This is the question that is on everyone’s mind when a customer has a unique application that requires a custody transfer meter to be the cash register of a business transaction. The seller and the customers need to trust these meters to be accurate. Always!
The preferred custody transfer flowmeter has changed with time, technology and application. Mechanical meters have long been used in most fiscal systems. Positive displacement flowmeters were commonly used for small pipelines, load racks and truck mounted applications. Larger transfer pipelines commonly used differential pressure orifice metering, pneumatic and later electronic systems.
In recent years, linear electronic flowmeters have replaced more and more traditional custody transfer systems. Modem flowmeters, like the Coriolis mass flowmeter, the Promass 84F, for example, has no mechanical moving components to wear or replace over time. The self-contained electronics and the multivariable aspects of Coriolis flowmeters can require less peripheral devices like temperature and pressure compensation sensors. In some applications, meters like the Promass can also eliminate the need for strainers and filters that would otherwise be used to keep metal burrs and solids from mechanical metering systems. Components like strainers, check valves, air and vapour eliminators may still be required regardless of metering technology in order to satisfy national regulations.
The requirements for flowmeters used in trade vary slightly around the world. However, there are requirements that are uniformly applied. Custody transfer, commercial or retail trade, flowmeters must have a redundant security method to prevent tampering with the calibration adjustments. The first level of security is commonly an internal switch or jumper which enables the access to the calibration physical mechanisms or electronic programming. The access to the internal switch, jumper or software programming must also be limited. The second level of security is a security seal that must be broken to show evidence of access. In the past it has been a wire seal applied through an eyelet screw and the housing or other mechanical system. Newer designs of electronic devices incorporate tamperproof reporting software called an audit trail that retains memory of changes made to the meter as well as the date and time of the event.
The custody transfer meters are tested by national metrology laboratories to verify the security of the design and the performance, this process is called type approval. The flowmeter is approved for use in suited applications. A marking label is applied to each meter containing the approval reference information. The flowmeter is then allowed to be sold into the appropriate applications.
Finally, critical service and custody transfer applications require that the flowmeter is proven against a traceable reference standard. In some countries the manufacturer’s calibration certificate is deemed acceptable. In other countries, the flowmeter must be proved as used. The performance requirements vary slightly with national regulations often dependent upon the value of the traded liquid or gas.
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