15
Appendix A
Standard Velocity vs. Actual
Velocity
Since thermal sensors are sensitive to changes in air
density and air velocity, all thermal anemometers
indicate velocities with reference to a set of
standard conditions. For TSI instruments, standard
conditions are defined as 70°F (21.1°C) and 14.7
psia (101.4 kPa). Other manufacturers may use
different values.
Standard velocity is the velocity the air would be
moving if the temperature and pressure were at
standard conditions. It is usually the most useful
measure of airflow because it defines the heat-
carrying capacity of the air.
Actual velocity is the velocity at which a
microscopic particle of dust would be traveling if it
were in the airstream.
Because actual air density is rarely equal to air
density at standard conditions, actual velocity
usually differs from standard velocity.
