Air flow pressure is typically not measured directly. Instead, what is often referred to as "air flow pressure" is actually air pressure or air velocity, and these are measured using different instruments and techniques. True pressure is measured with pressure sensors, while air flow velocity is often measured with anemometers.
Here's a breakdown of how air pressure and air velocity (which are related to what people often mean by "air flow pressure") are measured:
Measuring Air Pressure
Air pressure is the force exerted by air per unit area. Several instruments are used to measure air pressure, including:
-
Barometers: Used to measure atmospheric pressure. There are two main types:
- Mercury barometers: Measure air pressure by balancing the weight of mercury in a glass tube against atmospheric pressure.
- Aneroid barometers: Use a flexible metal box (aneroid cell) that expands and contracts with changes in air pressure. These changes are mechanically amplified and displayed on a dial.
-
Pressure Transducers/Sensors: These are electronic devices that convert pressure into an electrical signal. They are widely used in various applications due to their accuracy and ease of integration with data acquisition systems.
-
Manometers: U-shaped tubes filled with liquid (typically water or mercury) used to measure the pressure difference between two points.
Measuring Air Velocity (Related to Air Flow)
Air velocity refers to the speed of air movement. While not directly pressure, velocity is closely related to what people often consider "air flow pressure" because faster air flow often implies higher dynamic pressure. Common instruments for measuring air velocity include:
-
Anemometers: These devices measure wind speed and direction. Several types of anemometers exist:
- Hot-wire anemometers: As mentioned in the reference, a hot-wire anemometer measures air velocity by measuring the heat loss of an electrically heated wire placed in the air stream. The faster the air flow, the more heat is lost, and this is correlated to the air velocity. This is particularly useful in measuring turbulent flows.
- Vane anemometers: Use a rotating vane or propeller to measure air velocity. The rotation speed is proportional to the air velocity.
- Cup anemometers: Used primarily for outdoor weather applications.
-
Pitot Tubes: A Pitot tube measures the difference between the static pressure and the total pressure (static pressure + dynamic pressure) of the air flow. This difference is known as the dynamic pressure, which can then be used to calculate the air velocity. Pitot tubes are commonly used in aircraft to measure airspeed.
Relationship Between Air Pressure and Air Flow (Velocity)
It's important to understand the relationship between air pressure and air velocity. Bernoulli's principle states that for an inviscid flow, an increase in the speed of the fluid occurs simultaneously with a decrease in pressure or a decrease in the fluid's potential energy. Therefore, measuring air velocity can indirectly infer the pressure changes within a system, and vice versa.
In summary, air flow pressure is typically not a direct measurement. What is often meant by the term is either air pressure which is measured with barometers, pressure transducers or manometers; or air velocity which is measured using anemometers or Pitot tubes. The choice of instrument depends on the specific application and the desired level of accuracy.
