The custody transfer of petroleum worth $1 × 10~(12) / year is traced to the volume delivered from provers and liquid test measures. Mistakes as small as 0.02% in a custody meter calibration can lead to multi-million dollar corrections in bills. NIST presently uses the direct weighing method to measure the volume delivered from a test measure, by weighing (on a calibrated balance) the test measure when it is full of pure water and again after it has been drained. Recently, NIST reduced the uncertainty of its calibrations of volumes between 3.8 L and 40 L by constructing an automated substitution-weighing standard. The new standard reduces the mass measurement uncertainty by alternately placing 1) the test measure (unknown mass) and 2) approximately equal reference masses on the balance. Automated, pneumatically-driven hardware moves heavy liquid-filled volumes and reference masses on and off the weigh scale, thereby protecting the operator's safety and comfort. We describe the system's design, operation, uncertainty, and repeatability. The new standard was validated by comparison to NIST's well established direct weighing approach and by repeated calibration of a 38 L pipette during more than 2 years. The results are fully consistent with the 95% confidence level uncertainty estimate of 0.007% for the substitution weighing standard. Correlation between temperature and the volume of the 38 L pipette shows the need for improved environmental temperature control in the laboratory. We also describe the design of a set of four pipettes with an overflow filling system that can be used as references in an automated volume transfer standard.
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