

(an identified hazardous material). The user should exercise care and take steps to be familiar with the instructions

relative to first aid for cuts, contents of the "Material Safety Data Sheet" (MSDS) for mercury and the use of "mercury

spill clean up kits" for proper clean up and disposal of mercury. 

2 Reference Documents

2.1 NIST

2.1.1 NIST Handbook 44, Specifications, Tolerances, and Other Technical Requirements for Weighing and

Measuring Devices.

2.1.2 NIST Handbook 133, Checking the Net Contents of Packaged Goods. (Current edition.)

2.1.3 NIST Special Publication 250-23, Liquid-in-Glass Thermometer Calibration Service. 5

2.1.4 NIST Technical Note 1265, Guidelines for Realizing the International Temperature Scale of 1990 (ITS-90).

2.1.5 NBS Monograph 174, Thermometer Calibration: A Model for State Calibration Laboratories. 6

2.1.6 NIST Special Publication 819, A Procedure for the Effective Recalibration of Liquid-in-Glass Thermometers,

J.A. Wise, 1991. 

2.1.7 NIST IR 5341, Assessment of Uncertainties of Liquid-in-Glass Thermometer Calibrations at the National

Institute o Standards and Technology, J.A. Wise, 1994. 
NIST Handbook 105-6 October 1997 3

2.2 ASTM7

2.2.1 ASTM Standard E 1-95, Standard Specification for ASTM Thermometers,

2.2.2 ASTM Standard E 77-92, Standard Method for Inspection and Verification of Liquid-in-Glass Thermometers.

3 Terminology

Bulb. The reservoir for the liquid in a liquid-in-glass thermometer.

Ice Point. The fixed point between ice and air-saturated water under a pressure of 1 standard atmosphere (101 325 Pa)

and is 0 EC on the ITS 90.

Partial-Immersion Thermometer. A liquid-in-glass thermometer designed to indicate temperatures correctly when the

bulb and a specified part of the stem are exposed to the temperature being measured.

Reference Point (e.g., Ice point). The ice point is an example of a series of stable and reproducible temperature

reference points; selected to monitor changes in the bulb volume of liquid-in-glass thermometers.

Stem. The capillary tube of a liquid-in-glass thermometer through which the fluid moves with change of temperature.

Tolerance. Limit of maximum permissible error relative to the true value of the measured temperature.

Total-Immersion Thermometer. A liquid-in-glass thermometer designed to indicate temperatures correctly when just

that portion of the thermometer containing the liquid is exposed to the temperature being measured. 

Uncertainty. Parameter, associated with the result of a measurement, that characterizes the dispersion of the values that

could reasonably be attributed to the measurand.

Verification. The process of testing a thermometer for compliance with specifications.

4 Specifications

4.1 General

The specifications below provide key requirements to facilitate efficient thermometer use and tolerance testing. The

tolerance for Class 2 thermometers, provided in Table 1, is based on applications requiring the highest accuracy. This

will reduce the need for single purpose thermometers and the accidental use of thermometers unsuitable for a specific

application. Specifications are based on information available in applicable referenced documents.

4.2 Temperature Range

See Table 1 for temperature ranges needed for weights and measures activities. The actual range of purchased

thermometers may differ due to factors including local requirements, availability, cost, and type of thermometer desired.

4.3 Materials and Workmanship 

Type I thermometers must conform to specifications contained in ASTM E1-95, sections 4 through 13. Type II and 8

Type III thermometers must provide equivalent or superior performance to Type I thermometers.

4.4 Physical and Mechanical

All thermometer stems, probes or other portions of the thermometer that must be immersed in a product, environment

or thermometer well must fit the available space and be readable by the user. 

4.5 Graduations

The graduations and spacing for Type I thermometers must conform to specifications contained in ASTM E1-91,

sections 4 through 13. See Table 1 for minimum graduations.
NIST Handbook 105-6 October 1997 4

4.5.1 Readability

The readability for Type I thermometers must be at least 0.5 division with the unaided eye.

4.5.2 Type II and Type III 

Thermometers must provide resolution equivalent to Type I thermometers. In the case of Type II electronic digital

thermometers the last digit of readout must be 0.01 EF and/or EC for Class 1 and 0.1 EF and /or EC for Class 2

thermometers.

