Figure 1: Pitot Tube.

A Pitot tube is a device used to measure the velocity of fluid flow, commonly used in aerodynamics and fluid mechanics applications. It measures both the total (stagnation) pressure and static pressure of the fluid to calculate its dynamic pressure and subsequently the flow velocity.

Working Principle of Pitot Tube

The Pitot tube operates on the principle of Bernoulli’s equation, which relates pressure, velocity, and height in a steady flow of an incompressible fluid. When a fluid enters the Pitot tube, it comes to rest (stagnates) at the opening of the tube, converting the kinetic energy of the fluid into pressure energy. This enables the measurement of the fluid’s velocity by comparing the total pressure with the static pressure.

Construction of Pitot Tube

A typical Pitot tube, as shown in the figure 1, consists of the following parts:

Ellipsoidal nose: The front tip of the Pitot tube that faces the fluid flow.

Spacers: Used to maintain the position and alignment of the internal parts.

Static pressure holes: Small holes on the sides of the tube, away from the flow direction, to measure the static pressure of the fluid.

Supporting stem: The structure that holds the Pitot tube in position.

Alignment arm: Ensures proper orientation of the tube in the fluid flow.

Static and Total Pressure Connections: Tubes leading to a manometer or pressure measurement system.

Δh (Delta h): Represents the difference in pressure levels, which corresponds to the dynamic pressure.

Working of Pitot Tube

1. The Pitot tube is inserted into the fluid flow with its nose aligned to face the flow.
2. The fluid enters the open end, where its velocity reduces to zero (stagnates). The pressure at this point is the total pressure.
3. Static pressure is measured using the side holes, which are perpendicular to the flow and do not disturb it.
4. The total pressure and static pressure are recorded, and the difference between them gives the dynamic pressure.

Stagnation point

When the solid body is kept centrally and stationary, in the pipe line with fluid streaming down, the velocity of the fluid diminishes due to the presence of the body still it is reduced to zero in front of the body. This is what is known as stagnation point. The inner (stagnation) tube is open ended. It faces the incoming stream of fluid.

The fluid impinging this open end is brought to rest and its kinetic energy is converted into pressure head. This pressure head is called as “Velocity head”. Thus the pressure sensed by the stagnation tube (stagnation pressure) is greater than that in the free stream by velocity head.

Stagnation pressure consists of velocity head and the static pressure head of the free stem. The static tube is closed at the nose of the tube. It has ellipsoidal head at the nose of the tube. This is the facility is to avoid flow separations. Stream lines next to the nose are longer than in the undisturbed flow. This indicates increase in the velocity.

On the other hand right angled stem stagnates the flow. It tends to raise the static pressure in its vicinity. For an accurate result the pitot tube is moved across the entire diameter of the pipe to measure the velocity at different points so that average velocity is calculated.

Working Formula of Pitot Tube

The velocity of the fluid is calculated using the relationship:

Where:

\[ v = \sqrt{\frac{2 (\Delta P)}{\rho}} \]

• $v$: Fluid velocity (m/s)
• $\Delta$$P$: Dynamic pressure (Total pressure – Static pressure) in Pascals (Pa)
• $\rho$: Density of the fluid (kg/m³)

Advantages of Pitot Tube

1. Simple construction.
2. Less pressure loss.
3. Can be inserted in the pipe very easily.
4. Useful to check mean velocity of flow

Disadvantages of Pitot Tube

1. Accuracy is very less.
2. Not suitable for low velocity measurements (below 5 m/sec).
3. Sensitive to misalignment of the probe w.r.t free stream velocity.
4. Not suitable for measurement of fluctuating velocities.
5. Unsuitable for dirty, sticky fluids.
6. Use is limited to exploratory studies.
7. Not commonly used in industrial applications.

Applications of Pitot Tube

1. Useful in gas flow measurements.
2. Measurements of flow through large pipes and ducts.
3. Stream measurement where accuracy is not that important.

The post What is Pitot Tube? Working Principle, Diagram, Construction, Advantages & Applications appeared first on Electrical and Electronics Blog.

Figure 1: Turbine Flow Meter.

Working Principle of Turbine Flow Meter

The turbine flow meter is based on Faraday’s Law of Electromagnetic Induction. As the liquid flows through the pipe, it rotates the rotor. The rotor’s blades are equipped with permanent magnets, which create a magnetic field.

