A Dall tube is a differential-pressure primary element built to produce a strong flow signal while wasting very little energy. It squeezes the flow through a short converging section and a narrow throat slot, generating a large differential pressure, then lets the flow recover most of that pressure downstream so the permanent loss is far smaller than a classic venturi's. That combination of a good signal and low head loss makes it attractive on big lines where pumping energy is a real cost. This guide explains how the Dall tube works, why its pressure recovery is so high, and how its differential feeds a flow computer just like an orifice does.
Dall Tube in one line: A Dall tube is a shortened, high-recovery differential-pressure flow element in which a steep converging inlet and a throat slot create a large differential pressure for a comparatively small permanent pressure loss. It gives a stronger signal per unit of unrecovered head than a venturi and is far more compact, while its differential feeds a flow computer through the same square-root relationship as any DP meter.
Like every differential producer, the Dall tube works by accelerating the flow through a constriction and reading the pressure drop that acceleration creates. Fluid enters a steeply converging inlet, speeds up as the passage narrows, and reaches minimum area at a throat where a slot senses the low pressure. A tap upstream senses the higher approach pressure, and the difference between the two is the differential the meter reports. As with an orifice or venturi, flow is proportional to the square root of that differential, so a flow computer takes the square root to linearize the reading.
What distinguishes the Dall tube's geometry is how sharply it turns the flow and where it takes its low-pressure reading. The steep inlet cone and the abrupt shoulder near the throat produce a differential that is large relative to the actual restriction, so the meter yields a healthy, easy-to-measure signal even though it is short and does not choke the line as hard as its differential might suggest. The throat slot averages the pressure around the throat rather than sampling a single point, which helps the reading stay stable.
Because the operating principle is identical to other DP elements, the Dall tube needs the same supporting instrumentation: a differential-pressure transmitter, and typically pressure and temperature measurement for density compensation on gas or compressible service. Its coefficient, the factor relating measured differential to true flow, is established for its geometry and used in the flow calculation the same way a venturi or orifice coefficient is.
The defining virtue of a Dall tube is pressure recovery. In any differential producer the fluid gives up pressure as it accelerates through the restriction, but a well-shaped diverging section downstream lets much of that pressure come back as the flow slows again. The Dall tube's outlet is designed for efficient recovery, so only a small fraction of the differential it measures is lost permanently. An orifice plate, by contrast, throws away most of its differential to turbulence behind the plate, giving a high permanent loss for the same signal.
That low unrecovered head has a direct operating consequence: pumps and compressors have to work less hard to push flow through a Dall tube than through an orifice generating the same differential. Over the life of a large line running continuously, the saved energy can be substantial, which is exactly why high-recovery elements are chosen on major water, cooling, and pipeline services where the pumping bill dwarfs the cost of the meter. The Dall tube captures much of a venturi's low-loss benefit.
The Dall tube also does this in a compact package. A classic venturi is long, because its gently tapered recovery cone needs length to slow the flow smoothly, which makes venturis bulky and heavy on large diameters. The Dall tube achieves comparable recovery in a much shorter body, so it fits where a full venturi will not and costs less to fabricate and support on big lines. That shorter length, together with the strong signal and low loss, is the reason it was developed as a venturi alternative.
To a control system a Dall tube is just another DP flow point. Its differential-pressure transmitter outputs a standard 4-20 mA loop or a digital signal, and a flow computer, RTU, or PLC applies the square-root extraction and the tube's coefficient, along with density compensation from pressure and temperature where the fluid is compressible, to produce a flow rate. Nothing about the integration differs from an orifice or venturi; only the primary element upstream is different.
A cloud SCADA platform such as Merobix reads the flow rate, differential pressure, static pressure, and temperature from that field device over an industrial protocol, trends them, and totalizes the flow so an operator can see a large line's behavior from any browser. Because the Dall tube is often installed precisely on the big, energy-hungry lines where remote oversight is most valuable, pairing it with continuous monitoring gives a clear, ongoing picture of both throughput and the health of the metering point.
Trending the differential alongside the derived flow is useful for the same reasons it is on any DP meter: a slow drift, a step change, or a differential that no longer tracks expected flow can signal an impulse-line problem, a deposit in the tube, or a transmitter issue. Watching those trends remotely lets a team catch a developing fault on a critical line before it turns into a bad measurement, keeping the low-loss advantage of the Dall tube paired with dependable data.
Both are high-recovery differential producers, but the Dall tube uses a steeper inlet and a throat slot to get a large differential from a much shorter, lighter body than a classic venturi. It recovers most of the pressure it takes, like a venturi, so its permanent loss stays low, while fitting where a long venturi would not and costing less to fabricate on large lines.
The unrecovered pressure across a meter is head that pumps or compressors must supply, so it translates directly into energy cost over the life of the line. A Dall tube gives a strong differential signal while losing only a small fraction of it permanently, so it saves pumping energy compared with an orifice generating the same signal, which is why it is favored on large, continuously running services.
Yes. It is a differential-pressure element, so flow is proportional to the square root of the measured differential, and a flow computer applies that square-root extraction plus the tube's coefficient exactly as it would for an orifice. On compressible fluids it also uses pressure and temperature for density compensation. Only the shape of the primary element differs from an orifice installation.
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