Custody Transfer Mass Flow Meter

A custody transfer mass flow meter is a fiscal-billing flow meter built to the tightest accuracy class in its measurement family, with the supporting outputs and documentation needed for invoiced product movement between two parties. For liquid hydrocarbon and refined-product transfer this is almost always a Coriolis meter. Coriolis reads mass flow directly through the Coriolis force on a vibrating tube, reads density from the resonant-frequency shift on the same tube, and reads temperature from an embedded RTD. The custody build adds: ±0.1 % of rate accuracy grade, a dual-pulse output for prover loop integration, an audit-trail data logger that preserves the invoice basis for later retrieval, and build documentation traceable to the calibration rig for the buyer-arranged fiscal-regime submission.

On clean low-viscosity refined product (gasoline, diesel, jet fuel) at moderate pressure, a calibrated liquid turbine at ±0.5 % of rate is the lower-cost custody option and is widely accepted on certain fiscal regimes. Coriolis at ±0.1 % becomes the right pick when one or more of the following applies: the medium is mixed crude or refined-product blend (Coriolis reads density inline for net-volume correction; turbine needs a separate densitometer), the service includes batch-to-batch product changes (Coriolis density tracks the grade interface), the accuracy range must hold over wide viscosity swings or with paraffinic / waxy crude (turbine K-factor shifts above 10 cSt), or the regime requires direct mass measurement rather than inferred mass from volume + density. Cost trade-off: Coriolis is 3-5x the price of an equivalent turbine, justified by the inline density + temperature and the ±0.1 % spec.

The custody Coriolis ships with a dual-pulse output (two pulse channels with a fixed phase relationship, A leading B by 90 degrees or A and B reading opposite-direction pulses depending on the protocol). The dual-pulse pair lets a small-volume prover or ball prover detect any single-channel pulse-counting fault during the prove run, by cross-checking the A and B channels against each other in real time. Pulses are scaled at the transmitter (selectable kg/pulse or t/pulse) and terminate at the prover counter on a shielded twisted pair. The factory pre-configures the dual-pulse channel scaling at the order stage to match the buyer’s prover loop counter and the service flow range; field adjustment is available through the transmitter HART or RS-485 interface.

Refined-product invoicing is settled in net volume at a standard reference temperature, typically 15 °C for the metric world or 60 °F for the US. The custody Coriolis outputs live medium density (accuracy ±0.002 g/cm³) and live medium temperature (accuracy ±1 °C) on the same instrument as the mass flow value, through HART or RS-485 Modbus RTU. The fiscal flow computer (downstream of the meter, supplied by the buyer or by a flow-computer vendor) takes mass flow, density at service, and temperature at service; converts mass to gross volume at service using density; then converts gross volume at service to net volume at the reference temperature using the petroleum-industry volume correction tables. No external densitometer or thermowell installation needed at the meter run.

Custody transfer is governed by the local fiscal-billing regime, which varies by country and by industry. Common regimes include the European metrology directives on liquid measuring systems, the petroleum industry measurement standards published by trade associations in the US, the relevant ISO standards on hydrocarbon liquids, and the international metrology recommendations on measuring systems for liquids other than water. Type approval under any of these regimes is buyer-arranged with a notified body or designated calibration authority in the buyer’s jurisdiction. ProFlow ships each custody Coriolis with the build documentation needed for that submission: the ISO 9001 calibration certificate listing the K-factor and density coefficients, the calibration-rig traceability record, the construction drawing set, and the audit-trail logger access record. The factory does not hold ex-factory fiscal-regime certification; that is buyer-arranged in the buyer’s jurisdiction.

Standard lead time is 5 to 7 business days from order confirmation, factory direct. Each custody Coriolis ships with an ISO 9001 calibration certificate listing the K-factor (mass calibration coefficient), the density coefficient pair (D1 and D2 for the medium reference), the calibrated DN, the tested flow range, and the zero-point stability value for that serial. Calibration is performed on a gravimetric water rig at the factory at three flow points across the service range, with three repeat runs at each point. For high-value custody fits the buyer can arrange witness calibration at a third-party flow lab before shipment; the factory coordinates the lab booking and ships to the lab address. Hazardous-area certification is Ex d [ia] IIC T6 Gb (flameproof body with intrinsically-safe sensor connection) standard; IP67 ingress standard; ambient range −40 to +60 °C on the standard transmitter.