A pitot-static tube, also known as a Prandtl tube, is a single probe that measures both the total pressure and the static pressure of a moving fluid at the same point. A forward-facing hole senses total pressure where the flow is brought to rest, while small holes around the side sense static pressure, and the difference between them is the velocity head from which flow velocity is calculated. Because both pressures come from one self-contained probe, it needs no separate static tap in the pipe wall. This guide explains how the pitot-static tube works, how it differs from a plain pitot tube, and what to keep in mind when its signal feeds a SCADA system.
Pitot-Static Tube in one line: A pitot-static tube is a probe that combines a forward-facing total-pressure port with side-facing static-pressure ports in one body, so the differential between them is the dynamic pressure, or velocity head. Because it senses both pressures at the same location, that differential directly yields the local flow velocity through the Bernoulli relationship, without needing a separate wall static tap.
The pitot-static tube exploits a basic result of fluid mechanics: when a moving fluid is brought to rest at a point, the pressure there, the total or stagnation pressure, equals the ordinary static pressure plus a dynamic term that depends on the fluid's speed. The forward-facing port at the nose of the probe faces directly into the flow, so fluid stagnates against it and the port reads total pressure. The static ports, small holes drilled around the side of the probe body where the flow slides past undisturbed, read the static pressure.
Subtracting static from total leaves only the dynamic pressure, the velocity head, which by Bernoulli's equation equals one half the fluid density times the velocity squared. Rearranging that relationship gives velocity as proportional to the square root of the differential divided by density, so a differential-pressure transmitter connected across the two ports, together with a known or measured density, yields the local flow velocity. The square-root relationship is the same nonlinearity that appears in every dynamic-pressure measurement.
The elegance of the pitot-static design is that both readings come from one probe at effectively one location, so the transmitter sees a clean differential without needing a separate static connection somewhere else in the pipe. The probe geometry is shaped so the side holes sit where the local static pressure is representative and not disturbed by the nose, which is what makes the single-probe differential a faithful measure of velocity head.
A plain pitot tube has only the forward-facing total-pressure port. To get velocity from it you still need the static pressure, but that has to come from a separate static tap elsewhere, typically a hole in the pipe or duct wall. The pitot-static tube folds both measurements into one probe by adding the side static ports, so it delivers the differential on its own. That self-contained quality is why the pitot-static, or Prandtl, arrangement is the standard when a single insertion probe must provide a complete velocity reading.
Both devices are fundamentally single-point instruments: they sense the velocity at the one spot in the cross-section where the probe tip sits. That is very different from an averaging pitot tube, or annubar, which spans the full pipe bore with multiple sensing ports and reports an average velocity across the profile. A pitot-static tube reads a spot velocity, so turning its reading into a total flow rate through the pipe requires knowing how that point relates to the average velocity of the whole profile, which is why it is often used for a velocity traverse across the cross-section rather than as a fixed full-bore flow meter.
The practical upshot is that a pitot-static tube shines for spot-checking velocity, mapping a velocity profile, or measuring in large ducts and stacks, but it is not by itself an averaging flow meter. Where a continuous total-flow reading is wanted from a single insertion point, an averaging element that samples across the bore is usually the better fit, and the plain pitot-static probe is reserved for point velocity and traverse work.
In field use the pitot-static reading depends on a few things the SCADA integration has to respect. The probe must be aligned with the flow, because tilting it away from the streamlines lowers the total-pressure reading and biases velocity low; there is a limited angle of yaw over which the reading stays trustworthy. Fluid density must be known or compensated, since velocity depends on the differential divided by density, and for gas that means using measured pressure and temperature. A probe coefficient close to but not exactly unity is applied to account for the real geometry, and it is established by calibration.
Because the raw output is a small differential pressure, the loop is only as good as the transmitter's zero and span at that low range and the integrity of the impulse lines, which can plug or accumulate liquid and skew the reading. A remote terminal unit, PLC, or flow computer reads the differential-pressure transmitter as a 4-20 mA loop or a digital value, applies the square-root extraction, the probe coefficient, and the density term, and produces a velocity or a flow rate.
A cloud SCADA platform such as Merobix brings that derived velocity or flow up as a live tag, trends it, and totalizes flow where the point relationship to the average is known, so an operator can watch a stack, duct, or large line from a browser. Trending the raw differential alongside the computed velocity is worthwhile: a drifting zero, a plugged static port, or a probe knocked out of alignment shows up as an unexplained shift, letting a monitoring team flag a probe that needs field attention before the data goes quietly wrong.
A plain pitot tube has only the forward-facing total-pressure port and needs a separate static tap elsewhere to complete the measurement. A pitot-static, or Prandtl, tube adds side static ports to the same probe, so it provides the total-minus-static differential on its own. Both read velocity at a single point, but the pitot-static is self-contained.
It measures the velocity at the single point where the probe tip sits, not the average across the pipe. That makes it ideal for spot checks and velocity traverses, but to get a total flow rate you must relate that point to the profile average. An averaging pitot tube, or annubar, spans the bore to report an average directly, which a plain pitot-static tube does not.
The forward port only reads true total pressure when it faces directly into the flow. Tilting it away from the streamlines lowers the sensed total pressure and biases the velocity reading low, and each probe has a limited yaw angle over which the error stays acceptable. In field installations the probe must be aligned with the flow direction and secured so it does not rotate or vibrate out of position.
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