Measuring the Wind in a Pipe: The Pitot Tube
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Open the Flue Gas Velocity Pressure Calculator →How do you measure how fast gas is rushing up a stack, when you cannot see it and cannot slow it down? The answer, used in official emissions testing worldwide, is a beautifully simple device — the pitot tube — that reads the pressure of the moving gas and turns it into a speed. The same instrument tells an airplane how fast it flies.
Static and Moving Pressure
A moving gas carries two kinds of pressure. Static pressure is the ordinary push it exerts in all directions; velocity pressure is the extra push that comes from its motion — the force you feel when wind hits your hand. Velocity pressure represents the kinetic energy of the moving gas, and crucially, it is directly related to how fast the gas is going.
The Pitot Tube
A pitot tube is a small probe placed facing into the flow. By comparing the pressure it feels head-on against the static pressure to the side, it isolates the velocity pressure — the pressure due purely to the gas's motion. From this single measurement, the gas's speed can be calculated. It is an elegant way to weigh the wind inside a duct.
| Quantity | Meaning |
|---|---|
| Static pressure | Push in all directions |
| Velocity pressure | Push from motion |
| Difference | Reveals the speed |
Density Matters
The link between velocity pressure and speed depends on the gas's density: a denser gas moving at the same speed carries more kinetic energy and produces a higher velocity pressure. So accurate flue gas density — accounting for its real temperature, moisture, and composition — is essential to converting a pressure reading into a correct velocity. Get the density wrong and the speed is wrong.
The Regulator's Tool
The pitot tube is central to official stack testing, used in standardized methods to determine how fast exhaust leaves a stack and therefore how much pollution it emits per unit time. Regulators and operators rely on it to measure emissions flow. This same principle — reading speed from the pressure of moving fluid — is how an aircraft's airspeed indicator works.
Calculating Velocity Pressure
To compute velocity pressure from density and speed, use the Flue Gas Velocity Pressure Calculator. Find the velocity with the Stack Gas Velocity Calculator, and the density with the Flue Gas Density Calculator.
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