A small-volume prover displaces so little fluid that only a handful of meter pulses fall within its calibrated volume, and rounding to whole pulses would introduce huge error. Double chronometry solves this with two synchronized timers that let the system resolve fractional pulses. This guide explains the technique, why compact provers need it, and how the interpolated counts feed the meter factor calculation.
Double Chronometry (Pulse Interpolation) in one line: Double chronometry is a pulse-interpolation timing method that lets a small-volume prover measure a fractional number of meter pulses. Two synchronized clocks time both the whole meter pulses and the exact prover displacement window, and the ratio of those times converts a whole-pulse count into a precise fractional count.
Classic pipe provers work by whole-pulse counting: their swept volume is large enough that ten thousand or more meter pulses accumulate as the displacer travels between detector switches, so counting whole pulses gives a resolution better than a hundredth of a percent. A compact or small-volume prover, by contrast, has a tiny displaced volume, and only a small number of pulses may occur during a pass. Counting only whole pulses there would round away a fraction that represents a large slice of the total, making the result far too coarse for custody measurement.
Double chronometry, described in the ISO 7278-3 timing approach and its API equivalents, removes that limitation by measuring time instead of relying on pulse count alone. Rather than needing thousands of pulses, the prover can resolve the exact position of the prover volume relative to the pulse train, effectively recovering the fractional pulse that whole-pulse counting throws away. This is what makes small, portable provers viable for high-accuracy proving.
The method runs two timers off the same clock. The first timer measures the time for a whole number of meter pulses that comfortably brackets the prover's displacement. The second timer measures the time of the prover displacement itself, from the moment the displacer trips the first detector to the moment it trips the second. Because both intervals are timed against the same high-frequency reference, the ratio of the two times, multiplied by the whole-pulse count, yields the equivalent fractional number of pulses that corresponds exactly to the prover's calibrated volume.
In effect, the two timers let the system ask how far into a partial pulse the displacer was when it started and stopped, and scale accordingly. The accuracy of the interpolation depends on a fast, stable timer clock and on clean, jitter-free detector and pulse signals, since any timing error propagates straight into the interpolated count. This is why compact provers pair double chronometry with high-resolution timing electronics rather than the simple pulse counters a large pipe prover can get away with.
The interpolated fractional pulse count is what the proving computer uses in place of a raw whole-pulse count when it calculates the meter factor. The prover's certified base volume, corrected for temperature and pressure, is divided by the indicated volume the meter would have registered from that interpolated count, giving the meter factor for the run. From there the acceptance and repeatability logic is identical to any other proving: runs repeat until a consecutive set falls inside the accepted spread.
A cloud SCADA platform records the outputs of that timing computation - the interpolated counts, the run-by-run factors, and the temperature and pressure corrections - as tags read from the proving flow computer, rather than performing the microsecond-level chronometry itself, which stays in the dedicated prover electronics. Merobix's role is to preserve and present those results: an auditor can see that a compact-prover run used pulse interpolation, review the resulting factors, and confirm the run set met its repeatability criterion, all from the proving record without needing to touch the timing hardware.
A large pipe prover sweeps enough volume that thousands of meter pulses accumulate per pass, so whole-pulse counting alone gives fine resolution. A compact or small-volume prover displaces far less fluid, so only a few pulses occur and rounding to whole pulses would be too coarse. Double chronometry recovers the fractional pulse by timing, making the small prover accurate enough for custody work.
One timer measures the time for a whole number of meter pulses that brackets the prover displacement, and the other measures the time of the prover displacement itself between its two detectors. Because both run off the same clock, the ratio of those times converts the whole-pulse count into a precise fractional count matching the prover's calibrated volume.
Not inherently; it is designed to give small provers accuracy comparable to large whole-pulse provers. Its accuracy depends on a fast, stable timer clock and clean detector and pulse signals, since timing errors propagate into the interpolated count. With proper high-resolution timing electronics it lets a compact prover meet the same custody-transfer standards as a pipe prover.
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