A pitot tube measures flow by sensing the pressure a moving fluid exerts when it is brought to a stop, then converting that velocity pressure into a flow reading. It is a simple insertion probe that barely obstructs the line, which makes it useful for large ducts and flare gas where an orifice or venturi would be impractical. This guide explains the single-point pitot principle, its accuracy limits, and how it differs from the multi-port averaging Annubar.
Pitot Tube in one line: A pitot tube is a velocity-based flow sensor that measures the difference between the stagnation (impact) pressure of the flow and the static pressure of the pipe. That differential, called the velocity pressure, is proportional to the square of the fluid velocity, so measuring it gives the local flow velocity, which is multiplied by the pipe area to estimate flow rate.
A basic pitot tube is a small bent tube inserted into the flow with its open end facing directly upstream. Fluid rushing into that opening is brought to a halt, and the pressure it registers there is the stagnation pressure, the sum of the flowing static pressure plus the extra pressure that comes from stopping the moving fluid. A separate static tap, either a hole in the pipe wall or ports on the side of the probe, senses the static pressure alone. The difference between the two, the velocity pressure, is delivered to a differential-pressure transmitter. From that differential the velocity follows a square-root law, because velocity pressure rises with the square of speed, and multiplying the velocity by the cross-sectional area gives an estimate of the volumetric flow.
The key thing about a plain pitot tube is that it senses velocity at a single point in the pipe. Flow in a pipe is not uniform: it moves fastest in the center and slows to nearly zero at the walls, forming a velocity profile. A single-point pitot reads only the velocity where its tip happens to sit, so converting that one reading into an accurate whole-pipe flow rate assumes a known, well-behaved profile. Any distortion from an upstream bend, valve, or fitting throws that assumption off.
A pitot tube is an insertion device, so it barely restricts the line and adds almost no permanent pressure loss, and it can often be hot-tapped into an existing pipe or duct without a shutdown. Those properties make it attractive on very large lines and gas ducts where a full-bore element would be enormous, expensive, or impossibly lossy.
The classic home for a pitot-style probe is large-diameter, low-pressure gas flow, and flare gas measurement in particular. Flare headers are big, the gas velocity swings over an enormous range from a trickle to a blowdown surge, and there is very little pressure to spare, so a low-loss insertion probe is one of the few practical options. Large HVAC and combustion-air ducts, ventilation, and stack-gas measurement use pitot-type sensing for the same reasons. In all of these the appeal is simplicity, low pressure loss, and the ability to insert into an existing large line.
The limitations of a single-point pitot are what push many applications to more capable devices. Because it reads one point, its accuracy depends heavily on the flow profile, so it needs plenty of straight run and still struggles with turndown across a wide velocity range. Its small sensing openings can plug on wet, dirty, or particulate-laden gas, and low-velocity flows produce a tiny differential that is hard to measure precisely. A plain pitot is therefore best where the flow is reasonably clean, the profile is decent, and modest accuracy is acceptable.
This is exactly where the averaging pitot, such as an Annubar, improves on the single-point design. Instead of one impact opening, an averaging probe spans the whole pipe with several sensing ports and averages the impact pressure across the diameter, so it captures the profile rather than guessing it. That averaging makes it far less sensitive to profile distortion and gives a more representative flow reading than a single-point pitot, while keeping the same low-loss insertion advantage.
A pitot tube produces only a differential pressure; a differential-pressure transmitter converts it, and a flow computer, RTU, or PLC performs the square-root extraction and any pressure and temperature compensation to arrive at a flow rate. On a flare line that flow number matters well beyond process control, because flared and vented gas has to be tracked for emissions reporting and lost-product accounting, so the computed flow and totalized volume become important tags at the site.
A cloud SCADA platform like Merobix reads those flow and total tags from the controller over Modbus, DNP3, or OPC UA, letting operators trend flare and duct flow, watch cumulative flared volumes, and alarm on abnormal flow from any browser. Continuous remote trending is especially useful on flares, where an unexpected sustained flow can signal a relieving vessel or a passing valve somewhere upstream that would otherwise go unnoticed until a report is compiled.
Because a pitot reading is only as good as the flow profile and the small sensing ports staying clear, the historical trend also helps operators spot trouble. A signal that drifts, goes noisy, or flattens can indicate a plugged impact port or a shifting profile, and comparing the metered flow against expected process behavior flags a developing measurement problem before it corrupts an emissions or allocation figure.
A plain pitot tube senses flow velocity at a single point in the pipe, so its accuracy depends on assuming a known velocity profile. An Annubar is an averaging pitot tube with several sensing ports along a bar that spans the whole diameter, so it averages the impact pressure across the profile rather than guessing it. Both are low-loss insertion probes, but the averaging design gives a more representative and profile-tolerant flow reading.
Flare headers are large diameter and low pressure, with gas velocity swinging over a huge range, so a full-bore meter would be enormous and would waste too much of the little available pressure. A pitot-style insertion probe barely obstructs the line, adds almost no pressure loss, and can be inserted into an existing header, which makes it one of the few practical ways to measure flare flow. The flow figure is then used for emissions and lost-product accounting.
Because it reads velocity at one point, a single-point pitot depends heavily on the flow profile and needs long straight runs to be accurate, and it has limited turndown over a wide velocity range. Its small sensing openings can plug on wet or dirty gas, and low velocities produce a differential too small to measure precisely. These limits are why averaging pitot designs, which sense across the whole pipe, are often preferred.
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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