The transmitter has two pressure ports:
o High Pressure (HP) side
o Low Pressure (LP) side

• The measured differential pressure is:
  ΔP=Phigh−Plow\Delta P = P_{high} – P_{low}ΔP=Phigh−Plow
  Step-by-Step Working
  1. Pressure Application
• Process pressure is applied to both sides:
  o HP side → higher pressure
  o LP side → lower pressure
  2. Sensing Element (Diaphragm)
• Inside the transmitter, a flexible diaphragm (usually stainless steel) separates the two pressures
• The diaphragm deflects based on the pressure difference
  More pressure difference = more diaphragm movement
  3. Conversion to Electrical Signal
  Different sensing technologies convert this movement into an electrical signal:
  a) Capacitive Type
• Diaphragm movement changes capacitance
• Capacitance change ∝ pressure difference
  b) Piezoresistive (Strain Gauge)
• Strain changes resistance of sensor elements
• Resistance change converted into voltage
  c) Resonant / Silicon Sensor
• Frequency change in sensor due to pressure
  4. Signal Processing
• The raw signal is very small
• Internal electronics:
  o Amplify signal
  o Apply temperature compensation
  o Linearize output
  5. Output Signal
• Final output is:
  o 4 mA → Minimum DP
  o 20 mA → Maximum DP
• Can also provide digital output (HART, Modbus, etc.)
  Flow Measurement Principle (Important Use Case)
  When used with flow elements (like orifice plate):
  Q∝ΔPQ \propto \sqrt{\Delta P}Q∝ΔP
• Flow rate (Q) is proportional to the square root of differential pressure
• Higher flow → higher pressure difference
  Example Applications
  Flow Measurement
• Across orifice plate / venturi / flow nozzle
  Level Measurement
• In closed tanks:
  o Bottom pressure = liquid head
  o Top pressure = vapor pressure
  o DP gives actual level
  Filter Monitoring
• Measures pressure drop across filters
• High DP = clogged filter