Lecture 02 - Fluid Pressure PDF

Fluid Pressure and it’s Measurements Lecture 02 BY: Eng. S.D.R.Lakmal Lecturer Faculty of Technology Absolute, Gauge, Atmospheric and Vacuum Pressure 1.Absolute pressure The actual pressure at a given position is called the absolu...

Fluid Pressure and it’s Measurements Lecture 02 BY: Eng. S.D.R.Lakmal Lecturer Faculty of Technology Absolute, Gauge, Atmospheric and Vacuum Pressure 1.Absolute pressure The actual pressure at a given position is called the absolute pressure, and it is measured relative to absolute vacuum (i.e., absolute zero pressure). Absolute, Gauge, Atmospheric and Vacuum Pressure Absolute, Gauge, Atmospheric and Vacuum Pressure 2. Gauge pressure Gauge pressure is the pressure relative to the atmospheric pressure. In other words, how much above or below is the pressure with respect to the atmospheric pressure. Absolute, Gauge, Atmospheric and Vacuum Pressure Absolute, Gauge, Atmospheric and Vacuum Pressure 3. Vacuum pressure Pressures below atmospheric pressure are called vacuum pressures and are measured by vacuum gauges that indicate the difference between the atmospheric pressure and the absolute pressure. Absolute, Gauge, Atmospheric and Vacuum Pressure Measurement of Pressure The pressure of a fluid is measured by the following devices: 1. Piezometer 2. Manometers 3. Mechanical Gauges. 4. Pressure transmitters Fluid Pressure and it’s Measurements 01. Piezometer BY: Eng. S.D.R.Lakmal Lecturer Faculty of Technology Piezometer(Pressure Tube). Piezometer(Pressure Tube). As shown in the figure, one end of the piezometer is open to atmospheric pressure, and the other end is connected to the point A, where pressure is to be measured. The rise of liquid will be in accordance with the pressure at point A. If h is the height of liquid in the piezometer, pressure at Pressure = ρ × g × h point A is given by: Piezometer. Ex 01:- A pressure tube is used to measure the pressure of oil ( mass density, 640 kg / m3 ) in a pipeline. If the oil rises to a height of 1.2m above the centre of the pipe, what is the gauge pressure in Nm-2 at that point? (gravity = 9.81 m/s2) Piezometer. Fluid Pressure and it’s Measurements 02. Manometers BY: Eng. S.D.R.Lakmal Lecturer Faculty of Technology Manometers Manometers are pressure measuring devices which are based on the principle of balancing the column of liquid (whose pressure is to be found) by the same or another column of liquid. Manometers In its simplest form, a manometer is a U-shaped tube consisting of an incompressible fluid like water or mercury. It is inexpensive and does not need calibration. Manometers Applications ❖ Used in the maintenance of heating, ventilation, and air conditioning (HVAC) systems, low pressure pneumatic or gas systems. ❖ Construction of bridges, installing swimming pools and other engineering applications. ❖ Climate forecasting. ❖ Clinical applications like measuring blood pressure and in physiotherapy. ❖ Piezometers are used to measure the pressure in pipes where the liquid is in motion. Principle of Manometers Principle of Manometers Principle of Manometers (p1 - p2) = hρg p1 = pressure to be measured p2 = atmospheric pressure ρ = density of the liquid used in manometer Hence, ρg = specific weight of the liquid Principle of Manometers p1 = p2 + hρg Principle of Manometers Manometers Ex 02:- The pressure of a sample of gas is measured with a closed-end manometer, as shown to the right. The liquid in the manometer is mercury. Determine the pressure of the gas. (ρ=13600kgm-3) Manometers Ex 03:- The pressure of a sample of gas is measured at sea level with an open-end Hg (mercury) manometer, as shown to the right. Determine the pressure of the gas. (ρ=13600kgm-3 , Patm =1 x 105 Pa) Manometers The manometers may be classified as, a)Simple Manometers. b)Differential Manometers. Simple Manometers In general a simple manometer consists of a glass tube having ore of its ends connected to the gauge point where the pressure is to be measured and the other remains open to atmosphere. Simple Manometers Some of the common types of simple manometers are as noted below: I. U tube Manometer II. Well type or reservoir manometer III. Inclined manometer IV. Float type manometer  Fluid Pressure and it’s Measurements 03. Mechanical Gauges BY: Eng. S.D.R.Lakmal Lecturer Faculty of Technology Mechanical Gauges Mechanical gauges are those pressure measuring devices, which embody an elastic element, which deflects under the action of the applied pressure, and this movement mechanically magnifies, operates a pointer moving against a graduated circumferential scale. Generally these gauges are used for measuring high pressures and where high precision is not required. Some of the mechanical pressure gauges which are commonly used are as noted below: 1) Bourdon Tube Pressure Gauge 2) Diaphragm Pressure Gauge 3) Bellows Pressure Gauge 1) Bourdon Tube Pressure Gauge. Scan This 2)Diaphragm Pressure Gauge. Scan This 3.Bellows Pressure Gauge. Scan This Thank you