Velocity/Flow Measurements (Venturi, Pitot, Doppler)
A Venturi tube is one with a very gradual contraction and expansion
in the tube, in contrast to the sudden contraction of an orifice flow meter ( Fig.
30-18
). Because the contraction is smooth and gradual, the Bernoulli equation
(Equation 5) applies to this geometry. If we measure the pressure difference between
the widest and the narrowest parts of the Venturi tube, we can solve the Bernoulli
equation for velocity (see Appendix 5
).
The Venturi tube derives fluid velocity (U) from the pressure difference, not volume
flow (
). In laminar flow, fluid velocity is proportional to flow. The Venturi
tube is used in many industrial applications and is also used on some aircraft to
measure speed.
Figure 30-18
Venturi tube. By measuring the pressure difference between
two points in a laminar flow, the average flow velocity can be determined because
the mass flow must be the same (see Appendix
5
).
A Pitot tube is a cylindric tube whose open end is pointed directly
into the flow, that is, "upstream" ( Fig.
30-19
). The pressure measured in the Pitot tube is the stagnation
pressure, given earlier by Equation 5. If we also measure the static
pressure (p) (the p1
side port in the figure) and we know the fluid density
ρ, we can easily solve Equation 5 for the fluid velocity (U). Note again that
the Pitot tube derives velocity (U), not volume flow (). The Pitot tube is
simple and reliable and is almost universally used on aircraft to measure their speed.
In anesthesia, the Pitot tube is used in Datex Ultima monitors. To measure gas
flow in two directions, the Datex monitor incorporates two Pitot tubes, one facing
in each direction. Additionally, the monitor samples gas composition to correct
for the density and viscosity of the gas mixture.
Figure 30-19
Pitot tube. As flows increase, wall pressure decreases
as a result of the Bernoulli principle. The Pitot tube measures the difference in
pressure from the middle of the flow to the wall and converts this difference to
a flow measurement (see Appendix 5
).
(Adapted from Ehrenwerth J, Eisenkraft J: Anesthesia Equipment: Principles
and Applications. St Louis, Mosby-Year Book, 1993.