When fluid flows through the pipe, the rotor starts to rotate due to the kinetic energy of the flowing liquid. The speed of rotation of the rotor is proportional to the flow rate of the liquid.

As the rotor rotates, the magnetic poles (N and S) of the magnets pass near the coil. This induces voltage pulses in the coil.

The frequency of the voltage pulses corresponds to the rotational speed of the rotor. By counting these pulses, the flow rate of the liquid can be determined.

Construction of Turbine Flow Meter

Pipe (1): Through which the liquid or gas flows.

Rotor (Turbine) (2): Inside the pipe, a rotor or turbine with small permanent magnets is mounted on a shaft. The rotor blades are angled to make the turbine rotates as fluid flows through it.

Shaft (3): The rotor is connected to a shaft that holds it in place and allows free rotation.

Coil (4): A magnetic pickup coil is placed outside the pipe. It senses the magnetic field changes caused by the rotating rotor.

Working of Turbine Flow Meter

At the centre of pipe line a shaft is supported in the bearing. Turbine blades NS, NS, NS pairs are fitted on the shaft. Rate of rotation of rotor is proportional to the rate of flow of liquid through the pipe. At the surface of the pipe a coil having no. of turns is fitted firmly. Now rotor blades rotate as fluid flow is impinged on them. Rotor blades are alternately N-pole, S-pole and hence a magnetic field is around the poles.

When the rotor rotates the magnetic field produced by them also rotates. It is a rotating magnetic field. This flux is cut by the number of turns of the coil fitted on the surface of pipe. As per Faraday’s law therefore EMF i.e. voltage is produced in the coil. Value of this voltage depends on the rotational speed of rotor and rotation is due to the flow of liquid.

Voltage ∝ fluid flow rate

The scale is calibrated such that this voltage indicates the fluid flow rate.

Advantages of Turbine Flow Meter

1. Better accuracy.
2. Allows low pressure drop.
3. It provides excellent repeatability and rangeability.
4. Low maintenance.
5. Easy installation.
6. It gives good temperature and pressure ratings.
7. Accuracy range is from ± 0.25% to ± 0.50%.
8. Repeatability ranging is from ±0.25% to ± 0.02%.
9. Available in sizes from 6.25 mm to 60 mm.
10. Liquid flow rate from 0.1 to 50,000 gallons/minute.

Disadvantages of Turbine Flow Meter

1. Very costly.
2. Creates problems for non-lubricating fluids.

Applications of Turbine Flow Meter

1. For measurements of liquid, gas.
2. Measuring low flow rates.
3. In military operations.
4. Useful in blending system for petroleum industries.
5. Useful in airborne applications for energy fuel and cryogenic (liquid, Oxygen and Nitrogen) flow measurement.

The post What is Turbine Flow Meter? Working Principle, Diagram, Construction, Advantages & Applications appeared first on Electrical and Electronics Blog.

Figure 1: Venturimeter.

A venturimeter is a device used to measure the flow rate of a fluid (liquid or gas) through a pipe. It is based on the principle of Bernoulli’s equation, which relates the pressure, velocity, and elevation of a fluid in steady flow.

Working Principle of Venturimeter

The venturimeter operates on the principle of the Venturi effect, which states that when a fluid flows through a constricted section of a pipe, its velocity increases while the pressure decreases. By measuring the pressure difference between the wider section and the constricted throat, the flow rate can be determined.

Main Parts of Venturimeter

1. U-tube manometer
2. High pressure tap
3. Low pressure tap
4. Diameter at inlet section (D)
5. Diameter at throat section (d)
6. α₁ inclined angle (19 to 23°)
7. α₂ inclined angIe (5° to 15°)

Construction of Venturimeter

1. Inlet Section:
   • A large-diameter section where the fluid enters.
   • It is connected to a high-pressure tap for pressure measurement.
2. Converging Section:
   • A gradually tapering section reduces the pipe diameter.
   • This accelerates the fluid, increasing its velocity and lowering the pressure.
3. Throat Section:
   • The narrowest section where the fluid velocity is at its maximum, and the pressure is at its minimum.
   • A low-pressure tap is placed here.
4. Diverging Section:
   • A gradually expanding section that slows down the fluid, recovering some of the pressure.
   • Ensures the flow returns to normal conditions downstream.
5. Pressure Taps:
   • High-pressure and low-pressure taps connected to a U-tube manometer.
   • Measures the pressure difference (Δ$h)$ between the inlet and throat.

