The Bourdon tube is a mechanical pressure sensing element based on elastic deformation.

• It consists of a C-shaped, coiled, or helical tube that is oval in cross-section and sealed at one end.
• Principle: When pressurized fluid enters the tube, the oval cross-section tries to become circular due to the internal pressure.
• This change in cross-section forces the tube to straighten out (uncoil).
• The movement of the sealed tip is proportional to the pressure applied.
• A mechanical linkage connects this tip movement to a pointer on a dial, converting the physical deformation into a readable pressure value.

This uses the principle of electrical resistance change due to physical deformation.

• A strain gauge (a metallic foil pattern) is bonded to a flexible diaphragm.
• Process: When process pressure is applied to the diaphragm, it deflects or stretches.
• This stretching causes the strain gauge grid to elongate or compress.
• Result: The elongation increases the electrical resistance of the wire, while compression decreases it.
• This change in resistance is measured using a Wheatstone Bridge circuit, which converts the resistance change into an electrical signal (mV) proportional to the pressure.

Capacitance transmitters measure pressure by detecting changes in capacitance between a sensing diaphragm and capacitor plates.

• The sensor contains a central sensing diaphragm situated between two fixed metal plates (capacitor plates), filled with a dielectric oil.
• High pressure is applied to one side and low pressure to the other.
• Deflection: The pressure difference causes the sensing diaphragm to deflect towards the lower pressure side.
• Capacitance Change: This deflection changes the distance between the diaphragm and the fixed plates. As distance changes, capacitance changes.
• The electronics detect this shift in capacitance and convert it into a 4-20mA output signal.

Piezoelectric sensors are used primarily for measuring dynamic pressures (rapid changes) rather than static pressure.

• Material: They use specific crystals (like quartz) that generate an electric charge when mechanically stressed.
• Action: When pressure presses against the crystal, the crystal lattice deforms.
• Effect: This deformation creates a separation of charge, generating a voltage across the crystal faces.
• The voltage generated is directly proportional to the force (pressure) applied.
• These are self-generating sensors and require no external power source to produce the signal, though they need amplifiers for signal transmission.

A diaphragm seal isolates the pressure transmitter from the process fluid, used for corrosive, viscous, or high-temperature fluids.

• Isolation: A flexible isolation diaphragm is placed in contact with the process fluid.
• Transmission: The space between this diaphragm and the transmitter sensor is filled with a non-compressible fill fluid (like silicone oil) inside a capillary tube.
• Working: Process pressure pushes on the isolation diaphragm.
• The diaphragm pushes against the fill fluid.
• Because the fluid is non-compressible, it hydraulically transmits the pressure through the capillary directly to the transmitter's sensing element.

Orifice plates work on the principle of Differential Pressure (Bernoulli’s Principle).

• Restriction: An orifice plate is a metal plate with a hole (bore) placed in the pipe. It creates a restriction to the flow.
• Velocity Increase: As fluid passes through the hole, its velocity increases significantly to get through the smaller area.
• Pressure Drop: According to energy conservation, when velocity increases, static pressure decreases.
• This creates a pressure difference (differential pressure) between the upstream and downstream sides of the plate.
• The square root of this pressure drop is proportional to the flow rate. A DP transmitter measures this drop to calculate flow.

Magmeters work on Faraday’s Law of Electromagnetic Induction.

• Requirement: The fluid must be conductive (like water, acids, or slurries).
• Field Generation: Coils outside the flow tube generate a constant magnetic field across the pipe.
• Conductor: The conductive fluid acts as a moving conductor cutting through this magnetic field.
• Voltage Generation: As the fluid moves through the field, a voltage is induced perpendicular to both the flow direction and the magnetic field.
• Electrodes mounted on the pipe walls measure this induced voltage.
• The magnitude of the voltage is directly proportional to the velocity of the fluid.

Coriolis meters measure mass flow directly, independent of temperature or pressure changes.

• Vibration: The meter contains one or two tubes that are vibrated at their natural frequency by a drive coil.
• Flow Effect: When fluid flows through these vibrating tubes, the inertia of the fluid resists the vibration.
• Twisting: This resistance causes the tube to twist. The inlet side of the tube lags behind the vibration, and the outlet side leads the vibration.
• Phase Shift: Sensors measure the time difference (phase shift) between the inlet and outlet vibrations.
• The amount of twist (phase shift) is directly proportional to the Mass Flow Rate.
• Additionally, the frequency of the vibration is related to the fluid's density (denser fluids cause slower vibration).

