7. Variation of K with Re (cont)
At high Reynolds number K is
essentially constant, but as
Reynolds number is lowered, K is
also increased
K~maxm at Re= 200-600
8. Variation of K with Re (cont)
The lowering of Reynolds number
increases viscous action which causes a
decrease in Cv and an increase in Cc
The latter apparently predominates over
the former until Cc reaches its maximum
value (about to 1)
With further decrease in Re, ‘K’ then
becomes smaller because Cv continues
to decrease
10. Difference between an Orifice
meter and Venturimeter
For venturi tube, there is no
contraction-
Area of the jet is always equal to the area
of the throat and is fixed
While for the orifice the area of the jet
is a variable
(less than Ao, the area of the orifice)
11. Difference between an Orifice
meter and Venturimeter
For venturitube
Cd = Cv (Cc=1.0)
While for the orifice, it is much
more affected by variations in Cc
than it is by variations in Cv
12. Orifice meter
Disadvantage
Its practical disadvantage is that-
it offers greater frictional
resistances as compared to
venturimeter
13. Reference
Chapter 12, Fluid Measurements
FLUID MECHANICS with Engineering Applications
(SI Metric Addition)
Robert L. Daugherty
Joseph B. Franzini
E. John Finnemore
