A single differential-pressure transmitter across an orifice does a good job over part of a gas well's or pipeline's flow range, but it cannot hold accuracy across the whole range, because the flow spans too wide a band of differential pressures for one cell to cover well. The stacking practice puts two, or sometimes three, DP transmitters of different ranges on the same orifice and switches between them so the most appropriate one is always in use. This guide explains why turndown limits a single cell, how stacked DP transmitters extend the usable range, how the selection logic chooses between the low- and high-range cells, and how the arrangement holds accuracy where it matters.
Stacked DP Transmitters in one line: Stacked DP transmitters are two or three differential-pressure transmitters of different measuring ranges connected across the same orifice, used together to extend the usable flow turndown of the meter. At low flows the low-range cell, which is accurate at small differential pressures, is used; at high flows the meter switches to a higher-range cell that can read the larger differential pressure without being overranged. By always using whichever cell reads its portion of the range most accurately, the stack holds percent-of-reading accuracy across a far wider span than any single transmitter could manage alone.
In an orifice meter the flow rate goes with the square root of the differential pressure, and that square-root relationship is the root of the turndown problem. Because differential pressure is proportional to the square of flow, halving the flow quarters the differential pressure, and a modest range of flows translates into a very wide range of differential pressures. A transmitter that reads a healthy differential pressure at high flow will be reading only a tiny fraction of its range at low flow, where its accuracy is worst.
A transmitter's accuracy is best expressed as a percentage of its reading over the upper part of its range, but near the bottom of the range that specification degrades, and a fixed absolute error becomes a large percentage of a small reading. Combined with the square-root relationship, this means the flow error blows up at low flow: a differential pressure that is a small fraction of the cell's span carries a large relative uncertainty, and the square root does not rescue it. The practical consequence is that a single DP cell can only deliver good flow accuracy over a limited turndown - the ratio of the highest to the lowest flow it can measure well.
For gas measurement in particular, where custody transfer demands accuracy and where flows genuinely vary widely across a day or across the life of a well, this turndown ceiling is a real constraint. Sizing a single transmitter for the high flows leaves the low flows poorly measured, and sizing it for the low flows means it gets overranged at high flows. There is no single range that serves both ends well, which is exactly the gap that stacking is designed to fill.
Stacking solves the turndown problem by using more than one transmitter, each optimised for a different part of the range, all sensing the same differential pressure across the same orifice. A common two-cell arrangement pairs a low-range transmitter, which reads small differential pressures accurately, with a high-range transmitter, which can handle the large differential pressures at high flow without saturating. A three-cell stack adds a mid-range cell to bridge the two. The transmitters are manifolded to the same high- and low-pressure taps, so they all see the identical differential pressure and differ only in the range over which each reads it well.
The gain is that each cell only has to be accurate over the portion of the range it is responsible for. The low cell is used when the differential pressure is small, precisely where it is at its most accurate and where the high cell would be reading a negligible fraction of its span. The high cell takes over when the differential pressure climbs beyond where the low cell can go, keeping the low cell from being overranged and damaged and providing a reading that stays within the high cell's accurate band. By handing off between cells, the meter keeps whichever transmitter is currently in its accurate zone in service.
The net effect is a far wider usable turndown than any single transmitter provides, while holding percent-of-reading accuracy across the whole span rather than only near full scale. Instead of accepting poor low-flow accuracy or overranging at high flow, the stack lets the meter measure both extremes well. This is why stacked, or dual-range and triple-range, DP transmitters are a common practice on custody gas orifice runs where wide flow variation and high accuracy have to coexist.
The piece that makes a stack work is the logic that decides, at each moment, which transmitter's reading to trust. Typically implemented in the flow computer, the selection logic watches the differential pressures and chooses the cell that is currently within its accurate range. As flow rises and the low cell approaches the top of its range, the logic transfers to the next cell up; as flow falls, it transfers back down. The transfer points are set with overlap and hysteresis so that the meter does not chatter back and forth between cells right at a boundary, and so that the reading is continuous through the handoff.
Good selection logic also uses the redundancy the stack provides for validation. In the overlap region where two cells both read the differential pressure within their ranges, their readings should agree, and a disagreement is a useful diagnostic that one transmitter has drifted or failed. The logic can flag that mismatch and, in a well-designed scheme, fall back to the healthy cell so a single transmitter fault does not take the measurement offline. This turns the stack from merely a turndown extender into a more robust measurement, since there is a second cell available if one fails.
This is where SCADA and remote monitoring add real value, especially on remote gas sites that are visited infrequently. A cloud SCADA platform such as Merobix can trend all of the stacked differential pressures, show which cell is currently selected, and surface the agreement between overlapping cells, so an engineer sees at a glance whether the stack is switching correctly and whether the transmitters still track one another. An alarm on a low-versus-high cell mismatch in the overlap region gives early warning of a drifting transmitter long before it corrupts a custody total, and trending the selected cell against flow confirms the transfer points are set where the flow actually spends its time. Monitored this way, a stacked DP arrangement not only extends turndown but becomes a self-checking measurement whose health can be watched from anywhere.
Because in an orifice meter flow goes with the square root of differential pressure, a wide flow range maps to a very wide differential-pressure range that no single transmitter can cover accurately. A single cell sized for high flow reads poorly at low flow, where its accuracy is worst. Stacking two or three cells of different ranges lets each cell handle the part of the range it measures best, extending usable turndown while holding percent-of-reading accuracy across the whole span.
Selection logic in the flow computer watches the differential pressures and uses whichever cell is currently within its accurate range - the low-range cell at small differential pressures and a higher-range cell as flow and differential pressure climb. Transfer points are set with overlap and hysteresis so the reading stays continuous and does not chatter between cells at a boundary. In the overlap region the cells' readings can also be compared to detect a drifting or failed transmitter.
They are common on custody-transfer gas orifice meter runs, where flows vary widely - across a day or over the life of a well - yet high accuracy is required at both low and high flow. A single transmitter cannot serve both extremes well, so a dual-range or triple-range stack is used to keep accuracy across the range. Remote gas sites benefit further because the redundant cells make the measurement more robust and self-checking when combined with SCADA monitoring.
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