Working of Venturimeter

Fluid enters the venturimeter through the inlet, where the pressure is measured using the high-pressure tap. As the fluid passes through the converging section, its velocity increases, and the pressure decreases due to the reduced cross-sectional area. At the throat, the velocity is at its maximum, and the pressure is at its minimum. The low-pressure tap measures the pressure at this point. The U-tube manometer measures the pressure difference between the inlet and the throat. The height difference (Δ$h$) in the manometer corresponds to the pressure difference. The fluid slows down as it flows through the diverging section, recovering some pressure while avoiding flow separation.

Liquid flow route

Flow comes from pipe of diameter ‘D’ and introduces inside, then passed through throat of less diameter ‘d’ then comes in the diverging section of inclination of angle α₂.

Location of pressure taps from where the manometer tube is introduced – One tap at inlet section and second at middle of throat section. This arrangement provided to measure the pressure difference (P₁ – P₂) by manometer u-tube.

Derivation of Flow Rate for a Venturimeter

The derivation involves applying Bernoulli’s equation and the continuity equation.

From Bernoulli’s Equation

\[P_1 + \frac{1}{2} \rho v_1^2 = P_2 + \frac{1}{2} \rho v_2^2\]

Rearranging to find the pressure difference:

\[P_1 – P_2 = \frac{1}{2} \rho \left( v_2^2 – v_1^2 \right)\]

The continuity equation is:

\[A_1 v_1 = A_2 v_2\]

From this, the velocity at the inlet (\(v_1\)) is expressed as:

\[v_1 = \frac{A_2}{A_1} v_2\]

Substituting \(v_1 = \frac{A_2}{A_1} v_2\) into the Bernoulli’s equation for pressure difference

\[P_1 – P_2 = \frac{1}{2} \rho \left( v_2^2 – \left( \frac{A_2}{A_1} v_2 \right)^2 \right)\]

Also

\[P_1 – P_2 = \frac{1}{2} \rho v_2^2 \left( 1 – \left( \frac{A_2}{A_1} \right)^2 \right)\]

The pressure difference (\(\Delta P\)) is

\[\Delta P = P_1 – P_2 = \frac{1}{2} \rho v_2^2 \left( 1 – \left( \frac{A_2}{A_1} \right)^2 \right)\]

Rearranging for \(v_2\)

\[v_2 = \sqrt{\frac{2 \Delta P}{\rho \left( 1 – \left( \frac{A_2}{A_1} \right)^2 \right)}}\]

The volumetric flow rate (\(Q\)) is

\[Q = A_2 v_2\]

Substitute \(v_2\)

\[Q = A_2 \sqrt{\frac{2 \Delta P}{\rho \left( 1 – \left( \frac{A_2}{A_1} \right)^2 \right)}}\]

The final flow rate equation is

\[Q = A_2 \sqrt{\frac{2 \Delta P}{\rho \left( 1 – \left( \frac{A_2}{A_1} \right)^2 \right)}}\]

where,

A₁, A₂​: Cross-sectional areas of the inlet and throat.

ΔP = (P₁ – P₂) : Pressure difference.

$\rho$: Fluid density.

Advantages of Venturimeter

1. Accurate measurement of flow rate.
2. Low energy loss compared to orifice meters.
3. Suitable for large-diameter pipes.
4. Good characteristics.
5. Suitable for flow of suspended fluids.
6. More accurate.
7. Low permanent loss.
8. Suitable for high flow rate.
9. No tear or wear as surface is smooth.

Disadvantages of Venturimeter

1. Large size so occupies more space.
2. Higher cost.
3. Not easy for inspection.
4. It cannot be used for small pipe diameters.

Applications of Venturimeter

1. Used in water supply systems, oil pipelines, and chemical industries.
2. For measurements, liquids, slurries, dirty fluids etc.

Features of Venturimeter

1. The main venturi tube is of cast iron or steel.
2. Flow coefficients = 0.984.
3. Accuracy much more ± 0.25 to ± 3%.
4. Sizes 100 mm to 813 mm.

The post What is Venturimeter? Working Principle, Construction, Formula, Diagram and Applications appeared first on Electrical and Electronics Blog.

Figure 1: Orifice Meter.

