Verification for custody transfer: what to know before the project starts
When product passes through a measurement station — crude oil, petroleum products, natural gas, biogas or biomethane — every measurement becomes the basis for settlement between the parties. Verification of measuring instruments used in custody transfer is therefore never left to the discretion of the system owner. It is strictly regulated at three levels. The International Organization of Legal Metrology (OIML) sets requirements for measuring systems in its Recommendations. The European Union establishes them in the Measuring Instruments Directive 2014/32/EU (MID). Each country adopts its own metrology law: in Latvia this is the law “Par mērījumu vienotību” (“On Uniformity of Measurements”), which requires measurement results to be traceable to international standards.
The requirements exist, yet no single document describes the whole path from start to finish. The norms are spread across recommendations, standards, laws and procedures, and each project participant sees only their own part. In this material Pro meter sets out the complete verification chain with its main stages: what happens at each stage, where the risks arise and how to close them. The international and national norms, recommendations and laws are collected at the end.
This material is written for operators of measurement stations, design organizations, equipment suppliers and metrology laboratories — and above all for those building a biogas or biomethane metering system for the first time.
Measurement station and metering system: what you are actually ordering
A metering system is the set of measuring instruments, flow computers and software that produces the measurement result for product quantity and quality parameters. A measurement station is the facility as a whole: the metering system together with the process piping, filters, sampling, shut-off valves, the connection point for the reference standard, power supply and operating conditions. The supplier delivers the system. The result accepted by the parties comes from the measurement station as a whole — under the real conditions of a specific site.
In the Baltic States this question is new for many operators. Things often happen in this order: the operator orders a metering system as a package, the supplier delivers it, then a site is found, the system is installed — and only after that does certification begin and verification questions arise. At that point it turns out that there is no room for the reference standard, no connection points, and the measurement conditions do not match the procedure. Some of these issues are then settled administratively. An administrative decision does not make measurements more accurate: it postpones the problem until the first dispute over product quantity.
A measurement station is designed together with its verification procedure — before the equipment is purchased. More in the modules EM-3 “Measurement Station Engineering”, EM-5 “Pre-Design Stage” and EM-6 “Design”.
Error: the price of one percent
Measurement error is the deviation of a measurement result from the true value of the quantity. In custody transfer the maximum permissible error is specified for the system as a whole, in addition to each individual instrument. The system error is made up of the errors of all channels: flow or mass, pressure, temperature, product density or composition, and the flow computer. An accurate flowmeter in a system with an unverified temperature channel does not deliver an accurate result.
Without competent metrological analysis, hidden error can be several times larger — even when every measuring instrument works flawlessly. The methods for this analysis are collected in the module EM-14 “Methods for Improving Measurement Accuracy”.
Error and measurement uncertainty are also treated separately: international documents require uncertainty to be evaluated, while national procedures often work with error. The project must decide in advance in which terms the parties will calculate and prove the result.
More in the module EM-11 “Error and Uncertainty”.
Verification scheme: where confidence in the result comes from
The verification scheme shows from which standard, and through which steps, the unit of measurement is transferred to the working measuring instrument at your measurement station: from the national or international standard to the standards of accredited laboratories, from them to the working standards (prover, master mass flowmeter, pressure calibrator), and only then to the working measuring instruments. Transfer methods and permissible errors are defined for each step.
The verification scheme must be approved. This is a question of traceability: the law requires the measurement result to be linked to international standards through an unbroken chain of comparisons. If the chain is not documented, the measurement result is legally vulnerable, however accurately the equipment works. Most operators learn this only at the first verification, when it is too late to change the scheme.
More in the module EM-12 “Verification Scheme”.
The composition of the measurement station, the maximum permissible error of the system and the verification scheme are defined before the equipment is purchased. This is the stage at which a metrologist’s involvement costs least and brings the greatest benefit.
