3.Pressure Measurment-Bourden Tube,Diaphragm Guage,Bellow

 

There are four types of Mechanical pressure sensing elements which are explained below.

1) Bourdon Tube: Bourdon tubes are hollow, cross-sectional beryllium, copper, or steel tubes, shaped into a three quarter circle.

Operating principle is that when the input pressure is applied to the bourdon tube, it converts it in to displacement.

The outer edge of the cross section has a larger surface area than the inner portion and when a pressure is applied outer edge will have large force applied because of its large surface area, and it will lead to expand the outer edge.

Types of Bourdon tubes:    a. C type,     b. Helical,   c. Spiral.

a) C type: It is a simplest pressure measuring element.

It consists of a length of thin-walled metal tubing which has been flattened, to approximately an elliptical cross section and then rolled into a C shape, having an arc span of about 270°. This tube is supported by a socket.

Pressure-inlet passageway is connected to the tube though the socket. The free end of the tube is called as Tip and it is permanently sealed.

Displacement is amplified by the lever and given to the sector.

A pinion attached with a pointer is mounted on same sector which rotates the pointer on a pressure calibrated scale.

The deflection of the tip of the tube depends upon

The radius of the bend, The total tube length, The wall thickness of the tube,

The major and minor axes of the cross section, and Young's modulus of elasticity of the tube material.

Bourdon tubes will measure pressure up to 80,000 psi, it may be as low as 0 to 15 psi and 0 to 50 psi is a quite common range. The most common materials used for the Bourdon tube are trumpet brass, phosphor bronze, beryllium copper, alloy steel, and stainless steel.

       The selection of a material will depend on The maximum pressure, Modulus of elasticity, Cost and Corrosion resistance.

 

b) Helical type: In this type the sensing element is formed in helical spring shape.

It also converts applied Pressure to Displacement.

When input pressure is applied, pointer will rotate along with its axis and pointer end showing reading on a scale which is marked in pressure units.

The sensitivity of this type is more due to its angular length so no external gain amplification is required.

c) Spiral Type: In this type the sensing element is formed in the spiral spring shape.

The inner end of the tube is treated as reference and outer free end gives the displacement according to applied pressure.

If a pointer is attached to the outer free end of the tube, then it directly gives the pressure measurement on a scale which is marked in pressure units.

Errors in the Bourdon Tube:

1. Zero error: If zero-input is applied (minimum input) output should be minimum. It remains constant throughout the range.

2. Span Error: Span error or multiplication error is the mismatching of the input span and output span. Span adjustment will be done by changing the input lever length.

3. Angularity Error (non-linearity): Angularity error comes due to the link and lever mechanism because, same travel of the link, lever will not give same change in angle. Can be eliminated by making the tip travel minimum and change the input lever length.

4. Hysteresis Error: It occurs due to the property of material used. It cannot be removed, so that in designing it should be taken minimum.

2) Diaphragm Gauge: Diaphragm is a thin circular or rectangle type metallic or non-metallic disc.

It converts applied pressure in to force which in turn converted in to displacement due to the spring property of metallic diaphragms and the displacement of the diaphragm depends upon

1. Applied pressure, 2. Diameter of the diaphragm,  3. Thickness of the diaphragm, 4. Material of the diaphragm.

displacement of diaphragm is directly property to the applied pressure because rest of all are constant.

It is simplest form a diaphragm. It consists of a thin flexible material like beryllium copper, leather, or rubberized fabric. Suitable for measuring low pressure and vacuums. The applied pressure is sensed by diaphragm.

Displacement of the diaphragm is transmitted to a lever arrangement which is further amplified by sector and pinion arrangement.

A pointer is attached on pinion, give the measurement of applied pressure on the scale which is marked in terms of pressure.

Types of Diaphragm Gauge:

Based on the material of construction:

1) Metallic Diaphragm and     2) Nonmetallic Diaphragm.

Based on type of construction: 

        1) Flat Diaphragm.  2) Corrugated Diaphragm.

 

 

 

 

 

Advantages of Diaphragm Type Pressure Gauges:

1. Linearity is good and cost is less.

2. Can able to measure both absolute and differential pressures.

3. Available in several materials which are having good

   Corrosion resistance.

4. Suitable for slurry services pressure measurement also.

 

Disadvantages of Diaphragm Type Pressure Gauges:

1. Maintenance, repairing is difficult.

2. Not suitable for high pressure Measurement.

 

 

3)Bellow Type of Pressure Gauge: Bellow elements have a thin walled metal tube.

It permits axial expansion and compression when applied pressure is changing.

Bellow converts applied pressure in to displacement which is further measured in terms of pressure.

Materials commonly used are Brass, Bronze, Beryllium copper, alloy of Steel and Monel.

To increase the pressure range normally calibrated springs are used.

4)Capsule Type of Pressure Gauge: Capsule is the combination of two diaphragms, Pressure is applied on the center of both diaphragms.

One end is fixed & the other end is connected to a pinion & pointer.

Sensitivity is high so suitable for low pressure measurement.

 

Advantages of Bellows: 1. Large displacement can be obtained.     

  2. Moderate cost.           3. Linear displacement can be obtained.

  4. Best suitable for absolute and differential pressure Measurement.

Disadvantages:

1. Not suitable for High pressure measurement.

2. Required calibrated spring for accurate measurement.

Advantages of Bellows: 1. Large displacement can be obtained.     

  2. Moderate cost.           3. Linear displacement can be obtained.

  4. Best suitable for absolute and differential pressure Measurement.

Disadvantages:

1. Not suitable for High pressure measurement.

2. Required calibrated spring for accurate measurement.