This document discusses calibration considerations for orifice plate based gas flow computers used in custody transfer applications. Differential pressure meters are commonly used despite ultrasonic meters having advantages. Calibration is important due to the financial impact of errors in custody transfer measurements. Test equipment selection depends on the type of inputs to the flow computer. Procedures involve verifying and adjusting pressure, temperature, and current inputs through multiple measurement points. Safety, isolation, and proper setup are important to get accurate results and verify calibrations.

1. Differential Pressure
Meter Gas Custody
Transfer Calibration
CALIBRATION CONSIDERATION OF ORIFICE
PLATE BASED GAS FLOW COMPUTERS
Joel Hartel
Fluke Corporation

2. Agenda
• Why Calibration Matters in Custody Transfer
• Differential Pressure (D/P) vs. Ultrasonic
• Test Tools Considerations
• Pressure Calibration Test Tools
• Temperature Calibration Test Tools
• % Full Scale vs. % Reading + Floor
• Curriculum Topics
• UUT Considerations
• High Pressure Test Considerations
• Process Overview

3. Why Calibration Matters in Custody Transfer
• Custody Transfer is the cash register
• Calculated using volume or mass with density, and BTU value
• High volumes mean even small errors have significant financial impact
• Legal and Contractual Requirements
• Flow rate
• BTU value
• Turndown – ratio of max flow to min flow
• Accuracies better than 1% needed
• Repeatability: meters at a particular custody transfer point need to
agree
• If they don’t agree, calibration records are key inputs into settling the
dispute
• Focus of this talk is on the Flow Rate, not BTU value
• Many potential sources of error in gas sampling

4. Differential Pressure vs. Ultrasonic
• Ultrasonic well suited for custody transfers of pipeline quality dry
natural gas
• Lower pressure drop
• Less maintenance (especially wear) and built in diagnostics
• Fewer moving parts
• Better Turndown/Rangeability (50:1 typical, vs. 10:1 for turbine, 3:1 for D/P)
• However, 90% of all custody transfer meters sold world-wide are D/P
• Cheap and Simple
• Large installed base = large replacement market
• Well Understood
• Publicly available data sets and standards available:
• International Standard Organization, 5167 Part 2.
• American Gas Association Report No. 3, Part 1.
• Gas Processors Association GPA 8185-90, Part1.
• American National Standard Institution ANSI/API MPMS 14.3.1
• Relatively accurate with wet gas: correctable to +/- 2%1
1 Kinney, J. and Steven, R., “Effects of Wet Gas Flow on Orifice Gas Plate Meters”, Proceedings of the 48th ASGMT, 2013

5. Selecting test equipment
• Pressure calibration test tools
• Electronic calibrators
• Easy to set up and read
• Choice of % Full Scale vs. % Reading
• Pumps and Fittings
• Deadweight testers
• Maximum possible accuracy, under the right conditions
• Leveling
• Adjustment for local gravity
• Compensates for small leaks
• Harder to detect leaks in test setup
• Doesn’t account for pressure drop across
test hoses

6. Selecting test equipment
• Temperature calibration test tools
• Precision thermometers
• Deliver measurement accuracies adequate to verify flow computer
temperature sensor
• Probe considerations
• RTD probes deliver required accuracy
• Probes must be designed for rugged industrial applications
• Dry block calibrators
• Used to spot check test RTD probes
• Can be used to verify installed probes (if removed
from pipeline)
• Electronic simulators
• Simulate RTD probe signals into transmitters or
flow computers
• Do not verify accuracy of RTD sensor
• Many have accuracies equal to decade boxes

7. % FS vs. % Reading + Floor
• Analog sensors used to be specified in % FS
• Digital is a different tech, and gives the option to specify either way
• For units specified as %FS, accuracy increases as max
pressure decreases
• Important to pick the lowest range possible
• External pressure modules can cover unusual higher range needs
• Floor spec not always achievable in the field
• Cold weather – most units don’t specify below -10 C
• Bottom Line: a calibrator that is more accurate is good,
but the real requirement is for the calibrator to have a
sufficient Test Uncertainty Ratio for the Unit Under Test

8. 0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
0 100 200 300 400 500 600 700 800 900 1000 1100 1200 1300 1400 1500
SpecError(psi)
Measurement Pressure (psi)
Cal A
Cal B
% FS vs. % Reading + Floor Example 1
Cal A spec = 0.025% of full scale
Cal B spec = 0.05% of reading + 0.1 psi

9. % FS vs. % Reading + Floor Example 2
0
0.005
0.01
0.015
0.02
0.025
0.03
0.035
0.04
0.045
-14 -13 -12 -10 -9 -8 -7 -6 -5 -4 -3 -2 -1 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16
SpecError(psi)
Measurement Pressure (psi)
Cal A
Cal B
Cal A spec = 0.025% of full scale (16 psi)
Cal B spec = 0.25% of reading +.004 PSI (-14 to 0 psi), 0.05% rdg + .001 psi 0 - 16 psi

10. High-level procedure overview
• Set up and isolation
• Leak check
• Three measurements, direct input to transmitter or
flow computer:
• Differential Pressure (~100-250 inches H20 typical)
• Static Pressure (0 to 2000 psi typical)
• Temperature (-50 to 50 C typical for pipeline, wellhead can vary
more)
• Return to service and clean up

