Custody transfer measurement is the measurement that buyer and seller accept as the basis for invoicing the quantity loaded into a truck. For a custody transfer loading skid, the central question is therefore not how precise the meter is, but how the complete measuring system is verified, which reference conditions the result is converted to, and how the process is recorded and protected against unauthorized change. This page deals with measurement governance and proving rather than the pump-to-arm loading chain.
Process measurement, custody transfer and legal metrology
· Process measurement: used to control loading and for internal tracking; it has no commercial consequence between parties.
· Custody transfer: the result is the basis of the sale and the invoice; method, proving and record rules are set by contract and industry standards.
· Legal metrology: applies when the measurement is under statutory control; requirements such as type approval and initial and periodic verification of the measuring system follow national regulations.
The same loading line can fall under both contractual custody transfer and legal metrology, and which applies depends on national law and the contract. EI HM 11, the code of practice for proving gantry metering systems, states plainly that local conditions, regulations and contract terms may take precedence over its guidance.
Meter accuracy is not system uncertainty
Legal metrology documents set separate limits for the meter and for the complete measuring system. Annex MI-005 of the EU Measuring Instruments Directive (MID) places measuring systems for loading and unloading road and rail tankers in accuracy class 0.5. For that class, OIML R 117-1 sets a maximum permissible error of 0.5 % for the complete measuring system and 0.3 % for the meter itself. The margin between the two is there for contributions from gas elimination, temperature and density conversion, installation and calculation. A meter accuracy figure from a manufacturer's datasheet is a meter-level value under reference conditions and should not be presented as the performance of the skid. Some applications, such as liquefied gases under pressure, fall into different accuracy classes.
Components of a custody transfer skid
Component | Role in custody measurement | Design note |
Strainer | Protects the meter from solid particles | Monitor pressure drop and leave access for cleaning |
Gas eliminator | Keeps air or vapor out of the meter; gas elimination is listed among the elements of the measuring system | Critical on first loads after a line has drained and when suction conditions are poor |
Meter | Primary measurement of quantity | Chosen by viscosity, flow range, number of products and proving method |
Temperature, and where needed pressure and density measurement | Input for conversion to reference conditions | Measuring point close to the meter and representative |
Control valve and preset batch controller | Delivers and stops the quantity and records the transaction | Cut-off accuracy and closing behavior at the end of the batch |
Proving connections | Allow a prover or master meter to be connected | Leak-check capability on isolation valves and access |
Sealable adjustment and protected software | Prevents unauthorized change | Event log of parameter changes |
The key distinction between meter technologies is the quantity they measure. Coriolis meters measure mass and density directly and calculate volume from them; positive displacement and turbine meters measure volume and need temperature, and where required pressure and density, for conversion. Product viscosity, flow range, the number of products sharing a meter and the proving method are weighed together; no single technology is right for every product.
Conversion to reference temperature
Liquid volume changes with temperature, so custody quantities are usually converted from metering conditions to volume at a defined reference temperature, or to mass. OIML R 117-1 distinguishes metering conditions from base conditions and treats the conversion device as part of the measuring system. Temperature and pressure correction factors for crude oils, refined products and lubricating oils are given in API MPMS Chapter 11.1, which excludes LPG and NGL. API MPMS Chapter 12.2 standardizes calculation sequence and rounding so that the result is the same regardless of who performs the calculation. The reference temperature itself is set by national regulation and contract and must be fixed early in design.
How the quantity is verified
At defined intervals the meter reading is compared with a known reference to establish or confirm the meter factor. The main methods used at truck loading racks are:
· Meter prover: product passing through a section of known volume is compared with the meter reading; provers can be fixed or portable.
· Master meter: a proven meter is run in series with the duty meter; some skids include a dedicated master meter and crossover piping for periodic verification of duty meters.
· Volumetric proving tank: the metered quantity is delivered into a vessel of known volume and the two values are compared.
EI HM 11 provides good-practice guidance for proving metering systems at road and rail loading gantries and also covers verification of secondary instrumentation and flow computers. The chosen method shapes the skid: prover connection points, leak-checkable isolation valves and access for a portable prover must be planned from the start. The defined minimum measured quantity of the system also needs review where small compartments or low-volume deliveries are common.
Sealing, records and audit trail
Under OIML R 117-1 the meter adjustment device must be sealable, and adjustment by bypassing the meter is prohibited. In electronic systems the equivalent is authorization-protected metrological parameters and a time-stamped event log of every change. API MPMS Chapter 21.2 covers auditing, reporting, verification and security of electronic liquid measurement systems used for custody transfer. A delivery ticket typically shows product, gross and corrected quantity, average temperature, density where relevant, meter identity and transaction number; the final list of mandatory fields is set by regulation and contract.
Approval is not limited to one device
A type approval for the meter or the controller does not, on its own, show that the whole skid is fit for custody transfer. The measuring system is assessed together with gas elimination, conversion, installation and proving arrangements, and approvals are often tied to a specific configuration. A similar principle exists in explosive atmosphere legislation: in the EU, whoever combines equipment into a functional assembly must assess any additional ignition hazards arising from the combination. Which approvals are needed and who will be the applicant should be settled at project start.
How TLY Enerji contributes
Depending on project scope, TLY Enerji provides engineering support in selecting meter technology and proving approach to match product and commercial requirements, supplying meters, temperature and density measurement, valves and control equipment, and in system integration, installation, testing and commissioning. Legal metrology verification and calibrations requiring accreditation are carried out by authorized bodies; these steps are defined separately in the project plan.
Information needed for a technical review
· Products and the viscosity, density and temperature range of each
· Sales unit: volume at reference temperature, mass or another unit
· Applicable regulations, expected accuracy class and approval requirements
· Preferred proving method and frequency; any existing prover or master meter
· Flow range per arm and the smallest compartment volume
· Delivery ticket and record format; the automation system to be connected
· Whether a new skid is planned or an existing bay is being converted to custody metering
Related pages
· Truck loading systems: The loading chain in which the custody skid sits.
· Automated truck loading terminals: How load records connect to terminal automation and ERP.
· Fuel custody transfer metering systems: Volume correction and sales practice for fuel products.
· LPG custody transfer metering systems: Sales units and vapor return for liquefied gas under pressure.
· Custody transfer natural gas metering stations: Governance of custody measurement in the gas phase.
Frequently asked questions
Is a meter with type approval enough for custody transfer?
No. The meter's approval applies to the meter. What is assessed for custody transfer is the complete measuring system, including the gas eliminator, temperature and density measurement, conversion calculation, control valve and record system. Where legal metrology applies, the measuring system may also need its own assessment and verification, and the procedure for that differs from country to country.
Should a Coriolis meter or a positive displacement meter be chosen?
There is no single right answer. Because Coriolis meters measure mass and density directly, they suit applications sold by mass or where product density varies. Positive displacement meters measure volume and have a long track record within suitable viscosity ranges. Product properties, flow range, proving method, maintenance approach and the existing meter population should all be considered together.
How often should the meter be proved?
Frequency depends on regulation, contract and operating conditions; there is no universal interval. A product change, a significant shift in flow range, recommissioning after maintenance or an unexpected drift in meter factor can all call for additional proving. Industry guidance recommends trending proving results and periodically checking secondary instruments as well.
Why is custody transfer handled differently for LPG loading?
LPG is a liquid under pressure and can vaporize when pressure drops. OIML R 117-1 requires liquefied gases under pressure to remain liquid in the meter and allows a vapor connection between tanks only if the returned gas is measured and subtracted; for some system types the connection is prohibited altogether. The correction tables used for refined products also exclude LPG, so the LPG-specific page should be consulted.