This meter works on the Von Karman Vortex Street principle.

• Bluff Body: An obstruction (bluff body or shedder bar) is placed in the flow stream.
• Shedding: As fluid hits the obstruction, it separates and forms alternating vortices (swirls) on either side of the bar.
• Frequency: These vortices detach at a specific frequency.
• The frequency at which these vortices are shed is directly proportional to the fluid velocity.
• A sensor (usually piezoelectric) detects the pressure pulses created by each vortex passing by and counts the frequency to determine flow rate.

Ultrasonic meters use sound waves to determine fluid velocity. There are two main types:

• Transit Time (For Clean Liquids): Two transducers send sound waves back and forth—one with the flow and one against it. The signal traveling with the flow arrives faster. The meter calculates flow based on the time difference between the two signals.
• Doppler Effect (For Dirty/Aerated Liquids): Sound waves are transmitted into the pipe and reflect off particles or bubbles in the fluid. Because the particles are moving, the frequency of the reflected wave changes (Doppler shift). The magnitude of the frequency shift is proportional to the flow velocity.

A mechanical flow meter that uses the fluid's energy to spin a rotor.

• Rotor: A turbine rotor with blades is mounted in the flow path, parallel to the flow direction.
• Spin: The moving fluid impinges on the blades, causing the rotor to spin.
• Proportionality: The rotational speed of the rotor is directly proportional to the fluid velocity.
• Pickup: A magnetic pickup coil mounted outside the pipe detects the passage of each blade tip (via magnetic reluctance).
• The coil generates a frequency pulse train where the number of pulses corresponds to the volume of fluid passing through.

A Rotameter consists of a tapered vertical tube and a float.

• Setup: The tube is tapered, meaning it is narrow at the bottom and wide at the top. Fluid flows upwards.
• Forces: The fluid flow exerts an upward drag force on the float, lifting it. Gravity acts downwards.
• Equilibrium: As the float rises, the annular area (gap) between the float and the tube wall increases.
• This increased area allows fluid to pass more easily, reducing the lifting force.
• The float stabilizes at a height where the upward drag force equals the downward gravitational weight of the float. This height indicates the flow rate on a scale.

Thermal mass meters measure flow based on heat transfer (thermal dispersion).

• Probes: The sensor typically has two temperature sensors immersed in the flow. One is a reference sensor measuring fluid temperature; the other is a heated sensor.
• Cooling Effect: As gas flows past the heated sensor, molecules carry heat away, cooling the sensor.
• Measurement: The meter measures either the power required to maintain a constant temperature difference between the probes OR the temperature difference change while keeping power constant.
• Relationship: Higher mass flow removes more heat. The rate of heat loss is directly proportional to the mass flow rate of the gas.

RTDs operate on the principle that the electrical resistance of a pure metal changes with temperature.

• Material: Typically made of Platinum (Pt100 is common), Copper, or Nickel.
• PTC: Metals have a Positive Temperature Coefficient (PTC). As temperature increases, the metal lattice vibrates more, impeding electron flow, thus increasing resistance.
• Linearity: Platinum is chosen because its resistance change is very linear and stable over a wide range.
• Measurement: The instrument passes a small constant current through the RTD element and measures the resulting voltage drop to calculate resistance, which is then mapped to temperature.

Thermocouples work based on the Seebeck Effect.

• Junctions: Two dissimilar metal wires are joined at one end to form a "Hot Junction" (measurement point). The other ends form the "Cold Junction" (reference point) at the transmitter.
• Gradient: When there is a temperature difference between the hot junction and the cold junction, a small voltage (EMF) is generated.
• Composition: The magnitude of this voltage depends on the types of metals used (e.g., Type K is Chromel/Alumel) and the temperature difference.
• The transmitter measures this millivolt signal and uses a "Cold Junction Compensation" calculation to determine the actual process temperature.

This mechanical gauge relies on the differential thermal expansion of metals.

• Construction: Two strips of metals with different coefficients of thermal expansion (e.g., steel and copper) are bonded together.
• Heating: When heated, one metal expands more than the other.
• Bending: This unequal expansion forces the bonded strip to bend or curl towards the side with the lower expansion rate.
• Display: The strip is usually wound into a helix. As it curls/uncurls with temperature changes, it rotates a shaft connected to a pointer on a dial.

