A bidirectional prover is a specific piece of proving hardware: a U-shaped run of pipe with a sphere that is driven through a certified volume first one way, then the other, to check a flow meter's accuracy. Its defining features are that calibrated ball and the two detector switches that mark the ends of the known volume. This guide explains how a bidirectional prover is built, why it runs in both directions, and how a flow computer uses it - focusing on the prover hardware itself rather than the general idea of proving.
Bidirectional Prover in one line: A bidirectional prover is a pipe prover in which a sphere sweeps a certified base volume between two detector switches, and it is designed to run in both flow directions - forward and reverse - through the same calibrated section. A four-way valve reverses the flow to send the ball back the other way, and the forward-plus-reverse round trip averages out small directional differences, giving a robust known volume for proving custody meters.
The heart of a bidirectional prover is a length of pipe, often folded into a U or a long loop to fit the volume into a reasonable footprint, whose internal volume between two fixed points is certified to high accuracy by a water-draw calibration traceable to a physical standard. Inside that pipe runs an elastomer sphere, inflated slightly oversize so it seals against the bore and is carried along by the flowing liquid like a piston, sweeping the full volume ahead of it as it travels.
Two detector switches are mounted in the pipe wall, one near each end of the calibrated section. Each detector is a small mechanical or optical device with a plunger or sensor that the passing sphere trips as it goes by. The certified base volume of the prover is precisely the volume of pipe between those two detectors - it is this switch-to-switch volume, not the whole prover, that is calibrated and stamped on the prover's certificate. When the ball trips the first detector the flow computer starts counting meter pulses, and when it trips the second the count stops.
Around the prover barrel sits the launching and receiving chamber and, crucially, a four-way valve. This valve is what makes the prover bidirectional: it can route the flowing liquid through the prover in either direction, so after the ball has traversed the barrel one way it can be sent straight back the other way without removing or repositioning it. Temperature and pressure taps on the prover let the flow computer correct the prover's volume to the same conditions as the meter.
The reason for running in both directions is precision. A single detector pair and a single ball are never perfectly symmetric - the two detectors may trip at very slightly different effective positions, and the sphere may seal marginally differently depending on which way it moves. Running the ball forward through the volume and then reverse back through the same volume, and summing the two passes, cancels most of these small directional and detector-position asymmetries. The forward-and-reverse pair is treated as one complete round trip, giving a base volume that is more reliable than either single pass alone.
In practice a prove consists of several such round trips run back to back. The flow computer commands the four-way valve to launch the ball, times the pulses over the forward pass, reverses the valve to send the ball back, and times the pulses over the reverse pass, adding the two to get one round-trip result. It repeats this until it has enough consecutive round trips that agree within a tight repeatability tolerance, then averages them into the meter factor. The bidirectional design's inherent symmetry is a large part of why pipe provers are trusted as high-accuracy references.
This is what distinguishes a bidirectional prover from a unidirectional one. A unidirectional prover sends the ball through the volume in one direction only and returns it through a separate path to relaunch, so it does not get the self-cancelling benefit of the reverse pass but can offer a simpler flow arrangement. The bidirectional layout trades a four-way valve and a bit more complexity for the accuracy that both-direction averaging provides, which is why it is a common choice on high-value liquid custody transfer.
A bidirectional prover does not work alone - it is orchestrated by a flow computer that sequences the whole prove. The flow computer commands the four-way valve, watches for the detector-switch trips, counts the meter's pulses between them, reads the prover and meter temperatures and pressures to correct both to common conditions, and computes the resulting meter factor once enough repeatable round trips are collected. It also produces the prove report that documents the run for audit.
Because so much of a modern prove is automatic, the values that matter - the base volume in use, the latest meter factor, the repeatability of the last runs, and the pass or fail of the prove - all live as data in the flow computer. That makes the prove an event a remote system can observe rather than something only visible at the skid. Detector switch health and the number of runs needed to reach repeatability are useful diagnostics too, since a sticking detector or a worn sphere shows up as poor repeatability.
A cloud SCADA platform such as Merobix reads the prove results and the current meter factor from the flow computer over Modbus, OPC UA, or a similar protocol and trends them across proves. Measurement staff can then confirm a bidirectional prove completed and passed, watch the meter factor's history for drift, and be alerted if a meter is overdue - keeping oversight of a remote proving skid without standing next to it, while the flow computer and prover hardware do the physical proving.
Running the sphere forward through the calibrated volume and then reverse back through the same volume, and summing the two passes, cancels most small asymmetries between the two detector switches and any directional difference in how the ball seals. The forward-and-reverse round trip therefore gives a more accurate base volume than a single pass would, which is why the bidirectional design is trusted for high-value proving.
They are two sensors mounted in the prover pipe, one near each end of the calibrated section, that the passing sphere trips as it goes by. The certified base volume is exactly the pipe volume between the two detectors. The flow computer starts counting meter pulses when the ball trips the first detector and stops when it trips the second, so the switches define the known volume the prove is measured against.
A bidirectional prover sends the ball through the calibrated volume in both directions using a four-way valve, so forward-and-reverse averaging cancels small asymmetries and improves accuracy. A unidirectional prover runs the ball one way only and returns it through a separate path to relaunch, which is simpler in flow but does not get the self-cancelling benefit of the reverse pass.
This page references the protocol specifications published by the organizations below. Editions, product capabilities, and documentation change over time - confirm current requirements and specifications directly with the source.
Last reviewed: July 27, 2026. Merobix is not affiliated with, endorsed by, or sponsored by these organizations; their names are used only to identify the standards and products discussed.
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