Understanding Static and Dynamic Pressure

• Pressure is a fundamental thermodynamic parameter and is an intensive property
• Pressure is defined as the ratio of force applied to a unit area: p = F/A
• p is pressure, F is the force, A is the area over which force is applied
• Pressure measures how much force is exerted on a particular area
• Force causes movement, either accelerating or decelerating

Pressure of a Fluid (P)

• Fluid molecules are in constant random motion, called "Brownian motion"
• Brownian motion causes the fluid to exert force on the container walls, even at rest
• The total pressure of a fluid at a given point has two components

Static Pressure

• Also known as "hydrostatic pressure"
• Refers to the pressure exerted by a fluid at rest
• Measured by a gauge moving with the fluid
• Not influenced by the fluid's motion

Dynamic Pressure

• Pressure associated with fluid movement
• Measures the kinetic energy of the fluid
• Depends on the velocity and density of the fluid

History of Pressure Measurement

• Late 16th century: Galileo Galilei (1564-1642) found water rose no higher than 10 meters in a suction pump
• 1643: Evangelista Torricelli (1608-1647) invented the barometer to measure atmospheric pressure, discovering the vacuum with mercury columns
• Five years later: Blaise Pascal used the barometer to show air pressure decreases at higher altitudes, determining the weight of air and named it "pressure"
• In 1849: Eugène Bourdon patented the Bourdon tube, still used today for relative pressure measurement

Key Reasons for Measuring Pressure

• Safety
• Process Efficiency
• Cost Saving
• Inferred Measurement of Other Variables

Types of Pressure Measurement

Absolute Pressure

• Measured relative to a perfect vacuum
• The pressure difference at a specific point compared to vacuum pressure (absolute zero)
• Denoted with "ABS"

Differential Pressure

• Pressure difference between two points
• Measured when one point is not at vacuum or atmospheric pressure

Gauge Pressure

• Measured relative to ambient or atmospheric pressure

Manometers

• Simple devices used to measure pressure
• Work by measuring the height of a fluid column
• It is a fluid-filled tube, usually U-shaped
• Gas pressure is applied, the fluid height shifts in proportion to the pressure
• Differential pressure instrument
• Measures the difference between two pressures by observing shift in liquid height

Working Principle of Manometer

• The term "manometer" comes from the Greek words "manós" (thin or rare) and "métron" (measure)
• Operates on hydrostatic equilibrium, which states that the pressure at any point in a fluid at rest is equal to the weight of the overlying fluid
• A typical manometer is a U-shaped tube filled with an incompressible fluid, can be water or mercury
• Inexpensive, requires no calibration
• Measures static pressure exerted by a still liquid or gas

Types of Manometer

U-Tube Manometers

• Consists of a glass tube bent in a 'U' shape
• Balancing the liquid column is done by another column of the same or different liquid
• One end of the U-tube is connected to the point where pressure is measured
• The other end is open to atmospheric pressure
• Liquid levels on either side of the tube indicate the pressure difference

Well Manometers

• Features a well area larger than the tube's area
• Measures rise in liquid level in the tube
• Rise in the well is neglected
• Used to measure the difference between two absolute pressures applied at the ends of the tube

Raised Well Manometers

• Similar to well-type manometers in design
• Has a vertical column limb inclined at an angle (θ)
• Inclined angle allows for more accurate measurement of small pressures
• Ideal for precise applications

Sphygmomanometer

• Sphygmomanometer, is a type of manometer commonly used to measure blood pressure
• Systolic pressure shows when the pulse is first heard
• Diastolic pressure is when the pulse is no longer audible.

Digital Manometer

• Uses microprocessor and pressure transducer to detect slight pressure changes
• The pressure is then displayed on a digital screen
• Measures differential pressure across two inputs
• Outputs pressure in analog or digital form in proportion to the instantaneous pressure

Mechanical Pressure Elements

Bellows

• Bellows resemble a metal accordion
• Thin-walled metallic cylinders with deep convolutions
• One end is sealed and free to move, while the other is fixed
• When pressure is applied to the closed end, the bellows compress
• Moves the closed end upward
• Movement is transmitted via a link to rotate a pointer, indicating pressure.

Diaphragms

• Thin disks of material (usually metal) that bow outward under fluid pressure
• Can have spring-like qualities or be slack for use with external restraining mechanisms
• Fluid pressure pushes on and bends the membrane outward
• As external pressure increases, the membrane bends back
• The degree of bending corresponds to the pressure measurement

Bourdon Tubes

• Spring-like metal alloys bent into a circular shape
• Under internal pressure, the tube attempts to straighten
• When pressurized, the Bourdon tube straightens slightly
• Creates a movement that drives a pointer
• The stiffness of the tube determines its suitability for high-pressure applications

Electrical Pressure Elements

Piezoresistive Sensors

• Operates on the principle of resistance changes under applied force
• Strain gauges are a classic example
• Strain gauge measures strain by detecting resistance changes caused by deformation
• Deformation happens when a force is applied to an object
• Principles allow for measuring stress, strain, and related variables like pressure, force, and acceleration

Differential Capacitance Sensors

• Measures pressure based on capacitance changes between two metal plates
• Plates are separated by an insulating material
• When pressure is applied, the distance between the plates or the dielectric properties of the insulator change
• Results in a variation in capacitance
• Capacitance change is proportional to pressure and is used to infer its value

Differential Pressure Transmitters

• Designed to measure the pressure difference between two points
• Feature two ports, labeled "high" and "low", to sense the respective pressures
• Device outputs a signal that represents the difference between these pressures, not their absolute values