1. By : M. Dlamini D. Jonker E. P. Tarnow
Implementation of a New Calibration Method to Resolve Back-Pressure Effects on the Calibration of Gas Flow Meters
By : M. Dlamini D. Jonker E. P. Tarnow
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2. Overview Introduction
Intermediate Checks Between Two Flow Cells of a Piston Prover Reference Standard
Calibration Method – Split Method
Verification and Commissioning of the 100 l/min Flow Cell - Validation of the Split Method
Conclusion
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3. Introduction
The NMISA Gas Flow laboratory performed intermediate checks on their piston prover gas flow reference standard (ML-800) using a series method.
When the flow rate was above 25 l/min, an “over-pressure” message was displayed by the flow cell connected upstream.
This indicated that the pressure to the reference standard was higher than its limits, that is, it exceeded 1,244 kPa above atmosphere.
The ML-800 gas flow reference standard is designed to be used at atmospheric pressures only.
A new calibration method, Split Method, was implemented and validated to have both reference standards venting to atmosphere.
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4. Intermediate Checks Between Two Flow Cells of the Piston Prover Reference Standard
Intermediate checks were performed between two gas flow reference standards flow cells, ML (50 l/min) and ML (100 l/min).
This was done to maintain confidence in the calibration of the 50 l/min flow cell and to commission the 100 l/min flow cell.
Figure 1: Showing the in line (series) calibration method for two flow cells
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5. Intermediate Checks Between Two Flow Cells of the Piston Prover Reference Standard
The results were to be used to verify that the 100 l/min reference standard was not damaged during shipping from manufacturer.
Table 1: Measurement results for intermediate checks between the 50 l/min and 100 l/min flow cells using series method. ML
(ml/min) ML
Correction
Expanded
Unc.
5 017,4
5 034,3
-17,0
8,1
9 981,4
10 006
-24 17
15 004
14 992 12 23
20 045
20 032 13 32
25 087
25 073 15 42
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6. Calibration Method – Split Method
The Split Method calibration setup positions the reference standard and the UUT in a parallel configuration.
This ensures that both the UUT and reference standard vent to atmosphere.
Back-pressure is then eliminated.
Figure 2: Split Method calibration setup. The arrows show the direction of gas flow.
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7. Calibration Method – Split Method
Any flow perturbation due to the up/down movement of the reference standard’s piston is avoided.
The Split Method was validated using measurement results for the verification and commissioning of the 100 l/min flow cell.
The Split Method was validated against a series calibration method using a laminar flow system as the reference standard.
The 100 l/min flow cell was calibrated against the 50 l/min flow cell using the Split Method.
It was then calibrated against the laminar flow system using a validated Series method.
The results are discussed in the next section.
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8. Verification and Commissioning of the 100 l/min Flow Cell
Table 2: Shows summarized Split Method validation results.
Nominal Flow (ml/min)
Split Method Correction (ml/min)
Series Method Correction (ml/min)
Split Method Exp. Unc. (ml/min)
Series Method Exp. Unc. (ml/min)
Calculated |En| values
5 000
-25,8
-27,5
105
11,7
0,02
10 000
-39
-42 24
0,03
15 000 38 -8
112 34
0,39
20 000 29
-10
109 46
0,33
25 000 18
-58
110 58
0,61
30 000 17
-106
116 72
0,90
35 000 4
-109
117 81
0,79
40 000
-13
-133
121 92
0,78
45 000
-167
132
102
0,94
50 000 -9
-173
0,95
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9. Verification and Commissioning of the 100 l/min Flow Cell
The normalized error values (En) between the Split Method and the Series method were all less than or equal to 1.
The results are also shown graphically in figure 3.
The graphs shows overlapping uncertainties.
This indicates satisfactory performance of the Split Method.
Raw validation data is presented in the full paper.
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10. Verification and Commissioning of the 100 l/min Flow Cell
Figure 3: Graphical presentation of the Split method validation results.
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11. Verification and Commissioning of the 100 l/min Flow Cell
The measurements indicate good agreement between the 50 l/min and 100 l/min flow cells.
Using the Split Method, gas flow measurements up to 50 l/min can be performed without any over-pressure error.
This was because the reference standard and UUT were both venting to atmosphere eliminating any flow restriction which would cause back pressure.
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12. Conclusion
The ½”-¼” reducer created flow restriction which resulted in back-pressure in the flow cell upstream.
The Split Method has the two flow cells venting to atmosphere, hence the back-pressure was eliminated.
Thus, flow measurements up to 50 l/min could be performed.
The Split Method will also be validated to calibrate other types of instruments such as mass flow meters.
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13. The End
Thank You!
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