Verification scheme · Legal Shield
The verification scheme is a mandatory annex to a metering system project
It is not a formality. An approved verification scheme answers the key questions of metrological assurance in advance and becomes the evidence base of custody transfer.
On site or in a laboratory, with which means and at what intervals.
Which measurement standards and verification means are needed before the measurement station is put into operation.
Which accredited laboratories and metrology institutes to involve in advance.
The scheme must be approved. In practice it is often neglected, and in a dispute its absence becomes the other party's main argument: traceability is not proven — so the measurement results cannot be trusted. Why the scheme is the basis of protection is explained on the Legal Shield page.
Example from practice
Let us take a real metering system
A crude oil measurement station at one of the facilities of a transit corridor. Confidential data are omitted; the technical substance is preserved.
- 5 metering lines: 3 duty and 2 standby
- This arrangement keeps the flowmeters within their optimal operating range and allows lines to be switched to suit the pumping conditions.
- Coriolis mass flowmeters and P and t sensors on every line
- The mass flowmeter is the main measuring instrument for mass; the sensors provide corrections for measurement conditions and condition monitoring.
- Inline densitometer
- The source of density ρ for calculations and checks.
- Stationary compact prover
- Part of the measurement station. The mass flowmeters are verified against it on site, without removal.
- Flow computer
- Collects signals, calculates mass, generates the archive and reports. Two modes — custody transfer and technical/verification — so that transient and unstable conditions do not enter custody transfer reporting.
±0.25 %
During verification against the prover, mass is made up of volume and density, with corrections for pressure and temperature:
M = V · ρ
The traceability chain therefore runs along three branches at once: volume, density, electrical signals and P/t. Each branch contributes to the final ±0.25 %.
How such a station is verified step by step — metering system verification: a 7-stage chain. How the scheme is built into the design — metering system engineering and design.
Scheme
The verification scheme of this metering system
Three levels and three branches. Every arrow is a document: a verification certificate, a calibration certificate or a measurement record.
The metering system frame includes only the instruments directly involved in generating custody transfer data. The scheme shows the principle; for a specific project it is developed and approved separately.
Verification protocols for every link — module EM-13; the full verification scheme with analysis — module EM-12. Both are in the EM Technical Guide catalog. Who verifies the working standards and how — metrology laboratories.
Traceability channels
What is verified against what
Four channels generate the legally significant custody transfer result: volume, density, pressure and temperature, and the flow computer. For each — what it is verified with and how traceability is ensured.
Volume channel V
Compact prover · volume Vi
Density channel ρ
Inline densitometer · density ρ
Pressure and temperature channel P, t
P and t sensors · flow computer analog inputs
Pro meter rule. The flow computer's analog P and t inputs are calibrated together with the sensors — as a single measurement channel: sensor → line → analog input.
Flow computer
Signals and calculations
Pro meter rule. All inputs are calibrated except the analog P and t inputs — these are calibrated together with the sensors in their own channel.
This is a condensed version. The full list of procedures, protocols and the method for eliminating systematic components — on request.
The essentials
Error and uncertainty — two languages of one metering system
One verification protocol, one data set — and two different figures. This is not a mistake: this is how two regulatory systems work. What matters is knowing which figure goes into which document.
The language of custody transfer: contract, verification protocol, custody transfer report.
The language of the laboratory and analysis: calibration certificate, laboratory report.
The language of evidential defensibility: every figure is supported by its own document.
Error, MPE
- A normalised characteristic: sets the permissible limits of deviation of the result.
- Established at type approval, confirmed at verification, written into the contract.
- Regulatory basis: OIML R 117, GOST R 8.595, MI 3272.
- Documented by the service organization.
Uncertainty, U
- An estimate of how far a specific measurement result can be trusted.
- Stated as U, k, P — usually k = 2, P = 95 %.
- Regulatory basis: GUM (JCGM 100), ISO/IEC 17025.
- Documented by the accredited laboratory.
The mass flowmeter verification protocol from our example:
δ = 0.206 %basic relative error per MI 3272, P = 95 %
U = 0.037 %expanded uncertainty at k = 2, P = 95 %, from the laboratory certificate
The difference is more than fivefold. The metering system does not become more accurate — it is another language describing the same reality. The engineer's task is to apply the right method in the right document, not to choose the "better" figure.
