G-20—Biasing of calibration and error tolerances for diaphragm deters

The copy of this document located on Measurement Canada’s website is considered to be the controlled copy.

Issue date:
Effective date:
Bulletin: G-20
Category: Gas
Supercedes: L.M.B. Memorandum 87.30 – "Biasing of Diaphragm Meters" (1987-11-06)


Table of contents


1.0 Purpose

The purpose of this bulletin is to provide continued temporary permission for biasing the calibration of new and remanufactured diaphragm meters, and for the application of corresponding shifted error tolerances for the verification of such meters.

2.0 Scope

This bulletin applies to both new and remanufactured diaphragm-type gas meters, but not to reverified meters.

3.0 Authority

This bulletin is issued under the authority of section 18 of the Electricity and Gas Inspection Regulations.

4.0 Terminology

MADT

median absolute deviation from target

New meter

a diaphragm meter which has never been used in service

Remanufactured meter

a rebuilt diaphragm meter in which all internal components (excluding the valve seats on certain models) have been replaced with new parts, with only the cast aluminum case of the original meter being retained and reused. A remanufactured meter may or may not include replacement of the original meter's register (index), but in either case is to be considered a new meter for purposes of calibration biasing and verification.

5.0 Background

Due to the nature of their design, composition, or construction, many diaphragm meters are known to exhibit an initial drift or shift in calibration accuracy during the initial portion of their life. This effect will differ for each model of meter, and will vary over time. Manufacturers claim that this instability is due to changes in the elasticity of the diaphragm material that occurs when the diaphragms are exposed to air or air/gas mixtures, with exposure to air having a greater initial effect (as occurs before a meter is initially placed into service, or after a meter is removed from service and put into storage). Shifts in meter accuracy can also occur as a result of shipping, depending on the mode of transportation used.

To compensate for these observed shifts in calibration, and to ensure that the performance accuracy of the diaphragm meters will be as close as possible to 100 percent (zero error) once the meters enter into service, Measurement Canada (MC) has been permitting meter owners and manufacturers to bias the calibration of new (and remanufactured) diaphragm meters, subject to the conditions listed in section 6.0 below.

6.0 Policy

6.1 Meter manufacturers, meter service organizations, and gas contractors are permitted to purposely bias the calibration of their new and/or remanufactured diaphragm meters to a target value other than zero error, subject to the conditions in clauses 6.1.1 to 6.1.5.

6.1.1 The applied bias values shall be taken into consideration when determining the acceptability of a meter's calibration errors during verification testing, by applying a corresponding shift to the meter's normal error tolerance. Similarly, for meter lots inspected by means of statistical sampling using a MC authorized sampling plan, the sample meter test errors shall be shifted or transformed in the same manner, prior to calculating the MADT.

6.1.2 The chosen bias value for each test point shall be declared in writing to the local Measurement Canada district office prior to the meters being presented for verification, for each applicable meter model.

6.1.3 The declared bias value(s) should not be subject to frequent change, and should be chosen with regard to manufacturer's recommendations and any relevant statistical study data available on the performance of the particular meter model.

6.1.4 Bias values shall fall within the range of -1.0 percent to 0.0 percent, and may differ in magnitude for each flowrate test point (H.L. and L.L).

6.1.5 The applied bias values shall be noted on the meter inspection certificate or recorded in the accredited organization's record of verification test results for the meter(s).

6.2 This permission may be amended or withdrawn in future, once investigative studies have been conducted to determine the various causes, magnitudes, and variability of diaphragm meter calibration error shifts, and the associated uncertainties.

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