A Thermistor is a thermally sensitive resistor made of semiconductor material (ceramic/polymer).

• Sensitivity: They have a much higher resistance change per degree of temperature than RTDs, making them extremely sensitive to small changes.
• NTC: Most are Negative Temperature Coefficient (NTC), meaning their resistance decreases drastically as temperature increases (opposite of metals).
• Usage: While highly sensitive, they are extremely non-linear and generally have a limited temperature range compared to RTDs or Thermocouples.

Pyrometers measure temperature without physical contact by analyzing emitted radiation.

• Planck's Law: All objects above absolute zero emit thermal radiation (infrared energy).
• Intensity: The intensity and spectral distribution of this radiation depend directly on the object's temperature.
• Optics: The pyrometer uses a lens to focus the emitted infrared energy onto a detector (thermopile or photodiode).
• Conversion: The detector converts the thermal energy into an electrical signal.
• The device calculates temperature based on the signal strength and the object's Emissivity (ability to emit energy).

Both detect smoke but use different physics:

• Ionization (Fast Flames): Contains a tiny radioactive source that ionizes the air between two plates, creating a current. When smoke enters, smoke particles attach to ions, reducing mobility and disrupting the current flow, triggering the alarm.
• Photoelectric (Smoldering Fires): Uses a light source (LED) and a sensor aimed away from the light. When smoke enters the chamber, it scatters the light beam. The scattered light hits the sensor, triggering the alarm.

Flame detectors look for specific spectral signatures emitted by fire.

• Spectral Emission: Burning fuels (hydrocarbons) emit high levels of infrared radiation at specific wavelengths (specifically the CO2 spike around 4.3 to 4.4 microns).
• Flickering: Flames also flicker at a unique frequency (1-10 Hz).
• Detection: The IR detector senses radiation specifically in the CO2 band.
• Logic: To prevent false alarms (from sunlight or heaters), it checks for both the threshold intensity of the CO2 spike AND the characteristic flicker frequency before alarming.

Used for detecting combustible gases (LEL monitoring).

• The Beads: Contains two platinum coils embedded in ceramic beads: an active bead (coated with a catalyst) and a reference bead (no catalyst).
• Heating: Both beads are electrically heated.
• Combustion: When combustible gas meets the active bead, the catalyst causes the gas to burn (oxidize) on the surface, even at concentrations below the ignition point.
• Temperature Rise: This combustion generates heat, raising the temperature of the active bead only.
• Resistance Change: The temperature rise increases the resistance of the platinum coil. A Wheatstone bridge circuit detects the imbalance between the active and reference beads.

Used for detecting gas clouds over long distances (Line of Sight).

• Setup: Consists of a Transmitter unit and a Receiver unit placed meters apart (e.g., across a fence line).
• Beam: The transmitter shoots a beam of infrared light to the receiver.
• Absorption: Hydrocarbon gases absorb IR light at specific wavelengths.
• Attenuation: If a gas cloud crosses the beam, it absorbs some of the IR energy. The receiver detects a drop in light intensity at the gas-specific wavelength but not at the reference wavelength.
• The ratio of absorption determines the total amount of gas in the path (measured in LEL-meters).

Used primarily for toxic gases (H2S, CO) and Oxygen.

• Cell Structure: Contains a working electrode, a counter electrode, and an electrolyte.
• Reaction: The target gas diffuses through a membrane into the sensor.
• Oxidation/Reduction: The gas undergoes a chemical reaction (oxidation or reduction) at the working electrode surface.
• Current: This reaction generates or consumes electrons, creating a tiny electric current flow between the electrodes.
• The magnitude of this current is directly proportional to the gas concentration (parts per million - ppm).

It distinguishes between normal temperature fluctuations and fire-induced heat.

• Chamber: It typically contains an air chamber with a flexible diaphragm and a small calibrated vent (leak hole).
• Slow Heat: If air heats slowly (weather), the expanded air escapes through the vent, maintaining pressure equilibrium.
• Rapid Heat (Fire): If temperature rises quickly, air expands faster than it can bleed through the vent.
• Trigger: This creates positive pressure inside the chamber, pushing the diaphragm to close an electrical contact and trigger the alarm.

pH measurement is potentiometric (voltage-based).

