TE-LP-003—Laboratory Practice for the Selection, Calibration and Use of Direct Reading Thermometers

Version: 04

Record of Change

Version Date Description
01 2006‑06‑06 Original Release
02 2008‑05‑14 Added "controlled copy" statement in the document header.
03 2008‑09‑23 Reworked procedure in sections 1.2, 2.1 table 3, 2.5, 2.8, 2.10, 3.2, 3.4 and 3.6.
04 2009‑07‑08 New ambient & in service temperature requirement under 2.2 & 3.3, changed from 22 ± 5 to 23± 5 °C.

Table of Contents

1.0 Introduction

1.1 Purpose

This document is to provide recommended practice in the selection, calibration and use of direct reading digital thermometers for calibration and inspection work conducted by Measurement Canada staff or accredited registered organizations. The document will also serve as a framework and guideline in the technical evaluations of related activities conducted by accredited registered organizations.

1.2 Scope

This recommended practice applies to direct reading electronic digital thermometers used for calibration and inspection work under the W&M and E&G regulation.

1.3 Applicable Documents

Document Number Document Title
GN-LP-003 Vocabulary of Technical and Metrological Terms
  Weights and Measures Act and Regulations
  Guide to the Expression of Uncertainty in Measurement

1.4 Abbreviations and Symbols

NRC
National Research Council
E&G
Electricity and Gas Regulations
W&M
Weights and Measures Regulations
VIM
Vocabulaire International des Termes Fondamentaux et Généraux de Métrologie/International Vocabulary of Basic and General Terms in Metrology

2.0 Selection And Requirements

2.1 Allowable Uncertainty

The expanded uncertainty of the thermometer, including all sources of error and using a coverage factor of 2 (k=2) , shall not exceed ± 0.25 °C.

The expanded uncertainty of a thermometer used for inspection of temperature measuring devices shall not exceed 1/4 of the device tolerance.

2.2 Operating Conditions

The thermometer's operating conditions will be rated as per the following Table 1:

Table 1: Classification of digital thermometers operating conditions
Class Application
Laboratory conditions Indoors
23°C± 5°C
non-condensing
Field conditions -30°C to +40°C
non-condensing

2.3 Resolution

For the specified application, thermometers shall have a minimum resolution and range as indicated in Table 2.

Table 2: Recommended Range and Resolution for thermometers.
Application Minimum Range and Min. Resolution
General field inspection -30 °C to +50 °C x 0.1°C
Heated product inspection -30 °C to +125 °C x 0.1°C
Calibration of thermometers used in gas prover for temperature correction 0°C to +35 °C x 0.01°C
Temperature correction in gas measuring apparatus 0°C to +35 °C x 0.1°C
Bell prover air temperature(s) 0°C to +35 °C x 0.1°C

2.4 Stability

Thermometers shall remain within the tolerance set out in section 2.1 for the verification period.

2.5 Probe Design and Construction

The thermometer sensor shall be designed and constructed to operate without leakage or deformation over the expected range of operating pressures, temperatures and environmental conditions including liquid immersion when required. Where thermometers will be used to determine temperature stability such as in the testing of ATC dispensers, the time constance of the thermometer in flowing liquid will be less than 15 Sec.

2.6 Thermal Shock

Thermometers intended for field applications shall be able to withstand thermal shocks of 50 °C without loss of accuracy.

2.7 Field Thermometers Packaging Requirements

Thermometer intended for field use shall be equipped with a carrying case of a water resistant, dust- proof and dimensionally stable over the range of environmental conditions expected during the service life of the device.

2.8 Construction

The construction shall be mechanically and electrically sound, and the materials, finish, etc., shall be such as to provide assurance of long life and sustained accuracy.

The thermometer must be CSA approved for Class1, Division1, group D hazardous locations.

2.9 Battery Power

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

2.10 Markings

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

  1. manufacturer's name
  2. model number
  3. instrument serial number
  4. temperature range

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

3.0 Calibration

Calibration includes the following operations:

  1. visual inspection of the thermometer,
  2. calibration and adjustment as necessary,
  3. recording as found and as left results
  4. determination of thermometer uncertainty, and
  5. production of a calibration certificate.

3.1 Test Points

Each transducer, associated with a thermometer, will be calibrated at a minimum of eight evenly distributed test points across it's measuring range or the range given in Table 2.

Where a non linear deviation function is used to model the thermometer response, a minimum of four test points per equation variable shall be used.

As found values must be determined prior to adjustments if any. It is recommended that the ice point be included in both as found and as left test points.

3.2 General Requirements for Calibration Equipment

3.2.1 Thermometer Standard

Traceability

For certification as a local standard, the temperature standard shall be traceable to a "Reference Standard" listed in the Weights and Measures Act and Regulations. For certification as a measuring apparatus under the E&G regulations, the thermometer standard shall be traceable to a national standard maintained by the NRC.

Measuring Range

The thermometer standard shall not be used outside it's certified measuring range.

Monitoring

The thermometer standard will be monitored for stability in an ice bath, water triple point cell or other suitable equipment with similar accuracy and stability. Monitoring will be conducted on a monthly basis as a minimum. Control charts are recommended.

Calibration Bath

A temperature bath of suitable operating range with demonstrated stability and homogeneity throughout this range will be used.