What goes where
| Document | Method | Documented by |
|---|---|---|
| Custody transfer report | δ, MPE | Service organization |
| Metering system verification protocol | δ, MPE | Service organization |
| Laboratory certificate and report | U, k, P | Accredited laboratory |
| Engineering report, audit | Both — with the difference explained | Service organization, all parties |
The word "uncertainty" in verification protocols and custody transfer documents is a trap for lawyers. It must not appear there.
The standard deviation of a measurement series is not the uncertainty of the metering system, only one of its components. Such a substitution understates the real estimate more than tenfold.
Full analysis with formulas and the error budget — module EM-11 in the catalog. Terms in three languages — EM-20 glossary. How to find and eliminate systematic errors — module EM-14.
Engineering and design
The scheme is built into the project — at the metering system certification stage
For each facility its own, regional verification scheme is developed: for the specific equipment, procedures and conditions. In its metrological characteristics it must comply with the national schemes.
The base scheme is not dogma
Other instruments with metrological characteristics no worse than the approved ones are permitted. What matters is clear traceability from the figure in the custody transfer report to the 1 kg mass standard.
Requirements for the scheme
- Developed and agreed before custody transfer operations start.
- Approved by all parties: Supplier, Buyer, service organization, laboratory.
- Kept at the facility and available to commissions and auditors.
- Included in the contract as an integral annex.
Who holds the measurement standards
Reference instruments are expensive, so it is reasonable to distribute them among the parties without artificial restrictions. The distribution, the work schedule and the responsibility boundaries are fixed in the contract with the laboratory.
Pycnometers, equipment for flow computer verification, auxiliary means.
Set of weights, reference balances, analytical mass comparators.
Balances for verification of the field standard test measure, pressure and temperature calibrators.
Given good working relations — use the working standards of the national metrology centre. The scheme becomes one link shorter, and one systematic component leaves the error budget. This is a real engineering saving, not marketing.
How the scheme becomes part of the design documentation — metering system engineering and design. What to put into the technical specification — measurement station tender. The full scheme with traceability of every parameter — module EM-12 in the catalog.
Engineering value of the scheme
Every link is a source of systematic error. The chain is shortened deliberately
The scheme and the channel table are a working map for the lead engineer, metrologist, designer and metering system developer. The longer the unit dissemination chain, the larger the total contribution of its links to the systematic component of the result.
Practical goal
To see where the chain can be shortened and so reduce the total systematic component — including by removing hardware redundancy and unnecessary comparisons.
Scope of this section
Here systematic error is considered only in the context of the verification chain and work with measurement standards. Installation conditions, operating conditions, hydraulics and operational factors are separate topics.
A compact prover with a volume of 250 litres: the number of pulses between the "start–stop" signals turned out to be limited. To compensate, the factory configuration included an additional link — a turbine meter comparator.
In the project a technical solution was found that increased the pulse count without an additional measuring link. The turbine comparator became metrologically redundant: one hardware link left the verification chain — and with it, its systematic component. The released resources were directed, in agreement with the manufacturer, to other engineering and service work in the project.
The solution has been implemented and confirmed only for a 24-inch compact prover. For provers of other sizes and configurations — only after engineering analysis and consultation with Pro meter specialists.
The task of a verification scheme is to manage the length of the chain and its systematic error deliberately, not to add instruments "by default". Methods for eliminating systematic errors — module EM-14. To discuss the scheme for your facility — contact.
Legal Shield · risks
Without an approved scheme, the dispute is lost in advance
In a dispute, an audit or a joint commission, the chain is checked in reverse — from the custody transfer report to the measurement standard. The absence of a verification scheme is the other party's main argument: traceability is not proven, so the measurement results cannot be trusted.
- Custody transfer report
- Metering system reports
- Verification protocols
- Instrument certificates
- Laboratory certificates
- Measurement standards
A break in any link is a vulnerability that the other party's lawyer will find on the first day of the audit.
What the auditor checks first
- Existence of the verification scheme and its agreement by all parties. Traceability.
- Applied procedures and protocol forms: approval, agreement, registration number.
- Identification of the facility and instruments in protocols and certificates: S/N, type, installation point.
- Working conditions and reproducibility: temperature, pressure, flow rate, operating conditions.
- Archive: shift and daily reports, density, event logs, data during shutdowns.
- Contracts of the service organization and the laboratory: schedules, responsibility boundaries.
- No verification scheme at the facility, or it is not agreed by all parties.
- No agreed protocol forms. A protocol made "as convenient" is not an evidential document.
