Pressure Sensors Flashcards
It is the force exerted by gases and liquids due to their weight
Pressure
pressure formula
Force/Area
defined as the mass per unit
volume of a material.
Density
defined as the weight per
unit volume of a material.
Specific weight
It is defined as the density
of a material divided by the
density of water or it can be
defined as the specific
weight of the material
divided by the specific
weight of water at a
specified temperature.
Specific gravity
the pressure of fluids or
gases that are stationary or
not in motion.
Static pressure
the pressure exerted by a
fluid or gas when it impacts
on a surface or an object
due to its motion or flow.
Dynamic pressure
(total pressure) is the sum
of the static and dynamic
pressures on a surface or
object.
Impact pressure
Which is zero pressure or
lack of pressure, as would
be experienced in outer
space.
Total vacuum
Is a pressure measurement made between total vacuum and normal atmospheric pressure (14.7 psi).
Vacuum
Is the pressure on the earth’s surface due to the weight of the gases in the earth’s atmosphere and is normally expressed at sea level as 14.7 psi
or 101.36 kPa.
Atmospheric pressure
is the pressure measured with
respect to a vacuum.
Absolute pressure
is the pressure measured with
respect to atmospheric pressure.
Gauge pressure
is the pressure measured with respect to another pressure and is expressed as the difference between the two values. This would represent two points in a pressure or flow system
Differential pressure
is the pressure in a liquid. 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
states that the pressure at a given depth in
a liquid is independent of the shape of the
container or the volume of liquid contained.
The pressure value is a result of the depth
and density.
Hydrostatic paradox
states that the pressure applied to an
enclosed liquid (or gas) is transmitted to all
parts of the fluid and to the walls of the
container.
Pascal’s law
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
It is a good
examples of pressure
measuring instruments,
though they are not as
common as they used to be
because of the development
of new, smaller, more
rugged, and easier-to-use
pressure sensors.
Manometers
consist of U-shaped glass tubes partially filled with a liquid.
When there are equal pressures on both sides, the liquid levels will
correspond to the zero point on a scale
U-tube manometer
were developed to measure low pressures. The low-pressure
arm is inclined, so that the fluid has a longer distance to travel than
in a vertical tube for the same pressure change. This gives a
magnified scale
Inclined manometers
are alternatives to inclined manometers for measuring low
pressures using low-density liquids. In the _____, one leg has a much larger diameter than the other leg, When there is no pressure difference the liquid levels will be at the same height for a zero reading
Well-type manometer
These are a major group of
pressure sensor that measure
pressure with respect to
atmospheric pressure. These are usually devices
that change their shape when
pressure is applied. These
devices include diaphragms,
capsules, bellows, and
Bourdon tubes
Gauges
consists of a thin layer or film of a material
supported on a rigid frame and is shown in Fig. 5.8a. Pressure can be applied to one side of the film for gauge sensing or pressures can be applied to both sides of the film for differential or absolute
pressure sensing
diaphragm
are two diaphragms joined back to back, as shown in Fig. 5.8b.
Pressure can be applied to the space between the diaphragms
forcing them apart to measure gauge pressure
capsule
are similar to capsules, except that the diaphragms instead of
being joined directly together, are separated by a corrugated tuben or tube with convolutions, as shown in Fig. 5.8c. When pressure is
applied to the bellows it elongates by stretching the convolutions and not the end diaphragms.
bellow
These are hollow, cross-sectional beryllium, copper,
or steel tubes, shaped into a three quarter circle. They may be rectangular or oval in cross section, but 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.
Bourdon tubes
are used for measuring atmospheric pressure. It is now rarely used due to its fragility and the toxicity of mercury
Barometers
Piezoelectric 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 pressure gauge
use the change in capacitance between the sensing
diaphragm and a fixed plate to measure pressure
Capacitive devices
are used to measure pressures less than atmospheric pressure.
The Bourdon tube, diaphragms, and bellows can be used as vacuum gauges, but measure negative pressures with respect to atmospheric pressure.
Vacuum instruments
it can measure vacuums down to about 5 torr. 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 and special setups using thermocouples
can be used to measure pressures down to about 2 torr. The gas is ionized with
a beam of electrons and the current is measured between two electrodes in the gas.
Ionization gauges
is a device set up to measure very low pressures, i. e., from 1 to 50 torr. The device compresses the low-pressure gas so that the increased pressure can be measured
McLeod gauge
Application considerations (Pressure Sensors)
- Selection
- Installation
- Calibration
