PS-G-14—Provisional specifications for the verification of correction devices and linearization functions incorporated in meters and flow computers

Category: Gas
Specification: PS-G-14 (rev. 1)
Issue date:
Effective date:
Supersedes: PS-G-14


Table of contents


1.0 Scope

This document provides requirements for the verification of flow meter performance enhancement schemes based on the linearization or curve fitting of flow calibration data.

This document applies to schemes which are incorporated in meters, electronic flow computers, correction devices and supporting software external to the device.

2.0 Authority

These specifications are issued under the authority of section 18 of the Electricity and Gas Inspection Regulations (EGIR).

3.0 References

4.0 Application

4.1 Testing schemes

The testing protocol is dependent on the device type and the location of the linearization scheme.

Meters which contain electronics containing user-programmable linearization schemes (unique coefficients for each individual meter) must first be evaluated against the accuracy tolerances in the appropriate meter specification, with the meter's linearization function disabled. The meter shall then be retested in the manner described in the following sections, with the linearization function enabled.

Flow computers which contain user-programmable linearization schemes (unique coefficients for each individual meter) shall either be tested using simulated inputs, or, where they cannot be divorced from the flow meter, tested in the manner described in the above paragraph.

4.2 Verification requirements

In order to verify or reverify the performance of a point-to-point linearization scheme, the form or type of equation must be known. This information shall be listed in the Notice of Approval (NOA) and include:

  1. the form(s) of the equation(s) and/or method used for the interpolation,
  2. the type of meter(s) which the equation(s) is modelling, if the function resides in a flow computer or correction device.

5.0 Evaluation of linear interpolation schemesFootnote 1

This method is applied to meters which:

This method can also be applied to flow computers when that are included as part of the metering assembly during flow testing. This scheme would typically apply to some types of ultrasonic meters.

The process of evaluation of the linear Interpolation scheme in a flow meter or flow computer is broken down into two basic steps. The first step involves the calibration of the flow meter by a Measurement Canada-recognized facility, to characterize the flow meter's performance (performed at or near intended operating conditions using natural gas as the test medium). The second step is the verification of the application of the linearization scheme.

Step 1

Standard test protocols require the meter to be evaluated at a minimum of five (5) flow rates (10%, 25%, 50%, 75% and 100% of Qmax). Within practical limits for setting flow rates, these cardinal points shall be used as the calibration points produced for flow calibration. The distribution of these points can be varied by placing more emphasis on the lower flow rates. High pressure calibration data shall be provided which have an uncertainty not greater than ±0.33%. The values themselves shall be within a band of ±1%. Values outside of this band shall not be accepted.

In this method of linear Interpolation, the error values and the corresponding flow rates are entered into the flow computer or meter. The form of the entry (K-factor or meter factor) will depend on the specific device. Care shall be exercised to ensure that the form is suitable for the meter or flow computer, and that additional scaling factors and their influence on the meter or flow computer's output are understood.

Step 2

As noted above, the second step in the evaluation is to verify the application of the linearization scheme. This is accomplished by flow testing the meter at two additional flow rates (0.1Qmax and 0.5 Qmax). The result shall be a reduction of the meter error.

6.0 Alternative test method for linearized flow rates

This method is usually applied to flow computers and meters which incorporate linear interpolation schemes which can be evaluated using a simulated input.

This method assumes that the effects of other correction factors can be determined and isolated from the determination of the metered volume (or mass). This method can also be applied where the linearization value determined and applied by the flow computer cannot be directly displayed.

Step 1

Standard test protocols require meters to be evaluated at a minimum of five flow rates (10%, 25%, 50%, 75% and 100% of Qmax). Within practical limits for setting flow rates, these cardinal points shall be used as the calibration points produced for flow calibration. The distribution of these points can be varied by placing more emphasis on the lower flow rates. High-pressure calibration data shall be provided with an uncertainty of not greater than ±0.33%. The values themselves shall be within a band of ±1%. Values outside of this band shall not be accepted.

In this method of linear Interpolation, the error values and the corresponding flow rates are entered into the flow computer. The form of the entry (K-factor or meter factor) will depend on the specific device. Care shall be exercised to ensure that the form is suitable for the meter/flow computer.

Step 2

Here again, the second step of the evaluation is to verify the application of the linearization scheme.