The verification chain: seven stages
Below is the complete path of a new measurement station from the technical specification to the report. Each stage builds on the result of the previous one: an error at an early stage carries over into all the following ones.
Why we verify the whole channel
A temperature or pressure measuring channel consists of a sensor, a transmitter and a flow computer. The approved measurement and verification procedures issued by institutes describe these instruments separately: the sensor on its own, the flow computer on its own. The laboratory must follow the document strictly, so it verifies them separately.
On site the picture is different. When a calibrator is connected to the channel, the flow computer performs its own channel adjustment procedure provided by its firmware. After this procedure, the results of the separate verifications of the sensor and the flow computer no longer describe how the channel actually works. In practice, the characteristic of the whole channel is determined, although the documents are issued for individual instruments.
In our years of work with national regulatory frameworks, we have not come across an approved procedure for verifying a measuring channel as a whole. The instruments exist and so do the requirements for them; a document treating the channel as a single measuring instrument does not.
At the same time, this approach is consistent with international practice. OIML Recommendation R 117 provides for verification of the sensor together with the indicating device. In custody transfer metering systems, the role of the indicating device is performed by the volume corrector or the flow computer.
This is why Pro meter works to its own procedure: we verify the channel as a whole, from the sensor to the result in the flow computer, under real measurement conditions and without stopping the process. For each site we write a working procedure with a checklist for the entire operation, taking into account the installed equipment and local conditions. A laboratory does not always have a full set of reference standards for such work on site: Pro meter provides the missing standards with valid certificates, and the work is carried out jointly with the laboratory. The laboratory certifies the measurement results and thereby recognizes the procedure as well. A detailed analysis with protocols is in the module EM-21 “Metering System Verification”. We also support laboratories as a separate direction — metrological assurance for metrology laboratories.
Conclusions: where projects lose money
Each stage of the chain looks like a formality until the first dispute over product quantity. Simplifications at an early stage — a standard procedure instead of a project-specific one, instrument certificates instead of type approval of the system, protocols without formulas — shift the costs to rework, recertification and claims between the parties.
This is why three arguments must come first in a tender for a metering system. The first is certified service companies with proven experience. The second is a competent feasibility study (more in EM-2 “Tender and Procurement”). The third is sound technical solutions. First establish whether the system will live and work; only then compare prices.
The approach “win the tender first, find the specialists later” fails. Experience proves it: what is saved in the tender comes back as the cost of rework and disputes, and commissioning dates slip.
The opposite result is measurable too. Metering systems built and maintained according to these rules operate without major overhaul until the equipment becomes obsolete — 18 years of operation against an average service life of 4–5 years. This is confirmed by project documentation.
A separate side of our work is Legal Shield: metrological audits, private consultations and documents that hold up under scrutiny in a dispute. Customers who worked with us on the basis of these documents have not lost international disputes.
Ideally at the technical specification stage. But we join at any stage: we review the decisions, procedures and protocols already in place and show where the risks are. The full verification cycle — from preparation and measurement to acceptance, evidence and corrective action — is set out in EM-21 “Metering System Verification”. Modules in development can be ordered out of turn, as can a set of modules for your project — EM Technical Guide.
What we do beyond standard verification
Some of Pro meter’s work goes beyond standard procedures. Below is a short overview of what we apply on site. A detailed analysis with formulas and protocols is in the EM modules.
System error budget
Software-based determination of the metering system error budget — an assessment of each channel’s contribution to the overall result. It shows where the system loses accuracy before the customer sees it.
More in EM-11 “Error and Uncertainty” and EM-14 “Methods for Improving Measurement Accuracy”
Verification of measuring channels
Pro meter has developed procedures for verifying parameter measuring channels as a whole. How this works is explained above, in the section on channel verification.
Protocols for the task
We develop protocols for initial, periodic and unscheduled verification, for scientific work and R&D, and for building models of metering systems. Extended versions allow mathematical and metrological analysis.
