pressure measuring device

manometers

1. PRESSURE MESURING DEVICE BY:-SACHIN KARDAM

2. PRESSURE MEASUREMENT Pressure measurement is a very common requirement for most industrial process control system and many different types of pressure-sensing and pressure measurement systems are available. The absolute pressure of a fluid defines the difference between the pressure of the fluid and the absolute zero of the pressure. PRESSURE:- It is define as force exerted per unit area Units and scales of Pressure Measurement Pascal (N/m 2 ) is the unit of pressure .                               

3. Pressure is usually expressed with reference to either absolute zero pressure (a complete vacuum)or local atmospheric pressure. The absolute pressure: It is the difference between the value of the pressure and the absolute zero pressure.                              Gauge pressure: It is the diference between the value of the pressure and the local atmospheric pressure(p atm )                               

4. Hydrostatic law Hydrostatic law defines the variation of pressure with respect to the elevation of fluid particles under static condition and states that “as the distance from a datum surface level increases the pressure drops”. When an object pushed in a fluid to its bottom, it compresses the fluid. so there arise an increase in pressure force around it and is termed as “ Hydrostatic force .”

5. The cylindrical element will be subjected to pressure force at one end with height z 1 is P and at the other end of height z 2 is P+ dP and also the weight of the fluid element dw which acts vertically downwards. Under equilibrium of the element, the resultant of forces in any direction is zero and resolving the forces in the direction along the central axis gives

6. dw = p*g*dA*ds z 1 Cylindrical fluid element with an arbitrary orientation in a container ( P * dA) − [( P + dP)dA] − ( ρ g * dA* ds * cos θ )=0 ( dP) = −(ρ g * ds * cosθ ) By rearranging ⇒ dP     g * cos   ds Note: Case 1: If the cylindrical fluid element is vertical, then the angle of orientation θ = 0 o , then becomes dP = − ( ρ g )= − Pg dz RT − − −

7. Case 2: If the cylindrical fluid element is horizontal, then the angle of orientation θ = 90° then becomes and so pressure is constant (i.e. P = constant) throughout in a static liquid, at a given height. dP = dz _

8. TYPE OF PRESSURE MEASURING DEVICES MANOMETER MECHANICAL GAGES TYPE OF MANOMERTER PIEZOMETER U-TUBE INCLINED MANOMETER DIFFERENTIAL MANOMETER

9. Manometer Manometer is a device used to measure pressure at a single or multiple points in a single or multiple pipelines, by balancing the fluid column by the same or another column of fluid. Manometers can be categorized into two types, namely simple manometer and differential manometer. Simple manometric devices measure pressure at a single point in a fluid, whereas differential manometric devices measure pressure at two or more number of points, in a single or multiple flow lines. The simplest type of manometer is simple manometers, wherein we have three types of simple manometers like Piezometer , U-tube manometer and Single column/Micro manometer . Simple manometers consist of a glass tube straight or bent, whose one end is connected to the point at which the measurement of pressure is required and the other end is left open to the atmosphere

10. A simple manometer to measure gauge pressure A simple manometer to measure vacuum pressure Manometer is needed for measuring large gauge pressures. It is basically the modified form of the piezometric tube. A common type manometer is like a transparent "U-tube "

11. The pressures at two points P and Q (Fig .) in a horizontal plane within the continuous expanse of same fluid (the liquid B in this case) must be equal. Then equating the pressures at P and Q in terms of the heights of the fluids above those points, with the aid of the fundamental equation of hydrostatics we have Hence, where p 1 is the absolute pressure of the fluid A in the pipe or container at its centre line, and p atm is the local atmospheric pressure. When the pressure of the fluid in the container is lower than the atmospheric pressure, the liquid levels in the manometer would be adjusted as shown in Fig. . Hence it becomes,

12. Piezometer Piezometer is a simple manometric device which measures pressure at a point in a fluid, without balancing any other fluid column. Consider a cylindrical vessel having a fluid filled in it and is open to atmosphere. Connect a piezometer glass tube at the point where pressure is to be measured, such that the other end of the piezometer tube is open to the atmosphere. As soon as the piezometer glass tube is connected to the cylindrical tank, some amount of the fluid rushes into the tube, due to atmospheric pressure outside the cylindrical tank. But as the other end of the piezometer glass tube is exposed to atmosphere, atmospheric pressure acts at that open end also and hence fluid raises up to some level in the glass tube and does not overflow from the tube.

13. vertical height ,

14. U-tube manometer U-tube manometer is a simple manometric device used to measure pressure at a point in a fluid, by balancing the fluid column by the same or another column of fluid. It has a glass tube bent in “U” shape with some amount of same or other type of fluid, called manometric fluid like mercury ,

15. The axis of each connecting tube at A and B should be perpendicular to the direction of flow and also for the edges of the connections to be smooth. Applying the principle of hydrostatics at P and Q we have, where, ρ m is the density of manometric fluid and ρ w is the density of the working fluid flowing through the pipe. We can express the difference of pressure in terms of the difference of heads (height of the working fluid at equilibrium).

16. Differential/Single column/micro-manometers Single column/micro-manometers are just similar to the U-tube manometer, only with a difference of having a small reservoir in the tube Micro-manometer has a glass tube bent in “U” shape or inclined and has a small reservoir in the tube, having some amount of same or other type of fluid called manometric fluid . When an additional gauge liquid is used in a U-tube manometer, a large difference in meniscus levels may be obtained for a very small pressure difference.

17. Differential manometer consist of a U-tube containing a heavy liquid whose two ends are connected to the point whose diff. of pressure is required to be find out. The monometer contain monometric liquid of different densities. It consist of U-tube made of glass, the end of which are connected two enlarge transparent reservoir.

18. The equation of hydrostatic equilibrium at PQ can be written as From continuity of gauge liquid, where and are the densities of working fluid, gauge liquid and manometric liquid respectively. If a is very small compared to A

19. Inclined differential Inclined differential U-tube manometer is a device just as same that of U-tube differential manometer but is inverted in construction. It measures pressure difference  P at two different points “A” and “B” in a single pipe or different pipes, carrying two fluids of same or different kinds For the measurement of small pressure differences in liquids, an inverted U-tube manometer is used . The one are of the manometer is inclined at an angle of (5-10angle) with horizontal so, as to obtaining large reading.