An orifice meter is a device used to measure the flow rate of a fluid (liquid or gas) through a pipeline. Orifice meter is a thin plate with a central narrow aperture which is introduced in a pipeline. Thus, when a fluid stream passes through the narrow constriction of the orifice, the velocity (i.e., kinetic energy) of the fluid at the orifice plate increases in comparison with its velocity at the entry of the pipe. Thus, the corresponding pressure energy decreases. The reduction in pressure can be measured by the manometer.

Working Principle of Orifice Meter

According to the Bernoulli’s theorem, the increase in the velocity head with the decrease in pressure head can be correlated between the two points i.e., where the manometer is arranged in a pipeline. The velocity of the fluid at the point (before entering the orifice) can be neglected due to the smaller diameter of the orifice than pipe diameter. In such a case, the velocity of the fluid is directly read from the manometer. The velocity of the fluid at the orifice can be calculated from the following formula.

\[{{u}_{0}}=\sqrt{2{{g}_{c}}\Delta H}……(1)\]

Where,

u₀ = Fluid velocity at the point of orifice

C₀ = Constant of orifice meter

ΔH = Difference in pressure head, m.

This is the simplest and widely used pressure differential flow meter. It consists of a thin plate with a central narrow aperture, which is smaller in diameter than the pipeline in which it is introduced. To measure the pressure difference, the manometer is connected to the pipe. The orifice meter can be placed in the side or bottom of the pipeline.

Working of Orifice Meter

When the fluid is introduced in the pipe, it passes through the narrow constriction of the orifice meter. Two points A and B are chosen in the orifice meter to demonstrate the velocity of the fluid flowing through the orifice. The fluid leaves the constriction of the orifice meter at point B with a velocity higher than the velocity at point A. Thus, the corresponding pressure head at point B is less than the pressure head at point A. The pressure difference (ΔH) is read from the manometer which is arranged at the points A and B.

The Bernoulli’s equation is applied for the two points A and B in the orifice meter as follows,

\[\sqrt{u_{0}^{2}-u_{A}^{2}}={{C}_{0}}\sqrt{2{{g}_{c}}.\Delta H}\]

Where,

u₀ = Fluid velocity at the point of orifice meter, m/s

u_A = Fluid velocity at the point A, m/s

C₀ = Constant

ΔH = Pressure difference, m.

When the diameter of the orifice is very less compared to the diameter of the pipe, then velocity of the fluid at point A (u_A) is less compared to the velocity of the fluid at the constriction (u₀). Hence, u_A is ignored.

Thus, equation (1) becomes

\[\sqrt{u_{0}^{2}}={{C}_{0}}\sqrt{2{{g}_{c}}.\Delta H}\]

\[{{u}_{0}}={{C}_{0}}\sqrt{2{{g}_{c}}.\Delta H}……(2)\]

The value of ‘ΔH’ is read from the manometer and substituted in equation (2) to calculate the velocity of the fluid flowing through the orifice meter, provided that the cross section of the pipe is known. The volume of the fluid flowing per hour is calculated.

In the above stated experimental conditions, the Bernoulli’s theorem is applied as follows.

\[{{X}_{A}}+\frac{u_{A}^{2}}{2{{g}_{c}}}+\frac{{{P}_{A}}}{g{{\rho }_{A}}}-F+W={{X}_{B}}+\frac{u_{B}^{2}}{2{{g}_{c}}}+\frac{{{P}_{B}}}{g{{\rho }_{B}}}……(3)\]

In the above equation, the following assumptions can be made to the orifice meter.

1. X_A = X_B because the heights of the points A and B are same and hence the terms get cancelled.
2. F = 0, friction losses are not appreciable and considered negligible.
3. ρ_A = ρ_B = ρ, because the fluid flowing through the orifice is the same.
4. W = 0, no work is done by the liquid.

