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Experiment number 04

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 Bernoulliís 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

 

Where

††††††††††† 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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