The measurement used to compare air pressure between two points within a duct is called which?

The measurement used to compare air pressure between two points within a duct is called which?

• A. Static pressure
• B. Differential pressure
• C. Total pressure
• D. Velocity pressure

Answer: (B)

Differential pressure represents the difference in pressure between two locations in the duct. In HVAC work, you often want to know how much pressure has dropped (or gained) as air moves from one point to another, such as across a filter, damper, or a section of duct. Taking static pressure readings at two taps and subtracting them gives this differential value, usually expressed in inches of water. This focuses on the pressure change along a path, which is exactly what you’re assessing when you compare two points. Static pressure is the pressure at a single point, not a difference. Total pressure combines static pressure with velocity (dynamic) pressure, so it’s not about comparing two locations. Velocity pressure is the dynamic portion related to air speed, obtained from a different kind of measurement and also isn’t the two-point difference you’re after.

Differential pressure represents the difference in pressure between two locations in the duct. In HVAC work, you often want to know how much pressure has dropped (or gained) as air moves from one point to another, such as across a filter, damper, or a section of duct. Taking static pressure readings at two taps and subtracting them gives this differential value, usually expressed in inches of water. This focuses on the pressure change along a path, which is exactly what you’re assessing when you compare two points.

Static pressure is the pressure at a single point, not a difference. Total pressure combines static pressure with velocity (dynamic) pressure, so it’s not about comparing two locations. Velocity pressure is the dynamic portion related to air speed, obtained from a different kind of measurement and also isn’t the two-point difference you’re after.