The equation (3) is reduced to,

\[\frac{u_{A}^{2}}{2{{g}_{c}}}+\frac{{{P}_{A}}}{g\rho }=\frac{u_{B}^{2}}{2{{g}_{c}}}+\frac{{{P}_{B}}}{g\rho }\]

\[\frac{u_{B}^{2}}{2{{g}_{c}}}-\frac{u_{A}^{2}}{2{{g}_{c}}}=\frac{{{P}_{A}}}{g\rho }-\frac{{{P}_{B}}}{g\rho }\]

\[\frac{1}{2{{g}_{c}}}(u_{B}^{2}-u_{A}^{2})=\frac{1}{u_{B}^{2}}({{P}_{A}}-{{P}_{B}})\]

\[u_{B}^{2}-u_{A}^{2}=\frac{2{{g}_{c}}}{g\rho }({{P}_{A}}-{{P}_{B}})\]

\[u_{B}^{2}-u_{A}^{2}=\frac{2{{g}_{c}}}{g\rho }.\Delta P\text{          }\left[ {{P}_{A}}-{{P}_{B}}=\Delta P\text{ } \right]\]

\[u_{B}^{2}-u_{A}^{2}=2{{g}_{c}}.\Delta H\text{           }\left[ \frac{\Delta P}{g\rho }=\Delta H\text{ } \right]\]

To nullify the differences between the velocities at orifice and at vena contracta (point B) a constant, C₀ is included.

\[\sqrt{u_{B}^{2}-u_{A}^{2}}={{C}_{0}}\sqrt{2{{g}_{c}}.\Delta H\text{ }}\]

The velocity through the orifice is u₀.

\[\sqrt{u_{0}^{2}-u_{A}^{2}}={{C}_{0}}\sqrt{2{{g}_{c}}.\Delta H\text{ }}\]

u_A, is negligible as the orifice diameter is 1/5 th of the pipe diameter or less.

\[{{u}_{0}}={{C}_{0}}\sqrt{2{{g}_{c}}.\Delta H\text{ }}\]

Advantages of Orifice Meter

1. It is a simple machine having low cost.
2. It requires less space.
3. Easy to install and interchange.
4. Adjustable orifices are available.

Disadvantages of Orifice Meter

1. Permanent loss of pressure.
2. When the ratio of orifice to pipe diameter is above 0.75, the results are not accurate.
3. If the fluid contains solid particles, it may obstruct the orifice.
4. The orifice meter is not recommended above 1000 psi at 800ºF.

Applications of Orifice Meter

The velocity of the fluid at the two points (A and B) can be calculated. The volume of the fluid flowing per hour can be determined provided that the velocity of the fluid at point A (u_A) and cross-section of the pipe are known. Orifice meters are used in industries like oil and gas, water treatment, power, and HVAC for measuring fluid and gas flow. Applications include natural gas pipelines, boiler feed water, steam, chemical processing, slurry flow, and air systems. They’re cost-effective, reliable, and versatile for diverse industrial and commercial flow measurement needs.

The post What is Orifice Meter? Working Principle, Construction and Diagram appeared first on Electrical and Electronics Blog.

Figure 1: McLeod Gauge.

Working Principle of McLeod Gauge

It operates on the principle of compressing a known volume of low pressure gas to a higher pressure and measuring the resulting change in volume by a mercury manometer.

Construction and Working of McLeod Gauge

The construction of McLeod gauge is shown in Fig. 1. McLeod gauge comprises a system of glass tubing made of tough glass and mercury is used to trap the known volume of gas. The gauge is connected to the unknown gas whose pressure is to be measured.  The plunger moves up, lowers the mercury level to the cut off positions, entering the gas at unknown pressure through the tube.

This gas fills the tubes down to the cut off position of mercury level. Here the pressure is equal throughout the tubes.

Now the plunger is moved down to rise the mercury above the cut-off and traps the gas inside the bulb and measuring capillary.

Further pushing of plunger compresses the gas in the measuring capillary and mercury in the reference capillary reaches to zero reference line.

The pressure in the measuring capillary is higher than the measured pressure in the reference capillary. This difference in pressure causes the difference in mercury level in two tubes.

This difference in height represents the rise in gas pressure and unknown pressure is calculated.

Advantages of McLeod Gauge

1. It is very simple in use.
2. Measurement is independent of gas composition and it is related to physical dimensions of gauge.
3. It is a very accurate pressure measuring device.
4. It can be used as a standard to calibrate other low pressure gauges.

Disadvantages of McLeod Gauge

1. If the gas contains the vapour, it may not give correct result.
2. It is applicable to those systems where mercury is tolerable.
3. It does not give continuous output.

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Figure 1: Ionization Gauge.

Working Principle of Ionization Gauge

Extremely low pressures can be measured with ionization gauge (10^-3 torr and below). Ionization means process of producing free electron and a positively charged ions by knocking off an electron from an atom. A simple arrangement of hot filament ionization gauge is shown in Fig. 1.

