lesson 2 Flashcards
is the force per unit area that a liquid or gas exerts on its surroundings,
such as the force or pressure of the atmosphere on the surface of the Earth, and
the force that liquids exert on the bottom and walls of a container.
Pressure
is not only an important parameter for process control, but also as an indirect
measurement for other parameters.
Pressure
are a measure of force acting over unit area. It is
most commonly expressed in pounds per square inch (psi) or
sometimes pounds per square foot(psf) in English units; or
Pascals (Pa) in metric units, which is the force in Newtons per
square meter (N/m2 ).
Pressure units
The pressure at a specific depth in a liquid is termed _______. The pressure increases
as the depth in a liquid increases. This increase is due to the weight of the fluid above the
measurement point.
hydrostatic pressure
This increase is due to the weight of the fluid above the measurement point. The pressure p is given by:
where is the specific weight (lb/ft3 in English units, or N/m3 in SI units), and h is the distance
from the surface in compatible units (e.g., ft, in, cm, or m).
p =yh
is defined as the density of a material divided by the
density of water. _______ also can be defined as the specific weight of the material divided by the
specific weight of water at a specified temperature.
Specific Gravity (SG)
Both ________and _______ are
temperature-dependent parameters, so that the temperature should be specified when they are
being measured.
specific weight and density
The specific gravity of a gas is its density (or specific weight) divided by the density (or specific weight) of ____ air at 60°F and 1 atmospheric
pressure (14.7 psia)
air
Many industrial processes operate at pressures that are referenced to atmospheric pressure, and
are known as _____________.
gauge pressures
is not a fixed value, but
depends on factors such as humidity, height above sea level, temperature, and so forth.
Atmospheric pressure
is the upward force exerted on an object immersed or floating in a liquid. The weight is
less than it is in air, due to the weight of the displaced fluid.
Buoyancy
he upward force on the object
causes the weight loss, called the buoyant force, and is given by:
B = yV
B is the buoyant force in pounds, is the specific weight in pounds per cubic foot, and V is
the volume of the displaced liquid in cubic feet
Measuring Instruments
U-tube manometers
silicon diaphragm
Vacuum measuring devices
are good examples of pressure measuring instruments, although they are not as common as they
previously were, because of the development of new, smaller, more rugged, and easier to use
pressure sensors
Manometers
are a major group of sensors that measure pressure with respect to atmospheric pressure. _______ are usually devices that change their shape when pressure is applied.
These devices include diaphragms, capsules, bellows, and Bourdon tubes
Gauges
consist of a thin layer or film of a material supported on a rigid frame. Pressure can be applied to one side of the film for gauge sensing, with the other inlet port being left open to
the atmosphere.
Diaphragms
_________ diaphragms are now in common use. Since ______ is a semiconductor, a piezoresistive
strain gauge and amplifier electronics can be integrated into the top surface of the ________ structure.
Silicon diaphrams
are two diaphragms joined back to back. Pressure can be applied to the space between
the diaphragms, forcing them apart to measure gauge pressure. The expansion of the diaphragm
can be mechanically coupled to an indicating device. The deflection in a capsule depends on its
diameter, material thickness, and elasticity.
Capsules
are similar to capsules, except that instead of being joined directly together, the
diaphragms are separated by a corrugated tube or a tube with convolutions. parated by a
corrugated tube or a tube with convolutions, as shown in Figure 7.7. When pressure is applied to
the bellows, it elongates by stretching the convolutions, rather than the end diaphragms
Bellows
are hollow, flattened, or oval cross-sectional beryllium, copper, or steel tubes
Bourdon tubes
The operating principle is that the outer edge of the cross section has a larger
surface than the inner portion. When pressure is applied, the outer edge has a proportionally
larger total force applied because of its larger surface area, and the diameter of the circle
increases. The walls of the tube are between 0.01 and 0.05 in thick.
Bourdon Tubes
are used for measuring atmospheric pressure. A simple ____________ was the mercury
in glass _________ , which is now little used due to its fragility and the toxicity of the mercury. The
aneroid (no fluid) __________ is favored for direct reading [e.g., the bellows in Figure 7.7, or the
helical Bourdon tube in Figure 7.9(b)], and the solid state absolute pressure sensor is favored for
electrical outputs.
Barometers
____________ pressure gauge is shown in Figure 7.10. __________ crystals produce a voltage
between their opposite faces when a force or pressure is applied to the crystal. This voltage can
be amplified, and the device used as a pressure sensor
Piezoelectric
devices have good sensitivity, a wide
operating temperature (up to 300°C), and a good frequency response (up to 100 kHz), but are not well suited for low frequency (less than 5 Hz) or for dc operation, due to offset and drift caused by temperature changes (pyroelectric effect). _________ devices are better suited for
dynamic rather than static measurements.
Piezoelectric
are used to measure pressures less than atmospheric pressure. Bourdon
tubes, diaphragms, and bellows can be used as __________ gauges, but they measure negative
pressures with respect to atmospheric pressure
Vacuum instruments
is shown in Figure 7.11(b). With special setups and thermocouples, the ____________ can measure vacuums down to approximately 1 torr (10−3 atm). These methods are based
on the relation of heat conduction and radiation from a heating element, to the number of gas
molecules per unit volume in the low-pressure region, which determines the pressure.
Pirani gauge
is a device set up to measure low pressures (1 torr). The device compresses the low-pressure gas to a level at which it can be measured. The change in volume and pressure
then can be used to calculate the original gas pressure, providing the gas does not condense.
McLeod gauge
Application Considerations
When installing pressure sensors, care should be taken to select the correct pressure sensor for
the application. This section gives a comparison of the characteristics of the various types of
pressure sensors, installation considerations, and calibration
are chosen for pressure range, overload requirements, accuracy,
temperature operating range, line-of-sight reading, electrical signaling, and response time. In
some applications, there are other special requirements.
Pressure sensing devices
Parameters such as hystersis and
stability should be obtained from the
manufacturer’s specifications
is a good choice for direct visual
readings, and the silicon pressure sensor for the generation of electrical signals [8]. Both types of
devices have commercially available sensors to measure from 5 psi FSD, up to 10,000 psi (70
MPa) FSD.
Bourdon tube
The following should be taken into consideration when installing pressure-sensing
devices.
- The distance between the sensor and the source should be kept
to a minimum. - Sensors should be connected via valves for ease of
replacement. - Overrange protection devices should be included at the sensor.
- To eliminate errors due to trapped gas in sensing liquid
pressures, the sensor should be located below the source. - To eliminate errors due to trapped liquid in sensing gas
pressures, the sensor should be located above the source. - When measuring pressures in corrosive fluids and gases,
an inert medium is necessary between the sensor and
source, or the sensor must be corrosion-resistant. - The weight of liquid in the connecting line of a liquid
pressure sensing device located above or below the source
will cause errors at zero, and a correction must be made by
the zero adjustment, or otherwise compensated for in
measurement systems. - Resistance and capacitance can be added to electronic
circuits to reduce pressure fluctuations and unstable
readings.
Pressure sensing devices are calibrated at the
Factory
Accurately calibrated standards can be obtained from the
National Institute of Standards and Technology
Low-pressure devices
can be calibrated against a
liquid manometer
High-pressure devices can be
calibrated with a
deadweight tester