More in EM-13 “Verification Protocols”
Other types of measurement
Compact provers, reference tanks and volume measures, weighing instruments and weights, in-line and laboratory density meters, gas chromatographs, flow computers. For each type we have our own working methods, built up over years of work on transit corridors.
More in EM-8 “Laboratories” and EM-16 “Maintenance of Metering Systems”
Protocol templates with explanations are available on request.
Measurement station и metering system: что вы на самом деле заказываете
Metering system (система измерений) — это совокупность средств измерений, вычислителей и программного обеспечения, которая выдаёт результат измерений количества и показателей качества продукта. Measurement station (узел учёта) — это объект целиком: система измерений вместе с технологической обвязкой, фильтрами, пробоотбором, запорной арматурой, местом подключения эталона, электропитанием и условиями эксплуатации. Поставщик поставляет систему. Результат, который признают стороны, выдаёт узел учёта в целом — в реальных условиях конкретной площадки.
В Прибалтике этот вопрос для многих операторов новый. Часто всё происходит в таком порядке: оператор заказывает metering system как комплект, поставщик его поставляет, затем ищут площадку, устанавливают — и только после этого начинается сертификация и появляются вопросы поверки. Тогда выясняется, что для эталона нет места, нет точек подключения, условия измерений не совпадают с методикой. Часть вопросов пытаются решить административно. Административное решение не делает измерения точнее: оно откладывает проблему до первого спора о количестве продукта.
Узел учёта проектируют вместе с методикой поверки — до закупки оборудования. Подробнее — в модулях EM-3 «Инжиниринг узла учёта», EM-5 «Предпроектная стадия» и EM-6 «Проектирование».
Погрешность: цена одного процента
Погрешность — это отклонение результата измерений от действительного значения величины. Для коммерческого учёта нормируется допускаемая погрешность системы в целом, а не только отдельного прибора. Погрешность системы складывается из погрешностей всех каналов: расхода или массы, давления, температуры, плотности или состава продукта, вычислителя. Точный расходомер в системе с неповеренным каналом температуры не даёт точного результата.
Без грамотного метрологического анализа скрытая погрешность может быть в разы больше — даже при безупречной работе средств измерений. Методы такого анализа собраны в модуле EM-14 «Методы повышения точности измерений».
Отдельно различают погрешность и неопределённость измерений: международные документы требуют оценивать неопределённость, национальные методики часто работают с погрешностью. В проекте нужно заранее решить, в каких понятиях стороны будут считать и доказывать результат.
Подробнее — в модуле EM-11 «Погрешность и неопределённость».
Поверочная схема: откуда берётся доверие к результату
Поверочная схема показывает, от какого эталона и через какие ступени размер единицы передаётся рабочему средству измерений на вашем узле учёта: от национального или международного эталона — к эталонам аккредитованных лабораторий, от них — к рабочим эталонам (пруверу, эталонному массомеру, калибратору давления), и только затем — к рабочим средствам измерений. Для каждой ступени определены методы передачи и допускаемые погрешности.
Поверочная схема должна быть утверждена. Это вопрос прослеживаемости: закон требует, чтобы результат измерений был связан с международными эталонами непрерывной цепью сравнений. Если цепь не документирована, результат измерений юридически уязвим, как бы точно ни работало оборудование. Большинство операторов узнаёт об этом только при первой поверке, когда менять схему уже поздно.
Подробнее — в модуле EM-12 «Поверочная схема».
Состав узла учёта, допускаемая погрешность системы и поверочная схема определяются до закупки оборудования. Именно на этом этапе участие метролога дешевле всего и приносит больше всего пользы.
Norms, recommendations and laws
The documents on which the verification chain is based.
This is a basic list, current at the date of publication. Before starting work, check the editions in force and any new documents. In the biogas sector, for example, the legislative and metrological framework is only taking shape and is regularly being extended.
Tell us what stage your measurement station is at — we will review the decisions, procedures and protocols and show you where the risks are. Contact Pro meter.