• Glass Electrode: Uses a special glass bulb that is sensitive to Hydrogen ions (H+). Inside is a buffer solution of known pH.
• Potential Difference: When dipped in the process liquid, H+ ions interact with the outer glass surface. A millivolt potential develops across the glass layer depending on the difference in H+ concentration between the inside buffer and the outside process liquid.
• Reference: A reference electrode provides a stable baseline voltage.
• Nernst: The analyzer measures the voltage difference between the glass electrode and reference electrode. This voltage changes by roughly 59mV per pH unit.

Commonly used in flue stacks for high-temperature combustion control.

• Electrolyte: Uses a Zirconium Oxide ceramic cell coated with platinum electrodes.
• High Temp: At temperatures above 600°C, Zirconia acts as a solid electrolyte that allows oxygen ions to pass through it.
• Migration: If the oxygen concentration differs on the two sides of the cell (Reference Air side vs. Flue Gas side), oxygen ions migrate through the ceramic lattice.
• Voltage: This ion movement generates a voltage potential (EMF) inversely proportional to the oxygen concentration in the flue gas (like a battery). Lower O2 in the stack generates higher voltage relative to the air reference.

Conductivity is the ability of a solution to carry an electric current.

• Ions: Pure water is an insulator. Dissolved salts/acids/bases dissociate into ions, which conduct electricity.
• Contact Method: Two metal electrodes are placed in the fluid. An AC voltage is applied. The analyzer measures the current flow between them. Higher ion concentration = higher current = higher conductivity.
• Inductive (Toroidal) Method: Uses two coils encased in plastic (no metal contact). One coil induces a magnetic field, creating a current loop in the liquid. The second coil detects the magnitude of this induced current, which depends on liquid conductivity.

GCs separate a mixture into individual components to measure quantity.

• Injection: A precise sample of gas is injected into a stream of "Carrier Gas" (like Helium).
• Column: The mix travels through a long, thin tube (column) coated with a stationary phase material.
• Separation: Different components travel at different speeds based on their interaction with the column lining (boiling point/polarity). Lighter/non-interacting molecules exit first; heavier ones exit later.
• Detection: As components exit the column one by one, a detector (like TCD or FID) senses them.
• Chromatogram: The result is a graph of peaks. The time of the peak identifies what the gas is; the area under the peak identifies how much there is.

Used for high-precision Oxygen measurement.

• Unique Property: Oxygen is strongly "paramagnetic," meaning it is attracted to a magnetic field (most other gases are diamagnetic/repelled).
• Dumbbell: Two glass spheres filled with Nitrogen (inert) are suspended on a taut wire inside a non-uniform magnetic field.
• Displacement: When sample gas containing Oxygen enters the cell, the Oxygen molecules are attracted to the strongest part of the magnetic field.
• Rotation: The Oxygen pushes the Nitrogen spheres out of the field, causing the dumbbell assembly to rotate/torque.
• The force required to keep the dumbbells stationary (or the degree of rotation) is directly proportional to the Oxygen concentration.

Used to measure trace moisture (dew point) in gases.

• Sensor: Consists of an aluminum base, an anodized layer of aluminum oxide (porous), and a thin gold coating on top. This forms a capacitor (Metal-Insulator-Metal).
• Adsorption: Water vapor molecules from the gas permeate through the gold and adsorb onto the porous aluminum oxide layer.
• Dielectric Change: Water has a very high dielectric constant compared to the oxide. As water adsorbs, the capacitance of the sensor increases significantly.
• The analyzer measures this capacitance change, which corresponds to the partial pressure of water vapor (Dew Point).

A solenoid valve converts electrical energy into mechanical motion.

• Coil: It consists of a wire coil wrapped around a hollow tube.
• Plunger: Inside the tube is a movable ferromagnetic plunger (core) held in place by a spring.
• Energization: When electric current flows through the coil, it creates a magnetic field.
• Movement: The magnetic field pulls the plunger against the spring force.
• Valving: The movement of the plunger opens or closes a small orifice, allowing air or fluid to flow or stop. When power is cut, the spring pushes the plunger back to its original position.

This distinction determines the pressure capability of the valve.

• Direct Acting: The electromagnetic force of the plunger directly opens the main valve orifice. It works even at 0 bar pressure but requires a large, powerful coil for high pressures.
• Pilot Operated (Servo): The solenoid plunger controls a tiny "pilot" port.
  1. When the pilot opens, it releases pressure from the top of a main diaphragm/piston.
  2. The line pressure of the fluid itself pushes the main diaphragm up to open the valve.
  Note: These require a minimum differential pressure (e.g., 10 psi) to operate.

