EFM and API 21.1 Audit Trails for Oil and Gas Measurement
Key Takeaway
API MPMS Chapter 21.1 (Electronic Gas Measurement) expects flow computers to retain calculation inputs, events, and configuration changes in a tamper-evident audit trail. Producers and midstream teams need EFM archives that survive SCADA polls — not just live Modbus totals.
Quick Answer
Electronic Flow Measurement (EFM) for gas typically follows API MPMS Chapter 21.1: the flow computer must keep hourly/daily averages, accumulated volumes, and an audit trail of events and configuration changes that affect calculated results. SCADA can read live rates over Modbus, but the custody-grade evidence lives in the EFM archive.
What belongs in the audit trail
- Orifice/meter factor and plate changes
- Gas composition / heating-value updates
- Contract hour and pressure base changes
- Calibration, clock sets, and firmware events
- Alarm and cutout events that affect flow time
Liquid LACT systems lean on other API MPMS chapters; use the same discipline for prove factors and ticket integrity — see LACT PLC and meter proving.
SCADA vs EFM roles
- Flow computer owns AGA/API calculations and local archives.
- SCADA polls totals, rates, and status for operations.
- Measurement teams collect EFM reports on a schedule (or continuous EFM collection) for accounting and audits.
- Never “fix” a missing audit trail by editing a spreadsheet total.
Common failures
- Composition updates in SCADA but not downloaded to the EFM
- Clock drift between RTU and flow computer
- Archive overwrite before monthly close
- Unauthorized config changes without dual control
NFM integrates Totalflow, ROC, and other EFMs with SCADA while preserving API 21.1-style audit evidence for tickets and audits.
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Frequently Asked Questions
API MPMS Chapter 21.1 covers electronic gas measurement systems — how flow computers calculate, store, and protect measurement data, including audit trails for events and configuration changes.
Usually not by itself. Historians store what was polled; API 21.1 expects the flow computer (or collected EFM archive) to retain calculation inputs and config/event records used for custody volumes.
At least often enough that archives cannot wrap before retrieval — commonly daily or weekly for busy meters, aligned with accounting close. Continuous EFM collection systems reduce missed months.
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