Calibration of
venturimeter with pitot static tube
Objective:
To calibrate the venturimeter fixed with the
Experimental set up consisting of Polythene Ducts and an Axial flow fan.
Apparatus:
(1)
Experimental
Polythene duct set up fitted with an Axial flow fan at one and a Quantity
controlling device at the other.
(2)
Pitot-static
tube
(3)
Inclined
tube Manometer
(4)
Scale,
connecting tubes, etc.
Description of Apparatus:
(1)
The
set up consists of an axial flow fan and Polythene ducting with branched airways
as shown in the figure 1. The venturimeter incorporated in the set up is
illustrated in the figure 2.
(2)
Pitot-static
tube (figure 3)- this consists of two concentric tubes, the Pitot or Total head
tube placed concentrically inside the other static tube.
(3)
Inclined
gauge manometer- this is essentially a U- tube mounted on a inclined plane with
an arrangement for varying the inclination. This is used instead of the
vertical U- tube since it is much more sensitive. For this experiment an
inclination of 1 in 4 is used to give a sensitivity of 0.25-mm w.g. with water
as the manometric fluid.
(4)
Askania
Minimeter (figure 4)- this accurate Micro-Manometer with interconnected
containers filled with distilled water, one being the observation vessel and
the other the balancing vessel. The balancing vessel can be raised or lowered
by operating a knurled ring, the amount of movement being noted on vertical
scale graduated in millimeters and micrometers having a least count of 0.001mm.
The high-pressure side (from the Pitot tube) is connected to the observation
vessel the liquid level in which is brought back to a fixed level marked by the
pointer by raising the balancing vessel. The amount of rising gives the
differential pressure directly. The exact coincidence of the tip of the pointer
with the water level is obtained by observing the internal reflection of the
pointer from the water surface.
Theory:
The
velocity of air as measured by the Pitot-static tube can be obtained from the
velocity pressure measured by the Askania Minimeter by using the relation:
h = k * v2/2 * g where
h =
minimeter reading in mm w.g.
k =
Pitot static tube correction factor = 1
r = Density of air = 1.15 kg/m3
g =
acceleration due to gravity = 9.8 m/s2
v =
velocity of air in m/sec.
On simplification, the relation reduces to
v = 4 * h1/2
To
get the average velocity of the duct as single pitot static tube reading is
taken at the center of the duct and is multiplied by a method factor of 0.84.
The quantity flowing through the duct is the average velocity multiplied by the
cross- sectional area of the duct.
The
Venturimeter operates on the principle of the Bernoulli’s Theorem that the
total pressure along a flow channel remains constant. The pressure drop between
the inlet and the throat, P gives the quantity Q flowing through the meter by
the relation:
Q = a * A2
* {2 * g * P/(1-m2)}1/2
a = Discharge coefficient for standard
venturimeter = 0.993
m =
Area ratio = (cross- sectional area at the throat)
(cross-sectional area at the inlet )
A2
= m * A1, where A1 is the cross sectional area at the
inlet.
g =
9.8 m/s2
P =
pressure in mm of w.g.
From
the given duct m = 0.5 taking A2 = 0.5 * A1 , the above equation reduces to
Q = 0.172 P1/2
Procedure:
The
Pitot-static tube with its nose facing the air stream is placed at the center
of the duct at the affixed and the two tappings from it connected to the
minimeter after the latter is properly leveled and its initial reading taken.
The
pressure tappings from the venturimeter are connected to the inclined tube
manometer after suitably leveling it taking its initial reading. Simultaneous
Pitot-static tube and its venturimeter readings are taken for five regimes of
flow, by adjusting the duct-regulating device at the duct outlet. The
quantities as measured by the two devices are then calculated and compared by
plotting a graph.
Results and Discussions:
Proforma
for recording the results.
|
S.no |
Pitot-static Tube reading h, mm w.g. |
v = 4 * h1/2 |
Average v = 0.84 * v |
Q from Pitot-static tube, m3/s |
Venturi-meter reading P1 mm w.g. |
Q from Venturimeterin m3/s. |
Precautions:
(1)
the
Pitot-static tube should be firmly fixed and should be vertical. The angle of
jaw should be as far as possible be equal to zero.
(2)
The
minimeter and the manometer should be set firmly and leveled properly.
(3)
The
Minimeter reading should be taken when the pointer just touches the water
surface, i.e. the image of the pointer coincides.
(4)
All
the connections should be made airtight.
(5)
Kinks
in the rubber tube should be checked and corrected.
(6)
Zero
error of the manometers should be noted before taking readings with them.
(7)
The
inclined manometer should be calibrated before use.
Note: Throttle diameter of
Venturimeter = 208 mm
Inlet diameter of Venturimeter = 294 mm.
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