The numbers refer to Ports / Positions.

• 3/2 Way SOV: Has 3 Ports (Supply, Output, Exhaust) and 2 Positions.
  Use: Used for Single Acting Actuators (Spring Return). It sends air to the actuator to open it, and when de-energized, it exhausts the air so the spring can close the actuator.
• 5/2 Way SOV: Has 5 Ports (1 Supply, 2 Outputs, 2 Exhausts) and 2 Positions.
  Use: Used for Double Acting Actuators. It sends air to one side to open while venting the other side, and reverses the process to close. No springs involved.

Fail Safe refers to the position the valve takes when electrical power or air supply is lost.

• Fail Closed (NC - Normally Closed): When power is lost, the spring forces the plunger to block the flow. To open it, you must energize it.
• Fail Open (NO - Normally Open): When power is lost, the spring forces the valve open. To close it, you must energize it.
• ETS (Energize to Safe): Power is required to trip the system (unsafe if power is cut).
• DTS (De-energize to Safe): Power is required to run. If power is cut (wire cut/fuse blown), the system trips to a safe state. This is the industry standard for safety systems (ESD).

A latching solenoid maintains its position without continuous power.

• Permanent Magnet: It typically incorporates a permanent magnet alongside the electromagnet coil.
• Operation: A short pulse of electricity moves the plunger. Once moved, the permanent magnet holds (latches) it in that position.
• Reset: To return the plunger, a pulse of reverse polarity (or a pulse to a second coil) is required to cancel the magnetic hold.
• Benefit: Extremely low power consumption and low heat generation, as power is only applied for milliseconds during switching.

The most basic position sensor based on physical contact.

• Actuator: Has an external arm, lever, or roller.
• Contact: When a moving object (like a valve stem) hits the lever, it physically moves a plunger inside the switch housing.
• Snap Action: This plunger pushes a set of electrical contacts, snapping them from Open to Closed (or vice versa).
• Reset: A spring returns the contacts to their original state when the object moves away.

Detects metal objects without physical contact.

• Oscillator: The sensor face contains a coil that generates a high-frequency electromagnetic field.
• Eddy Currents: When a metal object (target) enters this field, "Eddy Currents" are induced on the surface of the metal.
• Damping: These eddy currents drain energy from the sensor's field, causing the oscillation amplitude to drop (dampen).
• Trigger: A trigger circuit detects this drop in oscillation strength and switches the output state (On/Off).

Capacitive sensors can detect both metal and non-metal objects (liquids, plastics, grains).

• Field: It creates an electrostatic field rather than a magnetic field.
• Capacitor: The sensor face acts as one plate of a capacitor. The target object acts as the second plate.
• Dielectric: As the object approaches, it changes the dielectric constant in the field.
• Oscillation: This change increases capacitance, which starts an oscillator circuit (opposite of inductive). Once oscillation reaches a threshold, the switch triggers.

Uses sound waves to detect presence or distance.

• Pulse: The sensor emits a high-frequency sound burst (above human hearing).
• Echo: The sound waves travel through the air, hit an object, and bounce back to the sensor.
• Timing: The sensor measures the time taken for the echo to return.
• Switching: If the echo returns within a specific time window (meaning an object is within the set zone), the output switches on. It works well on clear, transparent objects that optical sensors might miss.

A simple magnetically actuated switch.

• Construction: Consists of two thin ferromagnetic metal blades (reeds) sealed inside a glass tube filled with inert gas.
• Gap: Normally, the reeds are separated by a small gap (Open).
• Magnetism: When a magnet approaches the glass tube, the magnetic field induces opposite poles on the two reeds.
• Attraction: The attraction between the opposite poles pulls the flexible reeds together until they touch, completing the circuit.
• Removing the magnet allows the reeds to spring apart.

The "Current Loop" is the industry standard for several reasons:

• Immunity to Noise: Current signals are not affected by electrical noise (voltage spikes) or voltage drops over long cable runs like voltage signals are.
• Live Zero: We use 4mA as "0%" instead of 0mA.
  - If the signal is 4mA, the process is at zero.
  - If the signal is 0mA, the wire is broken. This allows easy detection of wire breaks.
• Power: The 4mA minimum current is enough to power the internal electronics of the transmitter (2-wire loop powered), eliminating the need for extra power cables.

