Stagnation pressure, also called total or impact pressure, is the pressure a moving fluid reaches at a point where it is brought completely to rest. At that stagnation point the fluid's kinetic energy is converted into pressure, so the reading equals the ordinary static pressure plus a dynamic term that depends on the flow's speed. It is the quantity a pitot tube's forward-facing port senses, and subtracting static pressure from it yields the velocity head that probe-based flow meters exploit. This guide defines stagnation pressure, explains the Bernoulli relationship behind it, and shows why understanding it helps you interpret probe-based flow signals in a SCADA system.
Stagnation Pressure in one line: Stagnation pressure is the total pressure at a point where a moving fluid is brought to rest, equal to the sum of the static pressure and the dynamic pressure at that location. It is what a pitot tube's impact port measures, and subtracting the static pressure leaves the dynamic pressure, or velocity head, from which flow velocity is calculated.
Imagine a fluid streaming toward a solid object with a small hole facing directly into the flow. Right at that hole the fluid decelerates to a stop, and the streamline that ends there is called a stagnation point. As the fluid stops, the kinetic energy it carried has to go somewhere, and by conservation of energy it converts into pressure, so the pressure at the stagnation point is higher than the pressure in the freely moving stream nearby. That elevated pressure is the stagnation, or total, pressure.
Bernoulli's equation for an incompressible flow makes the relationship precise: total pressure equals static pressure plus dynamic pressure, where dynamic pressure is one half the fluid density times velocity squared. The static pressure is the pressure the fluid exerts as it flows past without being stopped, the everyday pressure a wall tap would read. The dynamic pressure is the extra amount that appears only because the fluid is moving and gets converted when it stagnates. Add them and you have the stagnation pressure.
This is why stagnation pressure is sometimes called impact pressure: it is the pressure the flow would impose if it slammed into a surface and stopped. The higher the velocity, the larger the dynamic term and the more the stagnation pressure exceeds the static pressure, and it is that gap, not either pressure alone, that carries the velocity information a flow measurement wants.
A pitot tube is essentially a small tube with its open end pointing upstream, and its forward port sits at a stagnation point, so it reads stagnation pressure directly. On its own that reading is not enough, because stagnation pressure includes the static pressure, which also depends on how the system is pressurized. To extract the velocity information you must remove the static component, which is why every pitot-based velocity measurement pairs the impact reading with a static-pressure reading.
Subtracting static pressure from stagnation pressure leaves the dynamic pressure, the velocity head. From there Bernoulli's relationship gives velocity as proportional to the square root of the dynamic pressure divided by the fluid density. A pitot-static tube does the subtraction with its own side static ports; a plain pitot tube relies on a separate static tap. Either way, the differential the transmitter actually measures is stagnation minus static, and that differential is the dynamic pressure that becomes velocity.
An averaging pitot tube, or annubar, extends the same idea across the full pipe bore. It has several impact ports along a bar spanning the pipe, each sitting at a stagnation point in the flow profile, and it averages those stagnation pressures against an averaged static reading. The averaged dynamic pressure it produces represents the mean velocity across the cross-section, which is what turns a stagnation-pressure principle into a full-bore flow rate rather than a single-point velocity.
Understanding that the useful quantity is stagnation minus static, not stagnation itself, changes how you read a probe-based flow point. The transmitter on a pitot or annubar reports a small differential pressure, the velocity head, and everything downstream depends on that differential being clean. If the impact line plugs, the static line accumulates liquid, or the two impulse lines are not at the same elevation, the differential shifts even though the true flow has not, and the SCADA reading drifts. Knowing what the differential physically represents makes those failure modes easier to diagnose.
It also clarifies the square-root behavior operators see. Because dynamic pressure grows with velocity squared, the differential a stagnation-based meter produces is small at low flow and grows rapidly at high flow, so the meter loses resolution and turndown at the bottom of its range. That is not a fault; it is inherent in the physics of stagnation pressure, and it explains why a flow computer applies a square-root extraction and often a low-flow cutoff to the raw signal.
A cloud SCADA platform such as Merobix reads the derived velocity or flow from the flow computer or transmitter over an industrial protocol and brings it up as a live tag, ideally alongside the raw differential and any static and temperature values used for density compensation. Trending those together lets a monitoring team see when a differential no longer tracks the expected relationship to flow, an early sign that a stagnation or static port is compromised, so the field probe can be serviced before the measurement quietly goes wrong.
Static pressure is the pressure a fluid exerts as it flows past without being stopped, the value a flush wall tap reads. Stagnation pressure is the higher pressure reached where the fluid is brought completely to rest, equal to the static pressure plus the dynamic pressure. The difference between the two is the dynamic pressure, or velocity head, which carries the flow-speed information.
Yes, the terms are used interchangeably in flow measurement, along with impact pressure. All refer to the pressure at a point where the moving fluid has been brought to rest, which equals static plus dynamic pressure. A pitot tube's forward-facing port senses this total, or stagnation, pressure directly.
Stagnation pressure includes the static pressure, which depends on how the whole system is pressurized and carries no velocity information by itself. Only the dynamic component, stagnation minus static, reflects the flow speed. So a velocity measurement always subtracts a static reading from the stagnation reading, whether the static comes from a pitot-static probe's own ports or a separate wall tap.
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