4.6 Immersion

Type I, Class 1 thermometers (reference NIST Handbook 44) are total immersion types. Class 2 liquid-in-glass

thermometers, may be of the total or partial immersion type. If the procedure fails to recommend the use of a total or

partial immersion thermometer, then the decision should be governed by the physical limitations in thermometer

placement.

4.7 Applications

Based on temperature ranges and tolerances of the Class 2 thermometers covered by this handbook, it is expected that

the use of total versus partial immersion thermometers will have errors less than 0.1 EC [0.2 EF]. Additional information

on this subject can be obtained in NIST Special Publication 250-23.

5 Tolerance

5.1 Tolerance

Tolerance for thermometers tested in accordance with this handbook are provided in Table 1. The tolerance is + 1

division for all liquid-in-glass thermometers. The specifications permit, within the tolerance, the direct reading of

temperature, without the use of corrections.

5.2 Testing Failures

Thermometers that exceed the standard temperature values during testing, by more than the tolerance will be rejected;

or, if users are properly instructed, calibrated and then used with corrections. 

6 Verification Requirements

6.1 Legal Requirements

The specifications and tolerances herein specified are intended to permit the use of the equipment in normal field testing

operations as standards having nominal values. Weights and measures requirements, including but not limited to,

inspection, testing, and sealing, by a NIST recognized laboratory shall be followed.

NOTE: Some States have requirements not documented here. Check with the local jurisdiction for specific

requirements.

6.2 Traceability

Field standards used for legal metrology shall be traceable to national standards by calibration in a laboratory recognized

by NIST in that parameter, range, and scope and evidence shall be documented and provided.

6.3 Calibration Verification

The laboratory will issue a report document for those thermometers that are in compliance with the specifications herein

and that have been tested and found to be within tolerance.

6.4 Initial and Periodic Verification

Field standards must be verified prior to use and rechecked as often as regulations or circumstances require, especially

when damage is known or suspected. Consideration must be given to any State or local weights and measures

regulations. The retest of liquid-in-glass thermometers may be a simple verification of the ice point to determine

stability.
NIST Handbook 105-6 October 1997 5

6.5 Reverification Type I

After initial verification, a liquid-in-glass thermometer, may be reverified by determining the change in temperature at

the ice point 0 EC [32 EF] relative to the ice point reading during initial verification. The change in reading must be

applied to all the initial calibration data point readings to determine if the liquid-in-glass thermometer remains in

tolerance. If the liquid-in-glass thermometer was initially calibrated with reported corrections, the corrections must be

updated to account for the change in the ice-point reading.

6.6 Reverification Type II and Type III

Type II and III temperature sensing devices require complete testing at reverification.

6.7 Failing Tolerance

Thermometers failing to meet the specifications and tolerance will be returned to the submitter for repair or replacement.

If a thermometer is out of tolerance, a report may be issued to provide corrections that must be applied at time of use.

7 Test Methods

7.1 Documented Test Procedures

Verification or calibration of thermometers shall be by an approved NIST procedure, ASTM procedure, or

internationally recognized method if legal for trade measurements are to be made. Additional requirements may have

to be met depending on the jurisdiction in which the device will be used.

7.2 Tolerance / Calibration

In cases where the observed verification value, plus or minus the uncertainty, is within tolerance, a report stating that

the thermometer is “within tolerance” may be adequate. In addition, a verification tolerance test requires that the

expanded uncertainty be less than one third of the applicable tolerance. The complete calibration of thermometers,

including the reporting and use of corrections and uncertainties, is often required, especially for standard thermometers

used in grain moisture and liquid-in-glass thermometers that will be reverified through a test of the ice point. 

7.3 Temperature Span

Once determined to be operational and meeting the specifications in this handbook, thermometers should be verified

or calibrated at three temperature points: starting at the 0 EC [32 EF] reference point and at two additional points,

approximately equally spaced, maintaining 25 EC [50 EF] or less between points. Points should be selected by the user

such that the lowest and highest test points span at least 60 percent of the temperature range used in test (see Table 1).

