A flow-proportional composite sample is a single blended sample built up from many tiny grabs, where the grabs are taken in proportion to how much product is flowing rather than on a fixed clock. The idea is that the composite in the bottle should mirror the batch that actually passed through the meter, so that when it is tested for water content and gravity, the results represent the true average of everything delivered. Because grabs are paced by flow, a period of high flow contributes proportionally more sample than a slow trickle, which is exactly what you want when different portions of a batch may differ in quality. It is the sampling method custody transfer relies on to make the tested quality match the metered quantity.
Flow-proportional composite sample in one line: A flow-proportional composite sample is a composite built from small grabs taken in proportion to metered flow, so the blended sample represents the entire batch as it actually passed the meter. This flow-weighting is what makes it more accurate than fixed-interval or spot sampling for determining BS&W and gravity in custody transfer.
Custody transfer settles on two things: how much product moved, from the meter, and what quality it was, from the sample. For the settlement to be fair, those two have to describe the same batch. A meter integrates flow continuously over the whole transfer, so the sample needs to represent that same whole, weighted the way the meter weighted it. A flow-proportional composite achieves this by taking more sample when more product is flowing and less when flow slows, so each barrel that crossed the meter is represented in the bottle in proportion to its share of the total.
This matters because a batch is not always uniform. Water content can slug through unevenly, gravity can shift as different tanks feed the line, and the beginning and end of a transfer can differ from the middle. If the sample over-represents a low-flow period when water happened to be high, the tested water content will be biased even though the meter total is correct. Flow-proportional grabbing prevents that by tying sample contribution to flow contribution, so a brief high-water slug during low flow does not dominate the composite beyond its true share of the delivered volume.
The result is a composite whose measured water content and gravity are honest averages for the batch as metered. That is precisely what net volume math needs: the meter gives gross volume, the composite gives the fraction that is water and the gravity for temperature correction, and because the sample was flow-weighted, applying those quality numbers to the metered volume gives an accurate net. The whole scheme rests on the sample and the meter measuring the same weighted batch.
A spot sample is a single grab at one moment. It is fast and cheap, but it assumes the whole batch looks like that one instant, which is only true if the product is perfectly uniform throughout the transfer. Any variation in water or gravity across the batch is simply missed, and if the grab happens to land on an unrepresentative slug, the tested quality can be badly wrong while looking perfectly plausible. For custody transfer of a variable stream, a spot sample is a gamble on uniformity.
Fixed-interval sampling, taking a grab every so many minutes regardless of flow, is better because it spreads grabs across the whole transfer, but it still misweights the batch whenever flow is not constant. A slow period and a fast period get the same number of grabs, so the slow period is over-represented relative to how much product it actually delivered. If quality happens to correlate with flow rate, and it often does, fixed-interval sampling bakes a bias into the composite that flow-proportional sampling avoids by design.
Flow-proportional sampling is the method that removes both weaknesses. It spans the entire transfer like fixed-interval sampling, but it weights each grab by flow so the composite matches the metered batch rather than the clock. For determining basic sediment and water and for gravity, where the tested value multiplies against a large metered volume to produce the settled net, that weighting is not a refinement; it is what makes the quality determination defensible for money changing hands.
On a LACT unit, the flow-proportional sample is produced by an automatic sampler that takes its cue from the meter. As product flows, the flow computer accumulates volume and emits pulses representing quantity passed; the sampler is set to fire one small grab per a defined increment of metered volume. When flow is fast, that increment is reached quickly and grabs come often; when flow is slow, grabs space out. The grab rate therefore tracks the flow rate automatically, which is exactly what makes the composite flow-proportional without anyone timing anything.
In a SCADA context, the flow computer and sampler operate as a coordinated pair, and the platform oversees them. Merobix reads the meter totals, the grab count, and the sampler status, so the operation can confirm that grabs are actually being paced off flow and that the sampler kept up through the whole transfer. If the sampler stalls or the grab count falls out of proportion with the metered volume, that is visible as a discrepancy rather than a surprise discovered only when the composite is tested and looks wrong.
Historizing the grab pacing alongside the metered volume gives the transfer a verifiable chain of custody for its quality determination. An auditor can see that the composite behind a ticket was built from grabs paced by the metered flow, not by a clock or a single dip, which is the evidence that the tested water content and gravity genuinely represent the delivered batch. Tying the sampler to the flow computer and recording both is how the accuracy of the whole net volume, quantity times quality, is made to stand up.
A spot sample is a single grab at one moment and assumes the whole batch looks like that instant, so any variation in water or gravity across the transfer is missed. A flow-proportional composite is built from many grabs taken in proportion to flow across the entire batch, so it represents the metered volume as a weighted whole. That makes it far more reliable for custody-grade water and gravity determination.
Fixed-interval sampling takes a grab on a clock regardless of flow, so a slow period gets the same number of grabs as a fast period and is over-represented relative to how much it actually delivered. Flow-proportional sampling weights grabs by flow, so the composite matches the metered batch. When quality correlates with flow rate, fixed-interval sampling bakes in a bias that flow-proportional sampling avoids.
The flow computer accumulates metered volume and emits pulses representing quantity passed, and the sampler is configured to fire one grab per defined increment of that volume. When flow is fast the increment is reached quickly and grabs come more often; when flow is slow they space out, so the grab rate tracks flow automatically. A SCADA system oversees the pacing and records the grab count against the metered volume.
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