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Why periodic calibration of marine custody transfer measurement systems is vital

by , | Nov 22, 2023 | Marine, Measurement Instrumentation

Tom Josefsson

Tom Josefsson

Product Manager Cargo Systems • Marine Level

It is vital to measure the cargo volume with absolute precision on LNG carriers, as imprecise measurements will result in incorrect billing, which can further lead to financial disputes and a lack of trust between buyers and sellers. Radar technology is a proven solution for providing extremely accurate level measurements as part of an onboard custody transfer measurement system (CTMS), in compliance with marine regulations. However, as we explained in an article published by Riviera Maritime Media, extreme temperatures, humidity or accidental dam­age can all cause the measurement accuracy of instrumentation to drift over time, creating the need for calibration. The article states:

 

…To counteract any deviations that might occur, it is essential to calibrate instruments periodically to see if they have drifted. Based on the calibration result, the instruments can then be adjusted as nec­essary to return them to the required accuracy. A convenient method of performing such calibration is by using a reference standard, which in some cases is referred to as a test cable instrument. When using this method, the calibration of the instrument must be traceable to a higher measurement stan­dard and have the appropriate calibration certifica­tion.

 

Radar technology provides extremely accurate level measurements as part of a custody transfer measurement system onboard LNG carriers.

 

Worldwide measurement systems are coordinated by the Bureau International des Poids et Mesures/International Bureau of Weights and Measures (BIPM). The article states that the BIPM defines calibration as:

 

…an operation that “under specified conditions, in a first step, establishes a relation between the quantity values with measure­ment uncertainties provided by measurement stan­dards and corresponding indications with associated measurement uncertainties and, in a second step, uses this information to establish a relation for ob­taining a measurement result from an indication”.

 

In other words, calibration is the act of comparing a device under test of an unknown value with a ref­erence standard of a known value (a measurement standard). Importantly, the BIPM also notes that calibration is sometimes mistakenly considered to involve making corrective adjustments to the device under test. In fact, any necessary correc­tive adjustments are made following calibra­tion.

 

We go on to explain that measurement equipment and systems need to be recalibrated and recertified on a periodic basis. Performing and documenting calibrations for CTMS instruments involves lots of resources and can require significant investment. However, organizations should also consider the consequences of poor calibration or neglecting to maintain the calibration process, which can include contractual disputes and failure to comply with regulations. Therefore, in the long term it can be less costly to make the necessary investment and ensure that calibrations are performed as required.

 

The International Safety Management (ISM) Code – which is mandatory under the provisions of the International Convention for the Safety of Life at Sea (SOLAS) – recommends that all ships should carry a calibration procedure, and have confirmation of com­pliance with that procedure available onboard. Reference instruments must themselves be calibrat­ed at regular intervals at specialized facilities and carry a calibration certificate.

 

Calibration results must be traceable to the inter­nationally defined measurement units. Metrological traceability refers to an unbroken chain of compar­isons relating an instrument’s measurements to a known standard that is directly or indirectly related to national metrological institutes (NMIs), internation­al standards, or certified reference materials. Such traceability provides confidence in the accuracy of the reference instruments and gives assurance that the measurement results are correct.

The article continues:

…Traceability is typically visualized as a calibration traceability pyramid, for which the International System of Units (SI) is found on the top. Just below the SI level of the pyramid, the BIPM works directly with the NMIs of member countries to facilitate the promotion of the SI units within those countries. However, it is not always affordable or efficient for organizations to work directly with an NMI. The NMI-level standards are used to calibrate primary standards or instruments; primary standards are in turn used to calibrate secondary standards; secondary standards are used to calibrate working standards and working standards are used to cali­brate process instruments. In this way, references to the SI standards can be efficiently passed down the calibration pyramid through the NMI.

 

Traceability is typically visualized as a pyramid, demonstrating the calibration hierarchy from the realization of the SI units down to the measurements performed by process instruments.

 

Visit here to learn how Emerson’s marine custody transfer systems enable you to measure and report custody transfer with absolute precision.

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The opinions expressed here are the personal opinions of the authors. Content published here is not read or approved by Emerson before it is posted and does not necessarily represent the views and opinions of Emerson.