Pressure Measurement
• Pressure measurement is the analysis of an applied
force by a fluid (liquid or gas) on a surface. Pressure
is typically measured in units of force per unit of
surface area.
Barometer
Absolute pressure is zero-referenced against a perfect vacuum, using an
Absolute scale so it is equal to gauge pressure plus atmospheric
pressure.
Gauge pressure is zero-referenced against ambient air pressure, so it is
equal to absolute pressure minus atmospheric pressure. Negative signs
are usually omitted.
Differential pressure is the difference in pressure between two points
LOW PRESSURE MEASUREMENT
• It is impossible to measure very low pressure under
atmospheric pressure(<1.013 bar) with ordinary
pressure measuring technique
• Special devices have been developed for measurement
of pressures in the vacuum range below atmospheric
pressure
• These special devices include thermocouple ,
• the Pirani gauge
• the thermistor gauge
• the McLeod gauge and the ionization gauge.
What is Vaccum ?
• Vacuum is space devoid of matter.
• An approximation to such vacuum is a region with a gaseous pressure
much less than atmospheric pressure
• The quality of a partial vacuum refers to how closely it approaches a
perfect vacuum.
• Other things equal, lower gas pressure means higher-quality vacuum.
• For example, a typical vacuum cleaner produces enough suction to
reduce air pressure by around 20%.Much higher-quality vacuums are
possible. Ultra-high vacuum chambers, common in chemistry, physics,
and engineering, operate below one trillionth (10−12) of atmospheric
pressure (100 nPa), and can reach around 100 particles/cm3.Outer
space is an even higher-quality vacuum, with the equivalent of just a
few hydrogen atoms per cubic meter on average in intergalactic
space.
THERMAL CONDUCTIVITY
• The ability of a material to carry heat by
conduction is called “thermal conductivity”.
• As a real gas increases in density -which may
indicate an increase in pressure- its ability to
conduct heat increases.
• Temperature is dependent on the rate at
which the filament loses heat to the
surrounding gas, and therefore on the thermal
conductivity.
The relationship between changes in conductivity and changes in pressure work
over a pressure range from about 10^-4 torr upto about 10^-2 torr.
Working Principle
• Enclosing a wire filament in a chamber connected to the
Pressure source
• When voltage is applied to the filament, electricity flows,
making it hot. The rising temperature increases the
resistance of the filament.
• The filament then reaches an ‘equilibrium temperature’,
the temperature at which heat is produced in the filament
as fast as it is removed.
• The change in conductivity changes the equilibrium
temperature of the filament, which in turn causes the
change in the resistance.
• The change in resistance is used to indicate the pressure
change.
Thermal conductivity Gauges
• There are two types of thermal conductivity gauges
a) Pirani Gauge : It consists of two wire filaments.
b) Thermocouple Gauge : filament temperature is
measured by a thermocouple.
Pirani Gauge
STRUCTURE
The Pirani gauge consists of a metal filament (usually platinum)
suspended in a tube which is connected to the system whose
vacuum is to be measured. Connection is usually made either by a
ground glass joint or a flanged metal connector, sealed with an oring. The filament is connected to an electrical circuit from which,
after calibration, a pressure reading may be taken
Construction
The main parts of the arrangement
are:
• A pirani gauge chamber which
encloses a platinum filament.
• A compensating cell to minimize
variation caused due to
ambient temperature changes.
• The pirani gauge chamber and the
compensating cell is
housed on a wheat stone bridge
circuit as shown in diagram.
Applications of Pirani gauge
• Used to measure low vacuum and ultra
high
• vacuum pressures.
Advantages of Pirani gauge
• Good response to pressure changes
• Relation between pressure and
resistance is linear
for the range of use.
• Readings can be taken from a distance.
Limitations of Pirani gauge
• Pirani gauge must be checked frequently.
• Pirani gauge must be calibrated from
different gases.
• Electric power is a must for its operation.
THAT’S ALL FOR TODAY
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