The liters shown on a loading meter are not, by themselves, the invoice figure. Gasoline, diesel and jet fuel change volume with temperature, so a fuel quantity becomes commercially acceptable only when four conditions hold: the volume is converted to the contract reference conditions, both parties can repeat the calculation with the same result, the meter is verified regularly, and every step is recorded in a tamper-evident way.
The general make-up of a custody transfer loading skid is covered on the custody transfer truck loading skids page. This page focuses on what is specific to refined fuels: volume correction, billing practice and measurement governance at the terminal.
Why the billed quantity differs from the metered volume
Light petroleum products expand as they warm. A tanker loaded on a hot afternoon and one loaded on a cold morning can show the same metered volume yet carry different amounts of product. Fuel trading therefore uses two volumes: the gross volume registered at metering conditions, and the standard volume obtained by removing the effect of temperature, and where relevant pressure. The contract sets which one is sold; national regulations may make that choice binding for some sales channels.
The reference temperature differs between jurisdictions and contracts, so no figure is given here. What matters is that the reference conditions, product group definitions and calculation sequence used by the controller are fixed in writing at the start of the project.
Volume correction: from printed tables to an algorithm
API MPMS Chapter 11.1 is the widely used reference for correcting temperature and pressure effects on the density and volume of crude oils, refined products and lubricating oils. It defines the correction as an algorithm rather than printed tables and yields the temperature (CTL) and pressure (CPL) correction factors. LPG and NGL are outside its scope, so a terminal also handling LPG needs a separate calculation basis.
Because the correction depends on how a product's density responds to temperature, gasoline, jet fuel and diesel do not share one curve. If a product is assigned to the wrong group in the controller, every ticket will carry an error in the same direction even when the meter itself is reading correctly. For blends containing bio-components, the parties should agree explicitly which product group definition applies.
Commercial acceptance rests on the calculation giving the same answer whoever performs it. API MPMS Chapter 12.2 sets the equations, rounding rules and calculation sequence for measurement tickets; when parties round differently, trivial per-load differences become reconciliation problems at a busy terminal.
Mass, gross volume and standard volume
Coriolis mass flow meters measure mass and density directly and derive volume. If fuel is sold by volume, a mass meter must still deliver standard volume, so referring the measured density to base conditions becomes part of the calculation. Positive displacement and turbine meters measure volume directly with on-line temperature compensation, using either a defined product density or an on-line density meter. Matching meter technology to product properties is covered on the fuel metering skids page.
When does legal metrology apply?
Custody transfer is measurement that underpins ownership and payment between two parties; legal metrology applies where that measurement is under statutory control. National law decides which fuel loading measurements fall under it.
Internationally, OIML R 117-1 sets the requirements for dynamic measuring systems for liquids other than water. In the EU, annex MI-005 of the Measuring Instruments Directive (MID) places road and rail tanker loading and unloading in accuracy class 0.5. For that class, OIML R 117-1 allows a maximum permissible error of 0.5 percent for the complete system and 0.3 percent for the meter alone. The gap is intentional: a meter approval does not prove system compliance, because gas elimination, the conversion device and the installation are assessed as part of the system.
Proving: keeping the meter factor current
Loading meter performance can shift with product, flow rate and time, so custody transfer meters are proved against a reference at intervals and the meter factor is updated. Common methods are volumetric test measures, pipe or small volume provers and master meters; the choice depends on the number of products, flow rates, bay layout and what local regulations accept. Statutory re-verification, contract terms and the terminal loss control program together set the interval, and multi-product terminals may need a meter factor per meter and product.
Records, seals and tamper evidence
A commercial quantity stays defensible only if every parameter behind it is preserved. OIML R 117-1 requires a sealable meter adjustment device and prohibits adjustment by bypassing the meter; API MPMS Chapter 21.2 covers auditing, reporting and security of electronic liquid measurement. In practice, keep at least:
· The load ticket: Gross volume, average product temperature, density used, correction factor and standard volume.