Firstly, and when possible, the correct entry of the values in the flow computer or meter shall be manually verified. Flow testing shall then be simulated at flow rates of 10%, 25%, 50%, 75% and 100% of Qmax.Footnote 2

The test points shall be calculated based on the input pulse count and the associated frequency. From this data, the expected meter volume (or mass) indication and the value determined by the flow computer can be determined and compared. The specifics of the calculation will again be based on the methods used in the flow computer.

These methods can generally be broken down into two types:

Equation 1
(V expected) = Pulse count ÷ Calculated linearizing K factor
Equation 2
(V expected) = (Pulse count × Calculated linearizing MK) ÷ Base K factor

In both cases, the frequency of the simulated pulse input must be known in order to determine the value of the K-factor or meter factor which should have been used by the flow computer. Once the frequency has been determined, the corresponding value in Table 1 can be used in equation 1 or 2 to determine the expected indicated volume (Vexpected) from the flow computer. It is unlikely that the exact flow rate or pulse rate value in Table 1 can be achieved, and in these cases, the measured input frequency shall be used to calculate the corresponding K-factor or meter factor. The equation shall be the same linearization equation used in the flow computer. This procedure shall then be repeated for each value of flow rate in Table 1.

Table 1: Expected correction values as a function of Qmax
Test pointsFootnote 3
(% Qmax)
Expected value (%) Expected meter factor Expected K factor
5 -1.00 1.010000 1.01000 × base K
15 -0.33 1.003333 1.003333 × base K
35 0.80 0.992000 0.992000 × base K
60 0.30 0.996960 0.996960 × base K
95 0.05 0.999500 0.999500 × base K

The performance of the flow computer's linearization function can then be determined by comparing the mathematically determined volume throughput to the indicated throughput.

The error in the linearization shall not exceed ±0.1%.

Equation 3
(Linearization error %) = [(V indicated – V expected) ÷ V expected] × 100

7.0 Evaluation of curve fitting interpolation schemesFootnote 4

This method is usually applied to meters which incorporate curve fitting interpolation schemes which cannot be tested using a simulated input.Footnote 5 This method can also be applied to flow computers when they are included as part of the metering assembly during flow testing. This scheme would typically apply to some types of ultrasonic meters.

The evaluation of the flow computer's ability to determine the volumetric output, given a predetermined set of coefficients, is divided into two basic steps. The first step focuses on the evaluation of the generation of the flow coefficients using approved software, and the second focuses on the evaluation of the meter's ability to apply the coefficients during flow or a simulated flow testing.

Step 1

Here again standard test protocols require the meter to be evaluated at a minimum of five (5) flow rates (10%, 25%, 50%, 75% and 100% of Qmax).

The owner of the meter may wish to provide additional test points where the slope of the performance curve fluctuates dramatically (usually below 20% of Qmax). Generally, the distribution of the calibration points shall be logarithmically evenly distributed as a function of the flow rate.

Once the flow rate and error values from the calibration have been entered into the software package and the coefficient values have been predicted, the precision of the prediction shall be evaluated. This shall be done by comparing the values of the meter error predicted by the equation to those supplied on the certificate of calibration.

For each flow rate listed in the certificate of calibration, the predicted correction value is determined using the equation. The difference or residual between the predicted value and the value listed in the certificate of calibration is then determined. Two times the standard deviation of all the differences shall not exceed 0.2%. Also, any one value shall not exceed 0.5%. If the deviation is exceeded, then additional test points may be needed.

Step 2

As noted above, the second step in the evaluation is to evaluate the flow computer's ability to apply the coefficients during flow conditions. Once the previously determined values for the coefficients are entered into the flow computer or meter, and when available, the correct entry of the values in the flow computer or meter shall be manually verified. Again, the form of the entry will depend on the specific device. Care shall be exercised to ensure that the form is suitable for the meter/flow computer, and that additional scaling factors and their influence on the meter or flow computer's output are understood.

The meter shall then be retested at the flow rates of 10%, 25%, 50%, 75% and 100% of Qmax. The results shall demonstrate a reduction of the meter error and meet any other applicable tolerances.

Where it is practical to simulate the inputs to a flow computer, the method in section 6.0 above shall be used.

8.0 Revisions

The purpose of revision 1 was to:

Date modified: