Custody transfer measurement takes place where ownership of gas changes hands and payment is based on the measured quantity or energy. What sets it apart from process measurement is not so much the meter as the requirement that the result be acceptable to both parties, verifiable, and open to audit afterward. That requirement shapes every layer of station design, from meter run arrangement and calculation method to sealing, record keeping and periodic verification.
This page focuses on measurement governance and the uncertainty approach. Meter run architecture, meter technologies and gas analysis systems each have their own pages.
Process measurement, custody transfer and legal metrology
These three terms are often used interchangeably, yet they answer different questions:
Concept | Purpose | Who sets the requirements? |
Process measurement | Monitoring, control, efficiency and allocation within a facility | Mostly the operator's own engineering decisions; there is usually no statutory accuracy requirement |
Custody transfer measurement | Determining the quantity or energy of gas changing hands as the basis for billing | The sales or transportation contract, network codes and relevant industry standards |
Legal metrology | Ensuring instruments and systems meet accuracy and control requirements set by a public authority | National legislation, typically modeled on OIML Recommendations and, in the European Union, the Measuring Instruments Directive (MID) |
Whether a custody transfer point also falls under legal metrology depends on national law, station size and the contract. The first design step is therefore to establish which contractual provisions and statutory requirements apply.
Meter accuracy is not system uncertainty
Accuracy figures published by meter manufacturers generally describe the meter itself under defined calibration conditions, and some datasheets state explicitly that they exclude the uncertainty of the calibration facility. The quantity that appears on an invoice, however, is the output of a whole chain: meter, pressure and temperature measurement, compressibility calculation, composition or calorific value data, and the flow computer.
The International Organization of Legal Metrology (OIML) makes the distinction explicit, with separate Recommendations for gas meters and for measuring systems for gaseous fuel. The measuring-system Recommendation defines accuracy classes A, B and C, with maximum permissible errors on energy of ±1.0%, ±2.0% and ±3.0% respectively. These limits are separate from meter classes and apply to the system as a whole. Which class or uncertainty target applies is set by regulation and contract.
From volume to energy: the billing basis
Gas can be billed on volume or on energy. On a volume basis, the volume the meter registers at line conditions is converted to the contractual base conditions using pressure, temperature and the compressibility factor. On an energy basis, that base volume or mass is multiplied by a representative calorific value. The calorific value is usually calculated from gas composition, taken from an online process gas chromatograph or another source the contract accepts.
Two points in this chain deserve particular care. The first is base conditions: different reference temperature and pressure definitions give different volumes for the same gas, and overlooking the difference can cause serious disputes in custody transfer, so base conditions must be written into the contract. The second is the calculation method: both parties should agree which standard is used for compressibility and calorific value, and which composition data feeds it.
How custody transfer requirements shape the design
· Meter run redundancy: a standby meter run keeps measurement going during maintenance and verification; an unmetered bypass is either omitted or tied to sealing and logging rules.
· Check and comparison metering: a second meter in series (check meter) or an arrangement that allows runs to be compared against each other reveals meter drift early.
· Use of diagnostics: methods such as comparing the speed of sound measured by an ultrasonic meter with the value calculated from gas composition help monitor the consistency of the measurement chain continuously.
· Pressure and temperature measurement: because these feed volume conversion directly, redundant transmitters and test connections for field verification are considered.
· Flow computer and calculations: the calculation method, base conditions and fixed parameters are documented, and parameter changes are subject to access control.
· Audit trail: industry standards for electronic gas measurement call for auditable recording and reporting of measurement data, events and configuration changes.
· Sealing and access control: connections, valves and software parameters that could affect the result are protected by physical or electronic seals.
· Sample point: for energy-based billing, the chromatograph or sample point is located so that a representative sample is taken.
Design inputs fixed by contract and regulation
Parameter | Why it matters in design | Effect on selection |
Billing basis (volume, mass, energy) | Defines where the calculation chain ends | Need for calorific value measurement, flow computer configuration |
Base conditions | Different definitions give different volumes for the same gas | Flow computer parameters, report format |
Accuracy class or uncertainty target | Sets the permissible error for the whole system | Meter type, number of runs, transmitter selection, calibration approach |
Meter calibration and verification intervals | Limit performance drift over time | Flow calibration, in-situ verification capability, meter exchange arrangements |
Check metering and dispute resolution | Give both parties confidence in the result | Series meter, comparison runs, backup calculation |
Data retention and reporting | Needed for retrospective correction and audit | Retention period, event logging, remote reading infrastructure |
Witnessed tests and sealing | May require the counterparty or an authority to attend | Test procedures, seal points, access rights |
Verification through commissioning and operation
Verification at a custody transfer station does not end at commissioning. During commissioning, transmitter calibrations, flow computer parameters, calculation results and communications are documented, often witnessed by the counterparty or an authorized body. In operation, periodic transmitter checks, review of meter diagnostic logs, comparison against check metering and, where needed, meter recalibration keep the measurement reliable. How often and by which method is set by contract and regulation.
What TLY Enerji contributes
On custody transfer metering projects, TLY Enerji can provide engineering support for selecting measurement technology and instrumentation; supplying equipment such as ultrasonic or orifice metering systems, pressure and temperature instruments and process gas chromatographs; and for system integration, site installation, testing and commissioning. PLC- and SCADA-based data acquisition and reporting can be added to the scope. Responsibilities for legal metrology procedures, calibrations requiring accreditation and witnessed tests are agreed separately for each project.
What to clarify before requesting a quotation
· Contractual billing basis (volume or energy) and base conditions
· Applicable national regulations and the required accuracy class or uncertainty target
· Technical requirements of the counterparty or network operator
· Expectations on check metering, standby runs and bypass policy
· Source of composition data: online analysis, laboratory or network data
· Meter calibration and periodic verification expectations
· Data retention period, report format and remote reading requirements
· Flow range, operating pressure and temperature conditions
Related pages
· Ultrasonic gas metering skids: Meter runs built around multipath ultrasonic meters, widely used for fiscal metering.
· Orifice gas metering skids: Differential pressure metering based on standardized calculation methods.
· Natural gas quality analysis systems: Composition and calorific value measurement for energy-based billing.
· Natural gas metering stations: Meter run architecture and station components.
· LNG send-out metering: Custody metering of regasified gas delivered to the grid.
Frequently asked questions
Does using a meter with type approval make a station fit for custody transfer?
Not on its own. A meter's approval covers only the requirements defined for the meter, while the billed quantity also depends on pressure and temperature measurement, volume or energy conversion, composition data and the meter run installation. International legal metrology documents likewise set separate requirements for meters and for measuring systems. The whole system, including its calculation method and record keeping, therefore has to be assessed.
What is the difference between volume-based and energy-based billing?
With volume-based billing, payment is based on gas volume converted to base conditions. With energy-based billing, that volume is multiplied by the gas's representative calorific value, so gases of different quality are priced according to the energy they contain. Energy billing requires the calorific value to be determined reliably, which means online gas analysis or another composition source accepted in the contract.
Why use a check meter?
A check meter is installed in series with the main meter and measures the same gas a second time. Tracking the difference between the two readings over time reveals drift, fouling or a fault in either meter before billing periods close. Pairing meters based on different measuring principles can reduce the effect of common error sources. Whether check metering is needed depends on the contract and the importance of the station.
What does an audit trail cover?
An audit trail is a time-stamped record of every event that could affect the measurement result: hourly and daily quantities, alarms, instrument faults, parameter and configuration changes, and calibration and verification activities. These records make retrospective corrections possible in a dispute and allow the consistency of the measurement to be reviewed independently. Retention period and format are set by contract and regulation.