8 Uncertainties

8.1 Legal Applications

Uncertainties of the calibration must be evaluated according to the ISO Guide to the Expression of Uncertainty in

Measurement,9

1993 to ensure that the three-to-one accuracy ratio contained in NIST Handbook 44 is maintained. 

8.2 Sources of Variation

8.2.1 Accuracy Limit

The uncertainty reported by NIST for many primary liquid-in-glass thermometers is around 0.03 EC and is the 10

maximum level acceptable for calibration of Class 1 thermometers. Thermometers with wider temperature ranges and/or

greater accuracy shall be tested within the range and tolerance specified in Table 1. The uncertainty of the laboratory

standard must be less than one-third of the tolerance of the thermometer under test based on requirements in NIST

Handbook 44.

8.2.2 Repeatability

Testing should be conducted in accordance with guidelines provided in NIST Special Publication 250-23 and NIST IR

5341 and repeatability of the measurements may be used to evaluate measurement uncertainties in use.

Notes
NIST Handbook 105-6 October 1997 6

1. NIST Handbook 44, Specifications, Tolerances, and Other Technical Requirements for Weighing and

Measuring Devices. (See current edition.) NIST, Gaithersburg, MD, 20899.

2. NIST Handbook 133, Checking the Net Contents of Packaged Goods. (Current edition.) NIST,

Gaithersburg, MD, 20899.

3. The International Temperature Scale of 1990 (ITS-90). Metrologia 27, 3-10 (1990); Metrologia 27, 107

(1990).

4. NIST Technical Note 1265, Guidelines for Realizing the International Temperature Scale of 1990 (ITS90). NIST, Gaithersburg, MD, 20899.

5. NIST Special Publication 250-23, Liquid-in-glass Thermometer Calibration Service. NIST, Gaithersburg,

MD, 20899. 

6. NBS Monograph 174, Thermometer Calibration: A Model for State Calibration Laboratories. NIST,

Gaithersburg, MD, 20899.

7. ASTM, American Society for Testing and Materials, 100 Barr Harbor Dr., West Conshohocken, PA 

19428.

8. ASTM E1-95, Standard Specification for ASTM Thermometers. ASTM, American Society for Testing

and Materials, 100 Barr Harbor Dr., West Conshohocken, PA 19428.

9. ISO, International Organization for Standardization, Geneva, Switzerland.

10. NIST IR 5341, Assessment of Uncertainties of Liquid-in-Glass Thermometer Calibrations at the National

Institute of Standards and Technology, NIST, Gaithersburg, MD 20899.
NIST Handbook 105-6 October 1997 7

Table 1. Typical suggested ranges and associated tolerances

Devices Division Typical Range Tolerance

Maximum

Division 

Type I 

(liquid-in-glass)

Maximum

Type II (digital)

(EF) (EC) (EF) (EC) (EF) (EC) (EF) (EC)

Petroleum Products

Wholesale petroleum 1 0.5 0.1 0.1 0 to 0 to 50 1 0.5

meters 120

Liquefied petroleum gas 1 0.5 0.1 0.1 -20 to 0 to 50 1 0.5

meters 120

Hydrocarbon gas vapor 1 0.5 0.1 0.1 60 to 15 to 1 0.5

meters 120 50

Grain Moisture Testing

Accessory Thermometer 1 0.5 0.1 0.1 32 to 0 to 40 1 0.5

104

Standard Thermometer 0.2 0.1 0.01 0.01 32 to 0 to 40 0.2 0.1

104

Quality Control

Alcoholic beverages 1 0.5 0.1 0.1 -20 to -30 to 1 0.5

120 50

Compressed gas in 1 0.5 0.1 0.1 20 to -10 to 1 0.5

cylinders 100 40

Petroleum products 1 0.5 0.1 0.1 -20 to -30 to 1 0.5

120 50

Aerosol commodities 1 0.5 0.1 0.1 20 to -10 to 1 0.5

100 40

Frozen foods 1 0.5 0.1 0.1 -30 to 0 to 50 1 0.5

120

Note: Temperature ranges may not correspond exactly between EC and EF units. Fahrenheit units listed first in accordance with

field practice.