· A parameter change log: Who changed a meter factor, product group or reference condition, and when.
· An event log: Power failures, sensor faults, alarms and interrupted loads.
· Proving records: Date, method, the meter factor found and the value applied.
OIML R 117-1 notes that tanker loading systems may, by prior agreement, be designed so the transaction is not settled when the driver leaves. Where tickets are finalized later through the terminal automation system and ERP, the record chain must be designed for that from the start.
Reconciliation at terminal level
Measuring each load correctly does not mean the terminal balance will close. Static inventory from tank gauging is compared periodically with dynamic bay totals. Industry guidance on hydrocarbon management at distribution terminals separates apparent loss from measurement and accounting errors from physical loss, and covers stock control and the settlement of measurement disputes.
Design criteria for fuel custody transfer
Parameter | Why it matters in design | Effect on selection |
Billing basis (gross volume, standard volume or mass) | Defines the entire calculation chain | Meter type, need for density measurement, controller configuration |
Reference conditions and product groups | Correction accuracy depends on them | Controller or flow computer parameters, product recipes |
Scope of legal control | Determines which measurements need verification and sealing | Type approval status of components, sealing points, document set |
Proving method and interval | Keeps meter factors current and supports loss control | Prover connections, master meter location, bay layout |
Number of products and bio-component content | Separate correction and meter factors may be required | Parameter management and product-specific proving plan |
Records and data transfer | Provides traceability in audits and disputes | Event logging, access rights, TAS and ERP interface |
How TLY Enerji supports these projects
TLY Enerji provides system design, product supply, engineering, installation, commissioning and maintenance for fuel tanker loading and unloading facilities. For custody transfer metering, that can include selecting meters suited to the billing basis, installing and commissioning flow meters, configuring control and automation, and post-start-up support. Legal verification and sealing are carried out by the competent authorities; preparation duties and the scope of supply are agreed through the project specification.
Information to prepare for a technical review
· The quantity fuel is sold on and the reference conditions in your contracts
· Product list, product group definitions and bio-component content
· Whether the measurement is under legal control, and which national rules apply
· Current proving method, interval and availability of test measures or provers
· For upgrades, existing meters, controllers and their parameter structure
· Reconciliation differences observed between tank inventory and loaded totals
· The TAS or ERP system that receives tickets and reports
Related pages
· Custody transfer truck loading skids: Fluid-independent components and proving infrastructure of a custody transfer skid.
· Fuel metering skids: Choosing meter technology for viscosity and product changes.
· Fuel terminal automation systems (TAS): Linking load tickets to orders, delivery documents and ERP.
· Fuel truck loading systems: Product segregation, additives and static electricity on the loading side.
· Fuel truck unloading systems: Reconciling received quantities with tank inventory.
Fuel custody transfer FAQ
Why can the liters on the meter differ from the liters on the invoice?
The meter measures product at its actual temperature and pressure during loading. If the invoice is based on standard volume, that gross volume is converted to reference conditions using the correction factor for the product group. Warm product gives a standard volume smaller than the metered volume, cold product a larger one. The difference is not a measurement error but the intended result of the calculation defined in the contract.
Which reference temperature should we use?
The reference temperature is a commercial and legal definition, not an engineering preference. If national law sets it, that value applies; otherwise it must be written into the contract. The terminal system, the buyer and any auditor must use the same value, product group definitions and calculation sequence, because mixed reference conditions are a frequent source of disputes.
Does a Coriolis meter remove the need for temperature correction?
For sales by mass, yes, because a Coriolis meter measures mass directly. If fuel is sold by standard volume, the measured density still has to be referred to base conditions and the standard volume calculated. The correction does not disappear; it moves to another point in the calculation chain.
If the meter has a type approval, is the whole measuring system compliant?
No. Legal metrology documents set separate maximum permissible errors for the meter and for the complete measuring system. Gas elimination, temperature measurement, the conversion calculation, installation layout and sealing are all part of the system. Which certificates and verifications are needed should be established at the start of the project against the national rules that apply to the measurement.