11. Unit under test (UUT) considerations
• Pressure device under test
• Range
• Commissioned at time of manufacture
• Typically specified as % of input full scale
• Required test equipment accuracy
• Matching the range of the test equipment is optimal
• Especially for test equipment specified as % of full scale
• Since two pressure ranges are needed, two tools with different
pressure ranges or a dual range tool are needed for desired accuracy
• Test Uncertainty Ratio (TUR) – 4:1 common in bench testing, nuclear &
military, frequently not possible in the field
• 2:1 or 3:1 more commonly seen in field calibration
• Lower TUR means greater uncertainty and lower confidence when
you have marginal readings
• mA output considerations
• Flow computers with mA inputs rather than direct P or T inputs require
calibration of the transmitters mA signal

12. High pressure test considerations
• Cleanliness of test instrument
• Unwanted fluids can damage test equipment (seals)
• Important to properly clean UUT prior to connecting
• Drain unwanted liquids from calibration cavities
• High Pressure Portable Nitrogen Bottles
with Regulators
• Deliver regulated pressure up to 2000 psi without
hydraulics
• Faster and cleaner than hydraulic test pumps but
bottles need to be recharged
• Hydraulic Test Pumps
• Typically use De-ionized water or mineral oil as
test medium
• Portable, lightweight, don’t need recharging
• But: slower, require priming, need to clean
UUT after test complete

13. Accuracy considerations
Transmitter
Pressure reading 6 30 250 1500
Rosemount 3051C range Range 2 Range 3 Range 4 Range 5
Upper Range Limit (URL) 9.035 36.135 300 2000
Calibrated span (5:1) 7.228 28.908 240 1600
Ambient temp (°C) 23 23 23 23
Pressure uncertainty
Pressure range psi 16 36 300 3000
Reference uncertainty FS 0.025% 0.025% 0.025% 0.025%
Pressure accuracy floor (psi) n/a n/a n/a n/a
Reference pressure accuracy 0.004 0.009 0.075 0.75
Temperature effect 0.00 0.00 0.00 0.00
Total pressure accuracy 0.004 0.009 0.08 0.75
mA Measurement uncertainty
Full Scale (mA) 24 24 24 24
Reading (mA) 13.282 16.604 16.667 15.000
Accuracy (%rdg) 0.015% 0.015% 0.015% 0.015%
Accuracy floor (mA) 0.002 0.002 0.002 0.002
mA Accuracy 0.004 0.004 0.005 0.004
Temperature effect 0.000 0.000 0.000 0.000
Total combined mA accuracy 0.004 0.004 0.005 0.004
Total combined mA accuracy (equivalent pressure units) 0.002 0.008 0.068 0.425
Total Accuracy
Total pressure accuracy 0.004 0.009 0.075 0.750
Total combined mA accuracy (equivalent pressure units) 0.002 0.008 0.068 0.425
Total Uncertainty (psi) 0.004 0.012 0.101 0.862
3051 Total Performance ±0.15% span (psi) 0.011 0.043 0.360 2.400
Test Accuracy Ratio 2.500 3.600 3.600 2.800

14. Accuracy considerations

15. Orifice plate style flow computers
• Multiple manufacturers and measurement methods
• Direct input computers
• Pressures and temperature measurement made with direct inputs to
device
• Calibration requires laptop computer to make adjustment
• Indirect input computers
• Transmitters convert measured pressures and temperature to mA
signals applied to the input of the flow computer
• Requires accurate measurement of mA signals
• Both methods require tools for calibration of
pressure and temperature
• Direct input computers require a laptop to verify/adjust the flow
computer
• Indirect method may require HART communication for adjustment
and verification

16. Precautions and preparation
• Safety first
• Isolation and leak checking (shut in leak test)
• Close the process valves, leave the equalize valves shut
• At least 30 seconds
• D/P will go negative with high side leak, positive with low side
• Pre-test data recording
• As-found transmitter zero
• Typically record transmitter data immediately prior to performing
test
• Post test data recording
• Typically record transmitter data immediately after performing each test
/adjustment / verification

17. Differential pressure transmitter test
1. Pre-test Zero test
1. As-found transmitter zero
2. Verification
1. Connect calibrator to the low pressure side of flow computer
2. Connect PC to the flow computer (if needed)
3. Apply pressures as directed using the hand pump (small
pump)
1. Typically check 3 to 5 points from 0% to 100%.
4. Vent the transmitter or flow computer and disconnect
3. Adjustment
1. Adjust per local procedure
4. Post adjustment verification
1. Repeat Step 2
(Pressure source not shown)

18. Static pressure transmitter test
1. Verification
1. Attach the high pressure side of the calibrator to the
appropriate port on the transmitter or flow computer
2. Connect high pressure source (ex. N2 bottle or pump)
3. Test set by local procedure
1. Common tests is to check 0% and 50% of span and compare
with flow computer reading.
2. If out of spec (0.1% typical), then check 5 points from 0% to
100%.
2. Adjustment
1. Adjust per local procedure
3. Post adjustment verification
1. Repeat Step 1

19. Temperature transmitter test
1. Verification
1. Depends on flow rate
1. High rate typically checks are easier
2. Low rates require RTD simulator, dry block, or bath
2. For high flow rate, use RTD to check & compare with the transmitter
reading
1. Variance > 0.2 °F but < 0.5 °F, transmitter re-zero
2. Variance > 0.5 °F frequently requires adjustment
3. For low flow, connect RTD simulator and do 3
or 5 point check over the full span of the transmitter
1. Same variance criteria
2. Adjustment
1. Adjust per local procedure
2. 1524 Super Thermometer
3. Post adjustment verification
1. Repeat Step 1

20. Wrap up and resources
• Questions?
• For more details, see our application note:
• Calibrating Gas Custody Transfer Flow Computers (6002276A)