TE-LP-003—Laboratory practice for the selection and evaluation of direct-reading digital thermometers

Version 9.0

Version Date Description
1.0 Original release.
2.0 Added "controlled copy" statement in the document header.
3.0 Reworked procedure in sections 1.2 and 2.1, tables 3, 2.5, 2.8, 2.10, 3.2, 3.4 and 3.6.
4.0 Added a new ambient and in-service temperature requirement under 2.2 and 3.3 (changed from 22±5 °C to 23±5 °C).
  • Revised tolerance structure, new calibration tolerance and specifications for stability and effect due to environmental conditions.
  • Carried out a final review, combining content from the current tolerances and field calibration requirements, made editorial corrections and fixed equations.
  • Added option to include other sources of uncertainty in uncertainty calculation (needed for MC primary and secondary thermometers).
  • Changed recommended range for liquid heated product applications up to 120 °C instead of 125 °C.
6.0 Corrected the temperature symbols and updated the format.
7.0 Removed the requirement to include uncertainty sources on certificates in section 3.3.8.
  • Removed section 3.9 (Request for the designation of a local standard by accredited laboratories) and the appendix (Designation of local standard request).
  • Clarified ice point test requirement in section 4.
  • Reference thermometer monitoring is now as per TE-MP-001.
  • Clarified the title, purpose and scope of this document.
  • Updated document to reflect formatting changes in the Omni Assistant Guide.
  • Removed sections 3.3.1 to 3.3.4 as well as sections 3.3.6 to 3.3.8 (Omni Assistant Ticket Number 70); information has been captured in VOL-TP-022.
  • Added references to VOL-TP-022 and GN-LP-010 in section 1.3.
  • Clarified the thermometer case recommendation in section 2.3.5.
  • Clarified the difference of thermometer bias error for adjustable and non-adjustable thermometers in section 3.3.5.
  • Added clarification to section 4 concerning multi-probe and multi-channel thermometers.
  • Clarified section 3.1 to refer to working reference standards, thus differentiating from the national reference standard.
  • Simplified section 3.8 to reference GN-LP-010 and only include items specific to thermometry calibration.
  • Added a recommendation to contact ACSL for details when considering buying a new thermometer model.

Table of contents

1.0 Introduction

1.1 Purpose

This document describes the recommended practice for the selection and evaluation of direct-reading digital thermometers in order to determine whether they can be certified as local standards by Measurement Canada under subsection 13(1) of the Weights and Measures Act.

1.2 Scope

This document applies to direct-reading digital thermometers to be designated as local standards under subsection 13(1) of the Weights and Measures Act.

1.3 Reference documents

1.4 Abbreviations and symbols

ACSL: Approval and Calibration Services Laboratory

2.0 Specifications and requirements

It is recommended that the inspector contact the ACSL when considering the purchase of a new thermometer model for weights and measures applications.

2.1 Accuracy and measurement uncertainty requirements

The criteria for the certification of thermometers as local standards are specified in Schedule IV, Part XIX, of the Weights and Measures Regulations as an accuracy tolerance of ±0.25 °C for temperatures up to 50 °C and ±0.6 °C for temperatures above 50 °C.

To ensure the above tolerances are met, the following uncertainty components are applied:

  1. Maximum calibration uncertainty

    The maximum allowed expanded calibration measurement uncertainty (with k = 2) for the purpose of thermometer certification is 0.15 °C. The thermometer calibration measurement uncertainty is determined at the time of calibration by the laboratory performing the calibration of the thermometer. The calculation of the calibration measurement uncertainty is described in section 3.3.

  2. Thermometer model performance specifications

    Thermometer model performance specifications are detailed in section 2.3. This section addresses the sources of measurement uncertainty that are not included in the calibration uncertainty estimate. These include the thermometer stability and susceptibility to ambient conditions. Only those thermometer models meeting these criteria are eligible for certification.

2.2 Thermometer operating conditions and resolution

The thermometer's operating conditions classification will be rated as per Table 1.

Table 1: Classification of digital thermometer operating conditions
Classification Application
Laboratory conditions 23 °C ± 5 °C (indoors)
Field conditions −20 °C to 40 °C (outdoors)

The recommended resolution and minimum temperature calibration range of thermometers are dependent on the intended application (see Table 2).