3.3 Calibration Conditions and Method

The documented calibration process will ensure the following:

Before beginning the calibration procedure, the thermometer under calibration and the thermometer standard will be preconditioned at ambient conditions for at least 12 hours.

The instruments shall be installed in their normal operating position.

Environmental conditions will be maintained within 23°C ± 5 °C.

The thermometers sensors will be sufficiently immersed in the temperature bath to address immersion errors. If this cannot be achieved, immersion error will be considered in the estimate of the uncertainty.

Temperature will be allowed to reach the set point and stabilized before readings are taken.

3.4 Uncertainty Estimate

3.4.1 Sources of Uncertainty

The sources of uncertainty are categorised in two groups. Sources of uncertainty that can be determined at the time of calibration and sources of uncertainty which are dependent on the use of the thermometer and requires additional knowledge of the equipment which may not be available to a second party calibrator.

The combined uncertainty of a thermometer shall include those sources listed in table 3 :

Table 3: Sources of Uncertainty in Calibration and Use of Digital Direct Reading Thermometers
Group of uncertainty Symbol Source of uncertainty Typical Distribution Divisor
Calibration u(fit) Linearity and repeatability of the thermometer. It is estimated by calculating the Standard deviation of residual for the determined deviation function. Normal 1
Calibration u(bath) Stability and uniformity of the temperature bath.(see procedure TE-GCP-008) Normal 1
Calibration u(ref) Expanded uncertainty of the thermometer standard Normal 2
Calibration u(res) Resolution of the instrument under calibration for selected temperature units. Rectangular 3.46
Use u(drift) Drift of the thermometer under calibration . From manufacturer's data otherwise evaluated from historic of the results of ice point checks. (Unless otherwise evaluated assumed to be equal to +/-0.15°C as a result of ice point checks) Rectangular 1.73
Use u(amb) Variation of ambient conditions of temperature and humidity on the instrument during it's use. (Temperature coefficients obtained from the manufacturer) Arcsine 1.41
Use u(immersion) Immersion errors for the transducer during its use ( Can be neglected if the immersion of the sensor is 20 time its diameter ). Rectangular 1.73
Use u(self heating) Self heating of the thermometer which occurs when the measuring current additionally heats up the sensing element. Rectangular 1.73

The calibration certificate will report as a minimum the uncertainty resulting from the "calibration" sources of error. It is recommended that the combined uncertainty (both calibration and use sources of uncertainty) be reported in which case the calibration report will clearly indicate that the reported uncertainty estimate includes contributions due to long term effects and varying operating conditions. In the absence of such statement on the certificate, it will be assumed that these sources of error have not been included.

For each thermometer model, the values of the "Use" uncertainties in Table 3 will be obtained by the owner of the thermometer. These may be obtained by:

  1. measurement and test conducted by the owner
  2. manufacturer's data or compliance statement
  3. second party measurement and test.

3.4.2 Calculations

Uncertainty calculations shall be based on the recommendations of the Guide to the Expression of Uncertainty in Measurement. Uncertainty values will be reported at 95% confidence interval or with a value of k=2.

3.5 Calibration Frequency

Unless otherwise authorized, thermometers shall be calibrated at least once every two years.

3.6 Sealing

Where the instrument is capable of being calibrated or adjusted, the access to the adjustments shall be easily detectable.

The thermometer must be labelled to indicate the status of calibration. A verification mark or tag shall be affixed to the instrument indicating;

  1. the expiration date of the calibration certificate, and
  2. unique identifier of the laboratory which performed the calibration.

The thermometers that have physical access to adjustment, must be protected by a seal to prevent any adjustments that can invalidate the calibration.

In the case of thermometers received with broken seals, the technologist must indicate clearly on the certificate that the seals were broken.

Note : The technologists must notify the responsible Regional Accreditation Coordinator if:

  1. The seal is broken
  2. The seals is intact but the as found errors are out of tolerance.
  3. Standards are received significantly past the re-certification due date.

3.7 Certificate of Calibration

The following information shall be contained in the certificate of calibration for the instrument:

  1. name of the instrument's manufacturer
  2. name and address of the owner of the instrument
  3. model number
  4. instrument serial number
  5. date of calibration
  6. range of the calibration
  7. estimated uncertainty
  8. traceability of the standards used in the verification
  9. correction factors (if used)
  10. name and address of the laboratory which performed the calibration
  11. any special limitations for the use of the instrument
  12. the expiration date of the verification certificate
  13. title : calibration certificate
  14. certificate number
  15. page identifier
  16. identification of the method used
  17. the signatory

4.0 Application And Use

The thermometer shall not be used outside the calibrated temperature range of ambient temperature for which it is certified.

The thermometer shall be suitably rated for the electrical classification of the area of intended use.

Unless otherwise specified in this document, the thermometer shall be used as per the manufacturers recommendations.

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

A scheduled monthly check of the thermometer should be performed, because the transducers are susceptible to damage and the instrument is subject to drift. It is recommended that an ice point test be used. Where this is not practical, the thermometer may be calibrated at a single point ensuring that the uncertainty of the calibration equipment is 1/4 or less than the allowable drift of the thermometer.

In the case of instruments designed or used as temperature differential measuring equipment, zeroing of the equipment will be conducted, as a minimum, on a monthly basis. If the drift is >0.2°C an adjustment and recertification are necessary.

When a thermometer is subjected to damage or mechanical stress, the integrity of the thermometer shall be verified.