Custody Transfer: Introduction

The right to define weights, volumes, or lengths that serve as a reference for material goods and to mint coins accepted as payment for these goods is one of the privileges most jealously guarded by the rulers of the oldest societies.

Units of mass, volume, and length remain important today because they determine the price a seller will demand and a buyer will pay in any transaction involving tangible goods. In the past, diverse interests and regional and historical differences led to the development of different systems of units. However, the increasing importance of trade prompted the harmonization of these measurement systems. This development culminated in the introduction of the International System of Units (SI) (m, kg, s), which every country has since used as the basis for its own legal metric systems.

The following illustrates, as an example, how the custody transfer principle operates under the laws of the Federal Republic of Germany. Other countries have their own regulations, but the internationally accepted principles are based on those of the OIML committee.

The objective of the legal control of weights and measures

The purpose of legal control of weights and measures is to protect citizens from the consequences of incorrect measurements in commercial transactions or contracts with the administration, and to propose standard weights and measures in all aspects of the labor, health and environmental fields.

The legislation formulated to achieve these aims establishes the application requirements for measuring equipment and the procedures adopted for measurement and verification. In Germany, for example, these requirements are regulated by EEC and EC directives, the Standards Act, the Standards Ordinance, and the verification requirements and regulations issued by the Federal Physical-Technical Institute (Physikalisch-Technische Bundesanstalt, PTB). This is the country's highest technical authority for the control of weights and measures and engineering safety, and for official notifications related to standards, weights, and measures. The measures defined as a means to achieve these objectives can be divided into preventive and repressive measures.

– Preventive: UPO certification, initial calibration, sealing

– RepressivePeriodic recalibration during the useful life cycle, market control with warning measures or penalties for violations

Custody transfer operations in everyday life

The accountants who ensure compliance with legal custody transfer regulations are present in so many aspects of our daily lives that we often barely notice their presence:

– Scales in warehouses and markets (food products, etc.)

– Pumps at service stations (fuels, lubricants, etc.)

– Domestic water meters (drinking water, hot water, etc.)

– Meters in wastewater systems (wastewater treatment plants, pumping stations, etc.)

– Weighbridges (construction debris, waste, etc.)

In the case of liquid measuring equipment, OIML makes a clear distinction between meters for "water" and meters for liquids "other than water." Other devices are designed for measuring gas flow rates. The range of custody transfer meters is correspondingly broad.

Periodic recalibration during the service life cycle, exceptions to the standard. The validity of a calibration is generally limited to two years. However, the German Standards Ordinance includes a long list of exceptions in the section entitled "General Rules." The important time limits applied to flow meters are detailed in the list in Appendix B.

■ Measurement systems with counters for liquefied gases – 1 year

■ Measuring systems with volumetric counters for milk – 1 year

■ Volumetric meters for hot water – 5 years

■ Volumetric meters for cold water – 6 years (wastewater)

These timeframes are not valid for direct mass flow measurement systems. Coriolis flow meters generally need to be calibrated every twelve months when used to measure liquids other than water.

Meters not subject to mandatory periodic calibration (Standards Ordinance Schedule A):

■ Special volumetric meters and meters for wastewater.

■ Accountants in commercial transactions carried out through distribution lines between fixed partners.

■ Counters for:

– water with a maximum circulation rate of at least 2,000 m3/h,

– other liquids other than water with a maximum circulation rate of at least 600 m3/h,

– heat energy at a rate of at least 10 MW.