Table 2: Temperature calibration ranges and recommended resolutions for thermometers
Application Minimum calibration range and minimum resolution
General field approval inspectionFootnote 1 −30 °C to 50 °C × 0.01 °C
Liquid heated product approval inspectionFootnote 1 −30 °C to 120 °C × 0.01 °C
Calibration of thermometers used in a gas prover for temperature correction 0 °C to 35 °C × 0.01 °C
Temperature correction in gas measuring apparatus 0 °C to 35 °C × 0.01 °C
Bell prover air temperature(s) 0 °C to 35 °C × 0.01 °C

2.3 Performance specifications and design requirements

2.3.1 Thermometer stability

Thermometers must be designed to remain stable to within ±0.1 °C for the duration of the verification period.

2.3.2 Variation due to ambient conditions

For the rated temperature range, the thermometer measurement variations due to ambient conditions must be within ±0.1 °C. This performance characteristic may be derived from:

  • measurements and tests by MC
  • manufacturer's data and compliance statement
  • second-party measurements and tests

2.3.3 Probe design

The temperature sensor must be designed to operate without leakage or deformation over the expected range of operating pressures, temperatures and environmental conditions, including the immersion depth of the sensor in liquid when required.

Where thermometers will be used to determine flowing temperature stability, such as in the testing of automatic temperature compensation (ATC), the time constant of the thermometer in flowing liquids must be less than 15 seconds. The recommended probe diameter is 3 mm (or 1/8 inch) and the recommended lead wire length is approximately 6 feet.

2.3.4 Resistance to thermal shock

Thermometer probes intended for field applications must be able to withstand thermal shocks of 50 °C without loss of accuracy.

2.3.5 Field thermometer packaging recommendations

It is recommended that thermometers intended for field use are equipped with a carrying case which is water-resistant, dust-proof and dimensionally stable over the range of environmental conditions expected during the service life of the device.

2.3.6 Construction

The thermometer's construction must be mechanically and electrically sound. The materials, finish, etc., must be of a quality to provide assurance of long life and sustained accuracy.

2.3.7 Battery power

When power is supplied by a battery, the thermometer must be equipped with a low power indication.

2.3.8 Markings

The following information must be indelibly marked on, or be capable of being displayed by, the instrument:

  • manufacturer's name
  • model number
  • serial number

In the case of thermometers with multiple sensors, each sensor must be marked by a unique identifier.

2.3.9 Provisions for sealing

The thermometer's design must prevent inadvertent adjustment of the thermometer.

3.0 Calibration

Calibration of a thermometer includes the following operations:

  • visual inspection of the thermometer
  • adjustment, if applicable, and calibration of the thermometer
  • obtaining the "as found" and "as left" results
  • calculating the calibration measurement uncertainty
  • producing the calibration certificate

3.1 General requirements for the working reference thermometer and calibration equipment

3.1.1 Working reference thermometer Traceability

When used for the calibration and certification of thermometers as a local standard, the working reference thermometer must be traceable to a reference standard listed in the Weights and Measures Act and Regulations.

When used for the calibration and certification of thermometers as measuring apparatus under the Electricity and Gas Inspection Act and Regulations, the working reference thermometer must be traceable to a national reference standard maintained by the National Research Council of Canada. Measuring range

The working reference thermometer must not be used outside of its certified measuring range. Monitoring

The working reference thermometer must be monitored for stability with an ice bath, water triple point cell or other suitable methods with similar accuracy and stability. Monitoring must be conducted as prescribed in the laboratory maintenance procedure TE-MP-001.

3.1.2 Calibration bath

A calibration bath of suitable operating temperature range, with demonstrated stability and uniformity of U ( k = 2 ) 0.02 throughout the target calibration range, must be used to calibrate the working reference thermometers.

3.2 Calibration conditions and method

The documented calibration process (TE-CP-001 or TE-CP-005) ensures the following:

