Pressure Measurement Device

BELL DIFFERENTIAL PRESSURE GAUGETHERMIONIC IONIZATION VACUUM GAUGE

BELL DIFFERENTIAL PRESSURE GAUGE

Bell differential pressure gauge bell type of pressure gauge the force produced by the

difference of pressures on the outside and the inside of a bell is balanced against a weight, or against the force produced by the compression of a spring

The bell gauge is a type of pressure transducer that measures differential pressure between 0.06 Pa and 4 Kpa

The static pressure may be as high as 4 to 6 MPa

Schematic Diagram

Bell differential pressure gauge The instrument consists of a bell suspended

with the open end downwards in a sealed chamber usually made of cast iron, containing a liquid such as oil or mercury.

The liquid covers the open end of the bell and acts as a seal, so forming two chambers.

Working process (single element )When pressure p1 and p2 apply on the system The resulting force causes the bell to rise until equilibrium is

reached between the upward force and the apparent weight of the bell.

As the bell rises, there is less of it immersed in the sealing liquid, so that the up thrust on it due to buoyancy is reduced

The thickness and density of the material form which the bell is made, its cross sectional area, and the density of the sealing liquid, are determined by the range for which the instrument is to be used.

NIT CALICUT

Working process (single element)

Working process (single element )the pressure within the bell is greater than

that outside, it will cause the level of the liquid on the outside of the bell to be greater than the level on the inside, as well as causing the bell to rise

The movement of the bell is taken out by link and lever mechanism or by some electrical methods

Working process ( two element )two bells inverted in a bath of oil are usedThese bells are suspended from the balance beam which is

carried on pivot and socket type bearings having a very small surface contact giving small friction. The two pressures to be compared are led into the inside of the bells, and the pressure differential is indicated by the pointer which moves with the balance beam.

Since the restoring force is small (being produced by the change in position of the centre of Gravity of the balance beam it rotates) the instrument is sensitive to very small changes of pressure.

Working process ( two element )Both bells are subject to the same changes of

ambient temperatures, so that the instrument is unaffected by temperature changes.

Changes as small as 2.5 x 10⁻² mm of water gauge may be detected, and The instrument is used for controlling furnace pressure.

Working process ( two element )Pressure difference =

(p2 –p1)=wd/LA * sinα Where A = Area of bells L = Distance from pivot to bell support point α = angle of deflection of beam from

horizontal d = distance from pivot to counter weight

Thermionic Ionization Vacuum gauge

Thermionic Ionization Vacuum gauge In an Thermionic Ionization Gauge, the residual gas

molecules are ionized using an electron beam. A + eˉ ― A⁺ + 2e⁻ •In the above reaction, we have the following.Ais the gas

molecule from the residual gas.e-is the ionization electron beam.A⁺ is the ionize⁺d gas molecule.2e-are the electrons in the electric current.

Thermionic Ionization Vacuum gauge This reaction produces, two different types of current.

They are I+and I-. For the measurement of vacuum pressures, there are

two types of Ionization Gauges. They areThermionic Ionization GaugeCold Cathode Gauge These gauges operate accurately up to very low

pressures, typically, in the order of 10-³to 10-¹⁰ mbar.

Thermionic Ionization Vacuum gauge The schematic of a Thermionic Ionization Gauge is as shown in

the figure. It consists ofThermionic Filament FCylindrical open mesh grid GIon collector C The Thermionic Filament F emits the electrons to ionize

the residual gas The mesh grid G traps the electrons to measure

electronic current

Thermionic Ionization Vacuum gauge

Thermionic Ionization Vacuum gauge The filament is charged with a positive potential of +30

V. The grid is maintained at +180 V.

This large positive potential difference is required to accelerate the electrons in least possible time.

•The ion collector C is earthed, in order to maintain a zero potential.

•The electrons are emitted from F and are accelerated towards the grid.

Thermionic Ionization Vacuum gauge

A majority of electrons strike the grid.•However, a few of the electrons move beyond

the grid, due to porosity of the grid and high velocity of electrons.

•These electrons enter a region of decelerating field in between mesh grid Gand collector C.

Thermionic Ionization Vacuum gauge They oscillate back and forth and are eventually

collected on the grid.•During this phase, the electrons have a maximum

probability to hit the residual gas molecules, which produce ionic current.

•This ionization current represents the ions in residual gas. This is directly calibrated to read the gas pressure.

These gauges are used from 10-3to 10-7mbar.

Advantages

It offers a high reliability and ease of operation.

•It can be easily degassed by electron bombardment (Grid power = 35 W).

•These gauges offer a linear calibration current and pressure

Disadvantages

The use of hot filament increases the risk of burring out, when exposed to atmospheric air.

•As a result, an extra filament is provided as a standby.

•The chemical reaction within the residual gas at high temperatures, produces undesirable gases.

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