Working and Construction of Ionization Gauge

The gauge consists of triode vacuum tube i.e. cathode, grid and anode plate. Cathode serves as a heated filament. Grid is positive changed.  The anode plate is maintained at negative potential with respect to cathode.

Thus the cathode plate is positive ions collector and anode plate is electron collector. This assembly is kept in a vacuum system whose pressure is being measured.

The heated cathode emits the electrons which move past the grid. The positive grid accelerates these electrons where they collide with gas molecules causing its ionization.

The anode is negative so positive ions collect there producing plate current I_l the electrons and negative ions are collected by grid, produces grid current I₂ in the grid circuit.

This ratio of current I_l and I₂ gives the vacuum pressure measurement. The vacuum pressure is given by,

\[{{P}_{vaccum}}=\frac{{{I}_{1}}}{{{{I}_{2}}K}}\]

Where k is the proportionality constant known as sensitivity of the gauge.

Advantages of Ionization Gauge

1. Very low pressure measurement up to 10^-11 torr is possible.
2. It can give continuous pressure reading.
3. They have good linearity.
4. Best suited for the wide pressure range from 10^-3 torr to 10^-9 torr.

Disadvantages of Ionization Gauge

1. Few gases like oxygen carbon dioxide get decomposed by the hot filament.
2. Chances of burning the filament if exposed to air.

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Figure 1: Rotameter.

Working Principle of Rotameter

Rotameter consists of a vertical, slightly tapered, transparent tube, which consists of a plummet or float. The float is free to rise or fall in a tapered glass tube due to the variation in the flow of fluid.

Construction of Rotameter

The rotameter consists essentially of a tapered metering glass tube. Inside the tube is a float, which is the active element of the meter. The float material has a specific gravity higher than that of the fluid to be metered. The spherical slots cut into a part of the float cause it to rotate slowly about the axis of the tube and keep it centred. The stability of the float is ensured by employing a guide along which the float would slide.

Working of Rotameter

With an increase in the flow rate, the float rises in the tube and there occurs an increase in the annular area between the float and the tube. The float adjusts its position in relation to the discharge through the passes the float rides higher or lower depending on the flow rate.

Advantages of Rotameter

1. Low cost, direct indicating, minimum piping required.
2. Pressure loss in rotameter is nearly constant and small.
3. It can handle any corrosive fluid.
4. It has quite good accuracy especially at low now rates.
5. It provides linear scale.
6. It can be compensated for changes in fluid density and viscosity.
7. The capacity can be changed with relative ease by changing float, tube.
8. Condition of flow is readily visible.

Disadvantages of Rotameter

1. It must be installed in vertical position only.
2. For high pressure and temperature, it is expensive.
3. When opaque fluid is used, float may not be visible.
4. It cannot be used with liquid carrying large percentage of solids in suspension.

Applications of Rotameter

1. It is used for measurement of flow of liquid and gases.
2. The rotameters are used in large scale drug industries.
3. The rotameters are used in fermenters to control the supply of air.

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Figure 1: Bellows Pressure Gauge.

Working and Construction of Bellows Pressure Gauge

The bellows pressuree gauges are used for the measurement of absolute pressure. It is more sensitive than Bourdon tube gauges. It is generally used for the range down to 155.1 mm Hg (3 psi). It may be used for even lower pressures upto 40 mm Hg by making the bellows large enough.

The bellows are made of an alloy which is ductile, has high strength and retains its properties over long use i.e. has very little hysteresis effect. They are used in two forms.

In one arrangement, pressure is applied to one side of the bellows and the resulting deflection is counter balanced by a spring as shown in Fig. 1. This arrangement indicates gauge pressure. They are called as spring opposed bellow elements. They are very sensitive and are quite useful in working signaling and tripping devices because of the considerable amount of movement for a given change in pressure.

It is made of a metallic bellows enclosed in a shell which is connected to a pressure source. Pressure acting on the outside of the bellows compresses the bellows and moves its free end against the opposing force of the spring. A rod resting on the bellows transmits the motion to a pointer. Phosphor bronze is the commonly used material for bellows and the springs are of carefully heat treated metal.

In another arrangement, the differential pressure is also indicated.

In this device, one pressure is applied to the inside of one sealed bellow while the other pressure is applied to the inside of another sealed bellow as shown in Fig. 2. By suitable linkage and calibration of the scale, the pressure difference is indicated on the scale. They are extremely useful for larger static pressures upto 2000 psi and larger differential pressures upto 50 psi.

