## 1 Introduction
For instance, when a refinery or storage facility measures crude oil at a high ambient temperature, they use ASTM D1250 tables to adjust the observed volume to a standard reference temperature of 60 \({}^{\circ}\)F (15.6 \({}^{\circ}\)C) [16]. For consistent operations, uniform measurement methods and instruments of consistent accuracy must be applied, along with unified metrological supervision. However, while the application of standardized measurement methods and instruments is vital, it does not inherently resolve the challenge of method selection and application across the entire supply chain, especially in individual stages of oil purchase and sale, where methods may differ based on operational needs. The implementation of international, national standards is not mandatory until it falls within national legal acts (cited standards, standard identification number, name provided) or international agreements are signed for their implementation [17; 18]. Despite ongoing efforts by international standardization organizations and specialized bodies to harmonize metrology guidelines, national legislation often lacks consistent [19], clear requirements for mass in air or vacuum, as well as clear guidelines for conversion of calculations and presentation of results when using different measurement principles [20; 21]. In Europe, the standard pressure and temperature reference conditions for measuring petroleum products, oils, greases, and gases are 101.325 kPa and 15 \({}^{\circ}\)C [22; 23; 24]. In Canada and Australia, however, different standard pressures may be used when measuring petroleum products [25; 26]. Norway sets the accuracy with which mass or volume must be measured and regulates when corrections must be made; for instance, a correction is required if the deviation exceeds 0.02% of the total volume [27]. Legal requirements for expressing measurement results aren't always consistent or logically justified. For example, accounting rules [28] might demand converting liquefied petroleum gas (LPG) mass to volume at ambient temperature, even when LPG excise duties [29] are based on tonnes. This creates unnecessary recalculations and potential discrepancies. An analysis of measurement result protocols and applicable oil volume regulations at a specific company, conducted during a particular project, revealed an issue: mass isn't consistently converted to volume at ambient temperature, despite this being a requirement of petroleum product accounting rules [30]. These discrepancies can lead to inaccurate inventory management, erroneous financial reporting, and potential regulatory compliance issues. Regulatory and methodological uncertainty at the national level is also a shortcoming of current practice, preventing full transparency and comparability throughout the oil supply chain. Figure 1 illustrates a typical, generalized oil processing workflow for a medium-sized regional oil refinery. In Stage 1, the raw material is received through pipelines (where mass is determined using a volumetric method) as well as via rail and trucks (where mass is weighed, and volume is not calculated). Stage 2 involves processing activities, resulting in the production of fuel oil, diesel, LPG, and bitumen, which are then stored in tanks. The internal processes within Stage 2 use the volumetric method to measure volume. Some measuring instruments provide volume measurements, while others quantify in kilograms or tonnes (with a summary provided in tonnes). In Stage 3, petroleum products, LPG, and bitumen are sold by weight, which is determined through weighing. The volume of light petroleum is measured and then converted into mass for final sale and transport in tank trucks. As evident from this workflow, various measurement methods and expressions of quantity are employed. Therefore, it is crucial for all involved parties to reach a consensus on the measurement of quantity, submission of results, and the conversion between mass and volume.