Experiment No. 2. Properties of fluid like mass density, specific weight, specific gravity. Measurement of pressure in pipes

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Bathsheba Dorsey
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1 1.0 Title: Experiment No. 2 Determination of Pressure using different pressure measuring devices such as Piezometers, U-tube manometer and Bourdon's pressure gauge 2.0 Prior Concepts: Properties of fluid like mass density, specific weight, specific gravity 3.0 New concepts: 3.1 Propositions Proposition 1: Pressure Measurement Simple piezometers are used for measurement of pressure at a point, Where as simple U- tube manometer (tube bent in U shape) can measure both positive and negative pressures. A U-tube manometer having equal limbs connected to two pressure tapping (none open to atmosphere) is called a differential manometer. The differential manometer gives only difference of pressure at two connected pipes. Proposition 2: Need for inverted Manometer Whenever the manometric liquid is lighter in density than the liquid in the pipe, the differential manometer (U tube) is required to be held inverted. Proposition 3: Bourdon s Pressure guage Bourdon's Pressure gauge is a mechanical pressure gauge named after the inventer used to calculate gauge pressure from low to high intensity. 3.2 Concept structure Measurement of pressure in pipes 4.0 Learning objectives 4.1 Intellectual skill a) To develop the ability to select and use appropriate device for pressure measurement. b) To understand concept of various pressure measuring devices. 4.2 Motor skill a) To develop the ability to take correct manometer reading. 4 MAHARASHTRA STATE BOARD OF TECHNICAL EDUCATION

2 Experiment No Apparatus Piezometer tube, simple U tube, Differential U tube, Inverted manometer, Bourdon pressure gauge 6.0 Diagram a) Piezometer Tube: Piezometer is a simple transparent glass tube which is connected by flexible rubber tubing and pressure tapping to the pipe. Piezometric tubes are used for measurement of pressure at a point. A piezomeric tube is tapped into the wall of the container or conduit. b) Simple U tube Manometer: A manometer transparent tube of 10mmor more, bent in a U shape is called U-tube or simple manometer. U tube manometer can measure both positive and negative pressures. The manometric liquid used should fulfill the following requirements:- 1. It should not mix with the flowing fluid. 2. It should not chemically react with the flowing fluid or the pipe wall material. 3. It should have higher density than the flowing fluid. 4. It should not stick (adhere) to the tube. 5. Its boiling point should be much higher than the atmospheric temperature. 6. It should be clearly visible. One of the limbs of U-tube manometer is longer than the other and is kept open to the atmosphere. This is called open limb. The other limb is connected to the pressure tapping. The mercury level (bottom of the concave meniscus of the mercury) in each limb is read, difference is calculated; called deflection. (a) (b) MAHARASHTRA STATE BOARD OF TECHNICAL EDUCATION 5

3 b) Differential manometer: A U tube manometer having equal limbs, connected to two pressure tappings (none open to atmosphere) is called differential manometer. The two pressure tappings may be on two different pipes or on one pipe, at two places. Figure 2.3 Differential manometer c) Inverted manometer: A tube manometer having two equal limbs connected to be inverted position. Whenever the manometric liquid is lighter in density than the liquid in the pipe, the differential manometer (U-tube) is required to be held inverted. Use of lighter manometric liquid increases sensitivity of the manometer. 6 MAHARASHTRA STATE BOARD OF TECHNICAL EDUCATION