Converts an electrical 4-20mA signal into a pneumatic 3-15 psi output to drive control valves.

• Coil: The 4-20mA current flows through a coil located in a magnetic field.
• Force Balance: The magnetic force moves a flexure arm (flapper) relative to a nozzle.
• Flapper-Nozzle: As the flapper moves closer to the nozzle, it restricts air from escaping, increasing back-pressure.
• Amplification: A relay amplifies this small nozzle back-pressure into a high-volume pneumatic output (3-15 psi) proportional to the input current.

A positioner ensures the valve stem is exactly where the controller wants it to be, overcoming friction and line pressure.

• Input: Receives the 4-20mA command from the controller.
• Feedback: A mechanical arm measures the actual position of the valve stem.
• Comparison: It compares the Command vs. Actual Position.
• Correction: If there is an error (e.g., valve is at 40% but should be at 50%), the positioner adds or vents air supply to the actuator until the positions match exactly. It acts as a local closed-loop controller.

HART (Highway Addressable Remote Transducer) superimposes digital data on top of the analog 4-20mA signal.

• FSK: Uses Frequency Shift Keying.
• Overlay: It represents digital "1" as 1200 Hz and "0" as 2200 Hz sine waves.
• Average: Because these are high-frequency AC sine waves, their average value is zero. Therefore, they do not disturb the main DC 4-20mA analog signal.
• This allows two-way communication (configuration, diagnostics, tag names) while the process variable is still being transmitted via the analog loop.

PID calculates how to move a valve to keep a process at a Setpoint (SP).

• P (Proportional): Looks at the Present error. "The error is big, so I will make a big correction." Main drive but leaves an offset.
• I (Integral): Looks at the Past error. "The error has existed for a long time, so I will ramp up the output to eliminate it." Removes offset but can cause oscillation.
• D (Derivative): Looks at the Future rate of change. "The error is changing very fast, so I will hit the brakes to prevent overshooting." Adds stability.

Cavitation is a destructive phenomenon in liquid flow.

• Pressure Drop: As liquid passes through the valve restriction (vena contracta), velocity increases and pressure drops drastically.
• Vaporization: If pressure drops below the liquid's vapor pressure, bubbles form (flashing).
• Recovery: As the liquid moves past the valve, velocity slows and pressure recovers (rises).
• Collapse: When pressure rises back above vapor pressure, the bubbles implode violently.
• Damage: These implosions create microscopic shockwaves that pit and erode the valve trim and body.

• Sensor: A device that detects a physical parameter (temperature, pressure) and changes its physical state. (e.g., A thermocouple junction generates voltage; a Bourdon tube straightens out). It "senses" the change.
• Transducer: A broader term for a device that converts one form of energy into another. A sensor is often part of a transducer. (e.g., Converting the mechanical strain of a diaphragm into an electrical resistance change).
• In industry, "Transmitter" is the device that packages the sensor/transducer and conditions the signal to a standard output (4-20mA).

• Deadband: A range of input where there is no change in output. (e.g., A steering wheel that you can turn slightly left or right without the car wheels moving). It is a zone of insensitivity.
• Hysteresis: The difference in output for the same input depending on whether the input is increasing or decreasing. (e.g., A pressure switch might trip at 100 psi going up, but reset at 90 psi going down). It is path-dependent.

Two methods to prevent explosions in hazardous areas.

• Explosion Proof (Exd): "Contain the bang." The device housing is heavy and strong. If gas enters and explodes inside, the housing contains the pressure and cools the escaping hot gas so it doesn't ignite the atmosphere outside.
• Intrinsic Safety (IS): "Limit the energy." Uses Zener Barriers in the control room to limit the voltage and current going to the field device. The energy is kept so low that it physically cannot create a spark hot enough to ignite the gas, even if wires are shorted.

The most common pneumatic actuator for control valves.

• Chamber: Consists of two halves separated by a flexible rubber diaphragm.
• Forces: Air pressure is applied to one side of the diaphragm, pushing against a large spring on the other side.
• Movement: As air pressure increases, the force overcomes the spring compression, moving the actuator stem (and the valve plug).
• Equilibrium: The valve stops moving when the force of the air pressure exactly equals the force of the compressed spring.