Transcript Thermodynamics-review

Pressure Measurements
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Pressure measurements related to the fluid systems
are the topic of this chapter.
Absolute pressure refers to the absolute value of the
force per unit area exerted on the containing wall by a
fluid.
Gage pressure represents the difference between the
absolute pressure and the local atmospheric (atm)
pressure.
Vacuum represents the amount by which the
atmospheric pressure exceeds the absolute pressure
Pressure Measurements
P(absolute)
Positive gage pressure
Atmospheric pressure
Negative gage pressure or vacuum
Pressure Measurements
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The standard SI unit for pressure is the (N/m ) or
Pascal (Pa).
1 atm = 1.01325x10 Pa
= 760 mmHg
Fluid pressure results from a momentum exchange
between the molecules of the fluid and a containing
wall.
Pressure Measurements
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For an ideal gas, the pressure is given by:
P = (n)(k)(T)
where:
n: is the molecular density
k=1.3803x10-23 J/molecule
T: absolute temperature
The mean path () is the average distance a molecule
travels between collisions
 = 2.27x10-5 T/P, T in K and P in Pa
Mechanical PressureMeasurements devices
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Mechanical devices offer the simplest means for
pressure measurements.
The barometer is a device used to measure the
atmospheric pressure
Consider the U-tube manometer shown in figure 6.3.
A pressure balance of the tube columns dictates that:
P – Pa = gh(m - f)
Bourdon-Tube Pressure Gage
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Refer to figure 6.7
Diaphragm and Bellows Gages
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Diaphragm and bellows
gages measures pressure
based on sensing the elastic
deformation of materials as
a result of pressure.
The diaphragm deflection
will be according to the
pressure difference. The
deflection is measured by
appropriate displacement
transducers or strain gages
P1
Diaphragm
P2
Diaphragm and Bellows Gages
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The deflection generally follows a linear variation
with P when the deflection is less than 1/3 the
diaphragm thickness.
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Consider figure 6.12 for a bellows gage.
The pressure difference causes the bellows
movements which may be converted into electrical
or mechanical signal.
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LVDT can be used as a pressure gage…(figure 6.14)
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The LVDT
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The linear variable differential transducer (LVDT)
assembled with a diaphragm can be used as a
differential pressure gage. (see figure 6.14)
The displacement of the core is connected with the
diaphragm movement, which is in turn, indicates the
pressure difference P2-P1.
The Bridgman Gage
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It is known that the resistance of fine wires changes
with the pressure according to:
R = R1 (b + P)
where: R1: is the resistance at 1 atm
b: pressure coefficient of the resistance
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This gage can measure pressures as high as 100,000
atm
The Pirani Thermal-Conductivity
Gage
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It is known that, at low pressures, the effective thermal
conductivity of gases decreases with pressure. The Pirani
gage is a device that measures the pressure through the
change in thermal conductance of the gas.
The amount of heat loss-which depends on the gas
conductance- from a heated filament wire, located in a
vacuum space, indicates to the vacuum value.
To bridge circuit
To vacuum space
The Pirani Thermal-Conductivity
Gage
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Another way that is used to sense the vacuum is
through measuring the variation in resistance of the
filament material.
Pirani gages measure vacuums in a range of 0.1-100
Pa since the thermal conductance changes very little
above these pressures.
The Alphatron
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The Alphatron is a radioactive ionization gage, as
shown in figure 6.20.
A small radium source emits alpha particles which
ionize the gas inside the enclosure. The ionization
degree is determined by measuring the output voltage
Eo. The Eo is in fact linearly directed with the vacuum
connected to the gage enclosure.
The measuring range for this gage is 0.1 to 10 Pa.