4 Experiment No. 2 c) Bourdon Pressure Gauge : It cosists of a elastic metal tube, having elliptical cross-section. The tube is bent in circular shape one end of the tube is called tip which is sealed and connected by a link to a geared sector. A pointer is fixed to the pinion and the movement of the tip is communicated to the pointer. The pointer moves on scale and indicates pressure in Kpa or KN/m² 7.0 Procedure a) Piezometer 1) When the pressure is to be measured, the tap is put on. 2) The liquid in the pipe rises into the Piezometer till the time when the pressure inside the pipe is equal to the pressure exerted by h m of the liquid plus the atmospheric pressure. 3) When the liquid stop rising, the piezometer head h is measured by a scale or graduations on the tube, with its zero at the centre of the pipe. 4) Under the equilibrium condition.γ.h+ po, in terms of absolute pressure. γ.h in terms of gauge pressure. Where γ - specific weight of liquid S L specific gravity of liquid b) U-tube manometer 1. Pressure at A is positive (Refer fig. 2.2 a). The manometric equation can be written as p B = pc + S L γ. y1 = po + Sm.γ.h = 0 + Sm.γ.h = Sm.γ.h S L. where Sm specific gravity of manometric liquid 2. Refer fig 2.2 b The Manometric equation for this condition is p B = p C (Pascal s law) + S L. + Sm.γ.h = po = 0 (gauge pressure) = (-) [ S L. + Sm.γ.h] = (-) [ S L. + Sm..γ.h] MAHARASHTRA STATE BOARD OF TECHNICAL EDUCATION 7

5 c) Differential manometer (Refer fig. 2.3) 1. The point C and D are on the horizontal line, in continuos liquid; Even though points E and F are on horizontal line, the liquid is not continuous and hence pressure at E is not equal pressure at F 2. p C = + S L. p D = p B + S L. γ.y 2 + Smγ.h ( p C = p D ) p B. γ (y 2 y 1 ) + Sm. γ.h From geometry of the fig. y 1 + z = y 2 + h y 2 y 1 = z - h Sm Putting p B. γ.z S L. γ.h [ 1 ] 3. If points A and B are at the same level, z = o hence; Sm p B. γ.h [ 1 ] d) Inverted manometer (Fig. 2.4) 1. Applying Pascal law = p B p 1 S L. = p 2 S L. γ. y 2 Sm.γ.h p 1 p 2. γ ( y 1 y 2 ) Sm.γ.h 2. By geometry y 1 = y 2 + h i.e. y 1 y 2 = h 3. Substituting p 1 p 2.γ.h Sm.γ.h In step (1) Sm p 1 p 2. g.h ( 1 SL ) e) Bourdon s pressure gauge 1. The fluid under pressure enters the curved tube at the fixed end and tends to straighten out this tube. 2. The movement of the free end of the tube is transferred to the needle by means of levers, rack and pinion arrangements, and the needle moves to indicate the pressure on a properly calibrated scale. 8.0 Observations S L 1. Specific gravity of manometric liquid, (a) for differential manometer Sm = (b) for inverted manometer Sm = 2. Specific gravity of liquid flowing in pipe,s L = S L 8 MAHARASHTRA STATE BOARD OF TECHNICAL EDUCATION

6 Experiment No Sample Calculation 10.0 Results 11.0 Conclusion Note : Teachers should guide the students to write the conclusion keeping in view skills acquired and results obtained Questions Note : Distribute any 3-5 questions to each students for writing the answer in lab manual. 1) What should be the minimum diameter of the piezometer? 2) What is the reason to keep minimum diameter of piezometer? 3) What are the limitations of Piezometer? 4) What is the least count of Bourdon s pressure gauge? 5) Can we use bourdon pressure gauge to measure air pressure? 6) Why mercury is used in manometer? 7) In inverted manometer if mercury is used what will happen? 8) Define pressure and pressure head 9) State requirements of a liquid to be used as manometric liquid. 10) Why lighter fluids are used in inverted manometer? 13.0 (Space for Answers) MAHARASHTRA STATE BOARD OF TECHNICAL EDUCATION 9

7 (Space for answers) 10 MAHARASHTRA STATE BOARD OF TECHNICAL EDUCATION

8 Experiment No. 2 GRAPH MAHARASHTRA STATE BOARD OF TECHNICAL EDUCATION 11

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EXPERIMENTAL AERODYNAMICS FLOW TROUGH AN ORIFICE LAB SHEET

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