  • The thermometer model meets the specifications outlined in section 2 of this document.
  • The thermometer is inspected, and the inspection results as well as the seal condition are recorded.
  • The temperature of the calibration bath is allowed to reach the set point and is stabilized before readings are taken.
  • Before beginning the calibration procedure, the thermometer under calibration and the reference temperature standard are acclimatized to ambient conditions for at least 12 hours. Ambient temperatures in the calibration laboratory are maintained within 23±5 °C.
  • The thermometer sensors are sufficiently immersed in the temperature bath to eliminate immersion errors. If this cannot be achieved, immersion error is considered in the estimate of the uncertainty.
  • The "as found" results, at a minimum of two temperature points, are obtained prior to any adjustments. It is recommended that the ice point be included as a test point.
  • The thermometer is adjusted as per the manufacturer's instructions or other suitable method which will minimize the thermometer measurement error.
  • In the case of thermometers that cannot display the measured resistance and are adjusted based on measurement errors, a suitable deviation function is selected for the thermometer. The deviation function is the equation used to model the expected thermometer corrections over its calibration range. Often a linear correction deviation function will be suitable when the thermometer error is linear and can be characterized by a slope and an offset. Second or third-order equations may be used for the deviation functions where the thermometer error is not expected to be linear.
  • Each transducer associated with the thermometer is calibrated at a minimum of six (6) evenly distributed test points across its measuring range or, as a minimum, the range given in Table 2. Where a nonlinear deviation function or calibration equation is used to model the thermometer response, a minimum of three (3) test points per equation variable is used.
  • For each sensor, the coefficients of the deviation function or calibration equation are determined by using the least squares fit method.
  • In the case of thermometers that are to be certified for use with corrections, the deviation function is used to generate the correction table for the sensor.
  • If the thermometer is adjusted, an "as left" ice point check is included as well.

3.3 Calibration measurement uncertainty calculations

3.3.1 Thermometer uncertainty budget

Measurement uncertainty calculations must be based on the recommendations of the Guide to the Expression of Uncertainty in Measurement (GUM). Uncertainty values will be reported using a coverage factor k = 2 for a coverage probability of approximately 95 %.

3.3.2 Thermometer bias error (Error bias)

This section applies to thermometers that have a resolution less than or equal to 0.01 °C and are to be used without a correction table. The maximum bias error is taken as the maximum measured error over the measuring range. The maximum bias error must be less than ±0.05 °C for adjustable thermometers and ±0.75 °C for non-adjustable thermometers.

3.4 Calibration pass or fail

The thermometer must not be certified if the expanded calibration measurement uncertainty (with k = 2) is greater than 0.15 °C.

For non-adjustable thermometers that cannot be adjusted as close as possible to zero error, the thermometer must not be certified if the thermometer indication error, at one or more calibration points, exceeds ±0.75 °C.

3.5 Certification period

Unless otherwise authorized, thermometers must be calibrated at least once every two years as prescribed in Part III of the Weights and Measures Regulations.

3.6 Sealing

Thermometers that have physical access to any means of adjustment that can affect the thermometer calibration must be protected against inadvertent access to the adjustments by a physical seal in order to reveal any tampering.

The values of the programmable parameters that can affect measurement accuracy of the thermometers must be included in the calibration certificate notes.

3.7 Calibration status indicator

The thermometer must be labelled to indicate the status of its calibration by the application of a verification mark or tag indicating:

  • the expiry date of the calibration, and
  • unique identifier of the laboratory which performed the calibration.

3.8 Certificate of calibration

In addition to requirements in GN-LP-010, the following information must be contained in the certificate of calibration for the instrument:

  • the name of the instrument's manufacturer
  • a description and unambiguous identification of the instrument
  • the temperature range of the calibration
  • a correction table (as applicable)
  • any special limitations for the use of the instrument
  • the certificate number
  • the value of programmable calibration parameters that can affect measurement accuracy of the thermometers
  • when the thermometer is received with a broken seal, a note to this effect

4.0 Application and use of a thermometer certified as a local standard

The thermometer must not be used outside of its calibrated temperature range.

The thermometer must not be used outside the rated ambient temperature for which it is certified.

The owner is responsible to ensure that the thermometer is suitably rated for the hazardous location classification, as required.

Unless otherwise specified in this document, the thermometer must be used as per the manufacturer's recommendations.

The thermometer's long-term stability must be monitored by performing an ice point test as prescribed in bulletin V-24. The ice point test compares the thermometer's ice point reading to that of the "as left" ice point value recorded on the certificate of calibration. If the absolute value of the difference is more than 0.1 °C, the thermometer must not be used as a local standard and must be repaired and recertified before further use, unless all of the following conditions are met:

  • the thermometer has multiple channels and multiple probes
  • one or more probes remain in tolerance
  • the probe or probes not in tolerance are removed from service

Temperature measurement errors due to non-ideal location of the probe and dynamic effects will be taken into consideration when reviewing the suitability of a thermometer for a measurement application.