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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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