- The verification certificate has expired, yet the reports for that period have been signed.
- No approved procedure for force majeure: shutdowns, shifted maintenance and verification schedules.
- Water for the water draw without a certificate and analyses of density and salts — an additional systematic error of 0.1 % or more.
- The Supplier delivers by MPE, the Buyer accepts by U. In a transit corridor, the cost of the issue is hundreds of millions of dollars a year.
For lawyers and customs authorities, the absence of a verification scheme is a real gift. Audits have found facilities with no scheme at all. In transit corridors, customs issues of using foreign equipment and measurement standards come on top of this.
How the evidence base is protected — Legal Shield. Protocol forms and their legal function — module EM-13 in the catalog. Verification step by step — a 7-stage chain.
Traceability partners
Laboratories and metrology institutes: build the links before start-up, not after
An accredited laboratory verifies the working standards, issues reports and certificates, and takes part in dispute-resolution measurements. Its document must be fit to serve as part of the evidence base.
What the certificate must contain
- A correct, approved document form.
- Identification of the facility and instruments: type, S/N, installation point.
- Measurement conditions and the applied procedure.
- Data on the measurement standards and their traceability.
- The result in the U, k, P format. Without these three parameters the certificate is incomplete.
What to fix in the contract
- A work schedule agreed with the dispatch service.
- Responsibility boundaries of the parties.
- Mutual use of measurement standards.
- Procedure for force majeure and extension of certificates — approved in advance.
Initial verification at a border oil product measurement station required simultaneous agreement with the national metrology laboratory of Latvia and with the metrology institute of a neighbouring country. Direct working contacts with both organisations allowed verification without delay: issues that normally take a month or more were closed within 2–3 working days.
Without such links — months of administrative deadlock.
How to develop a laboratory and extend its scope of accreditation — metrology laboratories, module EM-8 in the catalog. Metrology across the full project lifecycle — metrological assurance.
Regulatory basis
The documents the scheme relies on
Public documents describing the logic of traceability and hierarchical schemes — from top-level documents down to working measuring instruments. The edition in force is checked in every project.
- InternationalOIML D 005:2022 — metrological traceability and hierarchical schemesPrinciples for building chains: measurement standards → working standards → measuring instruments, link to uncertainty.
- InternationalOIML R 117 · GUM (JCGM 100) · API MPMSMeasuring systems for liquids, evaluation of uncertainty, custody transfer practice.
- EUEURAMET — calibration guidelinesProcedure structure, reporting formats, best practice by measurement field.
- RUFederal Law No. 102-FZ "On Ensuring the Uniformity of Measurements"Legal basis of metrological assurance and traceability.
- RUGOST R 8.595 · MI 3272 · PR 50.2.006Mass of crude oil and oil products, metering system verification, organisation of verification.
A list of documents for your facility and field of application — on request.
Next step
We will develop the verification scheme for your project
The verification scheme is the technical and legal foundation of custody transfer. Pro meter develops it for your equipment and procedures, agrees it with the parties and the laboratory, and supports its approval.
What you get
- A project verification scheme with traceability of every parameter.
- Distribution of measurement standards among the parties and a verification schedule.
- Agreed protocol forms.
- Links with an accredited laboratory and a metrology institute.
EM Technical Guide modules
EM-11 — error and uncertainty, EM-12 — verification scheme, EM-13 — verification protocols. Modules in development can be ordered out of turn, and a set of modules can be ordered for your project.
Examples of work — without disclosing clients
- The verification chain was aligned with regulatory documents and instrument data sheets.
- Systematic mass discrepancies were eliminated to a level acceptable for custody transfer operations.
- The risk of claims during inspections and custody transfer operations was reduced.
- Audit of the source data and the logic of volume, mass and energy calculations.
- Documentation package: traceability scheme, procedures, control protocols.
- Clear responsibility boundaries and acceptance criteria for results.
- Traceability principles adapted to the client's process.
- Standardised procedures for checks and confirmation of instrument fitness.
- Fewer "grey areas" in documentation, better reproducibility of results.
The principles of measurement systems are universal: oil and gas, energy, trade measurements, medicine, laboratories — provided the facility's process flow diagram is available.
Discuss your project →To get a to-the-point answer, state the product type, the number of lines, the list of instruments and what currently raises questions.
This material contains Pro meter recommendations based on metering system operating practice. It does not replace mandatory regulatory documentation: always apply the current editions of documents.