The elastic pressure elements are primary sensing elements that are designed to operate under a pressure gauge by bending, deforming or deflecting or by moving some device an amount by which it depends on pressure change.
• These elastic members convert the pressure into mechanical displacement which is later converted into an electrical form using a secondary transducer.
• Commonly used elastic elements for pressure measurement are –
(i) bourdon Tube Pressure Gauge
(ii) Bellows Pressure Gauge
(iii) Diaphragm Pressure Gauge
Bourdon Tube
Due to effectiveness and reliability of bourdon tube pressure gauge it is used in stream plant, chemical processes and other industries.
Construction of Bourdon Tube Pressure Gauge
As shown in Fig. 1
• It consists of a seamless metal tubing hawing elliptical cross-section.
• The tube is enclosed in a socket which contains pressure inlet.
• The free end of the tube is called tip.
• The socket, tube and tip are welded, brazed or soldered together according pressure ranges.
• Tube diameter is 0.5 mm to 1.25 mm
Materials used are bronze – used upto 600 psi, beryllium copper – used upto 10,000 psi, stainless steel used up and above 10,000 psi, chrome-molybdenum etc. materials are selected according to maximum pressure to be measured, cost, possibility of corrosion.
Working of Bourdon Tube Pressure Gauge
When pressure is applied, bourdon tube tries to become circular and also tries to unwind pointer shows the deflection on circular scale, that is direct measurement of pressure in kg/cm² or psi deflection depends on
(i) radius of band,
(ii) total tube length,
(iii) wall thickness (diameter) of tube,
(iv) modulus of elasticity of tube material.
For example, for filling air in tyres of vehicles, cycles we go to the garage.
• They use the meter that checks the air pressure in tubes. that meter uses bourdon tube inside it.
Types of Bourdon Tube
There are four types of bourdon tube
(i) C shaped Bourdon Tube - pressure range is from 10 psi to 100,000 psi.
(ii) Spiral Bourdon Tube - below 1000 psi.
(iii) Helical Bourdon Tube - below 1000 psi.
(iv) Twisted bourdon Tube.
C type Bourdon Tube
• When pressure is applied at inlet of socket tip deflects when the pressure is maximum motion of tip is heardly 6 mm.
• There is arrangement for covering this linear displacement into a circular one usually the deflection of pointer is around 270° gauge is precision made particularly gear.
• Hardened bushings are used for pivot and spindle sector gear has an extension beyond pivot called lever.
• Lever is connected by adjustable link to tip.
• Motion of the tip is transmitted by means of line, lever, segment gear, pinion gear and lastly to the spindle and pointer. The obtainable size is from 5 cm to 35 cm diameter in many different indicating styles.
Spiral Bourdon Tube
Fig. 2 shows the spiral bourdon tube where the socket pressure connection is provided at the centre. Free end can be wound several times. When pressure is applied from the inlet (a) then the amount movement is proportional to the angle subtended by total are. For a given pressure the tip deflection is more so that it is quite sensitive.
Helical Bourdon Tube
A large instrument case is not necessary because the mean diameter of turns can be kept constant over a wide range of pressure. So that helical elements are frequently used where multiple recording on same chart are required. Helical elements are therefore frequently used where multiple recording on the same chart are required.
Twisted Bourdon Tube