Advantages of Bellows Pressure Gauge

1. Moderate cost.
2. Delivery of high force.
3. Adaptability for absolute and differential pressures.
4. Good in low to moderate pressure range.

Disadvantages of Bellows Pressure Gauge

1. Ambient temperature compensation needed.
2. Unsuitability for high pressures.
3. Limited availability of metals and work-hardening of some of them.
4. Unsuitability of its zero and the stiffness.

Hence, it is used only in conjunction with a reliable spring of appreciably higher stiffness for accurate characterization.

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Figure 1: Float Level Sensor.

Working and Construction of Float Level Sensor

The float type level indicator is shown in Fig. 1. A float rests on the surface of liquid and follows the changing level of liquid. The movement of the float is transmitted to the pointer by a stainless steel or phosphor-bronze flexible cable wound around a pulley, and the pointer indicates liquid level in the tank.  The float is made is made of corrosion resisting material and it rests on liquid level surface between two grids to avoid error due to turbulence. With this type of instrument, liquid level from 1/2 ft. to 60 ft. can be easily measured.

Figure 1: Hydraullic Transmission system for level indicator.

The liquid level can be transmitted to a distant place using a hydraulic transmission system as shown in Fig.1.  It consists of four bellows elements, two on the transmitter side (C and D) and two on the receiver side (A and B) and are fixed at the outer ends.

Bellows B, C and A, D are hydraulically connected through pipes filled with an oil. When the float moves up or down according to change in level, its position is transmitted by a lower arm to the bellows assembly A – B.

When the level rises, B is compressed and A expands, thus causing oil in the pipe to flow from B to C, and from D to A. The bellows are balanced so that the movement at the transmitter end is reproduced in reverse made at the receiving end i.e. C expands and D is compressed as the liquid level rises.

The two bellows C and D act on the compensating link pivoted on the pointer, in the same directions and will rotate the pointer in proportion to the level value. Any change in ambient temperature affects both the bellows i.e. both expand or contract.  Therefore level transmissions at the pointer is not affected with this type of instrument level transmission up to 250 ft. can be achieved.

Advantages of Float Level Sensor

1. Possible to read liquid levels, in a tank from ground level even if the tank is kept below the ground level.
2. Low cost.
3. Large temperature range.
4. Reliable design.
5. Possible to make it using corrosion-resistant material.

Disadvantages of Float Level Sensor

1. Limited to moderate pressure.
2. Tailored to tank geometry.

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Working and Construction of Diaphragm Pressure Gauge

In high precision instruments the diaphragms are generally used in a pairs, back to back, to form an elastic capsule. Metallica and slack are two types of diaphragm which are generally used.

1. Metallic diaphragm:

Figure 1: Metallic Diaphragm.

Metallic diaphragm gauge consists of a thin flexible diaphragm made of materials such as brass or bronze. It is shown in Fig. 1. A pointer is attached to diaphragm. The force of pressure against the effective area of the diaphragm causes a deflection of the diaphragm. In some cases the deflection of the diaphragm is opposed by the spring qualities of the diaphragm itself and in other cases a spring is added to limit the deflection of the diaphragm. The motion of the diaphragm operates an indicating instrument.

This type of gauge is capable of working in any position and is portable, and hence well adapted for use or for installation in moving equipment’s such as aircrafts.

2. Slack diaphragm :

Figure 2: Slack Diaphragm.

Slack diaphragm gauge is shown in Fig. 2. It is made of rubber or other flexible materials. It is used to measure pressure below the atmospheric pressure.  The full range from atmospheric pressure to a perfect vacuum is only 14.7 psi. As this gauge has slack diaphragm it can move by a large distance in response to a small pressure change. A slack diaphragm gauge with a weak spring and a large are can be used over pressure ranges as low as 0.01 to 0.4 mm Hg with accuracy of 1-2 %.

Advantages of Diaphragm Pressure Gauge

1. Moderate cost.
2. High over range characteristics.
3. Adaptability to absolute and differential pressure measurement.
4. Good linearity.
5. Availability in several materials for good corrosion resistance.
6. Small size.
7. Adaptability to slurry services.

Disadvantages of Diaphragm Pressure Gauge

1. Lock of good vibration and shock resistance.
2. Difficult to repair.
3. Limited to relatively low pressures.

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